USING FUSULINIDS TO EVALUATE THE PERIODICITY OF CYCLES IN A SEQUENCESTRATIGRAPHIC SECTION OF PENNSYLVANIAN-PERMIAN STRATA ANDTO SUGGEST POSSIBLE SUPPORT FOR LOW-FREQUENCY MILANKOVITCHPERTURBATIONS: CASPER FORMATION, SOUTHEASTERN WYOMING, U.S.A.

摘要

Sequence stratigraphic models were developed in the 1980s to predict lateral and vertical relationships among rocks derived from siliciclastic depositional paleoenvironments. The basic underlying assumption for sequence stratigraphy is that sea level fluctuates on a periodic basis as a result of a few overarching processes (tectonics, sediment supply, eustasy) and that changes in the lateral positions of the depositional paleoenvironments are expressed in the preserved stratal sequences. The original model was based upon a juxtaposed onshore terrestrial coastal plain, beach, and offshore passive margin siliciclastic basin. Subsequent studies have examined other types of coastlines, ones that differed from the initial model. The Pennsylvanian—Permian Casper Formation in southeastern Wyoming contains strata developed from an onshore dune field adjacent to an offshore carbonate ramp, a combination of paleoenvironments not previously examined in sequence stratigraphy. As with any research that is based on periodicity, an age constraint needs to be applied to the observed cycles. In the Casper Formation, there are no interspersed ash layers or many fossils in the section studied to provide these data. There are, however, a few limestone units that contain fusulinids. Given the recent developments in fusuline research, ages of rock layers which hold fusulinids are now datable to a very fine scale. Using the fusulinids that are contained in a few of the Casper Formation sequences, a timing of the periodicity of the cycles is preliminarily established. Additional investigation is needed to provide a more robust database. Interestingly enough, the periodicity that has been found is a longer frequency than the more commonly accepted Milankovitch cycles and provides some support for more recent studies that have proffered evidence for these lower-frequency Milankovitch perturbations.