| Summary: | The system of stratigraphic nomenclature used previously for the Lower Precambrian rocks in the western part of the Vermilion district, northeastern Minnesota, is replaced by a formal nomenclature based on increased data gained from recent geologic mapping. The resulting changes in stratigraphic nomenclature are the following: 1. Previously recognized lithostratigraphic units -- the Ely Greenstone, Soudan Iron-formation, and Knife Lake Group -- are redefined and restricted in usage. 2. The Lake Vermilion Formation is formally established for rocks exposed in the vicinity of Lake Vermilion that were previously considered part of the Knife Lake Group. Four informal lithologic members are recognized in the Lake Vermilion Formation. These include a metagraywacke-slate member, a feldspathic quartzite member, a volcaniclastic member, and a mixed metagraywacke-felsic conglomerate member. Each is characterized by dominant and distinctive lithologies and may contain several mappable rock units, such as iron-formation and pillowed metabasalt, that can be recognized and delineated on the ground, but whose geographic extent and/or stratigraphic relationships are not known completely. 3. A second unit -- the Newton Lake Formation, a metavolcanic formation -- also is formally recognized. It is inferred to stratigraphically overlie rocks assigned to the Knife Lake Group and to consist of two informal lithologic members, a dominantly mafic volcanic member and a dominantly felsic-intermediate volcanic member. The two members interfinger in the Vicinity of the type locality at Newton Lake. 4. A variety of hypabyssal intrusive rocks are intimately associated with all the volcanic and sedimentary rocks in the Vermilion district. The hypabyssal rocks were emplaced over an interval of time as a consequence of the igneous cycle in the district. Accordingly the time term "Laurentian" should no longer be applied to these rocks in the Vermilion district. 5. The recognition that (1) mafic volcanism was not confined to a single period and (2) that a major unconformity separating an effusive volcanic episode ("Ely Greenstone" of the older literature) from a younger sedimentary series ("Knife Lake Group" of the older literature) is lacking raises serious doubts about the validity ofregionaJ correlations previously accepted in northern Minnesota. Accordingly, it seems necessary to abandon the terms "Keewatin" and "Coutchiching" as time-stratigraphic units for strata of Early Precambrian age in northern Minnesota.
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