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

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52 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 redistribution. The thickest of the known sandstones is about 90 m, which approaches the limit of seismic-reflec- tion resolution. Mapping the channels may be possible only if additional borehole data become available. Certainly, the overall low density of exploratory drilling of the LCU interval allows for, or cannot preclude, such an occurrence. The lowstand and transgressive sandstones of sequence 1 were covered by mudstones of the pebble shale unit (Bird and Molenaar, 1992), which is also part of the transgressive systems tract. The pebble shale unit, an im- portant hydrocarbon source rock and seal along the North Slope (Bird, 1991), represents the youngest rock unit of northern derivation. The pebble shale unit was covered by distal mudstones of the southerly-derived Brookian (tectonostratigraphic) sequence of latest Jurassic(?) to Holocene age. These distal mudstones are variously assigned to the gamma-ray zone, the uppermost part of the pebble shale unit in the western part of the North Slope, or to the Hue Shale in the eastern part of the North Slope (Molenaar and others, 1987). These mudstones, which represent increasing amounts of time in a northeasterly di- rection, include deposits assigned to the highstand systems tract, but they probably represent so much time that they must include all systems tracts developed from the Albian to the Campanian in the eastern area, near the Arctic National Wildlife Refuge. Thus, sequence 1, extending from the LCU to the gamma-ray zone, spans the transition from the Ellesmerian (equivalent to the Beaufortian sequence of some authors) sequence to the Brookian (tectonostratigraphic) sequence. 0 0 50 150 MILES 100 150 200 KILOMETERS - EXPLANATION Thrust fault boundin northern limit of Endicott andbe Long A Mountains subterranes - Approximately located Areas exposed by erosion in Hauterivian underlain by: Sedimentary fades of upper Deformation front of the part of Ellesmerian sequence Romanzof and Tigara uplifts Lower part of Shallow-shelf fades Ellesmerian sequence .... Slope and basin facies Normal fault - Hachures on downthrown side *-*-.... Facies boundary Figure 4. Paleogeography and paleogeology of North Slope at time of maximum regression in Early Cretaceous (Valanginian and Hauterivian) time. This figure, modified from Moore and others (1992), shows interpreted lowstand and transgressive systems tracts sandstones associated with sequence 1 of this report. Exposed area shows extent of Lower Cretaceous unconfomity Pre-Mississippian rocks and underlying rocks. Lowstand deposits may include shelf sandstone in Tunalik #1 well, and Tingmerkpuk subunit of Ipewik unit of Crane and Wiggins (1976). Transgressive and partly lowstand sandstones consist of Walakpa sandstone of local usage, upper part of Kuparuk Formation, Put River sand of local usage, Thomson sand of local usage, and Kemik Sandstone.
DEPOSITIONAL SEQUENCES IN ATIGUN SYNCLINE AND SLOPE MOUNTAIN AREA SEQUENCE BOUNDARY 2 The exposure of the Fortress Mountain Formation along the north wall of Atigun Gorge is one of the few places where the lower contact of the Fortress Mountain may be examined. At this locality the Fortress Mountain Formation overlies at least 600 m of gray concretion-bear- ing mudstone and thin-bedded sandstone that has been mapped as the Neocomian Okpikruak Formation by BrosgC and others (1979) or the Albian Torok Formation by Mull and Harris (1989, fig. 99). No direct paleontologic determinations of the age of these strata at Atigun Gorge are known. Whereas the base of the Fortress Mountain Formation is clearly unconformable in areas to the west, such as at Ekakavik Mountain and Castle Mountain (Mull, 1985), the lower contact of the Fortress Mountain Forma- tion was not mapped as unconformable in the Atigun Gorge area (Brosgt and others, 1979). The implication is that sedimentation may have been continuous throughout Early Cretaceous time in this area. However, the lower contact of the Fortress Mountain Formation in the Atigun area was reported to be unconformable by Crowder (1989, p. 296). Our preliminary work suggests that sandstone and mudstone of the basal part of the Fortress Mountain For- mation appear to fill what may be a valley incised into the underlying mudstone of the Torok or Okpikruak Forma- tions. Figure 5 is a panoramic view of part of the north wall of Atigun Gorge illustrating these relationships. The possible incisement of marine mudstone and later fill by fluvial and marine deposits led us to place a sequence boundary at this position in Atigun Gorge. The lower part of the Fortress Mountain Formation may be nonmarine to shallow marine in origin. These deposits were then covered by marine mudstone and sandstone as transgres- sion progressed. This possible sequence boundary and the associated sedimentary deposits as exposed along Atigun Gorge require more study in terms of parasequences and depositional environments. The lower part of this sequence in west-facing slopes at Atigun syncline was re- ferred to as Unit A by Crowder (1989) and Lithosome 4 by Goff (1990), and was interpreted by them to be marine in origin. Sandstones of the Fortress Mountain Formation in a similar stratigraphic position in the west-central part of the North Slope were interpreted as deep marine in origin (Molenaar and others, 1988). The marine sandstones grade up into channelized conglomerate and conglomeratic sandstone (unit B of Crowder, 1989; Lithosome 6 of Goff, 1990; Ryherd, 1990) at Atigun syncline east of Galbraith Lake (fig. 1B). Some of the sandstones we examined in the west-facing expo- sures displayed excellent examples of lateral-accretion bedding, formed as the result of the migration of sinuous channels. Some of the conglomerate and conglomeratic sandstones containing lateral-accretion bedding were stacked, forming composite conglomeratic sandstone bodies. The near-lack of mudstone and shale in this part of the section was noted by Ryherd (1990), who interpreted these conglomerates and conglomeratic sandstones as fan-deltaic deposits. The lack of turbidites and shales interbedded with the channel sandstones and the abun- dance of plant debris suggest that the majority of channel sandstones in this interval are nonmarine in origin. These same deposits have been interpreted as being of submarine fan origin, which is easy to understand given the poor exposure and lack of definitive sedimentologic criteria (Goff, 1 990). Figure 5. Photomosaic illustrating possible position of sequence others (1 979), mudstones do not appear to be deformed for boundary 2 along south-facing slope, north side of Atigun several hundred meters below contact, thus the Torok(?) Fm. Gorge. Sandstones and mudstones of basal part of the Fortress designation for them in this figure. Photomosaic taken looking Mountain Formation appear to onlap mudstones of the Torok(?) north from south side of Atigun Gorge, approximately 7.5 km Formation. Although latter rocks were mapped as deformed east-northeast of Atigun River bridge. Bottom width of mosaic strata of the Okpikruak Formation in this area by Brosgt and represents approximately 2.5 km. 53
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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
Page 7 and 8: 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
Page 57: DEPOSITIONAL SEQUENCES IN ATIGUN SY
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 SYSTEMS
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EARLY CENOZOIC DEPOSITIONAL S YSTEM
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RESERVOIR FRAMEWORK ARCHITECTURE, C
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GEOCHEMISTRY OF OPHIOLITIC ROCKS FR
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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 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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