The Sequence Stratigraphy, Seismic Facies and Chronostratigraphy of the Jurassic-Cretaceous Knowles Carbonate Ramps, Northeastern Gulf of Mexico

D. Bradford Macurda Jr., Gary Jones

Carbonate ramps are important as reservoirs and, in some instances, source rock environments in many basins. Three carbonate ramps, which span the Jurassic-Cretaceous boundary in the northeastern Gulf of Mexico, provide insights into the stratigraphic evolution, facies variations, and plays within an evolving carbonate margin.

A large clastic wedge (Cotton Valley Fm.) built out into the northeastern Gulf of Mexico in the Late Jurassic. This delta platform provided the flat foundation for three successive carbonate ramps. The first (Knowles I - a highstand systems tract) prograded out to the edge of the former clastic wedge. It is the least variable of the three. Knowles II (a highstand) backsteps from Knowles I. Carbonate buildups formed at its margin. Both ramps change to clastics updip; the carbonates of Knowles II extend farther updip.

Knowles III is the most complex ramp. There is a distinct lowstand deposit in front of the Knowles III margin which has both onlap fill and slide deposits. The subsequent transgressive systems tract formed a regionally thick and uniform deposit. The following highstand is characterized by regionally extensive progradation with shoals within the platform and buildups and sand belts at the ramp margin. Updip, the ramp changes facies into a siliciclastic environment.

The three Knowles ramps are regionally extensive over thousands of square kilometers and provide the foundation for the rimmed shelf of the Lower Cretaceous Sligo Formation in the northeastern Gulf of Mexico. Rapid relative changes of seal level resulted in shifting facies belts which interlay different environments both horizontally and vertically, resulting in subtle traps and seals. Facies and chronostratigraphic analysis within the workstation environment highlight these subtleties.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995