STRUCTURAL CONTROL OF FLUVIAL GEOMORPHOLOGY AND VALLEY FILL STRATIGRAPHY FROM AN UNDERFIT STREAM IN VANDERBURGH COUNTY, SOUTHWESTERN INDIANA: PRELIMINARY RESULTS

Previous research identified a small creek near the campus of the University of Southern Indiana as an underfit stream, but left the reason behind this classification unexplored (Talley et al., 2013). This research examines the influence of a possible fault, identified from the topographic expression of the valley and the stratigraphic architecture of the valley fill. A half-graben structure controlling fluvial deposition is the likely mechanism behind this underfit stream valley, creating a wide valley with a small stream.

The valley possesses an asymmetrical transverse topographic profile, with the eastern wall sloping at a 15.5% gradient compared to a 45% gradient for the western wall. The valley fill stratigraphy, identified by previous maps, GPR and sediment cores, shows a series of westward migrating, cut and fill sequences, beginning with late Wisconsinan glaciolacustrine sediments to the east and terminating with the modern stream and its alluvium pinned against the western valley wall. Oil and gas well locations surrounding the study area indicate a pattern consistent with structural traps. Hundreds of oil and gas wells have been logged in the 3 sections west of the valley, but only a dozen wells have been logged in the 3 sections to the east. Thus, tracing of key strata across the valley that could indicate offset in the underlying bedrock at depth, are not available.

GPR data thus far do not show offset within the unconsolidated valley fill suggesting; 1) the motion occurs along the edge of the valley fill and does not cut through it; or 2) fault motion has not occurred during the past 15 kyr, and the valley fill architecture evolved under static structural conditions.

Work is currently progressing to constrain the timing of deposition of alluvium within the valley and to determine if fault motion is evident in the Holocene alluvium.