The Structural Geology of the Thomson Formation: Cloquet and Esko Quadrangles, East-Central Minnesota
1985-05
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The Structural Geology of the Thomson Formation: Cloquet and Esko Quadrangles, East-Central Minnesota
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1985-05
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Abstract
The Thomson Formation at the type locality in northeastern Carlton County is composed of a thick sequence of interbedded metagreywacke, slaty greywacke (met as iltstone) and slate. Minor amounts of phyllite and hornfels as well as numerous diabase dikes are also present in the study area. Despite the lower greenschist-facies regional metamorphism, primary sedimentary structures are very well preserved. Bedding generally strikes within 10 degrees of east-west and dips moderately to the north or more commonly to the south. Sedimentary features such as load structures, convolute bedding, small-scale cross-bedding, and graded bedding are excellently preserved at many locations. Sedimentological studies performed previously have noted a dominant north-south paleocurrent trend with a subordinate east-west paleocurrent trend and a northerly source area for the sediments. This is compatible with deposition in a deep-water basin with the coarser beds being deposited by southward-flowing turbidity currents and the muddy units slowly accumulating during the periods between turbidity currents. The entire study area has been affected by at least one period of major folding that formed generally open, upright, subhorizontal, symmetric or asymmetric, east-west trending folds that contain an axial-planar cleavage. A boundary between one Penokean deformation to the north and two Penokean deformations to the south extends across the southern portion of the study area with multiple deformation south of the boundary being indicated by the presence of a crenulation cleavage. Cleavage is very well developed in most of the Thomson Formation and ranges from a fine continuous cleavage in the slate to a coarser disjunctive or rough cleavage in the slaty greywacke. Three varieties of rough cleavage are seen in the study rocks: type A (with poorly-developed mica seams), type B (with well-developed mica seams) and type C (with only isolated zones of well-developed mica seams). Microscopic evidence such as shape-modified quartz grains suggests that the rough cleavage seen here probably formed by a pressure solution mechanism whereby quartz was preferentially dissolved allowing the phyllosilicates to be passively concentrated into mica seams around the detrital grains. A crenulation cleavage that ranges from a zonal to a discrete crenulation cleavage is present at several localities. The microscopic examination of the cleavage zones indicates that the phyllosilicates were passively concentrated into the cleavage zones as deformation proceeded. This is compatible with a pressure solution origin for the crenulation cleavage. Other effects of pressure solution are seen at several places elsewhere in the study rocks. Kink-bands are common at the type locality at Thomson Dam. Several models have been proposed to explain their origin; however, the kink-bands in this area have characteristics that seem to fit a combination of the rotation model and the joint-drag model of kinking. The Thomson kink-bands formed late in the structural history of the area and are probably the result of Keweenawan activity. Other features presumably related to Keweenawan events such as several cases of unusual bedding-cleavage relationships are seen throughout the study area. Intracratonic models and plate-tectonic models have been proposed for the Penokean orogeny in east-central Minnesota. The intracratonic models suggest that Penokean deformation was the result of vertical remobilization of basement rocks along the boundary between Archean gneiss and granite-greenstone terranes. Several converging plate models with south and/or north-dipping subduction zones have been proposed for the Penokean orogeny although several features of modern plate-tectonics are absent from the Proterozoic rocks. The recent identification of probable nappes in the Thomson Formation provides considerable support to the idea that plate-tectonics was responsible for the Penokean orogeny.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Richard C. Clark in partial fulfillment of the requirements for the degree of Master of Science, May 1985. There are 2 supplementary files also attached to this record, which contain Plates 1-2 referenced in the thesis.
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