Abstract

The Devonian Woodford and Mississippian Barnett formations document a long (approximately 70-80 million year) period of clay-rich sedimentation along the southern margin of the Laurentian paleocraton during the middle Paleozoic. As might be expected, these rocks display many general similarities, for example in thickness, mineralogy, organic carbon content, thermal maturity, organic matter type, etc. Both also display conspicuous and systematic changes in composition from more proximal to more distal areas. However, our studies of more than 75 cores across the Permian and Ft. Worth Basins demonstrate that dissimilarities between the two systems are perhaps even more common than similarities. Many of the differences can be related to paleogeography, basin hydrography, and global sealevel.

Woodford facies comprise silt-rich mudstones (detrital silica), siliceous mudstones (biogenic/authigenic silica), calcareous mudstones, and claystones. The distribution of these facies reflects both temporal and spatial controls on their accumulation. Silt- and carbonate-rich Woodford rocks are most abundant in the Lower Woodford (Middle Devonian) and in updip areas. Siliceous mudstones and claystones are most common downdip in the lower Woodford and in Upper Woodford (Upper Devonian). Conodont evidence suggests that these two successions are separated by a significant hiatus. All Woodford rocks contain mixtures of illite, kaolinite, chlorite, and mixed layer clays; total clay and chlorite abundance is lowest in distal Upper Devonian rocks. Although silica content is variable, Upper Devonian mudrocks typically contain more abundant biogenic silica, especially in distal parts of the basin, whereas Middle Devonian rocks are dominated by detrital silica. Both Woodford successions also display consistent differences in depositional facies. The silt-rich Middle Devonian section is cross-laminated, locally graded, and commonly bioturbated. Upper Devonian mudrocks, by contrast, are dominated by fine-scale, parallel laminations and show no evidence of infaunal activity. These rocks also contain common conodonts, radiolarians, spore bodies, and deep-water brachiopods. The data suggest that at least in updip areas, the lower Woodford was deposited in moderately deep, at least intermittently oxygenated water, whereas the upper Woodford accumulated under more distal, low energy, poorly oxygenated, hemipelagic conditions.

Barnett rocks are dominated by finely laminated, argillaceous and siliceous mudstones with subordinate calcareous mudstones. Argillaceous facies are most dominant in updip areas of the basin whereas downdip rocks are progressively more siliceous. Like the Woodford, clay mineral content in the Barnett decreases downdip. Updip facies are dominated by illite with subordinate mixed layer clays and chlorite whereas downdip Barnett rocks contain subequal mixed layer clays and illite. Unlike the Woodford, the Barnett contains relatively little detrital silica but much higher silica content overall. Barnett facies are much more similar across the basin. Updip area rocks contain more abundant phosphate and are locally burrowed but otherwise very similar to rocks in the basin center. All facies contain sedimentary features and allochems indicating accumulation in a dominantly unoxygenated, hemipelagic setting in which the dominant mechanism of sediment delivery was suspension settling and turbidity transport.

Both the Woodford and Barnett display complex variations in organic matter distribution. However, both mudrock systems contain admixtures of Type 2 and Type 3 kerogen in updip areas but are dominated by Type 3 organic matter downdip.