ABSTRACT

Three depositional sequences associated with cycles of relative sea level change and coastal onlap can be identified in the Mississippi interior salt basin of Alabama. These depositional sequences are here designated the LZAGC-3.1, LZAGC-4.1, and LZAGC-4.2 sequences and include Callovian through Kimmeridgian strata. In the Mississippi interior salt basin, the LZAGC-3.1 depositional sequence is bounded by a major composite basal unconformity developed on Paleozoic and early Mesozoic strata. The sequence includes Werner and Louann evaporites (transgressive deposits), Pine Hill anhydrites and shales (condensed section deposits), and Norphlet continental sandstones (progradational, highstand regressive deposits) and represents the initial incursion of marine waters into the basin during the Callovian in conjunction with early rifting of the Gulf of Mexico and subsequent sea level fall in the Oxfordian. The LZAGC-4.1 depositional sequence is bounded by a lower type 2 unconformity associated with this fall in sea level. This sequence includes Norphlet marine reworked sandstones (shelf margin deposits), lower Smackover packstones, wackestones, and carbonate mudstones (transgressive deposits), middle Smackover carbonate mudstones (condensed section deposits) and upper Smackover grainstones, Buckner anhydrites, and middle Haynesville carbonates, clastics, and evaporites (progradational, highstand regressive deposits). Smackover deposition was part of a major Jurassic transgressive event in the Gulf of Mexico area. The LZAGC-4.2 depositional sequence is bounded by a lower type 2 unconformity in the Kimmeridgian. This sequence includes upper Haynesville peritidal sandstones (transgressive deposits), upper Haynesville carbonate mudstones and shales (condensed section deposits), and upper Haynesville anhydrites, shales, and continental sandstones (progradational, highstand regressive deposits). These sediments are overlain by the marginal marine to continental deposits of the Cotton Valley Group. In these depositional sequences, progradational, highstand regressive deposits are the principal petroleum reservoirs; however, shelf margin sandstones and transgressive carbonates also are reservoirs. Condensed section and, in part, transgressive deposits have the potential to be source rocks if subjected to proper depositional and burial conditions; however, only the lower and middle Smackover carbonate mudstones were deposited and buried under conditions favorable for hydrocarbon generation and preservation. An understanding of sequence stratigraphy can serve as an aid to identifying potential hydrocarbon exploration targets.