The Geology and Sedimentology of the Archean Metasedimentary Rocks of the Virginia Horn Area, Northeastern Minnesota
1991-03
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The Geology and Sedimentology of the Archean Metasedimentary Rocks of the Virginia Horn Area, Northeastern Minnesota
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1991-03
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Abstract
The Virginia horn area contains Archean metasedimentary rocks of two types, interbedded graywacke and slate, and conglomerate. The rocks occur in three vertically dipping lithologic units that strike northeast, with two relatively thick graywacke/slate units on either side of a thinner conglomerate unit. All of the rocks are regionally metamorphosed to the greenschist facies. The area has been affected locally by intense sericite-carbonate alteration, as well as widespread folding and faulting on all scales. The stratigraphy has been largely destroyed through the metamorphic and structural activity, but some smaller-scale features are preserved. The intense alteration, present primarily in the northern part of the area, contains significant auriferous mineralization. The structural features include faults, shear zones, regional foliation, mylonitic zones, isoclinal (vertical) folds, (horizontal) open folds, sheath folds, and kink bands. These features are assigned to four successive stages of deformation, indicating be D1 to D4 Isoclinal folding and regional foliation belong to D1; shear zones, mylonites, and sheath folds are associated with D2; horizontal, open folds resulted from D3; and kink bands and faults belong to D4. The four different deformations represent only an observed temporal relationship between the features; completely separate deformational events are not implied. The northern and southern rock units are made of intercalated graded graywacke and slate. The thin central unit is composed of conglomerate. The graywacke units display Bouma-type sequences, with Bouma "A", "AB", and "E" beds present. The conglomerate unit contains predominantly polymict, clast-supported, thickly bedded conglomerates, with normal, inverse-to-normal, and disorganized-type grading in beds. Matrix-supported beds are common in the eastern part of the study area. There are some monomict, matrix-supported beds, with green, andesitic clasts and matrix; these may have been deposited as lahars. The three units are petrographically very similar, aside from grain-size. They are volcanogenic, with a preponderance of resedimented, felsic volcanic detritus. There are lesser contributions from other volcanic rock types, and very minor contributions from non-volcanic rock types. The source area for the sediments was probably a felsic volcanic center or series of centers, with some associated sedimentary rocks also uplifted and eroded. There is no evidence for any significant pre-existing continental crust. The site of deposition for the three units was probably a submarine fan complex. The dominance of Bouma "A" and "AB" beds indicates a proximal position, probably in and near a channel. The nature of the conglomerates further supports this model, because they are predominantly thickly bedded, and show disorganized-bed textures and inverse-to-normal grading. The apparent lateral change from disorganized beds in the east to predominantly normally and inverse-to-normally graded beds in the west may indicate a facies change westward from a source area located somewhere to the east.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Eric Ramon Levy in partial fulfillment of the requirements for the degree of Master of Science, March 1991. There is 1 supplementary file also attached to this record, which contains Plate 1 referenced in the thesis.
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