Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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104 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 0.5 percent for the Jonesville coal zone and from 0.2 to 0.5 percent for the Premier coal zone. For comparison, analyses of coals in the Wishbone Hill district show heats-of- combustion from 10,400 to 13,200 Btunb, ash contents from 4 to 22 percent, and sulfur contents from 0.2 to 1.0 percent. Coals in the Wishbone Hill district also have apparent rank of high-volatile B bituminous. LITHOTYPES Lithotypes from below the Eska to above the Jones- ville coal zones of the Chickaloon Formation are fine- grained dominated compared to those of the Wishbone Formation, which is coarse-grained dominated (fig. 4). Mudstone and siltstone are dominant over sandstone, coal, and carbonaceous shale in the Chickaloon Formation. In contrast, conglomerate is dominant over sandstone, siltstone, and mudstone in the Wishbone Formation. MUDSTONE AND SILTSTONE The mudstone lithotype is light gray to dark gray; the latter color reflects higher organic content. This lithotype Plan view displays "popcorn" texture in weathered surface, indicating smectitic clay content; however, fresh surfaces show nonbedded, blocky, and mottled appearance. Root marks, whole to partial fragments of plant stems, trunks, and leaves, and vertical to horizontal animal burrows are abundant. Crude laminations marked by macerated plant fragments and silty grains are common. Diagenetic mineralization in the form of carbonate and siderite concretions and spherules are common and range in diameter from a few centimeters to greater than 1.5 m. Siderite spherules often contain plant remains and weather dark brown to reddish. Vitrain lenses often mark codified roots, stems, and trunks; "coffee grounds" composed of finely macerated plant fragments form wispy lenses in the mudstone. The siltstone lithotype is light gray to dark gray with the latter color enhanced by dispersed fine organic material in the silty matrix. This lithotype is commonly parallel to subparallel laminated and ripple laminated (climbing and current microlaminations). These sedimentary structures are commonly intercalated with mud drapes. Vertical and horizontal animal burrows and root marks commonly destroy the original sedimentary structures. Carbonized woody fragments, leaves and stems, and coal spars are scattered in the siltstone. Diage- netic carbonate concretions, from a few to 30 cm in diameter, are scattered throughout the siltstone. Figure 3. Structural model of deformation of Riedel experiment showing Riedel shear (R) and conjugate Riedel shear (R'). W, width of shear zone. Modified from Tchalenko (1970).
EARLY CENOZOIC DEPOSITIONAL S YSTEMS, MATANUSKA COAL FIELD 105 SANDSTONE AND CONGLOMERATE The sandstone lithotype is light gray to buff colored, ranges from very fine to coarse grained, and may contain thin, basal lag grits or conglomerates. The sandstone is feldspathic and the accompanying basal lag conglomerates include pebble-dominated fragments of volcanic, metamor- phic, and sedimentary rocks (for example, coal, chert, mudstone, ironstone, and siltstone fragments); the latter rock types were probably recycled from Jurassic, Creta- ceous, and older Tertiary sedimentary rocks. The basal lag grits consist of granules to fine pebbles of vein quartz, feldspar, chert, mudstone, and coal. Sedimentary structures in the sandstone include trough crossbeds (as much as 1 m in height), tabular crossbeds (as much as 0.5 m in height and 15 m long), convolute laminations, and current ripple laminations. Sigmoidal macroforms are common and sepa- rated by mud drapes. Laminae of cross stratifications are defined by carbonaceous matter. The lithotype contains abundant plant fragments, and stems and trunk impressions along bedding planes. Trough-crossbed foreset measure- ments (n=24) show west-southwest dip orientation. The conglomerate lithotype occurs either as thin basal lag deposits (mainly in the Chickaloon Formation) or as thick bodies (as much as 15 m) that contain a few thin (up to several centimeters thick) sandstone beds. Thick conglomerate beds are common in the Wishbone Formation in Formation 560 rn Chickaloon Formation 910-1 500 m 0 . . Sandstone : STUDY INTERVAL EXPLANATION \ 1 Jonesville coal zone I Premier coal zone Midway = coalbed Conglomerate 1 Eska coal zone ._. . . . . . ... r 50 Mudstone and siltstone Coal and carbonaceous shale Figure 4. Composite Tertiary stratigraphic section showing study interval in Chickaloon and Wishbone Formations. the Wishbone Hill coal district. The conglomerate interval contains from 25 to 75 percent conglomerate beds (Clardy, 1974). This conglomerate consists predominantly of pebbles and cobbles, with some boulders. The phenoclasts include abundant volcanic rocks (60 percent), common sedimentary rocks (chert and jasper), and rare plutonic fragments (Clardy, 1974). The conglomerates are framework-clast supported and loosely to tightly packed. Sand fills pore spaces between clasts. Texturally, the clasts are poorly to well sorted and of unimodal pebble size. Clasts are rounded to subrounded and range from spherical to rodlike in shape. The conglomerates are crudely to well imbricated, exhibiting east-northeast dip orientation (A axis). Trough-crossbed foresets in the sandstone interbeds show west-southwest orientation (Clardy, 1974). COAL AND CARBONACEOUS SHALE The coal and carbonaceous shale lithotypes are the least common units within the total rock volume of the Chickaloon Formation. The apparent ranks of coalbeds in this formation range from subbituminous to anthracite, but are mainly bituminous in the study area (Barnes and Payne, 1956). As many as 30 single and dull- to bright- banded coalbeds, as much as 3.5 m thick, were recog- nized in the upper 450 m of the Chickaloon Formation by Barnes and Payne (1956) and Merritt (1986). These coalbeds are interbedded with bony coal, bone, carbonaceous shale, coaly mudstone-ironstone, and tonstein partings and thus are grouped as coal zones. Petrographic analyses by Rao and Wolff (1980) and Merritt (1985) in the Premier mine about 8 km southwest of the study area indicate that the coals are dominantly woody (92.7 percent vitrinite, 3 percent liptinite, and 4.3 percent inertinite). Carbonaceous shale, consisting of fissile, black mudstone laminated by abundant macerated plant fragments and as thick as 1.8 m, is the most common coalbed parting. Proportions of carbonaceous-shale impurities in the coalbeds yield various gradations from bony coal to bone. The bony coal contains evenly disseminated impurities yielding dull luster and dark-gray to black color as well as some carbonaceous-shale laminae, which yield dull bands of impure coaly material. Bone is dark gray to black and commonly contains alternating carbonaceous-shale laminae or coaly mudstone lenses in coal, yielding a heavier and tougher coaly material than the pure coal. Tonstein beds, which are white to light green and bentonitic in composi- tion, are as thick as 1.8 m and the least common type of parting. Ironstone or siderite (carbonate) partings occur as reddish-brown nodules, concretions, and irregular knobby lenses containing abundant plant fragments and tree trunks. The Premier coal zone, in particular, contains abundant carbonaceous shale and tonstein and ironstone partings.
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Geologic Studies in Alaska by the U
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CONTENTS Introduction Cynthia Dusel
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CONTENTS CONTRIBUTORS TO THIS BULLE
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GEOLOGIC STUDIES IN ALASKA BY THE U
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LATE HOLOCENE LONGITUDINAL AND PARA
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DEEP-WATER LITHOFACIES AND CONODONT
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Table 1. Locality register for key
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Table 1. Locality register for key
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LITHOFACIES AND CONODONTS OF CARBON
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LITHOFACES AND CONODONTS OF CARBONI
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Table 1. Conodont faunules and lith
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DEPOSITIONAL SEQUENCES IN ATIGUN SY
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DEPOSITIONAL SEQUENCES IN ATIGUN SY
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Page 65 and 66: U-Pb AGES OF ZIRCON, MONAZITE, AND
Page 77 and 78: APPEAL FOR NONPROLIFERATION OF ESCA
Page 85 and 86: FAVORABLE AREAS FOR METALLIC MINERA
Page 97 and 98: GOLD AND CINNABAR IN HEAVY-MINERAL
Page 107 and 108: EARLY CENOZOIC DEPOSITIONAL SYSTEMS
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Page 125 and 126: RESERVOIR FRAMEWORK ARCHITECTURE, C
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Table 1. Isotopic ages of intrusive
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Table 1. Isotopic ages of intrusive
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GEOCHEMICAL EVALUATION OF STREAM-SE
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Table 3. Summary of geochemical sig
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GEOCHEMICAL CHARACTER OF UPPER PALE
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RECONNAISSANCE GEOCHEMISTRY OF BASA
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OSTRACODE ASSEMBLAGES FROM MODERN B
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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RUBIDIUM-STRONTIUM ISOTOPIC SYSTEMA
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US. GEOLOGICAL SURVEY REPORTS ON AL
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U.S. GEOLOGICAL SURVEY REPORTS ON A
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U.S. GEOLOGICAL SURVEY REPOR TS ON
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REPORTS ABOUT ALASKA IN NON-USGS PU
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REPORTS ABOUT ALASKA IN NON-USGS PU
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Geologic Studies in Alaska by the U.S. Geological Survey, 1992

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