Refraction seismic investigations of the northern midcontinent gravity high

Eighty-seven seismic refraction profiles have been obtained to define the geologic structure in the upper crust associated with the midcontinent gravity high in Minnesota and Wisconsin. The seismic measurements were taken across a fixed spread of seven geophones from distances up to 13 km. A structural section was prepared for each profile by interpretation of the travel-time graph, and the individual sections were compiled onto regional cross sections. Measured seismic velocities in bedrock fall in the 2.5–7.1 km/sec range. Observed velocities can be assigned to seven categories corresponding to Paleozoic, upper, middle, and lower Upper Keweenawan strata, Middle Keweenawan volcanics, pre-Keweenawan felsic-to-intermediate intrusive rocks, and pre-Keweenawan mafic intrusive rocks. These groups display good continuity through the area and suggest the lithologic equivalence of rock bodies between geologic provinces. The St. Croix horst and its flanking basins underlie the midcontinent gravity high and its parallel gravity lows north of Minneapolis. Minimum throw along the western and eastern boundary fault zones reaches about 3.0 and 2.0 km. Sedimentary rocks in the eastern basin reach a thickness of at least 2.6 km. A complex horst-like structure also underlies the midcontinent gravity high in southern Minnesota; an uplifted basaltic body is bordered by sedimentary basins having strata about 3.0 km thick. Middle Keweenawan basalts are present locally in the eastern and western basins underlying the Upper Keweenawan strata. Rocks probably lithologically equivalent to the Oronto group are rare in the western basin, common in small basins on the St. Croix horst, and abundant in the eastern basin. Rocks probably lithologically equivalent to the Bayfield group are extensive in the western and eastern basins, but they have not been found on the St. Croix horst. The Bayfield group seems to be several kilometers thick across Douglas County north of the Douglas fault, and it does not appear to increase in thickness under the Bayfield peninsula.