5.4. SR 09-07-2004 - Laserfiche WebLink

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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 variable 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 gravels deposited by glacial meltwaters at the front of a retreating glacier. Ice contact deposits often result in specific geomorphic features such as the gravel rich esker complex which comprises the surficial geology of the gravel mining district. Eskers or more generally, ice contact deposits are the result of deposition by subglacial fluvial processes such as meltwater streams and rivers. These deposits are essentially reworked till from which the fines were removed by fluvial activity. The lack of fines provides for the relatively high permeability of these deposits." "An alternating sequence of till and glacio-fluvial deposits are present beneath the esker deposit. The uppermost unit beneath the esker appears to be a relatively continuous till unit that is variable in thickness. A sand and gravel unit is generally present below the till. Other sand and gravel units are interbedded within or between successively deeper till units. These sand and gravel units are important water supplies for local residential use. A majority of the residential developments west of the mining district utilize the first sand and gravel unit present below the esker deposit." "Beneath the glacial deposits the first bedrock encountered in the immediate vicinity of the mining district is the Eau Claire formation. This formation consists of alternating sandstone and shale, and is generally recognized as a regional confining layer. Beneath the Eau Claire formation are the Mount Simon and Hinckley formations. These two formations are generally described as fine to coarse grained quartzose sandstones which combine to form the Mt. Simon-Hinckley Aquifer. The Mt. Simon-Hinckley is an important water supply locally and throughout the Twin City Basin, as well. According to USGS Hydrologic Atlases, the basement rock beneath the Mt. Simon-Hinckley consist of undifferentiated Pre- Cambrian igneous and metamorphic rocks including quartz, diorite, granodiorite and quartz monzonite." "The bedrock elevation beneath the mining district and over much of the area to the west and south appears to be at an elevation of 800 to 850 feet National Geodetic Vertical Datum (NGVD). Slightly east of the mining district and extending along an approximate north- south trend is a reported major bedrock valley incised over 100 feet deeper than the bedrock elevation beneath the mining district. No major bedrock structures are documented west of the district and the bedrock elevation averages 800 feet NGVD." 13