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Abstract: Ichnocoenoses and Paleoenvironments of the Middle Jurassic Carmel-Twin Creek Seaway (Carmel Formation, Southwestern Utah)

KILBOURNE, KELLY H., and H. ALLEN CURRAN,, Smith College; MARK A. WILSON, The College of Wooster.

The Carmel Formation of southwestern Utah, near Gunlock, consists of carbonate siltstones and sandstones deposited under arid conditions in shallow marine to peritidal environments of the Carmel-Twin Creek Seaway. Unit D (Nielson, 1990) of the Co-op Creek Member is largely a grainstone of shallow-marine origin. It contains a rich trace fossil assemblage with at least two distinct ichnocoenoses that reflect changing conditions of water depth and salinity.

The lower beds of unit D are characterized by an ichnocoenosis dominated by the dense occurrence of Planolites/Paleophycos burrows and networks of large Thalassinoides burrows, with shaft and tunnel diameters averaging 3 cm. Ichnofabric index measurements average 3.5 (Droser-Bottjer scale) for these beds. This ichnocoenosis grades upward into an ichnocoenosis dominated by well developed and preserved trails of Gyrochorte comosa, with an average width of 4 mm. Also present but less common are small vertical burrows, Lockeia burrows of varying sizes, smaller diameter Thalassinoides burrows, and rare Asteriacites resting traces formed by asterozoans.

Both ichnocoenoses represent shallow shelf environments marginal to the Carmel-Twin Creek Seaway. The prolific bioturbation displayed by the first ichnocoenosis suggests a shallow subtidal environment with generally stable bottom conditions. As the environment shallowed, the Gyrochorte-dominated ichnocoenosis developed, indicating peritidal conditions. In the Gunlock area, the uppermost beds of unit D are marked by the occurrence of microbial mats, mudcracks, salt hopper casts, and thinly-bedded gypsum horizons as the seaway continued to shallow under the arid conditions. Here the Carmel Formation is unconformably overlain by beds of the Dakota(?) Conglomerate.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah