Subsurface Structures and Paleovalley Orientations Delineated from Distribution and Thickness Patterns of Carboniferous Rock Units along the Southern Part of the La Salle Anticlinorium, Illinois Basin, Lawrence County, Illinois

By

LUMM, DONALD K.

Kentucky Revenue Cabinet, Frankfort, KY

     Subsurface mapping 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) at the southern part of the La Salle anticlinorium, Illinois Basin, suggests that small-offset faults (less than 10 feet or 3 m) exist which are parallel or coincident with previously delineated flexures and paleovalleys.  Data from 720 geophysical logs derived from a 170 mi² (350 km²) area of Lawrence County, Illinois were used to produce a network of intersecting cross sections and distribution and thickness maps of selected rock units.  An informally named sandstone unit (“sandstone A”) tentatively correlated at the base of the Caseyville Formation (Pennsylvanian) and three informally named sandstone units (“sandstones B, C, and D”) within the overlying interval of the Caseyville Formation and Tradewater Formation were mapped.  An east-west trending paleovalley entreched in the Beech Creek (“Barlow”) Limestone separates an area of thin “sandstone A” in the north-central part of the study area from an area of thick “sandstone A” to the south.  On the western limb of the Bridgeport anticline, a local structural element of the La Salle anticlinorium, “sandstone A” is separated by a north-south trending “shale gap” facies with the Tar Springs Sandstone (Pope Group).  An abrupt westward thickness increase of “sandstone B” west of the hinge line of the La Salle anticlinorium suggests a depositional response to uplift of the anticlinorium or basinal subsidence.  A significant thinning of “sandstone C” across the axis of the Bridgeport anticline suggests continued uplift and antiform development.   The abrupt thickness changes, discordant dips, and linearity of these sandstone units, as well as the juxtaposition of erosional valleys and the “shale gap” facies suggests the presence of small offset faults that outline an ancestral structural block which is coincident with the Bridgeport anticline.  The uncertain correlation of “sandstone A” precludes the exact placement of the Mississippian-Pennsylvanian unconformity and the timing of corresponding structural events.