RI-44 Gravity Investigation for Potential Ground-Water Resources in Rock County, Minnesota

Abstract

The gravity method was used to investigate the thickness and potential ground-water resources of Cretaceous and Quaternary sediments in Rock County, Minnesota. This fill, which partially covers the Early Proterozoic Sioux Quartzite, is a major source of ground water for the county, but its deeper parts are very poorly known because of poor drill-hole control. Local gravity signatures reflecting the low-density fill were isolated from a smooth, regional field reflecting deep, intrabasement sources by a graphical cross-proftle procedure, which incorporated data from outcrops, drill holes, and seismic soundings, to define the regional field. At control points where fill thickness was known, the regional field value was determined by using a Bouguer slab approximation with a density contrast of 0.60 g/cm3 to strip out the local effect of the fIll. Additional control on the regional field was provided by iterative analysis of cross-profIles. Because it is assumed to be smooth, the regional field can be defmed by relatively few control points, and subtraction of this field from the observed gravity data produces a residual map of the fill signatures. The residual field was transformed into estimates of fIll thickness by using the same Bouguer slab approximation and density contrast of -0.60 g/cm3, and the elevation of the Precambrian bedrock was estimated by subtracting the fill thickness from the surface elevation. In southwestern, southeastern, and northeastern Rock County, the combined thickness of the Cretaceous and Quaternary deposits is interpreted to exceed 200 m (600 feet). The thick fill in southwestern Minnesota connects with a buried channel in South Dakota that contains several known aquifers. Potential ground-water resources may also be associated with several buried channels cut into the edges of a plateau of Sioux Quartzite in the northwestern and central parts of the county. Along the southern margin of this plateau, a buried and somewhat dissected escarpment is interpreted to be associated locally with at least 215 m (650 feet) of unconsolidated fill. Additional resources may lie within the fractured rock and thickened fIll in a northwest-striking fracture zone in the Sioux Quartzite, which may extend in the subsurface across the county. The results of this study indicate that the gravity method is an effective reconnaissance-scale tool for ground-water exploration in the Sioux Quartzite areas of southwestern Minnesota.