Ordovician System

Type locality and use of name: The Oneota Dolomite was named by McGee (1891, p 331-333) for exposures along the Oneota River (now called the Upper Iowa River), Alamakee County, Iowa, where 200 to 300 feet (61 to 91 m) of dolomite overlies the Jordan Sandstone and underlies the New Richmond Sandstone. In 1951 use of the name was extended to Illinois where the Oneota Dolomite overlies either the Gunter Sandstone or the Eminence Formation and is overlain by the New Richmond Sandstone (Willman and Templeton, 1951). The Oneota of Illinois was traced through the subsurface to Indiana by Droste and Patton (1985) but with modified stratigraphic relations. The Eminence Formation, the Gunter Sandstone, and the New Richmond Sandstone are not recognized in Indiana, so that the Oneota Dolomite makes up the basal unit of the two-unit Prairie du Chien Group.

Description: The Oneota conformably overlies the Potosi Dolomite and conformably underlies the Shakopee Dolomite except in northwesternmost Indiana where in the area of one well pre-Ancell erosion stripped completely the Shakopee and Oneota rocks, so that the Ancell rocks (St. Peter Sandstone) lie directly on the Potosi Dolomite. It is assumed, nevertheless, that in parts of this area yet untested by deep wells the Oneota unconformably underlies Ancell rocks.

The Oneota consists predominantly of fine-to medium-grained dolomite but includes chert and, particularly near its base in some places, sporadic quartz sand and thin interbeds of green shale. The color is dominated by light shades of gray and brown, some medium-grayish-brown colors are present, and very light gray to white medium-grained dolomite several tens of feet thick generally marks the basal part of the formation.

The chert in the Oneota is light colored and is variably vitreous, opaque, and tripolitic. Its color ranges from uniform to color-banded and oolitic-textured patterns. The chert is highly variable in abundance, ranging from small amounts of tripolitic quartz between well-formed dolomite rhombs to nodules and thin irregular beds in dolomite through zones several feet thick.

The thickness of the Oneota Dolomite increases from its eroded limit in northwestern Indiana to more than 500 feet (152 m) in southwestern Indiana.

Correlation: The age of the Oneota has traditionally been considered to be Canadian, and the base of the Oneota defines the base of the Ordovician System in Indiana. The Oneota Dolomite of Indiana equates with the Gunter Sandstone and the Oneota Dolomite collectively of Illinois, with the lower part of the undifferentiated Prairie du Chien Group of Michigan, the lower part of the upper undifferentiated Knox Dolomite of Ohio, and the Gunter Sandstone and the Gasconade Dolomite of Kentucky.

In Indiana it is the sandy dolomite in the basal Oneota that corresponds to the Gunter Sandstone of Illinois and that is traceable into eastern Ohio, where it becomes what Janssens (1973, p. 24-27) called informally the "Rose Run sandstone" (within the Knox Dolomite), and into eastern Kentucky, where it has been identified as the "Rose Run sand" (Freeman, 1949; McGuire and Howell, 1963). (See also Droste and Shaver, 1983, fig. 2, and Shaver and others, 1985.)