(Shinarump Conglomerate Member, Monitor Butte Member, Owl Creek Member)
Figure 1: Paleogeographic map of the Late Triassic, Chinle Formation. (Blakey, 2008)
Non-marine fluvial channels, floodplains, paleosols, marshes, and small lakes.
The Chinle Formation was deposited during the Late Triassic when the supercontinent Pangea had landmass on both sides of the equator. Utah lay at the paleolatitude of 15° N. On the western margin of the continent (the approximate location of California today) and in southern Arizona into Mexico, subduction complexes contributed the volcanic ash to the bentonitic beds in the Chinle Formation (Prochnow et al., 2006.) East of Utah, the Uncompahgre highlands was a sediment source for Chinle deposits. (Prochnow et al., 2006)
The beginning of Chinle deposition was dominated by wet environments such as stream systems, lakes, wetlands, and deltaic distributary channels. Eventually the climate shifted and dryer environments prevailed such as seasonal stream systems and floodplains. By the end of the Chinle time, eolian deposits (sand dunes) indicate arid conditions.
· Uranium-rich conglomeratic sandstones in the Shinarump Member.
This resistant basal unit is typically white, yellow, or gray in color. Sandstone structures within this subfacies include lenticular internal scour surfaces, large trough cross beds, and some horizontal laminations. The sandstone grades laterally into siltstone and mudstone lenses which contain organic carbon fragments as well as carbonized plant fossils (Dubiel, 1987). The Shinarump Member is a coarse-grained conglomeratic sandstone that represents a widespread fluvial channelbelt.
· Colorful variegated mudstones and bentonitic sediments in the Monitor Butte Member are gentle slope formers of the characteristic Chinle “badlands”.
There is a gradational contact between the Shinarump and overlying Monitor Butte Member which is purple, yellow, and white mottled sandy siltstone and sandstone. This unit is known as the purple mottled unit (PMU), the color variations occur from different concentrations of iron bearing minerals (Dubiel, 1987). This unit contains lungfish burrows and represents a fluctuating water table which formed oxidizing and reducing environments which redistributed the iron in the sediments (Dubiel, 1987), along with fragments of plant material. The black mudstone has abundant conchostracans, fish scales, fragments of fish bone, and lenses of coal. This unit represents lacustrine marsh bog and wetland environments. Limestone, bentonitic sandstone, and siltstone occur above the coal units. Overall, the Monitor Butte Member was an extensive system of fluvial (stream) and deltaic distributary channels and splays, lacustrine (lake), prodelta and deltaic deposits (Dubiel, 1987).
· Carbonized and Petrified Wood in the Monitor Butte Member.
Above the Moss Back Member, lavender and brown variegated mudstone and sandstone of the Petrified Forest Member (Dubiel, 1987 has bentonites (volcanic ashes), thin lenses of carbonate nodule conglomerate, and sandy units with large scale internal scour surfaces and large trough cross-stratification. Important fauna include abundant vertebrate remains, gastropods, lungfish tooth plates, and unionid thin shelled bivalves. This unit represents fluvial sandstone and floodplain mudstones and laterally restricted marsh mudstones. It was deposited by sinuous streams and had many avulsion (redirection of the stream) events.
The Petrified Forest Member interfingers with pink and green limestone and red to orange siltstone of the Owl Rock Member. The limestone has mottled coloration and contains lungfish burrows and ostracodes. This indicates lacustrine basins and lacustrine margin deposition (Dubiel, 1987).
Sites Best to See it:
Stop 2-9: Chimney Rock and Fault
Stop 2-10: Twin Rocks
Stop 3-2: Oyler Mine
For a complete list of references please go to the References page.
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Disclaimer: The information is property of the University of Utah. Unless cited, images and files found on this site have been taken or created by the Geology and Geophysics Department at the University of Utah. Any use of these images should be cited appropriately. The stratigraphic column is from: Mathis, A. C. 2000. Capitol Reef National Park and Vicinity Geologic Road Logs, Utah, in: P.B. Anderson and D.A. Sprinkel (eds.) Geologic Road, Trail, and Lake Guides to Utah’s Parks and Monuments Utah Geological Association Publication 29. http://www.utahgeology.org/uga29Titles.htm
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