RI-13 Sedimentology of the Middle Precambrian Thomson Formation, East-Central Minnesota

Abstract

The Thomson Formation is exposed in parts of Carlton, Pine, and southern St. Louis Counties of east-central Minnesota. The formation was folded and metamorphosed during the Penokean orogeny 1,700 million years ago, but primary sedimentary textures and structures are well-preserved in the Cloquet-Carlton area. The formation is characterized by intercalated slate, siltstone, and graywacke. In two measured sections at the type locality, graywacke comprises 34 percent, siltstone 35-43 percent, and slate 23-31 percent of each section. Most beds are less than one foot thick. Because of abundant graded beds, lateral continuity of individual beds, well-defined internal structures common to turbidite sequences and consistent directional structures, the graywacke and siltstone beds are interpreted as individual sedimentation units, apparently deposited by waning, sediment-laden turbidity currents. An analysis of cross-bedding suggests that much of the sediment was deposited by southward-flowing currents moving down a regional paleoslope. However, the presence of flute and groove casts which trend eastward and westward implies that some currents probably flowed parallel to the strike of the inferred paleoslope. X-ray and thin section studies reveal that the graywackes are composed of 4-35 percent quartz, 2-28 percent feldspar, 1-10 percent rock fragments, 15-85 percent matrix material consisting of muscovite, chlorite, and quartz, and 1-17 percent calcite. Mineralogically, the siltstones are fine-grained equivalents of the graywackes. The correlation of the Thomson Formation with other similar rocks in the Lake Superior region has been debated since Irving in 1883 first suggested a Middle Precambrian age, but the formation's physical isolation has left correlations in doubt. The marked similarity of the mineralogic and sedimentologic aspects of the Thomson Formation with those observed in the Middle Precambrian Rove Formation shows they were derived from a similar source terrane and were deposited by similar mechanisms. This, coupled with paleogeographic data, strongly suggests that they can indeed be correlated with each other.