The lower Proterozoic Virginia Formation, a thick sequence of argillite,
siltstone, and graywacke, forms the top of the Animikie Group on the
Mesabi range in northern Minnesota. It is overlain by thick Quaternary
deposits, but is exposed locally where the underlying Biwabik Iron Formation
has been mined. Drill cores available from four sites provide stratigraphic
and sedimentologic data, and the 450 m of core from a site south of Biwabik
and 342 m of core from south of Calumet are proposed as alternate reference
sections to the original type locality. The Virginia Formation can be
divided into two informal members--a lower argillaceous lithosome and an
upper silty and sandy lithosome. The argillaceous lithosome thickens toward
the west end of the Mesabi range where it also contains appreciable quanti
ties of chert and a discrete unit of cherty, sideri tic iron-formation.
The upper silty and sandy lithosome also consists predominantly of argillite,
but is characterized by intercalated beds of siltstone and finegrained
graywacke. Mineralogic and sedimentologic data indicate that the
graywackes were derived largely from granitic and low-grade metasedimentary
and metavolcanic rocks like those now exposed in the Archean greenstonegranite
north of the Mesabi range, and that they were deposited by turbidity
currents on a southward-dipping paleoslope into a basin south of the range.
In detail, the sedimentologic attributes of the formation resemble "thickening-
and coarsening-upward" turbidite sequences associated with the lower
and mid-fan parts of a prograding submarine fan complex.
Lucente, M.E.; Morey, G.B..
RI-28 Stratigraphy and Sedimentology of the Lower Proterozoic Virginia Formation, Northern Minnesota.
Minnesota Geological Survey.
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