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

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50 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 general lack of continuous outcrop, paucity of close bios- tratigraphic control in many units, general lack of borehole control, and structural overprint of many units. INTERPRETATION OF SEQUENCE BOUNDARIES Several stratigraphic units were examined during this study, including the Fortress Mountain Formation (Aptian? and Albian; Bird and Molenaar, 1992), Torok Formation (Aptian to Cenomanian; Weimer, 1987), the Nanushuk Group (Albian and Cenomanian; Mull, 1985), and the Gilead Creek sandstone unit (Albian? and Cenomanian?). This work, combined with a survey of available literature, forms the basis for the following discussion of sequences and the five sequence boundaries interpreted during this study (fig. 3). SEQUENCE BOUNDARY 1 The first sequence boundary considered for this study is the widely recognized Lower Cretaceous unconformity (LCU) and its equivalent down-dip conformable surface. In the foothills of the northeastern Brooks Range, the LCU cuts rocks as young as Valanginian and is overlain by rocks as old as probable Hauterivian (Molenaar, 1983). 0 SHALLOW MARINE SHELF AND SLOPE - paraseijuence transgressive surface \ Figure 2. Diagram of systems tracts and parasequence stacking patterns in type 1 depositional sequence (after Mitchum and Van Wagoner, 1991). Lowstand tract contains submarine fan deposits and prograding deposits of lowstand wedge, consisting of incised valley fill of fluvial and estuarine sandstones. These deposits are overlain by transgressive surface and retrogradational The unconformity bevels the Jurassic and Cretaceous Kingak Shale and older rocks in a northerly direction (Molenaar, 1983; Bird and Molenaar, 1987). To the south, the unconformity dies out and is represented by a conformable surface extending southward into the axis of the basin (fig. 4). Several sandstone units were deposited on this surface. Because of their quartzose composition, indicating a northerly provenance, and their close association with hydrocarbon source rocks, these sandstones are attractive exploration targets. The sequence model suggests the occurrence of lowstand shelf and deeper marine lowstand fan deposits south of the southern limit of the LCU. An apparent ex- ample of a shelf sandstone is the upward-coarsening sandstone beneath the pebble shale unit penetrated in the Tunalik well in the western part of the National Petroleum Reserve in Alaska (NPRA) (Magoon and others, 1988, plate 19.2). Furthermore, we speculate that the quartzose sandstones of the Tingmerkpuk subunit of the Ipewik unit of Crane and Wiggins (1976), as mapped in the northern part of the DeLong Mountains (Curtis and others, 1990; Ellersieck and others, 1990), represent a lowstand fan de- posit, perhaps derived from the Tunalik shelf region (fig. 4). Subaerial erosion creating the LCU is likely to have produced incised valleys, controlled in part by structure (Noonan, 1987). As sea level rose, the valley system would have aggraded by fluvial and estuarine depositional processes, whereas shallow-marine processes would have - TYPE 1 DEPOSITIONAL SEQUENCE BOUNDARIES - SYSTEMS TRACT BOUNDARIES parasequence set of transgressive system tract. In marine sec- tions, transition from upper part of transgressive systems tract to lower part of highstand tract commonly contains condensed sec- tion. Highstand tract contains progradational parasequence set (after Mitchum and Van Wagoner, 1991). Lower sequence boundary and transgressive surface can merge in updip direction.
DEPOSITIONAL SEQUENCES IN ATIGUN SYNCLINE AND SLOPE MOUNTAIN AREA predominated during thi overall transgression. Several sandstone units were deposited on the LCU, including the Put River sand, the Thomson sand, and the Walakpa sandstone (all of local usage), as well as the Kemik Sandstone (Melvin, 1986; Mull, 1987) and the upper part of the Kuparuk Formation (fig. 4). Additional unnamed sandstones at this stratigraphic position are penetrated by nu- merous wells, but their areal extent is unknown (Bird, 1986, fig. 7). The Put River sand (Jamison and others, 1980; Bird, 1988), located just west of Prudhoe Bay (fig. 4), has an areal extent of about 25 square kilometers, a maximum thickness of about 21 m, and an elongated north-south sinuous outline consistent with either a channel or bar de- posit (see isopach map in McIntosh, 1977, or the outline map in Bird, 1986, fig. 8). The Thomson sand (Gautier, 1987; Bird and others, 1987; Bird, 1988) consists of as much as 90 m of sandstone composed of more than 50 percent detrital dolomite grains, and locally includes pebble, cobble, and boulder conglomerate with boulders as large as 1.4 m (Gautier, 1987). These deposits are cer- tainly of local derivation and may represent a nonmarine channel sandstone deposited in an incised valley as part of sequence the lowstand wedge. Just south of Barrow (fig. I), the Walakpa sandstone (Schindler, 1988) is poorly known, al- though recent drilling in the area suggests that the Walakpa sandstone is more sheetlike than the Put River sand (R. Glenn, personal communication, 1992). The Kemik Sandstone (Mull, 1987) is one of the thickest and most widespread of these sandstones (Bird and others, 1987, fig. 7.11). The upper member of the Kuparuk For- mation (Carman and Hardwick, 1983) (equivalent to the Kuparuk River Formation of Masterson and Paris, 1987) overlies the LCU and was deposited in a marine shelf set- ting during a period of extensional faulting. This part of the Kuparuk displays considerable stratigraphic complexity including two transgressive episodes and two regressive episodes plus the cutting of an intra-unit unconformity. The interplay of eustatic sea-level change and tectonism has yet to be resolved for these rocks. The lowstand systems tract depositional model pre- dicts the occurrence of sets of sandstone-filled channels at the LCU. Their thickness and preservation depends on a number of factors, including competency of the fluvial system, a source of sediment, and preservation of the valley fill during transgression with limited reworking and NE A' Gilead Creek area Figure 3. Summary schematic diagram of five depositional sequence boundaries tentatively identified in this study. Sequence boundary 6 is early(?) Cenomanian depositional sequence boundary described by Phillips and others (1990). Nomenclature for units in the Nanushuk Group slightly revised from Huffman (1989, fig. 227). 51
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Geologic Studies in Alaska by the U
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CONTENTS Introduction Cynthia Dusel
Page 5 and 6: 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: DEPOSITIONAL SEQUENCES IN ATIGUN SY
Page 59 and 60: 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
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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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