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Depositional History of the Smackover-Buckner Transition, Eastern Mississippi Interior Salt Basin
Steven D. Mann, David C. Kopaska-Merkel
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. Relative sea level fell during late-Smackover progradation and sabkha deposition, then rose again to deposit peritidal carbonates. A sharp contact typically separates these peritidal carbonate deposits and the overlying massive Buckner Anhydrite in the MISB. Evidence for a significant sea-level change at the Smackover-Buckner Boundary is lacking.
Smackover sabkha deposits in the MISB 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, or oolitic peritidal dolostone. Smackover sabkhas formed on local paleotopographic highs concentrated along a north-south trending ridge produced by movement of the Louann Salt. Because 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 carbonate 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, and its extent 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, suggesting that evaporative drawdown was a contributing factor in the deposition of the massive anhydrite.
Smackover sabkhas are not physically connected to the subaqueous evaporite deposits of the Buckner nor did they form in the same way. 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.
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