Ordovician System

Type locality and use of name: The Shakopee Dolomite was named (Winchell, 1874, p. 138-139) from outcrops at Shakopee, Scott County, Minn., where Shakopee rocks lie on sandstone that was later correlated with the New Richmond Sandstone. From this original understanding, use of the term Shakopee was extended to Illinois, where this unit overlies the New Richmond Sandstone or the Oneota Dolomite and is overlain by the St. Peter Sandstone generally or by the Everton Dolomite in southern Illinois (Willman and Templeton, 1951, and earlier authors). The Illinois concept was then applied to Indiana but with some modification of its defined stratigraphic relations (Droste and Patton, 1985).

Description: The Shakopee Dolomite of Indiana is a pure to impure and generally very fine grained to fine-grained dolomite containing some chert and interbeds of shale, siltstone, and sandstone. In southern Indiana, where younger beds of the Shakopee are preserved, fine- to medium-grained dolomite increases in abundance upward. The color ranges from light shades of gray to light to medium shades of brown. Chert in the Shakopee is vitreous, opaque, and tripolitic; is uniformly colored, color banded, or colored with oolitic texturing; and ranges in abundance from sporadic nodules and thin irregular beds in dolomite through zones several feet thick.

Sandstone beds generally as much as several tens of feet thick are interbedded in dolomite through a total thickness of about 400 feet (122 m) in southeastern and south-central Indiana. Most of the interbedded dolomite is not the typical very fine grained to fine-grained rock but instead is medium grained and very light colored.

The Shakopee overlies the Oneota with gradational contact and is overlain unconformably by rocks of the Ancell Group generally except in southwestern Indiana where the Everton lies superjacent to the unconformity. The Shakopee is present everywhere in the Indiana subsurface except for an area in northern Indiana where pre-St. Peter erosion completely stripped the formation. Its only Indiana exposure is among the chaotically faulted rocks that are quarried at Kentland in Newton County (Gutschick, 1983).

The Shakopee increases in thickness from its eroded limit in northern Indiana to perhaps 2,000 feet (610 m) in southwestern Indiana. A significant amount of the thickening of the Shakopee results from the addition of younger beds southward, but a regional thickness increase southward indicates a major center of deposition in that direction.

Correlation: The Shakopee Dolomite of Indiana is equivalent to the upper part of the undifferentiated Knox Dolomite of Ohio; the upper part of the undifferentiated Prairie du Chien Group of Michigan; the Roubidoux Formation, the Jefferson City Dolomite, and the Cotter Dolomite of Kentucky; and the New Richmond Sandstone and the Shakopee Dolomite of Illinois (Droste and Shaver, 1983; Shaver and others, 1985).