Bedrock geologic map of the Ashland and the northern part of the Ironwood 30' x 60' quadrangles, Wisconsin and Michigan

Abstract

This map portrays the geology of part of the Midcontinent rift system along the southern limb of the Lake Superior syncline in northern Wisconsin and the upper peninsula of Michigan. The southeastern half of the map area is underlain by a great monoclinal succession of Middle Proterozoic rift-related rocks of the Keweenawan Supergroup. This steeply north-dipping structure, the Montreal River monocline, exposes 15 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 Early Proterozoic strata, including the Gogebic iron range, and about 10 km of Late Archean rocks, exposed in the southernmost part of the map area, are also part of the monocline. The northwestern half of the map area, north of the Douglas fault, is underlain by nearly flat-lying continental red beds of the Bayfield Group, the youngest strata of the Keweenawan Supergroup. In spite of the simple surface geology in this area, geophysical data indicate a complex deeper rift structure, which is shown in the accompanying cross sections. 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data exists for the area as a result of many individual studies over the last hundred years, but much has remained unpublished in theses, dissertations, and other reports of limited availability.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 1991 to 1994. Our studies were designed to fill gaps in existing data and reconcile conflicting interpretations on some aspects of the geology of the region. STRATIGRAPHY AND TECTONIC SETTING Three principal stratigraphic sequences, separated by profound unconformities, are present in the map area. Each sequence was deposited in a distinctly different tectonic setting as the region evolved from a Late Archean primitive oceanic terrane to a Middle Proterozoic craton. Late Archean Metavolcanic and granitic rocks constitute a poorly exposed Late Archean terrane in the southern part of the map area. They are among the southernmost exposures of volcanic rocks in the Superior province and are typical of granite-greenstone terranes of that province. Volcanic rocks consist mostly of basalt and lesser andesite and rhyolite. Dacitic breccia is the dominant rock type immediately south of the Gogebic iron range between Hurley and Mellen, Wisc. Where best exposed, near Gile Flowage, the volcanic rocks are as much as 6 km thick and are overturned, so that they face northwest and dip steeply southeast (Greathead, 1975). The volcanic rocks were intruded by granitic rocks of the Puritan batholith at about 2750 Ma (Sims and others, 1977). Map unit Wp consists mostly of massive and pegmatoid granitic rocks of the Puritan Quartz Monzonite that form the Puritan Batholith. The unit also includes gneissic rocks probably derived from volcanic rocks during emplacement of the batholith. Early Proterozoic Early Proterozoic strata of the Marquette Range Supergroup comprise a 2-to 3-km-thick sedimen...

Failed rifting and fast drifting: Midcontinent Rift development, Laurentia’s rapid motion and the driver of Grenvillian orogenesis

“…1, 3 and 8; Palmer and Halls, 1986;Nicholson et al, 1997). The Powder Mill Group is split into the Siemens Creek Volcanics, which are comprised of ∼2000 m of thin basalt flows, and the overlying Kallander Creek Volcanics, which are comprised of volcanic rocks ranging in composition from basalt to rhyolite (Palmer and Halls, 1986;Cannon et al, 1996). Volcanics is a quartz and feldspar-phyric rhyolite that is informally known as the Sheep Farm rhyolite (Cannon et al, 1996).…”

Tracking Rodinia Into the Neoproterozoic: New Paleomagnetic Constraints From the Jacobsville Formation

“…The Jacobsville Formation has been mapped to have a continuous extent from the Wisconsin and Michigan border northeast to the Keweenaw Peninsula and east to the Marquette region (Figure 1; Hamblin (1958); Cannon et al. (1995, 1996); Cannon and Nicholson (2001)). In the eastern Lake Superior region near Sault Ste.…”