Pennsylvanian Sandstones of the Eastern Interior Coal Basin

Abstract

ABSTRACT The Pennsylvanian sandstones of the Eastern Interior Coal Basin were studied petrographically and stratigraphically to get a better understanding of source areas, sediment transport patterns, and conditions of sedimentation. The petrographic study was extended to adjoining basins, the Appalachian, Michigan, and northern Mid-Continent, to obtain a regional picture of mineral composition. Stratigraphic patterns are described in terms of general character, lateral persistence, thickness, and contrasts between channel and sheet sandstones. The sheet sandstones are commonly thinner, finer grained, more argillaceous, and more widely persistent than channel sandstones. In the petrographic study, modal mineralogy, quartz variety, and rough size analyses were made of 176 samples and heavy minerals were estimated from 35 samples. The sandstones fall into three petrographic types that correlate with stratigraphic position: orthoquartzites in the basal zone (Caseyville Group), types intermediate between orthoquartzites and subgraywackes in the next higher zone (lower Tradewater Group) and subgraywackes in all higher groups. Although there are minor differences in composition bet een geographic provinces, the major aspect is one of homogeneity over the entire area of the northcentral and northeastern states. Current intensity, the primary factor in sedimentation conditions, varied greatly both locally and regionally, but the range of variability was more or less homogeneous throughout the stratigraphic section and in all of the geographic provinces. Conflicting evidence on marine or nonmarine origin of the sandstones is harmonized, to infer deposition in a combination of a variety of shallow water marine, deltaic, and coastal plain environments. Source areas and patterns of movement of detritus to sedimentary basins were largely inherited from early Pennsylvanian time. After Caseyville time, detritus from the east and northeast moved over the entire basin of deposition, vastly diluting material derived from the north and northwest. Continued mild erosion of source areas throughout Pennsyl anian time led to gradual but incomplete removal of a sedimentary cover and to the exposure of limited areas of igneous and metamorphic rocks.