Sixtymile Formation

Sixtymile Formation Stratigraphic range: NeoProterozoic, less than 742 Ma
Sixtymile Formation in Grand Canyon
Type	Geological formation
Unit of	Grand Canyon Supergroup
Underlies	Tonto Group separated by Great Unconformity
Overlies	Chuar Group
Thickness	120 feet (37 m), at maximum
Lithology
Primary	siltstone and sandstone
Other	intraformational breccia
Location
Region	The Chuar syncline, Grand Canyon, northern Arizona.
Country	United States of America
Type section
Named by	Ford et al. (1972), Ford and Breed (1973), and Elston (1979)

The Sixtymile Formation is the uppermost unit of the Grand Canyon Supergroup. It is a very thin accumulation of sandstone, siltstone, and breccia 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 during the formation of the Great Unconformity and prior to deposition of the Tapeats Sandstone.

Sixtymile Formation and the underlying Chuar Group are preserved only in a broad asymmetric fold comprising called the Chuar syncline. The Chuar syncline is a doubly plunging fold, which means that along the hingeline within the axis of the syncline, beds in some areas (Nankoweap Canyon) dip toward the south, and in other areas (Lava Chuar Canyon) beds dip toward the north. The Sixtymile Formation, Chuar Group, and Chuar syncline is bounded on the east by the Butte fault zone and on all other sides by the Great Unconformity and overlying Tapeats Sandstone. The hingeline of the Chuar syncline parallels the trace of the Butte fault, suggesting a genetic relationship between the syncline and the fault.

In descending order, the Sixtymile Formation is underlain by the Chuar group, Nankoweap Formation, and the Unkar Group. The Unkar Group lies unconformably overlies deeply eroded granites, gneisses, pegmatites, and schists of the Vishnu basement rocks.. The Nankoweap and Sixtymile formations together with the Chuar and Unkar groups comprise the Grand Canyon Supergroup.

The Sixty Mile Formation of Chuar Group was first recognized by Ford and Breed (1972). It was formally named the Sixty Mile Formation, upper formation of Chuar Group of Grand Canyon Supergroup by Ford and Breed (1973). They designated the type section of the Sixtymile Formation to be outcrops of breccia and sandstone on cliffs on the north side of the upper part of Sixtymile Canyon. Later it was changed to Sixtymile Formation of Chuar Group of Grand Canyon Supergroup and its spelling changed to Sixtymile.

The Sixtymile Formation is typically subdivided into three informal members. They are (1) a lower red siltstone, sandstone, and breccia member; (2) a middle cherty quartzite member; and (3) an upper breccia-bearing sandstone and conglomerate member. The lower member, which is 22 22 meters (72 ft) to 27 meters (89 ft) thick, consists of beds of a heterogeneous mixture of laminated hematitic sandstone; thin-bedded micaceous sandstone containing rock fragments; monomict and polymict breccia; crudely bedded sandstone; and thin-bedded soft sandy siltstone. Individual beds are discontinuous and local in extent and often grade into each other. Blocks of dolomite from the underlying Walcott Member often occur in the lower member.