SHORT NOTES OF{ALASKA GEOLOGY 1997 Professional Report 1 1 I I SHORT NOTES ON ALASKA GEOLOGY 1997 , Edited by James G. Clough and Frank Lahson Cover design fu Ann-Lillian C. Schell Professional Report 1 18 Recent research on Alaska geologt Front cover: View of central Kigluaik Motmtains on the Seward Pminsula. Peaks are composed of Thomp- san Creek Ortlwgneiss of Late Proterczoic ta Earb Precambian age and metamotplnsed at uppet on- phibolite facies conditions about 9I million years Fairbanks, Alaska ago. Pltotograph by J.M. Amato. 1998 EMSIAN (I-{rE EARLY DEVONIAN) FOSSIIS INDICAilE A SIBERIAN ORIGIN FOR THE FAREWETL TERRAT{E by Robert B. Blodgettl INTRODUCTION Recent years have witnessed the rise of the Permian), now have convinced the autJror that this hypothesis that nearly all of Alaska, as well as much of continental margin sequence originated from the the westem Cordillera of North America, is composed Siberian continent, rather than northwestern North of numerous discrete, accreted tectonostratigraphic America. terranes (Coney and others, 1980; Jones and others, 1981, 1982, 1986, 1987 ; Nokleberg and others, 1 994). DEVOI\IAN BIOGEOGRAPI{Y The only exception to this in Alaska is represented by the small triangular area of the Nation Arch in east- The study of Devonian marine biogeography has central Alaska (fig. l). This area is composed primarily been the subject ofconsiderable attention in the literature of Precambrian and Paleozoic rocks that represent a for nearly 30 years now. Most ofthese studies have relied continuation of similar rocks exposed farther to the east on three primary groups: brachiopods (Boucot, 1974, in the Ogilvie Mountains of the Yukon Territory. The 1975; Boucot and others, 1969; Johnson,1970, l97l; autochthoneity of these east-central Alaskan Paleozoic Johnson and Boucot, 1973; Savage and others, 1979; rocks is supported by both lithic continuity and Wang Yu and others, 1984); rugose corals (Oliver, 1973, paleontological evidence. Faunal evidsnce for this is 1976, 1977; Oliver and Pedde\ 1979,1984; Pedder and especially notable in Early Devonian age strata, as these Oliver, 1990); and trilobites (Ormiston, 1972, 1975; contain faunas typical of North American miogeoclinal Kobayashi and Hamada, 1975; Eldredge and Orrniston, and cratonic strata in northwestern and Arctic Canada. 1979). Other faunal groups have also been studiedto a In this paper, however, attention is focused on newly lesser degree [that is, gastropods (Blodgett, 1992; acquired faunal data of important biogeographic Blodgett and others, 1988, 1990; Forney and others, significance from late Early Devonian (Emsian) age 1981); and ostracodes (Copeland and Berdan, 1977; strata of two teranes, the Nixon Fork and Mystic, Berdan, 1990)], but are in virtually full agreement with situated in west-cenffal and southwestern Alaska. These established biogeographic patterns shown by the more terranes, along with Dillinger terrane, were recognized common and better studied faunal groups mentioned by Decker and others (1994) to be gatetically related above. and were reduced in rank to subterranes of a larger The highest level of provincialism in Devonian terrane, termed the Farewell terrane (fig. 1). Other marine faunas occurred during the later fwo stages terranes in interior Alaska such as the Minchumina, East (Pragian and Emsian) of the Early Devonian (Boucot, Fork, and Livengood are also probably genetically 1975, 1988; Blodgett and others, 1988). Both before related to the Farewell teffane, and together represent a and after, the levels ofprovincialism vary from moderate locally highly deformed continental margin sequence to low, with the Middle Devonian evincing steadily composed primarily of Neoproterozoic to Paleozoic declining levels of endemism from the Eifelian into the strata. Previous interpretation of the gross succeeding Givetian. The Late Devonian, including both lithosfiatigraphy and faunal affinities of the most well- the Frasnian and Famennian, is considered to be studied subterrane, tle Nixon Fork, has previously been characterized by a remarkably high level of suggested to indicate either fi:agmentation from or lithic cosmopolitanism at the generic level, although continuity with northwestem Canada @lodgett 1983; undoubtedly detailed future studywill still show distinct Churkin and others, 1984; Blodgett and Clough, 1985; biogeographic provinces on the basis of the disnibution RohrandBlodgett, 1985; Abbott, 195). Newly acquired ofboth species and some genera. biogeographic data from Emsian (late Early Devonian) Rich, abundaqt, diverse megafaunas are known from sfrata ofthe Farewell terrane, along with reconsideration both early Middle (Eifelian) and early Late (Frasnian) of the affinities of faunas and floras of other time Devonian faunas of the Farewell terrane. The Eifelian intervals (Middle Cambrian, Late Ordovician, and faunas were thought to show their sfrongest similarities lDeparment of Zootogy, Oregon State University, Cowallis, Oregon 97331. 53 s4 Snorr Nores or Alasra Geolocv 1997 wittr those of northwestem Canada (Blodgett, 1983, differentiate faunas biogeographically between these 1992; Blodgett and others, 1990), although strong areas, it is best to use evidence from the middle and late affinities are also noted to Siberia and the Urals @axter Early Devonian (Pragian and Emsian), when global and Blodgett, 1994). Frasnian faunas of the Farewell endemism was relatively high. Unfortunateln until terane show close biogeographic ties to western North recently virtually no biogeographically useful collections America, Novaya Zemlya, Taimyr, and Kolyma of these ages were available to the author from this (Blodgett and Gilbert, 1992a; W.K. Braun iz Blodgett, terrane. At this time, however, biogeographically 1983, p. 128). Differentiation between Siberia and distinctive Emsian collections have been examined from northwestem Canadian faunas during the Middle and two localities, which now indicate strong Siberian and Late Devonian is difficult, because both areas share Uralian-rather than westertr or Arctic Canadian- many of the s.une genera, due to the moderate to low affinities and origin for this terrane. level of endemism during this time intenral. To better ,r\r t \ lr\r /\-r, 'rlS'r./rlir'-\ jr \i li--\-\ \'\-\ \f ./-l \l )r r t lrtt l, /\-t '-,/ i .,,.r t- ', /\-\'-.t--tLtr. ir I r t r -l \ | 3r r \ lrt.r t \ \ /\-t /\ !," i,t'1_'1 i r' Frgrcl. Generalizcdmq slwwinglocationofrqjortec'tonostrdigraphicterranes inAlaskaandrnrthwstemCanda (modified frcrn Coney and otlen, 1980). Dasled pat@ra Notth American anuhlptotts basement. Bsbd arrcws irfrcde ditw'tion ofmajorstrike-slipmovements.F-Faravellterrane;I-Innoko;R-futry;C-Goodnews: P-Peninsular;Cg-Chugch; Ct-Chulitrn;W-Wrangellia;PM-Pingston&McKinley; Sp-SewadPenfusula;En-Endicott;Kv-Kagvik;NS-North Slope;W -Yakon-Tanma; M- Tracy Arm; Ax -Alqntder; T -Titht; St- Sfikine; Ch - Cacle Cteeh E -EastanAssemblage. Enasran (iane Earr-v Devor.uar) FossrLS TNDICATE a SresruAN oRrcrN FoR rse Fenewsu- TERRANE 55 BIOGEOGRAPHICALLY area), ornorthwestem and Arctic Canada. Another strong DISTINCTTVE EAREWELL Siberian linkage of the brachiopod fauna is shown by tle presence of the brachiopod genus Aldanispirifer, TERRAI\IE EMSIAN FAUNAS described origrnally from northeastem Siberia, and also The first locality is from the Nixon Fork subterrane. unknown from either nodhwestern or Arctic Canada. It consists of a number of collections from three In facg the now extensive collection of "Reef Ridge" measured sections of a distinctive marker unit of brachiopods assembled by the author shows not even limestone and minor shale of early Emsian age in the one species in common with equally extensive-but even more richly diverse-Emsian brachiopod fauna Medfra B-3 Quadrangle (fig. 2, loc. l). This unit, which is about 150 m thick, is well exposed in a north-tending gathered by the author from the Ogilvie Formation of ridge, long known informally as "Reef Ridge" by east-central Alaska and adjacent Yukon Territory. The mineral-company geologists. It is repeated twice by latter formation forms a western terminus of the thrust faulting along this ridge, which is located in extensive carbonate platform that then constituted part section 23,T.24 S., R. 23 E. of that quadrangle. The of the miogeoclinal assemblage of the North American entire succession is thought to represent shallow, subtidal continent during late Eady Devonian time. to open shelf depositional environments (Chalmers and Rugose corals identified by Pedder from the "Reef others, 1995). Carbonate rocks include packstone, Ridge" unit consist mostly of solitary forms and wackestone, andmudstone; locally, favositid coral heads dominated by relatively undiagnostic Pseudoatnplexus r:rre are cofirmon in the limy intervals, reaching up to I m in altaicus o Lithopltyllum spp., and Zonophyllum from diameter. Some channels ofprobable tidal origin bearing 21.3 to 57.3 m. From 65.2 to lll.9 m corals include reworked bioclasts and lithoclasts are present. Three R hi z ophy I lum s c hi s c h kat icum, Ly t h o p hy I I um spp., prominent, richly fossiliferous shale intervals occur Pseudoamplexus altaicus, new species of Zelophyllum, interbedded with the dominantly carbonate section; the Acathophyllum, and a new species. Pedder noted that shale intervals are thicker inthe upperpart ofthe sectiorq although undescribed, these higher faunas are related as is characteristic of deepening-upward shelf sequences. to early Emsian Kolymian loop coral faunas