Origin and Paleoecology of Free-Rolling Oyster Accumulations (Ostreoliths) in the Middle Jurassic of Southwestern Utah, USA

Ostreoliths are abundant in the limestones, siltstones, and shales of the upper Carmel Formation (Middle Jurassic) in southwestern Utah. These are circumrotatory accumulations of the oyster Liostrea strigilecula, along with Plicatula bivalves, the bereniciform bryozoan Eurystrotos, the boring Gastrochaenolites, the nestling bivalve Modiolus, and the encrusting inarticulate brachiopod Discinisca. The ostreoliths are mostly spherical to discoidal and range in diameter from less than two centimeters to more than half a meter. They are found in two laterally-extensive horizons, one directly on a hardground and the other passing through ooid shoal and lagoonal facies. The ostreoliths formed in two ways. One group started as Liostrea-encrusted hardgrounds that were eroded and fragmented on the seafloor. These oyster accumulations collected additional oyster recruits on their sides and undersides until final burial. The second group of ostreoliths developed on soft substrates in oolitic shoals where Liostrea encrusts mollusk valves. The oysters grew on the upward-facing convert surfaces; the downward-facing concave sides were encrusted by bryozoans and Plicatula. After the ostreolith rotated Liostrea again colonized the upper surfaces but died on the new undersides. The dead Liostrea valves on the undersides were encrusted by Plicatula and the bryozoans. Lithophagid bivalves bored the dead valves as well, and Modiolus nestled in vacant borings and between oyster valves. This accumulation process continued until the ostreoliths were buried in tempestites. These ostreoliths represent a mobile community in which faunal composition and diversity was controlled by the rate of overturning and the consistency of the substrate. Morphological features of Liostrea, such as a deep left valve, rapid growth and calcitic composition, were essential for ostreolith development. One laterally-extensive ostreolith horizon was formed when storm currents distributed them across facies boundaries.