In addition, thick, prominent, and dark-colored basaltic sills and dikes cut across the purple to red to brilliant orange strata of the Hakatai Shale.

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.

[3][8] First, the Hance Rapids (lower) member consists of purple to reddish-purple mudstone, interbedded sandy siltstone, and rare occurrences of thin-bedded subarkose- to quartz arenite.

Finally, the Stone Creek (upper) member of the Hakatai Shale consists of pale purple or lavender, fine- to coarse-grained, crossbedded coarse arkose.

Adjacent to the sills, the Hatakai Shale has been altered to knotted hornfels containing porphyroblasts of andalusite and cordierite that have been replaced by muscovite and green chlorite, respectively.

[3] In the eastern part of the Grand Canyon, the contact between the Hatakai Shale with underlying Bass Formation is typically gradational over an interval of a meter or so.

For example, in Red Canyon, the contact consists of an interval in which stromatolitic limestone of the Bass Formation is intimately interbedded with coarse deposits of the overlying Hakatai Shale.

Within the lower subarkose of the Shinumo Sandstone, a basal lag of conglomerate, which contains basement clasts up to 5 cm (2.0 in) across, lies on the eroded surface that forms this disconformity.

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.

At this outcrop, they occur in the transitional zone between it and the Bass Formation where stromatolitic carbonate beds are intimately interbedded with coarse deposits of the Hakatai Shale.

[11] In addition, the dark gray to black central cores of the reduction mottles found in the Cheops Pyramid (middle) member is possible of organic (stromatolitic?)

In addition, the 40Ar/39Ar dating of detrital muscovite from the basal, Escalante Creek Member of the overlying Dox Formation indicates that it is younger than 1,140 million years.