The Bright Angel Shale is one of five geological formations that comprise the Cambrian Tonto Group. It and the other formations of the Tonto Group outcrop in the Grand Canyon, Arizona, and parts of northern Arizona, central Arizona, southeast California, southern Nevada, and southeast Utah. The Bright Angel Shale consists of locally fossiliferous, green and red-brown, micaceous, fissile shale (mudstone) and siltstone with local, thicker beds of brown to tan sandstone and limestone. It ranges in thickness from . Typically, its thin-bedded shales and sandstones are interbedded in cm-scale cycles. They also exhibit abundant sedimentary structures that include current, oscillation, and interference ripples. The Bright Angel Shale also gradually grades downward into the underlying Tapeats Sandstone. It also complexly interfingers with the overlying Muav Limestone. These characters make the upper and lower contacts of the Bright Angel Shale often difficult to define. Typically, its thin-bedded shales and sandstones erode into green and red-brown slopes that rise from the Tonto Platform up to cliffs formed by limestones of the overlying Muav Limestone and dolomites of the Frenchman Mountain Dolostone.
Nomenclature
In 1875, G. K. Gilbert recognized that strata exposed near the bottom of parts of the Grand Canyon are similar to those known elsewhere in Arizona as the Tonto Group. Based upon this similarity, he assigned the strata that form the lowermost part of the Phanerozoic strata in the Grand Canyon to the Tonto Group. Without designating formal type localities, he also subdivided the Tonto Group into three subdivisions, from youngest to oldest, the Marbled limestone, the Tonto shale, and the Tonto sandstone.
In 1914, L F. Noble renamed Gilbert's subdivisions of the Tonto Group. He renamed the Tonto sandstone as the Tapeats Sandstone and the Tonto shale as the Bright Angel Shale. He named the Bright Angel Shale after Bright Angel Canyon because in the walls of this canyon, this formation is well exposed.
Later in 1922, the Marbled limestone was redefined and renamed as the Muav Limestone by L. F. Noble. According to Noble's definition, the Muav Limestone consisted of an upper set of dolomite beds and a lower set of limestone beds. Noble's lower set of limestone beds is now defined as the Muav Limestone and the Frenchman Mountain Dolostone is now defined as Noble's upper set of dolomite beds.
E. D. McKee and C. E. Resser largely retained Noble's nomenclature. However, they did subdivide the Bright Angel Shale into a number of subdivisions, members, based upon prominent beds of limestone and sandstone within it. However, these members have found to be confusing and unhelpful in understanding the stratigraphy of the Bright Angel Shale and are typically ignored and rarely used in the published literature.
Description
In the Grand Canyon, the Bright Angel Shale is typically a heterogenous, sometimes fossiliferous, mixture of interbedded shale, mudstone, siltstone, fine-grained sandstone, and isolated beds of limestone. The dominant lithology within the Bright Angel Shale is greenish shale that is composed largely of illite and varying amounts of chlorite and kaolinite. A reddish brown coloration is imparted to a number of the sandstone and siltstone beds by the high percentage of hematitic ooids and iron oxide cements that they contain. Isolated beds of coarse-grained sandstone with either basal conglomerate or conglomeratic sandstone also locally occur within the Bright Angel Shale. The gravel fraction of these beds contain quartz, minor amounts of potassium feldspars, sedimentary rock fragments, and glauconite. Glauconite also occurs in and imparts a greenish color to many of the siltstones and sandstones. Towards its base, the Bright Angel Shale becomes increasingly coarser with increasing frequency of sandstone beds until it grades gradually into the Tapeats Sandstone.
The siltstones and sandstones of the Bright Angel Shale exhibit sedimentary structures throughout the Grand Canyon. These sedimentary structures include horizontal laminations, small- to large-scale planar tabular and trough cross-stratification, and wavy and lenticular bedding. The isolated coarse-grained and conglomeratic sandstones are typically structureless and crudely stratified and overlie a locally scoured surface.