Lithostratigraphy: Hunton Limestone Megagroup >>Joliet Formation
Chronostratigraphy: Paleozoic Erathem >>Silurian System >>Niagaran Series
Allostratigraphy: Tippecanoe Sequence
H. B. Willman and Elwood Atherton
The Joliet Formation (Savage, 1926, p. 522) (figs. S-3, S-5) is named for Joliet, Will County.
The type section for the Joliet Formation is in the National Stone Company quarry on the south side of Joliet (NE SE 21, 35N-10E), where the formation is 68 feet thick.
Outside the area where the Joliet is differentiated, the position of its top is not well established, but in northwestern Illinois the Marcus Formation may in part be equivalent to the Joliet, and in southern Illinois the top of the Joliet correlates to a horizon within the St. Clair Limestone.
Extent and Thickness
The Joliet Formation is 70-80 feet thick in exposures along the Des Plaines River Valley, but only about 40 feet thick along the Kankakee River. It is 35-40 feet thick along the Mississippi River at Grafton in Jersey County, but farther west the top is truncated by Middle Devonian strata, and in southern Calhoun County the formation is entirely eroded.
The Joliet Formation is differentiated as the basal formation of the Niagaran Series in an area extending from Chicago to Calhoun County in western Illinois (fig. S-1).
In exposures in northeastern Illinois along the Des Plaines, Kankakee, Du Page, and Rock Rivers, the Joliet Formation is differentiated into a basal shaly, red, green, and gray member (the Brandon Bridge Member), overlain by nearly white, generally cherty dolomite that is silty at the base, grades to slightly silty at the top (the Markgraf Member), and is, in turn, overlain by nearly white, locally red-mottled, pure dolomite (the Romeo Member). In western Illinois, much of the Joliet is similar to the Romeo Member, although there is a weakly shaly zone at the base. The Romeo Member is prominent on electric logs and has been traced widely in subsurface.
The basal shaly zone of the Joliet Formation is characterized by an abundance of arenaceous Foraminifera, among which Ammodiscidae are prominent (Dunn, 1942). This zone is also present at the base of the Joliet in Jersey County (Collinson et al., 1954) and at the base of the St. Clair Limestone in southern Illinois. It has not been found in northwestern Illinois or in Wisconsin north of Milwaukee, but eastward it is well represented in the Osgood Formation of Indiana.
COLLINSON, CHARLES, D. H. SWANN, and H. B. WILLMAN, 1954, Guide to the structure and Paleozoic stratigraphy along the Lincoln Fold in western Illinois: Illinois State Geological Survey Guidebook Series 3, 75 p.
DUNN, P. H., 1942, Silurian Foraminifera of the Mississippi Basin: Journal of Paleontology, v. 16, p. 317-342.
SAVAGE, T. E., 1926, Silurian rocks of Illinois: Geological Society of America Bulletin, v. 37, p. 513-533.
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