Shinumo Quartzite

The Shinumo Quartzite also known as the Shinumo Sandstone, is a Mesoproterozoic rock formation, which outcrops in the eastern Grand Canyon, Coconino County, Arizona, (Northern Arizona). It is the 3rd member of the 5-unit Unkar Group. The Shinumo Quartzite consists of a series of massive, cliff-forming sandstones and sedimentary quartzites. Its cliffs contrast sharply with the stair-stepped topography of typically brightly-colored (orange, red, yellow, etc) strata of the underlying slope-forming Hakatai Shale. Overlying the Shinumo, dark green to black, fissile, slope-forming shales of the Dox Formation create a well-defined notch. It and other formations of the Unkar Group occur as isolated fault-bound remnants along the main stem of the Colorado River and its tributaries in Grand Canyon.

Typically, the Shinumo Quartzite and associated strata of the Unkar Group dip northeast (10°–30°) toward normal faults that dip 60+° toward the southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area (below East Rim).

The Shinumo Quartzite is a middle member of the Unkar Group. The Unkar Group is about 1,600 to 2,200 m (5,200 to 7,200 ft) thick and composed of, in ascending order, the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt.

The Unkar Group consists of a sequence of sedimentary rocks that accumulated in a variety of environments ranging from fluvial to shallow-marine. Overall, the strata within it are conformable with the exception of a significant disconformity between the Hakatai Shale and Shinumo Quartzite. The Unkar Group is overlain in ascending order by the Nankoweap Formation, about 113 to 150 m (371 to 492 ft) thick; the Chuar Group, about 1,900 m (6,200 ft) thick; and the Sixtymile Formation, about 60 m (200 ft) thick. The Unkar Group, as the base section of the Grand Canyon Supergroup, overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks.