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

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56 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY. 1992 Additional sequence boundaries may be present within the Chandler Formation at Slope Mountain, but given the relatively poor exposure of the lithologies between sandstones and the fair exposure of the sandstones, they are not outlined for this study. More definition of the environments represented by the Chandler Formation would help to define these sequence boundaries, if present. GILEAD CREEK SANDSTONE UNIT The informally named Gilead Creek sandstone unit (Reifenstuhl, 1989; Pessel and others, 1990) is exposed in the foothills about 60 km northeast of Slope Mountain (figs. 1, 3). This sandstone interval, 600-900 m thick, is anomalous in that it lies in close proximity to the Kingak Shale. Although exposures are poor, present mapping does not allow for hundreds to thousands of meters of mudstones of the Torok or Okpikruak Formations that typically occur in this position. Because this is a structur- ally complex area, it is possible that these mudstones, once present, are now faulted out. Such faulting, how- ever, would require either a normal fault (in a dominantly compressional regime) or a thrust fault that places younger over older strata. We have no evidence to sup- port either of these possibilities. We suggest that the Gilead Creek sandstone unit is the stratigraphic equivalent of part of the Torok Formation, or that it is somewhat younger and has scoured away an unknown amount of the Torok. Such relations could be explained by a lowstand fan deposit. During relative sea-level low stands, the depositional sequence model predicts the development of lowstand fans in a downdip position at the same time that canyons and valleys are cut in an updip position. We concur with the interpretation of Molenaar and others (1984) of the Gilead Creek sandstone.unit as composed mainly of turbidites. Such an interpretation is consistent with a lowstand fan deposit. The age of the Gilead Creek sandstone unit is con- strained by a marine pelecypod identified by W.P. Elder (personal communication, 1992) as Inocerarnus dunveganensis of Cenomanian age from the upper part of the Gilead Creek sandstone unit (R. Reifenstuhl, personal communication, 1991) and marine pelecypods Buchia of Valanginian age from the underlying Kingak Shale (Pessel and others, 1990). With such age constraints, the Gilead Creek sandstone unit could represent a Cenomanian lowstand fan that scoured away Albian and Aptian mudstones, or it more likely represents an amalgamation of lowstand fan deposits spanning several depositional sequences. DISCUSSION Five depositional sequence boundaries have been outlined from exposures of the Fortress Mountain Formation, Torok Formation, and the Chandler Formation of the Nanushuk Group in the area from Atigun syncline and Slope Mountain, and from examination of other sandstones including the Gilead Creek sandstone unit. These are not the only sequence boundaries possible in this stratigraphic interval; only these five are postulated from limited field observations. Other sequence boundaries may be present within the Fortress Mountain Formation and the Nanushuk Group, and their interpretation will have to be based on detailed field examination of these exposures. One of the questions that arose during our field work was the stratigraphic relationship between the Fortress Mountain Formation and the Nanushuk Group in the Atigun syncline-Slope Mountain area. In this area there is a strong similarity of lithologic composition between these stratigraphic units, and one could argue that the For- tress Mountain at Atigun Gorge is simply the more proxi- mal facies of the Nanushuk Group at Slope Mountain. Although the Fortress Mountain Formation-Torok Forma- tion-Nanushuk Group transition is illustrated across the North Slope in cross sections (Molenaar, 1988), actual field evidence showing this succession is limited, as far as we could ascertain, to the area of Igloo Mountain near the Kokolik River, which was described by Chapman and Sable (1960). In the Igloo Mountain area (fig. lA), the Fortress Mountain Formation appears to be in depositional contact with the Torok Formation, which is overlain by the Kukpowruk Formation (part of the Nanushuk Group). As the Kokolik River area is approximately 550 km west of the Atigun syncline area, the stratigraphic relationships between these three stratigraphic units are not clear. We speculate that the two sequences outlined in the Fortress Mountain Formation at Atigun syncline could be equivalent to two sequences of the Nanushuk Group at Slope Mountain, given the lack of good stratigraphic and biostratigraphic control. Although many seismic lines are available in the NPRA (Molenaar, 1985, 1988), none are properly positioned to resolve these stratigraphic relationships. We attempted to point out the possible sequence boundaries that were present in this part of the stratigraphic section based on limited field work. We did not attempt to relate these boundaries to a causal mechanism, such as tectonics or eustasy. The Albian to Cenomanian section in the Western Interior Seaway of the United States contains many sequence boundaries ascribed by some to eustatic mechanisms (Haq and others, 1988), or to purely tectonic effects in the active Colville basin (Noonan, 1987) or in similar-aged rocks in Arctic Canada (Embry, 1989). Others have developed hybrid tectonic-eustatic models for the formation of depositional sequences in the Brookian sequence of Alaska (Rudolph and others, 1990). Whatever the cause, relative base-level changes had a significant ef- fect on the depositional sequences as they developed in the Lower Cretaceous section of the Colville basin.
DEPOSITIONAL SEQUENCES IN ATIGUN SYNCLINE AND SLOPE MOUNTAIN AREA 57 CONCLUSIONS Limited field work in the Atigun syncline-Slope Mountain area of the North Slope has led us to interpret five possible depositional sequence boundaries. Many more such boundaries may be present in the rocks exam- ined, which were the Fortress Mountain Formation and Nanushuk Group, but require detailed sedimentologic examination of the entire section, and possible attempts to follow such boundaries regionally. The Gilead Creek sandstone unit was placed within a context of a lowstand submarine fan, based on its possible age, sedimentology, and geographic location relative to the Fortress Mountain and Nanushuk environments. Future work should focus on more closely defining the biostratigraphic and sedimentologic aspects of these units. REFERENCES CITED Bird, K.J., 1986, A comparison of the play analysis technique as applied in hydrocarbon resource assessments of the Na- tional Petroleum Reserve in Alaska and the Arctic National Wildlife Refuge, in Rice, D.D., ed., Oil and gas assessment- methods and applications: American Association of Petro- leum Geologists Studies in Geology 21, p. 133-142. -1988, Alaskan North Slope stratigraphic nomenclature and data summary for government-drilled wells, in Gryc, George, ed., Geology and exploration of the National Petro- leum Reserve in Alaska: U.S. Geological Survey Profes- sional Paper 1399, p. 317-353. 1991, North Slope of Alaska, in Gluskoter, H.J., Rice, D.D., and Taylor, R.B., eds., Economic geology of the United States: Boulder, Geological Society of America, Geology of North America, v. P-2, p. 447462. Bird, K.J., and Molenaar, C.M., 1987, Stratigraphy, in Bird, K.J., and Magoon, L.B., eds., Petroleum geology of the northern part of the Arctic National Wildlife Refuge, northeastern Alaska: U.S. Geological Survey Bulletin 1778, p. 37-59. --- 1992, The North Slope foreland basin, Alaska, in Macqueen, R.W., and Leckie, D.A., eds., Foreland basin and foldbelts: American Association of Petroleum Geolo- gists Memoir 55, p. 363-393. Bird, K.J., Griscom, S.B., Bartsch-Winkler, S., and Giovannetti, D.M., 1987, Petroleum reservoir rocks, in Bird, K.J., and Magoon, L.B., eds., Petroleum geology of the northern part of the Arctic National Wildlife Refuge, northeastern Alaska: U.S. Geological Survey Bulletin 1778, p. 79-99. Brosgt, W.P., Reiser, H.N., Dutro, J.T., Jr., and Detterman, R.L., 1979, Bedrock geologic map of the Phillip Smith Mountains quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Investigations Map MF 879-B, 2 sheets, scale 1:250,000. Carman, G.J., and Hardwick, P., 1983, Geology and regional setting of the Kuparuk oil field, Alaska: American Association of Pe- troleum Geologists Bulletin, v. 67, no. 6, p. 1014-1031. Chapman, R.M., and Sable, E.G., 1960, Geology of the Utukok- Corwin region, northwestern Alaska: U.S. Geological Sur- vey Professional Paper 3034, p. 47-167. Crane, R.C., and Wiggins, V.D., 1976, Ipewick Formation, significant Jurassic-Neocomian map unit in the northern Brooks Range foldbelt [abs.]: American Association of Pe- troleum Geologists Bulletin, v. 60, no. 12, p. 2177. Crowder, R.K., 1987, Cretaceous basin to shelf transition in northern Alaska: deposition of the Fortress Mountain For- mation, in Tailleur, I., and Weimer, P., eds., Alaskan North Slope geology: Bakersfield,Society of Economic Paleon- tologists and Mineralogists, Pacific Section, Book 50, v. 1, p. 449458. 1989, Deposition of the Fortress Mountain Formation, in Mull, C.G., and Adams, K.E., eds., Dalton Highway, Yukon River to Prudhoe Bay, Alaska: Alaska Division of Geological and Geophysical Surveys, Guidebook 7, Volume 2, p. 293-301. Curtis, S.M., Ellersieck, Inyo, Mayfield, C.F., and Tailleur, I.L., 1990, Reconnaissance geologic map of the De Long Moun- tains A-1 and B-1 quadrangles and part of the C-1 quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1930, 2 sheets, scale 1:63,360. Ellersieck, Inyo, Curtis, S.M., Mayfield, C.F., and Tailleur, I.L., 1990, Reconnaissance geologic map of the De Long Moun- tains A-2 and B-2 quadrangles and part of the C-2 quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Map 1-1931, 2 sheets, scale 1:63,360. Embry, A.F., 1989, A tectonic origin for third-order depositional sequences in extensional basins-Implications for basin modeling, in Cross, T.A., ed., Quantitative dynamic stratigraphy: New Y ork, Prentice-Hall, p. 491-501. Gautier, D.L., 1987, Petrology of Cretaceous and Tertiary reservoir sandstones in the Point Thomson area, in Bird, K.J., and Magoon, L.B., eds., Petroleum geology of the northern part of the Arctic National Wildlife Refuge, northeastern Alaska: U.S. Geological Survey Bulletin 1778, p. 117-122. Goff, R.A., 1990, Stratigraphy and sedimentology of the Fortress Mountain Formation at Atigun syncline, northern Alaska: Fairbanks, University of Alaska, M.S. thesis, 122 p. Haq, B.U., Hardenbol, J., and Vail, P.R., 1988, Mesozoic and Cenozoic chronostratigraphy and cycles of sea-level change, in Sea level changes-An integrated approach: So- ciety of Economic Paleontologists and Mineralogists Spe- cial Publication 42, p. 71-108. Huffman, A.C., Jr., 1989, The Nanushuk Group, in Mull, C.G., and Adams, K.E., eds., Dalton Highway, Yukon River to Prudhoe Bay, Alaska: Alaska Division of Geological and Geophysical Surveys Guidebook 7, v. 2, p. 303-309. Huffman, A.C., Jr., Ahlbrandt, T.S., Pasternak, I., Stricker, G.D., and Fox, J.E., 1985, Depositional and sedimentologic fac- tors affecting the reservoir potential of the Cretaceous Nanushuk Group, central North Slope, Alaska: U.S. Geo- logical Survey Bulletin 1614, p. 61-74. Hunter, R.E., and Fox, J.E., 1976, Interpretation of depositional environments in the Fortress Mountain Formation, central Arctic Slope: U.S. Geological Survey Circular 733, p. 30-31. Jarnison, H.C., Brockett, L.D., and Mchtosh, R.A., 1980, Prudhoe Bay-A 10-year perspective, in Halbouty, M.T., ed., Giant oil fields of the decade 1968-1978: American Association of Pe- troleum Geologists Memoir 30, p. 289-314.
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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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Page 19 and 20: DEEP-WATER LITHOFACIES AND CONODONT
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Page 37 and 38: LITHOFACIES AND CONODONTS OF CARBON
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Page 51 and 52: Table 1. Conodont faunules and lith
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Page 65 and 66: U-Pb AGES OF ZIRCON, MONAZITE, AND
Page 77 and 78: APPEAL FOR NONPROLIFERATION OF ESCA
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Page 107 and 108: EARLY CENOZOIC DEPOSITIONAL SYSTEMS
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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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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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