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

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34 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 5 , y ~ s > V) - g 9 U W V) = y 0 I + 268 264 f 5 o g -/ --/ - &ERA (not to scale) g z ap 9- ,o a0 ou , 5 2 ;: C,T(3) I- (I) T 0 z; 9 $3 $5 z3 nu : L5 EXPLANATION ROCK TYPES SEDIMENTARY STRUCTURES Shale and = Parallel lamination - - mudstone = Cross lamination Dolostone and A Graded bedding dolomitic mudstone 2 Bioturbation Chert and spiculitic mudstone Scoured base 0 lntraclast CARBONATEROCKTYPES FOSSILS M Mudstone Q? Brachiopod W Wackestone Bryozoan P Packstone Solitary coral SAMPLES C Conodont @ Crinoid ossicles f Foraminifer T Thin section R Radiolarian (number of X Sponge spicule thin sections)
LITHOFACIES AND CONODONTS OF CARBONIFEROUS STRATA IN THE IVOTUK HILLS, BROOKS RANGE 35 grains, chiefly fish debris, and 10 to 20 percent intraclasts of black shale and spiculitic mudstone occur in some packstone layers (figs. 4E, F). Dolomite has replaced matrix and bioclasts in many samples (fig. 4F), although crinoid grains are generally still calcite; locally, bioclasts have been replaced by silica (fig. 4E) and (or) pyrite. Sev- eral samples grade from coarse, sand-sized bioclastic packstone into silt-sized siliceous spiculite. The contact between the Kayak Shale and the overlying Lisburne Group is marked by an abrupt change from recessive, yellow-weathering, fissile shale to resistant, red- dish-brown to black-weathering, thin-bedded spiculitic mudstone, dolostone, and chert (fig. 3). lel and local cross-lamination. Clasts range from 0.5 to 7 mm and are predominantly crinoid ossicles, although a few samples also contain foraminifers, bryozoan fragments, brachiopods, phosphatic grains, and (or) mudstone clasts. Most beds are pervasively dolomitized and (or) silicified, but the original supportstone texture can still be discerned (fig. 5E). Some bioclasts are outlined or partly replaced by pyrite (fig. 5F). MIDDLE UNIT LISBURNE GROUP The middle unit consists of subequal amounts of dolostone and chert, chiefly massive, but with subordinate intervals a few meters thick of irregular 3- to 20-cm-thick beds (fig. 6A). The base of this unit is the base of a distinctive, 20-m-thick interval of massive, white to pale-bluish-gray-weathering chert with 10 to 20 percent dolostone; At least 225 m of dolostone, chert, and minor shale makes up the Lisburne Group in the Ivotuk Hills. The section can be divided into three subunits on the basis of bedding style and lithology. The lower 50 m consists of chiefly dark-gray to black, thin-bedded chert, dolostone, and mudstone, and subordinate, lighter colored, thicker bedded dolomitic pacwgrainstone containing abundant crinoid ossicles. The middle 80 m is more massive and resistant, white to medium-gray, dolomitized and (or) silicified skeletal packstone and lesser wackestone containing a variety of bioclasts. The upper 95 m is recessive, dark-gray to black, thin-bedded dolostone, chert, and shale. the dolostone fills irregular vugs, 0.5-1 cm long, and forms lenses 5 to 8 cm thick and as much as 30 cm long. Dolostone dominates the rest of the unit; some dolostone layers are vaguely parallel laminated and a few are obviously burrowed, but in general sedimentary structures are rare. A few intervals of massive dolostone contain vuggy porosity filled with solid hydrocarbons (fig. 6B). Dolomitization has locally obscured or completely obliterated original textures in this unit. Where depositional textures can be discerned, massive strata consist of crinoidal pacwgrainstone; thinner bedded zones comprise bryozoan wackestone (fig. 6C) and subordinate layers of bioclastic packstone. Bioclastic beds contain abundant crinoid ossicles and bryozoan fragments (fig. 6D-F), as LOWER UNIT well as rare solitary corals. Siliceous sponge spicules are About 60 to 70 percent of the lower unit consists of abundant in mud-rich intervals, but are rare overall comthin-bedded to laminated spiculitic mudstone, spiculitic chert, dolomitic mudstone, and dolostone, interlayered on a pared to overlying and underlying strata. scale of a few millimeters to 7 cm (fig. 5A). Sponge spi- UPPER UNIT cules are chiefly siliceous and 20 to 100 pn in diameter (figs. 5B, C); some spiculitic chert samples also contain rare radiolarians. Dolomite crystals range from 50 pm to 0.5 mm in size (fig. 5C). Beds 740 cm thick of bioclastic pacwgrainstone occur intercalated with these dark mudstones and are particularly abundant in the lower half of the unit. Some beds are clearly graded (fig. 5D) and display well-developed pard- The upper unit consists of dolostone, dolomitic and spiculitic mudstone, spiculitic chert, and black shale in even to slightly undulatory beds, 0.5 to 5 cm thick (figs. 7A, B). Only the lower third of the unit is well exposed. A distinctive zone of black shale, 10 to 50 cm thick, marks the basal part of the unit; the shale is fissile, slightly calcareous, and weathers into plates l to 3 cm thick. Dolostone and dolomitic mudstone make up most of the unit; these rocks are medium gray to black, weather tan = to pale yellow brown, and contain local wispy parallel laminae, starved ripples, and rip-up clasts. Some laminae Figure 3. Measured section, Kayak Shale and Lisburne Group, in Ivotuk Hills. Base of section is concealed and is probably a thrust fault (C.G. Mull, written commun., 1992); tundra covers top of section. Chesterian(?) age for uppermost part of section is based on a collection from an isolated outcrop nearby (see text). Conodont species numbered in figure are: 1, Bispathodus aculeatus anteposicornis; 2, Bispathodus utahensis; 3, Bispathodus sp. indet.; 4, Cavusgnathus sp. indet.; 5, Hindeodus crassidentatus; 6, Kladognathus sp.; 7, Polygnathus communis; have been partly disrupted by bioturbation. Samples consist of 50-99 percent euhedral dolomite in a mudstone matrix; dolomite crystals are 8-240 pn (chiefly less than 80 pn) in size. Dolostone is interlayered on a scale of millimeters to centimeters with black spiculitic mudstone and spiculitic chert (fig. 7C); these lithologies contain abundant, chiefly siliceous, sponge spicules, minor clasts of mudstone, and 8, Polygnathus lacinatus circaperipherus; 9, Pseudopolygnathus (particularly in the upper part of the section) rare radiolarsp. indet.; 10, Synclydognathus geminus. ians. Chert constitutes only a few percent of the lower
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 and 38: LITHOFACIES AND CONODONTS OF CARBON
Page 39: 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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136 GEOLOGIC STUDIES IN ALASKA BY T
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GEOLOGIC STUDIES IN ALASKA BY THE U
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1 64 GEOLOGIC STUDlES IN ALASKA BY
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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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