Sediment diagenesis, fossil preservation, and depositional environment in the Stone City/Lower Cook Mountain transgression (Middle Eocene, southeast Texas): a test of chemical taphofacies in the rock record

Abstract

The recognition of taphonomic loss in the fossil record is important because it provides information about the depositional environment, sediment geochemistry, and post-depositional bias present in the fossil assemblage. Because diagenetic mineral formation and fossil preservation are controlled by geochemical conditions in the sediment, the diagenetic minerals present may be useful indices of sediment geochemistry and the preservational condition of fossils. Most taphonomic studies are of modern sediments, and there is a great need to test the taphofacies concept on the rock record. The taphofacies concept has been tested in the Stone City/Cook Mountain formations, middle Eocene, in Southeast Texas. The strata consist of brown shales, pelleted green shales, green shales, and quartose event units, representing several depositional environments. Two diagenetic minerals, framboidal pyrite and green marine clay, as well as sediment texture and composition were compared to mollusk preservation. Additional information was obtained through the use of organic carbon data, carbon/sulfur ratios, and observations on concretions. Several factors thought to control shell dissolution were tested. Life position/habit was found to be of some importance in dissolution acquired during life. Shells of closely related taxa with the same composition and microstructure underwent differential taphonomic preservation, indicating that all the factors controlling shell preservation are not well known. Composition and microstructurewere not tested because they did not vary in the taxa analyzed. Shell size does not correlate with preservation, nor does valve type (for bivalves) control shell preservation. Shells preserved well in one environment frequently fared much worse than their cohorts in other environments, underlining the importance of depositional environment. Pyrite content, sediment composition, and total carbonate content were not linearly correlated with preservation. However, fossils did show differential preservation within lithologic type for all three variables. Data clustered within depositional environments when taphonomic condition was compared to any other variable. Brown shales containing pyrite but no green clay consistently possessed poorly preserved shells. Units containing both pyrite and green clay possessed the best shell preservation regardless of pyrite content, indicating that iron cycling may buffer pH and lead to good preservation.