Bedrock Geologic Map of the Port Wing, Solon Springs, and parts of the Duluth and Sandstone 30' X 60' Quadrangles, Wisconsin

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studies over the last hundred years, but much has remained unpublished in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.

[Summary provided by the USGS.]