Description of Mapping Units for Mineral Resource and Geochemical Maps: Endicott, Fairbury SW, and Reynolds Quadrangles, Nebraska R. M. Joeckel Conservation and Survey Division, School of Natural Resources, University of Nebraska-Lincoln C1 Industrial clay resource mapping unit 1 (lower Dakota Formation): primary economic importance Known and potential discontinuous distribution of red- and yellow-mottled industrial clay resources; also includes light gray to white silty claystones to siltstones, some of which are valued for buff-firing properties. “Clay resources” in this usage refers to claystones, silty claystones, clayey siltstones, and siltstones, all of which can be collectively referred to as mudstones. These mudstones are a high-priority resource, being essential to current and future brick and tile production in the area. The vertical and lateral distribution of mudstones is irregular and only locally predictable, but red-mottled claystones to clayey siltstones (hydromorphic paleosols) several meters thick are relatively continuous at lower elevations along Little Blue River, where they have been mined for many decades in the Endicott area. This mapping unit also includes: (1) very light gray to gray silt shales to clay shales, (2) light gray to white siltstones to very fine sandstones, and (3) red- and yellow-mottled siltstones to fine sandstones, all of which can be used in the production of brick and tile. Well-sorted very fine to medium sandstones appear throughout this mapping unit in large-scale lateral accretion units within ancient meanderbelts, in large-scale paleochannel fills, and as thin sheets. Siderite in common in red- and yellow-mottled and mudstones in this resource unit, particularly as millimeter-scale concretions (sphaerosiderites), which can create expansion problems during the firing of products. Calcite and pyrite occur locally in lesser quantities, but the latter mineral is not perceived to be as problematic as in C2 (described below). Relatively well-crystallized kaolinite dominates the clay-mineral assemblage in industrial clay resources of this mapping unit, and illite/smectite is a lesser component. Loss-on-ignition (LOI) is generally very low (1% or less) for clay resource materials in this unit. A limited number of X-ray fluorescence (XRF) analyses indicate that red-mottled mudstones have an iron content of 2.8-5.9%, which is sufficient to confer red-firing characteristics. The gray mudstone and shale samples from industrial clay resource unit 1 that were tested yielded iron contents of less than 1.6%. C2 Industrial clay resource mapping unit 2 (upper Dakota Formation): secondary economic importance Known and potential discontinuous occurrence of light gray to very dark gray industrial clay resources, some of which are valued for buff-firing characteristics. Also, localized areas of potential acid rock drainage in uppermost Dakota Formation. This mapping unit is dominated by low-chroma mudstones and shales, with variable organic-matter content. It also includes relatively rare red-mottled mudstones. The upper half of the stratigraphic interval represented by this mapping unit is notable in its comparatively high pyrite and organic-matter contents. Pyrite and organic matter in industrial clay resources will present problems in the manufacturing of brick and tile if such materials are not handled properly. A limited number of tests yielded LOI values of 1.2-2.7% in these materials. Relatively well-crystallized kaolinite dominates the clay-mineral assemblage in industrial clay resources of this mapping unit, and illite/smectite is a lesser component. Scumming in bricks or other ceramic bodies would be highly problematic in the use of both fresh and preweathered materials (those naturally weathered under the modern land surface) from this mapping unit. Both of these materials tend to develop developed secondary gypsum or other sulfate salts, particularly those of iron and aluminium. It should be noted that the weathering of fresh material in stockpiles is likely to produce efflorescent salts over time as materials are exposed to the atmosphere. Large pyrite nodules are locally common (1-3% or rock mass) in the uppermost Dakota and may have arsenic (As) contents exceeding 2,000 ppm, although levels of heavy metals in pyrite are likely to be low. The potential environmental and groundwater-quality implications of this pyrites have not been assessed. Acid rock drainage (pH 3 or lower) can occur in this mapping unit if materials are exposed to the atmosphere by excavations and engineering projects. Efflorescent, hydrous iron and aluminium sulfate salts (e.g., copiapite, alunogen) will form spontaneously during periods of dry weather in roadcuts and pit faces, and the acid content of these salts will readily be liberated on the land surface by precipitation events. Kg Graneros Shale potential industrial clay resource (quaternary economic importance) Shales with little economic value. Also, a zone of potential acid rock drainage. The Graneros Shale is a potential industrial clay resource of marginal value, and then only if it were to be used in combination with other, higher-quality materials. The Graneros Shale contains sufficient pyrite to produce acid (pH 3 or lower) drainage when freshly exposed; preweathered Graneros Shale (shale naturally weathered under the modern land surface since Late Pliocene to Holocene exposure) contains jarosite and still produces an acid pH in aqueous suspension. With exposure to the elements, both fresh and preweathered shales rapidly produce gypsum efflorescences and subsurface crystals in fractures and other voids. Smectite and illite/smectite contents in this unit are significantly higher than in materials from mapping units C1 and C2. The X-bentonite within the Graneros Shale, although thin, is a high-smectite clay that contains lesser amounts of illite/smectite. It is probably too thin to serve as an industrial clay resource, but it may have potential as a plasticizer for use in small quantities in craft ceramics. Gypsum formation and resultant scumming in bricks would be problematic if these materials were to be used as a clay resource. Their relatively high organic- matter and pyrite contents would also be of concern in manufacturing. A limited number of tests yielded LOI values of 2.0-3.2% in these materials. Ls1 Limestone resource mapping unit 1--middle to upper Greenhorn Limestone (secondary economic importance) Thinly-bedded limestone to chalky limestone at the surface or within 6 m of the land surface under easily-strippable loess and silty clay (Peoria Loess, Gilman Canyon Formation, and Sangamon Geosol). Limestone strata within this mapping unit are the highest-quality limestone resources within the Greenhorn Limestone, with CaCO3 content approaching 90% in some horizons within the mapping unit. A limited number of XRF analyses indicate that CaO content should be expected to fall between 40% and 50%, Al2O3 content should be below 5%, SiO2 content should vary greatly, but generally fall between 0% and 15%, and that K2O content should generally be expected to be less than 0.7%. Recent use of crushed Greenhorn Limestone elsewhere in eastern Nebraska as agricultural lime sets precedent for value of this material in the same application. In the 19th and early 20th centuries, materials from Ls1 were used for house and building foundations in the local area; Mid- 19th-century use of Greenhorn Limestone for quicklime production is probably does not translate to a potential use in modern industrial processes, but additional study of the matter is warranted. There is a potential for the use of Ls1 in wall rock, landscaping, and ornamental stone slabs (i.e., slabs covered with fossil clams) for gardens and similar settings, although these markets have not been investigated. Materials in this unit have strong acid-neutralizing capabilities and will tend to produce limited amounts of “hard” water from fractures. Ls2 Limestone resource mapping unit 2--lowermost Greenhorn Limestone (tertiary economic importance) Laminated limestone to shaly limestone at the land surface or under thin (less than 1.5 m) colluvium and/or loess (Peoria Loess). These strata are of lesser economic significance than Ls1, because of the higher clay content in Ls2. CaCO3 content may be 15% or less in some zones within this resource unit and may not exceed 60-70% in the most carbonate-rich parts. Historic use of crushed Greenhorn Limestone elsewhere in Nebraska as agricultural lime sets precedent for value of this material in the same application. Materials in this unit have strong acid-neutralizing capabilities and will tend to produce limited amounts of “hard” water from fractures. SG1 Sand and gravel resource mapping unit 1 (primary economic importance) Western-source fluvial sands and gravels at the land surface or at shallow depth amenable to economical stripping and/or wet-pit extraction. Gravels in this mapping unit are the most desirable ones in the mapped area for most applications, especially road material. Most easily-mined areas not under agricultural/pastoral use have already been occupied. Also an important source of groundwater: groundwater δ18O values are within the range of modern precipitation, indicating Holocene to modern recharge. SG2 Sand and gravel resource mapping unit 2 (secondary economic importance) Glacial outwash and/or older fluvial terrace sands and gravels at the land surface or at shallow depth amenable to economical stripping and/or wet-pit extraction. Gravels in this mapping unit are suitable for most applications, but the commonness of friable clasts (Dakota Formation sandstone and weathered crystalline cobbles of glacial origin, especially specially granite, granitic gneiss, and quartz monzonite) in some areas, particularly south of the Little Blue River (resource mapping unit SG2/C1), render these materials less desirable than SG1. Most easily-mined areas not agricultural/pastoral use have already been occupied. SG2/C1 Sand and gravel resource mapping unit 2 (generally thin) atop industrial clay resource unit 1 (primary economic importance) Thin glacial outwash and/or older fluvial terrace sands atop mottled claystones and other sediments of the lower Dakota Formation. Generally, the sand and gravel resources in this mapping unit are of minimal economic significance, whereas the underlying clay resources locally warrant stripping of SG2 and overlying materials for access. SG3 Sand and gravel resource mapping unit 3 (quaternary economic importance) Alluvial sands, gravels, and silts of marginal economic value. Sands and gravels in this unit are expected to be a mixture of materials from different regional sources, but western-source materials dominate. Silt and fine to medium sand are common constituents of the alluvial valley fill of the Little Blue River, and therefore gravel production is anticipated to be only minimally economical, particularly in the context of nearby SG1 and SG2 resources.
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