| Sixtymile Formation | Kwagunt Formation | Galeros Formation | Nankoweap Formation |
| Cardenas Lava
| Dox Formation
| Shinumo Quartzite
| Hakatai Shale
| Bass Limestone |
The Bass Limestone
The Bass Limestone 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 Limestone is predominately dolomite with lesser occurrences of arkose, sandy dolomite, shale and argillite. Breccias and conglomerates both underlie and occur throughout the Bass. Pictured above left and right are exposures of the Bass Limestone in the Phantom Ranch / Phantom Creek vicinity, near a saddle approximately half way up a route connecting Phantom Ranch to the Utah Flats vicinity atop the mesa bordering on the west, at approximately UTM 12S 0400981, 3996034, 3372 feet elevation. In this vicinity the thickness of the Bass is about 330 feet. Exposures of the Bass in other localities towards the northwest exhibit a general trend of thickening in that direction.
In the image at left an exposure of the Bass Limestone near the base of Cheops Pyramid tops the folded, crystalline core of the Vishnu as a ribbon of pinkish, vertical cliffs. A tremendous gap in geological time on the order of 500 million years is represented by this disconformity and records which are missing from the geologic column between the heavily eroded Vishnu and the Bass sediments. Atop the Bass cliffs are conformable, softer shale and mudstone members of the Hakatai Shale which have eroded to form receding, purplish colored, stair stepped mounds.
The sedimentary structures, thickness trends and lithologic composition of the Bass Limestone 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.
"Bass Formation" has been proposed by some technical workers as a more accurate description who contend "Bass Limestone" is really somewhat of a misnomer due to the variety of rock types within this formation. The diversity of rock types composing the Bass is not atypical as virtually all of the strata in Grand Canyon popularly designated and labeled as "Sandstones", "Limestones" and "Shales" exhibit complex structures and members composed of differing lithologic types.
Some grade smoothly into each other, creating a continuous record of deposition and geological and environmental conditions. Between others there are minor to major temporal gaps, unconformities where depositional process were interrupted by a change in geologic conditions that produced no net deposition or erosion instead.
The geologic record provided by the Bass Limestone and the overlying Hakatai Shale is thought to be continuous and without any significant temporal gaps, with the "contact" between these two formations conformable, being the most gradational in eastern outcrops and sharpest in western outcrops. An underlying layer of conglomerate known as the Hotauta Conglomerate Member deposited in low areas of the eroded and hilly Vishnu terrain composes the lowermost unit of the Bass Limestone. A switchbacky section of the Phantom Ranch / Utah Flats route up a wall above the stair stepped outcrops in the Bass traverses a number of interformational conglomerate layers, one of which is shown above with my hat to provide some scale for the size of its clasts.
Above are views detailing another conglomerate layer towards the upper portion of the Bass with a 55 mm lens cap providing scale. In this exposure the conglomerate clasts are not well sorted and tend to be more on the broken and angular side than well rounded, indicating they had not been transported very far from their source by the erosional processes depositing them here. Outcrops of interformational conglomerate bearing strata occur in a number of places in the upper Bass along the Phantom Ranch / Utah Flats route.
Further up along the Phantom Ranch / Utah Flats route are outcrops of the Bass alternating with a relatively high frequency between a hard, sandy dolostone and softer strata, some of which is also clast bearing. As the sea continued to generally regress towards the west the environment of deposition in the region changed to more marginal, deltaic conditions. The predominance of generally shallower and more marginal marine conditions marks a conformable horizon ending Bass time and the beginning of deposition of the overlying Hakatai Shale.