H. B. Willman and T. C. Buschbach
The Arsenal Member of the Oneota Formation is named for the Joliet Arsenal, Will County.
The type section for the Arsenal Member is located near Joliet Arsenal, where the type section is represented in a boring (NE NE SW 25, 34N-9E) by samples (sample set 6199) from 980-1085 feet deep.
Extent and Thickness
The Arsenal Member is recognized in the northeastern quarter of the state, where it varies from 90 to almost 200 feet thick.
The Arsenal Member of the Oneota Dolomite (Buschbach, 1964, p. 45) is the lower, very cherty member. It is a cherty to very cherty dolomite that is light gray with some brown or pink tinting. The dolomite is medium grained but has some coarsely crystalline zones. The chert is white or light gray, partly banded, and only rarely oolitic. Sand grains are rare, but thin shale partings are common near the base.
BUSCHBACH, T. C., 1964, Cambrian and Ordovician strata of northeastern Illinois: Illinois State Geological Survey Report of Investigations 218, 90 p.
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