AN INTEGRATED PERSPECTIVE OF PALEOENVIRONMENTAL CONDITIONS BEFORE AND DURING OAE-2 AT THE SOUTHERN GATEWAY TO THE WESTERN INTERIOR SEAWAY

The Cenomanian–Turonian Eagle Ford Shale was deposited at the intersection of the southern Western Interior Seaway (WIS) and the northern margin of the Gulf of Mexico. In contrast to other regions, the mudstones that predate oceanic anoxic event 2 (OAE-2) in the lower part of the Eagle Ford are more organic-rich and were deposited under more anoxic conditions compared to the mudstones that were deposited during OAE-2 in the upper part of the Eagle Ford. In this study, we investigate paleoenvironmental changes at the southern gateway to the WIS that facilitated differences in organic carbon enrichment before and during OAE-2. For this purpose, we integrated bulk organic geochemistry, major and trace element concentrations, organic biomarkers, and sedimentology from the U.S. Geological Survey (USGS) Gulf Coast #3 (GC-3) drill core, which was collected ~30–35 miles east of Del Rio, Texas and the Shell Iona-1 drill core.

The results reveal organic matter source and preservation differed before and during OAE-2 at the study location. The integrated inorganic and organic geochemical results show that excess sulfide in porewaters and bottom waters prior to OAE-2 at the bottom of the Eagle Ford were coupled with sulfidic conditions in the photic zone as well as extensive organic matter sulfurization — a process that promoted organic carbon preservation. However, some biomarkers for photic zone euxinia persist in intervals where other lines of evidence point towards declining sulfide concentrations in the porewaters and bottoms waters. Using the results from this study, this presentation demonstrates how to integrate organic and inorganic geochemistry with sedimentology to refine interpretations of biomarkers for photic zone euxinia in the sedimentary record.