The McCloud Limestone will be extensively featured during our mid-week geology trip for cavers. Led by Joel Despain and Pat Kambesis, this caving trip proves to offer an exciting option for those wishing a more in-depth geology experience. Click here for more details.
The McCloud Limestone
The McCloud Limestone is a cavernous formation of Lower Permian age located in the region near Shasta Lake in Northern California. The formation contains the commercial cave, Lake Shasta Caverns. The following is a brief review of the outcrop pattern, thickness, age, depositional history, and lithology of the McCloud Limestone.
The McCloud Limestone outcrops in a band about 34 miles long extending from south of the Pit River arm of Shasta Lake, along the eastern shore of the McCloud River arm of Shasta Lake, and to the north along the McCloud River and Squaw Valley Creek. Often, the limestone outcrops in prominent cliffs along the lake and rivers, and the terrain is very rugged, covered largely with trees and brush. The outcrop band is usually less than 1/5 mile wide and the formation is separated into discrete blocks bounded by transverse faults along its entire length (Wilson, 1982). Near both ends of the outcrop band, the frequency of faulting within the limestone becomes more prominent. Bedding within the McCloud Limestone dips to the east at 45 to 60 degrees, and is disconformably overlain south of Shasta Lake by Cretaceous and younger rocks, and disconformably overlain to the north by the volcanic flows off the southern flanks of Mount Shasta.
Caving in the McCloud Limestone. Photo: Bill Frantz, NSS.
The McCloud Limestone has a total thickness of about 5,500 feet, but, due to extensive faulting, the maximum observed thickness at anyone place is 2,400 feet (Skinner and Wilde, 1965). The formation is overlain to the east by the Nosone Formation and the Dekkas Andesite, both of which are Permian in age, and is underlain by the Baird Formation to the west, which is Mississippian to Permian in age (Wilson, 1982). The nature of the contacts between the McCloud Limestone and the overlying and underlying formations is very complicated. In some areas they are gradational and in others they are defined by faults. In general, most of the contact between the McCloud Limestone and the overlying Nosone Formation is defined by the extensive McCloud Fault. The fault is steeply westward dipping and has left lateral and reverse movement with an unknown amount of displacement (Albers and Robertson, 1961). In most places a quartz diorite of Permian age has intruded along this fault (Wilson, 1982). The contact between the McCloud Limestone and the underlying Baird Formation is generally gradational, although in some places it is defined by a Permian intrusive, the Pit Stock.
The McCloud Limestone contains abundant fossils of invertebrate faunas. These fossils have been used to correlate sections of the formation truncated by faults, and have been used to help define the age of the formation. The fossils which have been identified are: fusulinids, brachiopods, bryozoans, mollusks, and a large variety of corals (Skinner and Wilde, 1965; Watkins, 1974; Wilson, 1982). Based on the fossil assemblage, the McCloud Limestone has been determined to be entirely Lower Permian in age (Wilson, 1982).
The Cathedral Room - Lake Shasta Caverns. Photo: Dave Bunnell, NSS.
The McCloud Limestone is thought to have been deposited on or near the margin of a shelf which bordered a basin lying to the west or northwest of the present-day outcrop (Skinner and Wilde, 1965). The shelf is interpreted to have been relatively shallow (less than 200 feet deep) based on the presence of organisms that lived in shallow water. Because there are no biohermal or biostromal structures present that are typical of reef deposits, and because there appears to have been vigorous wave agitation due to the clastic and fragmented character of the rock, the McCloud Limestone should not be considered a reef deposit (Demirmen and Harbaugh, 1965). The location of the basin toward the west or northwest is based on facies identification work done by Skinner and Wilde (1965) which shows a grading of the facies westward and northwestward from shallow water facies of thick-bedded limestones, to intermediate facies of interbedded dark, thin-bedded limestones and siltstones, to basinal facies of siltstones and silty shales.
The McCloud Limestone is a light bluish gray to gray, fine grained limestone. It is generally composed of 95% limestone and 5% chert (Albers and Robertson, 1961). Less than 1% of the limestone is dolomitic. The individual beds range from a few inches to up to 10 feet in thickness and contain layers of fossils ranging from 1 inch to as much as several feet in thickness. Demirmen and Harbaugh (1965) have divided the lithology of the McCloud Limestone into three major divisions: (1) the calcarenite division, (2) the pisolite division, and (3) the conglomerate division. They based these divisions upon the principal lithologic types occurring within the formation. The calcarenite division is the largest, extending throughout the length of the outcrop belt with a thickness ranging from less than 100 feet to approximately 2,000 feet. A calcarenitic limestone is one that is dominantly composed of detrital calcite particles of sand size. The pisolite division is isolated in the southern part of the outcrop belt; it ranges in thickness from 0 to 1,300 feet. This division is distinguished by the localized dominance of pisolites. Pisolites are small, spherical grains of calcium carbonate (2 to 10 millimeters in diameter) thought to be produced by a biochemical algalencrustation process. The conglomerate division is restricted to the northern part of the outcrop belt and ranges from 0 to approximately 1,300 feet in thickness. This division is characterized by cobbles and boulders of calcarenite contained in a matrix of calcarenite. These cobbles and boulders are thought to be older McCloud Limestone deposits that have been eroded, transported, and redeposited to form younger McCloud beds.
- Albers, J. P., and Robertson, J. F. (1961) Geology and ore deposits of east Shasta copper-zinc district, Shasta County, California, United States Geological Survey, Professional Paper 338: pp. 1-107.
- Dimermen, J., and Harbaugh, J. W. (1965) Petrography and origin of Permian McCloud Limestone of northern California, Journal of Sedimentary Petrology, volume 35, number 1, pp. 136- 154.
- Skinner, J. W., and Wilde, G. L. (1965) Permian biostratigraphy and fusulinid faunas of the Shasta Lake area, northern California, University of Kansas, Paleontological Contributions, Protozoa, volume 6, pp. 1-98.
- Watkins, R. (1973) Carboniferous faunal associations and stratigraphy, Shasta County, northern California, American Association of Petroleum Geologists, Bulletin, volume 57, number 9, pp. 1743- 1764.
- Wilson, Edward C. (1982) Wolfcampian rugose and tabulate corals (Coelenterata: Anthozoa) from the lower Permian McCloud Limestone of northern California, Contributions in Science, number 337, pp. 1-90.
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