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Description
The Upper Pennsylvanian Finis Shale occurs within the Graham Formation of the Cisco Group and represents a mud-dominated environment along the eastern shelves of the subequatorial Midland Basin (Pierce et al., 2010). In a representative section of the Finis Shale, Forcino et al. (2010) documented a fundamental decrease in the abundance and biomass of the dominant species of brachiopod, Crurythris, within the faunal assemblage midway through the section. For this study, oxygen-isotope (δ18O) variations within the well-preserved calcite valves of Crurythris specimens were used to assess potential changes in paleodepth via (1) changes in ambient bottom water temperature (e.g., shallower waters being warmer) and seawater δ18O (e.g., shallower water being potentially more influenced by isotopically-light freshwater input). Fourteen similarly sized and presumably similarly aged whole specimens that showed minimal surface recrystallization were selected from a broader sample population of thin-sectioned specimens. The interior shell material of the selected specimens were microsampled using a Micromill and analyzed on a Micromass PRISM dual-inlet mass spectrometer. For five of the 14 specimens, sufficient shell material existed to sample their umbo and commissure regions separately to assess intraspecimen variability. For four of the five umbo-commissure-sampled specimens, δ18O was higher in the commissure than umbo. The weighted averages for each of these specimens were then calculated to allow comparison to the remaining “whole-shell” specimens. Stratigraphically, the sample population shows a weak up-section trend of decreasing δ18O. This δ18O decrease is consistent with two potential up-section environmental scenarios: increasing temperature and fresh water input. In turn, both of these scenarios are consistent with a sea-level regression through the section. Additional analyses at higher stratigraphic resolution are warranted to test these exploratory results and tentative interpretations.
