Geology of the Cypress, Hanson and South Arm of Knife Lake Area, Boundary Waters Canoe Area: Eastern Vermilion District, Northeastern Minnesota

Abstract

Archean volcanic and volcanogenic sedimentary rocks of the Cypress, Hanson and South Arm of Knife Lake Area are located within the eastern Vermilion district and lie in three of Gruner's (1941) structural segments. The dominant lithology within the Spoon Lake segment is dacite porphyry conglomerate, derived from a small lens of dacite porphyry. The dacite porphyry conglomerate is composed of dacite porphyry detritus and is interbedded with sequences of greywacke-argillite. The graywacke is of both the feldspathic and lithic type. Two small outcrops of greenstone, one of which is pillowed, occur within the segment and may be fault slices. Keweenawan diabasic dikes intrude both the igneous and the sedimentary rocks. Bedding within the sedimentary rocks indicates that the rocks within the Spoon Lake segment strike N 45°E and dip steeply to the southeast and northwest. The dominant lithology within the Knife Lake Greenstone segment is greenstone which is texturally and compositionally similar to the Ely Greenstone. Overlying the greenstone and containing abundant clasts of greenstone, is a greenstone pebble conglomerate. This is a thin unit and grades laterally into graywacke-argillite. Bedding within the graywacke-argillite sequences indicates the rocks within the Knife Lake Greenstone segment strike N 45°E and dip steeply to the southeast and northwest. The rocks within the Knife Lake Synclinorium segment are divided into two units; a tuff-mafic conglomerate-mixed conglomerate unit, and a younger graywacke unit. These two units are interbedded and gradational into one another. The graywacke of the Knife Lake Synclinorium segment is similar to the graywacke of the Spoon Lake segment and the Knife Lake Greenstone segment, but can be distinguished from them by a greater diversity of volcanic rock fragments. One Keweenawan diabasic dike is present in the segment. The rocks within this segment strike predominantly northeast and dip steeply to the northwest. The northern part of the segment is part of a large overturned syncline, the axis of which trends N 45°E and plunges 30° to the northeast. The southern part of the segment lies on the limbs of folds whose axes trend N 25°E and plunges 75° to the northeast. Longitudinal faulting has removed the axes of the folds from this part of the segment. Turbidite sequences within the Spoon Lake segment and the Knife Lake Greenstone segment are characteristic turbidites corresponding to the depositional lobe of the inner to middle portion of a submarine fan. Turbidite sequences within the Knife Lake Synclinorium segment are characteristic of proximal turbidites, and correspond to facies associated with the inner fan of the slope-fan-basin system of a turbidite basin. The structural information obtained from the Knife Lake Greens tone segment and the Spoon Lake segment is minimal, and few interpretations could be made. The structure of the Knife Lake Synclinorium segment however, can be accounted for in terms of three tectonic deformations. The first period of deformation produced isoclinal folds, which trend N 40° to 50°E and plunge 30° to the northeast. The second period of deformation produced a N 54° to 62° W cleavage throughout the segment. The third period of deformation occurred on a regional scale and produced major longitudinal faults which has divided the present area of study into discrete structural blocks. Transverse faulting of smaller dimensions transect the trend of the longitudinal faults and may have formed as a consequence of movements on the longitudinal faults. The volcanic and sedimentary rocks of the Cypress, Hanson and South Arm of Knife Lake Area belong to the basalt-andesiterhyolite association found in all of the greenstone belts of the Canadian Shield, which is also typical of continental orogenic belts or island arc systems.