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Lead Creek Limestone Member


Mansfield Formation,

Pennsylvanian System

Type area and use of name in Indiana: The term Lead Creek Limestone was applied by Crider (1913, p. 279) to exposures of limestone, shaly limestone, and shale along Lead Creek in Hancock County, Ky., and to similar exposures elsewhere in the Kentucky parts of the Tell City and Owensboro Quadrangles. Crider referred to three or four ledges ranging in thickness from 8 to 10 feet (2.4 to 3.0 m) through an interval of 30 to 40 feet (9 to 12 m). The Lead Creek of Kentucky was later described as 5 to 11 feet (1.5 to 3.4 m) of shaly limestone (above) and hard limestone (below) that lies 85 feet (26 m) below the Lewisport Limestone (Chisholm, 1931, p. 224-225).

Crider's three or four limestone ledges ranging through the stated interval have probably never been reverified for the Lead Creek area of Hancock County, Ky. (Thompson, Shaver, and Riggs, 1959; Thompson and Shaver, 1964); Chisholm's description of the Lead Creek as consisting of two ledges of limestone is the concept that Shaver and Smith (1974, p. 5-6) applied to the extension of this limestone and shale interval into Dubois, Spencer, and Perry Counties, Ind., and there assigned these rocks member rank in the upper part of the Mansfield Formation. Two reference sections were designated in the Kentucky type locality, and three Indiana reference sections were designated in Spencer and Parke Counties, Ind. shaver and Smith, 1974, p, 6-9 and 34).

Description: In the southernmost Indiana outcrop area of Mansfield rocks, the Lead Creek Limestone Member characteristically consists of a 15- to 24-foot (5- to 7-m) interval made up of three lithologic units: a lower dark dense shaly or argillaceous limestone that has a shaly parting in some places, (2) a middle shale and siltstone that generally is 10 to 15 feet (3 to 4.6 m) thick and that has thin coal and clay in some sections, and (3) an upper massive limestone that is light colored, fine grained, and cherry and that becomes nearly all chert in some places. The two limestones, ranging from less than 1 foot (0.3 m) to nearly 5 feet (1.5 m) in thickness, are named the Fulda Bed and the Ferdinand Bed.

North of Dubois County, Ind., the full Lead Creek, exhibiting two limestone ledges, is rarely seen, and physical continuity is questionable. Nevertheless, a single thin impure limestone bed and, less commonly, two closely spaced beds have been tentatively identified as the Lead Creek member in upper Mansfield rocks as far north as Warren County.

The Lead Creek is overlain and underlain by clastic rocks in the approximately upper 50 to 60 feet (15 to 18 m) of the Mansfield Formation and in places much nearer the top of the formation.

Correlation: At times in the past, the ledges of limestone in Indiana and Kentucky that are now called the Lead Creek have been confused, either as presumably traceable correlative or more strictly in the age sense, with limestones now called the Lewisport and Curlew Limestone Members (Tradewater Formation) in Kentucky and the Perth Limestone Member (Brazil Formation) in Indiana and with what was once called the Curlew Limestone Member of Illinois, ail of which are now known to be younger than the Lead Creek.

According to Hopkins and Simon (1975, p. 182), the Boskydell Marine Zone of southern Illinois is a possible correlative of the Lead Creek, but other than this possibility, no other direct correlative is known in the Illinois Basin.

The Lead Creek has an abundant and key micro fauna that has both regional and global significance in correlating Pennsylvanian rocks ranging stratigraphically on either side of the Morrowan-Atokan boundary and of the Bashkirian-Moscovian boundary (in global terms not quite the same as the Morrowan-Atokan boundary). The ostracod fauna, however, belongs to the Zone of Amphissites rothi and has long been described in many formations and members in Oklahoma especially, but also in Colorado, that classically have been considered to be late Morrowan in age. The same fauna is now known more widely, including in southwestern states, in the Poverty Run Limestone Member (Pottsville Formation) of the Appalachian Basin (Ohio), and in upper type Morrowan rocks of Arkansas. (See discussions in Shaver and Smith, 1974; Knox, 1975; and Shaver, 1984, for names of potentially correlative units and sources of literature on this subject.)

The key fusulinid in the Lead Creek is Profusulinella kentuckyensis, Profusulinella being the earliest of the complex, fusiform fusulinacean genera and the name giver to the globally applied Zone of Profusulinella. This zone has been repeatedly recognized in late Bashkirian rocks around the world, which have been equated on several bases with late Morrowan rocks of North America by an internationally composed committee acting on behalf of the project called "Correlation of Stratigraphic Units of North America." (See, for example, the chart produced by Shaver and others, 1985.)

This correlation (that is, of Lead Creek faunas and rocks and the Zones of Amphissites rothi and Profusulinella with late Morrowan rocks) has not been a popular one among American stratigraphers. (See, for example, the partly to wholly different correlations advocated by Cooper, 1946; Thompson, Shaver, and Riggs, 1959; Thompson and Shaver, 1964; Hopkins and Simon, 1975, p. 181-182; Douglass, 1979; and certain papers edited by Sutherland and Manger, 1984, in a volume on "The Atokan Series and Its Boundaries.") The problem is at least partly semantic, because the midcontinent type Morrowan-Atokan boundary has not yet gained a stable definition and because some stratigraphers have chosen to define the Atokan Series, not on the basis of a type section or locality of rocks but as beginning at its base with the Zone of Profusulinella.

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