A research institute of the OVPR
Scales Shale


Maquoketa Group,

Ordovician System

Type locality and use of name in Illinois: The Scales Shale was initially named the Scales Formation by Templeton and Willman (1963, p. 135) for the village of Scales Mound, Jo Daviess County, Ill., near which the formation is exposed in railroad cuts. The lower and greater part of the formation is dark-brown shale; at the top is a gray shale that contains thin beds of argillaceous limestone. The formation most commonly is 75 to 100 feet (23 to 30 m) thick and is widely recognized in Illinois (Buschbach, 1964; Kolata and Graese, 1983).

Use of name and description in Indiana: A three-part division of the Maquoketa Group (then a formation) in northwestern Indiana was recognized by Gutstadt (1958a, p. 76), but he did not name the divisions, and it remained for Gray (1972b, p. 20-21) to extend the name Scales Shale to the lowest of these. The formation is identifiable in most of northern, central, and southwestern Indiana (John B. Droste, oral communication, 1983). Throughout its extent it is underlain by the Trenton Limestone (Ordovician) and overlain by the Fort Atkinson Limestone (Ordovician). The upper contact is generally considered to be conformable, but the lower contact is thought by some authors to be disconformable (Rooney 1966; Willman and others, 1975, p. 84-85), whereas others (for example, Gray, 1972b, p. 6-7 and 14-15) question this interpretation and consider this contact to be essentially conformable. On the basis of a recent regional study of the Trenton Limestone, Brian D. Keith (oral communication, 1984) has suggested that the contact is a regionally time-transgressive discontinuity, but not a disconformity. Age relationships discussed below strongly support the latter view.

The Scales Shale is commonly about 100 feet (30 m) thick in northwestern Indiana, but it thickens strikingly eastward and southeastward. In much of eastern Indiana, where it makes up by far the largest part of the Maquoketa Group, it is 500 to 700 feet (150 to 200 m) thick. The upper part is gray shale containing thin beds of limestone that become more abundant southeastward. This part grades downward into the lower part, which is dominantly dark brown shale and is 100 to 300 feet (30 to 100 m) thick.

Correlation: The Scales Shale of the northwestern Illinois outcrop area was long considered to be Richmondian in age (Twenhofel and others, 1954) but later was thought to be "equivalent to all the Cincinnatian strata older than the Richmondian" (Templeton and Willman, 1963, p. 136). More recent opinion (Shaver and others, 1985) assigns the Scales of Illinois to the Maysvillian Stage and implies that the Scales-Galena (in Indiana, Scales-Trenton) contact descends in time eastward, so that much of the Scales in Indiana belongs to the Edenian Stage and is contemporaneous with higher parts of the Galena Group in Illinois.

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