Mississippian System

Type locality and use of name in Indiana: The Glen Dean Limestone was named by Butts (1917, p. 97-102) for exposures near Glen Dean, Breckinridge County, Ky., where, in descending order, the formation as then defined consisted of 40 to 100 feet (12 to 30 m) of gray and red shale and thin beds of limestone, 30 to 60 feet (9 to 18 m) of gray crinoidal limestone, and as much as 10 feet (3 m) of red and green shale.

In Indiana rocks equivalent to the lowest of the three parts described above have long been a part of the Hardinsburg Formation (Malott and Thompson, 1920, p. 521-522; Malott, Esarey, and Bieberman, 1948, pl. 2). Equivalents of the upper part, formerly at least in part assigned to the Glen Dean, were excluded from the Glen Dean by Gray, Jenkins, and Weidman (1960, p. 38) to make the upper boundary more readily determinable and consistent. Thus defined, the name Glen Dean is applied in Indiana only to the main or massive limestone unit of former usage. This differs somewhat from standard usage (for example, Swann, 1963) as explained under "Correlation" below.

Description and distribution: The Glen Dean Limestone is a thick-bedded skeletal to oolitic to biomicritic limestone 9 to 31 feet (3 to 9 m) thick. Its typical fauna consists of brachiopods, blastoids of the genus Pentremites, and bryozoans including Archimedes (Perry and Smith, 1958, p. 94). Many of the faunal elements described in the literature on the Glen Dean come from shaly beds once regarded as upper Glen Dean but now assigned to the Tar Springs Formation. The fauna of the Glen Dean of present usage is not well documented but seems to include most of the larger invertebrates reported from the basal Tar Springs.

The Glen Dean Limestone is known in surface exposures from south-central Greene County to the Ohio River and is recognized in the subsurface from Greene County southwestward. It conformably overlies the Hardinsburg Formation and is overlain with apparent conformity by the Tar Springs Formation or disconformably by the Mansfield Formation (Morrowan).

Correlation: Although the name Glen Dean is applied throughout the Illinois Basin, the upper boundary of the Glen Dean is not consistently defined. In the subsurface of southwestern Indiana, a widely recognized limestone unit about 10 feet (3 m) thick and a little above the so-called main Glen Dean is commonly called the upper Glen Dean. This unit is thicker in Illinois where it is assigned to the Glen Dean proper and in places is a major part of the formation. In Kentucky an upper unit of variable thickness consisting of shale and thin beds of limestone and sandstone is retained in the Glen Dean.

The Glen Dean Limestone and overlying marine beds of the Tar Springs Formation are within the Zone of Pterotocrinus acutus Wetherby and embrace the ranges of P. bifurcates Wetherby and P. spatulatus Wetherby (Horowitz and Strimple, 1974) and the lower part of the range of the blastoid Pentremites broadheadi Hambach (= P. spicatus Ulrich) and its variants. These distinctive forms are readily recognizable in the field. In a broadly based paleontologic study, Horowitz and others (1979, p. 206) correlated the Glen Dean of northern Tennessee and therefore the Glen Dean of the Illinois Basin with rocks in North American foraminiferal Zone 17 of Mamet and Skipp (1971) and in the lower part of the Namurian Series (Zone El) of European usage. The Glen Dean has been assigned to the Gnathodus bitineatus-Kladognathus mehli Assemblage Zone of the standard North American conodont sequence (Collinson, Rexroad, and Thompson, 1971).