Natural Resources Research Institute Technical Report

Plates 1-7B mentioned in the report are also attached to this record. Disks 1-4 have not been located yet.Minnesota has a variety of clays and shales that have potential as industrial clays. These clays are: 1) Precambrian clays; 2) Paleozoic shales; 3) pre-Late Cretaceous primary (residual) and secondary kaolins; 4) Late Cretaceous ball clays and marine shales; 5) Pleistocene glacial clays; and 6) Recent clays. Minnesota clays are currently used for brick and as a portland cement additive. Other potential uses include filler and coating grade kaolins, ceramic tile, refractory products, lightweight aggregate, sanitaryware, and livestock feed filler. Precambrian clays occur in the 1 .1 Ga Keweenawan interflow sediments of the North Shore Volcanic Group, the Middle Proterozoic Thomson Formation and in the Paint Rock member of the Biwabik Iron-Formation on the Mesabi Iron Range, all in northeastern Minnesota. The Paint Rock clays have potential as red coloring additives and glazes. Paleozoic shales in southeastern Minnesota are primarily kaolinitic and illitic shales that are interbedded with limestones. The Ordovician Decorah and Glenwood Formations are marine shales that, in the past, have been used to make bricks, tile, and lightweight aggregate. The thickness of these shales ranges from 10-90 feet. The Decorah Shale has the lowest firing temperature with the best shrinkage and absorption characteristics of all the Minnesota clays. The pre-Late Cretaceous primary and secondary kaolins are found in the western and central portions of Minnesota; the best exposures are located along the Minnesota River Valley from Mankato to the Redwood Falls area and in the St. Cloud area. The primary or residual kaolinitic clays are the result of intense weathering of Precambrian granites and gneisses prior to the Late Cretaceous. Subsequent reworking of these residual clays led to the development of a paleosol and the formation of pisolitic kaolinite clays. Physical and chemical weathering of the saprolitic kaolinite-rich rocks produced fluvial/lacustrine (secondary) kaolinitic shales and sandstones. Recent exploration activity is concentrated in the Minnesota River Valley where the primary kaolin thickness ranges from 0 to 200 + feet, and the thickness of the secondary kaolins ranges from 0-45 + feet (Setterholm, et al, 1989). Similar kaolinitic clays occur in other areas of Minnesota, e.g., St. Cloud and Bowlus areas. However, less information is available on their thickness, quality, and areal distribution due to varying thicknesses of glacial overburden. Cement grade kaolin is extracted from two mines in the residual clays in the Minnesota River Valley, and a third mine there yields secondary kaolinite-rich clays that are mixed with Late Cretaceous shales to produce brick. During the Late Cretaceous, Minnesota was flooded by the transgressing Western Interior Sea, which deposited both non-marine and marine sediments. These sediments are characterized by gray and black shales, siltstones, sandstones, and lignitic material. Significant occurrences of Late Cretaceous sediments are found throughout the western part of the state, with the best exposures located in Brown County, the Minnesota River Valley, and the St. Cloud area. In Brown County, the maximum thickness of the Late Cretaceous sediments is > 100 feet. These sediments thicken to the west and can be covered by significant thicknesses ( > 300 ft.) of glacial overburden in many areas. Current brick production comes from the Late Cretaceous shales in Brown County. In the past, the Red Wing pottery in Red Wing, Minnesota, used Cretaceous and some Ordovician sediments to produce pottery, stoneware, and sewer pipe. Glacial clays occur in glacial lake, till, loess, and outwash deposits, and these clay deposits range in thickness from 5 to 100 + feet. Much of the early brick and tile production (late 1800s and early 1900s) in Minnesota was from glacial clays. The last brickyards to produce from glacial lake clays, e.g., Wrenshall in northeastern Minnesota and Fertile in west-central Minnesota, closed in the 1950s and 1960s. There has also been some clay production from recent (Holocene) fluvial and lake clays that have thicknesses of 2-10 + feet. Both recent and glacial clays are composed of glacial rock flour with minor quantities of clay minerals. Carbonates can be a significant component of many of these clays. Glacial lake clays in northwestern Minnesota (Glacial Lake Agassiz - Brenna and Sherack Formations) begin to bloat at 1830 ° F due to the presence of dolomite and smectite clays. These clays are a potential lightweight aggregate resource. Geochemistry, clay mineralogy, particle size, cation exchange capacity (CEC), raw and fired color, and firing characteristics are useful in distinguishing different potential industrial uses for Minnesota clays. These physical and chemical characteristics help to distinguish potentially useful clays from those with less desirable characteristics, e.g., high quartz or silica content, high shrinkage or absorption upon firing, undesirable fired color, too coarse-grained, CEC of < 5 milliequivalents, etc. Certain clays, e.g., the bloating Decorah and Brenna Formation clays, and the high alumina, refractory, pisolitic clays of the Minnesota River Valley, have physical and chemical characteristics that indicate further exploration and product research are necessary to fully evaluate the potential of these clays.Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442; Funded by the Legislative Commission on Minnesota Resource

Natural Resources Research Institute Technical Report

The utility of gravity, magnetic and electrical resistivity methods for kaolin exploration was evaluated on a test-drilled 300-meter by 600-meter prospect in the Minnesota River Valley in eastern Redwood County, Minnesota. Seven Wenner soundings and three resistivity profiles were taken over the prospect, and interpretations were constrained by direct determinations at nearby bedrock exposures and by drill hole (regolith) data. High-precision gravity data also appear to reflect thickness variations in the low-density kaolin. The magnetometer is not sensitive to the kaolin itself, but it may be useful in detecting rocks in the protolith that yield chlorite-rich, weathered clays, such as diabase dikes.Natural Resources Research Institute, University of Minnesota Duluth, 5013 Miller Trunk Highway, Duluth, MN 55811-1442; Minnesota Geological Survey, University of Minnesota, 2642 University Avenue, St. Paul, Minnesota 55114-105