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

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32 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 sequence is included in the Brooks Range allochthon, which is the structurally lowest of seven formally named large-scale structural units. In the central Brooks Range this allochthon is also known as the Endicott Mountains allochthon (Mull, 1989). Armstrong (1970) described lithologies and mega- fossils of the Lisburne Group at Lisburne Ridge and Mount Bupto, but did not describe the section in the Ivotuk Hills. The Lisburne No. 1 well, drilled in 1979- 1980 on the northwest margin of the Ivotuk Hills during studies of the National Petroleum Reserve, encountered five repeated sections of Carboniferous and Permian rocks; this succession was interpreted as stacked thrust plates of the Ivotuk Hills sequence (Mayfield and others, 1988). Brief descriptions of lithologies and foraminifers from this well are given in Legg (1983). GEOLOGIC SETTING Rocks of the western Brooks Range were folded and thrust-faulted during the Jurassic to Cretaceous Brooks Range orogeny (Mayfield and others, 1988). The Ivotuk Hills lie along the northern margin of the Brooks Range foothills; they consist chiefly of Carboniferous strata exposed as a steeply southward-dipping homocline (fig. 2), bounded on the north by a major thrust fault (fig. 1). The EXPLANATION Contad, appmxirnately lxated Siksikpuk Formation (Permian) crops out along the south side of the hills in several small, discontinuous exposures and is presumed to depositionally overlie the Lisburne Group although the contact is generally concealed and was not observed during this study. Fault-bounded strata of the Otuk Formation (Triassic and Jurassic) and the Okpikruak Formation (Cretaceous) crop out north and south of the Ivotuk Hills (Mull and others, in press). NOTUK HILLS MEASURED SECTION The section described here (fig. 3) was measured with Jacob's staff and tape along the east side of Otuk Creek, at the western end of the Ivotuk Hills (Killik River B-5 quadrangle, T. 11 S., R. 16 W., western edge of sec. 20; fig. 1). The northern third of the section was measured at stream level; the rest was measured in cliffy outcrops several tens of meters above the creek. Through most of the section, beds strike N. 70-80" E. and dip 55-65" S. Color alteration indices (CAI's) of conodonts recovered from this section range from 1-1.5 to 1.5, indicating the host rocks reached at least 50-80°C (Epstein and others, 1977). Petrographic descriptions are based on field observations and examination of 80 thin sections. Identification of calcite and dolomite was made using the Alizarin Red-S and potas- sium femcyanide staining technique of Dickson (1966). A A A Thnrst fault, dashed where approximately located; sawteeth 0 5 KILOMETERS on upper plate I I I I I I Figure 1. Distribution of Lisburne Group and location of section measured in Ivotuk Hills; geology from Mull and others (in press). Township and range grid from Killik River B-5 quadrangle. See table 1 for geographic coordinates, conodont faunules, and lithologies. 155"30'
LITHOFACIES AND CONODONTS OF CARBONIFEROUS STRATA IN THE IVOTUK HILLS, BROOKS RANGE LITHOFACIES KAYAK SHALE About 45 m of shale, mudstone, silty shale, and dolostone assigned by Martin (1970) to the Kayak Shale is exposed beneath the Lisburne Group along Otuk Creek. The base of the section is concealed and is probably a thrust fault (C.G. Mull, written commun., 1992). The section consists chiefly of fissile, black shale and blocky mudstone, locally yellow- to red-weathering. Some samples, mainly in the lower third of the exposure, contain minor amounts of detrital quartz silt. Dark-gray, reddish- brown-weathering concretions, lenses, and layers of dolomitic mudstone to dolostone make up 10 to 20 percent of the lower half of the section (fig. 4A). Concretions reach 20 cm in diameter; lenses and layers range from 2 to 30 cm thick and display discontinuous, wispy laminae. In thin section, concretions consist of 25 to nearly 100 percent dolomite crystals, 6 to 60 pn (rarely, to 200 pn) in diameter, in a noncalcareous mudstone matrix (fig. 4B). Laminae reflect variations in the concentration of dolomite versus mud. Spiculitic layers a few centimeters thick make up about 10 percent of the section. Some samples consist of calcareous spicules (fig. 4C), or a mixture of calcareous and siliceous spicules, in a mudstone matrix; other samples contain only siliceous spicules in a matrix of microcrystal- line chert. Spicules are typically well aligned and may be concentrated into parallel or cross laminae; some spicules have been partly or completely replaced by pyrite. Medium- to dark-gray, orange- to reddish-brown- weathering, dolomitic bioclastic packstone occurs near the base and middle of the section in beds and lenses 10 cm to 1.7 m thick (fig. 40). Bedding surfaces are densely bioturbated; some beds are graded and display wispy parallel laminae. Clasts range from less than 1 mm to several centimeters in diameter; matrix consists of calcareous and noncalcareous mud. Most clasts are bioclasts, chiefly crinoid ossicles, brachiopods, and rare solitary corals (fig. 40- F). Many bioclasts are broken and abraded, and some have bored and micritized rims. Five to 25 percent phosphatic Figure 2. Carboniferous succession in Ivotuk Hills; view to east. Line marks contact between Kayak Shale below and Lisbume Group above. Meters correspond to thicknesses in figure 3. 33
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 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: LITHOFACIES AND CONODONTS OF CARBON
Page 41 and 42: 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 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
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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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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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