Genesis and Taphonomy of a Crinoid Lagerstatte in the Upper Pennsylvanian Barnsdall Formation of Northeastern Oklahoma
Type of Degreethesis
DepartmentGeology and Geography
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A 50-cm mudstone interval within the Upper Pennsylvanian Barnsdall Formation, exposed near Copan, Washington County, northeastern Oklahoma, records prodeltaic distal shelf sedimentation in an oxygenated, low energy setting. Three thin (3-8 cm thick) horizons within this section contain abundant articulated crinoid remains, with the lowest of these horizons containing the highest genus-level crinoid diversity identified within the Pennsylvanian System. These thin units alternate with thicker (8-15 cm thick) units lacking in articulated crinoids but containing abundant endobenthic bivalve fossils. Centimeter-scale microstratigraphy, fabric analysis of mudstone slabs, taphonomic analysis of crinoid specimens, assessment of fossil content and character derived from disaggregation of mudstone slabs, and evaluation of the morphology, distribution, and physical properties of associated siderite concretions have shed light on the biological, sedimentological, and geochemical processes necessary for the occurrence and preservation of this exceptional crinoid fauna. The crinoid-bearing units were deposited under conditions of sediment starvation associated with minor transgressive episodes. Long periods of slow sedimentation were interrupted by episodic distal storm events, which were characterized by rapid deposition of fine-grained sediment without erosion, winnowing, or strong unidirectional currents. As a result, the crinoid-bearing units are composed largely of stacked obrution layers. Individual burial layers were very thin and may have been spatially or temporally variable in thickness, as evidenced by taphonomic variability among crinoid specimens recovered from the same horizon. Sediment starvation, by allowing obrution horizons to become bundled, appears to explain the extremely high diversity and abundance of articulated crinoids within the thin units. This mechanism is better supported than burial of a single extraordinarily diverse crinoid community. The thicker units not bearing articulated crinoid fossils represent minor episodes of relative sea-level fall and consequent deltaic progradation. The increased proximity to sediment-source area resulted in higher sedimentation rates and more energetic storm events, as evidenced by a primarily endobenthic biofacies, thicker individual burial layers, winnowed lags, and evidence for minor erosion. These interpretations indicate that the stratigraphic expression of parasequences in distal shelf settings may be represented by such thin and subtle alternations of distinct taphofacies, biofacies, and concretion morphologies. Taphonomic variability among crinoids within the same subclass is shown to be significant at the genus level, but is also detected among individuals of the same species. This preservational heterogeneity results from the complex interactions between morphological, ecological, and depositional factors, including rate of burial, scavenger preferences, morphology and ethology of crinoids, size of crinoid individuals, and heterogeneity in burial layer thickness.
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