Historical:St. James Member

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Handbook of Illinois Stratigraphy
Series Bulletin 95
Author H. B. Willman, Elwood Atherton, T. C. Buschbach, Charles Collinson, John C. Frye, M. E. Hopkins, Jerry A. Lineback, Jack A. Simon
Date 1975
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Lithostratigraphy: Ottawa Limestone Megagroup >>Galena Group >>Kimmswick Subgroup >>Dunleith Formation >>St. James Member
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Champlainian Series >>Trentonian Stage
Allostratigraphy: Tippecanoe Sequence

Authors

H. B. Willman and T. C. Buschbach

Name Origin

The St. James Member of the Dunleith Formation (Templeton and Willman, 1963, p. 119), which overlies the Buckhorn Member, is named for St. James Cemetery in Stephenson County, 3 miles northwest of the type section.

Type Section

The type section of the St. James Member is located in the same exposure as the Buckhorn type section (a quarry at Buena Vista (NW SW NE 15, 28N-7E)), where the St. James is 13.3 feet thick.

Other Names

The St. James Member is called the "Gray" in the lead-zinc district.

Extent and Thickness

The St. James Member commonly is about 14 feet thick in the dolomite facies in northern Illinois and 8-12 feet thick in the southern limestone facies.

Description

In northern Illinois the St. James Member is light tan, pure, medium-bedded, vuggy dolomite, but thin green shale partings are common in the upper 2-4 feet. It has black speckles but not as many as the Buckhorn. In the southern area the St. James is a coarse-grained, gray to brown, faintly gray-speckled calcarenite that locally has a few inches of conglomerate at the base.

Fossils

Prasopora simulatrix (fig. O-5) is common in the upper few inches of the St. James Member in the extreme northwestern part of the state but has been found eastward only as far as Rockford, where it is rare.

References

TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation
7720
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