Sixtymile Formation

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The Sixtymile Formation is a very thin layer of sandstone, siltstone, and breccia that lies beneath the Tapeats Sandstone in the eastern Grand Canyon, Arizona. It is exposed in only four spots in the Chuar Valley, on top of Nankoweap Butte and in Awatubi and Sixtymile Canyons. The maximum preserved thickness is about 60 meters (200 feet), but the original thickness is unknown because much of it was eroded before the Tapeats Sandstone was deposited.

This formation sits above the Chuar Group and is cut by the eastward Butte fault zone, with Tapeats Sandstone over it. The Sixtymile, Chuar Group, and related rocks together form the Grand Canyon Supergroup. The hanging line of the Chuar Syncline (a large, doubly plunging fold) runs near the Butte fault, linking the syncline to the fault.

Three informal members make up the Sixtymile Formation:
- Lower member: about 22–27 m thick, a red mix of laminated hematitic sandstone, siltstone, and breccia; dolomite blocks from the underlying layer are sometimes present.
- Middle member: about 25 m thick, thin-bedded, very fine cherty quartzite that often shows chert and some signs of slumping.
- Upper member: about 12 m thick, fine sandstone and sandstone with small amounts of conglomerate, formed by rivers and fan-shaped deposits along the syncline.

Depositional environments shift from a lake or wet basin in the lower part, to a lake along the syncline axis in the middle, and finally to a river/stream environment with fan-delivered sediments in the upper part. The formation records active faulting along the Butte fault system. No fossils have been found in the Sixtymile.

Although the unit was once considered Precambrian, dating of detrital zircons in 2018 showed Cambrian age, around 520–509 million years ago. This places the Sixtymile Formation as the lowermost part of the Tonto Group, deposited in lacustrine, fluvial, and shallow-marine settings within relatively fault-controlled basins.