ABSTRACT

Lithofacies analyses of conventional cores indicate that the Smackover Formation is composed of three major shoaling-upward cycles in the north-central Gulf of Mexico (Mississippi, Arkansas, and Louisiana). In ascending order these are termed, the "C", "B", and "A" cycles. The "C" cycle (200 m) consists of basal lime mudstones that grade upward into wackestone and ooid grainstone (the Reynolds Oolite). "C" cycle lime mudstones comprise three lithofacies, each up to 50 m thick. Upward these are: (1) the laminated, (2) the thin-bedded, and (3) the burrowed lithofacies. The "B" and "A" cycles (each 50 m thick) are composed of lime mudstone and wackestone that grade upward into ooid grainstone.

Lithofacies patterns suggest that Smackover deposition began on a carbonate ramp which evolved into a carbonate shelf by the end of the "C" cycle deposition. Sedimentary characteristics indicate that the laminated lime mudstone was deposited in a basinal position and the thin-bedded lime mudstone in a steepened outer ramp ("slope"), both under anoxic conditions. The overlying burrowed lime mudstone was deposited on a well-oxygenated mid- to outer-ramp. The wackestone facies of the "C" cycle accumulated under low- to moderate energy conditions on the mid-ramp, while ooid grainstones constitute a shoreface-beach complex. This vertical lithofacies succession represents progradation of adjacent paleoenvironments following a rapid initial transgression. Relative sea-level fall exposed the shelf at the end of "C" cycle deposition.

"A" and "B" cycle grainstones formed prograding barrier shoal complexes near the "C" cycle shelf margin. The "C", "B", and "A" cycles are also recognized on the Wiggins Arch in southeast Mississippi. More rapid subsidence on the arch promoted aggradation, rather than progradation, of the cycles there.

Preservation of organic matter in the laminated and thin-bedded lime mudstones lithofacies of the "C" cycle created oil-prone source rocks. During deposition of these organic-rich lithofacies, the water column was stratified into an anoxic, hypersaline bottom layer separated by a pycnocline from an oxygenated, surface layer. Accumulation of organic matter was driven by high-productivity algal-bloom cycles related to climatic variations that coincided with high freshwater influx rates especially from the ancestral Mississippi River.