Isotope Stratigraphy and Paleodepth Changes of Pennsylvanian Cyclical Sedimentary Deposits
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abstract
Oxygen isotopes can record water depth changes because of the relationship between depth and temperature, and ice volume changes by its relation to the isotopic composition of sea water. Because of this, oxygen isotopes provide an excellent means of studying the conditions responsible for the formation of cyclical sedimentary deposits in the late Paleozoic. We have determined the isotope stratigraphy of two late Pennsylvanian units in north Texas, the Necessity Shale and the Colony Creek Shale, after having carried out detailed studies of the depositional environments and fossil biotas in these units. Isotope data were obtained from fibrous calcite in the shells of the brachiopod Crurithyris planoconvexa, which is present throughout both shales. The two cycles studied can be divided into as many as four depth zones based on sedimentology, fossil biota, and position within transgressive and regressive sequence. From shallowest to deepest, they are the myalinid zone, fusulinid zone, ammonoid zone, and gondolellid zone. The 18O of the brachiopods records a decrease in temperature associated with deepening of water. Temperature effects are the major component of the oxygen isotopic signal present in the cycles. -from Authors
