Midcontinent microcosm: Geology of the Atkins lake - Marengo falls area (Field trip 2)

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Archean and Proterozoic rocks exposed over about 16km2 between Atkins Lake and Coffee Lake in southeastern Bayfield County (Fig. 1) chronicle almost all of the major Precambrian geologic events in the history of the southern Superior Craton. The oldest rocks are part of a locally gneissic quartz monzonite complex, the Puritan Batholith, with an igneous Rb-Sr age of 2710+140 Ma (Sims et al., 1977). At the regional scale, this complex is part of one of the youngest Archean granite-greenstone belts in the Superior Province, and it intrudes greenstones of the Neoarchean Ramsay Formation. In the Atkins Lake – Marengo River area, the Puritan Batholith is nonconformably overlain by the Paleoproterozoic (ca. 2200 Ma) Bad River Dolomite. The Bad River Dolomite is in turn separated by an unconformity from rocks of the ca. 1875 Ma Menominee Group (Palms Formation and Ironwood Iron-formation), which locally contain mafic volcanic rocks and diabase sills (Cannon et al., 2008). These Paleoproterozoic rocks provide insight into climate and biogeochemical cycles during the transition to an oxidizing atmosphere (Bekker et al., 2006) and have deformational fabrics (folds, strong cleavage, local mylonite zones) that record the ca. 1850 Ma Penokean Orogeny. The youngest rocks in the area are Mesoproterozoic basaltic lava flows (Siemens Creek Volcanics, ca. 1110 Ma) and a layered mafic complex (the Mineral Lake Intrusion, also ca. 1100 Ma), both related to the Mid-continent Rift. All of the stratified units show static contact metamorphic textures near their contacts with the Mineral Lake Intrusion. Thus the area constitutes a microcosm of the regional bedrock geology, and the cross-cutting relationships among the units provide clear constraints on the relative timing of different phases of deformation and magmatism (Cannon etal., 2008, Bjørnerud, 2010a).

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Additional publication details

Publication type Conference Paper
Publication Subtype Conference Paper
Title Midcontinent microcosm: Geology of the Atkins lake - Marengo falls area (Field trip 2)
Year Published 2011
Language English
Publisher Institute on Lake Superior Geology
Contributing office(s) Eastern Mineral and Environmental Resources Science Center
Description 18 p.
Larger Work Type Book
Larger Work Subtype Conference publication
Larger Work Title Institute on Lake Superior Geology, Proceedings Volume 57, Part 2: Field Trip Guidebook
First page 31
Last page 48
Conference Title 57th Annual Meeting Institute on Lake Superior Geology
Conference Location Ashland, WI
Conference Date May 18-21, 2011
Country United States
State Wisconsin
County Bayfield County
Other Geospatial Atkins lake, Coffee lake, Marengo falls