Distribution of carbonate sediments in the Maverick Basin, Texas, during the Cretaceous Ocean Anoxic Event 2

The Ocean Anoxic Event 2 occurred at the Cenomanian-Turonian boundary (95-93 Ma) and is one of the most well defined OAEs of the mid-Cretaceous. It is visible in geological history by mass accumulations of organic-rich fine-grained sediment and a globally recognized positive isotopic carbon excursion. However, during the OAE-2 in numerous locations a period of oxygenation is observed. One of these locations is the Maverick Basin, Tx, situated on the Cretaceous Comanche Platform and between the Cretaceous Western Interior Seaway and the Proto Gulf of Mexico. The sediments deposited in this area prior to the OAE-2 exhibit properties more typical of an OAE, whereas during the OAE-2 sediments exhibit more oxygenated conditions. The Cenomanian-Turonian sediments here are well studied with regards to organic matter, but less understanding regarding the accumulation of the carbonates during the OAE-2. These sediments are known as the Cretaceous Eagle Ford Group. The Eagle Ford Group contains both organic matter and carbonates. The presence of both carbonate and high total organic carbon (TOC) in the Eagle Ford Group requires a balance between organic matter accumulation and preservation and the production and accumulation of carbonate. This is because organic matter preservation requires primarily low oxygen conditions on the sea floor, by contrast carbonate production requires the presence of oxygen for calcifier organisms to thrive. This study will investigate the effect of the oxygen content within seawater on organic matter and carbonate production, accumulation, and preservation. This can then directly be correlated to locally understanding the oxygen conditions of Maverick Basin in relation to what is occurring locally prior, during and post the OAE-2. We chose the Maverick because of [a] the abundance of existing outcrops and subsurface data, and [b] because of the semi-isolated nature of the basin allowing us to test our hypothesis. Our hypothesis being that bottom water conditions in the Maverick Basin is driven by local eustatic changes and the primary control is local fluctuations in sea-level causing opening and closing of the connection with this basin and platform to the Tethys Ocean. We test this hypothesis by constructing forward stratigraphic models using the diffusion based DionisosFlow Forward Modelling Software. Our strategy is to use simplified scenarios in which we directly input ocean oxygen content based on the controlling factors of each scenario. By calibrating our model results against published present day total organic matter (TOC) maps of the Maverick Basin and published carbonate content from geochemical analysis, we can put forward an explanation for the proposed decoupling of the carbon cycle observed in the Eagle Ford Group.