RI-30 Interflow Sedimentary Rocks in the Keweenawan North Shore Volcanic Group, Northeastern Minnesota

Abstract

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.