Mount Simon Sandstone


Potsdam Supergroup,

Cambrian System

Type locality and use of name: In a provisional classification given by Walcott (1914, p. 354), the Mount Simon Sandstone was credited to a manuscript by E. O. Ulrich. General consensus indicates that the name is taken from an escarpment called Mount Simon near Eau Claire, Eau Claire County, Wis. Walcott described about 235 feet (72 m) of coarse-grained to conglomeratic sandstone overlying Precambrian granite and underlying fine-grained sandstone of the Eau Claire Formation. The term Mount Simon Sandstone is generally used for the basal Cambrian formation in the upper Mississippi Valley and southern Great Lakes areas, including in Indiana.

Description: The Mount Simon Sandstone, the oldest known post-Precambrian unit in Indiana and the basal unit in the Potsdam Supergroup, is recognized only in the subsurface and was described in some detail by Becker, Hreha, and Dawson (1978). It consists of poorly sorted fine-grained to very coarse grained sandstones that are generally poorly consolidated. A general color change occurs downward from white to yellowish gray to grayish red below. Gray and maroon shale is present throughout the formation in beds ranging from less than a foot to tens of feet in thickness, and another prominent shale zone, as thick as 60 feet (18 m), occurs in the upper part of the Mount Simon in northwestern Indiana. A basal arkosic sandstone interval, as thick as 50 feet (15 m), is known from several wells that penetrate Precambrian rocks in Indiana.

The known thickness of the Mount Simon Sandstone ranges from more than 300 feet (91 m) in eastern Indiana to more than 2,000 feet (610 m) in northwestern Indiana. Although wells in southwestern Indiana have not reached Precambrian rocks, wells in Illinois and Kentucky indicate that the Mount Simon thins to about 750 feet (229 m) in that part of the state (Droste and Patton, 1985). The Mount Simon is overlain conformably by the Eau Claire Formation and is underlain, insofar as known, unconformably by Precambrian rocks.

Correlation: The Mount Simon Sandstone is known by this name in Illinois, Wisconsin, Michigan, Ohio, and Kentucky and is the equivalent of the Lamotte Sandstone of Missouri (Droste and Shaver, 1983 Shaver and others, 1985).

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