GRAND CANYON SUPERGROUP

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Fig. 7

The sedimentary units of the Grand Canyon begins with the Grand Canyon Supergroup. This in turn is divided into the Unkar Group, the Nankoweap Formation, the Chuar Group, and Sixtymile Formation. Within the Unkar Group there is the Bass Limestone, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Lava. These all have their own members too.

Within the Bass Limestone biscut-form and biohermal stromatolites can be found. Stromatolitic dolomite layers can be found within the Dox Formation. The Unkar Group records a major west to east trangression of the sea. Minor fluctuations of sea level or sediment surface elevation is recorded by features suggesting both subaerial and marine deposition throughout the sequence. Apparently the Unkar Group was deposited in a basin in which the rate of subsidence was approximately equal to the rate of deposition.