Keokuk chert

Keokuk chert comes from the Mississippian Keokuk Limestone, the unit just above the Burlington Limestone along the Mississippi River in Iowa, Illinois and Missouri. The lowest 30 feet in the type region are so cherty that they are named the Montrose Chert Member, and the rest of the formation carries beds and nodules of chert in light gray, crinoid-rich limestone speckled with darker gray, brown or black. The limestone is thinner bedded and darker than the Burlington and holds many bryozoans, corals and brachiopods. The formation is 60 to 80 feet thick over much of its extent and is well exposed along the Mississippi and Illinois rivers in western Illinois.

Keokuk Limestone, Mississippian (Valmeyeran Series): primarily a biocalcarenite, with the lower 30 feet in the type region very cherty (Montrose Chert Member), and the upper part fossiliferous crinoidal limestone interbedded with fine-grained limestone, argillaceous dolomite and shale. It overlies the Burlington Limestone and is overlain by the Warsaw Shale. The type section is at Keokuk, Lee County, Iowa, and the unit occurs through much the same area as the Burlington.