Historical:Mortimer 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 >>Mortimer Member
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Champlainian Series >>Trentonian Stage
Allostratigraphy: Tippecanoe Sequence


H. B. Willman and T. C. Buschbach

Name Origin

The Mortimer Member of the Dunleith Formation (Templeton and Willman, 1963, p. 121) overlies the Fairplay Member and is named for Mortimer Street in East Dubuque, Jo Daviess County, near the type section.

Type Section

The type section of the Mortimer Member is part of the Dunleith Formation type section (in an exposure in the Mississippi River bluffs on the north side of East Dubuque (SE 19, 29N-2W))(fig. O-2B), where the Mortimer Member is 11.3 feet thick.

Extent and Thickness

The Mortimer Member is commonly 10-13 feet thick.


The Mortimer Member consists of thick-bedded vuggy dolomite that contains bands of large white chert nodules. Its purity is between that of the pure Fairplay Member below and the argillaceous Rivoli above. It has a 2.2-foot argillaceous zone at the top in the type section, which thins to 5-6 inches in the Dixon and Rockford areas. It contains a few dense argillaceous beds. Thin lenses of bentonite occur along a prominent bedding plane at the top.


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