DEPOSITIONAL ENVIRONMENT AND SEQUENCE STRATIGRAPHY OF THE LOWER TUSCALOOSA SANDSTONE IN WASHINGTON PARISH, LOUISIANA

The Tuscaloosa Group is a complete second-order depositional sequence representing the fifth transgressive-regressive cycle recorded in the Gulf of Mexico basin. The Tuscaloosa Group hosts a potentially major oil play extending in the subsurface of south-central Louisiana and south-central Mississippi, and the sedimentary record of important climatic events such as the Oceanic Anoxic Event 2. The detailed depositional environment of the entire Tuscaloosa Group is poorly constrained in Louisiana. Here we analyze the depositional environment of the Upper Cretaceous (Cenomanian to Turonian) Lower Tuscaloosa Fm. We used two conventional cores recovered from hydrocarbon exploration wells drilled in the Washington Parish. We preliminary describe the cores lithostratigraphy to reconstruct their sequence stratigraphy and depositional environments.

The Lower Tuscaloosa as seen in the cores is an overall fining-upward unit composed of alternating sandstone and mudstone intervals. Individual medium-fine grained sandstone layers, most common in the lower part of the core, show the prevalence of cross lamination (ripple marks) overlying sharp erosional base contacts. The sandstone layers show evidence of glauconite minerals and are highly bioturbated. The mudstone layers show mostly undisturbed lamination. The cores also show the extensive occurrence of allochthonous clasts (up to c. 5 cm in diameter) composed of laminated mudstone and, occasionally, sandstone. These clasts are confined in discrete layers (10 to 50 cm thick) within the general stratigraphy and are interpreted as sourced from shallower areas of the margin.

The presented preliminary observations indicate abrupt changes in facies within the Lower Tuscaloosa Fm. They are tentatively interpreted as the occurrence of thin-bed turbidites and gravitative processes depositing coarser grained sediments in the lower slope region of the paleo Gulf of Mexico continental margin. This analysis provides new insight in the Lower Tuscaloosa facies and associated depositional environment, which may correspond to the regressive phase of the depositional cycle.