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Belle Plaine (Minn.); Geological maps


Combined geologic and gravity investigations in the vicinity of Belle Plaine, in the Minnesota River valley, have disclosed a hitherto unknown fault of possible major significance. The fault is inferred on the basis of available evidence to have a left lateral displacement of the order of 100 miles and later vertical movements of about 1,000 feet. The field work was supported by the Minnesota Geological Survey. The writers wish to express their great appreciation to the late Dr. Edward Thiel of the Department of Geophysics and to Dr. J. C. Craddock and Barton Gross of the Department of Geology, University of Minnesota for giving permission to use their unpublished gravity data in the area. We also wish to thank the Chicago and North Western Railroad for providing their right of way maps and elevation profiles; these aided greatly in the gravity data reduction. The geologic work on which this report is based was done by the senior writer in 1961 and 1962, as part of the compilation of a revised geologic map of the southeastern part of the state. This work indicated the probable presence of a fault near Belle Plaine. Because of the sparse exposures in this part of the state and the small amount of available subsurface geologic data, the area of structural discontinuity was investigated in the spring of 1962 by detailed gravity measurements. These data, obtained by the junior author, substantiated the geologic interpretation and further elucidated the structure and stratigraphy of the area. Gravity data and station locations are on file in the office of the Minnesota Geological Survey.

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