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

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110 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 siltstones. The middle coalbeds commonly contain bone in contrast to the uppermost coalbed, which commonly contains bony coal. The conglomeratic sandstones (see fig. 8) grade upward into a thick (as much as 10 m) mudstone with some siltstone interbeds, which in turn, is unconformably overlain by conglomerates of the Wishbone Formation. In the mudstone-siltstone dominated facies sequence between the Premier and Jonesville coal zones, the facies sequence consists of a wide, erosional-based, fining-upward sandstone complex at the west highwall and grades eastward to coeval narrow, thin, multierosional, fining- WEST upward sandstones associated with coarsening-upward mudstones, siltstones, and sandstones as well as thin to thick, discontinuous coal and carbonaceous shale beds (fig. 9). Abundant vertical petrified tree trunks, as much as 6 m in height, occur along the extent of this facies sequence. The tree trunks are primarily associated with fining-upward sandstones and a coarsening-upward mudstone, siltstone, and sandstone complex. The uppermost part of the facies sequence lying immediately below the Jonesville coal zone consists of an interbedded mudstone, siltstone, and silty sandstone complex that thickens eastward. This complex is parallel to ripple laminated where not 5, Trough crossbeds (large-scale) 1 / (11 t Jonesvilla - - . -. . . Mudstone, siltstone, Overbank and - -. Ed and silty sandstone 1 channel plug Coal, carbonaceous shale, }Mire and coal and carbonaceous shale & Trough crossbeds (small-scale) "-- - - Ripple laminations METERS 8 8 Ironstone B Bentonite O METERS AA Root marks 111 Fault-Arrows indicate relative horizontal displacement
EARLY CENOZOIC DEPOSITIONAL SYSTEMS, MATANUSKA COAL FIELD destroyed by nondescript poor to intense vertical and horizontal burrows. In the lowermost part of the conglomeratic Wishbone Formation over a distance of 1.4 km, a continuous erosional contact is recognized between mudstone of the Chickaloon Formation and the erosional-based conglomerate and sandstone of the Wishbone Formation (fig. 10). Conglomerates lie directly on top of the Chickaloon mudstones, however, interbedded gritty sandstones and conglomerates occur at the eastern part of the cross section. This conglomerate-gritty sandstone complex is overlain across the extent of the cross section by a thick, internally EAST 111 scoured conglomerate complex. Thus, the lowermost part of the Wishbone Formation may be divided into two major subfacies: (I) a lower conglomerate-sandstone-dominated subfacies merging with a conglomerate-dominated facies, both of which are locally distributed, and (2) an upper conglomerate-dominated subfacies that is regionally distributed. FACIES INTERPRETATIONS Facies characteristics of (1) the Eska-Midway-Pre- mier coal zone interval, (2) Premier-Jonesville coal zone interval, (3) the interval above the Jonesville coal zone of the Chickaloon Formation, and (4) the lowermost part of the Wishbone Formation indicate deposits of (1) low- gradient high sinuosity (meandering) bedload, (2) low- sinuosity anastomosed streams, (3) high-gradient braided streams, and (4) alluvial fans, respectively. The coal zones occur alternately above fining-upward- and coarsening-upward-dominated fluvial sequences suggesting development of topogenous mires in vertically and laterally aggrading fluvial systems. CHICKALOON FORMATION The Eska-Midway-Premier facies sequence, which consists of multiscoured, multistory, multilateral, finingupward, conglomeratic sandstones with crude sigmoidal macroforms, reflects fluvial deposits of high-sinuosity -1 - - - (meandering) u, streams. Dominance of sand bedload (less than 5 percent silt and clay channel-fill sediment), traction currents (trough crossbeds), and lack of channel bars suggest deposition in bedload streams affected by seasonal - discharge that favored preservation of sinuous-crested dune -------- ~- ~ - - - -. -. bedforms (Schumm, 1972, 1977; Ethridge and Schurnm, . . - .. -. _ _ _ . . - . .- - - .- . - .. - -..- - . - . . -.-: 1 1978). Amalgamation of these channel sandstones sug- - - .- - . _ . -- '1 -_ -. - . - _ - . - . . - - . gests a long-term development of a relatively stable mean- , .- .. - .-. .. - a - -. . _ _. - _.- - & -- der belt in which lateral aggradation bv downstream - - -- -- -- - - 0 -- - - .- - meander migration was dominant over channel cut-off~ and shifting (avulsion) of channels. Subsidence and slow rates of vertical aggradation in the surrounding flood plain a, prevented preservation of point bars (sigmoidal macro- > forms), which were successively eroded from older chan- a, -- - - - - - - - - - - - - - nels by younger migrating channels. Trough crossbeds in - - . --..-.. - _._ - .. .- - ._ ___ - -. - - .- .._ -_.__.__. - .. - -. -.. _ . - the channel sandstones indicate a west-southwest flow ' . - .. -.. _. ._._ -.. - .. - ..@- .. - .. - .. - .. -..-. direction for these meandering streams. Overthickening of -_ ..the Eska and Midway coal zones against laterally coeval, - Figure 8. Lateral-facies architecture of Jonesville coal zone of the Chickaloon Formation and overlying sedimentary interval. Architecture is restored from vertical component of movement of Jonesville left-lateral, oblique-slip fault whose location is 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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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
Page 111 and 112: EARLY CENOZOIC DEPOSITIONAL S YSTEM
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Page 125 and 126: RESERVOIR FRAMEWORK ARCHITECTURE, C
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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