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

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120 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 during the Pliocene by a regressive phase. The stillstand period resulted in deposition of the upper part of the Tyonek Formation and lower part of the Beluga Forma- tion. Cyclical sequences of fining-upward sandstone, underclay, and coal in these formations were interpreted by Merritt (1986) as deposits formed during periods of source-rock uplifts and rapid basin subsidence. Conse- quent to sea-level fall or uplift, regression followed with deposition of the upper part of the Beluga Formation and Sterling Formation. Uplift occurred in the Kenai-Chugach Mountains east of the Cook Inlet basin during deposition of the Beluga Formation (Hartman and others, 1972; Hayes and others, 1976). However, uplift along the Aleu- tian-Alaska Range provided detritus during deposition of the Sterling Formation (Hartman and others, 1972; Hayes and others, 1976). The Beluga and Sterling Formations were interpreted by Hite (1975, 1976) and by Hayes and others (1976) as deposits of small, high-gradient braided streams flowing on alluvial fans and broad alluvial plains during Miocene and Pliocene time; subsequently these were replaced by large, meandering streams on broad alluvial plains during Pliocene time. Flores and Stricker (1992) modified these interpretations to include anastomosed streams in the upper part of the Beluga Formation and small meandering streams in the upper and middle parts of the Sterling For- Stratigraphic section 9860-9862 USGS fossil plant localities from Wolf and others (1966) 0 2 KILOMETERS Figure 2. Map of study area in vicinity of Clam Gulch showing outcrops of the Sterling Formation along beach coast, locations of measured sections, and paleobotanical sampling localities of Wolfe and others (1966). mation. Flores and Stricker (1992) suggested that coal accumulation was better developed in mires associated with anastomosed streams as compared to those mires related to the small meandering streams. These meandering and anastomosed streams probably were an integral part of flu- vial networks of the river drainage basins, which were temporally related to the braided and large, meandering streams. Parts of the Beluga and Sterling Formations exposed along the east shore of the Cook Inlet and along the north shore of Kachemak Bay are, based on megaflora content, Homerian Stage or late Miocene in age (Wolfe and others, 1966). Type Homerian flora reflect a cool, temperate climate. The upper part of the Sterling Formation in the Clam Gulch area, from Happy Creek (13 km south of Ninilchik) northward to a point 6.4 krn north of Clam Gulch on the east shore of Cook Inlet, is Clamgulchian Stage or Pliocene in age (Wolfe and others, 1966). In this study this area is informally called the Clamgulchian type section. The Clamgulchian megafloras reflect a cool, temperate climate but one cooler than the Homerian climate and considerably warmer than that of Cook Inlet today. The Clamgulchian cool climate was not a significant factor in organic-matter preservation or coal accumulation, be- cause peat is preserved and accumulated in modem cool- climate mires (Martini and Glooschenko, 1985). Thus, the sparse coalbeds in the Clamgulchian type section of the Sterling Formation may be explained by depositional rather than climatic factors. GENETIC STRATIGRAPHIC ANALYSIS Genetic stratigraphic analysis of the Clamgulchian type section of the Sterling Formation may be described in a hierarchical approach that consists of the following, from the smallest to largest rock properties: (1) internal sedi- mentary structures, grain size, and color, (2) nature of con- tacts, (3) lateral and vertical facies organizations or architecture, and (4) cross-sectional geometry. This ana- lytical approach permits interpretation, at different scales, of the depositional systems of lithostratigraphic sequences and facies associations. A lithostratigraphic sequence is a rock type or a grouping of rock types that have similar grain size (or grain size variation) and lithic composition. Facies association is the vertical and lateral variations be- tween lithostratigraphic sequences. LITHOSTRATIGRAPHIC SEQUENCES Lithostratigraphy of the Clamgulchian type section of the Sterling Formation may be grouped into (1) a lower cOal~ and sandy sequence and (2) an upper coal-pOOr and sandstone-and-mudstone-dominated sequence. Volumetri-
RESERVOIR FRAMEWORK ARCHITECTURE, CLAMGULCHIAN TYPE SECTION, STERLING FORMATION 121 cally these sequences comprise as much as 75 percent sandstone, about 24 percent siltstone and mudstone, and 1 percent coal, carbonaceous shale, and tonsteins. SANDSTONE SEQUENCES Sandstone occurs in fining-upward sequences ranging from coarse to fine grained, light gray to buff, and locally include lag conglomerates of coal spars and pebble- to boulder-size mudstones. These sandstones are, in turn, amalgamated into multiscoured bodies with the internal scour bases locally marked by lag conglomerates. These multiscoured sandstones are up to 65 m in thickness and more than 3 km long. The sandstone bodies display I FORMATION THICKNESS (IN METERS) Unnamed deposits megaforesets as high as 3.5 m and common trough crossbeds as high as 1.2 m and 23 m long. A few planar crossbeds as high as 1.5 m occur in cross sets up to 3 m high. Convolute laminations as high as 3 m occur in zones as much as 6.7 m in height. Ripple laminations (climbing and current ripples) either locally cap sandstone complexes or occur within scoured sandstone bodies. Very fine grained sandstone occurs as tabular bodies with sharp to gradational bases and gradational tops. They commonly exhibit climbing and current ripple laminations. In places, a few narrow (a few millimeters across) and short (a few centimeters long) vertical and horizontal bur- rows have destroyed some or all of the ripple laminations. These sandstones, which are as thick as 2.5 m, are either DESCRIPTION Alluvium and glacial deposits PROPOSED STAGE (Wolf and others, 1966) Sterling Formation Sandstone, siltstone, mudstone, Clamgulchian --?---?-- 0-2.100 carbonaceous shale, and lignite Early Pliocene Homerian 'sandstone, conglom-\- Upper half of Miocene -? - - - -?- - eratic sandstone, silt- Beluga Formation stone, mudstone, Late early Miocene > 1,500 carbonaceous shale, 0' middle Miocene -, and subbituminous coal // Seldovian Tyonek Formation Sandstone, mudstone, Early Miocene - ?- - - ?- - siltstone interbeds, and- 1,200-2,350 subbituminous Late Oligocene coal Hemlock Conglomerate Sandstone and 90-270 conglomerate OLDER TERTIARY ROCI
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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 75 and 76: U-Pb AGES OF ZIRCON, MONAZITE, AND
Page 77 and 78: APPEAL FOR NONPROLIFERATION OF ESCA
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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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20 GEOLOGIC STUDIES IN ALASKA BY TH
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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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