Geologic Map Series 3. Geology of the Cloquet Quadrangle Carlton County, Minnesota

Abstract

The Cloquet quadrangle is bisected by the St. Louis River, a major stream that flows into the western end of Lake Superior. Precambrian rocks are extensively exposed in the river valley where overlying Pleistocene glacial deposits have been eroded. The Precambrian rocks are dominated by the Thomson Formation, which consists of interbedded slates, slaty graywackes, and graywackes. Small-scale cross-bedding, graded bedding, flute casts, load casts, clastic dikes, and other primary and penecontemporaneous structures are common, as are calcareous and siliceous concretions. The formation probably was deposited in a relatively deep-water basin, in part by turbidity currents. It has only one conspicuous marker bed, the Otter Creek unit, so the stratigraphic thickness across the intricate folds of the region is difficult to determine, but it probably is about 20,000 feet. Normal and reverse faults with displacements of a few tens of feet are common, as are steeply dipping conjugate joints of northwest and northeast trends. Cleavage is well developed in slaty units. The Thomson Formation is correlated with the Animikie Group. Abundant microgabbro dikes were intruded during subsidence of the Lake Superior syncline. The Pleistocene glacial history was marked by three phases of advance and retreat of the Superior Lobe, preceded by a phase of the Rainy Lobe. Drumlins, moraines, outwash plains, eskers, lake plains, and diversion channels constitute distinctive landforms. During final withdrawal of the Superior Lobe from the area, the St. Louis River, which carried the outflow from Glacial Lake Upham, was diverted to form prominent erosional channels leading to the St. Croix River at progressively lower elevations, until it finally flowed into the proglacial Lake Nemadji, whose outlet formed a final channel in the sequence.