Identification of OAE1B on the Comanche Shelf, Central Texas, and Implications for the Impacts of OAE on Shallow Water Carbonate Depositional Systems
The Cretaceous Comanche shelf of central Texas is a shallow water carbonate-dominated system whose stratigraphic architecture is partially controlled oceanic anoxic events (OAE). OAE 1A resulted in the shutdown of the Sligo rimmed shelf system, and OAE 2 eliminated the Mesozoic benthic carbonate factory entirely. These OAE are associated with a general increase in TOC, development mudrock facies, and excursions in δ13C. More cryptic are the occurrence of OAEs that do not lead to platform wide biotic crises, are not associated with substantial mudrock deposition, and do not well-developed carbon isotope excursions. OAE 1B is such an interval, and contains a very different lithologic and stratigraphic record as compared to OAE 1A or OAE 2. A study of OAE1B (approximating the Apto-Albian boundary) was performed using classical facies descriptions from both core and outcrop, along with nannofossil biostratigraphy and both isotopic and trace element chemostratigraphy. These measurements indicate that OAE 1B is positioned in the lower Glen Rose limestone rather than within the underlying Bexar shale, as previously hypothesized. The singular events within the OAE1B interval my be associated with thin (sub-meter scale) marls in more distal environments, may have δ13C excursions, and may include heightened concentrations of redox-sensitive trace metals (Mo, V, Co, Ni). In more shallow regions of the shelf, identification of OAE 1B beds is more problematic, as the events are not associated with platform-wide shale drapes. OAE1B does not have the dramatic effect of suppressing the benthic carbonate factory on the Comanche shelf as OAE 1A or OAE 2 do. Rather, it occurs as the benthic carbonate factory begins to re-colonize the shelf after OAE1A (Hammet / Pine Island shales) and subsequent clastic influx associated with the Bexar shale. Also, rather than occurring during a clear transgressive event, OAE1B occurs within the early regressive phase of early Albian Stuart City system. Further investigation into the links between OAE and their effects on shallow water carbonates will aid in better understanding the environmental feedbacks associated with these events, and ultimately in the development of predictive concepts for subsurface studies.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017