ABSTRACT: Depositional History of the Smackover-Buckner Transition, Eastern Mississippi Interior Salt Basin

MANN, STEVEN D., and DAVID C. KOPASKA-MERKEL, Geological Survey of Alabama, Tuscaloosa AL

Shoaling-upward cycles of the upper part of the Smackover Formation are locally capped by thin sabkha deposits adjacent to and on the crests of paleotopographic highs in the eastern Mississippi interior salt basin (MISB). These sabkhas are overlain by peritidal carbonates of the uppermost Smackover. A sharp contact typically separates these peritidal carbonate deposits and the overlying massive Buckner Anhydrite in the MISB. Smackover sabkha deposits consist of intercalated matrix-dominated nodular anhydrite and anhydritic dolostone in which the proportion of anhydrite commonly increases upward. They are overlain by micritic, pelletal, and oolitic peritidal dolostone.

Smackover sabkhas formed on local paleotopographic highs that were mainly concentrated along a north-south trending ridge that was produced by movement of the Louann Salt. Salt movement was variable in timing and intensity causing Smackover sabkhas to be discontinuous and probably not everywhere the same age. The carbonates that overlie the Smackover sabkhas resemble typical Smackover carbonates and include reservoir strata. Massive Buckner Anhydrite strata are predominantly subaqueous saltern deposits, though peritidal evaporite deposits occur on the margins of the MISB. Saltern deposits formed in a gypsum-precipitating lagoon in the eastern MISB. This region was a silled basin, sheltered from less-saline water in the main part of the MISB by a salt-cored anticline capped by carbonat grainstone shoals in eastern Mississippi. Saltern deposits are dominated by selenite and gypsarenite. Saltern deposits are locally interbedded with subtidal hypersaline to normal-marine carbonate deposits, which may record sea level fluctuations or changes in water circulation. A saltern, or evaporite lagoon, formed in the eastern MISB at the beginning of Buckner time as a result of restriction of water influx into the eastern MISB and resultant rapid increase in salinity to gypsum saturation. The salinity increase was a chemical event, hence the base of the massive anhydrite is a time plane, the extent of which approximates that of the Buckner evaporite lagoon. Subaqueous evaporites of the basal Buckner occupy a smaller area than do peritidal carbonates of the uppermost Smackover, sugg sting that evaporative drawdown was a contributing factor in the deposition of the massive anhydrite. Saltern deposits thicken away from paleotopographic highs and have a blocky density-log pattern. Smackover sabkhas typically have spiky density-log patterns and thin away from paleotopographic highs. Smackover sabkhas are not physically connected to the subaqueous evaporite deposits of the Buckner nor did they form in the same way. Relative sea level fell during late-Smackover progradation and sabkha deposition, then rose again to deposit peritidal carbonates. Evaporative drawdown during early Buckner deposition caused a subsequent minor sea level fall.

AAPG Search and Discovery Article #91014©1992 AAPG GCAGS and GC-SEPM Meeting, Jackson, Mississippi, October 21-23, 1992 (2009)