Geology of the Colorado Plateau: Grand Canyon National Park
by Morgan Haldeman and Mikaela Reisman
How did Arizona’s Grand Canyon Form?
Few places can come close to approximating the appearance of the Colorado Plateau. However, they are not nearly as large, nor can they replicate the conditions of its formation (borrowed from Ranney, 2012):
- Thick stack of stratified rock.
- Varied and vivid color within the strata.
- Widespread, gentle uplift such that the strata remain relatively flat-lying.
- The presence of large rivers and their tributaries.
- A modern arid climate.
- The Grand Canyon is carved into a series of flat, seemingly uninspiring plateaus.
- It averages ten miles across and one mile deep. The estimated material eroded from the Canyon is approximately one thousand cubic miles.
- Tectonic uplift of the Colorado Plateau and resultant erosion from the Colorado River, along with additional weathering due to freeze-thaw processes, has carved the Grand Canyon to the extent we see today (Karlstrom et al., 2012; Ribokas, 2009).
- Wind has removed sediment and other materials from the canyon walls, assisting in widening the canyon to its current width.
- The absolute age of the canyon is unknown and is a subject of great debate among scientists. For many years, the canyon was thought to be five to six million years old – very young in geologic time. This is still thought to be the case for at least part of the canyon. However, papers published throughout the years argue for a greater age of the canyon, ranging from 17 million years old to nearly 70 million years old (Karlstrom, K.E., Lee, J. P., et al., 2014).
- The Grand Canyon was originally thought to be carved by concurrent river erosion and tectonic uplift. Two hypotheses abounded: one, that the river cut through sedimentary layers as the Colorado Plateau uplifted, and two, that the river began after uplift and was attempting to reach isostatic equilibrium. However, over the past several years, new hypotheses have been created.
- The current thinking is that the Grand Canyon is the result of several smaller, much older canyons and their respective rivers being pulled together as a result of stream capture and knickpoint migration along faults. Some of these older canyons are up to 70 million years old, whereas other portions of the canyon are much younger, leading to current age debates (Karlstrom, et. al., 2014).
- Due to the slight dip of the Colorado Plateau (approximately 1°-2°), the North Rim is nearly 1200 ft (365 m) higher than the South Rim.
- The Grand Canyon is considered one of the best geologic exposures of the Paleozoic to Pre-Cambrian periods (about 250 ma to about 1800 million years ago; Mathis and Bowman, 2007) in the world.
- The Colorado River has carved through millions of years of Earth’s history. It has reached the Vishnu Schist and the Zoroaster Granite (both Pre-Cambrian in age; approximately 1700 ma; Mathis and Bowman, 2007) at the bottom of the stratigraphic column.
From youngest to oldest:
- Kaibab Limestone: Permian (~270 ma). Shallow marine shelf deposit. Fossiliferous (brachiopods, gastropods, crinoids, bryozoans). Represents the time of maximum eastward transgression of the Kaibab Sea (Condon, 1997). Fossiliferous in nature, with brachiopods, gastropods, crinoids and bryozoans present. Approximately 300 ft thick.
- Toroweap Formation: Permian (~273 ma). Contains of the most diverse rock varieties in the Canyon, i.e. sandstone, mudstone, gypsum, limestone. Originally deposited near the shore of a shallow sea in mudflats and evaporate deposits.
- Coconino Sandstone: Permian (~275 ma). Very large scale cross-stratified sandstone. Deposited by aeolian processes and consists of fine, well-sorted quartz grains (Mckee, 1945 and 1979). Thickness varies from 57ft to over 600 ft (Middleton et. al, 2002). Present are fossilized trackways and tail-drag marks of prehistoric reptiles.
- Hermit Shale: Permian (~280 ma). Comparatively nonresistant unit of siltstone, mudstone, and sandstone. Thickness varies between 100 ft to 900 ft. Low angle cross-bedding present, as well as poorly-preserved plant fossils (Blakey, 2003). Multiple, ill-preserved plant fossils present.
- Supai Group (315-285 ma) Created from oceanic transgressions and regressions, forming alternating conglomerates, siltstones, and sandstones (Blakey, 2003). Fossil examples of amphibians, reptiles, and terrestrial and marine plants present. (Lockley, M. G., and Hunt, A. P., 1995).
- Surprise Canyon: Mississippian (~320 ma). Considered a ‘channel fill’ formation of conglomerate, sandstone, limestone and siltstone (South Rim Stratigraphy Map). Deposited atop the horizontal Redwall Limestone. Environments during deposition were most likely a moist and humid with orangey soil similar to soil found in the tropics today.
- Redwall Limestone: Mississippian (~340 ma). Large red cliff-forming unit ranging from 500 ft to 800 ft in thickness. Thin-to-thick bedded limestone. Chert often present. Both its upper and lower contacts are unconformities (Beus, 2003; Mckee and Gutschick, 1969). Fossils of brachiopods, corals, and trilobites are present. Paleoenvironments of the Redwall Limestone most similar to those of the Surprise Canyon.
- Tonto Group: Represents the basal sections of the Paleozoic layers of the Grand Canyon. The Tapeats Sandstone provides the platform upon which less-erosion-resistant layers (Muav Limestone, Bright Angel Shale) are able to rest. Represents a marine transgression series (Rose, 2011). The Tonto Group has trilobites, brachiopods, and trace fossils of marine life (Lull 1918, and McKee and Resser, 1945).
- Muav Limestone: Cambrian (~505 ma). 350-600 ft thick. Light grey, brown, and orange-red limestone. Dolomite and calcareous mudstone also present, which indicates shallow seas and the likely presence of saline lagoons.
- Bright Angel Shale: Cambrian (~515 ma). Green and reddish-purple shales and siltstone, interbedded with red-brown sandstone.
- Tapeats Sandstone: Cambrian (~525 ma). Highly erosion-resistant red-brown sandstone and conglomerate unit. Paleoenvironments included marine transgression because it is the bottom layer of sandstone, with shale and limestone above. All were part of a transgressing paleo sea.
- Grand Canyon Supergroup: Precambrian (~740-1200 ma). Grouping of twelve units, including sedimentary and volcanic formations. These units were faulted and tilted approximately 10°-15°.
- Zoroaster Granite and Vishnu Schist: Precambrian (~1750-1680 ma). Crystalline basement rocks. Consist of granites and schists. (Karlstrom et. al., 2008).
Our Experiences on the Hermit Trail (3/13/14-3/14/14)
March 13, 2014. Three twelve-passenger vans and one Ford Explorer arrived at the Grand Canyon amidst mist and snow flurries. Twenty-six people clambered to the railing for the first view of the Canyon. While a few had seen it before, the majority had not, and we couldn’t help but stare. No matter how many times we were told how big the Canyon was, our minds could not establish the true magnitude of it. It stretched in front of us for miles, red and white and brown layers seeming to welcome us even as we threw echoes of our voices into the maw. The cold forgotten, we simply stared. It was hard to imagine that we would be trekking into it the next day.
Later that evening, after we had set up our campsite and gotten more appropriately dressed for the cold weather, a group of ten or so took off and returned to the edge. We clambered down onto a ledge and sat with our legs dangling off the sides. A few brave souls ventured further out onto a spar of rock sticking out from the side of the Canyon wall. Echoes of our exhilarated yells bounced from the walls and back to our ears as the sun began to set in the west, lighting up the red and white of the walls. It wasn’t until we realized we had no flashlights with which to get back that we reluctantly turned to return to our campsite.
March 14, 2014. Twenty-six people took off from the trailhead at Hermit’s Rest and began the hike down the Grand Canyon stratigraphic column.
The hike down took about an hour and a half, with a few stops along the way for educational purposes… mainly to look at fossils, differentiate between the strata, and view the trackways of the Coconino.
It wasn’t until we stopped and looked up at the layers above us that we started to realize how big this canyon actually was. A few people stopped at the end of the Coconino Sandstone and headed back up, unwilling to hike down any further and face an even longer climb to the top. The rest of us continued, some stopping at the Hermit Shale and others towards the end of the Supai Group.
Lunch in the Hermit Shale was pleasant. We ate lunch (consisting mostly of deli meat sandwiches, pickles, and jumbo bags of beef jerky) beneath a shade tree and tried not to sit on cactuses. Forty-five minutes of teasing, stories, and jokes about the rest of the group’s longer climb to the top, our own climb began.
Blasting ‘Eye of the Tiger’ and other inspirational music from our iPods, it took us about two hours to make it back to the top. We stopped at every other switchback, shouting obscenities about the large steps in the Kaibab and Coconino. Once we reached the top, it was one of the best feelings in the world. We whooped and celebrated and bought hot chocolate at the Hermit’s Rest visitor’s center, sitting at the edge of the canyon for a while as we caught our breath and lauded our accomplishments.
All in all, it was one of the best experiences of our lives.
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