September 7. Moenkopi Formation

Monument Valley, Arizona

You have to admit, the Triassic rocks have some pretty cool names. Chugwater, Spearfish – and today we’re with the Moenkopi. The Moenkopi formation is widespread, generally somewhat west and southwest of the area covered by the Chugwater. It’s another red and multicolored group of rocks found in Arizona and New Mexico, Utah and Nevada, extending into parts of California and Colorado.  

It’s another suite of rocks laid down in broad mud flats or river flood plains, probably near the ocean margin. The rock contains ripple marks, mud cracks, raindrop impressions, and salt crystal casts, all of which indicate episodic wetting and drying in a tidal, near-shore, or riverbank setting. The water deepened to the north and northwest, with much of west-central and northwestern Utah and adjacent Nevada under water, but it was most likely a large, restricted bay rather than the open ocean. [Link to map]   There are some thin limestones within the formation that may represent incursions of the sea, and some lenticular sand bodies may represent river channels that cut through the mud flats, but most of the rock is shale, mudstone, and siltstone – pretty fine-grained sediment, originally. Like the Chugwater and the Spearfish, there are some gypsum beds in the Moenkopi. Clearly the depositional setting was similar across western and southwestern United States during early Triassic time, around 230 to 240 million years ago.

But just maybe, the Moenkopi setting was more favorable to life, at least in places. There are more fossils in the Moenkopi generally than in the Chugwater. Freshwater sharks and lungfish, good numbers of amphibians, some reptiles, and dicynodonts have been found in parts of the Moenkopi in northern Arizona. The Moenkopi is a good place to find reptile and amphibian tracks, too.

The Moenkopi erodes pretty easily, but it is preserved in places like Monument Valley, Arizona and Utah, where it is overlain by more resistant units. Most of the massive, vertical cliffs at Monument Valley are Permian rocks of the De Chelly Sandstone, but the tops of some of them are formed by a thin, resistant conglomerate that protects the non-resistant Moenkopi, a relatively thin slope just below the tops of the monuments.

The name Moenkopi comes from a Hopi town in Arizona, whose name means “place of flowing water.” 

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Today’s geological birthday is Ferdinand Vandiveer Hayden, born September 7, 1829, in Westfield, Massachusetts. He is probably best known as the leader of the pioneering expeditions in the Rocky Mountains on behalf of the US Geological Survey, the first such explorations to include rigorous scientific documentation of the features they found. His crews were the first to document the area that became Yellowstone National Park, in 1871, a survey that helped convince congress to designate the first National Park in the world the following year, 1872.  And happy birthday to geologist Cheryl Hastings, too.

—Richard I. Gibson

Reference
University of Utah (excellent photos) 

Photo by Richard Gibson