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

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36 GEOLOGIC STUDIES IN ALASKA BY THE U.S. GEOLOGICAL SURVEY, 1992 E 0 0.5 MILLI\IETtR 0 0.5 MILLIMETER u F u
LITHOFACIES AND CONODONTS OF CARBONIFEROUS STRATA IN THE IVOTUK HILLS, BROOKS RANGE 37 half of the unit, but 30 to 50 percent of the upper strata. Shale makes up 10 percent of the unit and occurs as part- ings between beds and rare layers as much as 10 cm thick. Several beds of silicified bioclastic packstone, 5 to 8 cm thick, punctuate the section; these beds contain abundant disarticulated brachiopod valves and lesser crinoid ossicles, spicules, and phosphatic clasts. Some shells are rimmed with borings filled with brown or black mud (fig. 70). Tundra covers the highest beds of the Lisburne Group along Otuk Creek. The contact between the Lisburne and the overlying Siksikpuk Formation is not exposed along the line of our measured section, but is preserved in an isolated outcrop 100 m to the southeast (fig. 1, sample locality 85ASi14-18; see also "Age and Biofacies" below). The stratigraphic relationship between this outcrop and our measured section is uncertain, but it most likely represents strata at least in part equivalent to the uppermost beds of the measured section. The total thickness of the Lisburne Group in the Ivotuk Hills is probably no more than 250 m. AGE AND BIOFACIES KAYAK SHALE A sample from the 2-m-thick dolomitic packstone bed at the base of the section in the Kayak Shale yielded conodonts of late Early Mississippian (early Osagean) age (fig. 80, P-T). The fauna represents a polygnathid- bispathodid biofacies, which generally indicates a relatively shallow-water, normal-marine platform or shelf depositional environment. The two predominant species, however, are Polygnathus communis and Bispathodus sp. indet. (a single-row morphotype that is either B. stabilis (Branson and Mehl) or B. utahensis Sandberg and Gutschick). These are pelagic forms (Sandberg and Gutschick, 1984) that are widespread in most normal-marine Early Mississippian environments, but are most abundant in the more basinal and (or) anoxic to dysaerobic environments (Sandberg and Gutschick, 1984; Dumoulin and others, this volume). Figure 4. Sedimentary features of Kayak Shale. A, Dolostone lens in black shale, 6.1 m above base of section. B, Photo- micrograph of dolomitic rnudstone lens, 8.4 m above base of section. C, Photomicrograph of layer rich in calcareous sponge spicules, 22 m above base of section. D, Dolomitic bioclastic packstone layer containing abundant crinoid columnals (c) and solitary corals (r), 21 m above base of section. E, Photomicro- graph of bioclastic packstone lens shown in 40; note clast of spiculitic mudstone (s), and crinoid ossicles (c) replaced by silica. F, Photomicrograph of bioclastic packstone lens 0.5 m above base of section; note phosphatic fish debris (p), and crinoid ossicles (c) replaced by dolomite. The collection from the Kayak Shale also contains a few representatives of species characteristic of shallow- water (but not the shallowest water) biofacies such as Bispathodus aculeatus anteposicornis (fig. 8T) and Synclydognathus geminus (Hinde) (fig. 8D), as well as representatives of shallow- to slightly deeper water platform or shelf biofacies such as Hindeodus crassidentatus (fig. 8S), Pseudopolygnathus sp. indet., and Kladognathus sp. indet. Of note is the absence of deeper water (outer shelf or slope) early Osagean genera such as gnathodids and scaliognathids. Taken together, the species association and preservation characteristics suggest postmortem transport within or from a moderate to shallow depth platform or shelf environment. LISBURNE GROUP Two samples from the lower unit of the Lisburne Group in the Ivotuk Hills section produced conodonts of early Late Mississippian (late Merarnecian) age, as did a sample from the massive middle unit. Samples taken 54 and 66 m above the base of the section are from dolostone with relict crinoidal packlgrainstone texture. Conodonts are relatively sparse, virtually all are incomplete (the ratio of generically determinate to indeterminate elements ranges from 1:3 to 1:2) and bispathodids (fig. 814) and kladognathids (fig. 8M) predominate. The collections contain a few diagnostically shallow-water forms such as Synclydognathus geminus (fig. 8E, F) and cavusgnathids (fig. 8N, 0). Overall species association and preservation characteristics suggest postmortem transport within or from a relatively shallow-water platform or shelf environment. The sample taken from the middle massive unit at 115.4 m above the base of the section is from a dolomitic bryozoan-crinoidal wackestone. Conodonts are few, somewhat more complete than in samples from the lower unit, and consist chiefly of representatives of the pelagic species Bispathodus utahensis, with lesser numbers of Kladognathus sp. indet. and a few fragments of cavusgathids (fig. 8G, H, and L). The species association and preservation characteristics suggest a normal-marine depositional environment with somewhat less postmortem transport than in the two samples from the lower unit. Thin-bedded dolostone in the upper unit of the Lisburne Group, 190 m above the base of the section, was barren of conodonts. Another sample from the upper unit taken about 267 m above the base of the section produced only a few indeterminate conodont fragments. However, a sample collected by J.P. Siok (Terrasat, Anchorage, Alaska) from an isolated outcrop about 100 m southeast of our measured section provides a date for the uppermost part of the Lisburne Group in the Ivotuk Hills. This outcrop exposes a few tens of meters of the uppermost strata of the Lisburne Group, as well as strata of the overlying Siksikpuk Formation. A sample of thin-bedded, fine-
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 41: 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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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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GEOLOGIC STUDIES IN ALASKA BY THE U
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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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