CHEMOSTRATIGRAPHY OF THE MIDDLE-UPPER DEVONIAN BLOCHER MEMBER (LOWERMOST NEW ALBANY SHALE) AT GALLATIN, TENNESSEE

The Middle to Late Devonian (about 375 million years ago) is associated with increased environmental and ecological volatility, including ocean anoxia, global climate change, and biotic crises. In order to better constrain environmental volatility during this time, this study uses chemostratigraphy to reconstruct the environmental conditions through the deposition of the Givetian-Frasnian Blocher Member of the New Albany Shale. Samples come from an outcrop at Gallatin, Tennessee, located at the southernmost edge of the Illinois Basin and proximal to the Nashville Dome. X-ray fluorescence and loss-on-ignition techniques were used to provide a better understanding of the lithology of the samples, paleoredox conditions during deposition, and sedimentation rate/organic matter dilution. Bulk organic carbon isotopic analysis was used to differentiate marine from terrestrial organic matter input to the depositional setting and to put the outcrop within a chronostratigraphic framework. Geochemical data from the Blocher Member indicates that there was cyclicity in Total Organic Carbon (TOC) preservation in black shale facies, which corresponds closely with anoxic conditions (using Mo concentration as a paleoredox proxy) and somewhat to siliciclastic influx (represented by the proxy Ti/Al). This cyclicity, and the correspondence observed between TOC, Mo, and Ti/Al, suggests that organic matter preservation was related to siliciclastic input to the basin and sedimentation rate; at this time it is unclear if this pattern was a result of orbital and/or climate cyclicity. Bulk organic carbon isotopic results indicate a shift toward dominantly marine organic matter through time in the Blocher Member; this shift also corresponds to a global shift to lighter carbon in the ocean reservoir during the latest Givetian, though biostratigraphic control at the study locality is limited. Future work at this locality should aim to improve biostratigraphic control, better constrain the timing of the cyclicity observed and its cause, and expand the current dataset into overlying New Albany strata.