Abstract: New Insights on Depositional Environments and Organic Carbon Accumulation in the Smackover Formation, Northcentral Gulf of Mexico

Ezat Heydari, William J. Wade, Laurie C. Anderson

Lithofacies analyses suggest that the Smackover Formation is composed of two major shoaling upward cycles in the northcentral Gulf of Mexico. The lower cycle (200 m) consists of basal mudstones that grade upward into wackestone and then into ooid grainstone. The mudstones are composed of three lithofacies, each up to 50 m thick. In ascending order they are: (1) laminated, (2) thin-bedded, and (3) burrowed. Sedimentary characteristics indicate that the laminated and thin-bedded mudstones were deposited in basin and slope environments, respectively, under anoxic conditions, while the burrowed mudstone was deposited in a well-oxygenated mid- to outer-shelf area. Wackestone was deposited in low- to moderate-energy conditions on the mid-shelf. Ooid grainstone formed shoals clo e to the paleoshoreline. This vertical succession of lithofacies indicates lateral progradation of adjacent paleoenvironments following the rapid initial transgression. Relative sea-level fall may have exposed the shelf at the end of the lower cycle deposition. Production and accumulation of organic matter were linked to algal bloom cycles driven by climatic variations, changes in freshwater influx from the ancestral Mississippi River, and migration of the pycnocline within the water column.

The upper cycle (100 m) is composed of two or more shoaling-upward cycles. Each is composed of mudstone-wackestone that grades upward into ooid grainstone. These grainstones formed barrier shoals seaward of the lower cycle grainstone shoals and prograded basinward.

AAPG Search and Discovery Article #90955©1995 GCAGS 45th Annual Meeting and Gulf Section SEPM, Baton Rouge, Louisiana