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

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DEEP-WATER LITHOFACIES AND CONODONT FAUNAS OF THE LISBURNE GROUP, WEST-CENTRAL BROOKS RANGE, ALASKA By Julie A. Dumoulin, Anita G. Harris, and Jeanine M. Schmidt ABSTRACT Deep-water lithofacies of the Lisburne Group occur in thrust sheets in the western part of the foreland fold and thrust belt of the Brooks Range and represent at least three discrete units. The Kuna Formation (Brooks Range allochthon) consists mostly of spiculitic mudstone and lesser shale; subordinate carbonate layers are chiefly diage- netic dolomite. Predominantly shale sections of the Kuna that contain few sponge spicules occur in the western part of the study area. The Akmalik Chert (Picnic Creek allochthon) is mostly radiolarian-spiculitic chert; rare limy beds are calcitized radiolarite. The Rim Butte unit (Ipnavik River allochthon) consists chiefly of calcareous turbidites, derived from both shallow- and deep-water sources, interbedded with spiculitic mudstone. Much of the material in the turbidites came from a contemporane- ous carbonate platform and margin, but some fossils and lithic clasts were eroded from older, already lithified carbonate-platform rocks. All three units appear to be roughly coeval and are chiefly Osagean (late Early Missis- sippian) in age in the study area. Shallow-water lithofacies of the Lisburne Group exposed in the Howard Pass area (Brooks Range allochthon) are mostly of Meramecian (early Late Mississippian) age. Thus, these carbonate-platform rocks were not the source of the calcareous turbidites in the Rim Butte unit. Rim Butte turbidites could have been derived from older platform carbonate rocks such as those of the Utukok Forma- tion (Kelly River allochthon) exposed mainly to the west of the Howard Pass quadrangle. INTRODUCTION The chiefly Carboniferous Lisburne Group is a domi- nantly platform carbonate sequence exposed throughout northern Alaska in the foreland fold and thrust belt of the Brooks Range (fig. 1). Deeper water facies (used here to mean sediments deposited below the photic zone, probably at depths of 100 m or greater) occur within this group, particularly in the western Brooks Range, but have been little studied. In this paper we describe deep-water Lisburne lithofacies found in the Howard Pass and western Killik River quadrangles (fig. 2). Previous workers (for example, Mayfield and others, 1988) considered these facies to be largely younger than nearby outcrops of the platform facies, but our work suggests that, in the Howard Pass area, the deeper water facies are chiefly older than nearby platform facies. The deep-water carbonate rocks considered here be- long to three units: the Kuna Formation, the Akmalik Chert, and the Rim Butte unit. In the structural framework of Mayfield and others (1988), these units were assigned to three discrete structural sequences or allochthons. The allochthons are distinguished on the basis of inferred structural level and differences in lithologic succession; differences are most pronounced in the Carboniferous facies. The distribution of these allochthons in the study area is shown in figure 2. Platform facies of the Lisburne Group, as well as deeper water facies of the Kuna Formation, were assigned to the Brooks Range allochthon (equivalent to Endicott Mountains allochthon; Mull, 1989). Chert and limestone equivalent to the Akrnalik Chert is part of the Picnic Creek allochthon, and the Rim Butte unit is part of the Ipnavik River allochthon. The findings described here are based on measured sections and outcrop studies at localities shown in figure 2; petrographic descriptions are based on field observations and examination of about 200 thin sections. Identification of calcite and dolomite was made using the Alizarin Red-S and potassium femcyanide staining technique of Dickson (1966); the presence of dolomite was confirmed in some samples by X-ray diffraction analysis (Elizabeth Bailey, U.S. Geological Survey, written commun., 1992). KUNA FORMATION The Kuna Formation was examined across the study area (fig. 2); outcrops investigated include the type section (fig. 2, loc. 9) and one of the reference sections (fig. 2, loc.
DEEP-WATER LITHOFACIES AND CONODONT FAUNAS OF THE LISBURNE GROUP, BROOKS RANGE 13 13) established by Mull and others (1982). In the study area, the Kuna is at most 70 m thick; it depositionally overlies the Kayak Shale and generally underlies fine- grained sedimentary rocks of the Etivluk Group. Locally, particularly in the eastern part of the study area, the upper contact of the Kuna is a fault. As originally defined by Mull and others (1982), the Kuna Formation consists of black carbonaceous shale, black chert, fine-grained limestone, and dolostone. In the study area, some Kuna sections consist chiefly of black, carbonaceous, noncalcareous shale; such sections are generally poorly exposed and include only minor amounts of carbonate rocks and (or) chert (fig. 3A). These shale-rich outcrops are commonest in the western half of the Howard Pass quadrangle (fig. 2). Most sections of the Kuna Formation examined in this study, however, consist primarily of siliceous mudstone with subordinate (5-20 percent) shaly beds or partings (fig. 3B). Mudstone beds are even to irregular and 2-20 cm thick; shale intervals are generally a few centimeters or less, but may be as much as 10 cm thick. The thicker beds, called chert by some previous workers (for example, Mull and others, 1982), are siliceous mudstone, not true chert; they have an earthy rather than vitreous luster and do not fracture conchoidally. In thin section, they contain abundant biosiliceous material, chiefly sponge spicules but also subordinate radiolarians, in a matrix of organic-rich mud (fig. 30. Some mudstone layers have been locally STUDY gOw AREA -- --- silicified; silica for this process may have come, at least in part, from dissolution and remobilization of the biosiliceous component of these rocks. Fine lamination is the only sedimentary structure vis- ible in most outcrops of the Kuna Formation (fig. 30). Laminae consist of local concentrations of sand- to silt- sized clasts, mostly of mudstone, or alternating concentrations of sponge spicules and mud (fig. 3E). Most laminated intervals probably represent distal turbidites, or lags left by bottom currents. But cyclic changes in pro- ductivity and (or) detrital influx into the Kuna basin(s) could also have played a role in forming some laminae. Preservation of these laminae, as well as the appar- ently high organic content of much of the Kuna Formation, suggest that anoxic or dysaerobic bottom-water conditions prevailed during its deposition. Some intervals within the Kuna are burrowed, however, so oxygen levels at the sedi- ment-water interface were locally or periodically high enough to support a bottom fauna. Bioturbated intervals (fig. 3F) are most common in the easternmost exposures of the Kuna in the study area. Burrows are round to ovoid in cross section, about 0.5-1 cm in diameter, and generally richer in siliceous bioclasts than the surrounding mudstone. Carbonate forms 30 percent or less of most outcrops of the Kuna Formation that we studied. Carbonate occurs as concretions, 4-85 cm in diameter (fig. 4A), or more continuous layers a few millimeters to 30 cm thick (fig. 4B). Primary calcareous material, such as obvious f ALASKA 1 0 100 200 KILOMETERS I I / I I I Figure 1. Distribution of Lisburne Group (pattern) in northern Alaska (generalized from Armstrong and Mamet, 1978) and location of quadrangles mentioned in text.
Page 1 and 2: Geologic Studies in Alaska by the U
Page 3 and 4: 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 17: LATE HOLOCENE LONGITUDINAL AND PARA
Page 21 and 22: 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
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Page 51 and 52: Table 1. Conodont faunules and lith
Page 55 and 56: DEPOSITIONAL SEQUENCES IN ATIGUN SY
Page 65 and 66: U-Pb AGES OF ZIRCON, MONAZITE, AND
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U-Pb AGES OF ZIRCON, MONAZITE, AND
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APPEAL FOR NONPROLIFERATION OF ESCA
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FAVORABLE AREAS FOR METALLIC MINERA
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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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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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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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