|
|
See also Colorado Rocks, The Earth At WorkLast modified 10/18/04
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
The 'About Time' icon |
Geologic understanding requires careful attention to time as well as three-dimensional space, so we'd better stop here first. If you're unfamiliar with geologic time terminology and abbreviations, take that detour now.
With over 50 million modern human lifetimes elapsed since the planet formed around 4.5 Ga, geologic or deep time is, to say the least, difficult to grasp. But the effort pays, for with a feel for deep time comes a sense of its great power: Given enough time, almost anything energetically possible can happen—even at very large scales. And so it goes with the bending of seemingly rigid rocks, the cutting of majestic canyons, the raising and erasing of entire mountain ranges, the opening and closing of globe-girdling oceans, and the incessant splitting and regrouping of the dancing continents.
Our usually reliable day-to-day sensibilities tell us that such things can't happen, but they can and do happen because solid rock reveals its malleability only over time scales very long compared to human events—typically in spans of tens of thousands if not millions of years. Given enough time, cold surface rock will yield to flowing water and buried rock will bend or even flow rather than break. The planet's had ample time for all of this, even if it's out of our ken.
 |
Upturned sedimentary strata at the mouth of Boulder Canyon |
Under great confining pressures, or at depths where temperatures reach a significant fraction of their melting points (typically 10-15 km), rocks that are quite brittle at the surface become sufficiently plastic to deform without fracture at rates comparable to the rate at which fingernails grow (~10 mm/yr). Granted, that makes molasses look downright mercurial, but then relative viscosity is the whole idea here. My piano tuner's job security rests on the fact that properly tuned piano wire flows (stretches) at a similar rate and falls out of tune in a matter of months. Over a few Ma, fingernail speed is plenty fast enough to fold great thicknesses of sedimentary rock over the east edge of the relatively brittle Front Range basement block, as in the photo at right. Over 50 Ma, an entire mountain range like the Ancestral Rockies can rise up and vanish.
The pink feldspar bands in the severely deformed
metamorphic rock at right (a mylonite from the Homestake Shear Zone
in the northern Sawatch Range) show the kind of internal folding you'd expect
from warm taffy, but this rock didn't actually melt. The wavy fabric indicates
solid flow at mid-crustal temperatures of 400-500°C.
To recast barely conceivable geologic time intervals in a more familiar format, they'll also appear from time to time as military hours in a single 24-hour day (and all-nighter) of creation beginning at the planet's formation and ending with the here and now.
With the formation of the earth pegged at 00:00:00 h (midnight), our work day finishes at 24:00:00 h, just as we pull up to the present. We'll ignore seconds until we get close to 24:00 h. Each hour corresponds to 0.875 billion years (Ga), each minute to 3.125 million years (Ma) and each second to 52.1 thousand years (Ka). Conversely, 1 Ga takes 4 hours, 20 minutes out of the day; 1 Ma, 19.2 seconds; and 1 Ka, 19.2 milliseconds—literally the blink of an eye.
After an introductory section, Colorado's geologic history will unfold in chronological order below. As we come up on major divisions along the geologic timeline, a brief "About Time" header will summarize what event or fossil record breakpoint ushered in the upcoming division and show how it's subdivided. The table below collects the links to these headers.
Links to "About Time" Headers |
||
| Geologic Interval | Years Before Present | Hours Since Earth Formed |
| Precambrian Time | 4.5 Ga - 543 Ma | 00:00 - 21:06 |
| Proterozoic Eon | 1.6 Ga - 543 Ma | 15:28 - 21:06 |
| Phanerozoic Eon | 543 - 0 Ma | 21:06 - 24:00 |
| Paleozoic Era | 543-248 Ma | 21:06 - 22:41 |
| Mesozoic Era | 248-65 Ma | 22:41 - 23:39 |
| Cenozoic Era | 65-0 Ma | 23:39 - 24:00 |
| Pleistocene Epoch | 1.8 Ma - 10 Ka | 21:06 - 23:59:59.81 |
| Holocene Epoch | 10-0 Ka | 23:59:59.81 - 24:00:00 |
Note: Ga stands for a billion years; Ma, a million; Ka, a thousand; and 0, the present. |
||
Notice how even deep time can fly in proper perspective: The Precambrian was admittedly a slow start, taking until mid-evening to unfold on our day of creation, but then life as we know it appeared only in the last 3 hours. The entire Mesozoic Era (295 Ma long) shot by in under an hour, the dinosaurs departed just 20 minutes ago (thank goodness), and we've been comfy and relatively ice-free for only a mere 0.2 seconds. A healthy human female can expect to live just under 2 milliseconds in a developed country.
If you need more help with geologic time or terminology, check out these resources.
"I would have written a shorter letter if I'd had more time." — Benjamin Franklin
"It requires a very unusual mind to undertake the analysis of the obvious." — Alfred North Whitehead
"Science is a way of trying not to fool yourself." — Richard Feynman
"The universe is not only queerer than we suppose, it's queerer than we can suppose." — J. B. S. Haldane
"Civilization exists only with geologic consent subject to change without notice." — Will Durant
 |
Mount Sopris |
Colorado's geologic history is as complex as it is fascinating. The geology is in turn inextricably entwined with Colorado's human history. That's true everywhere, of course, but it goes double here. Colorado's generous endowments of accessible mineral wealth and fertile farmland were not inevitable birthrights. Things could have turned out quite differently.
This overview ventures a "to the best of our knowledge" summary of Colorado geologic evolution current as of late 2004. Colorado's story still includes many gaps and controversies, often around events and structures shrouded in deep time, deep earth or both. I've attempted to point out the most significant unknowns and disagreements, but often, rightly or wrongly, I've simply taken sides.
If you take nothing else away from this humble attempt, please consider this: Of Colorado's 55 Fourteeners, all but 2 (Longs Peak and Pikes Peak) lie either along the Colorado Mineral Belt or on the shoulders of the Rio Grande Rift. Most of them cluster around the intersection of these two profound lineaments, both which almost certainly cut the full thickness of the lithosphere. This telling elevation distribution reflects a powerful synergy between truly ancient plate processes driven by the cooling of the earth in the presence of gravity and ongoing mantle processes driven by Lord knows what.
Colorado spans three major North American geomorphic provinces roughly organized in distinct north-south bands:
|
The Rocky Mountains in the center, imposing by any standard; |
|
 |
Maroon Bells, Elk Range |
||
|
Tablelands to the west, including portions of the Colorado Plateau; and |
|
|
Echo Park, Dinosaur NP |
||
|
High Plains, ramping down to the east toward the Mississippi Valley. |
|
|
East of Kiowa |
This geologic overview focuses primarily on the Colorado Rockies and the Colorado Plateau, but adjoining portions of Wyoming, Utah and the High Plains as far east as the Black Hills of South Dakota also have important and related stories to tell. Throughout this article, I'll refer to Colorado and these adjoining areas as our region of interest, which closely coincides both with the area shown in the NASA photo at Bearings just below and also with the area affected by Laramide uplift.
 |
Western U.S. tectonic upland |
Colorado's also part of an immense, heterogeneous and longstanding tectonic upland covering the entire western third of the United States and ultimately can only be understood in that context. From the Middle Proterozoic on, much of Colorado's story also applies, with some notable exceptions, throughout the Southern Rockies* and the Colorado Plateau. Events in the Basin and Range and along the West Coast have also influenced Colorado's physical development, particularly from ~80 Ma on.
 |
Laramide Orogen from space, NASA |
Let's pause now to get oriented within and around our region of interest. If you haven't done so already, click on the true-color ^NASA Visible Earth satellite photo at right to open a larger version in a new browser window. Find Four Corners, where four state lines meet at right angles. Colorado holds down the northeast quadrant; New Mexico, Arizona and Utah run clockwise from there.
Snow caps highlight the highest of the green forested Rocky Mountain uplifts, all of which first rose in the region-defining Latest Cretaceous through Early Tertiary Laramide orogeny. The uplifts are highest and most densely packed in western Colorado, but they clearly extend well to the north and south. Important outliers include the Black Hills uplift of South Dakota (clipped in the NE corner) and the Uinta Mountains and the Wind River Range in the NW quadrant of the photo. The largest Laramide uplift, the Front Range, dominates the north central portion of Colorado. The highest Laramide uplift, the Sawatch Range, stands on the west shoulder of the Rio Grande rift, the narrow north-south gash between the two largest snow packs near the center of the photo. Further south, the rift is visible as the broad almond-shaped San Luis Valley of south-central Colorado. The narrower Rio Grande Valley, heading directly south from the San Luis Valley, is also part of the rift. (Keep your eye on the Rio Grande rift as we come up on post-Laramide events below. It's a key player in the development of the current landscape.)
In the southwest quadrant of the photo, exposed iron-stained sedimentary rocks nicely mark the Colorado Plateau (CP) in salmon pink. The CP saw action in the Laramide Orogeny but gained most of its elevation via post-Laramide epierogenic uplift. Four Corners lies near the east central margin of the Colorado Plateau.
You'll encounter many similar satellite photos and physiographic maps throughout this site because, in many important ways, the land speaks for itself. Anyone interested in learning to read the landforms will some find very powerful interactive tools at NASA's ^Visible Earth and ^Earth Observatory sites and at UNAVCO's ^Jules Verne Voyager site.
To a large extent, the Colorado Rockies and the Colorado Plateau have evolved together since 1.8 Ga or so, but since the onset of the Laramide Orogeny at around 72 Ma, the rigid, block-like Colorado Plateau has been something of a loose cannon. To complicate matters further, shared events have played out rather unevenly in the two provinces with somewhat disparate timing.
 |
Colorado in context |
The dominant earth processes shaping our region of interest provide a convenient framework for both discussion and understanding. Each set of events has overprinted preceding events in its own way, sometimes preserving and sometimes destroying traces of what came before. Disentangling these layers of process in both space and time makes for a fine puzzle. Geoscientists have gathered many of the pieces on the table and have even fit a good many of them together, but the geologic record is still full of holes, and more pieces have been lost to burial and erosion than anyone would like.
Plate tectonic processes have shaped Colorado since their inception at ~2.0 Ga, but Colorado hasn't been within 1,000 km of a credible plate margin for at least 1.4 Ga. The intracontinental deformations so beautifully in evidence now in Colorado reflect both far-field stresses generated at remote plate boundaries and cryptic upper mantle processes not directly related to plate interactions. One recent, strongly overprinting intraplate process to keep an eye on as we proceed is the Rio Grande rift, which has been busy rearranging the central and south central Colorado landscape in big ways since at least 28 Ma.
Another important influence to track throughout this overview is climate. It's importance is easy to overlook when you're focused on the rocks and how they've moved. Many earth processes play out at depth, well beyond the reach of the atmosphere and hydrosphere. But for processes like weathering, erosion, sediment transport, deposition, diagenesis, isostatic rebound and basin subsidence, the rubber meets the road at the surface, where the atmosphere, the hydrosphere and the land all interact strongly to shape both land and climate in a never-ending dance. Streams cut Colorado's deepest canyons in only the last 10 Ma. The relative contributions of wetter climate and uplift remain topics of contentious debate among geoscientists.
Defining Earth Processes in Colorado's Geologic History |
||
| Time |
Process |
Noteworthy Results |
|
1.78-1.65 Ga |
The southern margin of Laurentia, the nascent North American continent, accreted Colorado's oldest Precambrian basement rocks. |
|
|
1.45-1.35 Ga |
This mysterious and protracted event added copious 1.4 Ga granites, northeast-trending ductile shear zones and differential uplifts to the Precambrian basement throughout the Southwest. It also left behind basement-cutting rift faults that subsequent orogenies (mountain-building events) would reuse for their own purposes. |
|
|
1.2-1.1 Ga |
The voluminous Pikes Peak Batholith intruded the basement of the southern Front Range and the adjacent High Plains. |
|
| 1.4-0.6 Ga | Late Proterozoic continental rifting | Continental rifting related primarily to the Berthoud Orogeny and to the later breakup of the supercontinent Rodinia created enduring basement faults throughout the western 2/3 of the US. The compressive Ancestral Rocky Mountain and Laramide orogenies would later reactivate these faults in reverse. |
| 1.1 Ga - 510 Ma | Great Unconformity | Erosion planed off the differentially uplifted and rifted Precambrian basement during this immense gap in Colorado's rock record. With few exceptions, all pre-existing sedimentary cover was lost, and no new cover accumulated during this time. |
| 510-300 Ma | Early Paleozoic marine sedimentation | The planed-off Precambrian basement accumulated a 2-3 km-thick blanket of flat-lying tropical marine sediments. |
|
300-248 Ma |
The Ancestral Rockies rose along reactivated Proterozoic rift faults as two large island mountain ranges that were eventually buried in their own debris. |
|
| 248-72 Ma | Mesozoic sedimentation | Terrigenous sediments accumulated widely over Colorado through earliest Cretaceous time. Marine sediments blanketed the state from then on, right up to the Laramide orogeny. |
|
72-40 Ma |
Basement-cored uplifts rose again along reactivated Proterozoic rifts, this time in greater numbers as the current Rocky Mountains, the Colorado Plateau and, to a lesser extent, the High Plains. Associated volcanism left behind dozens of Laramide intrusions strongly clustered within the Colorado Mineral Belt (CMB) lineament. CMB mineralization had begun. |
|
| 36-5 Ma | Post-Laramide magmatism | Thick, widespread volcanic piles reorganized much of Colorado's topography and drainage patterns while intrusions peppered the Colorado Mineral Belt from below to complete its mineralization. |
|
28-0 Ma |
The Rockies and the Colorado Plateau rose to their present elevations with the onset of regional epierogenic uplift and extension, including the arrival of the Rio Grand rift. |
|
|
10-0 Ma |
Reinvigorated streams removed mass and incised channels to sculpt the uplands of the Rockies and the Colorado Plateau and eventually the range-front basins as well. |
|
|
1.8 Ma - 10 Ka |
Glaciers and their outwash streams deeply eroded the higher Laramide uplifts into their present dramatic topography but largely spared the topographically lower Colorado Plateau. |
|
| 10-0 Ka | Holocene uplift and erosion | Erosion- and mantle-driven regional uplift and stream erosion continue at slower paces; overall, the Rockies maintain their elevations in the face of erosion. |
The rocks and structures now exposed so beautifully at the surface in Colorado are important traces of all these processes, to be both admired and understood. (Far from diminishing the beauty, the understanding only enhances it, at least to my mind and eye.) Other less accessible but equally important clues come from geophysical investigations (tomography, magnetic and gravity surveys), geomorphic analyses (elevation models, stream courses and profiles), and mine, tunnel and borehole logs, among many other data sets.
Several recurring themes become apparent as one studies Colorado's evolution and current topography.
The interior section of the continent now graced by the Rockies and the Colorado Plateau has been a seat of unrest for nearly 1.8 Ga. Far-field plate tectonic stresses have reshaped the landscape here on and off since the Colorado Province mobile belt joined the continent in Early Proterozoic time; upper mantle stresses have been in on the act since at least 28 Ma.
 |
Four Corners from space, NASA |
Basement rocks and faults loom large here, controlling topography, sedimentation and even subsequent tectonics. Since at least the time of the Ancestral Rockies, reactivated basement-penetrating reverse and thrust faults of Proterozoic rifting ancestry have dominated the mountain-building style throughout the Rockies and the Colorado Plateau. These faults have typically uplifted large blocks cored with hard Precambrian crystalline rocks and have folded and occasionally even broken the overlying sedimentary cover. Thus, flat-topped highlands seldom showing more than 15° of tilt came to be the dominant large-scale landforms, not just in the tablelands of the Colorado Plateau, but even in the highest parts of the Rockies. Faulted sedimentary monoclines showing substantial and opposing dips typically flanked the uplifts.
Antecedent streams armed with steep gradients and generous fluxes of water and sediment entrenched their courses at a time (in the mid- to late Tertiary) when the uplands generally showed little relief, and the streams were free to meander. Later, as uplift and climate synergized throughout the region, the upland streams had no choice but to incise deep, narrow canyons through rather than around buried uplifts of resistant basement and sedimentary cover throughout Colorado and Wyoming. Colorado's most spectacular antecedent canyons — e.g., the Black Canyon of the Gunnison, Glenwood Canyon and Royal Gorge — have become world-famous scenic attractions, but lesser antecedent canyons riddle the Rockies and plateau country.
Mid- to Late Tertiary magmatism profoundly altered the landscape throughout the Rockies and the Colorado Plateau, but most of the direct evidence (the volcanic rock) has been lost to erosion, particularly in the Rockies. The indirect effects of this intense period of magmatism are still visible today in places like the Florissant Fossil Beds National Monument — if you know where to look. Associated intrusions added great wealth to the Colorado Mineral Belt and left us jewels like Mt. Sopris.
These recurring threads are worth keeping in mind as we now launch into the particulars of Colorado geology.
Most sections in this article close with a "Map Units" subsection describing how to find pertinent bedrock (surface rock) units on the Geologic Highway Map of Colorado.
To geoscientists, the "big picture" is a highly-prized and often hard-won 4D understanding of a landscape and its past at local to global scales. Here, I sketch the big picture in Colorado, as best I can put it together, from past to present. Subsequent sections will flesh out the details, also in chronological order.
 |
Modern Banda Sea |
The land we now call Colorado was first assembled in the Early Proterozoic between 1.78 Ga and 1.65 Ga (14:30-15:12 h) from a hodgepodge of island arcs, backarc basins and more mature continental fragments docking or developing against a backstop provided by the Archean Wyoming Province at the southern margin of the nascent North American continent. The mobile belt added to the continent during this time is known as the Colorado Province. Despite a long-standing intracontinental location, it's been unstable ever since.
The assembly of the Colorado Province resembled in some respects the Early Proterozoic assembly of northeast Australia, which has changed little since then and therefore has a history much easier to unravel than Colorado's oft-overprinted story. In other important respects, the gathering Colorado Province must have resembled the swarm of oceanic and continental elements now coalescing feverishly (by geologic standards) in the closing ^Banda Sea of Eastern Indonesia above.
 |
Buffalo Mountain |
Around 1.7 Ga, in the midst of this accretional frenzy, large quantities of granite and associated pegmatites intruded the middle to upper crust throughout the region, as at Buffalo Mountain (right) and ^Rocky Mountain National Park. Colorado intrusive rocks with radiometric dates in the 1.660-1.790 Ga range are sometimes referred to collectively as the Routt Plutonic Suite.
 |
Mount Evans from Denver |
A large number of granitic intrusions, ductile shear zones, differential basement uplifts and rifts peppered the Colorado Province, along with the rest of the continent, in the Berthoud orogeny at 1.45-1.35 Ga (16:16-16:48 h). Colorado's many intrusive rocks with radiometric dates in the 1.400-1.450 Ga range are sometimes referred to collectively as the Berthoud Plutonic Suite.
At 1.2-1.1 Ga (17:36-18:08 h), granite widely intruded the continent once again in a rather mysterious event often called the Grenville orogeny, but this time, Colorado took only one known hit — albeit a big one — in the Pike's Peak area.
The Berthoud and Grenville orogenies appear to have occurred in response to convergent plate interactions playing out far to the south. The many granitic intrusions at 1.4 Ga and the one massive batholith (very large intrusion) at 1.1 Ga added considerably to and effectively completed the Colorado basement as we know it today.
 |
Afar Region Rifting |
Over the long span of Proterozoic time, several major intracontinental rifting events repeatedly tried to pull the western two-thirds of the US apart, Colorado included, just as the East African Rift splits northeast Africa today. Some of the rifts may have originated during the Berthoud and Grenville orogenies, while others clearly developed in Late Proterozoic time at 0.9-0.6 Ga (19:12-20:48 h) around the breakup of Rodinia, the planet's earliest known supercontinent.
Although an ocean-forming continental break-up never ensued in or around Colorado, the recurrent rifting left behind a rhombic network of deep-seated intersecting normal (extensional) basement faults. The E-W and NE-SW extensional stresses driving the rifting produced primarily north- and northwest-trending normal faults and pull-apart basins. (The particularly deep west-trending Uinta rift basin was a notable exception.) Many of the rift faults probably cut the entire lithosphere.
[map coming] |
Rift map from Marshak |
During the Phanerozoic Ancestral Rocky Mountain and Laramide orogenies to follow, regional crustal shortening would exploit the old rift faults by reactivating them in reverse — hence the broad north- and northwest-trending tectonic grains still obvious in any shaded relief map of the modern Rockies and the Colorado Plateau. During the Laramide, two surviving rift basins filled with Precambrian sediments (now the Uinta and Uncompahgre formations) were squeezed and inverted to form the Uinta uplift and the southern (San Juan) portion of the Uncompahgre uplift as well.
Around 28 Ma, rifting returned to Colorado with the arrival of the Rio Grande rift.
Between 1.1 Ga and 510 Ma (18:08-21:17
h), erosion reigned supreme throughout Colorado. With
rare but notable exceptions, all Precambrian sediments were removed, along with ~10 km of
basement. This enigmatic period is marked by a 0.6-1.1 Ga-long gap in the
Colorado rock column known as the Great
Unconformity. Colorado spent most of the ensuing Early
Paleozoic underwater, accumulating great thicknesses of tropical marine sediments
(primarily sandstones, limestones and shales) laid down flat on the planed-off
Late Precambrian basement surface.
 |
Maroon Bells |
Around 300 Ma (22:24 h), the Ancestral Rocky Mountains rose quickly to moderate heights (probably no more than 10,000' above sea level) along reactivated Proterozoic rifts, apparently in response to a continent-continent collision between North America's predecessor Laurentia and the supercontinent Gondwana far to the south. The two largest island mountain ranges developed in Colorado—Frontrangia on the east and Uncompahgria on the west. By 250 Ma (22:40 h), just 50 Ma later, both ranges had completely succumbed to erosion, their debris piled high across half the state.
The mostly nonmarine Late Paleozoic to Early Mesozoic sediments shed off these highlands accumulated to great depths in large range-front basins—in the Denver basin to the east, in the Paradox basin to the west and in the Maroon Basin in between. We have the Ancestral Rockies to thank for the Maroon Bells (above) that now grace the Elk Range.
|
Four Corners, NASA |
Around 72 Ma (23:37 h), the Laramide Orogeny began to buckle up basement from southern Montana to central New Mexico—and to a lesser extent in the High Plains and the Colorado Plateau—along reactivated, basement-penetrating Proterozoic rift faults. The regional deformation apparently occurred in response to the onset of accelerated, low-angle (flat-slab) subduction of the young Farallon plate beneath the western margin of North America. The resulting basement-cored faulted uplifts and their flanking sedimentary monoclines continue to define the broad features of Rocky Mountain and Colorado Plateau topography. The ever stalwart Colorado Plateau moved ~100 km north as a rigid unit during the Laramide but saw less uplift and internal deformation than the neighboring Rockies.
As always, the Laramide uplifts began to erode as soon as they began to rise. By Oligocene time, their debris had filled the structural basins surrounding the uplifts to overflowing. Today, the stream-dissected syntectonic strata of the upper Denver Basin expose a clear record of the progressive unroofing of the rising Front Range block — a process repeated throughout the Laramide orogen.
The Laramide died out around 40 Ma (23:47 h), first in the north and finally in the south. Along the way, Laramide magmatism got the mineralization of the Colorado Mineral Belt off to a good start.
 |
West Elk Range |
Between 36 Ma and 5 Ma (23:48-23:58 h), intense regional magmatism boiled up through Colorado, leaving the Eocene erosional surface deeply buried under volcanic flows and ejecta and the subsurface shot through with intrusions that would elevate and metamorphose pre-existing sediments and inject mineral wealth all along the Colorado Mineral Belt. The causes of this fiery outburst remain obscure. Over most of the state, the far-flung Tertiary volcanics have been long lost to erosion, but the San Juan Mountains and the West Elk Mountains (right) are major exceptions. Nevertheless, the topographic legacy of the volcanic era lives on in the head-scratching courses of some of Colorado's largest antecedent streams. Before their removal by erosion, volcanic accumulations helped to reposition streams directly over several then-buried Laramide uplifts. The Black Canyon of the Gunnison is but one of the results.
 |
Front Range Pediment |
Starting around 28 Ma (23:51 h), in the Late Oligocene, epierogenic uplift probably related at least in part to the approach of the Rio Grand rift domed up the entire region to present average elevations with little tilting. This uplift added over 1.5 km (5,000') to the Colorado skyline. The greatest elevation gains centered on the intersection, near Leadville, of two of Colorado's most profound structural lineaments—the Colorado Mineral Belt and the Rio Grand rift.
Even with concomitant erosion, the nearby Colorado Plateau managed a net 6,000' elevation gain—much of it in the last 5 Ma alone. During the this time, the cohesive Colorado Plateau once again moved as a unit relative to the Rockies, this time to the northwest to accommodate the opening of the Rio Grand rift.
Today, with the help of erosion-driven isostatic rebound, mantle-driven regional uplift continues, albeit at a much slower pace, maintaining the height of the Rockies and the Colorado Plateau in the face of ongoing erosion.
 |
Painted Wall |
Exhumation of the Laramide Rockies and the Colorado Plateau began in the mid-Tertiary and kicked into high gear in the Late Miocene around 10 Ma (23:57 h). Stream incision was swift and deep, and intermontane basins were cleared of their sediments, thanks to continuing regional uplift and a wet Late Tertiary climate. Antecedent streams setting their courses on the low-relief Eocene erosional surface eventually found themselves cutting through buried uplifts that would become the ridges and even ranges of the current Colorado landscape. The range-front basins were uncovered and incised as well.
 |
Gore Range |
By Pliocene time, the Rockies and the Colorado Plateau had been thoroughly dissected by stream erosion but continued to stand high between the deep canyons. The devastating glaciations that would come next, primarily in Pleistocene time, around 1.8 Ma (23:59:25 h), took the project of exhuming the Rockies to new highs and new lows. Nearly everywhere above 8,000', the glaciers gouged out cirques, canyons and U-shaped valleys, dammed lakes, diverted streams, moved house-size boulders like so much furniture, and dispatched massive floods of outwash water and sediment to erode again at lower elevations. The less lofty Colorado Plateau largely escaped the ice.
 |
Maroon Bells |
When the last of the glaciations finally melted away at the close of the Tertiary around 10 Ka (23:59:59.81 h), Colorado looked much as it does today—and quite likely more beautiful than ever before. Purple mountain majesty indeed.
The sections to follow will attempt to flesh out the details of Colorado's most formative processes in chronological order.
The final condensation of planet Earth from the solar nebula at 4.5 Ga (00:00
hours on our metaphoric "day" of creation) kicked off the Hadean Eon (4.5-3.8
Ga, 00:00-03:44 h).
When the intense extraterrestrial bombardment defining the Hadean Eon fell off, the Archean
Eon (3.8-2.5 Ga, 03:44-10:40 h) of initial continent building began. The
long Proterozoic
Eon (2.5 Ga - 543 Ma, 10:40-21:06 h) followed. The current Phanerozoic
Eon (543-0 Ma, 21:06-24:00 h) began when complex life forms built on current
(not Edicarian) plans
first appeared explosively in the fossil record at the beginning of the Cambrian
Period (543-490 Ma, 21:06-21:23 h), which kicked off the Paleozoic
Era (543-248 Ma, 21:06-22:41 h) and ended Precambrian Time (4.5 Ga - 543
Ma, 00:00-21:06 h).
The Precambrian includes the Proterozoic, Archean and Hadean Eons, but most Precambrian rocks exposed in Colorado and elsewhere in the region date from the Proterozoic. At the surface of the earth today, Archean rocks are fairly uncommon and Hadean rocks are very rare.
Only a tiny sliver of Archean rock crops out in Colorado's northwest corner. But just across the Wyoming and Utah state lines, the stiff southern margin of a Late Archean craton (ancient continental core) known as the Wyoming Province served as a backstop for the Early Proterozoic assembly of the Precambrian basement now underpinning the entire southwestern United States, Colorado included.
 |
Structure of the Earth, source USGS |
Our knowledge of the earth in the Archean Eon (3.8-2.5 Ga, 03:44-10:40) is necessarily dim and incomplete, but parts of the puzzle are coming together, and they don't look all that familiar. As the Archean Eon opened, the earth was still piping hot if not frankly molten following its condensation from the solar nebula around 4.5 Ga and a merciless pummeling by stray solar system debris during the Hadean Eon (4.5-3.8 Ga, 00:00-03:40). Moreover, radioactive uranium, thorium and potassium isotopes concentrated primarily in the crust and upper mantle generated radiogenic heat at 2-3 times the current rate. Throughout the Archean, crust and mantle were both much hotter than today's, and earth processes operating then differed considerably from those at work in the last 2.0 Ga.
When and how the earth developed its current internal crust/mantle/core structure remains controversial, but evidence now points to rapid and thorough chemical differentiation of a largely molten earth very near the time of its formation, probably by ~4.4 Ga. This process released more heat, particularly with the fall of iron to form the planet's core. The important discontinuity between the upper and lower mantle at 650 km below the surface probably also developed at this time, although at a shallower depth in the hotter early mantle. The "650" has served as a fundamental barrier to material transport, both up and down, ever since.
Hydrated low-density sialic (silicon- and aluminum-rich) crustal residues floating on a global ocean of dense hydrated mafic (magnesium- and iron-rich) magma probably developed near the time of formation at 4.5 Ga and may have persisted well into the Archean. Buoyant patches of sialic crust eventually coalesced to form a number of large, durable, unsinkable rafts that eventually became the cratons (stable cores) of our modern continents—e.g., the Wyoming Province and the Canadian shield. Through fractionation and underplating of subjacent mantle, these early Archean cratons developed thick (>100 km) hulls and very deep (>250 km) keels of cool, strong, buoyant and refractory lithospheric mantle that to this day protect them from disruption by heat and currents in the deeper upper mantle.
Most of the planet's supply of sialic continental crust formed in the hot early Archean via high-temperature processes never repeated since. Much of that crust was recycled into the upper mantle prior to the advent of plate tectonics at ~2.0 Ga. The remaining Archean cratons have been tied up in continental cores ever since. Continental crust formed since that time, primarily at post-Archean collisional and convergent plate boundaries, has been much more mantle-like (mafic) than the surviving Archean cratons.
 |
LITHOPROBE map of North American basement rocks |
At the close of the Archean Eon, a nearby 3.2-2.1 Ga craton called the Wyoming Province stood at the southwest margin of Laurentia, a northern supercontinent that would eventually be trimmed down to North America. To this day, the Cheyenne Belt lineament of southern Wyoming and northeast Utah marks the southern margin of the Wyoming Province.
The stalwart Wyoming craton played a pivotal role in the Early Proterozoic assemblage of Colorado's Precambrian basement by serving as a backstop for a long series of juvenile Early Proterozoic island arcs and more mature terranes rafted in from the south along a 1,300 km-wide, east northeast-trending convergent plate margin active around 1.78-1.65 Ga.
In the ^LITHOPROBE map of North American basement rocks at right, the Wyoming Province appears in medium blue. The broad band of very light gray 1,800-1,600 Ma basement to its south will become the Colorado Province, which in turn will become the basement for the southwestern United States, including Colorado.
Only one small fault-bound sliver of the Wyoming Province manages to nip across the Utah state line into the northwest corner of Colorado—the 2.3 Ga Red Creek quartzite at the northern margin of the Laramide Uinta uplift north of CO318 near Browns Park.
The Red Creek quartzite is the closest any Colorado surface rock gets to an Archean age. The rest are all at least 500 Ma younger, including the nearby Uinta Mountain Group, a thick mass of unrelated 1.7-0.9 Ga Precambrian sediments preserved in an inverted rift basin.
Unfortunately, the Red Creek Quartzite is too small to show on the 1:1,000,000 Geologic Highway Map of Colorado, but on Ogden Tweeto's 1:500,000 Geologic Map of Colorado, it appears as a tiny purple wedge marked "Wr" against the Utah state line ~10 km (6 miles) south of the northwest corner of Colorado.
|
Colorado in context |
Nowhere on the planet do Archean rocks record clear evidence of the plate tectonic processes dominating earth dynamics today—no large thrusts, no overturned sedimentary piles, no large-scale rifting or collisions, no subduction. Instead, Archean cratons like the Wyoming Province tend to consist of large domal bodies of primitive granite intruding folded greenstone (metavolcanic) belts and covered with odd volcanic and sedimentary supracrustal rock sequences like the Red Creek quartzite. The planetary processes underlying this pattern remain largely unknown, but this much is clear—they haven't been repeated since anywhere on the planet.
Exactly when and how water-filled oceans formed is also unknown, but they were clearly present by the time plate tectonics began to operate with the onset of subduction in the Earliest Proterozoic, around 2.0 Ga (14:24 h). In fact, you must first understand the oceans to understand the land, or at least the basement, in and around Colorado.
Since its Early Proterozoic assembly via subduction, Colorado has been far removed from active plate boundaries, but far-field effects from subsequent plate interactions to the west, south and east continued to play an important role in Colorado's evolution, both at the surface and at depth.
I won't digress to introduce plate tectonics here, but you'll find more than you need to know to understand Colorado's tectonic past and present at The Earth at Work on this site. Many of the explanatory links in this article lead there.
The Proterozoic Eon followed the Archean Eon (3.8-2.5
Ga, 03:44-10:40 h). Many
pivotal geologic and evolutionary biological developments of Precambrian Time
(4.5 Ga - 543 Ma, 00:00-21:06 h) occurred in the Proterozoic, including the initial build-up of
oxygen in the atmosphere and the global oxidation and precipitation of iron from
the oceans.
 |
Red Canyon, Colorado National Monument |
The oldest rocks exposed in Colorado are 1.78-1.65 Ga (14:30-15:12 h) granites and metamorphic rocks of Early Proterozoic age. These basement rocks crop out throughout the Rockies, often on Fourteener summits, as at Mount Evans, and also in the deeper canyons of the Colorado Plateau, as at the bottom of Red Canyon (right). These rocks accreted to the Archean North American continent in a long series of subduction zone collisions known as the Colorado Orogeny.
The name Colorado Province has been given to the large Early Proterozoic mobile belt added to the continent in this protracted event. The Colorado Province joined the Archean Wyoming Province along the oldest of Colorado's Early Proterozoic sutures, an enduring east northeast-trending defect in the lithosphere known as the Cheyenne Belt.
 |
Cheyenne Belt, Southern Wyoming |
Around 1.78 Ga, a south-dipping subduction zone developed off the southern margin of the Wyoming Province and built an offshore magmatic arc now known as the Green Mountain arc. Shortly thereafter, this juvenile arc collided with the Wyoming Province, driving intervening 2.1 Ga oceanic lithosphere to the north, beneath the southern edge of the Wyoming craton. The resulting suture is still discernable in satellite images as an east northeast-trending lineament across southern Wyoming and northeast Utah known as the Cheyenne Belt (CB). (The CB barely nips the northwest corner of Colorado.) Just south of the CB, highly metamorphosed fragments of the Green Mountain arc still crop out in a band across northern Colorado known as the Green Mountain Block.
[diagram coming] |
from GSA Today |
A remnant of the oceanic plate subducted in the Wyoming-Green Mountain collision may be preserved in the lithospheric mantle along the suture. In tomographic (cross-sectional) images, the CB appears distinctly as a steeply south-dipping discontinuity of lithospheric scale accompanied by marked localized crustal thickening consisting entirely of a prominent downward bulge of the Moho to ~60 km. The crust here is some of the thickest in North America and well outside normal variation around the global mean. Isostasy would normally predict a large topographic swell over a crustal thickening of this magnitude. The absence of the expected mountain range means that the bulge (a "mountainless root" if you will) must overlie an unusually dense body—in this case, most likely eclogite replacing oceanic crust trapped in the lithospheric mantle below the basalt-to-eclogite transition (now at ~60 km) during the Wyoming-Green Mountain collision.
A number of other curious contrasts have been noted across the CB. To its south, mantle xenoliths and tomographic imaging reveal hydrated minerals, garnet-bearing rocks and tomographic layering in the lithospheric mantle, but none of these features occur north or the CB; nor do any of the 1.4 Ga Berthoud plutons so common to the south. These differences remain unexplained.
Bottom line: The Cheyenne Belt is a significant lithospheric boundary whose full import is still being unraveled.
Subduction zones reappeared many times off the growing southern margin of the nascent Colorado Province as additional arcs and other oceanic and small continental terranes of varying sizes, orientations and origins rafted in over the 150 Ma following the initial Wyoming-Green Mountain collision. Today, these accreted terranes form several broad northeast-trending belts or provinces that share common Early Proterozoic deformation histories but are far larger than any single island arc they might contain.
West of the Green Mountain arc, in Utah, a package of terranes known as the Mojave Province docked onto the Wyoming Province at 1.75-1.70 Ga. In Colorado, the Rawah arc-backarc complex (part of the larger Northern Yavapai Province) came in behind the Green Mountain arc, followed by the Salida-Gunnison arc (part of the larger Southern Yavapai Province). Terranes of the Mazatzal Province docked next at 1.65 Ga.
The Northern and Southern Yavapai Provinces account for nearly all the 1.7 Ga basement rocks found in Colorado. The Mazatzal Province now floors much of New Mexico, but its docking was clearly felt in Colorado, as evidenced by 1.65 Ga deformations recorded in the older Yavapai basement.
The Cheyenne Belt, the 1.78 Ga suture between the Green Mountain arc and the Archean Wyoming Province craton, barely grazes the NW corner of Colorado. Just to its south, the 1.75 Ga Farwell Mountain-Lester Mountain shear zones mark the suture between the Rawah block and Green Mountain arc. The Homestake shear zone of the northern Sawatch Range may overlie the suture between the Rawah block (part of the Northern Yavapai Province) and the Southern Yavapai Province. The Colorado Mineral Belt roughly follows the same suture, but its association with Early Proterozoic sutures remains controversial. The volcanically active Jemez Lineament of northern New Mexico gives surface expression to the 1.65 Ga suture between the Southern Yavapai and Mazatzal Provinces.
 |
Sunda Arc |
Today, the northbound Australian continent is busily sweeping up a swarm of Eastern Indonesian arcs in the Banda Sea (below) along its northern margin, just as the Wyoming craton gathered up oceanic terranes along its southern margin in the Early Proterozoic. Among the many arcs and continental fragments caught in this ever-tightening Australian-Eurasian vice are the ^Lesser Sunda Islands (right); the largest island shown is Timor.
| |
Modern Banda Sea |
An apparent reversal of subduction polarity attending the docking of the Green Mountain arc also finds a modern analog here: When the leading edge of the Australian craton nosed into the north-dipping subduction zone beneath the island of Timor (part of the ^Banda arc) at ~2.2 Ma (23:59:18 h), subduction ceased there but appears to be starting up again along a new south-dipping subduction zone just north of the arc. The extreme topography and bathymetry of the Banda Sea and nearby New Guinea reflect ongoing collisions with Australia as the Banda Sea closes.
Geologists use the term basement to refer to the igneous and metamorphic rocks between any sedimentary cover that might be present and the Moho, the boundary at the bottom of the crust. Colorado's oldest exposed rocks are hard, resistant granites and metamorphic rocks added to the continent in the protracted Colorado Orogeny lasting from 1.78-1.65 Ga (14:30-15:12 h), and these make up most of the basement in Colorado today. Many of them may be closer to 1.8 Ga in true age, but their radiometric signatures date instead from their alteration in and around the Early Proterozoic suture zones ca. 1.7 Ga. Later orogenies would add voluminous 1.4 Ga and rare 1.1 Ga Middle Proterozoic granites to complete Colorado's basement.
With each incoming Early Proterozoic magmatic arc came a typical package of 1.78-1.70 Ga arc rocks, as shown in the table below.
Arc-Derived Metamorphic Rocks of the Colorado Basement |
|||
| Raw Arc Materials | Original Rock Types | Metamorphic Products | Colorado Basement Example |
|
Volcanic and, to a lesser extent, intrusive igneous rocks, primarily of andesitic composition |
^Andesitic lava, breccia and tuff; gabbro | Gneiss, primarily of quartz and feldspar, sometimes with hornblende | Royal Mountain hornblende gneiss, Black Canyon Gneiss |
|
Aprons of primarily submarine volcaniclastic sediments |
Arkose sandstone and claystone | Mica and pelitic schist | Mount Evans schist |
|
Occasional fringing coral reefs |
Limestone | Marble | ?? |
|
Occasional mature sands |
Quartzose sandstone | Quartzite | Coal Creek Quartzite |
Occasional slivers of oceanic crust |
^Basalt, ribbon chert and turbidites | Gneiss, metachert and metaturbidites | Green Mountain Block |
|
Painted Wall |
With each collision, arc materials and other ocean floor edifices transferred to the building continent, often along with rumpled fragments of surrounding oceanic crust. Overthrusting during the arc-continent collisions buried arc rocks to depths of 11-16 km (7-10 mi) and severely folded and faulted them at high temperatures and pressures. Regional metamorphism accompanying nearly 200 Ma of protracted, recurring arc-continent collisions recrystallized raw arc materials into the suite of metamorphic basement rocks listed above. The dark 1.7 Ga Black Canyon Gneiss in the Painted Wall (right) in the north rim of the Black Canyon of the Gunnison is typical of Colorado's metamorphic basement.
 |
Buffalo Mountain |
Colorado's 1.8-1.7 Ga metamorphic rocks are shot through with slightly younger ~1.7 Ga granites and associated pegmatites, including the 1.72 Ga Boulder Creek batholith and the granites intruding the southern Gore Range at Buffalo Mountain (right). Similar granites occur throughout the southwestern United States. Some probably represent unerupted magma bodies within magmatic arcs or backarc basins. In their rise through the crust, some 1.7 Ga granites near Gunnison entrained 1.8 Ga zircons recycled from older, more mature crust of uncertain provenance. These inherited zircons imply that the Colorado Province accreted both magmatic arcs and older continental crust during its assembly. Where that older continental material originated remains a topic of considerable debate among Precambrian geologists.
In the Rockies, granites and other intrusive rocks with radiometric dates in the 1.660-1.790 Ga range are sometimes referred to collectively as the Routt Plutonic Suite. For convenience, I'll also use that nomenclature.
 |
Coal Creek Quartzite |
Mature light-colored cross-bedded quartz sands accumulated in substantial thicknesses in a number of places in the Colorado Province shortly after 1.7 Ga and now crop out as quartzite in several locations in Colorado and New Mexico. In the central Front Range, these include the Coal Creek quartzite (right) in ^Golden Gate Canyon State Park and a similar quartzite that contributes to nearby El Dorado Canyon's reputation for world-class rock climbing. These quartzites rest on granite of the 1.72 Ga Boulder Creek batholith intruded into even older pelitic (clay-based) metasediments. They imply a source region of elevated continental or felsic arc-generated crust located far overland from the site of deposition. How such crust found its way into the largely oceanic mix of the Colorado Province remains unclear, but the mix of oceanic arcs and continental fragments now forced together in the closing Banda Sea of Eastern Indonesia suggests some possibilities. Deformation of the Coal Creek quartzite in Golden Gate Canyon State Park probably relates to initial motion on the Ralston-Idaho Springs shear zone sometime after 1.7 Ga.
 |
Colorado Mineral Belt (blue) |
Colorado has been far removed from active plate boundaries since at least 1.6 Ga, but far-field stresses from distant plate interactions to the west, south and east continued to find expression in the Colorado Province mobile belt by exploiting persistent weaknesses left over from its assembly.
Never solidly healed, Early Proterozoic basement sutures and other structural defects have been reactivated many times, localizing subsequent deformation and magmatism throughout the Southwest to this day. Prominent examples include ongoing magmatism along Utah's St. George Lineament above the 1.75-1.70 Ga Mojave-Yavapai suture and along northern New Mexico's Jemez Lineament above the 1.65 Ga Yavapai-Mazatzal suture. Extensive Late Proterozoic rifting further cracked up the Colorado basement.
To what extent 1.7 Ga or 1.4 Ga structures have controlled the location of the NE-trending Colorado Mineral Belt remains under ^active investigation, but the important Idaho Springs-Ralston shear zone segment of the mineral belt seems to be a 1.7 Ga structure reactivated in the Berthoud Orogeny at 1.4 Ga and again during the Laramide Orogeny at ~72 Ma.
On the Geologic Highway Map of Colorado, the symbol "Xm" marks the 1.7 Ga metamorphics, while "Xg" marks the 1.7 Ga granites. Both units appear in light gray, the dominant map color throughout the Rockies. The Colorado Mineral Belt is easily found by following the diagonal band of dark maroon "Tki" Laramide intrusions and hot pink "Tmi" mid-Tertiary intrusions from Four Corners to Boulder.
Granites of Middle Proterozoic age (1.45-1.0 Ga, 16:16-18:40 h) intruding the Early Proterozoic basement completed the Colorado Province's Precambrian basement. NE-trending ductile shear zones and rifts ripped the basement as these granites were emplaced, while differential uplifts jostled basement blocks up and down across the state. Interestingly, none of these disturbances affected the Wyoming craton north of the Cheyenne Belt.
|
Mount Evans from Denver |
Colorado was well within the continental interior by 1.45 Ga (16:16 h), when a thus far unidentified convergent plate interaction of immense scale some 1,000 km to the south created ductile shear zones and emplaced numerous mid-crustal plutons (subsurface magma bodies) in a band stretching across the then closely apposed North American and Baltica (Northern European) plates. Intracontinental rifting accompanied these events. Far-field stresses related to transpression (combined transform and convergent plate motions) at the distant plate boundary also produced differential uplift of basement blocks throughout Colorado. This protracted event lasted 100 Ma, ending around 1.35 Ga (16:48 h).
Geoscientists now variably refer to this enigmatic episode as the Berthoud Orogeny, or simply as the 1.4 Ga event, the former after granite and shear zone exposures near Berthoud Pass on US40 in Colorado's Front Range. Indeed, in the Rockies and elsewhere in North America, intrusive rocks with radiometric dates in the 1.400-1.450 Ga range are often referred to collectively as the Berthoud Plutonic Suite. Berthoud granites are now known to dominate the basement across the midcontinental US and make up 20-30% of the basement throughout the Southwest as well. For convenience, I'll also use the Berthoud tag.
In the literature, Berthoud granites are also widely referred to as "A-type" and "anorogenic" granites because they're typically found without evidence of local mountain building. Such names are misleading, however. Berthoud granites are commonly found with evidence of significant regional tectonic stresses probably related to far-field plate boundary influences; in some areas, the Rockies included, these stresses have produced significant mountain belts and rifts.
Of the many Berthoud granites scattered through the crystalline Rockies, these stand out by virtue of size or beauty:
Some 1.4 Ga Granites in Colorado |
||||
| Granite | Age (Ga) | Location | Photo | Photo Notes |
1.422 |
North central Front Range |
|
Horseshoe Park from Trail Ridge in ^Rocky Mountain National Park. The domes adorning the Silver Plume walls of the park are common granite landforms. |
|
1.44 |
Central Front Range |
|
Mount Evans as seen from Denver. |
|
1.40 |
Northern Sawatch Range | n/a |
The St. Kevin granites rose along the ^Homestake shear zone, which shows signs of movement at both ~1.7 and ~1.4 Ga. |
|
The Mount Evans granite apparently rose along the Idaho Springs-Ralston shear zone, which appears to have been moving at the time but probably originated around 1.7 Ga. (Today, this shear zone, a segment of the Colorado Mineral Belt, is home to many of Colorado's richest mining districts.)
Later events exploited weaknesses left by the 1.4 Ga shear zones, as we'll see. To what extent 1.7 Ga structures located the 1.4 Ga granites and shear zones remains unclear, but that appears to have been the case at the Idaho Springs-Ralston and ^Homestake shear zones.
 |
Coarse pink 1.1 Ga Pikes Peak granite with green lichen near the summit of Devils Head, Rampart Range |
Another distant plate convergence far to the south at 1.2-1.1 Ga led to the emplacement of the 1.1 Ga Pikes Peak batholith (right), a huge granite body underpinning the southern Front Range and adjoining High Plains around Colorado Springs. They're common elsewhere in the southwestern US, but plutons of this age are rare in Colorado. (The ^Homestake shear zone and Mount Evans host two small occurrences, but the latter is probably a side lobe off the Pikes Peak batholith).
 |
Pikes Peak looms to the south from Garden of the Gods |
Little is known of the plate interaction triggering this intrusion, but the large Grenville Province of Texas and northern Mexico docked against the North American continent along the southern margin of the Mazatzal Province segment of the Colorado Province around this time. Some authors now refer to this event as the Grenville Orogeny, and for want of a better name, so will I.
Differential uplift of basement blocks accompanying the 1.45-1.1 Ga deformations brought rocks of widely varying crustal depths into juxtaposition along deep-rooted faults that may cut the entire lithosphere in places. These faults sometimes followed Early Proterozoic sutures and shear zones. Most of the basement rocks now exposed at the surface in Colorado appear to have formed at mid-crustal levels. Their mineral assemblages tell of time spent at depths of ~10 km or more prior to their arrival at the surface.
On the Geologic Highway Map of Colorado, 1.45-1.0 Ga granites carry the symbol "Yg" and appear in dark gray.
|
Four Corners from space, NASA |
The most visible legacies remaining from the black hole of time spanned by the Great Unconformity are the north- and northwest-trending tectonic grains of the Southern Rockies and the Colorado Plateau. This texture, clearly visible in any shaded relief or geologic map, follows a network of lithosphere-scale (extensional, normal) rift faults that accumulated throughout the western and midwestern United States from the Berthoud Orogeny at 1.4 Ga through the protracted breakup of the global supercontinent Rodinia between 0.9 and 0.6 Ga.
[diagram coming] |
Murphy et al., 2000, Fig.3 |
Western North America gained a Pacific coast when East Gondwana, a lesser supercontinent, rifted away from Rodinia's southwest margin sometime around 0.7 Ga. This long passive margin most closely approached Colorado in central Utah and remained stable through the Early Paleozoic. Inland, failed rifts would permanently scar the North American basement as far east as the midwestern United States.
After Late Proterozoic rifting subsided around or before 600 Ma, the region fell quiet for ~300 Ma. Its many dormant rift faults would later be reactivated in reverse (i.e., in compression rather than in tension) during Colorado's two most definitive Phanerozoic deformations—the Middle Pennsylvanian Ancestral Rocky Mountain Orogeny and the Latest Cretaceous to Eocene Laramide Orogeny. To this day, Late Proterozoic rift faults still influence the broad features of the Rocky Mountain and Colorado Plateau landscapes.
|
Afar Rifts |
At least twice during the Proterozoic, major rifting events driven by still poorly-understood mantle processes tried to pull apart the western two-thirds of the current North American continent—and Colorado along with it. (Large continents seem to draw such fire, as East Africa does today.) There were some close calls—notably, in the Uinta basin, the Midcontinent Rift and the Reelfoot Rift (the last the source of the ~M8 New Madrid, Missouri earthquakes of 1811-1812). Somehow, the western and midwestern United States managed to hold together against repeated rifting attempts, but the basement sustained severe damage in the process, perhaps through the full thickness of the lithosphere in places.
[diagram coming] |
Marshak et al., 2001, Fig. 2 |
The timing and number of Proterozoic rifting events remain a matter of debate. So far, four suspects have been proposed—a NE-SW extension at 1.5-1.3 Ga surrounding the Berthoud Orogeny, an E-W to NW-SE extension at 1.3-1.0 Ga coeval with the Grenville Orogeny and the opening of the Midcontinent Rift, an E-W to NW-SE extensions at 0.9-0.7 Ga attending the early breakup of Rodinia and again at ~0.7 Ga during East Gondwana's breakaway from Rodinia.
Whatever the timing, these deep-seated rifts left a rhombic network of intersecting north- and northwest-trending basement-penetrating normal (extensional) faults that would reactivate in reverse during the Ancestral Rocky Mountain and the Laramide deformations that have so strongly shaped modern Colorado.
 |
Afar tectonics, source USGS |
Current rift environments at various stages of development include
the Red Sea Rift, the Gulf of Aden and the Great Rift of East Africa (right and above)
our own Rio Grand Rift of New Mexico, Colorado and southern Wyoming (here we go again!)
The Rio Grande Rift is still developing, the East African Rift is more advanced, and the Red Sea and Gulf of Aden are at end-game—the sundering of a continent by a seaway floored with oceanic lithosphere. The East African, Red Sea and Gulf of Aden (right) rifts are three arms of a single ongoing large-scale continental rifting event centered over an upper mantle hot spot in the Afar Province of Ethiopia and Somalia. The East African rift may one day lop off the entire east coast of Africa in a colossal Madagascar recap.
Little is known about Colorado during the Late Proterozoic because nearly all the evidence has been culled from the geologic record by the long erosive interval marked by the Great Unconformity. Two important Precambrian sedimentary remnants have been preserved in western Colorado, however—the 1.4-0.95 Ga (16:32-18:56 h) Uinta Mountain Group and the 1.7-1.4 Ga (14:56-16:32 h) Uncompahgre Formation. Both were accumulated and subsequently sheltered in the deepest of the Proterozoic rift basins.
In Colorado, the only older sedimentary rocks still unaltered enough to be easily recognizable as such are preserved in the 2.3 Ga Red Creek quartzite, part of the Wyoming Province.
 |
Uinta Uplift, south flank |
 |
Steeply-dipping Mesozoic flatirons lean against their flat-lying counterparts atop the southern margin of the Uinta uplift |
From 1.4 Ga to the Laramide, a deep, 160 km-long west-trending rift of Berthoud Orogeny ancestry in western Utah and adjoining northwest Colorado sheltered nearly 7.3 km (24,000') of terrigenous sediments washed into it from surrounding highlands. These quartzites, conglomerates and shales comprise the Uinta Mountain Group, with ages ranging from 1.4 Ga at the bottom of the pile to 0.95 Ga at the top.
The Laramide Uinta Uplift is a classic example of rift inversion. As the Colorado Plateau plowed north during the Laramide Orogeny, it squeezed the Uinta Mountain Group up and out of its west-trending rift along reactivated rift faults and new parallel reverse faults that flared south toward the surface to thrust Precambrian Uinta strata over much younger pre-Laramide Paleozoic and Mesozoic strata deposited on the rift margin. The resulting Uinta Uplift, a larger than usual Laramide faulted anticline, now rises steadily to the west, culminating in the magnificent Uinta Mountains, with their still flat-lying layer-cake stratigraphy. The lower but rugged and deeply dissected east end of the Uinta Uplift provides a spectacular backdrop for ^Dinosaur National Monument of Colorado and Utah. There, Mesozoic strata still cover the southern margin of the uplift (right); the underlying Precambrian Uinta Group sediments are exposed only along the north flank.
The northern bounding fault of the Uinta Uplift (and basin) in southern Wyoming coincides with the southern boundary of the Archean Wyoming Province, which here recapped in a small way its major role of backstop during the Colorado Orogeny.
 |
Gently-dipping late Devonian Elbert sandstones rest unconformably on nearly vertical Uncompahgre Formation quartzites in Box Canyon near Ouray |
Like the Uinta Mountain Group to the north, the Precambrian sediments of the Uncompahgre Formation collected in a rift, but here the basin involved a pair of northwest-trending grabens (down-dropped fault blocks) bounded by ~1.6 Ga normal faults developing shortly after the Colorado Orogeny.
Unlike the Uinta Mountain Group, the originally flat-lying sandstones, shales and conglomerates of the Uncompahgre Formation are somewhere between 1.7 and 1.4 Ga in age and were tilted vertically and mildly altered into quartzites, slates and schists later in the Proterozoic. Just NNE of Durango, Uncompahgre rift sediments peek out from beneath the skirts of the SW margin of the massive mid-Tertiary San Juan Volcanic Field. Near Ouray, a smaller outcrop of vertical Uncompahgre beds appears unconformably in the walls of Box Canyon beneath gently dipping Late Devonian Elbert Formation sandstones.
The tough-as-nails quartzites and other metasediments of the Uncompahgre Formation hold up most of the highest topography as one leaves Ouray headed south on US 550.
In the Geologic Highway Map of Colorado, the Uinta Mountain Group and the Uncompahgre Formation are both subsumed under the symbol "Ym" and mapped in a medium gray with wavy dark gray stripes. Look for them near the northwest and southwest corners of the map, respectively.
An exceptionally long and effective
period of erosion held sway throughout
Colorado from sometime before 1.1 Ga to the Late Cambrian (~510 Ma). This profound gap in Colorado's geologic record is
known as the Great Unconformity. Where Early Cambrian sediments like the
Sawatch Formation rest on
the Precambrian erosion surface, the gap can cover as little as 600 Ma, but in many
places, it's much wider. At Red Rocks Park near
Denver, the gap spans 1.4 Ga where coarse ~300 Ma basal
Fountain
conglomerates rest unconformably on 1.7 Ga gneiss.
Late Proterozoic rifting must have interrupted the process at times, but by Late Cambrian time, statewide erosion had exposed a featureless expanse of planed off crystalline basement rocks originally hailing from a variety of mid-crustal levels, judging from their mineral assemblages. The sedimentary cover and at least 10 km of Precambrian basement were removed during this time. Where all the debris went is unclear, but little if any of it hung around Colorado.
Denudation proceeded unevenly across the state, with some basement horizons reaching the surface as early as 1.1 Ga and others closer to 600 Ma. In Colorado, the only Precambrian sediments to escape removal—the Uinta Mountain Group and the Uncompahgre Formation—were those sheltered in the very deepest of the Proterozoic rift basins. All other rift basins were planed off in toto. Any volcanism or surface topography developing in Colorado during or before this time had been thoroughly erased by the close of the Great Unconformity.
We're now left to rely on basement exposures to reconstruct what went on in Colorado prior to the Late Cambrian. Luckily, the Rockies expose a good bit of basement, but most of that rock had resided at depths of 10 km or below prior to the Great Unconformity and had long been out of touch with the surface. Surprisingly, the basement still has much to tell about the period from 1.7 to 1.1 Ga, but from there to the Late Cambrian, we have very little to go on.
On a geologic map with dip indicators, an unconformity between sedimentary units might show an abrupt change in dip along a depositional contact. Beyond that, unconformities have no explicit map representations. Their presence must be inferred from the absence of rock units that might otherwise have been expected to appear in a particular location.
The current Phanerozoic Eon (543-0 Ma, 21:06-24:00
h) began when complex life forms
built on current (not Edicarian) plans suddenly appeared in abundance in the fossil record at
the beginning of the Cambrian Period (543-490 Ma,
21:06-21:23 h). This "Cambrian Explosion" also kicked off the Paleozoic Era
(543-248 Ma, 21:06-22:41 h).
Geologists refer to the ~4.0 Ga interval prior to the Cambrian as Precambrian Time (4.5 Ga - 543 Ma, 00:00-21:06 h). The 210 Ma chapter about to unfold lasted a little more than an hour (1:07 to be exact) in our day of creation.
During the Paleozoic Era, the North American continent slid west 140° in longitude and rotated counterclockwise 90° in the process, but for all that, it never strayed far from the equator. The warm global climate had already melted the polar ice caps, leaving much of the continent awash. In Utah to the west was a passive continental margin with a growing continental shelf and the Pacific basin beyond.
Between Late Cambrian and Early Pennsylvanian time (510-300 Ma), shallow tropical epicontinental seas washed over Colorado Precambrian basement denuded during the Great Unconformity. For the next 210 Ma (1:07 h), flat-lying marine shelf sediments would accumulate to depths of 1.8-3.6 km (6,000-12,000') with few interruptions throughout the region.
 |
Sawatch Fm at river level in Glenwood Canyon |
 |
Sawatch Fm resting on Precambrian basement |
In the Late Cambrian (510-490 Ma), a shallow tropical sea spreading from west to east across the Precambrian plain of the Great Unconformity deposited a thick transgressional blanket of beach sands now known as the Sawatch Formation. This well-cemented, highly resistant sandstone is particularly well exposed along and above I-70 in Glenwood Canyon (upper photo at right), where it forms sheer thinly-bedded, buff-colored 150-180 m cliffs resting unconformably on 1.7 Ga Precambrian basement. (The other prominent cliff-former in Glenwood Canyon is the massive gray Mississippian Leadville limestone, not shown.) The Sawatch also crops out impressively along the east side of CO24 in the Eagle River Gorge south of Minturn.
Wherever it's found, the Sawatch Formation rests unconformably on 1.7 Ga Precambrian basement at the Great Unconformity (lower photo at right, also from Glenwood Canyon). Most good Sawatch exposures occur west of the Front Range. It's generally been lost to erosion along the eastern flank of the Front Range, but remnants have been preserved in the down-dropped ^Woodland Park half-graben on the Rampart Range Fault just west of Colorado Springs. In Woodland Park and Eldorado Canyon, Sawatch debris jammed into gaping Laramide faults to form unusually large clastic dikes. (Dikes are by definition slab-like crack-filling bodies of rock. Most are are igneous in origin, but the rare clastic dikes involve sedimentary fill instead.)
Some consider the Sawatch a sandstone, others a quartzite. Who's right may depend on the exposure. Some Sawatch outcrops may be cemented well enough to qualify as quartzites, but most are not.
Limy muds covered the Late Cambrian sands as the tropical sea deepened through Ordovician time (490-443 Ma). Near Colorado Springs, outcrops of the Ordovician Manitou Formation (cherty limestones), Harding Sandstone and Fremont Dolomite leaning against the Rampart Range record this environment. During Harding time, the sea shallowed enough for wave action to rework older rocks along the shoreline.
Silurian (443-417 Ma) sediments appear to have accumulated across Colorado as well, but they were eroded away in toto during the Early Devonian. A few limestone fragments preserved in kimberlite pipes near the Wyoming border are the only Silurian rocks ever found in Colorado.
Early in the Devonian (417-354 Ma), Colorado lost some of its older Paleozoic sedimentary cover to erosion during a brief rise above sea level. By Late Devonian time, however, a returning tropical sea had deposited the Parting Sandstone and Dyer Dolomite, which together make up the Chaffee Group. Near Ouray, Late Devonian Elbert Formation sandstones and Ouray Limestones are Chaffee equivalents.
During the Mississippian (354-323 Ma), a widened warm shallow sea deposited thick limy muds over most of Colorado. These muds would become the massive gray cliff-forming Leadville Limestone of 1870's Colorado mining fame, but much of the Leadville is actually dolomite. Regional uplift at the end of the Mississippian raised the top of the Leadville limestone above sea level, where it eroded into a karst landscape similar to those found in tropical limestone platforms today.
Interestingly, a long-running series of subduction-related collisions first hit the West Coast (then around the longitude of the Sierra Nevadas at the latitude of Colorado) in the Early Mississippian in an event known as the Antler Orogeny. Antler deformations didn't reach as far east as Colorado to my knowledge, but the timing of the Late Mississippian regional uplift makes me wonder about the effects of subsequent collisions there.
As Colorado continued to rise during the Early Pennsylvanian (323-290 Ma), the retreating sea left behind shales, sandstones and limestones of the Belden and Glen Eyrie Formations of Central and Eastern Colorado, respectively. Such are the sediments exposed in the southern Mosquito Range at Trout Creek Pass. Early Pennsylvanian uplift brought to a close over 200 Ma of widespread Early Paleozoic marine sedimentation in Colorado and ushered in another major mountain-building period, this time driven by far-field continental stresses related to the Middle Pennsylvanian assembly of the ^supercontinent Pangea.
On the Geologic Highway Map of Colorado, early Paleozoic sediments carry the symbol "MDOC" for "Mississippian, Devonian, Ordovician and Cambrian" and appear in dark purple.
By the end of Early Pennsylvanian time (~300 Ma), Colorado had taken up an
equatorial position and a climate to match and had acquired a fairly
simple structure: A thick blanket of flat-lying Paleozoic sediments covered a
planed-off jumble of differentially uplifted Precambrian basement blocks. But in the Middle
Pennsylvanian, continent-continent
collisions completing the assembly of the supercontinent Pangea (top right) well
to the south would soon change all that.
[diagram coming] |
Dickinson, 2003, Fig 1 |
Colorado had been solidly within the North American interior since the close of Early Proterozoic time (1.6 Ga), but around 300 Ma, far-field stresses propagating outward from the collision between Laurentia and Gondwana along the Ouachita Fold Belt of west Texas, Oklahoma and Arkansas rumpled up nearly a dozen large elongated northwest-trending island mountain ranges across what is now the North American south and southwest. The mechanics remain unclear, but many of these large basement-cored blocks apparently rose along Proterozoic rift faults reactivated in reverse. The collision zone involved portions of North America, South America and Africa.
Two of the largest of the Pennsylvanian island ranges rose in Colorado. These are now known collectively as the Ancestral Rocky Mountains (ARM). The eastern ARM uplift, now known as Frontrangia, came up in central Colorado a bit west of the current Front Range uplift, while Uncompahgria occupied southwestern Colorado. Both followed roughly NW-SE trends. The intervening lowland is now variably known as the Central Colorado Trough or the Maroon Basin.
The ARM ranges rose as faulted anticlines, their Early Paleozoic sedimentary covers draped over uplifted basement blocks bounded by steep reverse faults. Erosion eventually removed most of the sedimentary cover and wore deeply into the basement cores as well. Sediments shed to the east from Frontrangia accumulated in the long-lived Denver Basin of central and eastern Colorado. Sediments shed to the west from Uncompahgria accumulated in the Paradox Basin of southwestern Colorado. The intervening Maroon Basin, usually dry, received debris from both uplifts but mostly from Uncompaghria. It also accumulated thick evaporite beds (typically calcium sulfate or gypsum) left behind by seaways that sporadically flooded in and dried up.
 |
Fountain Conglomerate |
Most of the sediments shed by the ARM were terrigenous, with brief and scattered marine interludes. The first sediments to come off the rising highlands ringed them with coalescing alluvial fans now preserved in the coarse red arkosic conglomerates of the syntectonic Fountain (right), Minturn and lower Maroon Formations and the Hermosa Group. The red stains come from hematite (ferric oxide, Fe2O3) cements derived in turn from ferromagnesian minerals in the igneous and metamorphic rocks exposed by the Ancestral Rocky Mountain Orogeny.
As erosion progressed, the sediments became ever more fine-grained. By the end of the Permian, the Ancestral Rockies had been almost completely eroded away. With their roots now buried in their own debris, their direct influence on sedimentation in Colorado had come to an end.
 |
Bold red Fountain Formation flatirons jut skyward toward the Front Range below a ridge of white Lyons sandstone at the north end of Roxborough State Park |
Today in the east, the Fountain and Lyons Formations crop out spectacularly all along the Front Range—most notably in the Garden of the Gods Park near Colorado Springs, in the Red Rocks Park (right) and ^Roxborough State Park near Denver, and in the imposing Flatirons south of Boulder. Lyons sandstones, usually the pink variety, figure prominently in buildings, walls and walkways all along the Front Range.
|
Maroon Formation |
The Maroon Formation crops out rather drably throughout the central Colorado Rockies but finds its finest exposure by far in the oft-photographed Maroon Bells (right) of the Elk Mountains. Judging from the nature and extent of Maroon sediments, central Colorado must have been one big nasty mudflat in the Late Paleozoic.
How's that for an ugly duckling story?
 |
Deep Yampa River meanders cut in Weber Sandstone, Dinosaur National Park East |
In the northwest corner of Colorado and in adjoining portions of Utah and Wyoming, the flanks of the large Laramide Uinta uplift expose a 300 meter-thick deposit of handsome white Pennsylvanian aeolian dune sands known as the Weber Formation. The Weber dune field was coeval with the lower Maroon Formation, but its sands derived from Wyoming uplands to the north, not from Colorado's Ancestral Rocky Mountains. Deep antecedent meanders of the Green and Yampa rivers now dissect the Warm Springs monocline where the rivers meet at the east end of the Uinta uplift. The canyon walls are primarily of Weber sandstone. Harper's Corner in the Colorado side of Dinosaur National Park provides an excellent view of this spectacle; the 50-mile side trip off US 40 is worth it.
For reasons still shrouded in mystery, the Paleozoic Era closed with the greatest
known extinction of all time, the Permian Catastrophe at 251 Ma (22:40 h). Over 90%
of all marine species vanished from the fossil record in a geologic instant,
along with many terrestrial species. (Man, I hate it when that happens.) In fact, the
resulting break in the fossil record
defines the Paleozoic-Mesozoic boundary. The impact-related K-T extinction at the
Cretaceous-Tertiary boundary at 65 Ma (23:39 h), the one that killed off the dinosaurs, was
apparently a slap on the wrist by comparison. Many kill mechanisms have been
proposed for the Permian Catastrophe, including a large impact, a global loss of
atmospheric oxygen and a massive explosive release of ocean-floor methane
deposits, but no one knows for sure what went wrong.
On the Geologic Highway Map of Colorado, the Pennsylvanian through Permian strata discussed in this section appear in light blue.
The Mesozoic Era opened as new life forms—among them,
the dinosaurs and their predecessors—appeared in the aftermath of the global
extinctions of the Permian Catastrophe. The Triassic
Period (248-206 Ma, 22:41-22:54 h) kicked it off; the Jurassic
Period
(206-144 Ma, 22:54-23:14 h) and Cretaceous Period (144-65
Ma, 23:14-23:39 h) followed. The Mesozoic closed with the great Cretaceuos-Tertiary
"K-T" extinction that wiped out the dinosaurs.
This 176 Ma chapter lasted about 56 minutes in our day of creation.
As the curtain opened on the Mesozoic,
the Late Paleozoic Ancestral Rocky Mountains had been
erased from the landscape. North America was by this time well north of the equator.
In and around Colorado, an
arid Triassic gave way to a moist Late Jurassic and finally,
to a largely marine Cretaceous.
North America's slow traverse of our planet's great northern desert belt at 30-40° latitude was largely a Triassic (248-206 Ma, 22:41-22:54 h) affair but began in the Late Permian and extended well into the Jurassic. Colorado was entirely above water in the Triassic, but there was little surface relief. The final assembly of ^Pangea, the planet's most recent supercontinent, in the Late Permian also fostered arid continental climates well into the Jurassic.
 |
Triassic redbeds, Maroon Canyon, Elk Range |
Colorful Triassic redbeds, like those shown above along the north wall of Maroon Creek Canyon in the Elk Range, tell of the arid mudflats that dominated the scene statewide, as they do now along the southern coast of the Persian Gulf. Bentonite layers in the Chinle redbeds in western Utah's ^Canyonlands National Park record thick silicic ashfalls blown across the Southwest from magmatic arcs active along the West Coast in Late Triassic time.
|
Basalt-capped ridge south of Glenwood |
In eastern Colorado, Triassic muds accumulated in the upper Lykins Formation, which started in Late Permian time and now crops out extensively along the eastern margin of the Front Range. Minor marine limestone members embedded in the lower Lykins record two brief returns of the sea in the east. In the Denver area, thin upturned white limestone strata decorate red Lykins slopes cropping out below Red Rocks Park west of the Dakota Hogback. Near Basalt, North of Aspen, massive Triassic redbeds sport Late Tertiary basalt caps right).
In western Colorado, the Triassic redbeds make up the Moenkopi and Chinle Formations. (The Chinle is the red slope-former at right.)
 |
Monument Canyon |
Triassic mudflats eventually gave way to vast dune fields preserved in the thick, cross-bedded Wingate Sandstone, which tends to form massive cliffs and spires hundreds of feet high throughout the Colorado Plateau, as seen at left at Monument Canyon in Colorado National Monument. After the Wingate came the Kayenta Formation, with its easily recognizable irregular beds of buff to purple shale, sandstone, limestone and conglomerate. Silica-rich groundwater permeating the Kayenta after its burial cemented it into a hard and competent but improbable caprock often seen protecting imposing Wingate edifices, as seen here.
 |
Chinle and Wingate Fms |
Today, the Chinle, Wingate and Kayenta rest on Precambrian basement in the Colorado National Monument, where they are all beautifully exposed. The Kayenta/Wingate/Chinle triad is a visually distinctive combination responsible for very similar country throughout the Colorado Plateau, including ^Canyonlands National Park and vicinity in western Utah, where the Chinle also includes silicic ashfalls erupted from magmatic arcs hundreds of km away along the west coast of the time.
Arid Triassic conditions continued into the Early
Jurassic (206-?? Ma) in Colorado, at least in the west. Early Jurassic rocks are absent from
eastern Colorado, but in the west, the Navajo Sandstone, Carmel Formation and
Entrada Sandstone record yet another
transition from vast desert dune fields to vast red arid mudflats and back again. The
Entrada deserts would be the last regional dune environments Colorado has seen
to date. Colorado's ^Great Sand
Dunes National Monument is spit in the ocean by comparison, but it
exemplifies some of the processes responsible for dune fields in general.
By the close of the Jurassic, North America had reached its present latitude. Along the way, and with the Mid-Jurassic breakup of the supercontinent ^Pangea (right) also in progress far to the east and south, the Colorado climate became moist and the vegetation lush. The entire state was monotonously flat and barely above sea level.
Late Jurassic (??-144 Ma) sedimentation in Colorado began with muddy flood plain deposits now known as the Ralston Creek Formation in the east and the Summerville Formation in the west. Both contain minor lagoonal deposits and evaporites indicating occasional and brief marine transgressions. In places like the valley separating the Dakota Hogback and the Front Range foothills west of Denver, the soft Ralston Creek serves as a valley-former.
|
|
Morrison in the I-70 road cut west of Denver |
 |
... and at Dinosaur West |
|
|
... and in the Grand Monocline |
Next came the flood plain clays and stream channel sands of the statewide Morrison Form, which hosts many important dinosaur fossil finds, including the first intact sauropod skeleton at the town of Morrison in 1877. The thick, slope-forming Morrison is easily recognized by its colorful diagnostic mix of gray, green and maroon claystones, as seen at right in the distance in the Grand Monocline at Colorado National Monument.
The greens come from the clay minerals themselves, from pyrite and from iron silicate cements indicating bog-like reducing conditions at the time of deposition. Iron oxide cements provide all the other Morrison colors. Minor discontinuous lakebed limestones embedded in the Morrison also tell of a poorly-drained landscape dotted with lakes. The vast low-lying flood plain surrounding the Gulf of Mexico today is an analogous depositional environment.
In the upper photo at right, badlands erosions gives tilted but undeformed Morrison beds a chaotic look.
In the mid-Cretaceous, the oceans overflowed onto the continents. There were no
polar ice caps to store excess water, and unusually rapid seafloor spreading and
subduction rates attending the breakup of Pangea had together managed
to repave the much of the global ocean floor with young (thin and hot) oceanic
crust that floats considerably higher on the mantle than the thick old cold
crust it replaced. As the ocean basins shallowed, the excess
water washed over low-lying continental areas worldwide. With the tectonic help
of the Early Cretaceous Sevier Orogeny to its west, the
entire state ended up 600' beneath the waves of the Cretaceous
Interior Seaway, perhaps the largest continental sea in the global geologic
record.
In the Early Cretaceous (144-100 Ma, 23:14-23:28 h), new uplift in western Utah increased stream drainage across Colorado's Late Jurassic flood plains, leading to deposition of the coarse fluvial sands of the Lytle Formation in eastern Colorado and similar sands in the west. Small chert pebbles weathered out of Paleozoic limestones and dolomites exposed far to the west differentiate the Lytle from the upper sands of the Morrison Formation. The Lytle Formation would mark Colorado's last stand above water for the next 30+ Ma.
Cretaceous seas flooded onto the North American continent simultaneously from the Artic and the Gulf of Mexico. The waters met in SE Colorado in the mid-Cretaceous (~100 Ma, 23:28 h) and spread west from there, covering the entire state by ~85 Ma and for another ~20 Ma thereafter. As this Cretaceous Interior Seaway (CIS) swept across the land (transgressed), it left behind beach and barrier island sands and deltaic deposits. These shoreline sands were later covered by thousands of feet of shallow-ocean muds punctuated by occasional limes, sands and ashfalls blown in from magmatic arcs active all along the West Coast, which at the time ran through western California and Idaho.
The broad north-trending continental trough occupied by the CIS was at least in part a large foreland basin developed in response to the Latest Jurassic to Early Cretaceous ^Sevier Orogeny, a thin-skinned (basement-sparing) east-directed fold-and-thrust deformation affecting a broad swath of western North America from Alberta through western Montana and Nevada. As Sevier thrust sheets stacked up to the west, the lithosphere flexed downward beneath the added load. Colorado escaped direct Sevier folding and thrusting but sat in squarely in the foreland basin—the portion of the broad downwarp extending east of the Sevier mountain front. Sediments shed to the east from the mountain front accumulated in the CIS within Colorado and throughout the foreland basin. Interestingly, the Sevier Orogeny petered out just the Laramide Orogeny opened to its west. The causes of the Sevier Orogeny are still hotly debated, but yet another complication along the West Coast subduction zone of the time is likely.
In the Late Cretaceous (85-65 Ma, 23:33-23:39 h), as western Utah continued to rise with the last of the Sevier uplifts, the seas retreated eastward across Colorado state, leaving another but this time regressive layer of beach and barrier island sands in their wake. The resulting thick sandstone-shale-sandstone sandwich marks the Cretaceous throughout Colorado, but the formation names differ a bit from west to east.
 |
Dakota sandstone caps the east wall of the lower Animas canyon along US 550 just above Durango |
In western Colorado, the layers in the sandstone-shale-sandstone Cretaceous sandwich are fairly straightforward. The Early Cretaceous sands recording the marine transgression are known as the Dakota Sandstone. Counting its eastern equivalents, the Dakota is the most widespread stratum in Colorado.
At right, Dakota sandstone caps the east wall of the lower Animas canyon a few miles above Durango. Below it are the slope-forming, tree-covered Morrison Formation; the buff-colored Entrada, Navajo and Wingate sandstones; and the Permo-Triassic redbeds of the Chinle and Cutler Formations.
 |
Slopes of Mancos shale capped by Mesaverde sandstone make up the Book Cliffs, here along I-70 northeast of Grand Junction |
The marine muds and minor limes and sands deposited in the open Cretaceous sea are all lumped into the Mancos Shale, which crops out extensively in the Colorado Plateau, most notably as the corrugated "row of books" slopes of the Book Cliffs north of Grand Junction.
The Mancos shale also floors the broad Mancos River Valley surrounding Mesa Verde, the Grand Valley, the Gunnison Valley between Delta and Montrose, and many similar topographic lows surrounding the high plateaus of southwest Colorado.
 |
Mesaverde Fm along CO133 in coal country near Somerset |
|
"Cliff Palace" Anasazi dwelling at Mesa Verde NP nestles in a large Cliffhouse sandstone alcove |
The beach and barrier island sands and lagoonal deposits of the regressing Late Cretaceous sea dominate the Mesaverde Group. Between Somerset (right) and Paonia, the Mesaverde is heavily mined for its abundant reserves of highly valuable low-sulfur bituminous coal and its highly-prized anthracites, which were metamorphosed by heat emanating from mid-Tertiary intrusions in the nearby West Elk Mountains.
Mesaverde sandstones are common caprocks in plateau country. They cap the Book Cliffs throughout western Colorado and much of southern Utah as well. More famously, the uppermost sandstone member of the Mesa Verde Group, the Cliffhouse sandstone, caps Mesa Verde, where it hosts all the cliff dwellings, including the Cliff Palace at right.
In eastern Colorado, Cretaceous stratigraphy and nomenclature are a bit more confusing, but the basic sandstone-shale-sandstone sandwich structure remains.
 |
I-70 Road Cut |
In the east, it's more accurate to speak of a Dakota Group than a Dakota Sandstone. The lower member of the Dakota Group is the fluvial (river-laid) Early Cretaceous Lytle Formation already discussed. Above it are the transgressing mid-Cretaceous shoreline sands comprising the Dakota Group's upper South Platte Formation member, but I'll stick with the simpler and more common Dakota sandstone appellation in this web site. At right, the geologically famous I-70 roadcut west of Denver exposes the east-dipping South Platte (near), Lytle (middle) and Morrison (far) strata that make up the well-developed Dakota Hogback at Dinosaur Ridge.
|
Minor sandstone member capping a cuesta in Pierre shale at Kremmling |
Benton Formation, Niobrara Limestone and Pierre Shale
In eastern Colorado, Mancos-equivalent marine seaway sediments are subdivided, from oldest to youngest, into the Benton Formation, the Niobara Limestone and the dominant Pierre Shale. The gray upper Benton shales are rich in organic materials that, under proper burial and heating, release natural gas and petroleum into the underlying porous Dakota sandstone, where it can be recovered from anticlinal drag-fold traps created by Laramide faults flanking the Front Range. Black "coaly" organic layers also occur in the eastern Dakota, but true coal is rare there.
Laramie and Fox Hills Sandstones
Topping off the Cretaceous sandstone-shale-sandstone sandwich in the east are the Mesaverde-equivalent Fox Hills and Laramie Formations, both of which are predominantly sandstones with minor shales and coals. In Colorado, the Laramie coal lenses are economic in places but are no match for the massive seams of the Mesaverde Group in the west.
 |
Mt. St. Helens, 1980 |
Ashfalls blown over Colorado's ocean from distant prolific magmatic arcs along the coast to the northwest added hundreds of layers of bentonite and porcellanite to the marine Cretaceous ooze all across the state. These volcanic shales consist primarily of clay minerals weathered and compacted from the ash. The soft, dark bentonites contain expansile clays that wreak havoc with building foundations all along the eastern Front Range. Non-swelling clays make up the brittle white porcellanites.
The so-called "X" bentonite known from oil well logs in 6 states (Colorado, Kansas, Wyoming, Nebraska, South Dakota and Montana!) records a massive ash fall conservatively estimated to have been at least 15 m thick at the time of deposition! The "X" blew in from a Cretaceous vent traced back to southern Idaho near the west coast subduction zones of the time. Think about the "X" the next time you catch yourself complaining about rain or snow falling from the sky: Things could be a lot worse. Is it any wonder that all cultures have worshipped sky gods?
On the Geologic Highway Map of Colorado, Triassic sediments carry the symbol "Tr" and appear in teal; Jurassic sediments are marked "J" and appear in light green; Cretaceous sediments are marked "Ku1" (upper part of the Upper Cretaceous), "Ku2" (lower part of the Upper Cretaceous) and "Kl" (Lower Cretaceous) and appear in 3 more shades of green.
We're coming up fast on the Mesozoic-Cenozoic boundary at 65 Ma (23:39 h), but we'll defer a description of the Cenozoic Era until the next section. The Latest Cretaceous through Eocene Laramide Orogeny and its sedimentary fallout spilled well across the Mesozoic-Cenozoic boundary, but the Laramide is a region-defining event worthy of special treatment.
 |
Laramide orogen from space, NASA |
The profound Latest Cretaceous through Eocene mountain-building event known as the Laramide Orogeny first defined the broad features of the Rocky Mountain and Colorado Plateau landscapes as we know them today. The Laramide deformed the continent from northern Montana to southern New Mexico, and from the western High Plains to Utah and perhaps beyond. The affected region is known as the Laramide orogen.
The Laramide was first and foremost a regional shortening of the crust with maximum contraction to the east northeast. With shortening came uplift. Initially diffuse, the Laramide first raised the region up from the Cretaceous sea in a broad arch. Then came the Laramide's most conspicuous legacy—a swarm of discrete thrust-faulted uplifts most concentrated and elevated in western Colorado but scattered through every surrounding state. The uplifts were typically large, elongated, asymmetric basement-cored thrust-bound welts with a north or northwest trend. Range-front basins were often paired with the uplifts.
Many of the Laramide uplifts still stand high enough to take on a natural color-coding of forest and snow in the NASA satellite photo above. To this day, they give form to all the major ranges of the Southern Rockies—the Front Range, the Medicine Bow Mountains and the Laramie Range; the Sawatch and Mosquito Ranges; the San Juans and Sangre de Christos; the Park, Gore and Tenmile Ranges; the Wind River Range and the Uintas. Lesser Laramide uplifts dot Wyoming. The Black Hills of South Dakota (clipped at the top right corner of the photo) mark one of a number of High Plains basement uplifts of Laramide origin; the others are now buried.
The Colorado Plateau, the large pink region at lower left, also began to rise, rotate and move north as a rigid block during the Laramide, but it's by no means a typical Laramide structure. The Laramide-age and Laramide-like Uinta uplift at upper left owes its atypical east-west axis to stresses related to the approach of the Colorado Plateau and only indirectly to the Laramide per se.
Laramide deformations finally died out in the Eocene, around 40 Ma (23:47 h). After a period of tectonic calm and magmatic fury, a broad regional uplift very different in style from that of the Laramide set in around 28 Ma and continues unabated to this day. Some 70 million years of unrelenting erosion notwithstanding, the Rockies and the Colorado Plateau now stand higher than ever and continue rise at rates 2-3 times that expected from erosion-induced isostatic rebound. Where they'll stop, nobody knows, but something in the mantle seems to be pushing them up.
 |
East-side Front Range uplift, Red Rocks Park |
The photo at right shows a small but typical segment of the east side of Colorado's Front Range at Red Rocks Park west of Denver. The Front Range is by far the largest of the Laramide uplifts, and the second highest as well. The abrupt mountain front of Precambrian crystalline rock and the pronounced tilting away of adjacent Paleozoic and Mesozoic strata are common Laramide uplift features. These initially flat-lying strata once covered the uplifted basement block in the distance but were largely lost to erosion during its ascent.
If nothing else, the Laramide was a time of great complexity, and we seem to be just close enough to it to get really confused by the evidence left behind. The why, how, and how much remain controversial. Data sets from paleobotany, O18 paleoaltimetry, basalt vesicle paleobarometry, apatite fission track analysis and a host of other increasingly sophisticated earth science techniques seem to point in conflicting directions.
Before launching into a tentative Laramide story, let me outline some of the more important outstanding questions as of late 2003:
What really caused the Laramide? To what extent was the subducting Farallon plate involved?
How does intracontinental deformation on the scale of the Laramide even occur? The nearest plate margin was over 1,000 km to the west at the time!
How high were the Laramide mountains, and how far did erosion take them down?
How deep do Laramide reverse and thrust faults cut? Do they reach the brittle-ductile transition zone at the base of the upper crust? Did they cut their own paths through the top of the basement, or did they follow pre-existing faults?
The Rockies are as high as ever and still rising. How much of the cumulative Rocky Mountain uplift can the Laramide claim, and when did it occur, early or late in the orogeny?
How much have the Rockies risen since the Laramide, and why?
How much deformation, displacement and rotation did the Colorado Plateau undergo during vs. after the Laramide?
Many other nagging Laramide questions could have been listed. Since the Laramide remains a topic of intense research among geoscientists from around the world, answers will no doubt be forthcoming, but many new questions are bound to come up along the way.
The best story geoscience can muster for the Laramide is still in pieces as of late 2003, but I'll try to cobble together a reasonably coherent, defensible if not consensual saga in the next few subsections. Let's start with some observations no one disputes.
|
Laramide orogen from space, NASA |
Discrete Laramide uplifts are typically elongated north- or northwest-trending thrust-bound blocks of crust including both basement and sedimentary cover. Most are 50-300 km in length and all but one (the Elk Mountain uplift) are cored with hard crystalline Precambrian basement rock. Large structural basins developed alongside some of the uplifts—e.g., the Denver Basin against the Front Range. The paired basins deepened further as they accumulated sediments from the rising uplifts; vertical structural offsets of up to 10 km developed in some. In the ^NASA Visible Earth satellite photo at right, snow caps and green forests mark the highest of the Laramide uplifts, but many others remain inconspicuous here. The filled basins are not apparent.
Early on, crustal-scale Laramide faulting released basaltic magmas to the surface. Few of these early Laramide volcanics have survived in place (the Early Paleocene 62-63 Ma basalt flow capping South Table Mountain near Golden is a notable exception), but thick accumulations of volcaniclastic gravels like those of the syntectonic Denver and Arapahoe Formation tell of large volumes of erupted Laramide basalt early on.
These "leaky fault" basalts suggest a component of extension in the early Laramide, but it would eventually shorten the crust in a predominantly east northeast direction by as much as 15-20% across the combined width of the Southern Rockies and the Colorado Plateau. The shortening produced the Laramide uplifts.
Now
for some speculation. The Laramide Orogeny coincided at least temporally and
probably causally as well with a period of unusually shallow flat-slab subduction of the
Farallon plate beneath the western margin of North America, which at
the time lay over
1,000
kilometers west of the Laramide orogen. During this pivotal chapter in the development of
the western United States, subduction-related arc magmatism first shifted
progressively to the east as the Farallon plate rose up beneath the overriding
North American Plate and
then ceased altogether—presumably when the shallowing slab
had finally risen above melt-generating
depths.
 |
Nazca Ridge going down the Nazca-South American subduction zone off Peru |
Why the Farallon plate entered a protracted phase of subhorizontal subduction during Laramide time (72-40 Ma) remains uncertain, but the Nazca plate seems to have done the much same thing when a particularly large buoyant aseismic ridge (the Nazca Ridge, right) entered the Nazca-South American subduction zone off Peru around 8 Ma. Large buoyant structures (aseismic ridges, submarine basaltic plateaus, seamount chains, etc.) riding on down-going slabs have been implicated in other documented examples of shallow-slab subduction. Such edifices are common enough on the ocean floor that a large one presenting at the West Coast subduction zone in latest Cretaceous time might easily have initiated the shallowing of the Farallon slab. If so, the direct evidence has gone down with the slab.
Suspiciously, the Laramide Orogeny took off just as arc magmatism related to the Farallon plate snuffed out. The Rocky Mountain and Colorado Plateau region began to shorten, buckle and rise, even though they were far inboard of the West Coast subduction zone and its primary magmatic arc, the Sierra Nevadas.
The mechanisms underlying the Laramide Orogeny are still hotly debated, but most authors now favor a mechanical coupling of one kind or another between the shallow Farallon slab and the base of the North American lithosphere well inland of the West Coast subduction zone. Today, a similar process shortens the South American crust within and well east of the Andes in response to flat-slab subduction of the Nazca plate. Note that the Andes and their eastern foothills are first and foremost a compressional fold-and-thrust orogen; subduction-related arc magmas intrude the orogen and feed many large Andean volcanoes, but they don't make up the bulk of its volume. Undeformed forearc sediments along the Nazca-South American subduction zone again indicate that the responsible compressive stresses have developed not at but inboard of the subduction zone, presumably due to inboard coupling between the subhorizontal Nazca slab and the base of the South American plate.
A competing "end-loading" model denies inboard coupling in favor of the lateral transmission of compressive stresses from the West Coast subduction zone to the Laramide orogen via the North American lithosphere. Proponents of this model cite the occurrence of young (<5 Ma) volcanics derived from lithospheric mantle all across the western United States as proof that the lithospheric mantle has not been eroded away by direct contact with the Farallon plate during the Laramide. They conclude from that that significant mechanical coupling between the Farallon and the overlying North American plate occurred only at the subduction zone. This model ignores the possibility of a non-erosive viscous coupling mediated by chilled asthenosphere and seems mechanically untenable for a number of other reasons, chief among which is the lack of Laramide-age deformation in the forearc sediments deposited in California's Great Valley Sequence. End-loading of the North American plate sufficient to rumple up the Rockies would surely have rumpled these sediments as well.
I find inboard mechanical coupling with the flat Farallon slab much more compelling than the end-loading model, but either way, hydration reactions fed by watery fluids rising from the still-wet Farallon slab would amplify Laramide uplift and magmatism by fostering melting and expansion of the overlying mantle lithosphere.
Around 40 Ma, Laramide deformation ceased and an odd pattern of intense post-Laramide magmatism including the devastating ignimbrite flare-up ensued within and west of the Laramide orogen. These events are widely interpreted as manifestations of the break-up and falling away of the subhorizontal Farallon slab, an event referred to as the Farallon rollback. The highly correlated temporal and spatial pattern of post-Laramide magmatic fronts across the West suggests to some that a large section of the Farallon plate beneath the Basin and Range also folded up along an east-west axis as it sank. Whatever the details, the rollback once again allowed asthenosphere to come into direct contact with the base of the North American plate after tens of millions of years of shielding by the Farallon slab. The influx of hot, buoyant asthenosphere presumably generated the melts fueling post-Laramide magmatism and may also have kicked off the broad regional uplift that followed.
You won't find the Farallon plate marked on any current map, but it made a profound mark on the western United States before it disappeared down the West Coast subduction zone in the Miocene. Before that, it was the Pacific plate's eastern twin across the East Pacific Rise (EPR), a great north-trending mid-ocean ridge once rivaling the Mid-Atlantic Ridge in length. The gradual subduction of the northern end of the EPR along the California coast between 29 and 5 Ma brought the Pacific and North American plates into contact for the first time. Since the Pacific Plate had a northwesterly motion relative to North America, the EPR and the subduction zone had to be replaced with the broad mixed divergent-transform Pacific-North American boundary now familiar as the San Andreas Fault System and the Basin and Range Province.
The small Gorda and Juan de Fuca plates now subducting off Washington and Oregon and the Cocos and Rivera plates now subducting off Central America are nearly-consumed remnants of the Farallon plate. The ridges associated with them are EPR remnants.
 |
Williams Fork Thrust |
The Laramide block uplifts rose up along deep basement-rooted Y-shaped faults as the underlying crust shortened to the east northeast by as much as 15-20% across the combined width of the Southern Rockies and the Colorado Plateau. Few Laramide faults are well exposed, even after ~70 Ma of subsequent erosion, but the shallow Williams Fork Thrust (right) and the Elkhorn Thrust along west side of the Front Range block are notable exceptions.
Many if not all Laramide faults appear to represent reverse (contractional) reactivations of basement-penetrating normal (extensional) faults left over from Late Proterozoic continental rifting. Some Laramide faults, like the Gore fault, were active as reverse faults during the Pennsylvanian Ancestral Rocky Mountain orogeny as well. Laramide fault slips were partitioned among thrust, reverse, oblique and strike-slip motions according to the orientations of the old faults to the Laramide's ENE-trending horizontal regional compression, but low-angle thrust faulting generally prevailed near the surface while high-angle reverse faulting predominated at depth. As reverse motions approached the top of the basement, they appear to have abandoned the steep old rift faults to cut new lower-angle, mechanically-favored short-cut thrusts in some cases.
Whether reverse or thrust, most Laramide faults remain blind—i.e., they failed to cut the surface when they were active, and they were not uncovered during later exhumation. Most died out somewhere within the then flat-lying Paleozoic and Mesozoic sedimentary cover atop the basement harboring the reactivating faults. The monoclinal folding now seen in the cover along the flanks of many Laramide basement uplifts developed as the defining faults cut upward into the base of the cover from the top of basement.
[diagram coming] |
trishear |
Luckily, the shape of a fault-propagation fold provides valuable information about the geometry and movements of the responsible blind fault. Blind Laramide faults have been known from boreholes for years, but geoscientists are just now beginning to tease out their details with the help of a promising new technique known as trishear modeling. The trishear model of fault-related folding has been successfully and profitably applied to the many similar blind thrusts in Southern California's Los Angeles basin, including those responsible for the damaging 1987 M6.0 Whittier Narrows and 1994 M6.7 Northridge earthquakes.
Laramide uplifts are fairly monolithic, but the margins of their Precambrian cores show a good bit of splintering, particularly along the east flank of the Front Range. Near Colorado Springs, the southern end of the east flank of the Front Range block includes a substantial north-trending splinter, the Rampart Range, separated from the main Front Range block by the steep reverse Rampart Range Fault. During the Laramide, the Rampart block actually rose higher than its parent. Garden of the Gods Park straddles the Rampart Range Fault. Late Paleozoic strata there dip much more steeply to the east on the east side of the fault due to differential drag-folding along the fault. North of Garden of the Gods, the Woodland Park half-graben sags into the Rampart Range Fault, thereby preserving early Paleozoic strata found now in few other places along the east side of the Front Range.
With the Oligocene arrival of the Rio Grande Rift, things got even more complicated. The Sawatch Range and the southern portion of the Mosquito Range shared a common basement uplift before the rift split them apart along the deep and narrow upper Arkansas graben. The same thing happened to the originally united Precambrian cores of the San Juan and Sangre de Christo Mountains, which are now 50 km apart across the San Luis Valley, the broadest section of the Rio Grande Rift. Burying most of the San Juan uplift's Laramide core beneath the vast San Juan volcanic field doesn't help. Normal faulting related to but north and east of the main trace of the Rio Grande Rift now complicates many other Laramide structures, including the Gore Range and the lower Blue River Valley to its east.
A monocline is a fold that dips in only one direction, at least locally. The trishear model nicely fits the style of monoclinal range-front folding observed along one or both sides of virtually every Laramide uplift. When monoclines of opposite dips flank a single faulted uplift, it's also accurate to speak of a faulted anticline, even though the paired monoclines may be separated by many miles. Since the Dakota Hogback and the Grand Hogback flank the entire Rocky Mountain Province like gigantic bookends, it's not unreasonable to think of the entire province as one big faulted anticline.
As the reactivated Laramide reverse faults and short-cut thrusts propagated laterally and upward through brittle basement and into the more flexible overlying (usually Paleozoic) sedimentary strata, they died out, but along the way, they folded the sedimentary cover into variably faulted and often asymmetric range-front monoclines. Prime examples of range-front monoclines include
 |
Grand Monocline |
the Great Monocline seen at right along the NW flank of the Uncompahgre Plateau block at the Colorado National Monument
the Dakota Hogback and surrounding strata along the Laramide Golden Fault on the east flank of the Front Range block
Well, that's my Laramide story for the Rockies, and I'm sticking to it—at least until next month's issue of Geology.
|
Four Corners from space, NASA |
The Rockies (in the right half of the photo at right) and the Colorado Plateau (CP, the football-shaped salmon-colored area at left center) have been locked in a dance since at least Middle Proterozoic time, but they've been badly out of step from the Laramide Orogeny on.
Unlike the remainder of the Laramide orogen, the CP acts as a fairly rigid crustal unit. Relative to the Rockies, it moved north northeast over 100 km during the Laramide but rose and deformed less. Stresses related to it approach is likely responsible for the atypical east-west axis of the otherwise Laramide-like Uinta uplift in the upper left corner. The CP also rotated clockwise around a pole near its southeast corner during the Laramide; later it would rotate more. All these movements variously added left-lateral and right-lateral strike-slip components to the Laramide and later normal faults surrounding the CP.
The CP clearly contains basement-cored Laramide uplifts, albeit less dramatic than those in the Rockies. These include the jostled, fault-bounded High Plateaus—the Markagunt, Paunsagunt, Aquarius, Kaiparowits and Kaibab. All except the Aquarius bear Paiute names. At the western margin of the CP, the Kaibab Uplift set the stage for the cutting of the Grand Canyon, just as the Colorado's Gunnison Uplift set up the Black Canyon of the Gunnison.
Promising new paleoaltimetry work based on vesicle sizes in virgin CP basalt flows (Sahagian, 2002) suggests that the CP gained most if not all of its present 2.2 km average elevation during regional uplift after 25 Ma and particularly in the last 5 Ma. Other recent work (Pederson, 2002) relying on the analysis of a CP digital elevation model and reconstructions of the surfaces of the regressive 80 Ma Cretaceous Castlegate sandstone (the last known time the CP was at sea level) and the Eocene-Oligocene stratigraphic boundary favors Early Tertiary Laramide uplift instead. Go figure.
All other things being equal, it's mechanically much easier to overcome friction and generate sliding on a pre-existing fault than it is to rupture a new one—at least at the confining pressures found at upper crustal levels. Old normal (extensional, rift) faults tend to be lined with especially weak rock due to rock-water interactions invited by cross-fault tension and are particularly easy to reactivate.
Old rift faults can thus give expression to regional stresses that might be unable to break intact rock. That was exactly the setup that allowed the relatively weak intracontinental stresses of the Ancestral Rocky Mountain and Laramide Orogenies to generate the uplift, folding and faulting evident in Colorado's current topography and in her geologic record. Don't forget, these deep intracontinental deformations were the fallout of plate interactions playing out over 1,000 km to the south and west, respectively.
Some of the faults reactivated in the Laramide, like the Gore fault, were active as reverse faults in the Pennsylvanian Ancestral Rocky Mountain orogeny as well, but these may also have capitalized on even older rift faults.
The process of squeezing up basement and cover from deep in old rifts along reactivated normal faults is known as rift inversion. Examples of varying ages can be found all over the world, but some of the most striking are right here in our own back yard. The magnificent Laramide Uinta Uplift immediately comes to mind.
To understand how normal faults can reactivate in reverse, one must first understand their geometry and thermomechanical habitat. Few normal faults are truly planar. Most are concave upward instead—near-vertical at the surface, where the initial failure is mainly tensile, but increasingly horizontal at depth, where shear takes over as the rock becomes more ductile. For mechanical reasons, normal faults reaching the crust's brittle-ductile transition (usually around 10 km below the surface) tend to cross it at an angle near 45°. Normal faults extending deeper into the middle crust may flatten out to 30° or less, particularly if they link up to a low-angle regional detachment fault. (No one knows if such a detachment underlies the Laramide orogen, but regional detachments certainly link the normal faults of the highly extended Basin and Range at depth.)
When a previously extended region cut deeply by normal faults comes under compression, as Colorado did during the Laramide, the pre-existing faults are easily reversed at depths of 3-4 km or more, where they dip less than 60°. What happens at higher angles closer to the surface is less clear. At some point, it apparently becomes easier, mechanically speaking, to cut a new shallow short-cut thrust fault to the surface than to continue reverse motion upward along an ever-steepening old normal fault. In many Laramide uplifts, this point is reached below the top of the basement. When that occurs, the short-cut thrusts cut both basement and cover, the abandoned rift fault segments retain their original sense of offset, and the boundary between basement and cover becomes folded along with the overlying monocline of sedimentary strata. Laramide reverse and short-cut thrust faults fold up the monoclines as they propagate into the sedimentary cover, but for some reason, they usually die out before reaching the surface.
Rift inversion appears to an important mechanism behind the highly dependable style of deformation seen throughout the Laramide orogen—basement-cored uplifts bound by thrusts that tend to die out in the sedimentary monoclines flanking the cores. Less well-developed inverted-rift uplifts dot the mid-continent as far as south as the Ouachitas and as far east as the Appalachians. Thanks to the profound Late Proterozoic erosion marked by the Great Unconformity, expressed rift sediments are rarely found in Laramide country, but post-unconformity sedimentary strata manage to doll up the basement cores quite nicely.
The Laramide Orogeny brought magmatism to Colorado and indeed, to the entire region. In Colorado, Laramide volcanics have largely been lost to erosion, but the 62-63 Ma basalt caprocks atop the North and South Table Mountains near Golden are notable remnants. Thick syntectonic volcaniclastic gravel deposits like the Denver Formation on the east flank of the Front Range suggest that basaltic and andesitic Laramide volcanoes had come to dominate the Front Range uplands during Latest Cretaceous through Early Paleocene time.
Laramide intrusions (crust-invading magma bodies that never reach the surface) figure prominently in Colorado history, both geologically and economically. Nearly all fall along the Colorado Mineral Belt, particularly between Breckenridge and Leadville, with a handful of outliers in the northern Sangre de Christo Mountains and in the northern Sawatch Range south of Edwards. Together with a roughly equal number of post-Laramide intrusions, the Laramide intrusions pumped tremendous mineral wealth into the Colorado Mineral Belt, particularly near Leadville and along its Idaho Springs-Ralston shear zone segment. It's no accident that the ^National Mining Hall of Fame & Museum resides in Leadville.
Laramide intrusions dated at 75-55 Ma also mineralized many of the rich copper-molybdenum porphyry deposits found in southern Arizona, southwestern New Mexico, northern Mexico and west Texas. The Laramide had a long reach indeed.
The structural importance of the Colorado Mineral Belt can't be overstated. It receives more thorough coverage in the discussion of post-Laramide magmatism below.
At the onset of the Laramide around 72 Ma in Latest Cretaceous time, most of Colorado remained near sea level. The earth, considerably warmer than now, still lacked polar ice caps. A semi-tropical climate prevailed throughout Colorado, even though it already lay quite near its current latitude. Two major events would soon revise everything, however—the regional Laramide orogeny and the global K-T impact. Here, we'll focus on the sedimentary fallout attending Laramide mountain-building along the east flank of the Front Range.
Laramide uplifts began shedding sediments into intervening and peripheral basins as soon as they began to rise. The initial Laramide sands and gravels exposed in eastern Colorado record the initial rise of the Front Range — the first mountain-building to affect the region since the Ancestral Rocky Mountain orogeny. Late Oligocene and later sediments recorded the regional uplift commencing at 28 Ma and continuing to the present. These sediments include the Latest Cretaceous through Holocene formations listed below.
Latest Cretaceous conglomerates and sandstones collected along the rising Laramide range fronts in flat layers organized into alluvial fans built where streams draining the range front lost their steep gradients and dropped their loads. The Arapahoe rests discontinuously and unconformably atop the Laramie Formation. Prominent within the Arapahoe are secondary sediments eroded from the Paleozoic and Mesozoic sedimentary cover of the Laramide uplifts. It also includes chert nodules eroded from Paleozoic limestones and basement clasts of granite and gneiss.
Latest Cretaceous through Early Paleocene alluvial sediments continued to spread eastward from the range front and to coalesce into bajadas (alluvial fans merged into one continuous range-front surface). By Denver time, however, the sediments washing east off the Front Range uplift had become largely volcaniclastic (derived from volcanic debris), indicating that basaltic and andesitic Laramide volcanoes had come to dominate the Front Range uplands during this chapter of its rise. Fluvial beds containing clasts of andesitic volcanic material characterize the Denver Formation.
The Late Paleocene Green Mountain conglomerate is a still flat-lying collection of gravels composed almost exclusively of pink granitic Precambrian clasts shed from the Front Range to the west after its early Laramide volcanic cover had been breached by erosion.
 |
White outcrops of upper Dawson arkose (D2) dot the slopes of a mesa capped with Castle Rock conglomerate |
Feldspar-rich (arkose) Late Paleocene gravels, sands and muds spreading east over the Denver Basin from the Laramide range front make up the Dawson Arkose, named from outcrops on Dawson Butte ~7 miles south southwest of Castle Rock. The lower Dawson is stratigraphically equivalent to the Green Mountain conglomerate, but it accumulated farther out from the range front. Near Colorado Springs, white bluffs and hoodoos of upper Dawson Arkose clearly visible west of I-25 consist of debris weathered from Pikes Peak granite. The upper Dawson is so friable and easily eroded that, according to one local geologist, "it just melts away" once exposed.
Between the upper and lower Dawson members is a brightly-colored clay paleosol (fossil soil) developed on the floor of a Paleocene tropical rainforest. The paleosol once served as a source of pigments for Native Americans living in the area. It now serves as a readily recognized boundary between the lower D1 and upper D2 units of a much-needed simplified and mappable syntectonic stratigraphy of the Denver Basin recently proposed by Raynolds. Arapahoe, Denver, Green Mountain and lower Dawson strata comprise the D1 unit, which records the initial rise and progressive unroofing of the central portion of the Front Range block. The D2 unit, equivalent to the upper Dawson, records the rise of the Pikes Peak section of the Front Range 8 Ma later along the Ute Pass fault, which CO24 follows across the Front Range west of Colorado Springs.
 |
Castle Rock conglomerate with large clast of Wall Mountain tuff |
|
Castle Rock conglomerate in the east wall of Castlewood Canyon |
The resistant 34 Ma Oligocene Castle Rock conglomerate caps many of the mesas and buttes of the east central Colorado Piedmont between Denver and Colorado Springs, including the prominent butte along I-25 named Castle Rock. Like so much coarse concrete, it contains many granite, quartz and metamorphic cobbles, large and small, in sandy matrix strongly cemented with silica, with occasional ash layers from volcanic activity in the Rockies to east. Among the largest clasts are huge angular boulders of Wall Mountain tuff plucked from the walls of narrow canyons cut through the tuff to the west by powerful range-front streams and breakthrough floods roaring down from upcountry lakes (e.g., Florissant) temporarily dammed by volcanic flows and mudflows of the Middle Phase of Tertiary magmatism. The lower photo at right shows one such clast about the size of a large watermelon.
It is the Castle Rock conglomerate, not the older Wall Mountain tuff, that caps Castle Rock and most of the surrounding buttes and mesas in the east central Piedmont. At the lower photo at right, it forms the walls of scenic Castlewood Canyon south of Franktown. Note the lens of coarse cross-bedded conglomerate (a stream channel) wedged between two massive sandstone units.
The slightly older Larkspur conglomerate underlies the 37 Ma Wall Mountain tuff in the Colorado Piedmont near Larkspur. It contains no clasts of Wall Mountain tuff but is otherwise similar in composition to the Castle Rock conglomerate. It also caps some prominent Piedmont buttes and mesa, including Larkspur Butte (below right).
 |
Castle Rock butte |
As with most mesas and buttes in the east central Colorado Piedmont, Castle Rock's conglomerate caprock rests on easily-eroded Dawson Arkose. The whitish arkose forms the slopes below the caprock, but it's usually poorly exposed. Instead, it's covered over and stabilized by an armor of coarse debris spalled off the near-vertical caprock margins.
Castle Rock is a textbook example of topographic inversion: As a high-energy streambed deposit, its conglomerate caprock once occupied the very lowest spots in the local topography. With burial, it acquired a strong silica cement and locally unsurpassed resistance to erosion, so that its remnants now occupy the highest elevations in the local landscape. Collectively, Castle Rock and surrounding buttes and mesas provide a crude map of the local Oligocene drainage pattern.
 |
|
|
|
Butte collapse rings SW and SSE of Larkspur Butte |
But the inversions don't end there. Geologists have only recently recognized several odd Q-shaped hills in the Castle Rock-Larkspur area. Fully formed examples, like the one seen at right immediately southwest of Larkspur Butte, consist of a C-shaped circular ridge with a jagged profile and an alluvial fan of arkose debris with no obvious source emanating from the gap in the ridge. (The fan is the tongue of the Q.)
According to researchers from the ^Colorado Geological Survey, these odd landforms tell of yet another round of topographic inversion awaiting the buttes standing today — one that might be termed butte collapse. When a butte's caprock is finally breached, meteoric waters enter the central core of easily-eroded Dawson arkose from the top and begin to remove arkose from beneath the cap. Eventually, a small radial stream draining the breach cuts down through the outer slope and builds an alluvial fan of arkose at its base, all typically on just one side of the butte. Angular fragments of the progressively unsupported caprock tumble down both the original outer slopes and newly created inner slopes as well to form the jagged talus flatirons that give these landforms their sawtooth profiles. (Note the talus flatirons on the eastern inner slope in the lower aerial photo at right). Still protected by its armor around most of the butte, the relatively resistant outer slope is eventually left standing as a C-shaped ridge rising in some cases hundreds of feet above the arkose fan and the collapsed center, where the caprock once stood high.
In the topographic map and top aerial photo at right, the originally reported butte collapse ring sits immediately southwest of Larkspur Butte. The bottom photo shows a smaller ring with particularly large talus flatirons and a northwest-directed fan immediately north of Rattlesnake Butte. Just east of this 2nd ring may be yet another, larger and more deeply eroded with a west-directed fan, but preliminary field investigations there have proved equivocal so far.
You couldn't make this stuff up.
From the Latest Miocene on (7-0 Ma), alluvial fans along the range front have continued to build eastward and coalesce over the High Plains from southern Wyoming to New Mexico under the impetus of the regional uplift commencing at 28 Ma. These poorly cemented and loosely compacted gravel, sands and clays range up to 210 m in thickness, taper to the east, and host the famous aquifer of the same name. Ogallala strata were originally laid down flat but now dip to the east, particularly the older layers, in response to ongoing regional uplift. Ogallala deposition ceased in the Denver and Colorado Springs areas when tributaries of the Arkansas and South Platte Rivers beheaded the source streams during their excavation of the Colorado Piedmont.
Broad regional uplift typically precedes the arrival of a propagating continental rift, usually in the form of a broad dome or welt centered on the rift trajectory. Basement-penetrating rift-parallel normal faulting and basaltic volcanism soon follow. The north-propagating Rio Grande Rift has been no exception.
The east-dipping tilt of the High Plains Ogallala gravels flanking the Rockies decreases progressively toward the surface and toward the north. The pattern fits the development of the Rio Grande Rift well. The Ogallala rests unconformably on mid-Tertiary White River Group basin fill north of Denver but on progressively older rocks to the south, down to the Permian in northern New Mexico. This south-widening sub-Ogallala unconformity points to a south-to-north exhumation of the Front Range and its range-front basin as the Rio Grande Rift, not far to the west, uplifted and cracked its way north. Tertiary magmatic inflation of the Rockies probably played a role as well, particularly south of the Front Range, but the magmatism itself may also relate to the Rio Grande Rift, at least in part.
The thick and extensive Wasatch Formation and Green River Formation crop out impressively throughout the western portion of the state even though they tend to be rather poorly consolidated. Both are of Eocene age. The impressive Roan Cliffs west of Rifle on I-70 expose great thicknesses of colorful Green River and Wasatch sediments, which can also be seen all along the flanks of basalt-capped Grand Mesa to the south.
Equivalent eastern Colorado deposits no doubt formed, but none have been preserved.
 |
Roan Cliffs below Grand Mesa |
The older Wasatch Formation was deposited as a mix of fine-grained fluvial and alluvial sands and silts, mostly syntectonic, across a broad western lowland now known as the Piceance Basin (pronounced "pee-on'-see", not "piss ants"). Up to 1.5 km thick, the Wasatch rests conformably on the Late Cretaceous Mesaverde Group.
Fine-grained Green River lake sediments up to 1 km thick accumulated over the Wasatch in a large fresh-water lake now known as Lake Goshiute. The Green River hosts all of Colorado's famous oil shales. These organic-rich "shales" are actually for he most part shaley lacustrine limestones containing reserves of petroleum greater than any elsewhere in the US but devilishly difficult and expensive to extract.
Topographically speaking, a park is a flat-floored valley surrounded by mountains on all sides. In the Rockies, the larger parks are often faulted synclines (downwarps with younger strata bent toward each other) of Laramide origin. Colorado's four major parks include North, Middle and South Parks and the San Luis Valley. North, Middle and South Parks stand low because, for some reason, the Laramide uplifts left them behind while everything around them went up. The broad San Luis Valley, on the other hand, owes its low elevation to its status as the widest part of the Rio Grande Rift.
Unlike foreland basins, parks tend not to collect thick sedimentary covers. They're typically floored with soils or thin veneers of Late Tertiary alluvium or Mid- to Late Tertiary volcanics resting directly on variably deformed pre-Laramide bedrock.
 |
Looking west across South Park from Wilkerson Pass |
 |
Looking southwest into South Park from Boreas Pass |
South Park, for example, is a faulted syncline floored with early Paleozoic through mid-Cretaceous strata, andesitic flows and ash flow tuffs associated with the Thirtynine Mile Volcanic Field, and late Tertiary alluvium. An east-dipping, north-trending Dakota hogback, this time fronting the Mosquito Range, divides South Park at Hartsell. The Elkhorn Thrust, one of the Rockies' few exposed Laramide faults, separates South Park from the Front Range on the east near Wilkerson Pass (9,705'), just as the Williams Fork Thrust demarcates the Front Range along the east side of the Blue River Valley.
Parks typically shelter sedimentary and volcanic deposits long lost to erosion in the surrounding mountains. The Sawatch Formation has generally been eroded away all along the eastern flank of the Front Range, but the down-dropped ^Woodland Park half-graben just west of Colorado Springs preserves remnants. The Oligocene Thirtynine Mile volcanics preserved on the floor of South Park are another example.
On the Geologic Highway Map of Colorado, exposed basement cores of Laramide uplifts are shown in grays and generally bear the symbols "Xm", "Xg", "Ym" and "Yg". Laramide intrusions bear the symbol "TKi" and appear in dark maroon; nearly all fall along the Colorado Mineral Belt, which is easily found by following the diagonal band of dark maroon "Tki" and hot pink "Tmi" intrusions from Four Corners to Boulder. The Laramide basalts of North and South Table Mountain at Golden don't seem to be mapped.
Syntectonic Laramide sediments like Arapahoe conglomerates, Denver volcaniclasitics and Green Mountain conglomerates on Green Mountain near Denver are mapped in pale yellow as "Tl". So are the Wasatch and Green River Formations found in western Colorado.
Under construction
The Mesozoic Era (248-65 Ma, 22:41-23:39
h) and its final Cretaceous
Period (144-65 Ma, 23:14-23:39 h) closed with the great Cretaceous-Tertiary
("K-T") extinction that did in the dinosaurs and many other
successful species on the occasion of the splashdown of a large asteroid or comet near
Chixulub on the north shore of the Yucatan Peninsula. The Cenozoic Era
(65-0 Ma, 23:39-24:00 h) runs from there to the present.
Geoscientists find it convenient to divide the Cenozoic in a number of ways, depending on locale and subject matter.
The Tertiary Period (65-1.8 Ma) encompasses the Paleocene (65-54.8 Ma), Eocene (54.8-33.7 Ma) and Oligocene (33.7-23.8 Ma), Miocene (33.7-5.3 Ma) and Pliocene (5.3-1.8 Ma) Epochs—the first 97% of the Cenozoic.
The Quaternary Period (1.8-0 Ma) started with the Pleistocene Epoch (1.8 Ma - 10 Ka) and continues with the Holocene Epoch (10-0 Ka) from the end of the Ice Ages to the present. It represents the last 3% of the Cenozoic and the last 0.04% of earth history.
Paleogene time (65-23.8 Ma) covers roughly the first two-thirds of the Cenozoic, including the Paleocene, Eocene, Oligocene Epochs.
Neogene time (23.8-0 Ma) includes the Miocene, Pliocene, Pleistocene and Holocene Epochs—roughly, the final third of the Cenozoic.
Confused yet?
 |
Chixulub (red), Yucatan |
As the Cenozoic Era opened in Colorado at 65 Ma (23:39 h), Laramide uplifts had already dominated the regional topography. Syntectonic sediments were piling up around the uplifts throughout the Rocky Mountains and the Colorado Plateau. Basaltic volcanism played out here and there around particularly leaky Laramide faults like the Golden Fault.
Geologically speaking, it was a decidedly wild time around the state, but the climate was semi-tropical, and plant and animal life managed to thrive despite the turmoil. Unfortunately (or fortunately, depending on how you like your place in the food chain), something far wilder was headed their way.
The Mesozoic Era (248-65 Ma, 22:41-23:39 h) slammed shut with the splashdown of a large (~10 km) asteroid or comet near Chixulub on the north shore of the Yucatan Peninsula (large red circle near center in the image above). The ensuing great Cretaceous-Tertiary ("K-T") extinction killed off the dinosaurs and many other successful Cretaceous species, too.
 |
Colorado to Chixulub |
Like just about everywhere else in the world, Colorado literally burned to the ground in the aftermath of the impact. A rather inconspicuous clay layer devoid of fossils but rich in extraterrestrial iridium, impact-shocked quartz grains and elemental carbon (char) marks the K-T boundary all over the globe. In Colorado, the K-T clay and char crops out in only a few locations in the south, including a road cut on I-25 between Trinidad and Raton, where it's all of 5-10 cm thick—not much to show for a global catastrophe. It's also exposed at the south end of Lake Trinidad west of Trinidad in a 5 cm white clay over coal layer.
|
|
Brimming structural range-front basins (green) make up much of the state |
Most Laramide uplifts are paired with deep structural range-front basins, some of which eventually collected sediments to thicknesses of several kilometers. Basins like these form when crustal shortening stacks heavy thrust sheets on top of crust previously in isostatic balance. As the newly overloaded crust sags, its stiffness carries the flexure well beyond the mountain front. Abrupt mountain fronts are particularly effective in creating these range-front downwarps known as foreland basins or foredeeps, and Laramide uplifts typically have at least one steep flank. Similar foreland basins developed around the Frontrangia and Uncompahgria uplifts of the Ancestral Rocky Mountains over 200 Ma earlier, and some of these—most notably the Denver and Raton Basins—were reactivated during Laramide time.
Terrigenous syntectonic sediments began to collect in the foreland basins as soon as the Laramide uplifts emerged from the Cretaceous sea. As the sediments piled up, the receiving basins subsided further under their weight. The basement floor of the Denver Basin now sags nearly 14,000' (4.3 km) below the surface at Denver but sits at least 22,000' (6.8 km) below the same basement horizon in the adjacent Front Range. Not coincidentally, this largest of all Rocky Mountain foreland basins is paired with the largest of the Laramide uplifts. Its contents record the history of a large segment of the state over at least the last 300 Ma. Geologists and paleontologists at the ^Denver Museum of Nature and Science are working to read that record in unprecedented detail in their ambitions ^Denver Basin Project.
|
|
Castle Rock atop the Denver Basin |
By the close of the Laramide at ~40 Ma, the foreland basins had filled to overflowing and had become indistinguishable from surrounding tertiary pediment surfaces. With subsequent regional uplift and exhumation, they were uncovered and eventually incised by crossing streams, particularly in the last 10 Ma. The resulting exposures of syntectonic sediments are invaluable windows into the progressive unroofing of the source uplifts.
The table below lists past and present major foreland basins (as geologists often call them) in and around Colorado, in order of decreasing size within Colorado.
Foreland Basins, Rocky Mountains and Colorado Plateau |
||
| Basin | Source Range or Uplift, Flank | Notes |
|
Denver |
Front Range, east |
|
|
Piceance |
White River uplift, west; Uinta Mountains, south |
|
| Maroon | Uncompahgria, northeast; Frontrangia, southwest |
|
| Paradox | Uncompahgre Uplift, southwest |
|
|
Raton |
southern Sangre de Christo Range, east; Wet Mountains, southwest |
|
|
San Juan |
San Juan Mountains, south |
[] |
| Sand Wash | Uinta Mountains, northeast | [] |
| Laramie | Front Range, northwest; Medicine Bow Mountains, northeast | [] |
The Geologic Highway Map of Colorado marks Lower Tertiary Paleocene through Eocene strata "Tl" and maps them in yellow. This map unit includes the Wasatch and Green River Formations. Upper Tertiary Oligocene through Pliocene units, like the Ogalla, are marked "Tu" and mapped in tan. The anomalous K-T boundary layer doesn't show explicitly, but it might be preserved anywhere Lower Tertiary "Tl" and Upper Cretaceous "Ku1" units are found in contact, as they are around Trinidad at the east edge of the Raton Basin.
 |
West Elk intrusions |
The San Juan Mountains, the West Elk Mountains (right) and the 39 Mile Volcanic Field are imposing erosional remnants of a once vast volcanic blanket that covered much of central and southwest Colorado in mid to late Tertiary time. This post-Laramide magmatism came in three distinct waves. The Early and Middle phases were dominated by explosive volcanism that spread ejecta far and wide across the state. Most of the resulting volcanic deposits are now long lost to erosion, but in their time, they greatly influenced the present courses of some of Colorado's largest rivers and streams. The Rockies are famous for improbable canyons cut through rather than around impressive highlands. Prolific post-Laramide volcanoes shoving streams this way and that before final entrenchment were one of the factors controlling the courses of antecedent streams, particularly at the Black Canyon of the Gunnison. Less violent Late Phase basaltic flows ringed the margins of the Colorado Plateau and capped a number of central Colorado ridges and mesas, preserving them in the process.
In the literature, post-Laramide magmatic events are often referred to simply as "Oligocene". Since the Oligocene period (33.7-23.8 Ma) overlapped all three phases, it can be difficult to assign such events to a specific phase unless a specific date is given.
|
Spanish Peaks, NASA |
As volcanoes busily reworked the post_Laramide surface, the subsurface became riddled with igneous intrusions (magma reservoirs, dikes and sills) that eventually froze in place. The intrusions inflated surrounding host rocks and induced contact metamorphism via their thermal and mineral-rich fluid emanations, as in the Elk Mountains, on the west flank of the Sawatch Range around Aspen, and at ^Spanish Peaks (right) on the east flank of the southern Front Range. Gold, silver, lead, molybdenum, copper and zinc concentrated in ore bodies as the hot magmatic fluids permeated particularly receptive host rocks like the Mississippian Leadville Limestone of mining fame. Intrusive (and presumably volcanic) activity was particularly intense along a Proterozoic line of weakness now known as the Colorado Mineral Belt.
What brought on this hellish outburst is hard to say, but there are some likely suspects, and they're probably related:
Upwelling of hot, buoyant asthenosphere against the base of the North American plate as the Farallon plate broke up and fell away after its flat-slab subduction during Laramide time would uplift and stretch the North American plate from the Laramide orogen west as it produced copious melt within or below the lithospheric mantle. The melts would exploit pre-existing weaknesses like the Colorado Mineral Belt and new failures brought on by post-Laramide extension to reach the upper crust and the surface.
Resumption of normal-angle subduction and hinge rollback along the West Coast between 40 and 29 Ma would have put the backarc in extension once again. The earliest Basin and Range extension likely arose in this way, but how far east did the influence extend?
Additional extensional stresses developing in the crust as the Rio Grande Rift approached from the south and quickly cut north up the middle of the Laramide orogen would make the region even more permeable to the asthenosphere. Note, however, that continental rifting here may or may not have related to extensional processes to the west.
During the Middle Phase of Colorado magmatism at 30-26.5 Ma, the entire Basin and Range east of the Colorado Plateau extended east-west by 100% or more, and that cracking and stretching certainly contributed to the devastating Ignimbrite Flare-up at ~30 Ma. Colorado probably extended less than the Basin and Range, but ignimbrites lit up Oligocene Colorado at the same time.
The first post-Laramide eruptions blanketed large areas with ^andesitic lava flows, volcanic mudflows, ash flows and ash falls, particularly in the West Elk and San Juan Mountains. The far-flung West Elk Breccia dates from this phase.
[photo coming] |
Looking south from Dinosaur ridge, the tablelands on the right are for the most part capped by the WMT. |
Just south of Denver, erosional remnants of 37 Ma Late Eocene Wall Mountain tuff or ignimbrite (AKA Castle Rock ^rhyolite) up to 40' thick record one of the opening shots of the Early Phase in the east — the arrival of a devastating ash flow erupted from the Mt. Princeton area 138 km (86 mi) to the east in the Sawatch Range. The massive ash flow rolled down the Eocene erosional surface to blanket the western part of the Denver Basin incandescent ash at the time up to 400' thick. South Park and other intervening valleys were filled to brimming with syntectonic Laramide sediments at the time and had not yet been exhumed.)
[photo coming] |
[] cm WMT clast from Castlewood Canyon near Franktown, CO |
The gray- to buff-colored Wall Mountain tuff now caps many of the prominent buttes on the west side of the Denver Basin between Denver and Colorado Springs, but the younger Castle Rock conglomerate caps most of the buttes and mesas to the east, including its namesake butte on I-25 (right).
Explosive Oligocene eruptions seared the eroded Early Phase landscape with fast-moving incandescent ash flows (AKA pyroclastic flows, glowing cloud eruptions, nueés ardentes) depositing welded tuffs (ignimbrites) and unwelded tuffs over nearly a third of the state. Middle Phase tuffs apron the south flank of the West Elks to form the Palisades on the Gunnison. Today, rich volcanic soils help to make the Palisades region the premier fruit-growing region in the state, wine grapes included.
The Middle Phase coincides in both timing and style with the Ignimbrite Flare-up that turned the entire Basin and Range into a lifeless moonscape of welded tuffs around 30 Ma. The event probably signaled the return of hot asthenosphere beneath the western third of the continent after removal of the horizontally subducted Farallon slab, as detailed above.
 |
Pliocene basalts capping Grand Mesa |
|
Basalt-capped ridge of the northern Sawatch range |
Late Phase volcanism (25-5 Ma) blanketed the state with less violent basaltic lava flows. This final volcanic episode isn't well preserved, but Late Phase Pliocene basalts still cap Grand Mesa (upper right photo) and many ridges around Glenwood (lower right photo). Late Phase basalts also ring the margins of the Colorado Plateau. These readily distinguished, easily dated, originally flat-lying flows provide investigators with invaluable paleoaltitude and paleotopography indicators applicable to an number of important questions involving exhumation rates, regional tilting, paleoclimates, etc.
Long lineaments across the face of the earth are more often than not the work of deep-seated processes. The Colorado Mineral Belt (CMB) is such a lineament, and it goes down and back a long way. The ^Colorado Geological Survey map at right shows the CMB in blue.
|
Colorado Mineral Belt (blue) |
This legendary 50-mile-wide swath of mineral wealth cuts a straight path from Four Corners at the Colorado-Utah-Arizona-New Mexico boundary to Boulder, CO on the east flank of the Front Range—a distance of ~300 miles. Mineralization of the CMB came primarily by way of intrusions by Early and Middle Phase mid-Tertiary magmas, but Laramide intrusions also played an important role, particularly in the Idaho Springs-Ralston shear zone and continental divide segments at its east end.
The CMB appears to be a lithospheric-scale band of weakness that may well have developed over 1.6 Ga earlier during the Early Proterozoic assembly of Colorado. It may have started out as a suture associated with one of the terranes that docked against the southern margin of the Wyoming Province around 1.8-1.6 Ga to form the Colorado Province, but that conjecture has yet to be documented. CMB segments have been reactivated during the 1.4 Ga Berthoud Orogeny, most notably at Mt. Evans and in the Homestake shear zone of the northern Sawatch.
 |
Colorado Fourteeners |
Today, the CMB appears as a string of heavily intruded and variably mineralized Proterozoic shear zones stretching more than half the diagonal of the state. Geophysically, it's characterized by a major gravity low, low crustal seismic velocities and high heat flow—all suggestive of anomalously hot upper mantle and lower crust below the lineament. Large hot magma bodies related to the Laramide and mid-Tertiary intrusions of the CMB may well still reside in its lower crust.
Topographically, the CMB stands high, even by Colorado standards. The hot mantle and lower crust underlying the CMB probably buoy up the upper crust along its trace. Regardless of the explanation, it's no accident that over half of Colorado's Fourteeners lie within or near the CMB. Nor is it an accident that all but two of the remaining Fourteeners stand on the shoulders of the Rio Grande Rift.
The primary ores of the CMB were generally deposited as mixed metal sulfide veins containing pyrite (FeS2), galena (PbS), sphalerite (ZnS, FeS), and chalcopyrite (CuFeS2) with variable doping by gold, silver and copper impurities. Mineralized sedimentary rocks altered by contact metamorphism tend to host similar metal sulfides. When ground water reacts with pyrite, typically along the upper margin of a sulfide deposit, the sulphuric acid released in turn attacks other sulfide minerals to form secondary (usually oxide) minerals and occasional native gold and silver. Thus, miners often first encountered so-called "oxide minerals" like cerrusite (PbCO3), cerargyrite (AgCl), argentite (Ag2S) and chalcocite (Cu2S) as they dug toward the primary sulfide bodies.
Once weathered out of their host rocks, dense native metals often accumulated in placer deposits along stream beds. In many Colorado mining districts, the easily discovered and worked placers were the first big strikes. Hard rock mining typically ensued only when the sources of played out placer deposits could be located.
On the Geologic Highway Map of Colorado, Tertiary intrusions carry symbols "Tmi" and "Tui" and appear in shades of hot pink; Tertiary volcanics are marked "Tov" and "Tuv" and appear in brown and orange, respectively. To find the Colorado Mineral Belt, follow the discontinuous diagonal band of dark maroon Laramide intrusions marked "Tki" and hot pink mid-Tertiary intrusions marked "Tmi" from Four Corners to Boulder.
Broad regional uplifts with little tilting have a name—epierogenic. They bear little geologic or topographic resemblance to the discrete, fault-controlled uplifts of the Laramide Orogeny. Many lines of evidence now point to one or more episodes of post-Laramide epierogenic uplift affecting the Rockies, the Colorado Plateau, the High Plains, and to a lesser extent, surrounding areas. Since continuous epierogenic uplift starting at 28 Ma with a Miocene acceleration and some spatial variation seems to be the most likely scenario, that's the picture I'll present here. The story is one of hand-in-hand uplift and extension, with a definite chicken-and-egg flavor.
 |
Tertiary pediment, Front Range, looking south from Genesee Mountain |
The mid-Eocene die-out of Laramide deformation at ~40 Ma brought both tectonic calm and volcanic fury to the region. By late Eocene time, erosion and attendant basin filling had reduced the regional landscape to a high surface of at most modest relief punctuated here and there by the tallest remaining peaks and ridges of the Laramide uplifts. Remnants of this Tertiary pediment or Eocene erosional surface are still visible in the Rockies today (right). Thick blankets of post-Laramide volcanics erupted mostly in Oligocene time came to cover large portions of the Tertiary pediment, adding relief near volcanic centers (most notably, the San Juan, West Elk, Thirty Nine Mile volcanic fields) but reducing it everywhere else. With few obstructions and gentle gradients, streams meandered lazily across the pediment, the major rivers included.
Epierogenic uplift of this landscape began late in the Oligocene at ~28 Ma and kicked into high gear in the Miocene at ~10 Ma. Stream incision and exhumation of the by then largely buried Laramide uplifts ensued throughout the region, particularly after the Miocene acceleration, which set the stage for much of the Rocky Mountain and Colorado Plateau topography we see today. This kind of uplift is to slow to measure in real time, even with GPS techniques, but most studies confirm very recent and presumably current uplift at 1-3 times the rate expected from isostatic response to erosion alone.
 |
Big Dome Fourteeners and radial drainage pattern |
The effects of post-Laramide uplift aren't hard to spot. With 50+ peaks above 14,000', the Colorado Rockies boast the highest average regional elevation of any place in North America, Alaska included. They owe much of their extraordinary height to a huge and still rising dome-shaped post-Laramide uplift centered over the intersection of the Colorado Mineral Belt and the Rio Grande Rift near Leadville. I like to think of this mother of all post-Laramide uplifts as the Big Dome, but you won't find that term in the literature. As the map at right shows, most of the state's Fourteeners cluster around Leadville, and all of Colorado's major rivers (in clockwise order from north, the North Platte, Laramie, South Platte, Arkansas, Rio Grande, San Juan, Gunnison, Colorado, White and Yampa) flow off the dome in a radial drainage pattern of grand proportions.
 |
|
Mount Massive (14,421') at the top of the Big Dome, from Leadville Airport |
Deep Yampa River meanders cut in Weber Sandstone, Dinosaur National Park East |
The Colorado Plateau and surrounding uplands gained most of their present elevations in the Late Tertiary as well. These scenic tablelands are now famously dissected by rivers (the Colorado, the Green, the Yampa, the Escalante) with seemingly impossible wide-swinging meanders deeply entrenched in hard rock canyons—a hallmark of epierogenic uplift. In the Rockies, antecedent streams invigorated by rising headwaters and steepened gradients cut spectacular canyons (among them Royal Gorge and the Black, Glenwood, Gore and Wind River canyons) right through the highly resistant crystalline cores of many previously buried Laramide uplifts.
Subtler post-Laramide uplifts are also recorded. Stream course shifts and incision patterns in the High Plains over the Denver basin point to a broad domal uplift of Miocene age centered on the southern Front Range. Up-to-the-south tilting of the sub-Ogallala unconformity and up-to-the-west tilting of the Miocene Ogallala Formation itself are important examples that help geoscientists quantify recent uplift rates and geometries.
Extension went hand-in-hand with post-Laramide epierogenic uplift from the start and no doubt had a large hand in promoting the volcanism that accompanied it. (Volcanism thrives on extension and in many settings requires it.) Normal faults are the most conspicuous markers of extension, and their ages are often tightly constrained by the independently datable structures that they do and do not cut. Blocks down-dropped between opposing normal faults form structural basins (grabens and half-grabens) that accumulate younger sediments. Normal faults and their associated structural basins are much like stretch marks in skin, in that they align roughly perpendicular to the direction of maximum local extension.
 |
Map of Colorado faults showing late Cenozoic to recent movement: orange since 1882, red since 1.8 Ma, yellow since ~ 24 Ma |
A look at Colman's Map Showing Tectonic Features of Late Cenozoic Origin in Colorado or the Colorado Geologic Survey's ^map of faults with late Cenozoic to recent movement at right shows two families of post-24 Ma faults, all normal:
a larger swarm of subparallel north-striking normal faults clearly associated with the Rio Grande Rift— from onset to the present, the single most important focus of extension in the region and the single greatest source of earthquake hazard in Colorado. This family of faults stretches (pun intended) from southern New Mexico into southern Wyoming and is probably still cutting north.
a smaller set forming concentric partial rings around the Rio Grande Rift within Colorado, roughly along the flanks of the Big Dome.
Faults in the first family are involved in continental rifting, which we'll take up next. The second set of faults may be a direct response to uplift of the Big Dome, which must involve stretching of the crust. These faults and their associated structural basins serve as first-order guides to the distribution of post-Laramide extension in Colorado and also speak to its origins, as we'll see in the next section on the Rio Grande rift.
 |
Rio Grande Rift |
The Rio Grande Rift (RGR, right) is a young, large, active and regionally important tectonic feature splitting the Southern Rockies down the middle from New Mexico to southern Wyoming. This fast-moving continental rift first appeared in the geologic record in New Mexico at around 28-27 Ma, reached central Colorado by 26-25 Ma, left behind 10-8 Ma intrusions at the Colorado-Wyoming border and continues to cut northward through southern Wyoming today. The nascent RGR has yet to sunder the continent and produce oceanic crust in Red Sea fashion, but that day could well come. To what extent the RGR relate to extensional processes operating to the west is unclear.
Along the RGR, heat escapes the earth at a much higher rate than in typical intracontinental settings. A broad welt of uplift precedes the RGR's northern cutting tip and continues to rise along the shoulders of the rift well after the tip passes. These features are common among continental rifts the world over.
|
Colorado Plateau (pink), NASA |
The RGR has many direct and indirect surface manifestations in Colorado and New Mexico. In the NASA satellite photo of ^Four Corners at right, the Rio Grande River follows the RGR south out of the San Luis valley, a particularly wide RGR section. The upper Rio Grande valley marks the RGR from there through southern New Mexico. North of the San Luis valley, the RGR narrows through the deep graben of the upper Arkansas valley. The upper Arkansas River follows the RGR south through central Colorado high-country and the San Luis Valley before dog-legging east to cross the Front Range as an antecedent stream and exit the Rockies at Royal Gorge near Cañon City. The dogleg is consistent with the radial drainage pattern associated with the post-Laramide Big Dome uplift of central Colorado.
North of Leadville, the RGR lacks a discrete topographic expression, but geophysical studies show that its cutting edge has already crossed into southern Wyoming. Just east of the northern RGR trend, however, between the Gore and Front ranges, is the lower Blue River valley, a half-graben claimed to be the northernmost structural expression of the RGR. The much broader and more recently recognized Central Rockies Extensional Province (CREP) of northern Colorado and southern Wyoming may or may not be directly related to the RGR, but the extensional tectonics they share probably stem from the same processes.
|
The Rio Grande rift coincides with the swarm of NNW-trending faults at left center in this map of Colorado faults showing late Cenozoic to recent movement: orange since 1882, red since 1.8 Ma, yellow since ~ 24 Ma |
Shoulder uplifts are a universal feature among continental rifts, and the RGR is no exception. Riding high on the shoulders of the RGR are the Tenmile and Mosquito Ranges on the east and the Sawatch Range on west. Tenmile Creek and its impressive Tenmile Canyon appear to follow a northern extension of the Mosquito Fault, one of the major normal faults defining the east shoulder of the RGR in central Colorado, right through the heart of the once undivided Park-Gore-Tenmile-Mosquito basement uplift.
Unlike the Colorado Mineral Belt, which appears to follow ancient lithospheric weaknesses, the upstart Rio Grande rift follows its unswerving northerly course in complete disregard for older structures, no matter how large. In or around Early Miocene time, it sliced right through the eastern limbs of the massive Laramide-vintage Sawatch and San Juan basement uplifts. The upper Arkansas River and CO24 now occupy a deep rift valley between the Sawatch Range on the west and its much smaller fragment on the east, the Mosquito Range. Likewise, the San Luis Valley now occupies a 50 km gap between the San Juan uplift on the west and its Sangre de Christo fragment on the east. The RGR now appears to be plowing into the Wyoming Rockies with equal abandon.
[Part of the RGR story involves the Colorado Plateau. ??]
Causes for post-Laramide epierogenic uplift and extension in the Rocky Mountains and the Colorado Plateau remain obscure—in part because it's a chicken-and-egg question, but also because the mantle's still very difficult to observe. We won't be able to tease out which came first, extension or uplift, but it's important to recognize all the positive feedback loops involved. No less than four processes are involved.
Primary extension driven by divergence at the mixed Pacific-North American plate boundary, the closest one around by a long shot
Gravitational collapse of the elevated North American upland from the Laramide orogen west
Primary uplift driven by the upwelling of hot, buoyant asthenosphere
Isostatic response to erosion
Their combined actions and second-order effects may well have accelerated both uplift and extension.
Depending on scale, finite arch- or dome-like uplifts in the absence of compression must stretch and thin the overlying crust, if not the entire lithosphere—even with magmatism filling in gaps and adding volume as best it can. But primary extension can also lead to secondary uplift. A relatively clean example of the latter occurs along continental rifts like the Rio Grande rift. Substantial uplift nearly always occurs along the shoulders of such rifts. In fact, early sediments shed off shoulder uplifts are commonly used to date the onset of rifting. Upwelling of hot, buoyant asthenosphere into the rift zone is the usual suspect here, but shoulder uplifts affect modern rifts whether or not they show high heat flows, so there may be more to it than thermal inflation.
Consider now a less tidy case at larger scale pertinent to the question at hand. Thinning of the lithosphere across the Basin and Range as it stretches to keep the North American plate in contact with the partly diverging Pacific plate has led to a broad arch-like uplift of the entire province, again presumably due to the influx of hot asthenosphere beneath the thinning plate. But gravitational collapse driven by the uplift further thins the lithosphere, which in turn fosters more uplift by buoyant forces rooted in the mantle.
Complicating the picture further is the isostatic response to erosion. The higher surface elevation rises due to tectonic uplift, the faster erosion removes mass. But as erosion removes Rocky Mountain mass to the High Plains, the Basin and Range and beyond, the rocks of the Rockies rise via isostatic rebound. By itself, rebound can't raise or even maintain average surface elevation against erosion on a regional scale, but in combination with ongoing tectonic uplift, it has no doubt slowed the regional loss of surface elevation due to erosion. Gravitational collapse (a form of extension) can also trigger isostatic rebound.
[] Thankfully, at least one potential explanation can be ruled out. Intrusive and thermal inflation of the crust related to post-Laramide magmatism has clearly contributed to some local uplifts (e.g., in south central Colorado) but can't account for the regional uplifts at issue here. Excessive heat build-up beneath thermally insulating continental lithosphere might play a role at larger scale and greater depth, particularly in a mobile belt like the Colorado Province, but tomographic imaging shows lower regional upper mantle temperatures beneath the Rocky Mountains and Colorado Plateau than beneath the lower Basin and Range.
Post-Laramide machinations of the Farallon plate clearly brought uplift, extension and magmatism to the Basin and Range, but did its influences extend east to the Rockies and the Colorado Plateau? It's hard to say.
At the conclusion of Laramide deformation at ~40 Ma, the East Pacific Rise, the Farallon plate's ridge of origin, was about to be subducted along the west coast of the United States. The intervening and then subducting portions of the Farallon plate must have been young, hot and relatively buoyant from that time on, but some degree of normal-angle subduction appears to have resumed along the West Coast. Renewed hinge rollback put a broad, diffuse backarc region into extension once again. The earliest Basin and Range extensions date back to this time.
Meanwhile, the portion of the Farallon plate already beneath North American had been rolling back, breaking up and perhaps folding up as it pulled away from the underside of the continent to end the Laramide period of flat-slab subduction. Basin and Range magmatism related to this process lasted roughly from 54 to 21 Ma, presumably via a prolonged and complex influx of hot asthenosphere replacing the slab.
It would seem unlikely that asthenospheric upwellings related to the Farallon rollback would continue much beyond the die-out of Basin and Range magmatism at 21 Ma, but abnormal upper mantle does appear to be present under central Colorado today, roughly beneath the Big Dome and the Rio Grande rift. Tomographic imaging sees this so-called Aspen anomaly as a large blob-like region of anomalously low shear wave velocities presumably reflecting higher than normal temperatures and greater than normal buoyancy. Whether the Aspen anomaly resides in the lithospheric mantle or in the underlying asthenosphere remains under debate, but either way, the presumed buoyancy could help to explain the many high rootless mountains covering the Big Dome.
No one knows where the Aspen anomaly came from or how long it's been there, but it's not a good explanation for post-Laramide uplift in general. For one thing, it's too small relative to the region affected.
|
Central America plateau, south of Yucatan |
Now consider a somewhat analogous epierogenic uplift involving the Central American plateau of Honduras, well east of the magmatic arc associated with the Middle America Trench. Tomographic imaging reveals that the subducting Cocos plate broke off beneath the plateau and that hot asthenosphere has welled up beneath the plateau via the slab defect. The deeply entrenched river meanders dominating the topography of the Central American plateau indicate that its uplift occurred with little tilt. Note that the Colorado Plateau has similar topography. Note also that the plateau began to rise at ~10 Ma, about when the slab broke. The near-simultaneity of these events implies a degree of asthenospheric mobility that stands against lingering effects of the Farallon rollback in the Rocky Mountain region today. Uplift in the Carpathian Mountains has also been tied to slab detachment.
Plate-driven extension has clearly played a role in the Basin and Range since the inception of the partly divergent Pacific-North American plate boundary at 29 Ma. Have the Rocky Mountains and Colorado Plateau taken part? Again, it's hard to say.
Over the last 29 Ma, the nearest plate boundary, the Pacific-North American, has been at least 1,100 km away from the central Rockies. All other North American plate boundaries have been far too distant to have had any credible influence here.
Divergence along the Pacific-North American boundary has been driving Basin and Range epierogenic uplift and extension since at least 29 Ma. Recall that the Basin and Range stretched to the west by over 100% during the mid-Tertiary alone. Coeval gravitational collapse of the Basin and Range further drives its extension with mixed effects on regional uplift there.
How much of this plate-driven extension spilled east into the Rocky Mountain and Colorado Plateau region is uncertain, but it's unlikely to have stopped abruptly at with the Basin and Range. East of the Basin and Range, greater uplift and lesser extension have played out over the same period, the latter with a somewhat different style dominated by the Rio Grande rift.
Gravitational Collapse?
[] The pre-Laramide Creataceous Sevier orogeny to the west contributed to oversteepening of the Basin and Range but stopped short of the Rocky Mountains and the Colorado Plateau.
On the Geologic Highway Map of Colorado, Upper Tertiary sediments are marked "Tu" and appear in beige.
However grand, the Rocky Mountains and Colorado Plateau we see today are mere remnants. Erosion began its attack as soon as the Laramide uplifts began to rise (hence the syntectonic early Laramide gravels of Green Mountain near Denver) and accelerated through the mid- and Late Tertiary as regional uplift progressed and the climate became even wetter. Streams and later glaciers carried off massive amounts of material, particularly from the higher elevations, to reveal the surface we see today. The digging out of the Rockies and the Colorado Plateau is a process geologists call exhumation.
 |
The Arkansas River in Royal Gorge |
Today, dramatic Colorado landmarks like Royal Gorge (right), Glenwood Canyon and the Black Canyon of the Gunnison give the impression that ornery Rocky Mountain streams would rather chew their way through uplifts, ridges and ranges than go around them. Of course, no such thing ever happened, but what did happen is just as strange. Streams that persist in courses first set in long-gone topography are called antecedent, and antecedent streams, large and small, contribute heavily to topographic style throughout the Rockies and the Colorado Plateau.
 |
Tertiary pediment, looking south from Genessee Mountain, infrared telephoto |
By the time the regional Laramide deformations died out in Late Eocene time, around 40 Ma (23:47 h), the Laramide Rockies had already been eroded to a lofty surface of relatively low relief sloping to the east in rough continuity with the High Plains. The tallest peaks, many probably in well above 10,000', continued to stand high above this otherwise broad upland. Lesser peaks and ridges, on the other hand, ended up buried in their own debris as surrounding intermontane basins filled and eventually overflowed. Eventually, gentle gradients and a dearth of structural barriers in the Eocene landscape allowed even the largest streams to meander at will. At maturity, around 38 Ma or so, the landscape must have resembled today's Wyoming Basin. Many know the vast high plains and occasions ridges and peaks there from their travels along I-80.
|
Tertiary pediment, looking south from Genessee Mountain |
Remnants of this vast tertiary pediment or Eocene erosional surface are still easily spotted throughout the Rockies—e.g., along the east slope of the Front Range between Mount Evans and Pikes Peak (right and above) and also across the Front Range between Boulder and Kremmling.
Mid-Tertiary magmatic inflation, mid- and Late Tertiary regional uplift and extension and an increasingly wetter climate together reinvigorated the many streams meandering across low-relief uplands throughout the Rockies and the Colorado Plateau. By most accounts, regional uplift and climate changes aligned to kick stream incision into high gear around 10 Ma (23:57 h). By 6 Ma, Rocky Mountain streams were on average removing more material from their beds than they received from upstream.
 |
The Gunnison River in its Black Canyon |
Where stream courses were no longer subject to revision without notice by volcanic events, entrenchment ensued. Deep, steep-walled, hard-rock canyons began to dissect the Eocene erosion surface. As long as the water kept coming and gradients remained steep, entrenched streams had little choice but to cut down as best they could through anything they found in their beds. When the entrenched streams finally reached the hard sedimentary covers and even harder crystalline cores of long-buried the Laramide basement uplifts (as did the Gunnison at its Black Canyon, above right; the Arkansas at Royal Gorge, below right; and the Colorado at Gore and Glenwood Canyons), they simply rolled up their sleeves, narrowed their cuts and kept on sawing.
|
The Arkansas River in Royal Gorge |
As subsequent erosion stripped more and more alluvial, volcanic and sedimentary cover from the higher Laramide uplifts, their dissected cores were left standing high, complete with mind-boggling range-crossing antecedent streams. The high intermontane basins between the uplifts were also stripped of much of their fill during this time, with small remnants now left clinging to mountain slopes here and there like so many crusty bathtub rings.
In the Colorado Plateau and adjoining flat-topped uplifts of the West Slope, meanders deeply entrenched in resistant sedimentary rock deserve credit for the head-scratching scenery in places like Echo Park in ^Dinosaur National Monument and in Utah's completely improbable Escalante Canyon.
For stunning aerial photos of some of Colorado's more spectacular antecedent stream courses, visit geologist Louis Maher's Rivers that Cut Through Mountains. In fact, I'd recommend taking the time to study all of his aerial photo pages. Nothing imparts an appreciation of the geometry and provenance of a major landform as immediately as a well-chosen aerial view.
As the entire Rocky Mountain and Colorado Plateau region rose higher and higher, stream incision eventually extended downstream to dissect the by now brimming range-front basins as well. The South Platte River and its tributaries, for example, handily exhumed the western margin of the Denver Basin to create the range-front trough of rolling hills and tablelands now know as the Colorado Piedmont (AKA "I-25 Corridor"). The wet Late Tertiary climate provided ample hydraulic power for the project.
Above 8,000', particularly exuberant stream incision set the stage for the glaciations to come. The glaciers would take the exhumational ball and run with it, but much of the work had already been done.
|
The southern Gore Range |
Post-Laramide exhumation of the Rockies removed an immense mass from the highlands, the peaks and intermontane basins alike, and redeposited it across the High Plains and in the lower Colorado River basin. Just as a coal barge rises up out of the water as it's unloaded, so the thick Rocky Mountain crust has risen up out of the denser, plastic mantle on which it floats in response to erosion. Isostatic rebound due to erosion can never produce net surface uplift on a regional scale, but it can easily produce net rock uplift, which brings a specific rock horizon to a higher elevation even as the surface continues to drop. In a glacial setting, isostatic rock uplift can increase the net elevation of the highest summits in a range (but not average elevation of the region) as large valley glaciers gouge out wide and deep moats around them.
All credible paleotopographic indicators suggest that the highest portions of the Rockies have been standing near or above 10,000' since the onset of the Laramide Orogeny. Over 65 Ma of alpine elevation in the face of continuous and at times prodigious erosion can only mean ongoing uplift well beyond that attributable to isostatic rebound. By comparison, the Ancestral Rocky Mountains came and went from plain to plain in a mere 50 Ma.
Recent work by Margaret McMillan et al. documenting long wavelength tilting of the Ogallala and similar range-front alluvial deposits up to the east makes it very difficult to pin the exhumation of the Rockies on climate change alone. Most of the incision now observed was already in place when the climate turned decidedly wetter and colder in the Pliocene at ~4 Ma. Continuing uplift well beyond that attributable to exhumation-induced isostatic rebound is the recurring answer, and Colorado's restless upper mantle remains the prime suspect.
On the Geologic Highway Map of Colorado, Upper Tertiary sediments shed during exhumation are marked "Tu" and appear in beige.
The Tertiary Period (65-1.8 Ma, 23:39:12 - 24:59:25
h) ended and the Quaternary Period
(1.8-0 Ma, 24:59:25 - 24:00:00 h) and Pleistocene Epoch (1.8 - 10 Ka,
24:59:25 - 23:59:59.81) began with the onset of
the Ice Ages at 1.8 Ma. The ensuing Holocene Epoch
(10-0 Ka, 23:59:59.81 - 24:00:00 h) opened at the end of the Ice Ages and continues to unfold.
 |
Iceland |
Only a few small glaciers survive in Colorado today, indeed in the United States Rockies, but most Colorado uplands currently above 8000' underwent rapid glacial erosion during the Pleistocene and probably during the Pliocene as well. Like the Lake Creek Glacier at Twin Lakes, some of the larger valley glaciers managed to nose their way down to even lower elevations.
Most surviving glacial cirques, valleys, moraines, tills and outwash deposits in the Rockies date from North America's two most recent glacial periods—the Bull Lake (~160-40 Ka) and Pinedale (30-10 Ka). Continental ice sheets didn't reach the Colorado Rockies in Bull Lake and Pinedale times, but extensive alpine ice sheets formed as associated mountain glaciers coalesced. Deeply weathered Pre-Bull Lake moraines like those fronting the east slope of the Gore Range and east of ^Rocky Mountain National Park are found here and there in the Rockies. Pre-Bull Lake ice advances date from 5 Ma to 245 Ka, depending on the source. Sediment pulses recorded in the Wet Mountains document several well-known post-Pinedale cold periods, including the 1550-1860 A.D. world-wide glacial advance known as the Little Ice Age. Today, large glaciers like ^Iceland's retreating Breidamerkurjökull (right) survive only at high latitudes or very high elevations.
|
Southern Gore Range |
Alpine glaciers are by far the most efficient of all agents of erosion. Under the right circumstances, bedrock streams can erode spectacularly, as the Black Canyon of the Gunnison and the Glenwood and Grand Canyons of the Colorado attest, but glaciers are generally far more effective when it comes to local mass removal and relief generation. Flowing ice can pluck up massive volumes of bedrock and pre-existing valley fill and transport the load far beyond the mountains of origin in a relatively short period of time.
 |
Chicago Lakes, Mt. Evans |
Alpine glaciers typically remove the greatest volumes of rock well below their source areas, after individual ice flows have coalesced into immense valley glaciers like the one that carved South Willow Creek Canyon (right) into the southern Gore Range. In so doing, they initiate an interesting positive feedback loop that further enhances their erosive prowess. As large valley glaciers unload their host ranges, local isostatic rebound can produce net surface uplift in the high peaks above them.* As they elevate their own source regions and collect even more snow with even less summer melting, the valley glaciers grow and accelerate.
Alpine glaciers also enhance the erosive power of the bedrock streams below them. Glacial outwash streams pass large fluxes of water and abrasive sediment to draining fluvial systems at lower elevations. Glacial sediments may choke the lower streams at times, but in the long run, these glacial inputs only boost stream cutting power downslope. Glacially-induced isostatic rebound may steepen stream gradients as well.
Due to its lower average (2.2 km, ~7,200') and maximum elevations, the Colorado Plateau largely escaped glaciation.
On the Geologic Highway Map of Colorado, the glacial gravels preserved here and there at elevation are not shown explicitly, but glacial outwash contributes.
Under
construction
The Quaternary Period (1.8-0 Ma, 23:59:25 -
24:00:00 h) opened
with the Pleistocene Epoch (1.8 - 10 Ka,
23:59:25 - 23:59:59.81 h),
which coincided with the Ice Ages. The Holocene Epoch (10-0 Ka,
23:59:59.81 - 24:00:00 h) has been
in effect ever since.
Regional uplift due to ongoing mantle unrest and isostatic
rebound continue in Colorado and neighboring states to the north, south and
west, albeit perhaps at a slower pace than when the Rio Grande Rift
first cracked its way north through these parts.
Stream erosion continues through the Holocene as well, but with the much drier Holocene climate, but it's no match for the Neogene exhumation that began in earnest at 10 Ma. Our current climate is Colorado's driest since Early Jurassic Entrada desert dune fields dominated the state (at least in the west) around 200 Ma. Compared to the wet Pliocene and Pleistocene climates, the 12-14 inches of precipitation the Front Range averages annually now is a drop in the bucket.
 |
John Charles Frémont |
Tree-ring studies reveal a longstanding 40-year wet-dry periodicity in Colorado's Holocene climate. Some worried that the drought of 2001-2002 might signal the start of a decades-long dry cycle, but the late spring snows and rains of 2003 and expedition logs kept by explorer John Charles Frémont (right) in the 1840s remind us that wide swings in annual precipitation are the norm here now, regardless of longer term trends.
On the Geologic Highway Map of Colorado, Quaternary sediments are marked "QT" and appear in beige along with upper Tertiary ("Tu") rocks.
|
Royal Gorge |
Well, there you have it: Colorado—as gorgeous a place as you could ever hope for. And it's not just a another pretty face: It's been around the block, and it knows a thing or two.
So, what's next for Colorado? The short-range forecast calls for continuing uplift and exhumation across the state—in the Rockies, the Colorado Plateau, and the High Plains—at the urging of mantle unrest and isostatic rebound. For the foreseeable future, Colorado will continue to stand high while her valleys deepen and her flat-topped uplands are cut back by stream erosion.
 |
Baja Peninsula |
What the mantle has in mind is anybody's guess. If the region's dominant active geodynamic process, the Rio Grande Rift, continues the rapid progress it's made so far, the lower Rio Grande Valley could become a seaway akin to the Sea of Cortez (right) in a few million years. In a few million more, Leadville could find itself a beach resort with scenery second to none featuring towering Sawatch Fourteeners across the waves. I'd wouldn't buy up real estate there just yet, though. Continental rifts fail all the time, as the Uinta and San Juan basins will testify.
Climate has to be in the equation going forward, just as it's been in the past. If the Colorado climate continues to dry, the overall pace of erosion will fall off, but if global warming brings the predicted fewer but bigger storms per season, incision of the landscape could keep pace or even accelerate. (Most of the work of erosion is done by bank-full streams and 100-year gully-washers, not by average weather.) If another Ice Age should ensue, as some climatologists foresee, we could expect a return to the Pleistocene conditions prevailing a mere 10 Ka ago at 23:59:59.81 on the day of creation ending now.
The cooling of the earth in the presence of gravity drives plate tectonics.
Gravity's probably not going anywhere, but by some estimates, the earth has only
about 4-5 Ga worth of residual, chemical and
radiogenic heat to shed before cooling into a tectonically dead one-plate
planet like Mars and Venus. Real-estate will be a lot more dependable by then,
but the sun will enter its helium-burning red giant stage and engulf us in about
the same time frame, if not sooner, so things may get pretty hot. Even so, if we
can manage to turn ourselves away from the bright, dancing flame of
self-extinction, that still leaves time for plenty of surprises—including more
big impacts and a return of surf music. Maybe there's something to be said for
species-turnover after all.
In addition to the home page references, this article relies on the following sources, in alphabetical order by first author:
Allmendinger RW, Shaw JH, 2000, Estimation of fault propagation folding from fold shape: Implications for earthquake hazard assessment, Geology v. 28, p. 1099-1102 (December)
Axen GJ, Selverstone J, et al., 1998, If the Strong Crust Leads, Will the Weak Crust Follow?, GSA Today v. 8, no. 12, p. 1-8 (December)
Bennison AP et al., 1990, Geological Highway Map of the Southern Rocky Mountain Region, American Association of Petroleum Geologists, Tulsa, OK
Bergendahl MH, 1963, Geology of the Northern Part of the Tenmile Range, Summit County, Colorado, Geological Survey Bulletin 1162-D
Betts PG, Giles D, Lister GS and Frick LR, 2002, Evolution of Precambrian the Australian lithosphere, Australian Journal of Earth Sciences, v. 49, p. 661-695
Bowring SA and Karlstrom KE, 1990, Growth, stabilization, and reactivation of Proterozoic lithosphere in the southwestern United States, Geology v. 18, p. 1203-1206 (December)
Bump AP, 2003, Reactivation, trishear modeling and folded basement in Laramide uplifts: Implications for the origins of intra-continental faults, GSA Today v. 13, no. 3, p. 4-9 (March)
Collettini C, Sibson RH, 2001, Normal faults, normal friction?, Geology v. 29, p. 927-930 (October)
Colman SM, 1985, Map Showing Tectonic Features of Late Cenozoic Origin in Colorado, Miscellaneous Investigation Series, Map I-1566, USGS
Crosswhite JA, Humphreys ED, 2003, Imaging the mountainless root of the 1.8 Ga Cheyenne belt suture and clues to its tectonic stability, Geology v. 31, p. 669-672 (August)
Collins AG, Guralnick R, Speer BR, Web Geological Time Machine, University of California Museum of Paleontology, Berkeley, CA
CD-ROM (Continental Dynamics of the Rocky Mountains) Working Group, 2002, Structure and Evolution of the Lithosphere Beneath the Rocky Mountains, GSA Today v. 12, no. 3, p. 4-9 (March)
Dickinson WR, Lawton TF, 2003, Sequential intercontinental suturing as the ultimate control for Pennsylvanian Ancestral Rocky Mountains deformation, Geology v. 31, p. 609-612 (July)
Dann JC, 1991, Early Proterozoic ophiolite, central Arizona, Geology v. 19, p. 590-593 (June)
Durrheim RJ, Mooney WD, 1991, Archean and Proterozoic crustal evolution: Evidence from crustal seismology, Geology v. 19, p. 606-609 (June)
Erslev EA, 1991, Trishear fault-progation folding, Geology v. 19, p. 617-620 (June)
Fichter LS, 2000, ^Introduction to Sedimentary Rocks, ^Introduction to Igneous Rocks, ^A Very Simple Classification for Metamorphic Rocks
Giles D, Betts P and Lister G, 2002, Far-field continental backarc setting for the 1.80-1.67 Ga basins of northeastern Australia, Geology v. 30, p. 823-826 (September)
Gilmer AK, Kyle JR, et al., 2003, Extension of Laramide Magmatism in southwestern North America into Trans-Pecos Texas, Geology v. 31, p. 447-450 (May)
GTR Mapping, 1998, Colorado Geologic Highway Map, 1991 revision, scale 1:1,000,000 (available at ^MAPSCO, Denver, CO)
Hamilton WB, 1989, "Crustal geologic processes of the United States", in Pakiser LC and Mooney WD, eds., Geophysical framework of the continental United States, Geological Society of America Memoir 172, p. 743-781.
Heller, PL, Dueker K, McMillan ME, 2003, Post_Paleozoic alluvial gravel transport as evidence of continental tilting in the U.S. Cordillera, GSA Bulletin v. 115, p. 1122-1132 (September)
Hill BM and Bickford ME, 2001, Paleoproterozoic rocks of central Colorado, Accreted arcs or older extended crust?, Geology v. 29, p. 1015-1018 (November)
Humphreys ED, 1995, Post-Laramide removal of the Farallon slab, western United States, Geology v. 23, p. 987-990 (November)
Johnson KR and Reynolds RG, 2001, Ancient Denvers—Scenes from the Past 300 Million Years of the Colorado Front Range, Denver Museum of Nature and Science
Karlstrom KE, Daniel CG, 1993, Restoration of Laramide right-lateral slip in northern New Mexico by using Proterozoic piercing points: Tectonic implications from the Proterozoic to the Cenozoic, Geology v. 21, p. 1139-1142 (December)
Kellogg KS, 2002, Geologic Map of the Dillon Quadrangle, Summit County, Colorado, US Geological Survey Miscellaneous Field Studies Map MF-2390
Kellogg KS, Bartos PS and Williams CL, 2002, Geologic Map of the Frisco Quadrangle, Summit County, Colorado, US Geological Survey Miscellaneous Field Studies Map MF-2340
Kueker K, Yuan H, and Zurek B, 2001, Thick-Structured Proterozoic Lithosphere of the Rocky Mountain Region, GSA Today v. 11, no. 12, p. 4-9 (December)
Leonard EM, 2002, Geomorphic and tectonic forcing of late Cenozoic warping of the Colorado piedmont, Geology v. 30, p. 595-598 (July)
Livaccari RF and Perry FV, 1993, Isotopic evidence for preservation of Cordilleran lithospheric mantle during the Sevier-Laramide orogeny, western United States, Geology v. 21, p. 719-722 (August)
Marshak S, Karlstrom KE and Timmons JM, 2001, Inversion of Proterozoic extensional faults: An explanation for the pattern of Laramide and Ancestral Rockies intracratonic deformation, United States, Geology v. 28, p. 735-738 (August)
Marshak S, Paulson T, 1996, Midcontinent U.S. fault and fold zones: A legacy of Proterozoic intracratonic extensional tectonism?, Geology v. 24, p. 151-154 (February)
McCaffrey R, Abers GA, 1991, Orogeny in arc-continent collision: The Banda arc and western New Guinea, Geology v. 19, p. 563-566 (June)
Montgomery DR, 2002, Valley formation by fluvial and glacial erosion, Geology v. 30, p. 1047-1050 (November)
Murphy JB, Strachan RA et al., 2000, Proto-Avalonia: A 1.2-1.0 Ga tectonothermal event and constraints on the evolution of Rodinia, Geology v. 28, p. 1071-1074 (December)
National Geographic Society, 1998, Physical Earth (insert map)
Nyman MW, Karlstrom KE, et al., 1994, Mesoproterozoic contractional orogeny in North America: Evidence from ca. 1.4 Ga plutons, Geology v. 22. p. 901-904 (October)
Pederson JL, Mackley RD and Eddleman JL, 2002, Colorado plateau uplift and erosion evaluated using GIS, GSA Today, v. 12, no. 8, p. 4-9 (August)
Raynolds RG, 2002, Upper Cretaceous and Tertiary Stratigraphy of the Denver Basin, Colorado, Rocky Mountain Geology, v.37, p. 111-134 (November)
Raynolds RG and Johnson KR, 2003, Synopsis of the stratigraphy and paleontology of the uppermost Cretaceous and lower Tertiary strata in the Denver Basin, Colorado, Rocky Mountain Geology, v.38, p. 171-181 (May)
Reed JC, ??, Colorado Fourteeners, Trail & Timberline (Colorado Mountain Club newsletter), no. 956
Rogers, RD, Hrafnkell K and van del Hilst RD, 2002, Epeirogenic uplift above a detached slab in northern Central America, Geology v. 30, p. 1031-1034 (November)
Ryskin G, 2003, Methane-driven oceanic eruptions and mass extinctions, Geology v. 31, p. 741-744 (September)
Sahagian D, Proussevitch A and Carlson W, 2002, Timing of Colorado Plateau uplift: Initial constraints from vesicular basalt-derived paleoelevations, Geology v. 30, p. 807-810 (September)
Scott RB et al., 2001, Geologic Map of the Colorado National Monument and Adjacent Areas, Mesa County, Colorado, scale 1:24:000, Geologic Investigations Series I-2740, USGS (available at the Colorado National Monument Visitor's Center)
Shaw CA, Karlstrom KE, et al., 2002, ^Proterozoic Shear Zones in the Colorado Rocky Mountains: From Continental Assembly to Intracontinental Reactivation, printed handout from the GSA field trip of the same name
Shaw CA, Karlstrom KE, et al., 2001, Electron-microprobe monazite dating of ca. 1.71-1.63 Ga and ca. 1.45-1.38 Ga deformation in the Homestake shear zone, Colorado: Origin and early evolution of a persistent intracontinental tectonic zone, Geology v. 29. p. 739-742 (August)
Taylor AM, 1992, Guide Book to the Geology of Red Rocks Park and Vicinity, Cataract Lode Mining Company
Taylor AM, 1999, Guide to the Geology of Colorado, Cataract Lode Mining Company
Tweeto, Ogden, 1979, Geologic Map of Colorado, scale 1:500,000 (available at ^MAPSCO, Denver, CO)
Tyson AR, Morozova EA et al., 2002, Proterozoic Farwell Mountain-Lester Mountain suture zone, northern Colorado: Subduction flip and progressive assembly of arcs, Geology v. 30. p. 943-946 (October)
Varga RJ, Rocky Mountain foreland uplifts: Products of a rotating stress field or strain partitioning?, 1993, Geology v. 21. p. 1115-1118 (December)
Vorlink MW, ^Tectonics of the Banda Arc website.
Unless explicitly attributed to another contributor, all content on this site © Jeremy McCreary. Comments and corrections to colorado@cliffshade.com. Nothing is bought or sold here. Spam will be forwarded to the proper authorities. |
