4.4 EAW Attachment - Laserfiche WebLink

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Surficial Geology The surficial geology across Sherburne County is primarily sands and gravels deposited during the final stages of glacial retreat (1). The surficial geology identified on the Minnesota Geologic Atlas within the project area are of the Holocene and Pleistocene area, units are described below:  Qtu – Upper terrace – “Surface ranges in elevation from approximately 880 feet (268 meters) in the southeast corner to 1,020 feet (311 meters) in the northwestern part of the county. Patterned area indicates places where terrace sediments are thin (less than 10 feet [3 meters]) over underlying glacial till (units Qnt and Qct)” (1).  Qa – Sand, sandy loam to silt loam – “Generally coarse-grained sediment (sand and gravel) in the channels, and finer-grained sediment (fine-grained sand and silt) on floodplains; coarsens with depth. May be capped by and/or interbedded with organic- rich layers. Low area may be filled with thick silty to clayey sediment. Along the Mississippi River, unit consists of generally less than 6 feet (2 meters) of silt loam to loamy sand overlaying sand, gravelly sand, or cobbly gravel, with scattered wood and shell fragments. Sediment along smaller streams is generally finer grained. Floodplain alluvium” (1).  Qtl – Lower terrace – “Surface ranges in elevation from approximately 870 feet (265 meters) in the southeast corner to 950 feet (290 meters) in the northwestern part of the county” (1). Bedrock Geology “Sherburne County is almost completely covered by Quaternary glacial till, lacustrine sand, outwash deposits, terrace deposits, swamp deposits (peat), and floodplain alluvium” (1). The bedrock geology identified on the Minnesota Geologic Atlas within the project area are described below:  Ctc – Tunnel City Group (Upper Cambrian) – “ranges from less than 135 to approximately 180 feet (41 to 49 meters) in thickness in the map area and is missing because of erosion over much of the county. It is now divided into two formations: the upper Mazomanie Formation, and the lower Lone Rock Formation.” “Both the Mazomanie Formation and the members of the Lone Rock Formation were originally named and described by Berg (1954), who considered all to be members of the Franconia Formation. The uppermost non-glauconitic formation, the Mazomanie Formation, is dominantly white to yellowish-gray, fine- to medium-grained, cross- stratified, generally friable, quartz sandstone. Glauconitic grains typically are absent and never exceed 5 percent.” “Some beds contain brown, intergranular dolomite as cement. The Mazomanie Formation is about 60 to 80 feet (18 to 24 meters) thick. The Lone Rock Formation underlies and intertongues with the Mazomanie Formation. The uppermost member of the Lone Rock Formation is the Reno Member, which is pale yellowish-green, very fine- to fine-grained glauconitic, feldspathic sandstone with thin, greenish-gray shale partings. The sandstone is well sorted and contains thin zones containing dolomitic intraclasts. The Reno Member is underlain by the Tomah Member, a thin unit composed of interbedded, grayish-yellow-green, feldspathic siltstone, very fine-grained sandstone, and pale green shale. The Tomah Member is sparsely glauconitic. The basal Birkmose Member is grayish-yellow- green, fine-grained sandstone that is cemented by dolomite. There are dark red dolostone beds in the upper part of the Birkmose Member. The Birkmose Member is