Kennedy, Elaine G., Kablanow, Ray, and Chadwick, Arthur V.

Abstract of Paper:

The Tapeats Sandstone forms the basal Cambrian deposit of the Tonto Group and is generally interpreted as a shallow marine deposit. In the Grand Canyon, the sandstone was deposited on a low-relief pre-Cambrian surface broken by scattered remnant cliffs of Shinumo Quartzite and isolated granitic hills. Paleoslope measurements and sedimentological features were recorded at 63 sections from 21 localities in the Grand Canyon exhibiting significant pre-Cambrian topographic relief. At 24 of these sites, debris flows were apparently initiated by some catastrophic event that simultaneously broke and transported Shinumo clasts in a matrix of Tapeats Sandstone. These brecciated flows were deposited along the pre-Cambrian 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 indicates that deposition of even the shallowest material was below storm wave base. Thorium/uranium (Th/U) ratios from the breccia matrix in Ninety-one Mile Canyon indicate sediment deposition in a reducing environment. Such conditions are unlikely in a high energy, near shore facies. These submarine flows were deposited on a surface with over 140m of vertical relief and would have required water depths in excess of 200m below storm wave base. Sedimentary structures used to identify the Tapeats Sandstone as a shallow water marine facies need to be reevaluated. To explain the features documented in this research, we propose that the Tapeats Sandstone was deposited as a deep-water, submarine fan complex.

Published in: Proceedings of the Third Biennial Conference of Research on the Colorado Plateau, National Park Service Transactions and Proceedings Series NPS/NRNAU/NRTP-97/12. Charles van Riper III and Elena T. Deshler (eds.). 1997.

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