LATE CRETACEOUS OCEAN ANOXIC EVENTS II AND III IN THE NORTHEASTERN GULF OF MEXICO SHELF AREA

Two ocean anoxic events (OAE II and OAE III) have been reported to occur during the Late Cretaceous. The OAE II event is a global event that occurs near the Cenomanian-Turonian boundary. The OAE III event occurred during Coniacian-Santonian times and is believed to be more regional in distribution. This study is intended to identify possible OAE event sediments in the northeastern Gulf of Mexico area and determine their relationship with sea level changes in an integrated sequence stratigraphic framework.

Two black shale units were identified in five cores from southwest Alabama. The Marine Tuscaloosa Shale is a black, calcareous shale unit that can also be recognized regionally from well logs. It was deposited in a deep water, neritic environment. Calcareous nannoplankton analyses indicate a Late Cenomanian to Early Turonian age (CC10); therefore, the Marine Tuscaloosa Shale is interpreted as being associated with the OAE II event. Another black shale unit was identified between the Upper Tuscaloosa Formation and Selma Group in cores and logs. This shale unit graded into the lower Eutaw Formation updip and contains large quantities of terrestrial plant material and only a few calcareous nannoplankton. It was deposited in a lagoonal environment as part of a marine transgression. Nannoplankton analyses indicate that it is Coniacian-Santonian in age (CC15); thus, associated with the OAE III event.

The above observations were then integrated into a sequence stratigraphic framework established based on regional seismic data. The Marine Tuscaloosa Shale was found to be associated with the maximum flooding surface in the UK1 sequence (Middle Cenomanian-Upper Turonian), while the Lower Eutaw shale unit was determined to be part of the early transgressive systems tract of the UK2 sequence (Coniacian to Late Campanian).

Therefore, two types of OAE events can be identified in the northeastern Gulf of Mexico based on sequence stratigraphic relationships. The OAE II event occurred in association with a sea level highstand, and the black shales associated with this event were deposited in a deep water, open shelf environment. The OAE III event was part of the early phase of a marine transgression where large areas of the coast was inundated and lagoons and estuaries which trapped large quantities of terrestrial organic carbon formed.