The Tonto Group is a name for an assemblage of related sedimentary strata, collectively known by geologists as a Group, that comprises the basal sequence Paleozoic strata exposed in the sides of the Grand Canyon. As currently defined, the Tonto groups consists of the Sixtymile Formation, Tapeats Sandstone, Bright Angel Shale, Muav Limestone, and Frenchman Mountain Dolostone. Historically, it included only the Tapeats Sandstone, Bright Angel Shale, and Muav Limestone. Because these units are defined by lithology and three of them interfinger and intergrade laterally, they lack the simple layer cake geology as they are typically portrayed as having and geological mapping of them is complicated.
Muav Limestone-(broader-based gray cliff supporting tall-reddish Redwall Limestone cliff) and Bright Angel Shale-(greenish & extensive slope-former), resting on Tapeats Sandstone-(short, dark vertical cliff on gorge rim) and the Tonto Platform, inner canyon, Granite Gorge (the two units are easily seen below the red-stained Redwall Limestone (~550 ft thick))
View of Tower of Set peak and sub-unit cliff section from Tonto Trail, Granite Gorge, north of Mohave Point, Grand Canyon Village, South Rim. The peak is behind and separated from a cliff unit (with small prominence), in front-(photo center, right, Tower of Set (peak) to its left). Vertical erosion in cliff of Redwall Limestone, upon horizontal Muav Limestone cliff. The Tapeats Sandstone sits in foreground on Granite Gorge, and is seen as thinly-bedded. The
close-up and far views of the Tonto Group formations. Note in far view the cliff-run of the Tapeats Sandstone cliff below the (whitish)-greenish Bright Angel Shale-(often dull-greenish, but even yellowish in northeast Grand Canyon).
The Sixtymile Formation is a very thin accumulation of sandstone, siltstone, and breccia underlying the Tapeats Sandstone that is exposed in only four places in the Chuar Valley. These exposures occur atop Nankoweap Butte and within Awatubi and Sixtymile Canyons in the eastern Grand Canyon, Arizona. The maximum preserved thickness of the Sixtymile Formation is about 60 meters (200 ft). The actual depositional thickness of the Sixtymile Formation is unknown owing to erosion prior to deposition of the Tapeats Sandstone.
Sixtymile Formation in Grand Canyon
