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

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122 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 randomly interbedded with mudstone and siltstone, or cap coarsening-upward sequences of mudstone and siltstone. MUDSTONE AND SILTSTONE SEQUENCES Mudstone is dark to light gray and commonly displays popcorn-weathered texture indicating that it is composed of smectitic clay (Moore, 1976; Reinink-Smith, 1990). It contains abundant macerated plant fragments that are frequently concentrated as discontinuous laminae. The mud- stones are mainly massive but locally contain plant root marks and some nondescript vertical and horizontaI burrows. This lithologic unit is as much as 7 m thick and is volumetrically more common than siltstone. Siltstone is dark to light gray, sandy, and contains cur- rent and climbing ripple laminations. These ripple laminations are frequently crosscut by root marks of coalified plants and tubelike vertical burrows. It contains abundant plant fragments and "coffee ground" (finely macerated or- ganic matter) laminations. This lithologic unit is as much as 3 m thick. The mudstone and siltstone occur either as (I) randomly interbedded sequences with silty to very fine- grained sandstone or (2) coarsening-upward sequences of mudstone, grading upward into siltstone and tabular, very fine grained sandstone. These two types of sequences are, in turn, commonly interbedded with each other. I SWANSON RIVER FIELD 0- \c KENAl FIELD Figure 4. Map showing locations of the Kenai and Swanson River gas fields. COAL, CARBONACEOUS SHALE, AND TONSTEIN SEQUENCES Coal in the Clamgulchian type section of the Sterling Formation consists entirely of lignite (average vitrinite re- flectance is 0.32 percent; Memtt and others, 1987) with abundant carbonaceous shale partings and a few tonstein partings. Woody fragments with well-preserved internal structures and tree stumps are the common composition of the coal. Merritt and others (1987) described the maceral composition of Sterling coal on the eastern side of Cook Inlet as comprising mainly 85.3 percent huminite, 9.6 percent liptinite, and 5.1 percent inertinite. These workers in- dicated that Sterling coalbeds have increased levels of inertinites compared to coalbeds of the Beluga Formation. The coalbeds in the Clamgulchian type section range from a few centimeters to 1.4 m in thickness and thin upward with increased carbonaceous shale interbeds. The thickest coalbed in the lowermost part of the Clamgulchian type section is exposed for more than 3.1 km. Carbonaceous shale is dark-gray to black, fissile mudstone mixed with abundant macerated plant-fragment laminae and occurs as single beds or as partings in the coalbeds. It commonly contains coalified plant root marks, woody fragments, and tree stumps. The carbonaceous shale laterally grades into coalbeds and commonly replaces coalbeds in the uppermost part of the Clamgulchian type section of the Sterling Formation. A few, nonbedded, white to yellowish tonstein beds up to a few centimeters thick occur as partings in the coalbeds. A rippled tonstein bed, which is bentonitic in composition and mixed with mudstone, and as thick as 1.5 m, is interbedded with the coalbed in the lowermost part of the Clarngulchian type section. This tonstein bed is similar to the tonstein partings described by Reinink-Smith (1990) that are interbedded with Sterling coalbeds, which she in- terpreted to be volcanic ash. Sequences of coal, carbonaceous shale, and tonstein occur as interbeds in mudstone and siltstone sequences. FACIES ASSOCIATIONS Lithostratigraphic sequences may be organized verti- cally into facies associations, as displayed in figures 5-7. These facies associations pass from one to the other either in a preferred or random order. A facies association may grade into another or be bounded by an erosional surface. The facies associations are grouped in order to interpret them as genetically related neighbors. Thus, the Clamgulchian type section of the Sterling Formation was grouped into three facies associations according to their similarities and differences. The lower part of the composite Clamgulchian type section of the Sterling Formation consists of (1) interbedded fining-upward sandstone, (2) mudstone and
RESERVOIR FRAMEWORK ARCHITECTURE, CLAMGULCHIAN TYPE SECTION, STERLING FORMATION 123 siltstone, and (3) coal, carbonaceous shale, and tonstein- facies associations (fig. 5). The fining-upward sandstones, which average 20 m in thickness, are interbedded with mudstone and siltstone sequences that average 9.5 m in thickness, and with coal, carbonaceous shale, and tonstein sequences that average 2.8 m in thickness. The lowermost coal, carbonaceous shale, and tonstein sequences are as thick as 5.2 m and contain as many as four coalbeds; how- ever, they are interbedded with the mudstone and siltstone sequences. The fining-upward sandstones are composed of as many as three tiers (multistory) of amalgamated multiscoured (as many as four internal scour bases) bodies. These sandstone bodies are separated by thick mudstone and siltstone sequences that occasionally contain coarsening-upward sandstone. The multiscoured sandstones, in turn, exhibit a series of stacked bodies separated by a few thin, scour-based mudstone-siltstone lenses (see lowermost - .- + EXPLANATION V) I Mud I l~andl~ebble Conglomerate Sandstone Siltstone Mudstone Coal and carbonaceous shale - ---a - Trough crossbeds d & Megaforesets 4 l Convolute laminations -& - Ripple laminations -- ' 30 AA Root marks T Tonstein METERS sandstone complex in fig. 5). In addition, these multiscoured sandstone bodies are laterally offset, display- ing a multilateral architecture. .- I Mud /Sand~ Pebble - - 4 EXPLANATION Conglomerate Sandstone Siltstone Mudstone Coal and carbonaceous shale v'- - - b- Trough crossbeds @ Planar crossbeds Megaforesets Convolute W - laminations - Ripple laminations - /-., Ah Root marks T - 30 Tonstein METERS Figure 5. Composite vertical facies association in lower part of Figure 6. Composite vertical facies association in middle part Clamgulchian type section of the Sterling Formation. of Clarngulchian type section of the Sterling Formation.
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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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U-Pb AGES OF ZIRCON, MONAZITE, AND
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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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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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