Colorado National Monument

Our September, 2002 Colorado geology road trip included a blustery rainy morning in the Colorado National Monument (CNM). The CNM stands on the northeast margin of the Colorado Plateau, a vast, haunting and geologically important physiographic province roughly centered on the Four Corners area of the southwestern US. In the NASA satellite photo of ^Four Corners at right, the central salmon-colored rocks mark the Plateau fairly accurately.

The Rise of the Colorado Plateau

Prior to the onset of the Laramide Orogeny in Latest Cretaceous time, the Colorado Plateau had evolved more or less in step with the Southern Rockies to its north and east. Since then, it's shown a bit more independence. For starters, the Colorado Plateau shifted north by some 100-170 km relative to the Rockies during the Laramide but sustained less intense Laramide faulting, folding and uplift. Then it spurted up to its present elevation during the last 15-25 Ma, mostly in the last 5 Ma. Something's clearly been going on in the upper mantle beneath the Colorado Plateau since at least 25 Ma, but exactly what, no one's sure.

For the most part, Mesozoic sediments like those at right now exposed in the Colorado National Monument accumulated near sea level. They now stand up to 7500' higher atop the Uncompahgre Uplift, another faulted anticline initiated in the Laramide and raised with little tilt to its present elevation by regional uplift from the mid-Tertiary on. On top of the uplift, the sedimentary strata rest horizontally and unconformably on 1.7 Ga Precambrian basement rock exposed in the floors and lower walls of the deeper canyons. As they fold and ultimately break over the basement-penetrating reverse and thrust faults beneath the NE flank of the uplift, the Mesozoic sediments form a spectacular monocline—a textbook example of the Laramide structural style.

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Colorado National Monument and Grand Junction
Colorado National Monument
	East Entrance: As we approached the East ( Grand Junction) Entrance to the Colorado National Monument, we left behind the Book Cliffs, here seen rising in the distance to the NE in the 1st frame. From the same vantage, the 2nd frame looks up and SW into the heart of the Uncompahgre Plateau, where the thin, hard Triassic Kayenta Formation caps massive cliffs of Triassic Wingate sandstone. Climbing up onto the plateau now along Monument Road in the 3rd frame, we pass the soft, deep red slope-forming Triassic Chinle Formation at the base of the Wingate Formation above the road in the right foreground. The Chinle rests unconformably on the 1.7 Ga Precambrian basement here. Intact exposures of the weak Chinle rock were hard to come by here, but we found a decent one near the West Entrance. The Chinle is thicker and better preserved at ^Canyonlands National Park in nearby western Utah.


	Melon-sized clasts of Wingate sandstone rest against black 1.7 Ga mica schist bedrock here. This is a colluvial, not a depositional contact. (Colluvium is fallen loose debris at the foot of the slope.) The soft, easily eroded Chinle should be in between, but red Chinle debris can be seen all around these clasts.
	Morrison cuesta: The 1st frame is a view to the SW along the steep, winding climb up onto the plateau from the East Entrance shows a bold NE-dipping Dakota/Morrison cuesta on the flank of the Great Monocline. In the middle distance, diagnostic green, gray and maroon claystones of the fossil-bearing Late Jurassic Morrison Formation form the slope beneath the Early Cretaceous Dakato Sandstone caprock. In the foreground (2nd frame), a buff-colored Wingate outcrop shows the thin cross-bedding identifying it as a dune field deposit.

	Red Canyon: View to the north shows contrasting incisions made by Red Canyon Creek. The small V-shaped notch in the far canyon floor is the best the creek could do in the resistant 1.7 Ga Precambrian gneiss it encountered beneath the Chinle Formation. The stream was much more effective in removing the softer sediments above this exposure of the Great Unconformity. In the 3rd frame, the north rim of Red Canyon reveals the erosional habits of three of the Monument's most recognizable formations: From top to bottom, the salmon-colored bull-nosed Early Jurassic Entrada Sandstone; the tough, irregularly-bedded white to purple Triassic Kayenta Formation; and the massive cliff-forming cross-bedded buff-colored Triassic Wingate Sandstone. The 4th frame gives a better view of the Great Unconformity, here marking the passage of ~ Ga at the base of the red, rubbly Triassic Chinle Formation. A Close Call: Not far from here, we noticed the snap-snap sound of electrical arcing around the metal stays of our open umbrella as static electricity built up around us in preparation for a lightning strike that thankfully never materialized. Once we figured out what was going on, we took down the umbrella and dashed for the car as fast our rubbery knees would carry us. We never actually saw lightning that morning.



	Plateau country flora: John hobnobs with the flora of the Uncompahgre Plateau at the rim of Red Canyon during a lull in the rain: From back to front, pinyon pine, white-berried Utah juniper, spindly bright green Mormon tea and woody sage.
	Monument Canyon: The thin but tough Triassic Kayenta Formation caps many of the Wingate mesas and spires along the NE margin of the CNM. Streams flowing to the SW over Wingate dunes deposited the irregularly-bedded Kayenta muds, sands and gravels. Silica- and calcite-rich groundwater later permeating the Kayenta and firmly cementing its grains hardened it into a highly resistant caprock serving in that capacity throughout the Colorado Plateau. The slopes skirting the bottoms of the high vertical Wingate cliffs are Chinle mudflat deposits. I couldn't make out any Precambrian basement exposures through the alluvial cover in this series. The Kayenta/Wingate/Chinle triad is a visually distinctive combination responsible for very similar plateau country topography in ^Canyonlands National Park and vicinity in western Utah, where the Chinle also includes silicic ashfalls blown hundreds of km across the Southwest from magmatic arcs active along the west coast of the time. I can't recall the name of the prominent spire near center in the 1st frame, but note its hard Kayenta cap. Once the cap succumbs to erosion, the spire will be rounded into a bullet shape like the towers at the Coke Ovens (not shown).



	Independence Monument: Looking north from Independence Monument View over 550-foot-high Independence Monument, Monument Canyon and Fruita to the Book Cliffs in the distance. The top two images in this series were made from visible light—an original in in color and a copy converted to B&W in post-processing. The bottom 2 frames are made from near infrared (NIR) light through a Hoya R72 NIR-pass filter—an original in color and a copy converted to B&W in post-processing. ^Infrared digital photography isn't for everyone, but I get a big kick out of it—mainly for the otherworldly tonality and exceptional clarity it brings to the scene, even on hazy days like this one.



	Wind erosion: Wind carved graceful caves and arches into upper Entrada sandstone near Grand View. Before the Colorado River acquired its present name, it was aptly known as the Grand River. Grand Junction stood at the confluence of the Grand and the Gunnison.
	Great Monocline: In this panoramic sweep from east to SE (top to bottom) taken near the West (Fruita) Entrance, the Great Monocline on the north flank of the Uncompahgre Plateau flexes up over the reverse basement fault that uplifted the plateau in Laramide time. In the 1st frame, the Book Cliffs rise on the left in the far distance to the east. Hulking behind them is Grand Mesa. In the middle ground (zoomed in 2nd frame), buff to red Triassic Kayenta/Wingate cliffs slant above a thin deep red Triassic Chinle Formation resting 1.7 Ga Precambrian basement rocks not exposed here. On the back of (upsection from) the Kayenta in the middle distance rides the Early Jurassic Entrada Formation, a salmon-colored cross-bedded dune field deposit that tends to form bull-nosed layers rather than the square-shouldered vertical cliffs seen in the Wingate.


Book Cliffs From Near And Afar
	Book Cliffs: On my first to the Book Cliffs, the weather was less than idea for viewing from any distance. A subsequent trip through Colorado and Utah segments of the Colorado Plateau provided many photogenic encounters with the Book Cliffs, which seem to be everywhere in plateau country. In west central Colorado, the imposing Book Cliffs form a remarkably straight 1,000-2,000' high, 75 km-long wall (frames 1-4) trending northwest from Grand Junction. In fact, they form both the north wall of Grand Valley—a broad floodplain of the Colorado River (formerly known as the Grand)—and the southwest edge of the base of the Roan Plateau. Frame 1 in this series is a telephoto of the Book Cliff seen to the east across the top of the Uncompahgre Plateau from Independence Monument View. Frame 2 is a wide-angle of the cliffs standing on the horizon behind Monument Canyon at Colorado National Monument. Grand Valley is just this side of the cliffs. Frame 3 shows the cliffs at far left over the base of the Great Monocline at Colorado National Monument. Frame 4 shows the cliffs in the distance north of the East Entrance to Colorado National Monument. Now for a closer look taken along I-70 just east of Grand Junction. The caprock atop the Book Cliffs (frame 5) is resistant buff-colored sandstone of the Late Cretaceous Mesaverde Group. Badlands-style weathering in the soft Mancos Shale below it form vertically corrugated yellowish slopes that from a distance apparently reminded someone of books lined up on a shelf. The Mesaverde and Mancos formations correlate respectively with the Laramie and Fox Hills sandstones and the Pierre shale of the eastern Rockies. All these sediments rest on the state-wide Dakota sandstone. The Book Cliffs outcrop widely throughout the Colorado Plateau. Wherever they're found, the Mesaverde caprock displays its distinctive "hog nose" erosional habit, whether in Colorado (frame 6) or Utah (frame 7). Book Cliff strata underlie the Tertiary Green River shale and Wasatch Formation sediments that make up the Roan Cliffs, as seen in frame 8. The same Tertiary sediments underlie Grand Mesa, with its obvious protective cap of Pliocene Late Phase basalt.



On a later trip...





Grand Junction
	Back in town: We took lunch at the Rockslide pub in Grand Junction and then shopped the Main Street mall for spousal appeasements, extra hiking pants and some leg warmers for bicycling. At left are but two of the many intriguing metal sculptures dotting the downtown mall. The graceful bends in the massive train axles suggested an impossible pliability. The lifelike girl on the bike seemed about to wheel off at any moment. We'd intended to spend the afternoon cycling in the Monument, but when the rain escalated, we decided stay in town instead. John found a workout room while I hunkered down with a briefcase full of maps, geology books and hiking guides to choose a fitting hike the next day, for our last full day on the road. The winner, Cathedral Lake off Castle Creek Canyon south of Aspen, would prove to be an excellent choice. Grand Junction stands on soft Mancos Shale flooring the Grand Valley.


	John and his long-time friend and former partner Dick tussle over who gets the next slug. We spent the night at Dick's before heading out for the Black Canyon of the Gunnison early the next morning. We appreciated his hospitality.

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Gallery Note: All the images on this page were taken in September, 2002 unless otherwise noted.

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References

In addition to the references cited on the home page and in supporting articles, this article relies on the following sources, in alphabetical order by first author:

Colorado Geology Photojournals

A Tribute to Colorado's Physical Past and Present

Right: Trees and snow mark major Laramide uplifts in green and white while salmon pink marks the Colorado Plateau in this true-color satellite image of Colorado and surrounding states, courtesy NASA, ^Visible Earth

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