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

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54 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 SEQUENCE BOUNDARY 3 Sequence boundary 3 was interpreted on the basis of sedimentologic discussions of Crowder (1987, 1989) and Goff (1990), who divided the Fortress Mountain Formation at Atigun syncline into various units based on gross sedimentology (fig. 6). Approximately at midpoint in the ex- posure of the Fortress Mountain Formation are thin coal beds overlain by sandstone and mudstone interpreted as marine in origin (Goff, 1990). The coals could belong to the highstand systems tract of sequence 2 (below sequence boundary 3), or they may belong to a valley fill in the lowstand tract of sequence 3 (above sequence boundary 3). We include the coals tentatively as part of the highstand tract of sequence 2, based on our preliminary observations, 4 upper part of Formation SEQUENCE 3 3 SEQUENCE 2 lower part of -- although many coals in the Cretaceous of the Western In- terior are part of lowstand systems tracts (Van Wagoner and others, 1990). We recognize that more work needs to be done to define the placement of a sequence boundary in this position. The mudstones containing the marine pele- cypods that overlie the coals were placed in the transgressive systems tract of sequence 3. In sections that are proximal to the basin margin, as the Fortress Mountain Formation probably is, the first deposits above the sequence boundary are commonly part of the transgressive systems tract (fig. 2). The marine sandstones and mudstones are overlain by a thick sequence of channelized conglomerates and con- glomeratic sandstones interpreted generally as fan-deltaic or fluvial in origin. The sedimentologic characteristics of Multiple, intraformational truncation surfaces I Upward coarsening I Large scale, mutually erosive channel fills C P Shale and minor coal beds Bedded conglomerate separated by thick shale intervals Fortress Mountain Poorly exposed sandstone Formation turbidites with minor pebbly sandstone beds (From Crowder, 1989) --- -- - Figure 6. Schematic diagram showing general sedimentology of the Fortress Mountain Formation along west-facing slopes of Atigun syncline (from Crowder, 19891, to which sequence boundaries have been added. Sequence boundary 3 is tentatively placed where sandstone and coal are overlain by marine sandstone and mudstone, as reported by Crowder (1989) and Goff (1990). Sequence boundary 4 is tentatively placed at the top of Fortress Mountain Formation, but below Torok Formation.
DEPOSITIONAL SEQUENCES IN ATIGUN SYNCLINE AND SLOPE MOUNTAIN AREA the sandstones, as described by Crowder (1989) and Goff (1990), suggest that the sandstones formed in low-sinuos- ity channels on an extensive braidplain, possibly on a fan delta. This is Unit C of Crowder (1989) and the upper part of Lithosome 6 of Goff (1990). The sandstones are overlain by more sheetlike, finer grained channel sandstones that possibly represent deposits of sinuous fluvial channels. SEQUENCE BOUNDARY 4 The fourth Early Cretaceous sequence boundary is postulated on the basis of regional considerations. This boundary in the Atigun syncline area is placed either within the uppermost part of the Fortress Mountain Forma- tion, where depositional conditions change from fluvial below to marine above as observed at Ekakevik Mountain (Hunter and Fox, 1976; Molenaar and others, 1988), or higher in the section where the Fortress Mountain Forma- tion is overlain by the Torok Formation, as is the case in the Igloo Mountain area (Chapman and Sable, 1960, fig. 8). The Atigun Gorge-Slope Mountain area is typical of much of the foothills of the Brooks Range in that the uppermost part of the Fortress Mountain Formation and the lowermost part of the Torok Formation are not preserved. At Atigun syncline, for example, the uppermost part of the Fortress Mountain Formation is composed of upward- coarsening conglomerate, sandstone, and siltstone record- ing fluvial deposition (Crowder, 1989). SEQUENCE BOUNDARY 5 Sequence boundary 5 at Slope Mountain is tentatively placed within the lower part of the Killik Tongue of the Chandler Formation of the Nanushuk Group (fig. 7), as de- fined in this area by Huffman and others (1985) and Huffman (1989). The placement of the sequence boundary in this position is tentative and requires more sedimentologic definition of the enclosing lithologies and extent of erosion on this possible sequence boundary, as is being done in the Upper Cretaceous Seabee Formation in this area (Phillips and others, 1990). Channels cutting into the lower part of the Killik Tongue were observed on south- facing exposures at Slope Mountain. These channels are either part of an incised valley fill or they simply represent distributary channels in a prograding deltaic shoreline. More work needs to be done on defining the magnitude and downcutting relationships of these channels to determine if they indeed are distributaries or the lower part of an incised valley fill. The upper part of the Torok Formation and the lower part of the Tuktu Formation at Slope Mountain contain several stacked progradational parasequences in shelf deposits, suggesting that they represent part of the highstand systems tract of the underlying sequence. Figure 7. View northeastward of the Nanushuk Group at Slope Mountain (Dalton Highway Mile Marker 297) to which sequence boundary 5 has been added. Placement of sequence boundary 5 is speculative, and needs to be fully documented in the field to determine extent of channeling at this position. Sequence boundary is placed within lower Killik Tongue of Huffman (1989) of the Chandler Formation. Other sequence boundaries may be present in this section, but their interpretation awaits further field investigation. Structure at bottom is the Trans-Alaska pipeline. 55
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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
Page 9 and 10: LATE HOLOCENE LONGITUDINAL AND PARA
Page 19 and 20: DEEP-WATER LITHOFACIES AND CONODONT
Page 25 and 26: Table 1. Locality register for key
Page 27 and 28: Table 1. Locality register for key
Page 37 and 38: LITHOFACIES AND CONODONTS OF CARBON
Page 49 and 50: LITHOFACES AND CONODONTS OF CARBONI
Page 51 and 52: Table 1. Conodont faunules and lith
Page 55 and 56: 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
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EARLY CENOZOIC DEPOSITIONAL S YSTEM
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EARLY CENOZOIC DEPOSITIONAL SYSTEMS
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RESERVOIR FRAMEWORK ARCHITECTURE, C
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GEOCHEMISTRY OF OPHIOLITIC ROCKS FR
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136 GEOLOGIC STUDIES IN ALASKA BY T
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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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