Abstract

A sequence of semi-brittle deformational tectonic events in gently dipping middle Precambrian argillite and graywacke, and late Precambrian sandstone and volcanic rocks on the northwest coast of Lake Superior is interpreted from detailed geologic mapping in the Pigeon Point, Cook County, Minnesota area. The earliest tectonic event was broad, open folding in the middle Precambrian Rove Formation along N 35 °E axes and the development of two sets of joints (North and N 70° E). The second event was eastward trending, high-angle faulting in the Rove Formation and the disconformably overlying late Precambrian Puckwunge sandstone and North Shore Volcanic Group; the south sides moved upward relative to the north sides. These faults and associated joints were the loci of emplacement of 'early mafic' dikes of ilmenite-bearing diabase, which probably correlate with the 'Logan intrusions'. Subsequently, east-northeastward trending olivine diabase sills and dikes were emplaced in the Rove Formation and the North Shore Volcanic Group, and they cross-cut the early mafic dikes and sills. The third event occurred after cessation of igneous activity; two sets of regional joints (N 14 °W and N 53 °E) were formed, apparently as a result of fracturing accompanying initial subsidence of the so-called 'Lake Superior syncline' to the southeast. The fourth event was the development of a zone of cataclastic rock trending N 65 °E from the Pigeon Point area northeastward at least 60 km. To the north of this fracture zone, bedding in the Rove Formation dips 15–25 °SE. Copper and silver mineralization was subsequently emplaced within this fracture zone.The first three tectonic events appear to represent reactivation of structures of early Precambrian age, and appear to control the general outcrop pattern. The fourth event is an entirely Keweenawan feature, and marks the beginning of subsidence of the 'Lake Superior syncline'. It may correlate in time with the deposition of the Copper Harbor Conglomerate, and other late middle Keweenawan events.

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