MODELING BLACK SHALE DEPOSITION IN THE UPPER DEVONIAN WOODFORD FORMATION OF WEST TEXAS

A key question pertaining to black shale deposition is whether hypersaline bottom waters are required to maintain anoxia for extended periods of time. We developed a three-dimensional, silled-basin model to begin to quantify some of the variables controlling the preservation of organic carbon during deposition of the Upper Devonian Woodford Shale preserved in the Permian Basin of West Texas. The model assesses surface productivity, sedimentation rate, carbon sediment flux, dissolved oxygen, and sea level. The model employed two-layer estuarine circulation, driven by simple mass balance considerations and found the input variables to have a cumulative influence on the preserved carbon content of shale-forming sediments. The timing of organic-rich deposition was governed in the model primarily by sea level and its influence on the ventilation of the deep water of the basin. Dissolved oxygen was the sole electron acceptor considered in the model and the oxidation of organic matter was determined by a mass ratio of carbon and oxygen delivered to the deep water of the basin. Although the modeled basin was capable of producing sediment layers with up to 20% organic carbon, organic-rich deposition was discontinuous, as the basin failed to consistently maintain a significant volume of anoxic water. The results of the model suggest that hypersaline conditions may have facilitated the uninterrupted, organic-rich deposition of the Woodford Shale. Hypersaline bottom waters may have been necessary to limit vertical mixing of basin waters and continuously support a significant volume of stagnant deep water.