PALYNOMORPHS FROM THE JURASSIC SMACKOVER FORMATION, CONECUH EMBAYMENT ALABAMA, U.S.A.: IMPLICATIONS FOR PALEOCLIMATE, TECTONICS AND BASINAL HISTORY

The marine Oxfordian Smackover Formation is an important oil- and gas-producing carbonate unit that subcrops in the northern Gulf of Mexico. The formation consists of three carbonate dominated units, each separated by thin (up to 15 m), black, laminated shales and siltstones. The siliciclastic units are thought to mark lowstand systems tracts (LST) representing fourth or fifth order regressive intervals within a third-order transgressive sequence responsible for deposition of the Smackover and overlying Haynesville package. Here we report on the first palynological assemblages to be described from the Smackover Formation. They were collected from four sampled intervals of the shale (‘Shale C’) separating the two lower Smackover carbonates, derived from cores drilled in the Little Cedar Creek Field (T4N, R13E, Conecuh County, Alabama). Samples from the lower shale (late HST) are dominated by an impoverished dinoflagellate flora consisting primarily of Pareodinia ?ceratophora and Gonyaulacysta spp., but also containing abundant Classopollis (from the extinct xerophilous conifer family Cheirolepidiaceae), bisaccate pollen grains and diverse spores. Dominance of Classopollis in this unit could represent strand vegetation, or conversely, dominant trees of the more arid interior (‘Neves’ effect). Samples from the upper unit are dominated by bisaccate pollen (?stem Pinaceae, ?Podocarpaceae) and Araucariaceae, and also include Classopollis. Spores include representatives of bryophytes, club-mosses and at least seven to eight fern families, and probable fresh water algal cysts. Pollen morphospecies reflect the cosmopolitan nature of Late Jurassic floras, but a significant percentage of spore species lack exact matches elsewhere, suggesting some degree of endemism. Spores are relatively abundant and diverse, suggesting paleoclimate could be more humid relative to evaporite dominated basins in the western Gulf Coast, and supporting an imputed plate position for the Conecuh embayment below 30° north latitude. The late Jurassic transgressional event in the Gulf of Mexico is apparently related to post-rift tectonic cooling and subsidence; however, the presence of higher order regressional fluctuations within this sequence suggests a more complex history whose cause remains uncertain.