GENESIS OF A CRINOID LAGERSTATTE IN THE UPPER PENNSYLVANIAN BARNSDALL FORMATION OF NORTHEASTERN OKLAHOMA
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.
Such subtle alternations of biofacies, taphofacies, and sedimentary properties may represent the record of minor sea-level fluctuations in otherwise monotonous mudrock-dominated sequences in the North American midcontinent and elsewhere.