This Open-File Report provides digital data (shapefiles and .e00 files) for the bedrock geology in the Port Wing, Solon Springs, and parts of the Duluth and Sandstone quadrangles in Wisconsin.  A Miscellaneous Investigations Series map (I map) is currently in review with analogous data in paper format.

This map portrays the geology of part of the Midcontinent rift system (MRS) along the southern extension of the Lake Superior syncline in northern Wisconsin.  The map area contains the St. Croix horst, a rift graben filled with Mesoproterozoic rocks of the Keweenawan Supergroup that was subsequently inverted.  The horst exposes about 15 - 20 km of strata that record the opening of the Midcontinent rift, its subsequent transition to a thermal subsidence basin, and eventual inversion.  About 3 km of underlying Mesoproterozoic strata, including the Gogebic iron range, and about 10 km of Neoarchean rocks, exposed in the southernmost part of the map area lie to the southeast of the horst.

The nearly flat-lying continental red beds of the Oronto and Bayfield Groups, the youngest strata of the Keweenawan Supergroup, overlie the volcanic rocks.

A wealth of geologic data exists for the area as a result of many individual studies over the last hundred years, but much has remained unpublished in theses, dissertations, and other reports of limited availability.  This map has incorporated most of that data (see list of data sources) and includes results of our investigations conducted from 1992 to 2000.  Our studies were designed to fill gaps in existing data and reconcile conflicting interpretations on some aspects of the geology of the region.

The purpose of this map is to complete digital coverage of quadrangles with significant exposure of rocks of the Midcontinent rift in Wisconsin and Michigan at a scale of 1:100,000.  Other maps in this series include the bedrock geologic maps of the Ontonagon-Wakefield, the Ashland-Ironwood, and the Keweenaw Peninsula areas (USGS Open-File Reports 01-088, 99-546, and 99-149, respectively).