Interflow sedimentary rocks occur as lenticular bodies between, and crevice fillings within lavas of the Keweenawan North Shore Volcanic Group. These rocks consist of reddish, fine-grained, well-sorted, arkosic sandstones, with lesser amounts of conglomerate, breccia, shale, and tuff. They were deposited primarily by streams flowing across lava surfaces. This petrologic, sedimentological, and structural study of the interflow sediments investigates the relationship between the North Shore Volcanic Group and the surrounding borderland during Keweenawan rifting. In addition, evidence for tectonic relationships within the Keweenawan basin is considered. The combined thickness of 380m of interflow sedimentary rock is lithologically divided into conglomerates, volcanic and flow-top breccias, shale, elastic crevice fillings, and sandstone sequences which may contain any of the previous as thin intercalated or basal units. Individual elastic units vary in thickness from a few centimeters to 75m. Sedimentary structures include tabular and trough cross-bedding, planar bedding, ripple marks, and desiccation cracks. In coarse-grained deposits, grading and clast orientation or imbrication are common. Most interflow sandstones are lithic arkose or feldspathic lithic arenite. The major framework constituents are plagioclase, mafic to felsic volcanic rock fragments, and opaque minerals. In addition, quartz, pyroxene, and agate, chert, and shale fragments occur in minor percentages. Heavy minerals include pyroxene, altered olivine, apatite, zircon, and sphene. Opaque and non-opaque heavy minerals combined average 15% of the sample volumes analyzed. Burial of the sediments beneath 2000-12,000m of younger volcanic and elastic material produced a variety of metamorphic effects. Feldspars and rock fragments are commonly replaced by, or cemented with zeolite, calcite, prehnite, epidote, chlorite, hematite, and/or quartz. Four zones of burial metamorphic assemblages are delineated for sedimentary rocks, from minerals indicating low greenschist facies in most deeply buried sediments to zeolite facies minerals in higher level interflows. Sedimentological and paleocurrent data indicate that most sediments were deposited in a fluvial environment, and that streams flowed toward the present Lake Superior basin. Paleocontours constructed from these data show that deposition probably occurred in a northeast-trending, two-part basin. Stratigraphic and areal variations in sediment thicknesses and environments of deposition give evidence for the two-part basin concept. Sediments are thicker and generally reflect steeper stream gradients in younger deposits in the southwestern portion of the basin. In the northeastern portion, low energy fluvial and fluvial-lacustrine deposits indicate that deposition occurred on a relatively flat terrane. The area between the two parts of the basin is occupied by sediments of vent-type volcanic and alluvial fan origin, suggesting that intermittent uplift occurred at some time during the construction of the flow-interflow sequence. Although the predominant source for sediments was the Middle Keweenawan lavas, some Middle Precambrian, Archean, and Lower Keweenawan rocks contributed minor detritus. Most of the stratigraphic and lateral variations in petrology reflect differences in lava conposition. Extrabasinally derived constituents, including chert, microcline, rutile, sphene, and some slate, are most abundant in lower level interflow sediments near the present extremities of the Keweenawan exposures. This suggests that less extensive volcanic cover existed on pre-Keweenawan rocks during earlier episodes of sedimentary deposition. Clastic dike orientations were analysed in an attempt to determine tectonic stress directions at the time of deposition. The majority of the dikes are oriented northwesterly, and a less well-developed northeasterly trend is apparent. These trends are not inconsistent with the tensional (rifting) hypothesis. The sedimentologic, petrologic, structural, and volcanologic data are integrated to produce one sequential depositional-tectonic model. This model depicts deposition in a two-part depocenter. The southwestern portion, occupied by most strata of the southwest limb, subsided relatively rapidly; the northeastern portion appears to have subsided only gently during sedimentary deposition. During deposition of some of the younger interflow sediments, the area between the two parts of the basin had high relief and may have been a site of periodic uplift and explosive volcanism. This volcanic center may have collapsed to provide a depression into which the youngest flows on the North Shore were deposited.
A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Mark Alan Jirsa in partial fulfillment of the requirements for the degree of Master of Science, February 1980.
Jirsa, Mark Alan.
The Petrology and Tectonic Significance of Interflow Sediments in the Keweenawan North Shore Volcanic Group of Northeastern Minnesota.
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