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