The deeper part of the Illinois basin had water depths of as much as 1,000 feet but received little or no sediment during early Middle Mississippian (Valmeyeran) time. Burlington and Keokuk Limestones were deposited in shallow water at the west edge of the basin while little or no sediment reached the deep part; the Borden Siltstone delta complex then filled part of the basin and extended on to the adjoining western shelf, but only thin equivalent Springville Shale was deposited in deeper water beyond thick deltaic accumulation; Fort Payne Formation and Ullin Limestone filled the remaining deep water areas. Progradation of foreset slopes several hundred feet high was effective in basin filling and is likened to present deposition along continental margins of the sediment-starved ocean basins.

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