The Kaibab Limestone is a resistant cliff-forming, Permian geologic formation that crops out across the U.S. states of northern Arizona, southern Utah, east central Nevada and southeast California. It is also known as the Kaibab Formation in Arizona, Nevada, and Utah. The Kaibab Limestone forms the rim of the Grand Canyon. In the Big Maria Mountains, California, the Kaibab Limestone is highly metamorphosed and known as the Kaibab Marble.
Nomenclature
The Kaibab Limestone was named by Darton in 1910 for the Kaibab Plateau, which is on the north side of Grand Canyon in Coconino County, Arizona. In his definition of the Kaibab Limestone formation, no type locality was designated. He also designated the Kaibab Limestone as the upper formation of the Aubrey Group, a now-abandoned stratigraphic unit. In 1921, Bassler and Reeside revised Darton's work and defined the Harrisburg Member of the Kaibab Limestone. In his 1938 monograph on the Toroweap Formation and Kaibab Limestone of northern Arizona, McKee split Darton's original Kaibab Limestone into the currently recognized Kaibab Limestone and Toroweap Formation. He also revised Kaibab Limestone's lower contact and divided it into informal (descending) alpha, beta and gamma members. Later in the 1970s, its upper contact was revised and its areal extent were defined. Also, unsuccessful attempts were made to raise the formation to group rank and divide it into several formations. In 1982, Warren Hamilton renamed it as the Kaibab Marble and determined its areal extent for California. In 1991, Sorauf and Billingsley subdivided the Kaibab Limestone into (ascending) Fossil Mountain Member (new) and Harrisburg Member. They designated the strata comprising McKee's alpha (or upper) member as the Harrisburg Member and the strata comprising McKee's beta (or middle) member as the Fossil Mountain Member. The Fossil Mountain Member was named for Fossil Mountain along the south rim near the Bass Trail. McKee's gamma member is considered to be part of the Fossil Mountain Member. Later research has further redefined the regional extent of the Kaibab Limestone.
Description
The Kaibab Limestone is an assemblage of sedimentary rock types. It consists of a complexity of inter fingering and inter bedded carbonate and siliciclastic sedimentary rocks. In addition, intense post-depositional (diagenetic) changes have produced a more composition variation by the alteration of limestone to dolomite and the silicification of limestone to form chert. In the western Grand Canyon region, the Fossil Mountain Member consists of fossiliferous and cherty limestone with an abundant and diverse normal-marine fossil fauna. Further east in the Coconino Plateau region, the lithology, mineralogy, and fauna of the Fossil Mountain Member changes drastically where it grades laterally into sandy dolomite and dolomite that contains a restricted-marine fossil fauna and subordinate amounts of sandstone. The Harrisburg Member, which forms the uppermost cliffs and receding ledges along both north and south rims of the Grand Canyon, consists of an assemblage of gypsum, dolomite, sandstone, redbeds, chert, and minor limestone.
Contacts
Within the Grand Canyon region, the Kaibab Limestone overlies gypsum and contorted sandstones of the Toroweap Formation. Originally, geologists interpreted the lower contact of the Kaibab Limestone to be an unconformity based on the presence of local intraformational breccias and erosional surfaces. However, additional research has concluded that these local intraformational breccias and erosional surfaces are the result of collapse following the dissolution of evaporite deposits within the upper part of the Toroweap Formation. As a result, this contact is inferred to be conformable or only locally a disconformity. South and east of the Grand Canyon, the evaporites and contorted sandstones (sabkha deposits) of Toroweap Formation interfinger with and are replaced by cross-bedded sandstones of the Coconino Sandstone. As a result, the Kaibab Limestone directly overlies the Coconino Sandstone in the Mogollon Rim region. The Kaibab Limestone directly overlies the White Rim Sandstone in northeastern Arizona and southeastern Utah.