rockhounds.com: The Bass Formation is the stratigraphically lowest and oldest of the Supergroup formations, succeeded in age among the rocks of Grand Canyon only by the metamorphic basement rocks of the Vishnu Group. The Bass Limestone is believed to have been deposited approximately 1250 million years ago during the later part of the Proterozoic Eon. It lies disconformably atop the Vishnu terrane, which had eroded to a topography of low relief by Bass time.

The lithology of the Bass Formation is predominately dolomite with lesser occurrences of arkose, sandy dolomite, shale and argillite. Breccias and conglomerates both underlie and occur throughout the Bass.

The sedimentary structures, thickness trends and lithologic composition of the Bass Formation suggest deposition of its lower members by an easterly transgressing sea and of its upper members during a regressive phase as the sea retreated back towards the west. During the sea's maximum incursion carbonates and deep water mudstones accumulated in western Grand Canyon, while in its eastern regions shallow water mudstones were deposited. The dolomites and mudstones present in the middle portions of the Bass exhibit textural features indicating deposition in relatively deep water. Evidence of a regression and deposition in marginal marine environments in upper members includes ripple marks, mud cracks and deposits of oxidized shales. The Bass typically erodes to form terraces of stair-stepped cliffs, with the harder dolomites forming the risers and the softer mudstones and shales forming steep treads.

Other sedimentary structures to be found in exposures of the Bass Limestone include interformational breccias and conglomerates, and graded beds which are associated with stromatolite fossils. Especially noteworthy within the Bass Limestone are characteristic occurrences of stromatolite beds, the most ancient fossils at Grand Canyon.