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When interpreting depositional environments of dominantly clastic sequences, thin carbonates, if present, often are overlooked or given short shrift. Detailed study of the carbonates, however, can be instrumental in environmental interpretations of enclosing clastics. This is particularly true if the clastic units lack fossils or environmentally significant sedimentary structures. This hypothesis is supported by 2 examples from Pennsylvanian and Permian strata of southeastern Wyoming.
The Permian Goose Egg Formation consists of thick, red siltstone and mudstone with interbedded thin, widespread carbonates. The clastic units have been interpreted by various workers as deep-water marine, shallow marine, deltaic, specialized marine, or continental deposits. Petrographic examination of the carbonates suggests that they were deposited in shallow subtidal, intertidal, and supratidal environments. The facies mosaic exhibited by the carbonates suggests that enclosing siltstone and mudstone were deposited in nonmarine environments.
Festoon cross-stratified sandstone which characterizes the Casper Formation (Pennsylvanian-Permian) in the extreme southern Laramie basin has been interpreted as marine, subaerial, or fluvial in origin. Carbonate beds in the Casper Formation are thin, lenticular lithosomes of limited geographic extent. Petrologic studies of these limestones suggest that they were deposited in small lakes or ponds which periodically were emergent. The inferred environment of carbonate deposition supports a subaerial dune environment for the festoon cross-stratified clastics.
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