Lithostratigraphy: Ottawa Limestone Megagroup >>Galena Group >>Kimmswick Subgroup >>Dunleith Formation >>Sherwood Member
Chronostratigraphy: Paleozoic Erathem >>Ordovician System >>Champlainian Series >>Trentonian Stage
Allostratigraphy: Tippecanoe Sequence
H. B. Willman and T. C. Buschbach
The Sherwood Member of the Dunleith Formation (Templeton and Willman, 1963, p. 123) overlies the Rivoli Member and is named for Sherwood Street in East Dubuque, Jo Daviess County, near the type section.
The type section of the Sherwood Member is part of the Dunleith Formation type section (in an exposure in the Mississippi River bluffs on the north side of East Dubuque (SE 19, 29N-2W))(fig. O-2B), where the Sherwood Member is 13.6 feet thick.
Extent and Thickness
The Sherwood Member is 10 feet thick in central northern Illinois.
The Sherwood Member consists of relatively pure, vuggy dolomite with a strong argillaceous zone in the top 3.6 feet in the type section. Chert is erratic in occurrence, except for an almost continuous bed of chert just above the base in some areas. One of the most persistent bentonites, as much as 3 inches thick, occurs about 3 feet above the base, and another occurs locally at the base of the upper argillaceous zone.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
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