The Tapeats Sandstone forms the basal Cambrian deposit of the Tonto Platform and is generally interpreted as a shallow marine deposit. In the Grand Canyon, the sandstone was deposited on a low relief Precambrian surface broken by scattered remnant cliffs of Shinumo Quartzite and isolated granitic hills. Paleoslope measurements and sedimentological features were recorded from 63 localities in the Grand Canyon exhibiting significant Precambrian topographic relief. At these localities, debris flows were apparently initiated by some catastrophic event that simultaneously broke up and transported Shinumo clasts in a matrix of Tapeats Sandstone. These brecciated flows were deposited along the Precambrian surface topography from the cliff-faces basinward. Widespread preservation of the breccias along topographic relief during the deposition of the entire thickness of Tapeats Sandstone and much of the overlying Bright Angel Shale suggests deposition of even the shallowest material was below wave base. Th/U ratios from the breccia matrix in Ninetyone Mile Canyon indicate sediment deposition in a reducing environment. Such conditions are unlikely in a high-energy, nearshore facies. These submarine flows were deposited on a surface with over 140 m of vertical relief and would have required water depths in excess of 200 m.

Published in: The Proceedings of the 3rd Biennial Conference of Research on the Colorado Plateau, October 1995, p.43.