The Grand Canyon Supergroup, of which the Unkar Group is the lowermost part, overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks.

However this name, which was originally applied to the dikes and sills intruding strata underlying the Cardenas Basalt has been formally abandoned in the geological literature.

The brown, maroon, purple sandstones consist of texturally immature, planar-bedded, poorly sorted quartz and feldspar in a matrix of mica and clay.

[4][7] The basal contact of the Cardenas Basalt with the underlying Dox Formation is smooth, planar, parallel to bedding and locally interfingering.

Locally, the contact is a low relief erosional surface associated with a thin weathering zone developed in the lavas of the Cardenas Basalt.

The lowest part of the Nankoweap Formation consists of a basal conglomerate that is composed chiefly of gravel derived from the Cardenas Basalt.

[3][4] The contact between the Tapeats Sandstone and the Cardenas Basalt and rest of the folded and faulted Unkar Group is a prominent angular unconformity.

These monadnocks served locally as sources of coarse-grained sediments during the marine transgression that deposited the Tapeats Sandstone and other members of the Tonto Group.

The interbedded sandstones and hyaloclastites provide evidence that these eruptions occurred in wet coastal environments such as river deltas or tidal flats.

The current interpretation is that the deposition of the overlying Chuar Group in a marine setting disrupted the potassium-argon (K-Ar) radiometric system.

Apparently, fluids associated with the deposition of the Chuar Group have altered the older Cardenas Basalt, partially degraded the minerals, and therefore disrupted the K-Ar systematics.

New data acquired using newer dating techniques and approaches, indicate that the Cardenas Basalt erupted about 1,104 million years ago.