Elgin Shale Member
Willman, H. B., Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, and Jack A. Simon, 1975, Handbook of Illinois Stratigraphy: Illinois State Geological Survey Bulletin 95, 261 p.
H. B. Willman and T. C. Buschbach
The Elgin Shale Member of the Scales Shale (Calvin, 1906, p. 60, 98).
Named for Elgin, Fayette County, Iowa.
Extent and thickness
The Elgin Member of the Scales Formation forms the major part of the formation and is partly exposed in many localities in northwestern Illinois and locally in the other outcrop areas of the Maquoketa Group. Although dominantly shale, and in some areas almost entirely shale, the member contains beds of dolomite, limestone, siltstone, and sandstone. The lower two-thirds of the shale is commonly dark gray or dark brown, and locally in eastern Illinois it is nearly black. The shale generally becomes lighter in color to the northwest, and in the Scales type section only a few beds are dark.
Well log characteristics
The Lower Depauperate Zone consists of one or several thin depauperate beds at the base or in the lower few feet of the Elgin Shale. At Valmeyer, Monroe County, depauperate beds occur at the base of the shale and as much as 9 feet above the base. A similar depauperate bed, the Upper Depauperate Zone, occurs in the upper part of the Elgin Shale Member. It was formerly exposed in the Goose Lake clay pit in Grundy County but is now covered. At Goose Lake it is 60 feet above the base of the Scales Shale, is 20 feet below the top, and is overlain by shale and argillaceous limestone containing Isotelus, which forms the top of the member. The Upper Depauperate Zone has been encountered in borings only in central and northeastern Illinois.
Age and correlation
Environments of deposition
CALVIN, SAMUEL, 1906, Geology of Winneshiek County: Iowa Geological Survey, v. 16, p. 37-146.
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