Historical:Proviso Siltstone Member
T. C. Buschbach
The Proviso Siltstone Member of the Eau Claire Formation (Buschbach, 1964, p. 32), the uppermost member, is named for Proviso Township, Cook County.
The type section of the Proviso Siltstone Member is located 2 miles west of Proviso Township, Cook County, and is at a depth of 1385-1535 feet (sample set 15,336) in the same boring as the Elmhurst Member.
Extent and Thickness
The Proviso is widely present in the northern part of Illinois. It is 150-300 feet thick, except near the northern boundary of the state where it grades to sandstone and is not differentiated.
The Proviso Siltstone Memeber is dominantly dolomitic, sandy, firm, orange to pinkish gray, feldspathic, slightly glauconitic siltstone that commonly contains beds of greenish gray, pink, or red shale. Along its northern limit it is sandstone and shale, but southward it becomes more dolomitic. The upper few to 40 feet consists of fine-grained dolomitic sandstone or sandy dolomite that is transitional to the overlying Galesville Sandstone.
BUSCHBACH, T. C., 1964, Cambrian and Ordovician strata of northeastern Illinois: Illinois State Geological Survey Report of Investigations 218, 90 p.
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