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Impacts of Apto-Albian OAE on the Comanche Shelf, Central Texas

Abstract

The Cretaceous Comanche shelf of central Texas is a shallow water carbonate-dominated system whose stratigraphic architecture is partially controlled by oceanic anoxic events (OAE). During OAE, the broader shallow water benthic carbonate factories of the Comanche shelf were effectively shut down. The Apto-Albian OAE 1A/1B events appear to relate to the shutdown of the Hauterivian-Barremian Sligo rimmed shelf system. The subsequent recovery, colonization, and growth of the Albian Stuart City / Edwards carbonate system was then eliminated by events stratigraphically associated with Cenomanian-Turonian OAE2. A recent organic geochemical study of mudrock extracts from OAE1B on the Comanche shelf indicates that during the Apto-Albian, the majority of the preserved organic material originated from marine archaea. This discovery is in agreement with contemporaneous organic geochemical measurements from Tethyan and Atlantic examples, and implies that expansion of marine archaeal biomass during the Apto-Albian may have been global in reach. This is in marked contrast to organic material recovered from OAE2, which is primarily algal in origin. In addition, both organic and inorganic geochemical evidence is indicative of persistent anoxia during these events. We postulate that such drastic environmental change must have had a marked impact on marine ecology and the carbonate system as a whole, and may relate to observed changes in infauna around these events. Taken in simple terms, tectono-eustasy itself cannot account for the sedimentological, paleontological, and organic and inorganic geochemical changes we observe in carbonate systems around OAE. Instead, these events and their recoveries should be seen as periods of complex, and often dramatic environmental change that we are working to better understand. It appears increasingly likely that OAE exert an extrinsic control on Mesozoic carbonate systems and may be of fundamental importance to understand if we hope to affect predictability of both source and reservoir systems in the subsurface.