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 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 most complete, readily accessible, and easily studied, exposure of the Nankoweap Formation occurs in Basalt Canyon.

Within the Grand Canyon, the lower member of the Nankoweap Formation outcrops at only two locations adjacent to the trace of the north-south-trending Butte Fault.

First, it outcrops just north of the Colorado River in the eastern side of the Basalt graben at Tanner Canyon Rapids.

Finally, it outcrops 2 km due south of the Colorado River near and at the southern limit of preservation of the Cardenas Basalt and Nankoweap Formation.

Typically, the sandstone sections are thin-to-medium bedded and exhibit planar tabular and trough cross-bedding, ripple marks, mudcracks, numerous soft-sediment deformation structures, and rare salt pseudomorphs.

West of Tanner Canyon, erosion has locally removed as much as 300 m of Cardenas Basalt before the deposition of the Nankoweap Formation.

The deeply weathered lavas retain their original textures but have been pervasively stained and altered to earthy hematite and siderite.

The upper part of the Nankoweap Formation consists of cliff-forming sandstones that grade irregularly upward – from red to white.

Except for wide shallow channels cut in the Nankoweap Formation and filled with dolomite, the upper contact lacks any stratigraphic evidence that indicates a large erosional or temporal hiatus.

[6][7][10] Van Gundy[3][4] identified a structure found in a sandstone bed of the Nankoweap Formation in Basalt Canyon as a trace fossil impression of a stranded jellyfish.

Paleontologists, who were unconvinced by Cloud's interpretation, reinterpretated this structure to be a burrow (trace fossil), known as “Asterosoma,“ made by a sediment feeding, worm-like organism.

Both specimens are very similar in morphology to sedimentary structures initially interpreted to be fossil jellyfish and named “Astropolithon.” Like Brooksella canyonensis, Astropolithon is now regarded to be the result of the venting of fluidized sand into surficial sediments blanketed by microbial mats that were typical of Precambrian sea- and lake-bottoms.