Tuesday, April 17, 2012


Since my initial geological contact with the Colorado Plateau in 1967, I have been impressed and “excited” with the Moenkopi Formation.  It may not be as colorful or fossiliferous as other red rocks of the Plateau (i.e. Chinle Formation with the magnificent fossilized wood), nor as cliff forming as the Wingate Sandstone, nor as majestic as the Navajo Sandstone.  However, it has a certain undescribed cachet to which I have bonded!

The Lower and Middle Triassic Moenkopi, and its stratigraphic equivalents, crop out over much of the Colorado Plateau but are especially colorful and well exposed in many of the National Parks and Monuments in Utah.  Most of the unit is red or red-orange in color and the original sediments were deposited in a wide variety of depositional environments including floodplains, tidal flats, stream channels, dunes, beaches, near-shore marine, sabkhas.  The streams, both large and small, generally flowed west or northwest from highlands such as the Ancestral Rockies and perhaps even those associated with the Ouachita Front (southern supercontinent slamming into U.S.)  To the west these streams met marine waters and in places near-shore marine rocks are interbedded with terrestrial rocks of the Moenkopi.  The dominant red colors most likely are the result of oxidation of iron rich minerals found in Precambrian rocks of the exposed highlands.
Tilted and brightly colored beds of the Moenkopi Formation resting on flatirons (darker beds with vegetation) along Green River at the mouth of Split Mountain Canyon, Dinosaur National Monument, Utah.  The Park City overlies massive sandstone beds of the Weber Formation.

The Moenkopi seems to be bounded by unconformities; however, the amount of missing geological time is difficult to determine.  Most likely the entire Middle Triassic is missing between the Moenkopi and Chinle while “some” of the Early Triassic and “most” of the Middle and Late Permian is missing in the lower unconformity (the “Permo-Triassic Unconformity”).  At Dinosaur National Monument (northeast Utah) the Moenkopi rests on the Lower Permian Park City Formation (phosphatic and marine) and is overlain by the conglomeritic Gartra Grit Member of the Chinle (Shinarump equivalent).  At Zion National Park (southwest Utah) the unit rests on the Permian Kaibab Formation (marine) and is overlain by the Shinarump Member of the Chinle.  At Canyonlands National Park the Moenkopi overlies the Permian Cutler Group/Formation (wind, stream, marine) and is overlain by the Chinle.
Most of the Moenkopi is rather unfossiliferous; however, I rather enjoy looking at ichnofossils and trace fossils, and they are quite common and somewhat spectacular. Various sorts of ripple marks are spectacular and the burrows and crawling marks seem enough to get anyone excited.  There are vertebrate footprints in some areas; however, I have not experienced these in the field.

So my advice for what it is worth.  If you are in the Colorado Plateau where the Moenkopi is exposed, take a hike along the outcrop and keep your eye on the rocks!  
Dessication cracks in a mudstone, San Rafael Swell.
Mudcracks, and "perhaps" raindrop impressions.

Current ripple marks.

Dessication cracks near Moab.

Bioturbation: burrows, large and small, and crawling trails.

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