Petrology and Sedimentation of Early Precambrian Graywackes in the Eastern Vermilion District, Northeastern Minnesota

A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Mark John Severson in partial fulfillment of the requirements for the degree of Master of Science, August 1978. Plate I referenced in the thesis is also attached to this record.Archean metasedimentary rocks of the eastern Vermilion district consist primarily of "volcanigenic" graywackes, siltstones, slates, and conglomerates with minor interbeds of reworked tuffs and iron-formations. Most of the metasediments indicate a dominantly volcanic source area consisting of basalt-andesite-rhyolite piles typical of modern continental orogenic belts or island arc systems. However, clasts and detritus of Saganaga tonalite indicate that it was also an important source rock. The Saganaga batholith, located at the eastern terminus of the Vermilion district, may be compared with more recent batholiths which have been described as intruding and unroofing their own volcanic ejecta. Bouma sequences and other sedimentary structures typical of turbidites are common in the graywackes indicating deposition in a deep water basin or trough. Detritus in the graywackes and conglomerates probably originated as temporary accumulations on the slopes of volcanic piles. Periodic slumpage of the accumulations generated turbidity currents which transported the detritus to submarine fans. The graywackes were deposited as overbank spills while the coarser material was confined to long ·sinuous channels forming lerises of conglomerate. Each congiomerate unit crudely represents, from bottom to top, coarser conglomerate beds grading upward into finer conglomerate beds indicating gradual channel abandonment and a migration of channels within the fan system. The transport direction was to the southwest along the present tectonic strike, away from the Saganaga batholith with most of the supply being at the northeastern end of an elongate basin or trough. Slates and siltstones represent the background sediment of the basin but their deposition was repeatedly blotted out by the arrival of short-lived turbidity currents. The present structural pattern appears to have resulted from a combination of soft sediment deformation, at least two periods of tectonic folding along northeast and northwest axes, and late phase faulting. Folds, resulting from downslope soft sediment slump movements, range from a few centimeters to over a meter across and are varied in their style and attitude, often becoming chaotic. The major folds, defined by reversals in top· directions, were formed during the first period of tectonic deformation and trend northeast with steep axial planes and near-horizontal plunges. These folds appear to have been deformed by a later tectonic event. The later deformation formed minor northwest-trending folds with steep axial planes and plunges. Late phase faulting occurred on a regional scale deforming local areas of all rock bodies and divided the district into several separate segments. Deformation of the volcanic-sedimentary belt is attributed to Algoman tectonism, 2.7 b.y. ago. Since the Saganaga batholith appears to have been emplaced slightly earlier than the other Algoman granites of the Vermilion district, it may have acted as a buttress against which the younger sediments were folded, as suggested by Gruner (1941)