GEOLOGY OF THE MIDCONTINENT RIFT IN WESTERN LAKE SUPERIOR

Exposed rocks south of Lake Superior in Michigan, Wisconsin, and Minnesota preserve a nearly complete record for the Midcontinent rift. Rifting in the region began around 1108 Ma with eruption of flood basalts of the Siemens Creek Volcanics onto lacustrine sandstones of the Bessemer Quartzite. The Siemens Creek Volcanics are overlain by the Kallander Creek Volcanics, which consist of a lower flood basalt sequence (~1107 Ma) and an upper sequence dominated by andesite and capped by a large rhyolite flow dated at 1099 Ma. Anorthositic gabbros and granites of the Mellen Intrusive Complex (~1102 Ma) intrude the lower section of the upper Kallander Creek, and may have been shallow magma chambers feeding eruptions at a volcanic center. In eastern Minnesota and western Wisconsin, the Chengwatana Volcanics (~1102 Ma), composed of basalt, andesite, and minor rhyolite flows, and the Amnicon gabbroic and granophyric intrusive complex may also represent parts of another volcanic center. The Minong Volcanics, a sequence of flood basalts that overlie the Chengwatana Volcanics along a low angle disconformity, likely are equivalents to flood basalts of the Portage Lake Volcanics farther east in Michigan and Wisconsin. The Minong and Portage Lake Volcanics represent a second major phase of flood basalt eruptions within central rift grabens around 1096 to 1094 Ma. The Porcupine Volcanics (~1092 Ma) erupted from a volcanic center that formed a prominent volcanic edifice along the rift margin. Contemporaneous with the Porcupine Volcanics, rapid extension and subsidence were replaced by broader thermal subsidence, ending voluminous flood basalt activity. Over the next 30 to 40 million years, clastic and lacustrine sediments of the Oronto and Bayfield Groups were deposited within deep subsiding basins over the central rift grabens and within shallower flanking basins. Coincident with the final stages of sedimentation, regional compression reversed the sense of motion along early extensional faults, inverting the central rift grabens and creating a system of uplifted horsts along the length of the rift. Hydrothermal processes active in the later phases of the rift deposited copper throughout rift rocks resulting in development of the native copper deposits of the Keweenaw Peninsula and the copper sulfide deposits at White Pine.