PALEOECOLOGY AND PALEOENVIRONMENTS AT THE SOUTHERN END OF A MIDDLE JURASSIC INLAND SEAWAY: THE CARMEL FORMATION (BAJOCIAN) OF SOUTHWESTERN UTAH

The Co-op Creek Limestone Member of the Carmel Formation (Middle Jurassic, Bajocian) was deposited at the terminus of a long, narrow epicontinental seaway in southwestern Utah, which was at about 20° N latitude. This restricted end of the seaway had a variety of depositional environments ranging from supratidal carbonate flats to ooid shoals with numerous lagoons between them. These rocks represent a tropical Jurassic marine sequence relatively rare in North America. Key indicators of environmental conditions include sedimentary structures such as desiccation cracks, anhydrite nodules, current ripples, carbonate hardgrounds, and stromatolites. Lagoonal sediments are primarily silts and clays. The ooid shoals are locally abundant as cross-bedded grainstones approximately 1-5 meters thick. The ooids are radial calcitic and exceptionally well preserved, with a variety of biotic (carbonate shell fragments) and abiotic (quartz sand grains, intraclasts) nuclei. They show intervals of microboring and rejuvenation. The paleocommunities were dominated by low to middle tier bivalves. Where hard substrates were available, the oyster Liostrea strigilecula is abundant, forming thick shell layers on hardgrounds and occasionally producing circumrotatory ostreoliths in large numbers. Gastropods are present but rarely preserved well enough for identification. Thus far only two poorly preserved ammonites have been recorded. Crinoids are sporadic in the Co-op Creek Limestone Member, being found as scattered ossicles or the occasional thin encrinite. They are almost entirely one species (Isocrinus nicoleti). Echinoids are found mostly as rare scattered spines and test fragments. A moderately diverse sclerobiont community is present with mostly encrusting cyclostome bryozoans, bivalve borings and rare tubeworms and encrusting brachiopods. The communities in these restricted, shallow marine environments are of relatively low diversity and mostly generalists. Work continues to place these organisms in biogeographic and paleoecological context for comparison to the rest of the Jurassic world.