Sedimentology of the Pleasantview Sandstone, Southern Iowa and Western Illinois

Abstract

ABSTRACT In both Iowa and Illinois, the Pleasantview Sandstone exhibits a thick, variably bedded, elongate channel phase and a thin, uniformly, thin-bedded, widespread sheet phase. The sheet phase occurs laterally to the upper part of the channel phase and is believed to be unconformably truncated by, and hence older than, the channel phase at any one place. Ripple marks and local organic reworking are present in the sandstone of the sheet phase, which rarely exceeds a thickness of 10 feet. Sands of the sheet phase are considered to have been deposited initially as small distributary mouth bars in shallow marine water of a deltaic fringe environment. Subsequent coalescence of bars and partial redistribution of sand by coastal currents formed a delta-front sheet sand prograding seaward (westwar in western Illinois, south-southwest in southern Iowa), generally conformably, over delta fringe and distal prodelta silts and clays. Channels carrying the Pleasantview detritus were eroded, predominantly subaerially, to depths of 70 to 80 feet, as distributaries across the swamp- or marsh-covered deltaic plain. Channel filling (formation of the channel phase) was by point bar deposition and (or) nonmarine infilling after channel abandonment. Medium-scale crossbedding was formed within the basal point bar sands, whereas current ripple marks predominate in the upper portion of the channel fill. Texturally, the very fine-grained, silty sheet phase sandstones of the Pleasantview are generally finer grained than the sandstones of the channel phase (fine- to very fine-grained). Pleasantview channel sands in southern Iowa, however, contain noticeably more fine-grained sand and are coarser than the channel sands in western Illinois. Carbonaceous siltstone acid silty shale are common to dominant in the upper, thin-bedded section of the channel phase. Sediment transport within channels was parallel to channel margins, being toward the west in western Illinois, and to the southwest in southern Iowa. Excluding matrix, the modal constituents of the Pleasantview Sandstone show negligible variation from Iowa to Illinois and between sheet and channel sands. Moreover, the composition of the Pleasantview (subfeldspathic lithic wacke) differs but little from the other major Pennsylvanian sandstones of the same or younger ages in the Eastern Interior Basin. The garnet content of the Pleasantview Sandstone, however, is several times greater in southern Iowa than in western Illinois. A garnet-contributing source north, northwest, or northeast of Iowa is implied. This source, believed to have been primarily a Cambrian and (or) Precambrian sedimentary terrane, supplied the Pleasantview of Iowa and Illinois with compositionally stable modal and heavy mineral constituents. Sedimentary recycling is especially evident in the heavy minerals. Although there is no conclusive evidence, some garnet-free detritus may have entered Illinois from the east or northeast, diluting the garnet-bearing sediment coming from the north. Apparently no barrier to sediment transport existed between the Eastern Interior and Mid-Continent basins during Pleasantview Sandstone deposition.