TAPHONOMY OF AN UNUSUAL FRESHWATER SHELL BED IN THE UPPER CRETACEOUS KAIPAROWITS FORMATION, SOUTHERN UTAH

Dense fossil shell beds are well documented in marine settings, yet similar deposits in freshwater environments are relatively unexplored. Here we report on an extensive freshwater shell bed, dominated by a paucispecific assemblage of unionid bivalves, from the Upper Cretaceous Kaiparowits Formation in the Grand Staircase-Escalante National Monument. The locality is hosted within a fluvial channel sequence within the middle unit of the Kaiparowits Formation. Post depositional erosion prohibits exact spacial reconstruction, however the bed minimally extends for 56 x 48 m and ranges in thickness from 0.4 to 2.55 m. Shell density is extraordinarily high, ranging from 1000 to 2000 individuals per cubic meter, suggesting a population of >20 million bivalves. Aside from the volume of shells, the most stiking characteristic of this deposit is the articulated (closed valves) nature of the bivalves, which are randomly oriented and commonly in shell-to-shell contact. Shells range in length from 40 to 115 mm and fall into two main size populations. Based on these characteristics, obtained from detailed grid sampling, the deposit is interpreted to represent a mass mortality event (i.e., a census deposit). The cause of this catastrophic event is uncertain, however we present two hypotheses: (1) it is the result of a high-intensity storm event, such as a tropical storm or tornadic activity, which caused intense winnowing and build-up of the shallow, infaunal bivalves; or (2) it may represent an extraordinary fluvial flooding event.