The Stratigraphy and Lithology of the Glaciogenic Sediments of the Two Harbors Area, Northeastern Minnesota
1982-12
Title
The Stratigraphy and Lithology of the Glaciogenic Sediments of the Two Harbors Area, Northeastern Minnesota
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1982-12
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Abstract
Five texturally distinct sedimentary units are differentiated in the Quaternary deposits of the Two Harbors and Whyte quadrangles in Lake County, northeastern Minnesota. Their specific properties are attributed to differing (1) bedrock sources, (2) conditions of glacier flow, and (3) methods and environments of deposition. These glacially derived sediments, including a clay-rich diamicton, record the advance, stagnation, and retreat of the Rainy and Superior Lobes, two appendages of the Laurentide Ice Sheet, sometime after 30,000 years B.P. The dominant surficial deposit of the northwestern half of the Whyte quadrangle is a gray to brown, sandy to stony till, the Sullivan Lake Formation. The average sand:silt:clay ratio is 76:21:03. The clasts and surface boulders are predominantly granophyre, granite, greenstone, basalt, and gabbro-diabase. The source of sediment for this formation is the underlying Duluth Complex and other Precambrian igneous and metamorphic sources cropping out to the north and northeast. The Sullivan Lake Formation was deposited by an actively moving, wet-based glacier. For the most part, it was deposited in the basal zone as a lodgement till. This till is attributed to the Rainy Lobe advance from the northeast about 20,500 ± 400 years B.P. (Wright, et al., 1973). Geomorphic features attributed to this ice advance are the northeast- southwest trending Toimi drumlins, small eskers, and a tunnel valley, partially occupied by Sullivan Lake. The glaciogenic sediments that comprise the southeastern half of the Whyte quadrangle and the northern part of the Two Harbors quadrangle (the Cromwell Formation) are markedly different from the Sullivan Lake Formation. They are a distinct reddish brown in color and rich in rock fragments eroded from the late Precambrian North Shore Volcanic Group (basalt and rhyolite) and "red" sandstone of Keweenawan age from the Lake Superior basin to the east. The Cromwell Formation resulted from the advance, stagnation, and retreat of the Superior Lobe approximately 20,000 to 16,000 years B.P. (Wright, et al., 1973), as it moved westward and southwestward out of the Lake Superior basin. Supraglacially derived debris of the Cromwell Formation is mainly contained within the Highland Moraine. It is texturally quite variable with an average sand:silt:clay ratio of 66:30:04. Topographically, the debris forms a hummocky kettle and kame terrane. A variety of depositional processes were active in the terminal zone of the glacier. These include melt-out, as well as the secondary processes of sediment gravity flow, slump, and sheet and rillwater flow. Subglacially derived debris, exposed southeast of the Highland Moraine, is differentiated from the supraglacial debris by a finer texture and a fluted topographic expression. It has an average sand:silt:clay ratio of 57:32:11. The presence of the Highland flutes suggests deposition by lodgement processes from an actively flowing, wet-based glacier. A clay-rich (loam) facies of the Cromwell Formation occurs in an isolated area southward from the flutes. It has an average sand:silt:clay ratio of 40:39:21. Overriding of proglacial lake deposits and incorporation of fine-textured sediment into the basal zone could explain this textural change. Sediment supplied from stagnant ice sources and meltwater from the retreating Superior Lobe formed large deposits of outwash in the form of fans, braided stream deposits, deltaic deposits, and resedimented deposits of till. They occur randomly on the till surface, and along a northeast-southwest trending zone at about 350 to 366 m (1150 to 1200 feet). The stratified sand and gravel deposits along this zone are interpreted as deltas and other shoreline features of Glacial Lake Duluth and its immediate predecessors. The Wrenshall Formation, a clay-rich diamicton in the southeastern half of the Two Harbors quadrangle, forms another texturally distinct unit. The clay is reddish brown with an average sand:silt:clay ratio of 14:16:70. It is typically massive, jointed, and locally laminated. It ranges from clast-rich to clast-poor and commonly occurs in association with sand and silt sequences. Distinguishing between a subglacially deposited clayey till and a lacustrine clay in the Lake Superior region is problematical. This clay- rich unit is differentiated from the upland glacial deposits by its fine texture, sedimentary structures, the presence of ice-rafted material, and its restricted occurrence below the 350 to 366 m (1150 to 1200 feet) high level strandlines. This suggests deposition from suspension in Glacial Lake Duluth, formed during the final retreat of the Superior Lobe after 10,800 years B.P. (Wright, et al., 1973).
Description
A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by Laura Blanche Gross in partial fulfillment of the requirements for the degree of Master of Science, December 1982. Plates I-II referenced in the thesis are also attached to this record, along with the Legend that explains both plates.
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