Interflow sedimentary rocks occur as lenticular units and crevice fillings
between and within lavas of the North Shore Volcanic Group (Keweenawan
Supergroup, middle Proterozoic age) of northern Minnesota. Individual sedimentary
units range in thickness from a few centimeters to 75 m. They consist
of reddish, fine-grained, well-sorted sandstone, and lesser amounts of
conglomerate, breccia, shale, and tuff.
Much of the sandstone is either lithic arkose or feldspathic lithic
arenite. Major framework constituents include calcic plagioclase, mafic to
felsic volcanic rock fragments and pyroxene. Fragments of agate, chert, and
shale are present in minor amounts. Heavy minerals include magnetite, pyroxene,
apatite, altered olivine, zircon, and sphene. The major cement and
replacement minerals are zeolites, calcite, quartz, chlorite, epidote,
prehnite, and hematite. The distribution of secondary minerals is zonal and
presumably related to burial metamorphic processes.
Although the predominant source for the sedimentary rocks was the intercalated
lava flows themselves, some Archean, lower Proterozoic, and older
Keweenawan rocks contributed minor amounts of detritus. Detritus derived
from pre-Keweenawan rocks is most abundant in the lower interflow units near
the present extremities of Keweenawan exposures. This implies that the
volcanic rocks may not have extended very far past the present outcrop limits
during the early part of sedimentary deposition.
Sedimentary structures and paleocurrent measurements indicate that most
units were deposited by streams that flowed generally toward the present
Lake Superior basin. Paleocontours and stratigraphic and areal variations
in rock type imply that some of the deposition occurred in two northeast trending
sub-basins, probably separated by a basement high.
Jirsa, M.E., 1984, Interflow Sedimentary Rocks in the Keweenawan North Shore Volcanic Group, Northeastern Minnesota, Minnesota: Minnesota Geological Survey Report of Investigations 30, 20 p.
Jirsa, Mark A..
RI-30 Interflow Sedimentary Rocks in the Keweenawan North Shore Volcanic Group, Northeastern Minnesota.
Minnesota Geological Survey.
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