Illinois Basin--Sediment-Starved During Mississippian

Abstract

The deeper part of the Illinois basin contained water as much as 1,000 ft (305 m) deep and was sediment-starved during early middle Mississippian (Valmeyeran) time. During the earliest part of the Valmeyeran, the Burlington and Keokuk Limestones were deposited in shallow water at the west edge of the basin, whereas little or no sediment reached the deep part. Later, the Borden Siltstone delta complex extended from the east across part of the deep basin. The Warsaw Shale, a tongue of the delta complex, extended westward across the shallow shelf above the Keokuk Limestone. Thin Springville Shale was deposited in the deep water between and beyond the thick deltaic siltstone bodies. Dark-colored siliceous rocks of the Fort Payne Formation and the succeeding light-colored foss liferous Ullin Limestone filled the remaining deep-water areas prior to deposition of the Salem Limestone. Most of the pre-Salem Valmeyeran sediments in the Illinois basin were deposited on narrow foreset or foresetlike slopes several hundred feet high, which separated the shallow shelf environment from the deep-water areas. Little sediment reached the center of the deep-water areas and the basin was filled by progradation of the foreset slopes. Foreset slopes in starved basins resemble foresetlike bedding along the present continental margins, because the ocean basins are also sediment-starved.