KYLE, J. RICHARD, Department of Geological Sciences, The University of Texas at Austin, Austin, TX

ABSTRACT: Winnfield Salt Dome Cap Rock, North Louisiana Basin: Evidence for Late Mesozoic to Early Tertiary Cold (Warm?) Seeps

Salt dome cap rocks (CR) are the products of complex evolutionary processes associated with dissolution of the rising halite diapir, the accumulation of relatively insoluble components, and the alteration of these materials, largely by bacterially mediated diagenetic processes that involve petroleum degradation. The CR of the Winnfield salt dome in northern Louisiana appears to represent the products of an environment that was generally similar to currently active systems associated with petroleum seeps overlying shallow salt diapirs in the offshore Gulf.

The calcite and anhydrite CR zones of the Winnfield Dome are well exposed within and along the margins of an active quarry. Paleomagnetic data and stratigraphic analysis indicate that the principal period of diapir growth and anhydrite CR formation was during latest Jurassic. The calcite CR consists of an upper detritus-rich glauconitic fossiliferous limestone zone with irregular calcite-healed fractures; a lower banded calcite zone with subhorizontal cm-scale calcite layers overlies the sulfate CR. Fe-Zn-Pb sulfide (+/- barite) concentrations occur as stratiform lamina and mounds within the anhydrite CR and as veins and lenses within the calcite, particularly near the base of the calcite CR. These metals are believed to have been supplied to the CR-forming environment by heated forma ion waters. The banded calcite CR has a (delta){13}C range from -40 to -50 o/oo that is indicative of a methane source. The upper calcite CR is more variable with (delta){13}C values ranging from -22 to -45 o/oo. The Eocene Cane River limestone is locally exposed along the flanks of the Winnfield dome; limited data for this fossiliferous unit indicates a (delta){13}C range of -26 to -35 o/oo, values that also are indicative of a petroleum source. These relationships indicate that Winnfield dome was the site of prolonged fluid interactions at or just below the contemporaneous sea floor during the late Jurassic to early Tertiary.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.