6.1. SR 03-17-2003 - Laserfiche WebLink

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Describe any of the following geologic site hazards to groundwater and also identify them on the site map: sinkholes, shallow limestone formations or karst conditions. Sinkholes or other karst conditions, or shallow limestone formations do not exist in the area of the Site. The general geologic and hydrogeologic conditions in the vicinity of the Site were described in the Environmental Impact Statement (EIS) prepared for the Elk River Gravel Mining District, Elk River, Minnesota, prepared by Maier Stewart and Associates, Inc (MSA), dated May 1994 (the 1994 EIS). The following discussion is a summary of the geology and hydrogeology of the Elk River Gravel Mining District, repeated from the 1994 EIS. "The mining district is situated on the northwestern edge of a major regional structural feature termed the Twin Cities Basin. The general geology of the mining district, and vicinity consists of a sequence of glacio-fluvial deposits overlying an erosional surface of Paleozoic sedimentary bedrock units. Bedrock formations in the area dip southeastward, towards the center of the basin." "The surficial deposits and topography result from at least two major glacial events. The glacial depositional event responsible for many of the larger landforms in the region, occurred as a result of advance of the St. Croix phase of the Superior Lobe (Helgeson, Ericson and Lindholm, 1975). This glacial advance occurred from the northeast and deposited a sandy till that is characteristically red in color. The terminal moraine, formed by stagnation of the St. Croix lobe, trends northwest-southeast through the region and forms the core of many of the regions larger geomorphic features. The Grantsburg Sublobe of the Des Moines Lobe subsequently advanced through the region from the northwest and deposited a characteristic calcareous grey till with shale fragments. The Grantsburg till was the last major glacial depositional event in the region. Typically, the St. Croix glacial features are reworked with, and overlain by, a relatively thin veneer of Des Moines Lobe outwash sands and till. The Anoka sand plain is situated east of the mining district and was formed during wastage of the Grantsburg Sublobe. It was during this period of wastage that an esker complex was formed. This esker complex provides the thick sand and gravel deposits of the mining district (Wright, 1972)." "In the mining district three major classifications of glacial sediments are present, till, outwash and ice contact deposits, with the later being most prevalent. Each of these deposits has a unique although interrelated depositional history. The till units typically consist of var/able unstratified material ranging in size from silts and clays to large cobbles. Typically, till units are of low permeability and as such impede groundwater flow. The tills in the region consist of primarily silts and clays with a lesser sand and gravel fraction. Smaller lenses of sand and gravel are often present within the tills and evidence small scale fluvial activity during deposition. Glacial outwash deposits, such as those of the Anoka sand plain are relatively permeable consisting of stratified fine sands to gavels deposited by glacial meltwaters at the front of a retreating glacier. Ice contact deposits often result in LIESCH ASSOCIATES, INC. Hydrogeologists · Engineers · Environmental Scientists · Jan-2003 · - Page 12 ·