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Microstratigraphy and Comparative Taphonomic Analysis of the Upper Core Shale of a Pennsylvanian Cyclothem: Keys to the Recognition of Subtle Cyclic Deposition
The Upper Pennsylvanian Barnsdall Formation as exposed near Copan, northeastern Oklahoma, represents a low energy, prodeltaic distal shelf environment and constitutes part of the highstand phase (core shale) of the Missourian (=Kasimovian) Stanton cyclothem. Directly overlying a dark grey, phosphatic shale representing maximum highstand, an interval of green mudstone records initial regression and breakdown of water column stratification. Within this upper core shale is a diverse and abundant benthic fossil assemblage, including the highest levels of crinoid diversity recognized in the global Pennsylvanian System. Detailed microstratigraphic and comparative taphonomic analysis reveals that this lithologically monotonous succession records three cycles, each approximately 15 cm (6 in) thick. Each cycle consists of a distal portion (a thin, very time-averaged unit bearing large siderite concretions and a diverse, abundant, and well-preserved crinoid assemblage) overlain by a more proximal interval (a thicker unit bearing small siderite concretions, discrete sideritized burrows, winnowed skeletal lags, and burrowing mollusks preserved in living position). These cycles appear to be arranged into a progradational pattern, fitting the overall regressive trend of this portion of the cyclothem. This indicates that seemingly monotonous intervals may indeed be capable of revealing sea-level dynamics, although the evidence is inconspicuous and requires detailed investigation.
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