Information Circular 39. Scientific Test Drilling, 1989-1992: Descriptions and Interpretations Pertinent to the Bedrock Geology and Quaternary Hydrogeology of Southwestern Minnesota

Abstract

In 1980 the Minnesota Geological Survey (MGS) started a program of scientific test drilling designed to acquire a better understanding of the complex Precambrian bedrock of Minnesota. A research approach that combined selective test drilling with geophysical surveying of the Precambrian terrane was dictated by the fact that the Precambrian rocks are covered by substantial thicknesses of quaternary glaciogenic sediment in most places and therefore are not amenable to standard outcrop mapping techniques. This circular is the seventh to report basic results from the drilling program. It contains data from 20 holes drilled for three separate studies undertaken in southwestern Minnesota in 1989-92. The drilling for two of the three projects reported here was closely coordinated with geophysical surveys, and the selection of drilling targets was guided by geophysics to the maximum extent practicable. The third study, consisting of the single hole PR-90-1, was somewhat abnormal in that drilling was targeted on a topographic rather than a geophysical feature (see discussion below for details). Apart from this one exception, our standard procedure was to formulate regional interpretations of the buried Precambrian geology from the geophysical data and then drill to verify, modify, or otherwise constrain the geophysical interpretation. Earlier applications of this approach have contributed to regional-scale geologic maps of the Penokean orogen (Early Proterozoic) in east-central Minnesota (Southwick and others, 1988) and an Archean greenstone belt in north-central Minnesota (Jirsa and Boerboom, 1990; Jirsa, 1990; Jirsa and others, 1991). The data tabulated in this circular will eventually contribute to a new geologic map for southwestern Minnesota.