Cincinnatian and Champlainian Series

Ordovician System

Type locality and use of name: The Maquoketa Shale was named by White (1870, p. 180-182) for exposures of blue and brown shale that aggregate 80 feet (25 m) in thickness along the Little Maquoketa River in Dubuque County, Iowa. Since its first use, the term has spread gradually eastward, in the process becoming a group that embraces several formations. It is now used throughout Illinois (Willman and others, 1975, p. 84-85), was extended into northwestern Indiana by Gutstadt (1958a, 1958b), and was adopted for use in a group sense throughout Indiana by Gray (1972b).

Description: As described by Gray (1972b), the Maquoketa Group in Indiana is a westward-thinning wedge, 1,000 feet (300 m) thick in southeastern Indiana and 200 feet (60 m) thick in northwestern Indiana. It consists principally of shale (about 80 percent); limestone content is minimal throughout most of Indiana but increases prominently in the southeast, so that parts of the group are in places dominantly limestone. The lower part of the group is everywhere almost entirely shale, and the lower part of the shale is dark brown to nearly black.

As a consequence of this pattern of rock distribution, two schemes of nomenclature are used in subdivision of the Maquoketa Group in Indiana. In most of western, northern, and central Indiana, the component formations in descending order are the Brainard Shale, the Fort Atkinson Limestone, and the Scales Shale. Presently available data (John B. Droste, oral communication, 1983) indicate that these three formations are more widely recognizable than was indicated by Gray (1972b). In and adjacent to the area where rocks of the Maquoketa Group crop out in southeastern Indiana (Gray, 1972a; Gray and others, 1972), the component formations in descending order are the Whitewater Formation (limestone with minor amounts of shale), the Dillsboro Formation (shale with subequal to minor amounts of limestone), and the Kope Formation (principally shale).

In most of Indiana the Maquoketa Group overlies the Trenton Limestone. In a small area in the subsurface in southeasternmost Indiana the group overlies the Lexington Limestone, which is laterally equivalent to but not continuous with the Trenton, and there is a narrow linear area in which neither Lexington nor Trenton is present and in which the Maquoketa Group overlies the Plattin Formation of the Black River Group, which elsewhere underlies the Trenton and the Lexington. Within the Lexington there are shale beds that could be considered to be tongues of the Maquoketa, but a similar relationship with the Trenton has not been observed (Brian D. Keith, oral communication, 1984).

According to Rooney (1966) and Templeton and Willman (1963), the Maquoketa-Trenton contact is a regional disconformity. Gray (1972b, p. 14-15), however, found it difficult to reconcile this view with the Trenton-Kope-Lexington relationships described above, and in Kentucky, Black, Cressman, and MacQuown (1965) found contacts of the Lexington and its members to be conformable. Recent work by Keith (oral communication, 1984) suggests that the basal contact of the Maquoketa with the Trenton is a discontinuity involving submarine exposure of the Trenton but does not represent a prolonged period of subaerial exposure and erosion as postulated by Rooney (1966).

The upper contact of the Maquoketa Group, which marks the top of the Ordovician System, is disconformable throughout Indiana. The immediately overlying rocks, all of which are Silurian in age, include different formations in different areas. In most of the outcrop area in southeastern Indiana the Maquoketa Group is overlain by the Brassfield Limestone, but in small areas the Brassfield is absent, and the shaly Osgood Member of the Salamonie Dolomite, which elsewhere overlies the Brassfield, directly overlies the Maquoketa (Foerste, 1904a; Brown and Lineback, 1966; Nicoll and Rexroad, 1968). The Brassfield also overlies the Maquoketa in the subsurface of east-central Indiana, but in western Indiana, where rocks that are laterally equivalent to the Brassfield are assigned to the characteristically cherty Sexton Creek Limestone, that formation overlies the Maquoketa. In northeastern Indiana the Maquoketa is overlain by the Manitoulin Dolomite Member of the Cataract Formation (Rexroad, 1980).

Former nomenclature: In the outcrop area of southeastern Indiana the Maquoketa Group is nearly synonymous with the Cincinnatian Series. Few areas of the world have become so well known paleontologically as the tristate area of Indiana, Ohio, and Kentucky that surrounds Cincinnati, type area of the Cincinnatian Series. During the heyday of stratigraphic paleontology the naming and the correlating of Cincinnatian formations were accomplished mainly by describing the contained species and their ranges. Lithology, apart from fossils as lithologic constituents, was ignored deliberately in the definition of many stratigraphic units.

Many of the faunal zones described during this period have proved to be valuable for correlation and have contributed to the widespread recognition of the stages into which the Cincinnatian Series is conventionally divided: in descending order, Richmondian, Maysvillian, Edenian. Some of the better known and more useful fossil zones are those of the brachiopod Platystrophia for middle and upper Cincinnatian rocks (Cumings, 1903; McEwan, 1920), those of various bryozoan species for most of the Cincinnatian Series (Nickles, 1902; Cumings, 1908; Cumings and Galloway, 1913), such zones as are summarized for the Cincinnati area by Caster, Dalve, and Pope (1955, fig. 3), and the assemblage zones of the Richmond Group of Fox (1962, p. 633-637). More recently, a numbered series of useful conodont assemblage zones has been established in much of the central United States for the Maquoketa, equivalent Cincinnatian rocks, and other Ordovician rocks. (See Sweet, Ethington, and Barnes, 1971, and Shaver and others, 1985.) Most of these zones, however, have not yet been precisely delineated in Indiana.

Faunal zones are not formations, however, and in recent years most of the old, faunally dependent terms have been abandoned. (See the miscellaneous column on plate 2E; table 7 in Burger and Shaver, 1970, p. 47; and Brown and Daly, 1985, p. 1-8.) Among efforts to establish a truly lithostratigraphic classification for these rocks are reports by Fox (1962), Weiss and Sweet (1964), Brown and Lineback (1966), Ford (1967), Gray (1972b), Hay (1981), and numerous maps and reports generated by the U.S. Geological Survey in the Kentucky mapping program.