RI-37 Geologic Map (Scale 1 :250,000) of the Penokean Orogen, Central and Eastern Minnesota, and Accompanying Text

Abstract

The geology of the Animikie basin and contiguous rocks of Early Proterozoic age in central and eastern Minnesota (collectively constituting the western end of the Penokean orogen) is reinterpreted in terms of plate-tectonic theory on the basis of new geophysical and geological data. Fundamentally the Penokean orogen is inferred to consist of a deformed foreland basin or tectonic foredeep, encompassing the main bowl of the Animikie basin together with the newly recognized Long Prairie basin and Nimrod outlier, and a structurally and stratigraphically complex fold-and-thrust belt, encompassing the Cuyuna district, the Glen Township area, and adjacent areas to the south and east. The fold-and-thrust belt contains tectonically dismembered and reassembled sequences of volcanic and sedimentary rocks that are difficult to correlate with assurance from one structural panel to another. These greatly deformed rocks are unconformably overlain by less deformed sedimentary strata of the Animikie Group, which are interpreted to have been deposited after initial deformation of the fold-and-thrust belt but more or less concurrently with emplacement of the fold-and-thrust mass onto the margin of the Archean Superior craton. The southern panels of the fold-and-thrust belt contain deep-seated metamorphic and plutonic rocks characteristic of the internal zones of a collisional orogen. Iron-formations appear to have been deposited at different times and under differing geologic conditions during tectonic evolution of the Penokean orogen. They are associated stratigraphically with volcanic rocks in the South range of the Cuyuna district, with euxinic black shale and argillite in the North range of the Cuyuna district, and with tidally deposited sandstone and siltstone on the Mesabi range. The Biwabik Iron Formation of the Mesabi range is younger than the various iron-formation units of the Cuyuna district, and the correlation of the Biwabik with the Trommald Formation is no longer valid.