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.
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| 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!
mike
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| Dessication cracks in a mudstone, San Rafael Swell. |
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| Mudcracks, and "perhaps" raindrop impressions. |
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| Current ripple marks. |
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| Dessication cracks near Moab. |
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| Bioturbation: burrows, large and small, and crawling trails. |
