Geneva Dolomite Member
  

(JBD & RHS)

Jeffersonville Limestone,

Devonian System


Type locality, reference sections, and use of name: The name Geneva Limestone was first used by Collett (1882, p. 63, 81, 82) for exposures of a buff dolomitic limestone exposed along the Flat Rock River near Geneva, Shelby County, Ind. The same rocks were later called the Shelby Bed (Foerste, 1898, p. 234-235). Most early usage, however, was "Geneva," and in accord with the advice of Kindle (1901, p. 536), the term Shelby was abandoned. The term Geneva Formation has also been used (Dawson, 1941), and this unit was most recently assigned as the Geneva Dolomite Member to a lower part of the Jeffersonville Limestone (Droste and Shaver, 1975a, p. 403-404)

No type section was ever designated, but reference sections were suggested by Burger and Patton (1970, p. 62), one of which (quarry near Hanover, Jefferson County) is now under water. Two additional reference sections, here designated, reveal the differing relations that the Geneva has with the classic biozones described in the Jeffersonville at the Falls of the Ohio: (1) section along Big Camp Creek, Jefferson County, Ind. (NE¼NE¼SW¼ sec. 13, T. 4 N., R. 8 E.; described by Dawson, 1941, p. 38, and Shaver, 1974a, p. 4; Geneva underlies a part of the Coral Zone) and (2) section in the Berry Materials Corp. quarry at North Vernon, Jennings County, Ind. (NE¼ sec. 34, T. 7 N., R. 8 E.; described by Patton and Dawson, 1955, p. 24-96, and Droste and Shaver, 1975a, p. 405; Geneva underlies the Amphipora Zone; Coral Zone not recognized).

Description: Typically a calcareous dolomite that is buff to chocolate brown, rather soft, granular, and vuggy and that contains bands and partings of carbonaceous material, the Geneva Dolomite Member is massive to thick bedded in its lower part and more commonly thin bedded in its upper part. The distinctive colors are due to a high organic content, and near-surface beds are commonly oxidized to pale tan, cream, or even white. White crystalline, coarsely cleavable calcite masses (spar) ranging from 1 inch to more than 1 foot (0.3 m) in cross section, resulting from calcification of fossils, are scattered through the fine-grained dolomite matrix. Some calcite masses make up beautifully preserved, taxonomically identifiable fossil casts. Chert is present in some sections, and quartz sand is especially common in basal rocks.

Although dolomitization is pervasive, pelletoidal, burrowed, and bioclastic textures (including those referred to grainstones and packstones) and molds, casts, and other evidences of many kinds of highly altered fossils are recognized.

The Geneva variably overlies other Jeffersonville rocks (Dutch Creek Sandstone Member, subsurface area) conformably and Silurian rocks unconformable that range stratigraphically from the upper part of the Salamonie Dolomite to the lower part of the Wabash Formation. It is overlain variably and conformably by other Jeffersonville rocks, including those of the Coral and Amphipora Zones and of the Vernon Fork Member. It ranges in thickness from zero foot to more than 60 feet (18 m) in its roughly semicircular sea of distribution in the parts or all of about 30 counties of central and central western Indiana (Droste and Shaver, 1975a, fig. 8). Along the northern and eastern limits of the Geneva, the zero thickness is due to erosion, but the southern zero limit is a nondepositional and facies effect. (See also Perkins, 1963, and Becker, 1974.)

Correlation: Despite the abundance of fossil material in the Geneva, age assessment and correlation have long been controversial for at least three reasons: (1) preservation for taxonomic purposes is poor, (2) outcrop and subsurface studies tend to support different conclusions, and (3) age and correlation are partly matters of different definitions of what shall constitute the Geneva.

Early 20th century geologists variably thought that the Geneva was a facies of the Jeffersonville Limestone or was a facies of both the Jeffersonville Limestone and the overlying North Vernon (Sellers burg) Limestone, or that it was Schoharie (pre-Onondaga, New York standard) in age on the basis of its fossils. (See Wilmarth, 1938, p. 810.) In a middle period of study, Sutton and Sutton (1937, p. 331) considered that the Geneva, on the basis of faunal study, was a northward facies of the Jeffersonville. Patton and Dawson (1955, p. 37) and, by inference, Dawson (1941, p. 25-27), however, considered that Sutton and Sutton's (1937) sections and Kindle's (1913) fossils belonged to the Jeffersonville rather than to the Geneva. The semantics of definition, therefore, seem to have been part of a continuing problem.

Subsurface studies have supported the idea of a Geneva-Jeffersonville facies relationship, for example, those of Meents and Swann (1965), who correlated the Geneva with lower Grand Tower (= Jeffersonville rocks in Illinois, and those of Becker (1974). (Becker's statements, p. 29, were deliberately neutral see, for example, his pl. 1 and compare statements on p. 29 and 38.) Droste and Shaver (1975a), taking the already extant subsurface view, acknowledged (p. 404) that definition of what rocks should be called the Geneva enters into a solution to the correlation problem and presented evidence in itself for a Geneva-lower Jeffersonville relationship.

In their understanding, the Geneva correlates not only with other lower Jeffersonville and Grand Tower rocks but also with a lower to middle part of the Detroit River rocks in the Michigan Basin part of northern Indiana and adjacent Ohio and with lower Onondaga rocks in the Devonian standard of New York (= early Erian also, early Eifelian, global standard). (See also under "Jeffersonville Limestone" for a possible age as great as late Emsian, global standard.)






 
 
 
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