The Vishnu Basement Rocks is the name recommended for all Early Proterozoic crystalline rocks exposed in the Grand Canyon region. They form the crystalline basement rocks that underlie the Bass Limestone of the Unkar Group of the Grand Canyon Supergroup and the Tapeats Sandstone of the Tonto Group. These basement rocks have also been called either the Vishnu Complex or Vishnu Metamorphic Complex. These Early Proterozoic crystalline rocks consist of metamorphic rocks that are collectively known as the Granite Gorge Metamorphic Suite; sections of the Vishnu Basement Rocks contain Early Paleoproterozoic granite, granitic pegmatite, aplite, and granodiorite that have intruded these metamorphic rocks, and also, intrusive Early Paleoproterozoic ultramafic rocks.
The Vishnu Basement Rocks were deposited as mafic and felsic volcanic rocks and sediments but were later metamorphosed and intruded by igneous rock.
The Bass Formation, also known as the Bass Limestone, is a Mesoproterozoic rock formation that outcrops in the eastern Grand Canyon, Coconino County, Arizona. The Bass Formation erodes as either cliffs or stair-stepped cliffs. In the case of the stair-stepped topography, resistant dolomite layers form risers and argillite layers form steep treads. In general, the Bass Formation in the Grand Canyon region and associated strata of the Unkar Group-rocks dip northeast (10°–30°) toward normal faults that dip 60+° toward the southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area. In addition, thick, prominent, and dark-colored basaltic sills intrude across the Bass Formation.
Stromatolites in Bass Limestone, Grand Canyon
Bass Formation (multiple & thinly layered) below colorful Hakatai Shale-(orange), sitting on Granite Gorge-(Upper Inner Gorge, the blackish Vishnu Basement Rocks). Cheops Pyramid (prominence), Grand Canyon (Isis Temple region, Bright Angel Canyon)
Close-up showing a light rock color, but the multiple layers of bedding in the Bass Formation.
From Komo Point / Komo Point Trail, a few sections near Granite Gorge of black Vishnu Basement Rocks and a section of the layered Bass Limestone (note horizontal layering & dk blackish-brown color) – visible below the cliffs of Tapeats Sandstone. (high resolution expandable photo)