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Redwall Limestone
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Redwall Limestone
The Redwall Limestone is an erosion-resistant, Mississippian age, cliff-forming geological formation that forms prominent, red-stained cliffs in the Grand Canyon. these cliffs range in height from 150 m (490 ft) to 244 m (801 ft). It is one of the most fossiliferous formations exposed in the Grand Canyon region.
In 1875, Gilbert recognized and named the Redwall Limestone for the red coloration of its escarpment on either side of Grand Canyon. As originally defined by him, it included some strata younger and older than as it is currently defined. Later in 1910, Darton selected a canyon that he named the Redwall Canyon in the Shinumo drainage basin, on north side of the Grand Canyon, as the type section of the Redwall Limestone. At this location, it consists mostly of the usual heavily bedded massive limestone and is circa 244 m (801 ft) thick. Noble subsequently redefined the Redwall Limestone in its present definition, which includes all strata of Mississippian age. As a result of studies in Yavapai County, Arizona, Gutschick recognized four informal members within the Redwall Limestone and McKee later formally named them. The most comprehensive study of the Redwall Limestone is the History of the Redwall Limestone of northern Arizona by McKee and Gutschick.
Redwall Limestone consists predominantly of light-olive-gray to light-gray, fine- to coarse-grained, thin- to thick-bedded, often cherty, limestone. Its lower part consists of brownish-gray, interbedded with finely crystalline dolomite and fine- to coarse-grained limestone with layers of white chert lenses and yellowish-gray and brownish-gray, cliff-forming, thick-bedded, fine-grained dolomite.
In ascending order, the Redwall Limestone is divided into the Whitmore Wash, Thunder Springs, Mooney Falls, and Horseshoe Mesa members. All four member have their type locality in the Grand Canyon or its tributaries. They are recognized throughout the Grand Canyon area, northern Arizona, and southern Utah. They are all Mississippian in age.
The Whitmore Wash Member is the basal, oldest, member of the Redwall Limestone. It typically forms a high, resistant cliff standing on a narrow bench or series of ledges typical of underlying strata. This member consists of nearly pure limestone and dolomite, which contains less than 2 percent insoluble gypsum and iron oxides. This member is predominately composed of thick-bedded, ranging from 0.6 to 1.22 m (2.0 to 4.0 ft) and locally thicker, limestone in western Grand Canyon and changes to mostly very thick-bedded 1.2 to 4.5 m (3.9 to 14.8 ft), fine-grained dolomite in central and eastern Grand Canyon. The limestones are composed mostly of pelletal, skeletal or oolitic wackestones and packstones. In a few places, this member exhibits conspicuous medium-scale crossbedding. In the Grand Canyon, the Whitmore Wash Member varies in thickness from about 9 m (30 ft) in the eastern Grand Canyon to nearly 30 m (98 ft) at Iceberg Ridge, 8 km (5.0 mi) beyond the western end of Grand Canyon. The overlying Thunder Springs Member lies conformably on Whitmore Wash Member and its base is easily recognized by the lowest appearance of thin, dark, chert beds alternating with thin beds of lighter gray limestone or dolomite.
The Thunder Springs Member is the most distinctive member of the Redwall Limestone, because it forms cliffs that exhibit prominent black and light-brown banding. Its light and dark banded appearance is imparted by thin beds of either light gray limestone or light gray dolomite alternating with thin beds of dark reddish brown or dark gray beds or lenses of chert. Most of the limestone is fine to very coarse, thin-bedded, crinoidal grainstone or packstone. The carbonate beds vary from being predominately limestone in the western Grand Canyon and to predominately dolomite in the eastern Grand Canyon. The thin chert beds in this member consist of silicified bryozoan wackestones and lime mudstones. The Thunder Springs Member gradually increases in thickness from 30 m (98 ft) in eastern Grand Canyon to about 46 m (151 ft) in the western Grand Canyon. The contact of the overlying Mooney Falls Member with the underlying Thunder Springs Member is disconformable except in the extreme western end of Grand Canyon. Locally, this contact is a low-angle unconformity. This is indicative of a period of emergence, minor tectonic activity, and erosion between the deposition of the Thunder Springs and Mooney Falls members.
The Mooney Falls Member forms a major part of the high, sheer escarpment for which the Redwall Limestone is named. It consists predominantly of pure limestone, except locally where it is dolomitized. It contains less than 0.5 percent insoluble residue. These limestones consist of oolites, pellets, and a variety of skeletal fragments dominated by crinoid plates. In its upper part near its contact with the overlying Horseshoe Mesa Member, one or two zones containing thin beds or lenses of chert occur. The Mooney Falls Member is normally thick-bedded and looks massive in outcrop. The upper third of this member at several localities in central and eastern Grand Canyon reportedly exhibits large-scale, tabular-planar cross-bedding. The Mooney Falls Member is the thickest member of the Redwall, ranging from about 61 m (200 ft) in eastern Grand Canyon to nearly 120 m (390 ft) at the western end. The overlying Horseshoe Mesa Member lies conformably on the Mooney Falls Member and its contact can be difficult to define in outcrops. Typically, the boundary is located at the change from vertical, cliff-forming, medium- and coarse-grained, and thick- or massive-bedded limestone to a receding-ledge-forming, fine-grained, and relatively thin-bedded, limestone.
The Horseshoe Mesa Member is the youngest, thinnest, and of the least areal extensive member of the Redwall Limestone. This member is normally composed of thin-bedded, light gray, fine-grained, limestone, typically a mudstone to wackestone that commonly contains encrusting and sediment-binding algal structures. It contains some chert lenses in its lower part. Also, crossbedding, ripple marks, and oolite beds occur locally. The Horseshoe Mesa Member typically forms weak receding ledges in contrast to the massive cliff that characterizes the Mooney Falls below. Within the Grand Canyon it varies from 14 to 38 m (46 to 125 ft). It normally thinnest in the eastern Grand Canyon. Because of erosion, this member wedges out 48 to 64 km (30 to 40 mi) south of the Grand Canyon. The Horseshoe Mesa Member is also absent from the Redwall Limestone in most of central Arizona.
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Redwall Limestone
The Redwall Limestone is an erosion-resistant, Mississippian age, cliff-forming geological formation that forms prominent, red-stained cliffs in the Grand Canyon. these cliffs range in height from 150 m (490 ft) to 244 m (801 ft). It is one of the most fossiliferous formations exposed in the Grand Canyon region.
In 1875, Gilbert recognized and named the Redwall Limestone for the red coloration of its escarpment on either side of Grand Canyon. As originally defined by him, it included some strata younger and older than as it is currently defined. Later in 1910, Darton selected a canyon that he named the Redwall Canyon in the Shinumo drainage basin, on north side of the Grand Canyon, as the type section of the Redwall Limestone. At this location, it consists mostly of the usual heavily bedded massive limestone and is circa 244 m (801 ft) thick. Noble subsequently redefined the Redwall Limestone in its present definition, which includes all strata of Mississippian age. As a result of studies in Yavapai County, Arizona, Gutschick recognized four informal members within the Redwall Limestone and McKee later formally named them. The most comprehensive study of the Redwall Limestone is the History of the Redwall Limestone of northern Arizona by McKee and Gutschick.
Redwall Limestone consists predominantly of light-olive-gray to light-gray, fine- to coarse-grained, thin- to thick-bedded, often cherty, limestone. Its lower part consists of brownish-gray, interbedded with finely crystalline dolomite and fine- to coarse-grained limestone with layers of white chert lenses and yellowish-gray and brownish-gray, cliff-forming, thick-bedded, fine-grained dolomite.
In ascending order, the Redwall Limestone is divided into the Whitmore Wash, Thunder Springs, Mooney Falls, and Horseshoe Mesa members. All four member have their type locality in the Grand Canyon or its tributaries. They are recognized throughout the Grand Canyon area, northern Arizona, and southern Utah. They are all Mississippian in age.
The Whitmore Wash Member is the basal, oldest, member of the Redwall Limestone. It typically forms a high, resistant cliff standing on a narrow bench or series of ledges typical of underlying strata. This member consists of nearly pure limestone and dolomite, which contains less than 2 percent insoluble gypsum and iron oxides. This member is predominately composed of thick-bedded, ranging from 0.6 to 1.22 m (2.0 to 4.0 ft) and locally thicker, limestone in western Grand Canyon and changes to mostly very thick-bedded 1.2 to 4.5 m (3.9 to 14.8 ft), fine-grained dolomite in central and eastern Grand Canyon. The limestones are composed mostly of pelletal, skeletal or oolitic wackestones and packstones. In a few places, this member exhibits conspicuous medium-scale crossbedding. In the Grand Canyon, the Whitmore Wash Member varies in thickness from about 9 m (30 ft) in the eastern Grand Canyon to nearly 30 m (98 ft) at Iceberg Ridge, 8 km (5.0 mi) beyond the western end of Grand Canyon. The overlying Thunder Springs Member lies conformably on Whitmore Wash Member and its base is easily recognized by the lowest appearance of thin, dark, chert beds alternating with thin beds of lighter gray limestone or dolomite.
The Thunder Springs Member is the most distinctive member of the Redwall Limestone, because it forms cliffs that exhibit prominent black and light-brown banding. Its light and dark banded appearance is imparted by thin beds of either light gray limestone or light gray dolomite alternating with thin beds of dark reddish brown or dark gray beds or lenses of chert. Most of the limestone is fine to very coarse, thin-bedded, crinoidal grainstone or packstone. The carbonate beds vary from being predominately limestone in the western Grand Canyon and to predominately dolomite in the eastern Grand Canyon. The thin chert beds in this member consist of silicified bryozoan wackestones and lime mudstones. The Thunder Springs Member gradually increases in thickness from 30 m (98 ft) in eastern Grand Canyon to about 46 m (151 ft) in the western Grand Canyon. The contact of the overlying Mooney Falls Member with the underlying Thunder Springs Member is disconformable except in the extreme western end of Grand Canyon. Locally, this contact is a low-angle unconformity. This is indicative of a period of emergence, minor tectonic activity, and erosion between the deposition of the Thunder Springs and Mooney Falls members.
The Mooney Falls Member forms a major part of the high, sheer escarpment for which the Redwall Limestone is named. It consists predominantly of pure limestone, except locally where it is dolomitized. It contains less than 0.5 percent insoluble residue. These limestones consist of oolites, pellets, and a variety of skeletal fragments dominated by crinoid plates. In its upper part near its contact with the overlying Horseshoe Mesa Member, one or two zones containing thin beds or lenses of chert occur. The Mooney Falls Member is normally thick-bedded and looks massive in outcrop. The upper third of this member at several localities in central and eastern Grand Canyon reportedly exhibits large-scale, tabular-planar cross-bedding. The Mooney Falls Member is the thickest member of the Redwall, ranging from about 61 m (200 ft) in eastern Grand Canyon to nearly 120 m (390 ft) at the western end. The overlying Horseshoe Mesa Member lies conformably on the Mooney Falls Member and its contact can be difficult to define in outcrops. Typically, the boundary is located at the change from vertical, cliff-forming, medium- and coarse-grained, and thick- or massive-bedded limestone to a receding-ledge-forming, fine-grained, and relatively thin-bedded, limestone.
The Horseshoe Mesa Member is the youngest, thinnest, and of the least areal extensive member of the Redwall Limestone. This member is normally composed of thin-bedded, light gray, fine-grained, limestone, typically a mudstone to wackestone that commonly contains encrusting and sediment-binding algal structures. It contains some chert lenses in its lower part. Also, crossbedding, ripple marks, and oolite beds occur locally. The Horseshoe Mesa Member typically forms weak receding ledges in contrast to the massive cliff that characterizes the Mooney Falls below. Within the Grand Canyon it varies from 14 to 38 m (46 to 125 ft). It normally thinnest in the eastern Grand Canyon. Because of erosion, this member wedges out 48 to 64 km (30 to 40 mi) south of the Grand Canyon. The Horseshoe Mesa Member is also absent from the Redwall Limestone in most of central Arizona.
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