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Sedimentology and New Fossil Occurrences of the Upper Triassic Chinle Formation, Southeastern Utah
The Upper Triassic Chinle Formation in southeastern Utah was deposited in a complex fluvial-deltaic-lacustrine system. Lithofacies include planar- and trough-crossbedded sandstone; bentonitic mudstone and sandstone; black, organic-carbon-rich mudstone; foreset-bedded siltstone, sandstone, and mudstone; and thick-bedded, bioturbated limestone and siltstone. These lithofacies are interpreted to have been deposited in fluvial channels, on adjacent floodplain crevasse splays and mudflats, in deltaic distributary channel systems, and in lacustrine marshes, deltas, and basins.
Chinle depositional systems evolved in response to tectonic and climatic controls. Paleovalleys were initially eroded into the underlying Lower and Middle(?) Triassic Moenkopi Formation by Chinle degradational fluvial systems. Subsequent deposition by aggrading Chinle depositional systems filled the paleovalleys. Fluvial, marsh, and deltaic complexes supplied sediment to lacustrine basins whose margins fluctuated in response to changes in climate, sediment supply, and tectonic subsidence.
The Chinle strata contain numerous trace fossils that include lungfish burrows, the trace fossil Scoyenia, and both small and large coprolites. Lacustrine margin deposits and fluvial floodplain units exhibit color-mottled gleyed paleosols that indicate fluctuating water tables. Five fossil localities discovered in the Chinle of southeastern Utah contain conchostracans and ostracodes, both small and large vertebrates, gastropods, and pelecypods. The presence of specific faunal assemblages in certain lithofacies contributes to our understanding of depositional environments, paleoclimatology, and paleoecology of the Chinle. Lithofacies variation, fauna and flora, and paleosols indicate that the climate during deposition of the Chinle can be described as tropical monsoonal.
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