Petrographic and Geochemical Constraints on the Deposition and Diagenesis of the Haynesville Formation (Upper Jurassic), Southwestern Alabama

EUSTICE, RACHEL, University of Texas, Austin, TX

The Haynesville Formation in Clarke County, southwestern Alabama, is a 250 m thick, halite-dominated evaporite rock composed of four vertically stacked evaporite facies. The different facies present include a basal chevron-dominated unit, a gray cumulate unit, a unit dominated by brown, organic-rich cumulates, and a unit composed of halite and anhydrite interbedded with sand and mud. The facies are defined by halite textures, the presence of anhydrite laminae and dissolution surfaces, and the relative amount of terrigenous material. These criteria were used because they provide some constraint on the brine depths present during precipitation of the salt.

The integration of geochemical data with petrographic observations has been used to formulate a model for the deposition and diagenesis of the deposits. The bromide concentrations within the basal chevron zone systematically rise from 36 ppm to 101 ppm, while the bromide concentrations within the overlying cumulate zone rise more rapidly from 121 ppm to 440 ppm. The strontium isotopic composition of the salt over this interval systematically increases from 0.7068 to 0.7084. Bromide concentrations, strontium isotope ratios, and other chemical parameters, in combination with petrographic observations, constrain the relative importance of depositional and diagenetic processes. Processes that are important in controlling the geochemistry of the deposits include the influx of seawater and eteoric fluid into the basin, synsedimentary dissolution and recycling of solutes, the reflux of brines within the basin, and burial diagenetic processes.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)