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Tonto Group
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Tonto Group
The Tonto Group is a name for an assemblage of related sedimentary strata, collectively known by geologists as a Group, that comprises the basal sequence Paleozoic strata exposed in the sides of the Grand Canyon. As currently defined, the Tonto groups consists of the Sixtymile Formation, Tapeats Sandstone, Bright Angel Shale (or Formation), Muav Limestone (or Formation), and Frenchman Mountain Dolostone. Historically, it included only the Tapeats Sandstone, Bright Angel Shale, and Muav Limestone. Because these units are defined by lithology and three of them interfinger and intergrade laterally, they lack the simple layer cake geology as they are typically portrayed as having and geological mapping of them is complicated.
In 1874 and 1875, G. K. Gilbert named the Tonto Group. Although it was mentioned that it outcropped in the Tonto Basin and the headwaters of Tonto Creek of south-central Arizona, he did not indicate which of these landforms, for which this group is named. Although the word tonto is translated as silly or foolish in Spanish, this place name for either Tonto Basin or Tonto Creek is derived from the Apache word, Koun’nde. This word means wild, rough people and refers to the indigenous Dilzhe’e Tonto Apache.
G. K. Gilbert also recognized that rocks similar to those found in the Tonto Basin are exposed near the bottom of the Grand Canyon and designated them also as the Tonto Group. From youngest to oldest, Gilbert subdivided the Tonto Group found in the Grand Canyon into the Marbled limestone (now subdivided into the Muav Limestone and Frenchman Dolostone); the Tonto shale (now the Bright Angel Shale); the Tonto sandstone (now the Tapeats Sandstone). However, he did not specify a specific type locality. Decades later, R. C. Rose proposed an exposure in Backtail Canyon for a type locality.
In 1914, L F. Noble officially redefined and renamed the formations comprising the Tonto Group. The Tonto sandstone was officially renamed the Tapeats Sandstone and the Tonto shale was officially renamed the Bright Angel Shale. The Marbled limestone was later officially renamed the Muav Limestone, which by Noble's definition consisted of an upper set of dolomite beds and a lower set of limestone beds.
Later in 1945, E. D. McKee and C. E. Resser subdivided the formations into a number of members in their recognition of the laterally gradation, time transgressive, and interfingering nature of the formations comprising the Tonto Group. In addition, they subdivided the Muav Limestone of Noble into undifferentiated dolomites, overlying an Muav Limestone composed entirely of the lower limestone strata. Furthermore, they removed the undifferentiated dolomites from the Tonto Group. Recently, the undifferentiated dolomites, now known as the Frenchman Mountain Dolostone, have been restore as a formation within the Tonto Group. In addition, along with the Frenchman Mountain Dolostone, the Sixtymile Formation is regarded to be part of the Tonto Group.
Throughout the majority of it extent, the Tonto Group lies unconformably on underlying Precambrian rocks forming the Great Unconformity. This contact is either an angular unconformity truncating tilted strata of the Grand Canyon Supergroup or a nonconformity cut into the crystalline Vishnu Basement Rocks. The base of the Tonto Group consists of discontinuous beds of the Sixtymile Formation at Nankoweap Butte and within Awatubi and Sixtymile Canyons in the eastern Grand Canyon.
As currently defined Tonto Group consists of the Sixtymile Formation, Tapeats Sandstone, Bright Angel Shale, Muav Limestone, and Frenchman Mountain Dolostone.
The Sixtymile Formation is a very thin accumulation of sandstone, siltstone, and breccia underlying the Tapeats Sandstone. It is exposed only atop Nankoweap Butte and within Awatubi and Sixtymile Canyons in the eastern Grand Canyon, Arizona. Sixtymile Formation is preserved only in a broad asymmetric fold, called the Chuar syncline. The maximum thickness of the Sixtymile Formation is about 60 m (200 ft). The actual depositional thickness of the Sixtymile Formation is unknown owing to erosion prior to deposition of the Tapeats Sandstone. The Sixtymile Formation unconformably overlies the Chur Group. No fossils have been reported from the Sixtymile Formation.
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Tonto Group
The Tonto Group is a name for an assemblage of related sedimentary strata, collectively known by geologists as a Group, that comprises the basal sequence Paleozoic strata exposed in the sides of the Grand Canyon. As currently defined, the Tonto groups consists of the Sixtymile Formation, Tapeats Sandstone, Bright Angel Shale (or Formation), Muav Limestone (or Formation), and Frenchman Mountain Dolostone. Historically, it included only the Tapeats Sandstone, Bright Angel Shale, and Muav Limestone. Because these units are defined by lithology and three of them interfinger and intergrade laterally, they lack the simple layer cake geology as they are typically portrayed as having and geological mapping of them is complicated.
In 1874 and 1875, G. K. Gilbert named the Tonto Group. Although it was mentioned that it outcropped in the Tonto Basin and the headwaters of Tonto Creek of south-central Arizona, he did not indicate which of these landforms, for which this group is named. Although the word tonto is translated as silly or foolish in Spanish, this place name for either Tonto Basin or Tonto Creek is derived from the Apache word, Koun’nde. This word means wild, rough people and refers to the indigenous Dilzhe’e Tonto Apache.
G. K. Gilbert also recognized that rocks similar to those found in the Tonto Basin are exposed near the bottom of the Grand Canyon and designated them also as the Tonto Group. From youngest to oldest, Gilbert subdivided the Tonto Group found in the Grand Canyon into the Marbled limestone (now subdivided into the Muav Limestone and Frenchman Dolostone); the Tonto shale (now the Bright Angel Shale); the Tonto sandstone (now the Tapeats Sandstone). However, he did not specify a specific type locality. Decades later, R. C. Rose proposed an exposure in Backtail Canyon for a type locality.
In 1914, L F. Noble officially redefined and renamed the formations comprising the Tonto Group. The Tonto sandstone was officially renamed the Tapeats Sandstone and the Tonto shale was officially renamed the Bright Angel Shale. The Marbled limestone was later officially renamed the Muav Limestone, which by Noble's definition consisted of an upper set of dolomite beds and a lower set of limestone beds.
Later in 1945, E. D. McKee and C. E. Resser subdivided the formations into a number of members in their recognition of the laterally gradation, time transgressive, and interfingering nature of the formations comprising the Tonto Group. In addition, they subdivided the Muav Limestone of Noble into undifferentiated dolomites, overlying an Muav Limestone composed entirely of the lower limestone strata. Furthermore, they removed the undifferentiated dolomites from the Tonto Group. Recently, the undifferentiated dolomites, now known as the Frenchman Mountain Dolostone, have been restore as a formation within the Tonto Group. In addition, along with the Frenchman Mountain Dolostone, the Sixtymile Formation is regarded to be part of the Tonto Group.
Throughout the majority of it extent, the Tonto Group lies unconformably on underlying Precambrian rocks forming the Great Unconformity. This contact is either an angular unconformity truncating tilted strata of the Grand Canyon Supergroup or a nonconformity cut into the crystalline Vishnu Basement Rocks. The base of the Tonto Group consists of discontinuous beds of the Sixtymile Formation at Nankoweap Butte and within Awatubi and Sixtymile Canyons in the eastern Grand Canyon.
As currently defined Tonto Group consists of the Sixtymile Formation, Tapeats Sandstone, Bright Angel Shale, Muav Limestone, and Frenchman Mountain Dolostone.
The Sixtymile Formation is a very thin accumulation of sandstone, siltstone, and breccia underlying the Tapeats Sandstone. It is exposed only atop Nankoweap Butte and within Awatubi and Sixtymile Canyons in the eastern Grand Canyon, Arizona. Sixtymile Formation is preserved only in a broad asymmetric fold, called the Chuar syncline. The maximum thickness of the Sixtymile Formation is about 60 m (200 ft). The actual depositional thickness of the Sixtymile Formation is unknown owing to erosion prior to deposition of the Tapeats Sandstone. The Sixtymile Formation unconformably overlies the Chur Group. No fossils have been reported from the Sixtymile Formation.
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