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

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WEST GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 Channel and Sandstone { & Megaforesets crevasse splay Jonesville coal zone Mudstone, slltstone, and sandstone Coal and ) Mlre carbonaceous shale ~~~~~h Crossbeds & - /V ,-- , R~pple lamlnat~ons -,-li)i] Srneot~tc mudstone a 1 Upright tree trunks ' V u X An~mal burrows Covered METERS 0 0.2 KILOMETERS 1 11 Fault-Arrows indicate relative horizontal displacement
EARLY CENOZOIC DEPOSITIONAL SYSTEMS, MATANUSKA COAL FIELD 113 multiscoured, conglomeratic sandstones, deposited where left-lateral, oblique-slip faults were developed, suggests that the vertical component of movement during deposition (growth faulting) controlled the position of fluvial channels and equivalent mires. That is, fluvial channels were captured and incised within downthrown areas, while mire development was prolonged on upthrown areas. A series of growth faults may also account for the overthickening and splitting of the Premier coal zone by diversion of fluvial drainages into the downthrown areas. Here, the greater proportion of clastic sequences and coal splits suggests a dominance of crevasse-splay deposition that was localized by the fault structures. Similar fault-controlled fluvial and mire development was described by Flores (1984) and Weisenfluh and Ferm (1984). In contrast to the facies of the interval above the Jonesville coal zone, the multiscoured, multistory, multilat- eral, fining-upward, conglomeratic sandstones with well- developed sigmoidal macroforms represents fluvial deposits of high-sinuosity (meandering) streams. Channel width-to-depth ratios of greater than 40 (60 to 90 for the well-developed sigmoidal macroforms or point bars) indicate deposition in stable bedload channels (Ethridge and Schumm, 1978). However, the occurrence of less than 5 percent silt and clay channel-fill sediments associated with the point-bar sandstones suggests a mixed sediment load. Amalgamation of channel deposits suggests that cut-offs (reflected by mixed-load or channel-plug deposits) and shifts (represented by the bedload or point-bar sandstones) of channels were the dominant process of lateral aggradation within the confines of the meander belt. Vertical aggradation in the surrounding flood plain due to sediment compaction influenced preservation of well-developed point-bar sandstones. Stream flow, based on trough crossbeds, was towards the west-southwest. Facies between the Premier and Jonesville coal zones, characterized by coarsening-upward sequences associated with coeval, thin, narrow, fining-upward sandstones and enclosed by thick muddy and silty sediments, indicate fluvial deposits of anastomosed streams and related flood basins. Coarsening-upward sequences were formed by crevasse splays that spilled into low-lying flood basins during high discharges of a low-sinuosity stream. Cre- vasse channels drained the splays, and continued pro- gradation of splay lobes permitted bifurcation and merging of the channels. Subsidence and confinement of these crevasse channels produced vertically aggrading, low-gradient channels and wooded levees. Low thickness-to-width ratios of the fining-upward channel sandstones, which Figure 9. Lateral-facies architecture of interval from Premier to Jonesville coal zones of the Chickaloon Formation. Architecture is restored from vertical component of movement of left-lateral, oblique-slip faults whose positions are shown in upper part of diagram.
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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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Page 137: GEOCHEMISTRY OF OPHIOLITIC ROCKS FR
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1 64 GEOLOGIC STUDlES IN ALASKA BY
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166 GEOLOGIC STUDES IN ALASKA BY TH
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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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