Chadwick, A. and Kennedy, E.

The Tapeats Sandstone forms the basal Cambrian deposit of the Tonto Platform and generally has been 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, sedimentological features and trace element distributions have been documented and analyzed from 91 Mile Canyon and Red Canyon in the Grand Canyon. Debris flows containing Shinumo clasts with a Tapeats sand matrix were catastrophically eroded and transported along the Precambrian surface topography from the cliff-faces basinward. These submarine flows were deposited on a surface with over 140m of vertical relief. Sedimentary structures and contacts indicate that even the shallowest material was deposited in excess of 200m below storm wave base. Th/U ratios from the breccia matrix and primary glauconite in the Tapeats Sandstones at both localities indicate sediment deposition in a reducing/low oxygen environment. Such conditions are highly unlikely in a high-energy, near shore facies. A reevaluation of sedimentary structures used to identify the Tapeats Sandstone as a shallow water marine facies reveals these features to be consistent with the deep water model. To explain the features documented in this research, we propose that the Tapeats Sandstone was deposited as a deep-water, submarine fan complex.*

*The model was revised for the presentation of the paper at the IAS Congress in Alicante: We proposed that the Tapeats Sandstone was deposited when a continental shelf collapsed, generating high energy movement of sediments down slope, blanketing the Precambrian surface.

Published in: 15th International Sedimentological Congress, Alicante, p. 247-248, 1998.

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