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Last modified 10/22/04Top Page IndexDinosaur Ridge and Vicinity
Twenty miles or so west of Denver on I-70 is ^Dinosaur Ridge (DR), a particularly prominent segment of the Dakota hogback heavily visited for its famous, easily accessible dinosaur trackways and fossils, striking rock exposures, panoramic views and outstanding ridge-crest trail, and many nicely-done interpretive markers. After umpteen visits, DR remains a family favorite, and my kids don't even like to hike. On moving here from California, it was a most welcome introduction to the natural wonders of the greater Denver area, and it's the one place we strive to take all our out-of-town guests. Geologically, the DR area has been heavily studied, early on because of its excellent and tantalizing natural exposures, its proximity to Denver, long a hotbed of geoscience talent, and a number of nearby oil seeps. More recently, much of DR's interior has been laid open to direct inspection by the immense road cut I-70 uses to punch through its north end. Accordingly, the DR area is as well worked out as anyplace along the east flank of the Front Range, but it still holds many secrets, the most valuable of which may be the exact subsurface geometry of the adjacent Front Range Precambrian core. Working out this geometry has become a holy grail of sorts for geophysicists from far and wide — in no small part because of the great bounty oil explorationists have placed on its head. There are many reasons to believe that below 10,000' or so, the Precambrian core of the Front Range overhangs a triangle of upturned petroleum-bearing strata (most notably the elsewhere highly productive "J" sandstone of the upper Dakota Group) to form a fault trap containing potentially huge volumes of oil and gas, but no one's ready to gamble the many millions it would take to drill the technically challenging overhang once, much less to find its production sweet spot. Since DR is also a very prominent range-front landmark identifiable at long distances from many directions, it will serve as the focal point for this article. Unless otherwise specified, all references to the Front Range, east flank of the Front Range, mountain front, range front, and so on refer to the Dinosaur Ridge area in this article. Caveat ViatorA word of caution to the Front Range traveller: The east flank of the Front Range is exceedingly complex. It varies substantially in both geology and geometry along strike and resists all but the broadest of generalizations. Understandings gained at DR don't necessarily translate even 10 miles to the north or south. I'll try to highlight some of the more interesting variations encountered within the range-front segment covered here — roughly from Morrison to Rocky Flats. Nearby Attractions []Near Dinosaur Ridge are several other mountain-front scenic and recreation areas popular for good reason with locals and tourists alike:
All these areas offer worthwhile views and hiking and biking trails with many fascinating exhibits on local geology and paleontology. In fact, you'd have a hard time finding another spot exposing as much of Colorado's geologic history in one easy day trip. The exposures in the area provide excellent opportunities to observe first-hand evidence of two of the region's defining events of the last 300 Ma — the Ancestral Rocky Mountain and Laramide uplifts. These uplifts commenced over 200 Ma apart but share many similarities, some visible right here. Together, Red Rocks Park and Dinosaur Ridge expose an uninterrupted sequence of Late Paleozoic to early Cretaceous sedimentary rocks tilted up against the crystalline Precambrian core of the Front Range. For the most part, these were shed to the east from the Ancestral Rocky Mountains starting around 300 Ma. Just across C-470 to the east is Green Mountain, an erosional remnant of a much larger apron of alluvial fans shed to the east off the Front Range block as it rose during the Laramide Orogeny starting around 72 Ma. Along its western base are exposures of late Cretaceous sandstones marking the beginning of the Laramide uplift. The area also offers stunning views of and access to the abrupt eastern front of the Rocky Mountains, here embodied by the Front Range foothills. At nearly 320 km in length (from Cañon City to southern Wyoming) and up to 80 km in width, the Front Range is the largest and 2nd highest of the Laramide Rocky Mountain uplifts. Among many other attractions, the Front Range is home to Mount Evans, Pikes Peak and Longs Peak—three of Colorado's tallest Fourteeners. The last two are the only Fourteeners in the Southern Rockies located off both Colorado's main structural lineaments — the Colorado Mineral Belt and the Rio Grande Rift.
A Tale of Two Uplifts And a Sea Dragged In By the Thrust Belt Next DoorThe rocks in and around DR tell a dramatic story of two mountain-building uplifts and an intervening invasion by the planet's largest known inland sea. And who best to summarize a dramatic tale but a bona fide drama expert? The Short Version, As Told Over the Phone By Someone Else's Teenage DaughterWell, it got like reeeeeally boring around here and there was like nothing to do and OMG, it seemed like forever!!! But then like there was this really big concert happening down south with all the big continents coming together and you know, like jamming, and Africa tried to park next to Arkansas but it like didn't slow down, duh, and so it smacked right into it instead and it even got Texas, New Mexico, Colorado and even Utah all rumpled up, too! So Colorado got 2 really big lumps that time, but after a while they just fell apart and made this really big mess with gravel and sand and mud all over the place. But then it like started raining a lot and all these big weeds started growing everywhere in the mud and OMG, then a bunch of big dinosaurs just came in and started stomping all around like they owned the place. And then something else smacked into California and everything over there just got like shoved over and all piled up on Utah, and Utah got like too thick or something, it made Colorado sink, it was so heavy. So then this really big sea just barged in like it owned the place or something, and there was all this water everywhere, and it left a bunch of sand and mud and then some more sand all over the place. And then that crazy Farallon plate, well it like flapped up under Colorado instead of going down like it was supposed to. Can you believe it???? And it just started shoving stuff around, so the big sea just left and Colorado got all rumpled up again, but this time, there were like lots of lumps everywhere. No, duh, it like made these big mountains and they were covered with all these volcanoes all over the place at first but then the volcanoes just fell apart and made another big mess of volcano pieces everywhere, but the mountains, they like didn't fall apart completely so they're still hanging around and OMG they're still getting bigger, but they keep leaving these really big messes, you know, like gravel and sand and mud all over the place. So whazzup with you? Now for the longer version... The Great Unconformity
It is upon this stage that our tale of two uplifts and a large sea unfolds. The Ancestral Rocky MountainsDuring Late Pennsylvanian time, around 300 Ma, plate tectonic stresses emanating from a continent-continent collision related to the assembly of the Gondwana supercontinent far to the southeast began to buckle up a dozen or so discrete fault-bound northwest-trending uplifts stretching in a band from Arkansas to Utah. The two largest ranges rose in Colorado and are now collectively known as the Ancestral Rocky Mountains (ARM). The eastern range, here called Frontrangia, stood roughly where the Front Range stands today; Uncompahgria, the western range, was centered roughly where the Uncompahgre Plateau lies today. Following the initial ARM uplift, Colorado entered a long and relatively quiet period of continuous terrigenous, marine and transitional sediment accumulation lasting nearly 200 Ma. As Frontrangia rose and then completely eroded away between 300 and ~220 Ma, it shed thousands of feet of largely terrigenous debris onto the surrounding Precambrian basement surface in all directions. These sediments were laid down flat — first as syntectonic range-front alluvial fans and finally as broad coastal flood plain muds similar to those being deposited now in the Gulf states — and remained flat until the Laramide orogeny some 200 Ma later tilted them to the 30-110° dips observed along the east flank of the Front Range today.
Progressively finer Frontrangia debris shed to the east contributed entirely to the coarse syntectonic gravels and sands of the Late Pennsylvanian Fountain Formation and substantially to
The Mid-Jurassic breakup of the supercontinent ^Pangea brought a wet temperate climate and lush vegetation to a previously arid Colorado between Ralston Creek and Morrison time, and with the moisture and vegetation came burgeoning populations of dinosaurs. Dinosaur-wise, the Morrison is one of the most fossiliferous formations on the planet. In addition to the debris washing off its own ARM, Colorado from time to time supplemented its Triassic and Jurassic sedimentary piles with both fine and coarse sediments derived from highlands far to the west in Utah, where troubled plate interactions along the west coast of North America threw up high mountain chains on a recurring basis. In the earliest Cretaceous, renewed uplift in Utah spread a pulse of distinctive chert-bearing conglomeratic fluvial sands across the state. of the Lytle Formation across the state. The Cretaceous Interior SeawayThe Lytle sands marked Colorado's last stand as dry land for the next ~30 Ma, for close on their heels came the largest inland sea (an extensive body of salt water floored by continental rather than oceanic crust) the planet has ever known. For much of its stay, between 100 Ma and 72 Ma, the Cretaceous Interior Seaway (CIS) had most of Colorado under 600' of saltwater. It flooded in simultaneously from the Arctic Ocean and the Gulf of Mexico to fill a broad trough-like downward flexure of western North America representing at least in part an isostatic response to the combined weight of the many thrusts sheets already stacked to the west by the early to late Cretaceous Sevier orogeny, and it receded only with the onset of Laramide uplift. By the time the Colorado was once again above water, the seaway had left behind a thick region-wide blanket of originally flat-lying marine and transitional Cretaceous sediments, including the
The Niobrara's small hogback east of Dinosaur Ridge was quarried away shortly after the settlement of Denver, and Early Jurassic sediments are for some reason absent throughout central and eastern Colorado. Otherwise, all 200 Ma-worth of the post-ARM, pre-Laramide sediments deposited east of Frontrangia are exposed in the Dinosaur Ridge area today. The Laramide OrogenyAround 72 Ma, a deep-seated regional deformation known as the Laramide orogeny began to push up the Rockies and to a lesser extent the Colorado Plateau in a patchwork of discrete north- and northwest-trending block-like uplifts stretching from southern Wyoming to central New Mexico. Resistant 1.7-1.4 Ga metamorphic and granitic Precambrian basement rocks cored the uplifts.
For the most part, the Laramide regional deformation reactivated basement-penetrating normal faults left over from a series of Late Proterozoic continental rifting events affecting the entire western two-thirds of the US, including the Colorado Province. Thousands of feet of Mesozoic and Paleozoic pre-Laramide sediments resting on basement prior to the Laramide were bent upward along steep reverse and and shallower thrust faults flanking the rising Laramide blocks even as erosion stripped the same strata from block summits. Today, eroded Laramide uplifts still dominate the regional topography: Forests and snow nicely color the Laramide uplifts green and white in the satellite photo of Four Corners at right. They underpin Colorado's most prominent highlands, including the Medicine Bow Mountains; the Front, Park, Gore and Tenmile, Mosquito and Sawatch Ranges; the San Juan Mountains; and the Uncompahgre and White River Plateaus, and the Uinta Uplift. East To West, Young To Old
A drive west along I-70 from Denver to the eastern foothills of the Front Range is a trip back into deep time, as this roughly west-trending section just north of I-70 through the town of Golden shows. West and older are both to the left in the diagram. The bump left of center is the Late Jurassic through Cretaceous Dakota Hogback. The hogback is less pronounced at Golden than it is at the geologically famous "I-70 road cut" (right) just east of the Morrison Exit, No. 259.
At Golden, the Paleocene basalts atop North and South Table Mountains cap prominent mesas of sediments trapped in the Denver basin, a large and deeply sagging range-front basement depression brimming with ~13,000' of late Paleozoic to late Cenozoic sediments shed eastward from the current and Ancestral Rockies. The vertical structural relief between the bottom of the Denver basin (below the diagram and off to its right) and the top of nearby Mount Evans (off to the left) is at least 22,000'.
Off to the right and a little south of this section, Denver nestles within the Colorado Piedmont, a 1,000' deep range-front trough excavated by the North and South Platte, Arkansas and Canadian Rivers and their tributaries in Late Tertiary time. In its entirety, the Piedmont stretches from southern Wyoming to northern New Mexico. At Denver, Early Paleocene Denver Formation and Late Paleocene Dawson Formation gravels, sands and muds floor the Piedmont trough as the uppermost strata of the Denver basin. Green Mountain
West of Lakewood, the topography along I-70 gets more interesting as the deeper strata of the Denver basin bend upward against the Precambrian core of the Front Range uplift. Starting at Green Mountain (right), hillside exposures begin to reveal their structure. Green Mountain is the erosional remnant of a once extensive flat-lying syntectonic apron of alluvial fans deposited all along the east side of the rising Front Range during the Laramide mountain-building event. These gravels rest unconformably on east-dipping Late Cretaceous strata, including older syntectonic gravels, tilted upward to varying degrees during the uplift of the east side of the Front Range block during the Laramide Orogeny. Cropping out along the lower western slopes of Green Mountain are the Latest Cretaceous through Early Paleocene Arapahoe Formation and Denver Formation. These originally flat-lying syntectonic conglomerates and sands reflect different levels of erosion in the rising Front Range. The Arapahoe is mostly debris from the Mesozoic and Paleozoic sedimentary cover, while the Denver is largely composed of basaltic clasts (rock fragments) coeval with the 65-63 Ma Early Paleocene basalt flows preserved as a caprock on North and South Table Mountains just a few miles north of I-70 at Golden. Erupted early on during the Laramide, Front Range volcanics probably found their way to the surface along leaky Laramide faults cutting the full thickness of the crust, if not the entire lithosphere. The Arapahoe and Denver Formations now dip to the east along with older Denver basin strata. Capping Green Mountain is the Late Paleocene Green Mountain conglomerate, a flat-lying collection of gravels composed almost exclusively of pink granitic Precambrian clasts shed from the Front Range to the west after its volcanic cover had been breached by erosion. Its lack of tilt indicates that Laramide uplift of the Front Range had largely subsided by the Late Paleocene. Visit geologist Dick Gibson's ^Green Mountain page site for more information. Laramie and Fox Hills Formations
Older near-vertical beds of light pink beach sands of the lower Laramie Formation (70-75 Ma) crop out along the west margin of Green Mountain Park, where they underlie both the Denver/Arapahoe Formation and the Green Mountain conglomerate. Nearly vertical light yellow beach and bar sands of the Fox Hills sandstone crop out at the northwest base of Green Mountain along the south side of I-70, where they underlie a horizontal unconformity covered with white alluvium. These barrier island sands were deposited by the retreating Cretaceous Interior Seaway as the earliest Laramide deformations broadly lifted Wyoming and Colorado out of the water.
Why the Laramie and Fox Hills strata dip more vertically than those in and west of the Dakota Hogback is unclear, at least to me. The standard explanation is "drag-folding" due to movement on a west-dipping reverse Golden Fault during Laramide time, and that may well be so. But the nature and timing of activity on the Golden Fault is not so clear, as discussed in the following section. West of Green Mountain is the Laramide Golden Fault, the Dakota Hogback, Red Rocks Park and finally the Front Range foothills. We'll visit them all in the gallery below. Top Page IndexThe Elusive Golden Fault
The north-trending Golden Fault (GF) runs along the east flank of the Front Range for ~24 km (15 miles). It's known primarily from well logs and seismic profiles, all of which consistently show a subsurface vertical offset of some 3.4 km (11,000') in the Precambrian-Fountain contact along the mountain front. This offset is the GF. It has few verifiable surface traces, and details of its subsurface geometry remain largely speculative, but most authorities show it as it appears in the cross-section above — a steep, west-dipping reverse fault thrown up on the west. This configuration explains how the GF brought late Paleozoic through early Cretaceous sediments deposited on Precambrian basement up and over early Cretaceous through late Tertiary sediments at the top of the Denver Basin.
I-70 divides the GF roughly in half. The southern half runs beneath soil cover just east of the C-470 freeway (right), between Green Mountain and the Dakota Hogback. North of I-70, the GF runs west of North and South Table Mountains at Golden, where it swings west toward the mountain front to cut out the Dakota Hogback and adjacent strata. In the geologic map of Golden and North Table Mountain below, note how the westward bend in the GF truncates the Dakota, Benton and Niobara strata at top center. The bend may well reflect a change in the geometry of the Precambrian core of the Front Range in the subsurface at Golden, but no one knows for sure why the GF veers west here. A Golden Mystery
To this day, no one seems to know quite what to make of the Golden Fault (GF). At only 8% the length of the 320 km-long Front Range, the GF has neither the length nor the offset (well under half the total estimated Front Range uplift) to claim the starring role in the raising of the east side of the Front Range. It's often been cast as a local splinter off a deeper and much longer blind thrust or reverse fault, but seismic profiling stands against that, at least at Rocky Flats (see below). Areas of controversy still surrounding the GF include its type, dip and subsurface geometry, but a Laramide age is fairly well established. One way to bracket the timing and direction of the last movement of a fault is to identify datable rocks that it did and did not deform or cut. Differential folding and tilting of rock units across the Golden Fault indicate that it moved up to but not during or after eruption of the still flat-lying Table Mountain basalts, which are radiometrically dated with considerable reliability at 62-63 Ma. This strong evidence puts the GF active during Laramide time but doesn't exclude pre-Laramide movement. Most authors consider the GF a steep west-dipping reverse fault, up to the west, but some still argue for a dip to the east, and others have even proposed that it's actually a pre-Laramide normal (extensional) fault. Sure enough, if you restore the strata it cuts to their original horizontal attitude, its geometry and motion fit an east-dipping normal fault, but this last view is not widely held. Things are much more straightforward on the west flank of the Front Range, where the well-exposed east-dipping Williams Fork Thrust is clearly responsible for most if not all of Laramide uplift and western displacement of the block's western edge. But there are no exposed west-dipping reverse or thrust faults likely to have been responsible for the uplift of the eastern edge of the Front Range anywhere along its ~320 km length. Many lines of evidence point to the existence of such east-side faults, but they have yet to show themselves directly, and the GF may or may not be one of them. Seismic imaging or profiling is one such line of evidence. Two profiles shot across the mountain front just north of the town of Golden at Golden Gate Canyon and Rocky Flats (the latter particularly well controlled) show the Golden Fault as merely the inboard (west) member of a pair of roughly parallel west-dipping range-front faults. The outboard (east) member, dubbed the Basin Marginal Fault (BMF, not shown on the map at right) also shows up in mountain-front profiles shot at several locations up and down the east side of the Front Range. Surface projections of the GF and BMF would lie ~3-4 miles apart near Golden, but neither is directly exposed. Between the GF and BMF is a block of highly deformed and rotated sediments variably exposing vertical to overturned Niobara, Fox Hills and Laramie strata at the surface along the mountain front between Denver and Boulder. This geometry explains, for example, the abrupt increase in dip observed between the ~60° Dakota beds of the Dakota Hogback at Dinosaur Ridge and the ~90° Fox Hills and Laramie strata at the base of Green Mountain just across C-470, which roughly marks the inferred trace of the Golden Fault. At Golden, the GF has cut out the Dakota Hogback, and the ~70° Fox Hills, Laramie and Pierre beds there are actually overturned. If the BMF is indeed the master Laramide fault on the east side of the Front Range, the GF could represent a splinter joining it at some unimaged depth. The relatively shallow Golden Gate Canyon profile allows this possibility, but the deeper Rocky Flats profile shows the GF dying out some 6,000-7,000' below the surface, well above the BMF. Geophysicists work hard to overcome the technical difficulties inherent in the seismic imaging of steeply dipping structures like the GF and BMF, but these relationships must still be taken with a pinch of salt. Paired range-front faults turn out to be fairly common elsewhere in the Laramide orogen, Wyoming's Wind River range included. The cause for the pairing remains unclear, but some think of the outboard fault as a zone of heavily sheared sedimentary strata in the outboard limb of a tightly folded syncline paralleling the master inboard fault. In that view, the GF would be the master. Confused yet? You've got some very good company. Top Page IndexBottom of Gallery
Top of GalleryTop Page IndexReferencesIn addition to the references cited on the home page and in the supporting articles, this article relies on the following sources, in alphabetical order by first author:
Top Page IndexURL: http://www.cliffshade.com/colorado/dakota_hogback/index.htm
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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. |
By
600 Ma, most of Colorado and surrounding areas had been reduced to a vast
featureless plain of bare 1.7-1.4 Ga metamorphic and
granitic 
