M. E. Hopkins and J. A. Simon
The McLeansboro Group (DeWolf , 1910, p. 181 ; Weller, 1940, p. 36) is named for McLeansboro, Hamilton County.
The type section consists of strata in a diamond drill core from a boring near McLeansboro (SE SW SW NE 25, 4S-5E), where 811 feet of Pennsylvanian strata was encountered above the top of the Danville (No. 7) Coal, which defines the base of the group.
The McLeansboro Group extends into Indiana, and is equivalent to all but the lowest part of the Sturgis Formation of Kentucky.
The McLeansboro includes all Pennsylvanian rocks in Illinois above the No. 7 Coal and is made up of three formations- the Modesto, Bond, and Mattoon (fig P-2). DeWolf originally applied formation status to the McLeansboro, but Weller (1940) elevated it to a group and Willman and Payne (1942) defined it as including all the Pennsylvanian strata in Illinois above the base of the Copperas Creek (Anvil Rock) Sandstone Member. About 400 feet of additional Pennsylvanian strata younger than those encountered in the type section drill hole are present in the deeper part of the Illinois Basin in Jasper County, and even younger Pennsylvanian rocks are present in western Kentucky.
Lithologically the McLeansboro is similar to the Kewanee Group, but it contains more marine members, including thicker, less argillaceous limestones. Coals are not as thick and probably not as extensive in the McLeansboro. Most are less than 1 foot thick, although some coals are up to 4 feet thick. Variegated claystones, many of them dominantly red, occur several feet above and below some of the limestones. They are much less common in other Pennsylvanian groups. Light olive or tan argillaceous limestone units up to 3-4 feet thick and bearing only ostracodes, Spirorbis, and very few pelecypods are present in the McLeansboro, particularly in the upper part. They are similar to thicker limestones in the upper part of the Pennsylvanian in the Appalachian Basin. The Dix Limestone is the only one of these units that has been named.
DEWOLF, F. W., 1910, Studies of Illinois coal-Introduction: in Year-book for 1909, Illinois State Geological Survey Bulletin 16, p. 178-181.
WELLER, J. M., 1940, Geology and oil possibilities of extreme southern Illinois: Illinois State Geological Survey Report of Investigations 71, 71 p.
WILLMAN, H. B., and J. N. PAYNE, 1942, Geology and mineral resources of the Marseilles, Ottawa, and Streator Quadrangles: Illinois State Geological Survey Bulletin 66, 388 p.
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