Nachusa Formation

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Lithostratigraphy: Ottawa Limestone Megagroup >>Platteville Group >>Plattin Subgroup >>Nachusa Formation
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Champlainian Series >>Blackriveran 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

Nachusa Formation (Templeton and Willman, 1963, p. 87).

Derivation

Named for the village of Nachusa, Lee County.

Other names

History/background

Type section

Type location

The type section is a quarry on the east edge of Dixon (SE SE SW 33, 22N-9E), where the Nachusa is 17.8 feet thick.

Type author(s)

Type status

Reference section

Reference location

Reference author(s)

Reference status

Stratigraphic relationships

The Nachusa Formation overlies the Grand Detour Formation.

Extent and thickness

The Nachusa is 15-20 feet thick in the northern part of the southern outcrop area, but south of Ste. Genevieve in the Cape Girardeau area it thickens rapidly to 75 feet. It thins to less than 5 feet in subsurface in central western and extreme northwestern Illinois. The Nachusa is absent in the Mississippi Valley northwest of Illinois, but it occurs in northern Michigan.

Lithology

The Nachusa consists largely of fine- to medium-grained, vuggy dolomite in northern Illinois (fig. O-2F) and dolomite-mottled lithographic limestone in the southern area (fig. O-2D). It is pure to slightly argillaceous, cherty, fucoidal, and thick bedded to massive. The weathered surface is deeply etched. In parts of northern Illinois the Nachusa resembles the dolomite of the Galena Group, and in some areas it has been miscorrelated with the Galena. In the southern outcrop area it contains layers of bentonite, calcarenite, and conglomeratic limestone. A persistent argillaceous and thin-bedded zone in the Nachusa is used to subdivide the formation into three members; the Eldena (at the base) is thick bedded and slightly argillaceous; the Elm is thin bedded and argillaceous; and the Everett (at the top) is massive and pure. The Nachusa has a sharp contact with the overlying Quimbys Mill Formation but a gradational contact with the Grand Detour below.

Core(s)

Photograph(s)

Contacts

Well log characteristics

Fossils

The Nachusa is moderately fossiliferous; corals, brachiopods, and gastropods are the most common fossils. In the northern area, the Nachusa is characterized by the abundance of Foerstephyllum halli, but a few specimens also are found in upper Grand Detour strata. Foerstephyllum is present but not common in the southern outcrop area.

Age and correlation

The Nachusa is correlated with the Chaumont Formation of New York and Ontario because of striking similarities in fauna and lithology, with the upper part of the lower member of the Tyrone Formation in Kentucky, and with the lower member of the Carters Limestone in Tennessee.

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
7910
Oqm