Carbonate Factory Recovery Following Oceanic Anoxic Events: A Closer Look at the Cow Creek Member of the Pearsall Formation in South Texas
Ocean anoxic events (OAEs) are major carbon cycle perturbations that occurred multiple times in the Mesozoic. These events are most commonly associated with the shutdown of precursor benthic carbonate factories and deposition of black shales, some of which are known economic source rocks. In central Texas, OAE-related source intervals include the Pearsall and into the Glen Rose Formations (OAE-1a through OAE-1b), the Eagle Ford Formation (OAE-2), and the Austin Chalk (OAE-3). Recovery of the benthic carbonate factory following OAEs sets up the potential for encapsulated source-reservoir play systems. Post-OAE Gulf of Mexico reservoirs are known to occur in shelf interior grainstone belts (James Limestone Member of the Pearsall Formation) and in association with patch reefs (e.g.,James Limestone, Bexar Shale, and lower Glen Rose Formations). A lot of academic research has focused on identifying mechanisms that are responsible for the onset of OAEs, however, less research has focused on understanding how carbonate platforms recover from these perturbations and return to an environment that fosters a healthy carbonate factory. This study investigates the Early Cretaceous (Aptian) OAE-1a that is recorded by the Pine Island Member of the Pearsall Formation in South Texas, with a particular focus on shelf sequences preserved in a transect from the San Marcos Arch to the Pearsall Arch. To accomplish this, selected cores of interest have been scanned using pXRF at 5cm spacing. When linked to detailed study of ichnofossils and macrofossils, stratigraphy, and organic carbon chemistry, this coupling of core characterization with field observations yields understanding of the evolution of carbonate facies along this transitional interface during carbonate recovery. This work clarifies both the stratal architecture of the Pearsall Formation and refines the understanding of the relative timing of events from the peak of biotic crisis preserved the Pine Island Member during OAE-1a through the partial recovery phase recorded in the deposition of the Cow Creek Formation until the platform witnesses a second major biotic perturbation marked by OAE-1b in the Lower Glen Rose Formation. Identifying the mechanisms that dominate during recovery and transition from stressed OAE environments to sustained carbonate deposition will allow for better characterization of both conventional and resource plays by predicting spatial variability of source and reservoir rocks.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019