Historical:Walnut Grove Limestone Member
Charles Collinson and Elwood Atherton
The Walnut Grove Limestone Member of the Lingle Formation (North, 1969, p. 29-30), the uppermost member, is named for Walnut Grove Church, Union County, 1 mile southeast of the type section.
The type section of the Walnut Grove Member is an outcrop near the head of a small tributary of Clear Creek (SE SE SW 22, 11S-2W), where the member is 29 feet thick.
Extent and Thickness
The Walnut Grove occurs only in and near the outcrop area in Jackson, Union, and Alexander Counties (fig. D-15), and its maximum thickness is about 50 feet.
The Walnut Grove Member is cherty, very silty, glauconitic, spore-bearing, fine-grained, fossiliferous limestone locally interbedded with very dark brown, calcareous shale. The Rendleman Oolite Bed near the base is a distinctive key bed.
Fossils include crinoids, corals, brachiopods, and sporangites.
NORTH, W. G., 1969, Middle Devonian strata of southern Illinois: Illinois State Geological Survey Circular 441, 45 p.
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