The Geology of the Middle Precambrian Thomson Formation iIn Southern Carlton County, East-Central Minnesota

A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Marc Robert Connolly in partial fulfillment of the requirements for the degree of Master of Science, December 1981. There is 1 supplementary file also attached to this record, which contains Plate 1 referenced in the thesis.In southern Carlton County, east-central Minnesota, the Thomson Formation provides evidence for major deformational and metamorphic events. These events were associated with the Penokean Orogeny which terminated, or closely followed, Middle Precambrian sedimentation in the Animikie Basin approximately 1870 m.y. ago (Morey and Lively, 1980). The following eight Thomson Formation rock types were found in the study area: metasiltstone, phyllite, calcareous metasiltstone and phyllite, graphitic phyllite, volcaniclastic metasiltstone and phyllite, and metabasalt. Stratigraphic relationships and thickness estimates of these units were indeterminable due to lack of stratigraphic control, poor and discontinuous exposures, and complex regional folding. The phyllitic rock types were deposited slowly as shales in quiet water below wave base. Deposition of this material was intermittently interrupted by deposition of silt beds (since metamorphosed to metasiltstones) by southward flowing distal turbidity currents. Local calcareous silts and limy muds were later metamorphosed to calcareous metasiltstones and phyllites, respectively. The graphitic phyllites probably developed from organic-rich muds. Also, hypabyssal basaltic dikes and/or sills (since metamorphosed to metabasalts) occur locally and may have been a source rock for detritus for the volcaniclastic sediments found nearby. Volcanic vents associated with these intrusives may also have been a source of pyroclastic detritus for the volcaniclastic units. Three Penokean deformations and their resulting structures have been recognized in the study area. During the first Penokean deformation, beds of all Thomson Formation units were folded into overturned or recumbent isoclinal folds. These folds have axes that plunge at low angles to the east and west, and axial planes that are horizontal or dip at low angles (up to 40 degrees) to the south. A regional, continuous schistosity developed during this deformation as an axialplanar foliation to these isoclinal folds. As a result, the schistosity is found parallel to bedding at most (but not all) outcrops. Also associated with this event was the development of oblate quartz boudins which lie in the plane of the axial-planar schistosity. During the second Penokean deformation, bedding and the essentially bedding-parallel schistosity of the first deformation were together folded into two distinct and contemporaneous fold geometries. Folds of one geometry are tight and asymmetrical or overturned. These folds have axes that plunge at low angles to the east and west, and have axial planes that dip 10 to 40 degrees to the south. Folds of the other geometry are open and symmetrical or asymmetrical. These folds also have axes that plunge at low angles to the east and west, but their axial planes are approximately vertical. A discontinuous crenulation cleavage developed during this deformation as an axialplanar foliation to folds of both geometries. The third and final Penokean deformation was minor relative to the first two. No folds developed during the third deformation, but a nearly vertical crenulation cleavage that strikes north-south, mineral lineations that plunge at low angles to the south, and four joint sets were produced. All Thomson Formation units in the study area have been regionally metamorphosed. The grade at culmination of metamorphism was uppermost greenschist/lowermost amphibolite facies (garnet zone). Regional metamorphism and tectonism were contemporaneous events. Metamorphic culmination was reached before, and continued during and after, the first Penokean deformation. Metamorphism then continued during the second and third Penokean deformations at relatively lower greenschist facies grades