Ordovician System

Type area and use of name: The name Everton Limestone was first given by Ulrich (1907, p. 251-252) to exposures in deep ravines a few miles south of the Missouri-Arkansas state line near Everton, Boone County, Ark. In the type area the Everton exposures are bounded above and below by what Ulrich called the Upper and Lower St. Peter Sandstones. The Everton, as traced to Illinois, was first classified as a group (Templeton and Willman, 1963) and then with formation rank as the Everton Dolomite (Willman and Buschbach, 1975). Still later, this unit was recognized in Indiana (Droste, Abdulkareem, and Patton, 1982 Rexroad, Droste, and Ethington, 1982) and was there also designated the Everton Dolomite. And still later, in 1985, the Everton of Indiana was assigned to the Knox Supergroup (Droste and Patton).

Description: In Indiana the Everton Dolomite occurs only in the subsurface in the southwestern part of the state and is known from only two wells that penetrate the upper part of the formation before reaching total depth. Well samples and core from one of these wells indicate that the Everton is a medium-gray to brownish-black fine- to medium-grained dolomite inter bedded with shaly dolomite and fine- to medium-grained quartz sandstone (Droste, Abdulkareem, and Patton, 1982). Core from an interval about 25 feet (8 m) thick shows breccia zones throughout.

The Everton is unconformable overlain by the St. Peter Sandstone, and on the basis of data from wells in Illinois and Kentucky, it is thought to overlie unconformable the Shakopee Dolomite.

Correlation: Conodonts recovered from an Everton core from southwestern Indiana are dominated by Paraprioniodus costatus and Leptochirognathus quadratus and indicate a Whiterockian age (Rexroad, Droste, and Ethington, 1982), that is, an age younger than that traditionally assigned to any part of the Knox of Indiana (previously assigned a Trempealeauan and Canadian age). This association is indicative of Middle Ordovician faunas 2-4 of Sweet, Ethington, and Barnes (1971). The upper Knox assignment was made, however, as one possible solution to nomenclatural problems arising from the incompletely known stratigraphic relations of the Everton. Although these lower relations are unknown in Indiana, the Everton of Indiana is probably closely correlatable with the formation by the same name in its type area in northern Arkansas and in Missouri, Illinois, and Kentucky. The Everton of Indiana very likely also correlates with subsurface rocks in Michigan that in 1984 were newly discovered to be early Middle Ordovician in age and that were being newly named at that time (James H. Fisher, oral communication, 1984). (See Droste and Shaver, 1983, fig. 2, and Shaver and others, 1985.)