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

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126 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 these sandstone reservoir systems contain all four orders of bounding surfaces, their extent and continuity are less pre- dictable than those of the reservoir system in the lower part of the study interval. For example, numerous second- order internal scour surfaces bounding the stacked sandstone bodies truncate each other more commonly and are shorter than those in the reservoir system of the lower part of the Clamgulchian type section. Also, haphazard ar- rangement of the megaforesets, trough and planar crossbeds, convolute laminations, and ripple laminations produces random patterns of the third- and fourth-order bounding surfaces, resulting in poor interconnectedness and chaotic internal plumbing within the multistacked sandstone bodies. Heterogeneity of the reservoir systems is enhanced by random inclusion of numerous lenses of scour-based mudstone and siltstone. These fine-grained lenses disrupt communication between multistacked sandstone bodies as well as acting as a. local impermeable layer. Paucity of potential source rocks (coal, carbonaceous shale, and mudstone sequences) juxtaposed with these sandstone reservoir systems make them less signifi- cant as targets for exploitation. DISCUSSIONS OF FACIES AND ENVIRONMENTS The lithostratigraphic sequences and facies associations of the Clamgulchian type section of the Sterling For- mation provide a knowledge of their depositional facies, environments, and systems. Depositional facies are inter- preted from the facies associations, relying to a great extent on sedimentary models (Schumm, 1977; Galloway NORTH and Hobday, 1983; Reading, 1986). Comparison of physi- cal attributes of the depositional facies to those formed by modem processes defines environments of related depositional systems. Thus, environmental interpretations result from a range of process-response models in the context of the genetic stratigraphy and facies. In general, the lithostratigraphic sequences and facies associations in the study interval reflect various fluvial environments. The fluvial environments are represented by (1) the fining-upward sandstone sequence deposited in a fluvial channel, (2) the mudstone and siltstone sequence deposited in an overbank-flood-plain environment, and (3) the coal, carbonaceous shale, and tonstein deposited in a mire environment. These sequences, grouped as facies associations, suggest that the fluvial environments were overlapped through time and space. That is, a fluvial system composed of these discrete environments changed in character through time during deposition of the Clam- gulchian type section of the Sterling Formation. More specifically, the fluvial-channel morphology changed tem- porally (fig. 11) from low-sinuosity, bedload stream resulting in deposition of homogeneous sandstone reservoirs to high-sinuosity mixed bedload and suspended-load streams resulting in deposition of heterogeneous sandstone reservoirs. The lithostratigraphic sequences and facies associations in the lower part of the type section suggest that the fluvial system comprised well-developed mires that were associated with the flood plain. However, interbedding of coal, carbonaceous shale, and tonstein sequences with mudstone and siltstone sequences indicates that the mires were low lying or topogenous to permit detrital influx during floods. Relatively thin mudstone and siltstone sequences indicate diminished suspended load of the fluvial Figure 9. Sandstone reservoir framework architecture in middle part of Clamgulchian type section of the Sterling Formation. SOUTH
RESERVOIR FRAMEWORK ARCHITECTURE, CLAMGULCHIAN TYPE SECTION, STERLING FORMATION 127 channels; thus, fewer fine-grained sediments were avail- able to choke organic accumulation in the low-lying mires. The presence of a few coarsening-upward sandstones (for example, the units below coalbeds in fig. 5) in mudstone and siltstone sequences represents crevasse splays during floods, which are possible avenues for the limited detrital influxes to the mires in distal areas of the flood plains. Point sources of crevasse splays were moderately low- sinuosity bedload streams in which downstream migration by lateral and mid-channel bars was the dominant process of channel infilling as indicated by the multistory, multilateral, fining-upward sandstones. In the middle part of the study interval the fluvial system was dominated by fluvial channel and overbank- flood-plain environments. The absence of coal, carbonaceous shale, and tonstein sequences in this facies association suggests that mires were not developed or were outside these fluvial channel, overbank, and flood- plain belts. The multistory, multilateral fining-upward sandstones with a few scour-based mudstone and siltstone sequences represent active bedload and passive suspended load fluvial channel fills. In the active-bedload fluvial- channel fill, alternating lateral and mid-channel sand bars form downstream-migrating bars. This is indicated by scour-based sandstone bodies with megaforesets and trough crossbeds. In the passive suspended-load fluvial- channel fill, channel diversion or avulsion is punctuated by deposition of volumetrically minor channel plugs consisting of thin, discontinuous, scour-based mudstone and siltstone sequences. This fining-upward sandstone complex, which was formed in relatively low-sinuosity - J -- U w Mudstone and channel plug) braided channels, grades laterally and vertically into mudstone and siltstone sequences formed in channel margins and interchannel flood plains by bank overflow or bank breaches. Thin mudstone and siltstone sequences suggest redistribution of fine-grained suspended load farther into the outlying flood plains, which may have contributed to choking organic accumulation in topogenous mires. Alter- nating braidbelt, channel margin, and interchannel flood- plain deposits in the middle part of the Clamgulchian type section of the Sterling Formation reflect reoccupation and consequent stacking of channel-overbank-flood-plain sediments. This may be due to influence by either local subsidence resulting from tectonism, sediment auto- compaction, or confinement by relatively resistant fine- grained flood-plain sediments. In the upper part of the Clamgulchian type section of the Sterling Formation, the lithostratigraphic sequences and facies associations represent fluvial braidbelt- overbank-flood-plain deposits similar to those of the middle part of the interval. However, a difference lies in the occurrence of minor coal, carbonaceous shale, and tonstein sequences in the lower part of the interval and in the framework architecture of the fining-upward sandstone complex in the upper part of the interval. The coal, carbonaceous shale, and tonstein sequences probably represent accumulations in topogenous mires that formed in distal parts of the interchannel flood plains, which consequently encroached on abandoned braidbelt- overbank-flood-plain deposits. However, thick mudstone and siltstone sequences with accompanying coarsening- upward sandstone suggest abundant suspended-load && Megaforesets %- Ripple laminations LlU Trough crossbeds @ AA Root marks Figure 10. Sandstone reservoir framework architecture in upper part of Clamgulchian 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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APPEAL FOR NONPROLIFERATION OF ESCA
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APPEAL FOR NONPROLIFERATION OF ESCA
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Page 85 and 86: FAVORABLE AREAS FOR METALLIC MINERA
Page 97 and 98: GOLD AND CINNABAR IN HEAVY-MINERAL
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Page 180 and 181: 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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