Mortimer Member

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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

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)

H. B. Willman and T. C. Buschbach

Name

Original description

The Mortimer Member of the Dunleith Formation (Templeton and Willman, 1963, p. 121).

Derivation

Named for Mortimer Street in East Dubuque, Jo Daviess County, near the type section.

Other names

History/background

Type section

Type location

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.

Type author(s)

Type status

Reference section

Reference location

Reference author(s)

Reference status

Stratigraphic relationships

The Mortimer Member of the Dunleith Formation overlies the Fairplay Member.

Extent and thickness

The Mortimer Member is commonly 10-13 feet thick.

Lithology

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.

Core(s)

Photograph(s)

Contacts

Well log characteristics

Fossils

Age and correlation

Environments of deposition

Economic importance

Remarks

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
7680
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