Nankoweap Formation

Nankoweap Formation Stratigraphic range: Neoproterozoic, <770 Ma
Venus Temple landform, NNW of Lipan Point (East Rim); tilted (striped & multilayered) Nankoweap Formation above tilted (blackish) Cardenas Basalt, above (tilted and reddish) Dox Formation.
Type	Geological formation
Unit of	Grand Canyon Supergroup
Underlies	Chuar Group and, as part of the Great Unconformity, the Tapeats Sandstone
Overlies	Cardenas Basalt
Thickness	370 feet (110 m) approximate maximum
Lithology
Primary	sandstone, siltstone & shale (red beds)
Location
Region	Arizona east Grand Canyon Lava Butte region on Colorado River, near Lipan Point
Country	United States of America
Extent	(eastern) Grand Canyon, Colorado River region
Type section
Named for	Nankoweap Canyon
Named by	Van Gundy (1934), Van Gundy (1951), and Maxson (1961)

The Neoproterozoic Nankoweap Formation (pronounced Nan' coe weep), is a thin sequence of distinctive red beds that consist of reddish brown and tan sandstones and subordinate siltstones and mudrocks that unconformably overlie basaltic lava flows of the Cardenas Basalt of the Unkar Group and underlie the sedimentary strata of the Galeros Formation of the Chuar Group. The Nankoweap Formation is slightly more than 100 m in thickness. It is informally subdivided into informal lower and upper members that are separated and enclosed by unconformities. Its lower (ferruginous) member is 0 to 15 m thick. The Grand Canyon Supergroup, of which the Nankoweap Formation is part, unconformably overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks.

The strata of the Nankoweap Formation are exposed in a small area that occupies an area from just south of Carbon Canyon to Basalt Canyon on the west bank of the Colorado River to around Comanche Creek and Tanner Canyon on the east bank of the Colorado River within the eastern Grand Canyon, Coconino County, Arizona. It and associated strata of the Unkar and Chuar groups are preserved in a prominent syncline and fault block. The most complete, readily accessible, and easily studied, exposure of the Nankoweap Formation occurs in Basalt Canyon.

The nomenclature of the Nankoweap Formation has changed over time. Originally, the strata of the Nankoweap Formation was included in-part in the top of the "Unkar terrane" (Group) and in-part in the basal “Chuar terrane” (Group) by Walcott in 1894. Van Gundy first recognized the thin sequence of red beds unconformably overlying basaltic flows of the Unkar Group as a separate stratigraphic unit, which he called the “Nankoweap Group.“