Mississippian System

Type locality and description in Illinois: This unit was first named the Menard Formation by Stuart Weller (1913, p. 128) for exposures near Menard, Randolph County, Ill. Later describing the unit as 80 to 120 feet (24 to 37 m) of dark-gray thin-bedded fine-grained limestone, Weller (1920b, p. 202, 205-206) changed the name to Menard Limestone. As further redescribed by Swann (1963, p. 38-39, 74), the Menard is 30 to 150 feet (9 to 46 m) thick and consists of three named limestone members and three unnamed shale members. The Menard Limestone is a unit in the standard Chesterian section (Willman and others, 1975, p. 160).

History of name in Indiana: Application of the term Menard in Indiana must be viewed separately from surface and subsurface points of view. On the surface, the name Siberia Limestone was first applied to a tongue of this unit in an abstract (Malott and Thompson, 1920, p. 521). Later Malott (1')25, p. 109-110) defined the unit, and still later he correlated the Siberia with the Menard of Illinois (Malott, 1981, p. 222). With the expansion of drilling activity in the Illinois Basin, subsurface usage of the name Menard came to be common.

The name Menard (lithology and rank unspecified) was applied for the first time to outcropping rocks in southern Indiana by Malott and Esarey (1940); Malott, Esarey, and Bieberman (1948) first applied the full name, Menard Limestone. The name was changed, for Indiana usage, to Menard Formation by Rexroad and Nicoll (1965), who recognized the Siberia Limestone Member as a part of the formation. In none of these usages, however, were the thickness, rock content, or boundaries of the unit ever made clear. (See the discussion of upper Chesterian boundary problems under "Tar Springs Formation.") partly because of this, Gray (1978) devised a classification for outcropping upper Chesterian rocks that avoided the name Menard he assigned the Siberia Limestone Member to the Tobinsport Formation, introduced the Leopold Limestone Member of the Branchville Formation, and regarded both members as outcropping tongues of the Menard Limestone.

Description in Indiana: The Menard, here redesignated with the lithologic term limestone in recognition of its most consistent and widespread usage, is restricted in Indiana to the subsurface, where the unit is recognized from Dubois County southwestward. Three subunits, informally designated "upper," "massive," and "little" Menard, are recognized. Where the upper Menard is recognizable, it is a limestone 10 feet (3 m) thick. The massive or main Menard consists of 30 to 60 feet (9 to 18 m) of limestone. Beneath this are 15 feet (5 m) of dark-gray shale that is not named and the little or lower Menard, which is limestone and is 8 to 10 feet (2 to 8 m) thick. The principal type of carbonate rock in all three limestone members is light-gray micritic limestone some dark-gray fossiliferous limestone is present in the upper part of the main Menard.

The Menard Limestone overlies the Waltersburg Sandstone and is overlain in most places by the Palestine Sandstone. Both contacts are probably conformable. Where uppermost Chesterian formations have been removed by pre-Pennsylvanian erosion, the Mansfield Formation (Morrowan) disconformably overlies the Menard. For Indiana usage the Menard Limestone is here assigned to the Buffalo Wallow Group.

Correlation: Apparently the Menard Limestone correlates with rocks that are near the boundary of North American foraminiferal Zones 17 and 18 of Mamet and Skipp (1971) and within the Namurian Series, near the boundary of Zones El and E2, of European usage. The Menard Limestone and all its members have been assigned to the Kladognathus-Cavusgnathus navicuIus Assemblage Zone of the standard North American conodont sequence (Collinson, Rexroad, and Thompson, 1971).