Abstract

The paleotectonic setting of the Tonganoxie Sandstone (Upper Pennsylvanian, Virgilian) incised-valley system in the Forest City Basin of northeastern Kansas was analyzed using isopachous maps of the stratigraphic interval containing the paleovalley-fill and lineaments maps based on previous analyses of the configuration of the present-day Precambrian basement. Because structure maps of the present-day configuration of the Tonganoxie Sandstone reveal nothing about the structural framework that influenced paleovalley erosion and sedimentation during the Pennsylvanian, isopachous maps were used to detect subtle changes in local topography. These local topographic changes were the result in part of concurrent structural displacements.

Isopachous maps of intervals bounding and containing the Tonganoxie Sandstone were compared with lineaments interpreted from a map of the 2nd-order trend residuals of the basement gravity-field intensity and a map of the configuration of the present-day Precambrian surface. Previous studies along the eastern portion of the Tonganoxie paleovalley system used isopachous maps to identify a large trunk valley with smaller tributary valleys exhibiting a rectilinear or trellised drainage. Persistent linear thickness trends were recognized that coincide with the trends of interpreted lineaments. The coincidence of these trends suggests that Upper Pennsylvanian structures that influenced the location of paleovalley incisions were developed when preexisting basement faults were reactivated. The reactivation of these basement faults may have occurred in response to the Ouachita tectonic event far south of the Forest City Basin. Variations in thickness of these stratigraphic intervals are related, in part, to differential subsidence within distinct multi-km scale structural blocks with boundaries oriented predominately northwest-southeast and southwest-northeast. Movement along the structural blocks caused differential subsidence and influenced the subsequent drainage pattern of the Tonganoxie paleovalley. The results of this study suggest that exploration for incised valleys should focus on regional mapping of stratigraphic intervals that enclose the valleys and related basement information to delineate structural blocks that may have influenced the locations of valley incisions.