Wednesday, April 07, 2021

Geological Destination - Monument Valley Tribal Park

Back in November of 2019, while driving back from Arizona to the wife to do one of her Iron Man races, we drove through one of the Navajo Nation Tribal Parks, Monument Valley Tribal Park. As a runner, my wife wanted to get a shot of her running up the Forest Gump hill, while as a geologist, I just like looking at the pretty rocks.

Slightly downhill from us but essentially the same view in Forest Gump

As can be seen in the image, this is easily an icon geological location. The rock units within Monument Valley are essentially the same as the rock units within the nearby Canyon de Chelly National Monument, however the landscape is a bit different. Instead of the rocks being isolated within a canyon system like at the park, they are now elevated above the surrounding landscape. This is likely an effect of the Colorado Plateau, where this region had been slightly elevated compared to the areas further down south. A breakdown of the rock units within each of the mesas seen in the background is as follows:

Monument Valley geology. Image courtesy of the UGS.

The Shinarump, is part of the Chinle Formation, a Late Triassic (~225 million years old) yellow-grey river-deposited sandstone and conglomerate. 

Below the Shinarump is the Moenkopi formation. The Moenkopi Formation is an Early to Middle Triassic formation (~245 million years old) that is is predominantly made up of the reddish-brown shale. The Moenkopi was deposited within an intertidal environment, with alternating sea levels producing thinly bedded layers of mud (shale) and sand (sandstone). 

Below the Shinarump is the De Chelly Sandstone. The De Chelly Sandstone is a Permian age (~200 million years old) aeolian sandstone. Aeolian means that it is formed by blowing wind, in particular sand dunes, or a desert environment. When sand dunes are frozen in time, such as when they become rocks, and eroded you can see features termed cross-bedding. These rock preserve an ancient sand sea desert, known as an erg, that used to be located here. Sandstones are also frequently extremely hard rocks that are resistant to weathering. When they weather, they fracture into regular joints. Those are the vertical line patterns of the rocks as seen in the image above. It is also what produces the shear-walled rock mesas as we know them today. 

Below the De Chelly Sandstone, is the more erodible Organ Rock Shale. You can tell it erodes much more easily by the smooth slope that forms from the edge of the overlying sandstone. If the sandstone wasn't there to protect the shale, the shale would have eroded long ago. The Organ Rock Shale is another Permian formation (~270 million years old), that mainly comprised of mudstone (shale) and siltstones. They were deposited by streams within a tidal flat environment. The Organ Rock Shale then underlies much of the surrounding landscape which is then covered over with much, much younger (Quaternary) sediment (known as alluvium) transported in by winds and water from these and other surrounding rock formation.

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