Information Circular 37. Scientific Core Drilling in Parts of Itasca, St. Louis, and Lake Counties, Northeastern Minnesota, 1989-1991

Abstract

This information circular is the fifth since 1980 to report on the basic results of test drilling (Fig. 1) by the Minnesota Geological Survey (MGS). As with the previous projects, this test drilling was conducted primarily to provide a better understanding of the Precambrian geology in areas of Minnesota where bedrock is covered by variable thicknesses of glaciogenic sediment, and thus could not be thoroughly mapped by standard outcrop mapping and sampling procedures. The drilling summarized here was conducted during 1989-1991 in areas where the bedrock is locally well exposed, but where exposures are separated by covered terrane. Thus, the selection of drill targets was constrained largely by the interpretation of geophysical surveys (Chandler, 1983a, b). Our procedure is to formulate a regional geologic interpretation based on geophysical maps, new and existing outcrop data and the archived records of previous drilling, and then to constrain and test the interpretation by selective drilling. The results have been incorporated into new geologic maps of the region including Jirsa and others (1991) and Southwick (in prep.). An important secondary objective of this project is to describe the Quaternary deposits that overlie bedrock, though no systematic regional interpretation of those deposits has been undertaken to date. This report summarizes the results of studies in two areas of vastly different geology: 1) Archean and Early Proterozoic rocks of the Cook area in west-central St. Louis and eastern Itasca Counties, and 2) Middle Proterozoic rocks of the central Duluth Complex in parts of St. Louis and Lake Counties. The geology of each area is summarized in the "results" section of this report.