Late-Wisconsin Stratigraphy and Glacial History of Southwestern St. Louis County Minnesota
1991-11
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Late-Wisconsin Stratigraphy and Glacial History of Southwestern St. Louis County Minnesota
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1991-11
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The stratigraphy and landforms of southwestern St. Louis County indicate that during Late-Wisconsin time, the area was affected by glacial advances of the Rainy Lobe, the Superior lobe, and the St. Louis Sublobe, as well as two glacial lake phases. Sedimentologic signatures, stratigraphic relationships, fabric analyses, thin sections of till, and landforms were studied in an effort to monitor the interactions of each of the ice advances in relation to previous deposits and to reconstruct the depositional environment and origin of landforms associated with each lobe. Modern developments and theories based on the deformable nature of sediment at the base of glacier ice as well as the role of glaciotectonics in the glacial environment provided insight and assistance in interpreting important sedimentary features and landforms. The earliest Late-Wisconsin ice advance deposited the gray-brown, sandy (sand/silt/clay: 62/32/6) Independence Till of the Sullivan Lake Formation during the St. Croix phase of the Rainy Lobe, approximately 20,000 yrbp. Rock fragments indicate a northern provenance, and geomorphic features related to this advance include drumlins, eskers, and tunnel valleys. Recumbent folds and boudinage structures discovered in beds of sand exposed in a drumlin indicate the stress present at the base of glacier ice. Approximately 18,000 yrbp the Superior Lobe advanced in a west-southwest direction in the Automba phase and deposited the Upper Cromwell Formation. In the study area this formation is made up dominantly of red-brown, sandy (sand/silt/clay: 60/31/9) supraglacial material and is exposed in the massive Highland Moraine. Exposures of subglacial sediment are rare although one exposure in the moraine revealed intensely deformed glacial strata and stone fabric parallel to ice movement. An up-glacier depression and a related down-glacier ridge suggests the formation of a hill-hole pair. This, combined with deformed subglacial strata and the massive amount of material in the Highland Moraine reflects the compressional nature of ice flow at the glacier margin. The new rock-stratigraphic name "St. I.ouis Formation" is here introduced for till and outwash associated with the St. I.ouis Sublobe. This sublobe, a southeast extension of the Des Moines Lobe, advanced approximately 12,000 yrbp. The Alborn Till Member contains a gray, sandy, calcareous component (sand/silt/clay: 60/22/18) of western provenance and a red-brown, fine-textured mixture (sand/silt/clay: 35/34/31) of far-transported and locally eroded sediment typically displaying laminations. As the sublobe advanced from the northwest, it encountered earlier deposited fine-grained lake sediments. Consequently, the lake sediments were deformed and incorporated into the glacier as the ice thinned and became overextended. Evidence for this theory includes deformation structures, laminated till, a lack of major outwash features, and the small size of the Culver Moraine. The sublobe's anvil shape is also a possible consequence of the deformable substrate. The rock-stratigraphic term "Meadowlands Formation" is here introduced for lacustrine sediments contained within the Glacial Lake Upham Basin. Rhythmites, lake clay, wave modified till, and beach deposits are included in this formation.
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A Thesis submitted to the faculty of the Graduate School of the University of Minnesota by John William Ballantyne in partial fulfillment of the requirements for the degree of Master of Science, November 1991.
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