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Chulitnacula, a New Paleobiogeographically Distinctive Gastropod Genus from Upper Triassic Strata in Accreted Terranes of Southern Alaska

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Chulitnacula, a New Paleobiogeographically Distinctive Gastropod Genus from Upper Triassic Strata in Accreted Terranes of Southern Alaska Journal of the Czech Geological Society 46/1ñ2(2001) 213 Chulitnacula, a new paleobiogeographically distinctive gastropod genus from Upper Triassic strata in accreted terranes of southern Alaska (3 figs) JIÿÕ FR›DA 1 ñ ROBERT B. BLODGETT 2 1 Czech Geological Survey, Kl·rov 3, 118 21 Praha 1, Czech Republic; e-mail: [email protected] 2 Department of Zoology, Oregon State University, Corvallis, Oregon, 97331 USA; e-mail: [email protected] A new protorculid gastropod genus, Chulitnacula, is based on Protorcula alaskana Smith, 1927, from the Chulitna terrane of south- central Alaska. The type species also occurs in two other accreted terranes of southern Alaska: 1.) in the Farewell terrane (Nixon Fork subterrane) of southwestern Alaska; and 2.) in the Alexander terrane of southeastern Alaska. This taxon is a common to dominant element in shallow-water, near-shore late Norian age strata of all of these terranes. It appears to be biogeographically significant in that while it so abundant in these southern Alaskan accreted terranes, it is completely unknown elsewhere in Upper Triassic para-autochthonous rocks of western North America (i.e. Nevada, Sonora). The occurrence of Chulitnacula alaskana (Smith, 1927) in the Chulitna, Farewell (Nixon Fork subterrane) and Alexander terranes suggests that they were in close reproductive communication in the Late Triassic. The absence of this taxon in either the Wrangellia terrane of southern Alaska and British Columbia, as well as in the related Wallowa terrane of northeast- ern Oregon and adjacent western Idaho is worthy of note. This absence plus many other major differences in their respective Late Triassic gastropod faunas indicate that the Chulitna-Farewell-Alexander terrane association was far removed from the Wrangellia-Wallowa terrane couplet. While both faunal associations are characteristically tropical in aspect, their biogeographically differing faunas indicate that they were probably very distantly separated from one another in the Panthalassa Ocean at this time. Key words: Triassic, Gastropoda, Alaska, Chulitna terrane, Farewell terrane, Alexander terrane, new taxa Introduction this species are deposited in the University of Alaska Museum (UAM) in Fairbanks, Alaska, USA and in the On-going study by the two authors of Upper Triassic gas- Geological Survey (Prague), Ji¯Ì Fr˝da collection (CG⁄ tropod biogeography of western North America has re- JP). vealed a number of biogeographically significant gastro- pod taxa from different accreted terranes comprising the Paleobiogeographic significance western margin of the North American continent. Our pur- pose here is to demonstrate the utility of Upper Triassic The State of Alaska is for the greater part composed of gastropods as a tool in paleobiogeographic analysis of a number of tectonostratigraphic terranes (also referred accreted terranes. Although previously poorly known from to as accreted terranes, lithotectonic terranes, etc.), which western North America (the only described faunas are to have emplaced by means of sea-floor spreading and tran- be found in Smith 1927 ñ Erwin 1994), this group holds scurrent fault movements long distances from their pre- great potential for biogeographic studies due to the fact vious points of origin. The only exception to this is the that this molluscan class is well represented in faunas of roughly triangular portion of Alaska (Fig. 1), bounded these terranes. In addition, due to the very restricted na- on the northwest by the Porcupine River and to south- ture of larval distribution in certain gastropod groups, west by the Yukon River. The latter region represents the great potential exists for the fine delineation of low-level western terminus of the Paleozoic-Triassic North Amer- paleobiogeographic units. This has been more than ade- ican continental margin. A number of differing terrane quately exemplified by our earlier studies of Paleozoic nomenclatures exists for Alaska (contrast Coney et al. gastropod biogeography (Blodgett 1992; Blodgett et al. 1980 with Jones et al. 1987 and Nokleberg et al. 1994). 1988, 1990, 1999; Blodgett ñ Fr˝da 1999; Fr˝da ñ Rohr Triassic rocks from southern Alaskaís accreted terranes 1999). In this paper we draw attention to a Norian gas- contain thick carbonate successions with tropical marine tropod species, Chulitnacula alaskana (Smith, 1927), faunas, in contrast to the much cooler-water Triassic fau- which characterizes near-shore, shallow-water marine nas associated with the North American portion of Alas- strata of several accreted terranes (Chulitna, Farewell, and ka (lower part of the Glenn Shale of east-central Alas- Alexander terranes) of southern Alaska (Fig. 1). In addi- ka) and from the Shublik and Otuk formations of the tion to indicating close spatial relationships between these Arctic Alaska terrane of northern Alaska. Determination three terranes within the Panthalassa Ocean during Late of probable Triassic paleolatitudes of the southern Alas- Triassic time, this species is also important taxonomical- kan terranes has been the subject of many studies (Pa- ly because it represents a new genus of the Protorculidae, nuska ñ Stone 1985; Hillhouse 1977, Hillhouse ñ Grom- which appears to include several related species in the mÈ 1980, 1984; Stone 1982; Haeussler et al. 1992), many eastern Tethys region. The newly illustrated specimens of of which are contradictory to one another. Although these 214 Journal of the Czech Geological Society 46/1ñ2(2001) C o o P h o u 170 150 130 e k n o i n t s s Canada Basin k u l a Chulitna terrane Precambrian 170o shield East g fault lim Kalta it o f C o Farewell r d Den i o a l l l e 60 terrane i f a r u a l Wrangellia t n d e f o r Alexander m Fig. 1. Generalized map a terrane t i showing location of the Fa- o n rewell, Chulitna, Alexander, and Wrangellia terranes in T in southern Alaska and nor- t t in us a thwestern Canada (modified thr ega fa Aleutian m u from Coney et al., 1980). lt Lined pattern ñ North Ameri- can autochthonous basement. Wrangellia Chulitnacula occurrences: 1 ñ Nixon Fork subterrane of Q u Farewell terrane (Taylor e e o PACIFIC PLATE n Mountains D-3 quadrange); C 50 ha 2 ñ Chulitna terrane (Healy rlo tte A-6 quadrangle); 3 ñ Alexan- f 500 km aul der terrane (Port Alexander t D-1 quadrangle). accreted terranes have diverse, rich fossil Upper Trias- succession of dominantly shallow-water carbonate rocks. sic biotas, relatively little is known of their contained fau- Triassic strata are recognized in only two areas within nas with the exception of few groups such as the flat the subterrane, one in the northern part in the Medfra C-3 clams (Silberling 1985, Silberling et al. 1997, Grant- 1:63,360-scale quadrangle, and the other in southernmost Mackie ñ Silberling 1990), bivalves in general (Newton part of the subterrane in the Taylor Mountains D-2 and 1983) and corals (Stanley 1979; Montanaro Gallitelli D-3 quadrangles (Blodgett et al. 2000, McRoberts ñ et al. 1979). In these paper our attention is focused on Blodgett, in press). the biogeographic significance of one species of gastro- This species is also very abundant in late Norian strata pod, Chulitnacula alaskana (Smith, 1927), from three of the Chulitna terrane (its type area) of south-central Alas- separate terranes (Chulitna, Farewell, and Alexander ter- ka (Fig. 1). This terrane is characterized by a distinctive ranes) of southern Alaska. succession of Upper Devonian ophiolitic rocks; upper Pa- The Farewell terrane of southwestern Alaska (Fig. 1) leozoic chert, tuff, volcanic conglomerate and sandstone, was established by Decker et al. (1994) to include three flysch, and limestone; Triassic limestone, basalt, redbeds, previously defined terranes (Nixon Fork, Dillinger, and sandstone and shale; and Jurassic argillite, sandstone, and Mystic), which were recognized to be genetically relat- chert (Jones et al. 1980). The presence of a thick redbed ed to one another. The latter were all reduced in rank to unit within the Triassic succession, containing late Nori- being subterranes of the Farewell terrane. The Farewell an fossils in uppermost strata, has been frequently cited terrane has recently been interpreted to represent a rift- as evidence for a more southerly origin of this terrane dur- ed continental margin sequence derived from the Siberi- ing Triassic time (i.e., Jones et al. 1980, 1982). Ammo- an paleocontinent (Blodgett ñ Brease 1997; Blodgett nites from a Lower Triassic limestone unit also indicate a 1998; Blodgett ñ Boucot 1999). Chulitnacula alaskana more southerly origin for this terrane (Nichols ñ Silber- (Smith, 1927) is found in Norian strata of the Nixon Fork ling 1979). Norian age gastropods are especially common subterrane of the Farewell terrane in the Taylor Moun- in transitional strata between the Trrb unit and overlying tains D-3 1:63,360-scale quadrangle. The Nixon Fork JTrs unit of Jones et al. (1980) in the southern part of the subterrane is characterized by a succession of strata rang- Healy A-6 1:63,360 scale quadrangle. Chulitnacula alas- ing in age from Neoproterozoic to Triassic, with the kana (Smith, 1927) is the most abundant gastropod in three Neoproterozoic-Devonian portion representing a thick collections made in 1997 from these two units by mem- Journal of the Czech Geological Society 46/1ñ2(2001) 215 bers of the Alaska Divison of Geological & Geophysical Coelostylinidae to belong to the Loxonematoidea, and Surveys during mapping of this quadrangle. placed additional gastropod genera in this family, extend- Chulitnacula alaskana (Smith, 1927) is also present ing its stratigraphic range from the Devonian to the Ju- in late Norian strata of the Alexander terrane of south- rassic. Knight et al. (1960) briefly discussed the family- eastern Alaska (Fig. 1). This terrane includes a variety level classification of Loxonematoidea. These authors of rocks of latest Precambrian? to Middle(?) Jurassic age divided the Loxonematoidea into four families: Loxone- (Gehrels ñ Berg 1994).
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