Distribution and thickness patterns of an interval of Carboniferous rock units from the Beech Creek ("Barlow") Limestone of the Pope Group (Mississippian) to the Springfield Coal Member of the Carbondale Formation (Pennsylvanian) in Lawrence County, Illinois, were mapped in order to determine the effect of structural control of the southern part of the north-northwest trending La Salle anticlinorium, Illinois basin, during deposition of these units. Data from 720 geophysical logs from a 170 sq mi (350 sq km) area were used to produce a network of cross sections and distribution and thickness maps. Abrupt westward thickness increase of "sandstone B", an informally named unit of the Caseyville Formation, west of the hinge line of the anticlinorium suggests a depositional response to uplift of the La Salle anticlinorium or basinal subsidence. A significant thinning of "sandstone C", an informally named unit of the upper part of the Caseyville Formation across the axis of the Bridgeport anticline, suggests antiform development during the latter part of the Caseyville deposition. The abrupt thickness changes, discordant dips, and linearity of these sandstone units, as well as the juxtaposition of erosional valleys that truncate Pope Group units, suggest the presence of small offset (less than 10 feet or 3 m) faults that outline an ancestral block which is coincident with the Bridgeport anticline. An enigmatic problem is the apparent lateral continuity of the Tar Springs Sandstone (Pope Group) and "sandstone A", the basal unit of the Caseyville Formation in part of the study area. The uncertain correlation of "sandstone A" precludes the exact placement of the Mississippian-Pennsylvanian unconformity and the timing of corresponding structural events.
AAPG Search and Discovery Article #90925©1999 AAPG Foundation Grants-in-Aid