3: Status of the Keweenawan as a stratigraphic unit in the Lake Superior region
G. B. Morey, John C. Green, 1982. "3: Status of the Keweenawan as a stratigraphic unit in the Lake Superior region", Geology and Tectonics of the Lake Superior Basin, Richard J. Wold, William J. Hinze
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The term “Keweenawan” has been used for over 100 years to designate a suite of dominantly mafic igneous and derivative sedimentary rocks that crop out around the shores of Lake Superior. Although the rocks have been extensively studied and described, there is no generally accepted stratigraphic definition of the term in the geologic literature. It has been used both as a lithostratigraphic and as a chronostratigraphic unit.
This dual usage has resulted in confusion and consequent recommendations from time to time that the term be abandoned completely, but there is a growing consensus that the term “Keweenawan” best describes a lithostratigraphic package having supergroup status. As a chronostratigraphic term, however, no “Keweenawan Series” that accords with accepted stratigraphic procedures can be defined, because the rocks themselves are bounded by non-isochronous surfaces.
Since 1911, the Keweenawan has been subdivided into the lower, middle and upper Keweenawan. These subdivisions, as first proposed by Van Hise and Leith (1911), are not chronostrati-graphic units, but rather are rock units with either time-transgressive boundaries or boundaries that cannot be traced for any appreciable distance. Recent suggested revisions in these lithostratigraphic boundaries, intended to make the boundaries more useful and more nearly isochronous, have had wide acceptance in the United States.
The Keweenawan also has been subdivided into three paleomagnetic polarization sequences. Although several of the sequence boundaries coincide with unconformities representing hiatuses of unknown duration, the sequences have considerable value in intraregional correlations of rock units around Lake Superior. Furthermore, paleomagnetic pole positions associated with the polarization sequences should be recognizable worldwide and therefore should make excellent reference points for long-distance correlation. However, existing nomenclatural schemes using names such as lower Keweenawan and middle Keweenawan to designate paleomagnetic polarization sequences should be abandoned. These names can be confused with traditional lithostratigraphic terminology and should therefore be replaced by names that accord with accepted magnetostratigraphic procedures.