ABSTRACT

The so-called Mid-Cretaceous Sequence Boundary (MCSB), also known as the Mid-Cretaceous Unconformity (MCU), is recognized as a distinct, high amplitude seismic event that runs uninterrupted from the Campeche Bank across the abyssal Gulf of Mexico to disappear below the Sigsbee Escarpment and merge into the uppermost carbonates of the West Florida Bank. In the mid-1970's an effort was made to understand the age of this characteristic seismic event, and it was dated as Mid-Cretaceous using somewhat tentative correlations with well data from the perimeter of the Gulf or with dramatically thinned section in DSDP holes drilled in the abyssal plain. Newer well data from the Mississippi Fan Foldbelt refute this age. Specifically, the AT336 #1 well (Showboat Prospect) encountered an event mapped predrill as the MCU and found it to be the K/T Boundary.

The event at this location is caused by 50 ft of Upper Cretaceous micritic limestone capped by 12 ft of anomalous red shale. The limestone overlies a thick package of alternating kerogen-rich, deepwater shales and white micrite so regularly laminated as to suggest periodic deposition from algal blooms in Late Cretaceous time. The red shale is in turn overlain by deepwater Tertiary clastics starting with the Paleocene. Although some marls and carbonates are present in the Tertiary section, whatever ecology was responsible for the kerogen-rich shales never appears again above the boundary.

Dating of the MCU seismic event as the K/T Boundary makes a difference in predictions of the depth to anticipated source rocks, as well as the predicted depth of potentially shallow water carbonate facies in the Lower Cretaceous.