Late Pennsylvanian shales of north-central Texas : an assessment of their depositional environment
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Certain horizons in Late Missourian-Early Virgilian shales of north-central Texas contain an unusual mixture of marine invertebrates, terrestrial plant fragments, and phosphate nodules. These dark shales form part of the cyclothemic series of limestones, shales, and sandstones that result from the interplay of marine transgressions and regressions, uplift and erosion, deltaic development, and climatic variations. Classic models for phosphate genesis place these deposits near the oxygen minimum zone in a slope or off-shelf location, in relatively deep-water. Such an interpretation, however, suffers from: (1) the need for off-shelf transport of terrestrial material, (2) the repetition of this dark shale facies, often with obviously shallow-water algal limestones or deltaic deposits, and (3) the inferred deeper habitat for shallow-water invertebrates producing isolated patches of the mollusc-dominated Modern Evolutionary Fauna far offshore, a fauna that otherwise dominates nearer-shore habitats in the late Paleozoic. This study seeks a more viable explanation for these particular dark shales. Sixty-three sites were characterized for both fossils and enclosing sediment. Conventional paleontological procedures are supplemented with statistical cluster analysis and various analytical techniques (XRD, SEM, microprobe, INAA/ICP, rock-eval pyrolysis, stable & radiogenic isotopes) to suggest that the shales were intermittently deposited in a proximal shelf or nearshore environment. Encrustation and boring of fossils and nodules indicate that reworking, fluctuation of redox levels, and variations of sedimentation rates were all characteristics of the phosphate-bearing shale horizons of the Wayland Shale and Finis Shale. Shelly layers in the Placid and Finis shales record event horizons. Both plant roots and taxonomic congruence of miospore and plant fragments in the Bluff Creek/Necessity Shale are indicative of proximal shelf deposition. Phosphate middle rare earth element enrichment and radiogenic ⁸⁷Sr/⁸⁶Sr signatures do not support a deep water hypothesis. The proposed dysaerobic, shallow-shelf, depositional model resolves the sedimentary and other incongruities in the earlier deep-water depositional model by eliminating the need to explain the offshore transport of terrestrial material and the interbedding of shallow- and deep-water deposits. In particular, it clarifies the anomalous late Paleozoic occurrences of the Modern Evolutionary Fauna by assigning them to a more appropriate shallow-water setting.