Proviso Siltstone Member

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Lithostratigraphy: Knox Dolomite Megagroup >>Eau Claire Formation >>Proviso Siltstone Member
Chronostratigraphy: Paleozoic Erathem >>Cambrian System >>Croixan Series >>Dresbachian Stage
Allostratigraphy: Sauk Sequence

Primary source

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.

Contributing author(s)

T. C. Buschbach


Original description

The Proviso Siltstone Member of the Eau Claire Formation (Buschbach, 1964, p. 32).


Named for Proviso Township, Cook County.

Other names


Type section

Type location

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.

Type author(s)

Type status

Reference section

Reference location

Reference author(s)

Reference status

Stratigraphic relationships

The Proviso Siltstone Member is the uppermost member of the Eau Claire Formation.

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.




Well log characteristics


Age and correlation

Environments of deposition

Economic importance



BUSCHBACH, T. C., 1964, Cambrian and Ordovician strata of northeastern Illinois: Illinois State Geological Survey Report of Investigations 218, 90 p.

ISGS Codes

Stratigraphic Code Geo Unit Designation