TIME-AVERAGED ACCUMULATION OF LATE CRETACEOUS (CENOMANIAN-TURONIAN) OYSTERS (PYCNODONTE NEWBERRYI) IN THE TROPIC SHALE OF SOUTHERN UTAH

The Tropic Shale is a late Cretaceous (Cenomanian-Turonian) formation in southern Utah that was deposited along the western margin of the Cretaceous Western Interior Seaway. Recently, this formation has become well-known for its high diversity of vertebrate fossils, including fish, turtles, sharks, plesiosaurs, and a dinosaur. Extensive oyster beds have also been discovered in the lower sections of the Tropic Shale. These beds are dominated by oysters (Pycnodonte newberryi), but also contain gastropods (Turitella), belemnites, coiled ammonites, unidentified bivalves, and serpulid worm tubes.

The study area consisted of two large oyster beds which are located in Glen Canyon National Recreation Area and are approximately 13 kilometers apart. The western bed has approximately one km² of exposed oysters, while the eastern bed has less than 1/10 km² of exposed oysters. The oyster beds are located on top of Bentonites A and B, the lowest bentonites in the shale. However, oysters are not continuous between the two bentonites and are not found in high concentration elsewhere in the shale. The surface exposure of the oyster beds seems to be largely controlled by modern erosional processes (the beds are concentrated on more resistant bentonites, seem to continue into hills, and are cut by stream channels). Fourteen samples of surface sediment were collected in 50 cm by 50 cm squares on both beds. The total fossil weight of each sample after screenwashing ranged from 130.31 g to 457.96 g, with an average of 243.19 g. The percent of fossil weight to sediment weight in the samples ranged from 6.39% to 22.46%, with an average of 12.83%. These calculations suggest an extremely high density of oysters located at the surface. The high concentration of oysters does not continue at depth into the shale, as oysters are in much lower abundance directly beneath the surface. Additionally, oysters on the surface are heavily fragmented, while those found in place beneath the surface tend to be nearly complete. Isolated oysters found higher up stratigraphically in the shale also tend to be nearly complete. The high density accumulation and fragmentation of oysters at the surface but not beneath suggests a time-averaged accumulation of the oysters rather than an in-place, life accumulation.