Niagaran and Cayugan Series

Silurian System

Type area and history of name: The term Bainbridge Limestone was first given (K. O. Ulrich as published by Buckley and Buehler, 1904, p. 110) to all the Silurian carbonate rocks that lie above what was then designated as the Girardeau Limestone and below the Bailey Limestone along a several-mile stretch of the Mississippi River bluffs above and below Bainbridge, Mo., and Thebes, Ill. (Ulrich. 1911, pl. 28) redefined the lower boundary as being with the Brassfield Limestone, which meant that he restricted the Bainbridge to what were then considered to be Niagaran rocks. During that period of study, what is now considered to be the Bailey part of the Bainbridge was thought to be Early Devonian in age.

By 1949 Lowenstam (p. 12) elevated the rank and assigned two formations to the Bainbridge Group, the St. Clair Limestone below and the Moccasin Springs Shale above, and also defined the relations of this group in the Illinois Basin. This arrangement was continued in 1974 (Becker, p. 14) for the Indiana part of the basin. By 1978 (Becker and Droste, fig. 2 and p. 4) and 1980 (Droste and Shaver, fig. 4), however, full realization of a Silurian age for all but a small upper part of the Bailey Limestone brought about further redefinition of the Bainbridge Group to include the Bailey as its uppermost formation of three. In Indiana the uppermost part of what had been called the Bailey was reassigned to the New Harmony Group (Lower Devonian), although in Illinois the Bailey is still considered to include some Devonian rocks. (See this relationship in Shaver and others, 1985.)

In this understanding the Bainbridge Group has a rather vaguely defined vertical cutoff boundary in southwestern Indiana (as shown by maps in sources cited above) against rocks ranging upward from the Salamonie Dolomite through the Wabash Formation. This boundary is in the area of extensive deposits of reef and reef like rocks included in the feature called the Terre Haute Bank.

Description: Three rather distinctive lithologies of regional scope characterize the Bainbridge from the bottom part upward: (1) relatively pure white to pink, red, gray, and brown granular echinoderm-rich limestone (St. Clair Limestone); (2) multihued dense argillaceous to shaly, silty limestone and some shale (Moccasin Springs Formations); and (3) white, gray, and brown, very fine grained, cherry, and partly argillaceous limestone and some dolomitic limestone (Bailey Limestone).

A fourth distinctive lithology of the Bailey consists of rather pure light colored granular carbonate rocks in reefs and banklike deports that may extend vertically (for example, in pinnacle reefs as thick as 800 or 900 feet [244 or 274 m]) from within the St. Clair through the Bailey. (Besides the Indiana sources cited above, see Becker and Keller, 1976, and Droste and Shaver, in preparation.)

The Bainbridge rests unconformably on the Sexton Creek Limestone and underlies the New Harmony Group (Lower Devonian) probably unconformably and conformably. In its nonreef facies the Bainbridge thickens southwestward from about 200 feet (61 m) to about 400 feet (122 m) in the area of its southwestern Indiana distribution in the parts or whole of 10 counties.

Correlation: As noted above, the Bailey part of the Bainbridge Group was long correlated with Lower Devonian rocks, and as late as 1974 and 1975 (Becker, p. 14; Willman and others, fig. S-16 and p. 107-108) common practice in Indiana and Illinois was still to assign all or nearly an the Bailey to the Devonian. Nevertheless, Collinson and others (1967) and Schwalb (1975, as reported by Becker and Droste, 1978, p. 4) by that time had discovered Silurian conodonts and ostracods within the Bailey of Illinois and Kentucky further, the geophysical correlations at the base of Becker and Droste's (1978) and Droste and Shaver's (1980) Indiana work pointed to a Silurian assignment for nearly all the Bailey as it was previously understood

In these considerations, then, the Bainbridge Group ranges from early Niagaran (late Llandoverian) in age through latest Silurian and can be correlated approximately with a great many midwestern and midcontinent Silurian formations as shown in part by Shaver and others (1985). Direct-age data remain very sparse, however, and much of the suggested correlation is based on direct tracing and geophysical logging.