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
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
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
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
Morey, G.B.; Green, J.C.; Ojakangas, R.W.; Sims, P.K..
RI-14 Stratigraphy of the Lower Precambrian Rocks in the Vermilion District, Northeastern Minnesota.
Minnesota Geological Survey.
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