Information Circular 42. Scientific Test Drilling and Mapping in East-Central Minnesota, 1994-1995: Summary of Lithologic Results

Abstract

This report summarizes the results of test drilling, outcrop mapping, and geophysical analyses conducted by the Minnesota Geological Survey (MGS) in east-central Minnesota (Fig. 1). It is a companion to MGS Open-File Report 95-1 Jirsa, and others, 1995a). The results reported here, along with information from water-well records; exploration, engineering, and scientific test drilling; and existing geophysical maps, were used to construct the open-file map and the geological description in the following section. The western part of the map relies heavily on the interpretations of Boerboom and others (1995). Because geophysical data and interpretations played an essential role in targeting drill-hole locations and creating the geologic map, they are presented in a separate publication (Chandler and Jirsa, in preparation). This study focused on the Archean and Early Proterozoic geology of east-central Minnesota, which was some of the least understood in Minnesota. Previous interpretations depicted the area as largely Penokean granitoid intrusions. This work indicates that much of the area is indeed granitic; however, gravity and aeromagnetic data, and results given here, provide new insight about diverse rock types and complex structures that are also present. In addition, the study provided valuable stratigraphic information about the Middle Proterozoic (Keweenawan), Paleozoic, and Cretaceous bedrock, Quaternary glacial sediments, and the nature of boundaries between these strata.