POSSIBLE DEBRIS FLOWS IN THE UPPER CRETACEOUS CAPPING SANDSTONE, WAHWEAP FORMATION, GRAND STAIRCASE – ESCALANTE NATIONAL MONUMENT, UTAH

The capping sandstone member of the Late Cretaceous Wahweap Formation is interpreted as a braided stream system deposit with limited preservation of overbank fines. At Henrieville Creek and Bull Flat localities, pebbly conglomerate and pebbly sandstone deposited in various types of braid bars characterize the middle part of the section. However, within the upper portion of the conglomerate zone in both sections, graded and sandy siltstone to pebbly siltstone horizons occur, and these are best interpreted as debris flow deposits.

The debris flow deposits extend laterally for up to 100 meters before being truncated by the overlying channel complexes. The preserved thickness is up to 70 cm with a basal relief of up to 30 cm. Internally the bed fines upward from a pebbly sandy siltstone with ripped up clasts of conglomerate, pebbly sandstone and sandstone from the underlying bed, to a pebbly siltstone. Dewatering structures are present at the base and top of some beds. Some tops of beds are rooted with possible burrows. Within one bed at the Bull Flat locality two depositional units are recognizable.

A debris flow interpretation is supported by: 1) the lack of internal primary structures, 2) floating pebbles in a sandy siltstone matrix, 3) ripped up clasts of hydrodynamically heavier particles, 4) dewatering structures at the base and tops on some beds and 5) weak normal grading. The scoured bases of some debris flows indicate that some flows were initially turbulent followed by matrix-supported flow. Dewatering structures indicate rapid sedimentation and fluid expulsion. These debris flows most likely followed topographic lows, such as channels between braid bars. Superposition of two flows and rooting and burrowing at the tops of flows indicate that a significant time span occurred between truncation by younger fluvial processes.