The Upper Cretaceous Panther Tongue of east-central Utah is exceptionally well exposed in the Book Cliffs and Wasatch plateau and offers an unusual opportunity to study, in three dimensions, the morphology and internal structure of a nearshore depositional complex. Lithology, sedimentary structures, geometry, and trace fossil assemblages, considered collectively, permit recognition of seven depositional environments during Panther time.

Transition from the underlying Mancos Shale to the coarser terrigenous clastic sediments of the Panther Tongue is represented by densely mottled gray siltstone in which primary lamination has been destroyed by the activity of detritus-feeding organisms. Overlying the gray siltstone is a very fine-grained sandstone which has a high matrix content and contains an abundant and varied trace fossil assemblage. This very fine-grained sandstone is present in bar and backbar depositional environments, but is divided into two areas of occurrence by an elongate sandstone body believed to be a longshore bar. The wedge-shaped forebar contains wavy laminated sediments near the base which grade upward into thin-bedded mottled sediments. Trace faunas in the forebar change in nature vertically and la erally away from the longshore bar. In the backbar, sediments are more poorly sorted and stratification is thin- to thick-bedded. The trace fossil assemblage in the backbar shows very little lateral change and is less variable than in the forebar.

The longshore bar strikes northeast-southwest, and its cross section is well exposed in the western Book Cliffs and northern Wasatch plateau. The bar is characterized by massive bedding, lenticular shape, and asymmetrical flanks. Large-scale, low-angle (10°) foreset beds at the Panther type locality north of Helper, Utah, represent the seaward (southeast) face of the bar. These foreset beds have ripple-marked surfaces and contain an abundant and varied suite of sole marks. The dip and predominant current direction of sole marks are southwest, indicating that longshore currents flowed along the seaward side of the bar. Current ripples and oscillation ripple marks, however, developed in response to tidal action, as is indicated by orientation of their crests parallel with the longs ore bar. The bar and bar shore-face contain a characteristic trace fossil assemblage which is dominated by filter-feeding organisms and vagrants which left various surface trails on bedding planes. Behind the bar, a series of short, low- to medium-angle (5-20°), cross-stratified sandstone beds built landward over the backbar environment.

A marine transgression late in Panther time truncated the previously deposited sediments, and a sequence of horizontal to subhorizontal strata was laid down across the erosional surface during a second regression of the sea. Panther deposition ended with the return of the Mancos sea and the deposition of clay and mud.

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