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Oceanic anoxic events (OAE's), Biotic Crises, and Demise of the Cretaceous Passive Margin Platforms, Western Gulf of Mexico (GOM)


The Cretaceous greenhouse world, though not impacted by high-amplitude eustatic events, is profoundly influenced by oceanic anoxic events (OAEs) that override normal biotic and depositional patterns in carbonate systems. In broad, shallow, passive margin settings like the Cretaceous of western GOM, OAE's drive regional to global transgressions by reducing sedimentation rates over long time scales. These are recorded lithologically as organic-rich condensed sections. OAE's influence inner shelf through slope/basin facies and drastically reduce the carbonate factory's ability to keep pace with even modest tectono-eustatically generated accommodation, forming biotic-eustatic rather than tectono-eustatic supersequences. Because OAE's impact accumulation rates of organic material as well as carbonate, and as they impart a predictable pattern of source-reservoir parings, it is essential that Cretaceous greenhouse carbonate platform models integrate biotic events as a key element of the sequence architecture. Phelps et al's (2013) study of South Texas Hauterivian-Coniacian strata is expanded to incorporate Berriasian-Valanginian strata, delineating 10 sequences averaging 6.5 m.y. The Berriasian-Late Barremian is characterized by regressive to aggradational stacking patterns (Cotton Valley, Knowles, Sligo) driven by tectonic accommodation. Biotic forcing of the platform record begins with OAE 1A in the Late Aptian. The first biotic crisis is the Pine Island (La Pena) OAE 1A shut-down followed by partial recovery and deposition of the Cow Creek/James oyster/stromatoporoid/oncoid banks. OAE 1B is marked by the Bexar shale biotic crisis followed by healthy Glen Rose-Edwards-Stuart City coral-rudist platforms. The less extensive and diachronous end-Albian OAE 1D crisis recorded by the Georgetown transgression and partial recovery in the mixed benthic/pelagic Buda follows. The global OAE II Eagle Ford shut-down followed by a weakened pelagic Austin Chalk calcitic deep shelf marks the final demise of the platform. This biotically driven demise of the Cretaceous megabank is characteristic of epicontinental greenhouse platforms. However, the western Tethys record differs from open-marine shelves (e.g. Tethyan of Central Italy) that recover after repeated Cretaceous OAE's. The spectrum of carbonate suppression and recovery events related to OAE's will prove critical concept for understanding source rock distribution and for predicting variability in resource plays.