SULFUR AND ORGANIC CARBON RELATIONSHIPS WITHIN THE FANCY HILL BARITE DISTRICT, MISSISSIPPIAN STANLEY FORMATION, OUACHITA MOUNTAINS, ARKANSAS

The Mississippian Stanley Formation is the basal formation of an extensive Carboniferous flysch sequence exposed in the Ouachita Mountains of west-central Arkansas and southeastern Oklahoma. Bedded barites occur near the base of the organic carbon-rich Stanley Formation and are exposed in the Fancy Hill district along the north rim of the Mazarn synclinorum. The barite-rich zone is between 18 and 30 meters thick and is generally within 30 meters of the basal contact with the Arkansas Novaculate (Devonian).

Sulfur and organic carbon distributions suggest a variable history of the sulfur within the Stanley Formation that is closely tied to the presence and nature of the organic matter in the system, and the association of sulfur to the more mobile organic compounds. The mean isotopic value for the d ³⁴S of the pyrites in the barite zone is -0.65 permil, roughly 15 permil lighter than coeval seawater. The sulfur to organic carbon relationship in the Stanley Formation reflects an increase of sulfur in the shale portion of the formation in the areas adjacent to the bedded barites, and a slight depletion of sulfur in the shales in areas outside the enrichment zones. The resulting pattern observed in the data is likely due to a two stage process which initially entrained excess sulfur through biologic processes and trapped it as barium sulfate, early diagenetic pyrite, and as organic sulfur. The second stage of the process is marked by remobilization of the organically bound sulfur, accompanied by migration of hydrocarbons, during burial and low grade metamorphism. Organic carbon contents of the altered Stanley samples rarely drop below 0.5 wt% organic C which is thought to represent the remnants of the resistant terrestrial material brought into the basin along with the flysch sediments. The relationship between sulfur and organic carbon content when combined with sulfur isotopic data suggests primary biologic activity may have been a significant factor in the development and long term preservation of the Fancy Hill deposits.