Archean Geology of an area between Knife Lake and Kekekabic Lake, eastern Vermilion district, northeastern Minnesota

Abstract

Sedimentary and volcanic rocks of the Kekekabic Lake area, which is located within the eastern Vermilion district, northeastern Minnesota, comprise a portion of the Lower Precambrian Knife Lake Group and lie in three of Gruner's (1941) structural segments. The dominant lithology within the Knife Lake greenstone segment is arkose. The arkose is largely composed of plagioclase feldspar, is approximately 250 feet thick, and underlies the Amoeba Lake member of the Knife Lake Group. Interbedded with the arkose is black slate. Rocks within the Knife Lake greenstone segment trend N 74°E and dip 60° to the southeast. The dominant lithology within the Spoon Lake segment is graywacke. The graywacke samples studied are equally divided between the lithic and feldspathic subtypes. Rock fragments within the lithic graywackes are predominantly andesite and dacite. Plagioclase is the predominant feldspar within the feldspathic graywackes. Interbedded with the graywackes are green slates, mafic (basalt or andesite) crystal tuffs, volcanogenic conglomerate, and very minor iron-formation. The rocks within the Spoon Lake segment are approximately 800 feet thick, and have been deformed into a syncline which trends S 45W and plunges 35° to the southwest. The dominant lithology within the Kekekabic Lake segment is graywacke. The graywacke samples studied are equally divided between the lithic and feldspathic subtypes. Lithic and feldspathic graywackes of the Kekekabic Lake segment are similar petrographically to lithic and feldspathic graywackes of the Spoon Lake segment. However, the graywacke samples of the Kekekabic Lake segment, in general, contain more detrital K-feldspar (although it is still a minor component) and hornblende grains than those of the Spoon Lake segment. Interbedded with the graywackes of the Kekekabic Lake segment are green slates, mafic (basalt or andesite) and felsic (trachyte to latite) crystal tuffs, and very minor ironformation. Graywackes and associated interbedded rocks of the Kekekabic Lake segment are approximately 1000 feet thick, and have been deformed into a syncline which trends S 50°W and plunges 30° to the southwest. The graywackes and associated interbedded rocks contained in both the Spoon Lake and Kekekabic Lake segments comprise the Amoeba Lake Member of the Knife Lake Group (Gruner, 1941). The eastern portion of the Kekekabic Lake syncline is occupied by three subaerial flows. The oldest of these flows, stratigraphically, is a porphyritic green augite-hornblende andesite which is exposed at the nose of the syncline and is approximately 225 feet thick. To the west, the augite-hornblende andesite is conformably overlain by a red porphyritic hornblende andesite which is approximately 300 feet thick. The red hornblende andesite is overlain conformably, to the west, by a green porphyritic hornblende basalt which is approximately 300 feet thick. The three subaerial flows apparently plunge under a green hornblende-rich tuff and agglomerate unit. The tuff is composed exclusively of hornblende grains and is bedded and cross-bedded. The agglomerate clasts are accidental lamprophyre rock fragments. The hornblende-rich tuff and agglomerate is approximately 200 feet thick. The tuff and agglomerate and the three subaerial flows comprise the Kekekabic Lake Member of the Knife Lake Group (Gruner, 1941). Turbidite sequences within the Kekekabic Lake area are characteristic of distal turbidites, and correspond to facies associated with the depositional lobe of the mid-fan portion of a submarine fan (Walker and Mutti, 1973). Two periods of deformation have occurred in the Kekekabic Lake area along with broad folding and longitudinal and transverse faulting. The first period of deformation produced isoclinal folds, trending S 45°-50°W, with vertical to overturned fold axes that plunge to the southwest. The second period of deformation produced a pervasive N 62°-70°E cleavage throughout the area. Subsequently, broad folding warped the beds of the eastern Vermilion district on an axis trending N 60°W. Following folding, longitudinal faulting divided the Kekekabic Lake area into three distinct segments. Concurrent with or subsequent to longitudinal faulting, transverse faulting locally offset rock contacts. Sedimentary and volcanic rocks of the Kekekabic Lake area are representative of the middle portion of a calc-alkaline basalt-andesite-rhyolite volcanic pile accumulation which presumably developed within an island arc or continental orogenic system.