The Thomson Formation is a graywacke-slate turbidite sequence of Middle Precambrian age (Goldich, 1968) which crops out sporadically throughout eastcentral Minnesota. At the type locality near Thomson, Minnesota, the formation consists of 34 percent graywacke, 39 percent siltstone, and 27 percent slate (Morey and Ojakangas, 1970). Although mostly graywacke and slate, cherty units, graphitic slate and andesitic porphyry (?) also crop out within the area of study. The graywackes in the study area are quite finegrained and are texturally immature although mineralogically submature due to the high percentage of detrital quartz present. Quartz makes up an average of 49 percent of detrital grains and plagioclase 9 percent. Matrix amounts range from 29 to 56 percent. Compositionally the sediments plot as feldspathic graywackes (Pettijohn, Potter and Siever, 1973). Primary sedimentary structures found within graywacke beds suggest transport and deposition of sediments were mostly by turbidity currents. Crossbeds show that the paleocurrents flowed in a southeasterly direction probably perpendicular to the inferred paleoshoreline to the north. Evidence for reworking bottom sediments, possibly by longshore currents, was also found. Petrographic and paleocurrent evidence seem to indicate that most sediments were derived from Lower Middle Precambrian granites and granodiorites to the north. The Thomson Formation was deformed and metamorphosed during the Penokean Orogeny 1.85 b.y. ago (Goldich, 1968). Structural analysis indicates two periods of deformation, both probably associated with the Penokean Orogeny. The first deformation (D1) resulted in large scale northward-facing east-west (?) trending isoclinal-recumbant folds with axial planar s1 cleavage. This deformation may have been caused by a northward-dipping Middle Precambrian subduction zone located to the south as proposed by Van Schmus (1976). Subduction to the south possibly compressed the backarc (Animikie) basin. Downwarp of the back-arc area may have produced overturned and recumbant folds as sediments were moved northward by gravity tectonics into the back-arc basin away from the rising orogenic belt at the convergent plate margin area. The D2 deformation was probably a result of further compression of the back-arc basin. The timing of this second event is unclear. The D2 deformation resulted in open to tight, asymmetric to symmetric east-west trending F2 folds of varying wavelengths. Associated with these folds are east-west trending s2 axial planar crenulation cleavage and crenulation lineations. A uranium study of this area of the Thomson Formation indicated two genetically different types of uranium occurrences. The "Arrowhead Mine" locality appears to be related to the metamorphic-hydrothermal type of unconformity-vein uranium model. The Section 4 uranium occurrence appears to be syngenetic uranium within apatite grains. Neither occurrence found was in economic quantity.
A thesis submitted to the faculty of the Graduate School of the University of Minnesota by Gerald Lee Hyrkas in partial fulfillment of the requirements for the degree of Master of Science, May 1982.
Hyrkas, Gerald Lee.
The Sedimentology and Structural Geology of the Middle Precambrian Thomson Formation, Central Carlton County, Minnesota.
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