CLASSIC OSTREOLITHS REVISITED: THE ORIGIN AND DEVELOPMENT OF OYSTER BALLS FROM THE MIDDLE JURASSIC CARMEL FORMATION OF SOUTHWESTERN UTAH, USA

Ostreoliths are mobile circumrotatory accumulations of encrusting oysters around a nucleus. The most numerous and impressive ostreoliths are found in the Co-op Creek Limestone Member of the Carmel Formation (Middle Jurassic, Bajocian) in southwestern Utah, USA. These ostreoliths, colloquially known as “oyster balls”, are integral to understanding the restricted shallow marine conditions of Carmel deposition in the southernmost end of the long Twin Creek-Carmel Seaway. These ostreoliths were last studied in the 1990s and we can now add more details about their development, paleoecology, and taphonomy. They are composed primarily of the encrusting oyster Liostrea strigilecula, with occasional Plicatula bivalves, cyclostome bryozoans, disciniscid brachiopods, and boring mytilid bivalves (producing the trace fossil Gastrochaenolites). There are thus far two stratigraphic horizons of ostreoliths known in the Carmel. The upper layer consists of numerous smaller ostreoliths (principal dimensions average 98x73x50 mm; n=146). About 13 meters stratigraphically below is a larger ostreolith layer (230x181x112 mm; n=21). The smaller ostreoliths show a shape change from flattened and tabular to nearly spherical. This difference is apparently due to environmental energy that overturned the spheroids more often than the flatter varieties, but the surrounding sediments show no discernable differences. The larger ostreoliths are flattened and multi-lobed, suggesting they were overturned only infrequently by very energetic storm events. The smallest ostreoliths are preserved in a biosparite-grainstone matrix, indicating energetic bottom conditions near fair weather wavebase. The remaining ostreoliths are in lower-energy siltstones and shales. Previous studies suggested oolitic units would be associated with the ostreoliths, but thus far the only ooids found are trapped grains within borings. We suggest that the ostreoliths of the Carmel Formation formed in shallow, restricted marine waters with frequent storms and a relatively low biotic diversity. They are thus very common in a few places but virtually unknown elsewhere in the Twin Creek-Carmel Seaway.