Abstract: Fine-Grained Sediment Transport and Accumulation on a Cretaceous Prodelta: The Tropic Shale of Southern Utah

Elana L. Leithold

The Tropic Shale in southern Utah accumulated in the Cenomanian-Turonian on a muddy sea floor near the western margin of the Cretaceous Western Interior seaway. The U.S. Geological Survey 1 Escalante Core contains a complete, approximately 250-m-thick section through the fine-grained succession. The lower quarter of this section is composed primarily of hemipelagic facies, which accumulated at relatively slow rates during the Greenhorn second-order sea level rise. The upper part of the Tropic Shale accumulated during the Greenhorn highstand and sea level fall, as prodeltaic environments migrated rapidly toward the east. Several third-order sea level cycles were superimposed on the longer term Greenhorn cycle.

Sedimentologic analysis of prodeltaic facies in the Escalante core reveals a remarkably detailed record of processes of fine-grained sediment transport and accumulation near the margin of the seaway. Previous study of the Tropic Shale and correlative deposits over a broad region in southern Utah indicates that this margin was characterized by a clinoform morphology. Sediment accumulation occurred most rapidly in the foresets, and more slowly in the bottomsets and topsets. In the Escalante core, trends in the thickness and character of laminae and beds, in the abundance and type of evidence for soft-sediment deformation, and in the compaction of fecal pellets have been used to delineate these subenvironments. Physical sedimentary structures in the core reveal the variable roles that cu rents, waves, and sediment-gravity flows played in the transfer of fine-grained sediments from the shoreline to deeper-water environments of the Western Interior seaway.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994