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Successive facies changes in Chesterian through Wolfcampian strata illustrate five distinct carbonate to clastic transitions. These result from differing combinations of depositional environment, climate, sea level changes, and differential subsidence. Comparative stratigraphy of successive facies change across the craton-miogeosyncline boundary, combined with analysis of sedimentary structures and lithology, enhances interpretation of the causes of these facies changes.
A Chesterian to Morrowan transgressive facies change illustrates a carbonate to clastic transition resulting from shallow marine to subaerial environments, wherein the climate was tropical and marine current energy was low. A Morrowan to Atokan regressive facies change closely resembled the underlying transgressive facies change and results from similar circumstances, except that this transition has the addition of eolian quartz silt units and diagenetic overprints resulting from periodic emergence during the regression.
A subsequent Atokan transgression illustrates a contrasting facies change. Sandstones change to carbonates more gradually. Rocks containing equal amounts of carbonate and quartz sand occur commonly. Fewer varieties of carbonate texture and fossil species occur in corresponding facies than in the Morrowan transition. Cross-bedding of similar style occurs in sandstones as well as in carbonates. The facies change results from a broad shallow marine environment in which there was an eastern detrital source in the eastern Grand Canyon region. These environments existed in conditions resulting from more open marine conditions and/or greater aridity than during the Morrowan sequence.
The distribution of Desmoinesian strata is more easily explained by reciprocal sedimentation rather than by facies. Virgilian facies appear to result from a combination of the factors involved in the Morrowan and Atokan strata. Beginning in Wolfcampian time faulting and attendant differential subsidence control the distribution of clastic and carbonate facies.
Carbonate buildups along the upthrown side of faults serve as clastic sediment traps. Also erosion may occur on the upthrown side of faults. Both of these result in abrupt facies changes in Wolfcampian sequences.
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