Branchville Formation, and Vienna Limestone,

Mississippian System

Type locailty and description in Illinois: The Vienna Limestone was named by Stuart Weller (1920a, p. 396-398) for exposures near Vienna, Johnson County, Ill. As originally described, the lower part of the formation was cherty limestone, and the upper part was black fissile shale. In discussing the Vienna as a unit in the standard Chestenan section, however, Swann (1963, p. 38, 84-85) restricted the name to the limestone unit, which is commonly 5 to 10 feet (1.5 to 3 m) thick.

History of name in Indiana: When names were first assigned to upper Chesterian rocks in the state (Malott and Thompson, 1920), all Mississippian rocks above the Tar Springs Sandstone were assigned to the Buffalo Wallow Formation. This was said to include at its base the Siberia Limestone, then thought to be equivalent to the Vienna (Cumings, 1922, p. 518). A few years later, Malott (1925) assigned the Siberia to a higher position but failed to identify the Vienna. His manuscrjpt map of Perry County (Malott, 1923) suggests that some outcrops of the Vienna were misidentified as Glen Dean outcrops.

The first Indiana reference to the term Vienna Limestone was in a guidebook by Malott and Esarey (1940). As later described by Malott, Esarey, and Bieberman (1948, p. 25), the Vienna was prmcipally olive-green shale and was 40 to 60 feet (12 to 18 m) thick. So defined, the formation had obscure boundaries (see the discussion of boundary problems under "Tar Springs Formation") and was at variance with the prevailing concept, in which the Vienna is a dominantly limestone unit that is commonly only 3 to 10 feet (1 to 3 m) thick (Swann, 1963, p. 38, 84-85; Willman and others, 1975, p. 159). For these reasons, Gray (1978), in a restudy of outcropping upper Chesterian rocks in Indiana, redefined the Vienna as a thin member at the base of the newly named Branchville Formation. The Vienna Limestone is retained at formation rank in subsurface usage, and it is here assigned to the Buffalo Wallow Group.

Description: On the outcrop in southern Indiana, the Vienna Limestone Member is commonly a single bed of light-colored fossiliferous limestone 1 to 5 feet (0.3 to 1.5 m) thick (Gray, 1978, p. 8-10). It can be traced through Perry County and western Crawford County but has not been recognized farther north, although the equivalent horizon extends to southwestern Orange County where it is disconformably tramsected by the base of the Mansfield Formation (Morrowan). In the subsurface the Vienna Limestone is recognized from Pike and Dubois Counties southwestward, although there too its honzon extends somewhat farther north (Swann, 1963, p. 38). Boundaries of the Vienna are conformable, and apparently restrictions on its extent were depositionally controlled.

Correlation: Despite lapses in its occurrence, the Vienna is one of the more confidently traceable units in the upper Chestenan section. Continuity with the type section in southern Illinois is well established. On the basis of the conodonts it contains, the Vienna was assigned by Collinson, Rexroad and Thompson (1971) to the Kladognathus primus Assemblage Zone of standard North American usage, but in a later study Rexroad (1981) found those fossils more indicative of restricted and specialized environment than helpful in precise age determination. The Vienna Limestone Member eorrelates with rocks within North Amencan foraminiferal Zone 17 of Mamet and Skipp (1971) and within the lower part of the Namurian Series (Zone E1) of European usage.