Palynological and sedimentological basin analyses of lower to middle Pennsylvanian deposits in Rock Island County, Illinois, and Muscatine County, Iowa
Isbell, John L.
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Lower to Middle Pennsylvanian strata in Rock Island County, Illinois, and Muscatine County, Iowa, are identified as the Caseyville, Abbott, and Spoon Formation on the basis of spore assemblages. Spore distributions within coals and organic-rich shales allow for local and regional correlation. The Morrowan Caseyville Formation is equivalent in age to the Caseyville Formation in southern Illinois. Within the investigation area, Morrowan assemblages consist predominantly of Lycospora pellucida; other prevalent genera include Cingulizonates, Radiizonates, Waltzispora, Cyclogranisporites and Densosporites. One new spore, Granulatisporites rosalynensis, is proposed. The Abbott Formation of Atokan age is identified by the abundance of Cristatisporites indignabundus and Densosporites anulatus. In the Spoon Formation (Desmoinesian), the Rock Island Coal is represented by an assemblage composed almost entirely of the genus Laevigatosporites. Lower to Middle Pennsylvanian strata in Rock Island and Muscatine counties were deposited along the northwestern margin of the slowly subsiding Illinois Basin. The western limit of strata within this basin is delineated by the Mississippi River Arch. In northwestern Illinois and eastern Iowa, Lower to Middle Pennsylvanian sediments record the interaction between the arch, fluvial-deltaic dispersal systems, and changes in the elevation of base level. Within Illinois, pre-Pennsylvanian paleoslope orientation was towards the southwest. Formation of the Mississippi River Arch in Late Mississippian time resulted in northward deflection of southwest dispersal systems. Lower Caseyville sandstones, derived from sedimentary source terrains in Illinois and adjacent areas, were deposited in north-flowing braided streams. A change in fluvial architecture from braided to meandering in the Upper Caseyville Formation and ultimately to deltaic distributaries in the Abbott Formation is associated with westward shifts in local paleoslope. Depositional environments and paleoslope orientation indicate progressive burial and/or submergence of the Arch during the Lower Absaroka rise in sea level. Complete submergence of the Mississippi River Arch during Desmoinesian time allowed for the deposition of the Seville Limestone and for later southwestward deltaic progradation. Paleocurrents and sandstone composition in the Spoon Formation indicate a sedimentary and metamorphic provenance located in the northern Appalachian and eastern Canadian Shield regions to the northeast.