ABSTRACT

Jurassic systems tracts of southwest Alabama reflect greater clastic influx and less thermal subsidence compared to the central and western Gulf Coast. These factors resulted in expansion of continental portions of systems tracts at the expense of marine and marginal marine sections.

Werner Anhydrite and Louann Salt represent the earliest marine incursion onto the Gulf rim following rifting; because basin-wide evaporative drawdowns overprint even higher order eustatic sea-level fluctuations, transgressive (TST) and highstand (HST) systems tracts are indistinguishable. Anhydrite and shale capping the sequence accumulated via halite dissolution. Norphlet siliciclastics form a continental lowstand systems tract (LST). The marine-reworked uppermost Norphlet sandstone unit forms the base of a subsequent TST which includes lower Smackover laminated lime mudstones. The upper Smackover HST is characterized by formation of rimmed shelves upon which algal mounds and ooid grainstone parasequences accumulated. Shallow lagoonal carbonate and (Buckner) evaporite saltern deposition occurred behind ooid shoals; fine-grained siliciclastics accumulated in updip areas. The Buckner Formation straddles three type(2) sequence boundaries, and includes TST, HST, and marine and continental LST components. Precise equivalency to Buckner A, Buckner B, Gilmer, and Bossier sequences of the western Gulf is uncertain due to different subsidence histories and masking by undifferentiated Cotton Valley continental clastics.

Lack of biostratigraphic data precludes definitive age assignments to Alabama Jurassic sequences. Jurassic sequences of southwest Alabama are atypical and should not be used as templates for Gulf-wide sequences.