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

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114 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY. 1992 range from 1:4 to 1:30, are within the range for anastomosed streams suggested by Smith (1983). The common occurrence of mud drapes between steeply dipping silty sandstone foresets and the vertical stacking of these channel-fill units with low thickness-to-width ratios (1:4 to 1:6) are also characteristic of anastomosed streams (Galloway and Hobday, 1983). Widespread bioturbated mudstone, siltstone, and silty sandstone deposited in lacustrine set- tings suggest a close association of the anastornosed channels with flood-basin lakes. Coal zones immediately below and above deposits of these fluvial systems represent organic accumulation in mires which developed on abandoned meander (Premier coal zone) and anastomosed (Jonesville coal zone) belts. The high ash content (as much as 25 percent) and abun- dant carbonaceous shale and mudstone interbeds associated with the coalbeds indicate that these mires were low lying or topogenous. Although mires formed on topographically high meander belts such as that below the Premier coal zone, base level must have been lowered periodically to permit drowning by detrital influxes. However, detrital in- WEST - L fluxes were probably short lived, allowing for the reestab- lishment of the mires, until a final sustained influx termi- nated peat accumulation. Since the meander-belt deposits represent deposition in relatively stable, laterally aggrading channels probably controlled by stable tectonic condition, we suggest that the lowering of base level was caused by local subsidence due to sediment compaction. Further- more, splitting of the Premier coal zone by meander-belt- overbank deposits and the corresponding thinning of the underlying meander-belt deposits indicates that shifts or avulsion of stream courses promoted differential compaction of laterally offset channel sandstones and fine-grained flood-plain sediments. In addition, channel avulsion and diversion of crevasse splays may have been enhanced by vertical components of movement along contemporaneous left-lateral, oblique-slip faults. In contrast, the lowering of base level in association with the mires in which the Jonesville coal zone accumulated may have been controlled by regional tectonic subsidence. Regional lowering of base level probably started during sedimentation of the anastornosed streams, whose vertical aggradation is related .- + z t L o LL - EXPLANATION - Conglomerate -'- Trough crossbeds I METERS I Sandstone Planar crossbeds 0 . - Ei Mudslone and siltstone -- -- Ripple laminations relative horizontal L - A 0 Fault-Arrows indicate displacement METERS Figure 10. Lateral-facies architecture of lowermost part of Wishbone Formation. Architecture is restored from vertical component of movement of left-lateral, oblique-slip faults whose positions are shown in upper part of diagram. Reconstruction based in part on stratigraphic data of Clardy (1974). 300
EARLY CENOZOIC DEPOSITIONAL SYSTEMS, MATANUSKA COAL FIELD to rapid basin subsidence (Smith, 1983). Thus, the vertical repetition of coal zones in the Chickaloon Formation reflects an intrabasin interaction of autocyclic (avulsion and abandonment of stream courses influenced by local sediment compaction) and allocyclic (regional basin subsidence) processes. WISHBONE FORMATION Facies characteristics of the lowermost part of the Wishbone Formation, which consists mainly of framework-supported conglomerates and subordinate gritty sandstones, indicate deposition in high-gradient braided streams and alluvial fans. Steepening of stream courses at this time reflects extrabasinal or provenance uplift (Clardy, 1974). The multiscoured, imbricated, pebbly conglomerates were deposited as gravel bars by traction currents in channelized flows. Fining-upward conglomerate suggests gradual infilling of active channels by migrating gravel during floods. Waning stages of floods are reflected by ripple-laminated gritty sandstone caps on these gravel bars. Interbedded conglomerates and sandstones reflect the flashy discharge af the braided streams. Erosional-based sandstones interbedded with the conglomerates, which are trough and planar crossbedded, are attributed to sand-bar accretions marginal to or on top of gravel-bars during waning flows. Vertical stacking of the conglomerates suggests their deposition in laterally shifting, diverging, and con- verging channels such as in a broad-belt braided stream. The lower part of the study interval in the Wishbone Formation probably reflects a dichotomy of braiding by sand and gravel bars. The sandy braid bars suggest development of a passive belt of braided streams contemporaneous to gravelly braid bars, which represent active belts of braided streams. This dichotomous relationship of bedload sediments in a depositional continuum reflects gravel-bar sedimentation during flood periods succeeded by pro- longed gradual waning periods, which resulted in infilling of gravel belt margins of braided streams by sand bars. Waxing and waning depositional events in the braided streams are in contrast to the upper part of the study interval, which was deposited during perennial flood periods. This facies sequence indicates sedimentation at distal and proximal areas of humid alluvial fans (Clardy, 1974). Pebble imbrication and crossbed directions (Clardy, 1974) suggest westerly flow of the braided streams and south- westerly flow of streams within the alluvial fans. SUMMARY Facies architecture indicates that the fluvial deposition of west-southwest-flowing meandering to anastomosed streams in the Chickaloon Formation was 115 succeeded by westerly flowing braided streams and south- westerly prograding alluvial fans in the Wishbone Forma- tion. The meandering and anastomosed streams probably were a part of a flow-through, trunk-tributary, fluvial sys- tem that drained the "Matanuska geosyncline" (fig. 11). These streams may have been propagated under varying base-level conditions influenced by basin subsidence (allocyclic), avulsion due to sediment compaction (autocyclic), and growth faulting. When subsidence rates were low, lateral aggradation by meandering streams occurred in stable, confined meander belts. This was accompanied by flood-plain clastic accretion and organic accumulation in topogenous mires, which developed on abandoned meander belts removed from detrital influx. In areas dissected by left-lateral, oblique-slip faults, vertical movements localized aggradation by streams captured on downthrown areas. In addition, probable diversion of crevasse splays on downthrown areas caused splitting of the coalbeds accompanied by overthickened interburden intervals. In contrast, mires on upthrown areas, where thick coalbeds accumulated, were undisturbed by detrital influx. Flood-plain sedimentation by crevasse splays during flood discharges from meandering streams initiated the process of anastomosis. Progradation of crevasse splays into the flood plain permitted bifurcation and merging of crevasse channels. This process, influenced by a high rate of basin subsidence, expanded areas of anastomosis and flood-basin lake development, and enhanced vertical aggradation of crevasse channel-overbank sediments. Topogenous mires formed in flood basins enlarged onto abandoned sites of anastomosis. As anastomosis matured, a crevasse channel was transformed into a major conduit and through time, developed from a low- to a high-sinuos- ity stream, leading to the onset of a meandering stream and repetition of the fluvial cycle. However, we propose that these streams occurred alternately through time and geo- graphically coexisted in the drainage basin. Vertical and lateral aggradation of these streams were affected by local subsidence due to sediment compaction (autocyclic) and regional basin subsidence (allocyclic). During raising and steepening of the regional base level (due to extrabasinal factors), the mire-prone, meandering and anastomosed fluvial systems were replaced by high-gradient, braided streams and transverse alluvial fans. Uplift of the Talkeetna Mountains along the Castle Moun- tain fault north of the Matanuska Valley controlled the gradient of the streams and their sediment supply (Grantz, 1966; Clardy, 1974). Mountain-front, alluvial-fan conglomerates represent IocaIized deposits from intermittent movements along the fault. These alluvial fans supplied sediments to the lower-gradient, non-mire-forming, braided streams. Bimodal sediment loads in the braided streams reflect either episodic flash floods due to seasonal climatic changes or periodicity of provenance uplift.
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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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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
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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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