RI-11 Seismic Studies Over the Midcontinent Gravity High in Minnesota and Northwestern Wisconsin

Abstract

Seismic refraction data from 87 profiles contribute to the delineation of the geologic structures which cause gravity and magnetic anomalies associated with the northern part of the Midcontinent Gravity High. Interpretations of the seismic data provide knowledge about structure within the sedimentary section as well as depth to the igneous basement, which reaches a maximum of about 10,000 feet in this area. The area investigated extends from the Minnesota-Iowa border on the south to Lake Superior on the north, and includes parts of eastern Minnesota and northwestern Wisconsin. Gravity and magnetic maps compiled from published data and 12,000 additional gravity stations occupied as part of the present investigation were used in combination with geologic control from outcrops and drill holes as a guide in the location and later interpretation of the seismic profiles. The seismic data were obtained by shooting across a fixed geophone spread for shot distances up to a maximum of eight miles. Results for each profile are presented separately and include observed seismic travel-time data, an inferred geologic structure section, and a discussion. The discussion describes the geologic setting and purpose of the seismic profile, the reliability of the seismic interpretation, and the correlation of the seismic results with gravity, magnetic, and geologic control. Tabulated values for the observed seismic velocities fall in the range of 9,000-23,000 feet/second. The velocities can be assigned to seven groups of geologic strata, corresponding to Paleozoic, upper, middle, and lower Upper Keweenawan sedimentary strata, Middle Keweenawan volcanics, pre-Keweenawan felsic intrusives, and pre-Keweenawan mafic intrusives. Good velocity correlations can be established between similar strata in different geologic provinces. A second paper (Mooney and others, 1970) synthesizes data reported here into a regional geologic picture in the form of six structural cross-sections across the Midcontinent Gravity High.