ISSN 08695938, Stratigraphy and Geological Correlation, 2014, Vol. 22, No. 2, pp. 190–201. © Pleiades Publishing, Ltd., 2014. Original Russian Text © A.G. Konstantinov, 2014, published in Stratigrafiya. Geologicheskaya Korrelyatsiya, 2014, Vol. 22, No. 2, pp. 77–89.
Zonal Correlation and Boundaries of the Lower Carnian Substage in Northeastern Asia A. G. Konstantinov Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, pr. Koptyuga 3, Novosibirsk, 630090 Russia email:
[email protected] Received September 25, 2012
Abstract—The stratigraphic distribution of ammonoids was analyzed in the Daxatina canadensis Subzone of the Trachyceras Zone distinguished in the Dolomites of Italy. It was established that ammonoids of the Dax atina and Trachyceras genera are confined to the lower and upper parts of the canadensis Subzone in the StuoresWiesen section, which was suggested as a global stratigraphic section of the lower boundary of the Carnian Stage. Owing to discreteness of the ammonoid complex and absence of the Trachyceras genus, the lower part of the canadensis Subzone was excluded from the Trachyceras generic zone and is considered as the independent Daxatina canadensis Zone, which overlies the Frankites regoledanus Zone. On the basis of the principle of priority and similar ammonoids of the canadensis and regoledanus zones, the lower bound ary of the Carnian Stage was accepted in the basement of the Alpine Trachyceras aon Zone and coincides with appearance of the Trachyceras genus. The main problems of the Boreal–Tethyan correlation of the Lower Carnian and adjacent stratigraphic levels are reviewed. The composition and distribution of the Lower Car nian ammonoids of northeastern Asia are specified taking into account the results of the revision of the Early Carnian trachyceratids of this region. Being the traditional biomarkers of the basal beds of the Carnian Stage in the Tethys, the ammonoids of the Trachyceras genus, which were unknown before in the Boreal Realm, were identified for the first time in the Lower Carnian of northeastern Asia. The Lower Carnian rocks of northeastern Asia, British Columbia, and the Alps were zonally correlated and the Lower Carnian boundaries were substantiated in the Boreal Realm. The Boreotrachyceras omkutchanicum Zone correlates with the Alpine Trachyceras aon Zone by the presence of the Trachyceras genus and stratigraphic position over the Stolleyites tenuis Zone and its analogs in British Columbia. The ammonoid complex of the Neosirennites armiger Zone includes Sirenites s.s., in particular, Sirenites ovinus Tozer species, known in the upper zone of the Lower Carnian of British Columbia (Sirenites nanseni), which allows comparison of the armiger and nanseni zones and, through it, with the upper part of the Alpine Austrotrachyceras austriacum Zone. Keywords: Upper Triassic, Carnian, biostratigraphy, zones, ammonoids, northeastern Asia DOI: 10.1134/S0869593814020051
INTRODUCTION The Boreal–Tethyan correlation of the upper parts of the Ladinian and Lower Carnian and determination of the position of the boundaries of the Middle and Upper Series of the Triassic and Lower–Upper Car nian in the Boreal Realm are related to a series of problems and discussible questions. Such a position is caused by the decreasing role of ammonoids in subma rine invertebrate communities of the high latitudes in the beginning of the Late Triassic (Dagys et al., 1979), scarcity, monotony, and high degree of their ende mism in the Boreal latitudes. The Boreal Carnian ammonoids are devoid of many taxa, the specific sequence of which was the base for the standard scale of the upper parts of the Ladinian and Carnian. The sections of British Columbia and, locally, Arctic Can ada with both Tethyan and Boreal elements are impor tant for correlation of the zonal charts of various paleo biochores (Dagys et al., 1979; Bychkov and Polu
botko, 1984; Dagys and Tozer, 1989; Konstantinov and Sobolev, 1999b; Konstantinov, 2000). Recently, we compared the biostratigraphic schemes of the upper parts of Ladinian and lower parts of Carnian of Boreal and Tethyan Realms and substantiated the lower boundary of the Carnian Stage in northeastern Asia in the basement of the “Protrachyceras” omkut chanicum Zone, using the modern data on ammonoids and biostratigraphy of the Ladinian–Car nian boundary of the Southern Alps, Himalayas, Brit ish Columbia, northeastern Asia, Svalbard Archipel ago, and Arctic Canada (Konstantinov, 2008a, 2008b; Konstantinov and Klets, 2009). The detailed morpho logical studies of the ammonoids and specification of their systematic composition and distribution are signifi cant for the Boreal–Tethyan correlation of the individual zones of the Carnian Stage. As a result of the revision of the Early Carnian trachyceratids and related groups of northeastern Asia, for the first time, we established the
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taxa which allow the direct Boreal–Tethyan correlation of host rocks and substantiation of the boundaries of the Lower Carnian Substage in the region. VOLUME AND BOUNDARIES OF THE LOWER CARNIAN SUBSTAGE The Carnian Stage was established by Mojsisovics (1869) in the volume of beds with Trachyceras aonoides distinguished in the Hallstatt limestones of Salzkammergut (Carnic Alps) (Fig. 1). Later, the beds with Trachyceras aonoides were transformed into a zone which overlies the Bucephalus (=Tropites) sub bulatus Zone (Mojsisovics, 1874). Once the fallacy in interpretation of the sequence of these zones was revealed, the Carnian Stage was added from the bot tom by the Trachyceras aon Zone, which became its lower zone (Mojsisovics, 1882). By the end of the 19th century, the volume of the stage was specified and three (lower, middle, and upper) substages were distin guished in its structure (Mojsisovics, 1893). Later, they were named Cordevolic, Julic, and Tuvalic (Mojsisov ics et al., 1895). Each substage corresponded to one ammonoid zone: Trachyceras aon, Trachyceras aonoides, and Tropites subbulatus, respectively. In 1911, Frech (1911) introduced the Trachyceras austri acum Zone, which was distinguished on the basis of the ammonoids of the “upper marls member” of the Veszpremer Marlstone (Bacony Mountains, Hun gary), into the zonal chart of the Lower Carnian Sub stage. This author indicated the close relations of ammonoids of this zone with those from the lens with Trachyceras austriacum of Feuerkogel, which was included by Mojsisovics in the aonoides Zone, and considered the fauna of the austriacum Zone in the same rank with fauna of limestones with T. aonoides of Sandling. Significant progress in elaboration of the zonal chart of the Carnian Stage emerged only in the 1970s after the works on creation of the Northern American standard of the Triassic System (Tozer, 1967; Silber ling and Tozer, 1968). Similar ammonoids of the Alpine aon and aonoides zones and the absence of data on the real sequence of these zones in Alpine sec tions allowed the American researchers to suggest their partial synchronism and to unite them into the Lower Carnian Substage, which is equivalent to the Cord evolic and Julic of Mojsisovics et al. (1895) and Tra chyceratan subdivision of Spath (1934). The Trachy ceras obesum and Sirenites nanseni zones of British Columbia, Trachyceras dasatoyense Zone of Nevada, Californian beds with Sirenites, and Alaskan beds with Sirenites hayesi were included in the Lower Carnian in North America (Silberling and Tozer, 1968). The Upper Carnian Substage was supplied with the Tro pites dilleri, Tropites welleri, and Klamathites mac rolobatus zones, whose sequence was established in the southwest of the United States (Silberling, 1956, 1959). These subdivisions are characterized by various STRATIGRAPHY AND GEOLOGICAL CORRELATION
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tropitids, which makes them faunistically similar to the Alpine subbulatus Zone. The lower boundary of the Carnian Stage in North America corresponded to the first occurence of representatives of the Trachy ceras genus sensu stricto in the layers “which are sim ilar to the desatoyense Zone of Nevada and correlate with the aon Zone of the Southern Alps” (Silberling and Tozer, 1968, p. 14). These works stimulated the revision of the zonal chart of the Carnian Stage in its typical locality, viz., in the Eastern Alps. The Austrian paleontologist Krystyn (Krystyn et al., 1971, 1980; Krystyn, 1973, 1974, 1978) studied the Carnian sections in the Hallstatt limestones of Salzkammergut, analyzed the strati graphic distribution of ammonoids in them, and elab orated the zonal chart of the Alpine–Mediterranean Lower Carnian (Krystyn, 1978). It consists of the Tra chyceras aonoides and T. (Austrotrachyceras) austri acum zones, each of which includes two subzones. The important result of these studies was, in particular, the proof of the real and discrete stratigraphic levels which correspond to the aon and aonoides zones of Mojsisovics; however, owing to similar ammonoids from these subdivisions (Trachyceras s.s., Sirenotra chyceras, Klirsteinia, and Badiotites genera), their rank was decreased to subzones in the aonoides Zone. In regard to amalgamation of the Cordevolic and Julic into one zone, Krystyn supported the twomember division of the Carnian Stage and chose the Julic name for the lower substage, because initially (Mojsisovics, 1869) and later the aonoides Zone was always ascribed to the Carnian Stage. In the overlying austriacum Zone, the ammonoids are significantly renewed on the generic level with appearance of the Neoprotrachy ceras and Sirenites genera and the Austrotrachyceras subgenus, to which the index species of the zone belongs. The aon and aonoides subzones of the aonoides Zone and the lower austriacum Subzone of the same zone are easily distinguished and widespread both in the Alps and in other regions of the Tethyan Realm. The distinguishing of the upper Sirenites Sub zone of the austriacum Zone was conditional and based on a single locality in Yugoslavia. It corresponds to the section part between the austriacum Subzone and the lower zone (Tropites dilleri) of the Upper Car nian and is vaguely paleontologically characterized (coarseribbed Sirenites close to Neosirenites irregu laris (Kiparisova) and, probably, Neoprotrachyceras genus), which is why the boundary of the Lower– Upper Carnian in the Alps is insufficiently clear and coincides with appearance of tropitids in the basement of the Tropites dilleri Zone. This zonal chart has undergone insignificant modifications until now and was accepted by the Interdepartmental Stratigraphic Committee (Bychkov and Polubotko, 1984; Oleinikov and Gavrilova, 2008). The results of elaboration of the zonal charts of the Carnian Stage of North America and the Eastern Alps promoted the unification of its lower boundary and Vol. 22
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substage division. The twosubstage division of the Carnian Stage is currently commonly accepted and the boundary between the Ladinian–Carnian or Mid dle–Upper Triassic series was drawn by most research
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ers in the basement of the aon Zone (or subzone) and its equivalent in North America (desatoyense Zone). The first representatives of the Trachyceras genus in North America appeared at this level, whereas, in the
Fig. 1. Evolution of the points of view on the volume and zonal division of the Lower Carnian Substage of the Alpine region and North America.
1874
Trachyceras aonoides Zone
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Beds with Trachyceras aonoides
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Shevyrev, 1986, 1990
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Sirenites nanseni
Austrotr. obesum
T. aonoides
This work
Lower Carnian Trachyceras aon
Austrotrachyceras austriacum
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Austrotrachyceras austriacum
This work
Trachyceras aon
Lower Carnian
Trachyceras aon Daxatina canadensis
Trachyceras
Trachyceras aonoides
Austrotrachyceras austriacum
Julic
The revision of the boundary layers of the Ladinian and Carnian stages in the typical Cordevolic locality, viz., in the Southern Alps (Dolomites) of Italy (Mietto and Manfrin, 1995), has shown that some species of the Trachyceras genus (T. bipunctatum (Münster) and T. muensteri (Wissmann)) appear in the layers which underlie the aon Zone somewhat higher in the section than the first representatives of the Daxatina genus and Clydonitaceae superfamily (Clionitites genus). This stratigraphic level, which is located higher relative to the Frankites regoledanus of the Upper Ladinian and lower than the basal zone (Trachyceras aon) of the Lower Carnian, was ascribed to the Trachyceras generic zone by the presence of the Trachyceras genus and was distinguished as its lower subzone (Daxatina cf. canadensis) (Fig. 2). In the basement of the latter, it was proposed to draw the lower boundary of the Car nian Stage, which is marked by the appearance of the new ammonoid taxa, viz., Daxatina genus and Cly donitaceae superfamily (Clionitites genus). The Prati diStuores (or StuoresWiesen) section located on the southern slope of the ridge, which divides the valleys of the Badia and Cordevole rivers in the Dolomites, was chosen as a global stratotype (Broglio Loriga et al., 1999). The lower boundary of the Carnian is in the basement of the SW4 layer, which is located 45 m above the bottom of the San Cassiano Formation; the Daxatina genus and D. canadensis species appear at this level. The finesculptured trachyceratids, which were primarily (Mietto and Manfrin, 1995; Broglio Loriga et al., 1999; Mietto et al., 2007a) determined as Trachyceras bipunctatum (Münster) and, after revi sion, were ascribed to the Daxatina laubei Tozer (Mietto et al., 2007a, 2008), were found 16.5 m up the section. However, precisely the layer small in thickness which divides the findings of the first Daxatina and Trachyceras allowed Broglio Loriga et al. (1999) to suggest, taking into account the greater thickness of the canadensis Zone (about 150 m) and high sedimen tation rate, the approximate coincidence of the appearance of the Daxatina genus and Trachyceras genus, which was the commonly accepted biomarker of the lowermost parts of the Carnian Stage before now. According to the latest data (Mietto et al., 2007b, 2008), the first representatives of the Trachyceras genus (T. muensteri (Wissmann)) were found in the bed PSR2, 33.8 m above the suggested lower boundary of the Carnian Stage. On the other hand, the upper limit of distribution of Daxatina cf. laubei Tozer and Daxatina sp. is resticted in the section by the beds SW6 and PSR4, respectively, which corresponds to the lev els 16.5 and 34 m above the boundary. Against this back ground, only the index species of Daxatina canadensis
Mietto and Manfrin, 1995; Mietto et al., 2007a, 2007b, 2008
Trachyceras aonoides
Alps, the boundary was combined with the first occurence of the index species of the Trachyceras aon (Münster) lower zone (subzone), which did not exclude the origination of individual species of the Trachyceras genus on the lower stratigraphic levels (Krystyn, 1978).
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D. Ladinian canadensis
Fig. 2. Zonal division of the uppermost Ladinian–Lower Carnian of the Dolomites and adjacent areas of the Alpine region.
(Whiteaves) strongly stands out: one part of its findings comes from the lower subzone part (beds SW4–PSR3, 0–33.9 m higher than boundary), another part origi nates from the beds SW9 and SW10 (11.2 and 112.8 m higher than the boundary, respectively), and a big gap in findings corresponds to the beds PSR4–SW8. How ever, judging from the images of D. canadensis (Whit.) of the upper subzone part (Mietto et al., 2008, pl. 6, figs. 12, 14, 17), they are sharply distinct from the Daxatina genus by progressively growing whorls and strongly curved sicklelike ribs with bulges elongated along the spirals (Mietto et al., 2008, pl. 6, fig. 17). The lobe lines of the D. canadensis (Whit.) drawn from the fragments of the shell septa (Mietto et al., 2008, textfigs. 9f, 9h) apparently do not correctly reflect the contours of the line on the core surface; however, text fig. 9f clearly shows the deep hacking on the internal Vol. 22
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slope of the first side fold. Thus, in our opinion, the authors’ interpretation of the lobe line, including the contours of the V/L fold, is incorrect. These forms probably belong to the Sirentotrachyceras sp. indet. rather than to the Daxatina one. So, summing up the analysis of the stratigraphic distribution of the Daxatina and Trachyceras genera in the StuoresWiesen section, we may draw the follow ing conclusions. First, the Daxatina genus is confined only to the lower part of the canadensis Subzone of the Italian authors (Mietto and Manfrin, 1995), namely, to the beds SW4–PSR4 (45–79 m higher than the basement of the San Cassiano Formation), whereas the Trachyceras genus appears in the section signifi cantly higher, in the bed PSR2 (78.8 m higher than the boundary of formation), and characterizes the upper subzone part. Second, the absence of the Trachyceras genus in the lower part of the canadensis Subzone and the discrete ammonoid complex of this level, includ ing Daxatina canadensis (Whit.), D. laubei Tozer, D. sp., Sirenotrachyceras thusneldae (Mojs.), Frankites apertus (Mojs.), F. johnstoni Mietto et Manfrin, Zesto ceras barwicki (Johnston), and Z. lorigae Mietto et Manfrin, allow exclusions of this part from the Tra chyceras generic zone and consideration of it as the independent Daxatina canadensis Zone, which over lies the regoledanus Zone. Third, taking into account the absence of the Trachyceras genus in the canadensis Zone, a generic commonness of ammonoids of this zone, which underlies the Ladinian regoledanus Zone (Konstantinov, 2008a), and the principle of priority, the lower boundary of the Carnian Stage is accepted in the basement of the Trachyceras aon Zone and coin cides with the first appearance of the Trachyceras genus. The last reservation is related to the fact that the aon Zone is interpreted in the given work, in contrast to the scheme of Krystyn (1978), as an independent complex zone, which also includes the upper part of the canadensis Zone of the Dolomites, where the Tra chyceras genus and T. muensteri species appear. Such understanding of the volume of the aon Zone is prob ably similar to the interpretation of Mojsisovics (1882), who considered T. meunsteri (Wissmann) as a younger synonym of the T. aon (Münster) species, and agrees well with modern data on distribution of the ammonoids in the Stuores–Wiesen section. For example, the T. muensteri (Wissmann) species transits into overlying rocks, where it is associated with T. aon (Münster) (Ulrichs, 1994). In addition, the complex of the aon Zone includes T. bipunctatum (Münster), Klipsteinia cf. achelous (Münster), and Rossiceras armatum (Münster) ammonoids, which were found in the upper part of the canadensis Subzone (Mietto et al., 2007b) and indicate that the host rocks belong to this zone. Thus, we accept the following sequences of the ammonoid zones in the Lower Carnian Substage of Alps: Trachyceras aon, Trachyceras aonoids, and Aus trotrachyceras austriacum (Fig. 1). The upper bound
ary of the Lower Carnian is marked by disappearance of the trachyceratids of the Trachyceras and Austrotra chyceras genera and appearance of the tropitids in the basement of the Tropites dilleri Zone. BOREAL–TETHYAN CORRELATION OF THE LOWER CARNIAN Fundamental Approaches in the Boreal–Tethyan Correlation of the Triassic Rocks Three welldeveloped zonal charts of the Triassic system are known presently: Alpine, North American, and Siberian. Traditionally, taking into account the principle of priority and more diverse Tethyan ammonoids, the Alpine zonal chart is considered as a bizonal standard (Dagys et al., 1979; Bychkov and Polubotko, 1984; Shevyrev et al., 1986, 1990). The North American zonal chart links the Alpine and Siberian charts, because different territories of the Pacific eastern coast in Triassic were located in the area of the paleobiogeographical ecoton and charac terized by mixed Boreal and Tethyan fauna. The pale ontological and biostratigraphical data on northeast ern Asia are principally significant for the solution of problems of the Boreal–Tethyan correlation of the Triassic rocks, in particular, the upper parts of the Ladinian Stage and Lower Carnian Substage, because the most complete sequence of the Boreal ammonoid complexes of the upper parts of the Ladinian–Lower Carnian was identified in this region and detailed zonal charts of this stratigraphic level were elaborated (Dagys and Konstantinov, 1992; Bychkov, 1995; Kon stantinov and Sobolev, 1999a, 1999b). The specifica tion of composition and distribution of the ammonoids as a result of the detailed morphological studies and revisions of individual groups and new materials from the sections of northeastern Asia is also significant for the Boreal–Tethyan correlation of the individual zones of the Carnian Stage. Systematic Composition and Distribution of Boreal Lower Carnian Ammonoids The systematic composition and stratigraphic dis tribution of the Boreal Lower Carnian ammonoids have significantly been specified (Tozer, 1994; Bych kov, 1995; Konstantinov, 1995, 1999, 2006, 2012). In Arctic Canada, the Lower Carnian ammonoids were identified in the beds with Discophyllites (=Astro phyllites) cf. taimyrensis, Halobia cf. zitteli, and Arcto phyllites cf. taimyrensis (Popow) (Tozer, 1961, 1994), and in the Sirenites nanseni Zone with only the index species. The “Protrachyceras” (=Boreotrachyceras) omkutchanicum Zone, beds with Seimkanites aculeatus, and Yanosirenites buralkitensis, Okhototrachyceras seimkanense, and Neosirenites armiger zones are dis tinguished from the bottom to top in the Lower Car nian Substage of northeastern Asia, taking into account publications on specification of the Ladin
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CHUKCHI SEA
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Koly ma R .
ka R. Indigir
Yana R.
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Fig. 3. Location of ammonoids Trachyceras sp. indet. and Sirenites ovinus Tozer in the scheme of northeastern Asia. (1) Lower reaches of the Lena River, TaasAryy Island, outcrop 11; (2) Northern Okhotsk region, Vtoraya Sentyabr’skaya River, outcrop C2. The sections are described in (Konstantinov and Sobolev, 1999a; Konstantinov et al., 2007).
ian–Carnian boundary (Konstantinov, 2008a, 2008b). The representatives of the Trachyceratidae and Sireni tidae families were dominant among the Lower Car nian ammonoids. A significant amount of Boreal (Yakutosirenites) and endemic Siberian (Seimkanites, Yanosirenites, Neosirenites) genera were identified among sirenitids (Popov, 1961; Bychkov, 1982; Kon stantinov, 1999). The trachyceratids from the Lower Carnian rocks of northeastern Asia were first described by Bychkov (1973) and were initially ascribed to the Protrachyceras Moisisovics, 1893 genera (Protrachy ceras omkutchanicum Bytschkov and Protrachyceras seimkanense Bytschkov) by the absence of the rib split ting on the ventral edge and by single bulges of the ven tral spirals. Once the volume of the Protrachyceras genus was specified on the Tethyan material and its forms were excluded which occur in the Carnian rocks or transit into the Lower Carnian from the Ladinian and are distinguished in the new Neoprotrachyceras and Sirenotrachyceras genera (Krystyn, 1978), the native specialists came to ascribe the Protrachyceras seimkanense (Bytschkov) species to the Neopprotra chyceras Krystyn, 1978 genus (Bychkov and Alabush eva, 1982; Bychkov and Polubotko, 1984; Bychkov, 1995; Dagys et al., 1996). The revision of typical Early Carnian trachyceratids of northeastern Asia has shown that the sculpture of thin rare sicklelike curved ribs of the P. omkutchanicum Bytschkov and P. seimkanense STRATIGRAPHY AND GEOLOGICAL CORRELATION
Bytschkov species strongly differ from both the Protra chyceras Mojsisovics, 1983 and Neoprotrachyceras Krystyn, 1978 genera (Konstantinov, 2012). In addi tion, these species could not be ascribed to the latter genus because of the less compound lobe line and the absence of twinning of the bulges of the ventral spirals on the late growth stages. Significant differences in morphology of the early growth stages were estab lished between P. omkutchanicum and P. seimkanense, which indicates that they belong to the independent new Boreotrachyceras gen. nov. and Okhototrachyceras gen. nov. genera. During the study of the ammonoid collection from the lower parts of the Osipai Forma tion, which are exposed in the section of the Taas Aryy Island in the lower reaches of the Lena River (outcrop 11, bed 19, 3.5 m higher than the basement; Konstantinov et al., 2007) (Fig. 3), along with Boreotrachyceras omkutchanicum (Bytchkov), we found a part of the living chamber of the involute shell with quickly overgrowing and strongly equitant whorls with elongated–oval section (Figs. 4a, 4b). A series of features (elongated twoapex ventral bulges, multiple bulge spirals on closely located ribs, the rib splitting from the bulges of the umbilical edge, first side spirals and marginal spiral, and an obtuse angle between the ventral bulges) indicates that this form undoubtedly belongs to the Trachyceras Laube, 1869 genus and is the first finding of this genus in the Boreal Realm Vol. 22
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(c)
(a) (b)
(d)
Fig. 4. Ammonoids of the Trachyceras and Sirenites genera from the Lower Carnian of the lower reaches of the Lena River and Northern Okhotsk region. (a, b) Trachyceras sp. indet., specimen 17/920 (×3 magnification), D = 20 mm: (a) side view, (b) ventral side view, lower reaches of the Lena River, TaasAryy Island, Osipai Formation, Lower Carnian, Boreotrachyceras omkutchani cum zone, collection of N.I. Kurushin, 1980; (c, d) Sirenites ovinus Tozer, specimen no. C24/11.5p (×1 magnification ), D = 40 mm, W = 7 mm: (c) side view, (d) ventral side view, Northern Okhotsk region, Vtoraya Sentyabr’skaya River, outcrop C2, Sen tyabr’skaya Formation, lower subformation, Lower Carnian, Neosirenites armiger Zone, collection of A.G. Konstantinov and E.S. Sobolev, 1988.
(Konstantinov, 2012). Although we failed to study the lobe line, this specimen could not be ascribed to the Daxatina genus because of the rib splitting on the marginal spiral that is typical (Ulrichs, 1994) of the Trachyceras genus and the obtuse angle between the double ventral bulges and their clear individualiz ing from other bulges. I determine it as Trachyceras sp. indet.; however, by the amount of bulge spirals on the ribs (13), it is close to the species known in the Alpine aon Subzone. The ussuritids from the Boreal Arcto phyllites genus frequently occur and are typical of other ammonoid groups in the Lower Carnian of northeastern Asia (Konstantinov, 1995); specific endemic arpaditids (Siberioklipsteinia genus) were found only in one location (Konstantinov, 2006). The cosmopolitic genera, which include, as a rule, the local species, were identified among the sirenitids (Sirenites) and arcestids (Proarcestes). Thus, 11 genera are currently known among the Lower Carnian ammonoids of northeastern Asia and their majority (Yakutosirenites, Arctophyllites, Boreotrachyceras, Okhototrachyceras, Seimkanites, Neosirenites, Yano sirenites, Siberioklipsteinia genera) belong to the Boreal and endemic Siberian forms in contrast to cos mopolitan Sirenites and Proarcestes. The first finding of the Tethyan Trachyceras genus along with Boreal ammonoids is important to reveal the relations between the paleowater basins of different latitudes and for the Boreal–Tethyan correlation of the basal beds of the Carnian Stage.
Boreal–Tethyan Correlation of the Lower Carnian The recent new data on systematic composition and distribution of ammonoids in both the boundary interval of the Ladinian–Carnian of the Southern (Dolomite) Alps and Lower Carnian rocks of north eastern Asia allow us to directly correlate the individ ual zones by common taxa and to substantiate the boundaries of the Lower Carnian Substage in the Boreal Realm. The lower boundary of the Trachyceras desatoy ense Zone of British Columbia was typically combined with the lower boundary of the Alpine Trachyceras aon Zone (Dagys and Tozer, 1989; Dagys et al., 1993; Konstantinov and Sobolev, 1999b; Kazakov et al., 2002) on the basis of similar Trachyceras desatoyense Johnston index species and T. aon Mojsisovics, which was indicated by many authors (Johnston, 1941; Sil berling and Tozer, 1968; Krystyn, 1978; Ulrichs, 1994). The nathorstitids of the Stolleyites genus were found in layers which were included by Tozer (1994) in the desatoyense Zone. This allowed comparison of the Stolleyites tenuis Zone of northeastern Asia and the Svalbard Archipelago with the desatoyense Zone. However, these correlations did not take into account the stratigraphic distribution of the ammonoids in Canada. In reality, the Stolleyites genus, as follows from the distribution of the ammonoids in the British Columbia sections (Konstantinov, 2008a), was found along with Daxatina and Clionitites in the lower part of the desatoyense Zone, which is devoid of the Trachy
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ceras genus and T. desatoyense Johnston species as yet. Taking into account that the lower boundary of the aon Zone and Carnian Stage in the Alps coincides with appearance of the Trachyceras genus and similar T. aon and T. desatoyense species, only the middle part of the desatoyense Zone with the Trachyceras genus and index species of the zone could be compared with the aon Zone (Fig. 5). The T. aonoides Mojsisovics species are known in the upper part of the Canadian zone. These beds are equivalent to the aonoides Zone of the Alps. Thus, the chronoequivalents of the aon and aonoides zones of the standard scale are present in the middle and upper parts of the desatoyense Zone. On the basis of the common Daxatina and Clionitites genera, the lower part of the desatoyense Zone and the upper Asklepioceras laurenci Subzone of the Frankites sutherlandi Zone of British Columbia with abundant Daxatina canadensis (Whiteaves) species are equiva lent in total to the Daxatina canadensis Zone of the Dolomites in Italy. The correlation of the laurenci Subzone with the candensis Zone is also confirmed by the presence of other common ammonoid taxa (Fran kites and Zestoceras genera), which, according to (Mietto et al., 2008), include the same species in these subdivisions. In a recently published review on the modern con dition of the Triassic ammonoid charts (Balini et al., 2010), the lower part of the Carnian Stage of the Dolo mites (Daxatina canadensis = Daxatina canadensis Subzone of the Trachyceras generic zone) and the ter minal Alpine zone of the Ladinian Stage (Frankites regoledanus) were compared to the Trachyceras desa toyense Zone of British Columbia and Canadian Frankites sutherlandi Zone, respectively. This is in contrast to data on distribution of the Daxatina can densis (Whit.) species, which was described in Canada (Whiteaves, 1889) from the sutherlandi Zone (Tozer, 1967, 1994). If we accept the correlation of the bound ary zones of the Ladinian–Carnian of the Dolomites and British Columbia suggested by Balini et al. (2010), then we should state that ammonoids Daxatina canadensis (Whit.) chosen as the biomarker of the Carnian basement (Mietto and Manfrin, 1995) are widespread in the upper zone of the Ladinian and lower zone of the Carnian in Canada and the Alps, respectively. Such prochoresis of species was unlikely and it is evident that this correlation scheme has no grounds and is erroneous. The Stolleyites tenuis Zone of northeastern Asia and the Svalbard Archipelago is comparable by the common Stolleyites genus with the lower part of the desatoyense of British Columbia. The findings of ammonoids of the Daxatina genus in the tenuis Zone (Korchinskaya, 2000) at the Svalbard Archipelago confirm such a correlation. It follows, taking into account the abovenoted comparison of the Canadian and standard schemes, that the tenuis Zone of the Boreal Realm corresponds to the upper part of the Daxatina canadensis Zone of the Alpine chart and is STRATIGRAPHY AND GEOLOGICAL CORRELATION
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Late Ladinian. The Nathorstites macconnelli and Nathortstites lindstroemi zones of the Boreal Realm are confidently compared with the lower part of the Alpine canadensis Zone through the sections of Brit ish Columbia. For example, nathorstites Nathorstites macconnelli (Whiteaves) are abundant in the entire Asklepioceras laurenci Subzone of the British Colum bia Frankites sutherlandi Zone from its very lowest up to topmost layers, where they were found both jointly with the Daxatina canadensis (Whiteaves) species and stratigraphically higher than the findings of the latter (GSC loc. 68260 with Nathorstites macconnelli (Whiteaves) is higher in the section than GSC loc. 68272 with Daxatina canadensis (Whiteaves)). The correlation of the canadensis and lindstroemi zones of Bear Island and northeastern Asia is based on the findings of Daxatina canadensis (Whiteaves) at this level of the Svalbard Archipelago (Böhm, 1903; Dagys et al., 1993). The first finding of the ammonoids of the Trachy ceras genus in northeastern Asia is exceptionally important to substantiate the age of the basal beds of the Carnian Stage in the Boreal Realm. If previously we conditionally compared the omkutchanicum Zone with the aon Zone by its stratigraphic position higher than the tenuis Zone and its correlatives in British Columbia (Konstantinov, 2008a), then the finding of the form similar in amount of bulges to representatives of the Trachycers genus from the aon Zone in the lower parts of the omkutchanicum Zone allows direct corre lation of the these zones. The syncronism of the lower boundaries of these zones is emphasized by the appearance at this level of the Trachycers genus, which we consider as a biomarker of the lower boundary of the Carnian Stage. The synchronism of their upper boundaries is substantiated by the appearance of the nautilids Proclydonautilus goniatites (Hauer) in the overlying rocks, in the Alpine aonoides Zone, and in beds with Seimkanites aculeatus of northeastern Asia (Konstantinov and Sobolev, 1999b). The position of the lower boundary of the Carnian Stage in northeastern Asia in the basement of the omkutchanicum Zone corresponds to the first point of view (Dagys et al., 1979) and, in our opinion, is sub stantiated by both correlations and the strongest evo lutionary transformations of the Boreal ammonoids at this level (Konstantinov, 2008b). The extinction of the last nathorstitids Stolleyites and appearance of the first trachyceratids Boreotrachyceras and Trachyceras are confined to this boundary. It divides two large evolu tionary stages of the Boreal ammonoids. The Ladinian stage was remarkable by the low taxon diversity of ammonoids, high degree of endemism, and preva lence of specific Boreal taxa at the levels and families with dominant tsvetkovitids and nathorstitids, which separated from them. At the beginning of the Car nian–Early Norian, the first trachyceratids and siren itids reached the Boreal Realm and became dominant in the ammonoid communities of that period. Vol. 22
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Stage
Carnian
Ladinian
Substage
Upper
Lower
Upper
STRATIGRAPHY AND GEOLOGICAL CORRELATION
Frankites regoledanus
Daxatina canadensis
Trachyceras aon
Trachyceras aonoides
Nathorstites naclearni
Nath. lindstroemi
Frankites glaber
Beds with Nathorstites
Stolleyites tenuis
Stolleyites
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Nath. lindstroemi
Stolleyites tenuis
Bear Island (Dagys et al., 1993)
Fig. 5. Comparison of biostratigraphic schemes of the upper part of the Ladinian and Carnian stages.
Nathorstites s. str.
Stolleyites tenuis
Boreotrachyceras omkutchanicum
Beds with Lima, Myophoria and Pleurophorus
Spitsbergen Is. (Korchinskaya, 1982, 2000; Konstantinov, 2000)
Svalbard Archipelago
Trachyceras
Nath. macconnelli
Y. buralkitensis S. aculeatus
O. semkanense
Neosirenites armiger
Yakutosirenites pentastichus
Sirenites yakutensis
Kedonosirenites kedonensis
Northeastern Asia (Dagys, Kons tantinov, 1992; Konstantinov and Sobolev, 1999)
T. desa toyense Daxatina limpida, Stolleyites Asklepio ceras laurenci
T. aonoides
Austrotr. obesum
Sirenites ovinus
Sirenites nanseni
Austrotrachyceras austriacum
Sirenites yakutensis Yakutosirenites pentastichus Arctosirenites canadensis Proarcestes winnenae
1
2
Common ammonoid taxa
Tropites dilleri
Tropites welleri
Klamathites macrolobatus
British Columbia (Tozer, 1994)
Tropites dilleri
Tropites subbulatus
Anatropites spinosus
Alps (standard scale)
Trachyceras desatoyense Frankites sutherlandi
Beds with Nathorstites
Beds with Arcto phyllites taimyrensis and Z. cf. zitteli
Sirenites nanseni
Beds with Arcto sirenites canadensis
Beds with Jovites borealis
Canadian Arctic Archipelago (Tozer, 1961; 1994)
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The single findings of the ammonoids Sirenites s.str. in the Neosirenites armiger Zone are important for the zonal correlation of the upper parts of the Lower Carnian of northeastern Asia, Arctic Canada, British Columbia, and a standard chart. For the first time, they were found in a section of the Northern Verkhoyansk region (Darky River) and Omolon massif (Omkuchan River) (Konstantinov and Sobolev, 1999b) and more recently in the stratotype of the armiger Zone at the Vtoraya Sentyabr’skaya River in the Northern Okhotsk region and in the lowermost parts of the pentastichus Zone (C2 outcrop, bed 1, depth of 34.5 m higher than the bottom–top, bed 2, bed 3, lower 1.5 m; Konstantinov and Sobolev, 1999a, p. 7) (Fig. 3). The sculpture of these forms with five umbonal and ventral spirals of the bulges on the ribs, eminent keels from both sides of the ventral sulcus with very small and frequent oblique dents, and the proportion of the amount of bulges of the ventral and marginal spirals of 2 (Figs. 4c, 4d) corresponds to the Sirenites senticosus group of Mojsisovics (1893) (i.e., to the typical representatives of the Sirenites genus) and to the Sirenites ovinus Tozer species, which is typ ical of the upper zone (Sirenites nanseni) of the Lower Carnian of Canada. Thus, the armiger Zone is confi dently compared with the nanseni Zone by the pres ence of Sirenites ovinus and with the austriacum Zone of the Alpine scale with characteristic Sirenites of the senticosus group. Taking into account that the last S. ovinus were found at the level of 1.5 m higher than the basement of the pentastichus Zone and the com plete thickness of the latter in the section is 120 m, it is possible to conditionally combine the boundary of the Lower and Upper Carnian substages in northeastern Asia with the boundaries of the armiger and pentasti chus zones. The conditional upper boundary of the Lower Car nian Substage in the Boreal sections is caused by the absence of findings of the tropitids and Tropites genus in the rocks which directly overlie the Lower Carnian and, consequently, the absence of the reliable correla tives of the lower zone of the Upper Carnian (Tropites dilleri). Recently, new interesting data for the Boreal– Tethyan correlation of the rocks and substantiation of the age of the Upper Carnian lower layers were obtained by systematic composition of the ammonoids of the Yakutosirenites pentastichus Zone of Kotel’nyii Island (Bragin et al., 2012). In the previous correlation schemes (Konstantinov and Sobolev, 1999b; Kazakov et al., 2002), the pentastichus Zone was confidently compared only with Jovites borealis beds of Arctic Canada and upper subzone of the Tropites welleri Zone of British Columbia by the presence of the index species. The correlation of the pentastichus Zone with the Arctosirenites canadensis beds of Arctic Canada and dilleri Zone of North America was conditional. Currently, the findings of the ammonoids Arctosireni tes canadensis Tozer and Proarcestes winnemae Smith, which were determined in the collection of N.Yu. Bra STRATIGRAPHY AND GEOLOGICAL CORRELATION
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gin and A.B. Kuz’michev, employees of the Geologi cal Institute of the Russian Academy of Sciences, indicate the presence of correlatives of the Arctosiren ites canadensis beds of Arctic Canada at different lev els of the pentastichus Zone of Kotel’nyi Island and the Tropites dilleri Zone of California. The Seimkanites aculeatus beds and Yanosirenites buralkitensis and Okhototrachyceras seimkanense zones with endemic Siberian genera and species are hardly comparable with rocks of the same age from other regions and may be correlated only by their stratigraphic position higher than the omkutchanicum Zone and lower than the armiger Zone and their cor relatives with the upper part of the desatoyense and obesum zones of British Columbia and with the aonoides Zone and lower part of the austriacum Zone of the Alpine chart. CONCLUSIONS (1) The trachyceratids of the Daxatina genus in the StuoresWiesen section of the Dolomites are confined only to the lower part of the Daxatina canadensis Sub zone of the Trachyceras generic zone, whereas the Trachyceras genus characterizes the upper part of the subzone. The absence of the Trachyceras genus in the lower part of the canadensis Zone and a discrete complex of ammonoids of this level, including Daxa tina, Frankites, Sirenotrachyceras, and Zestoceras gen era, allow exclusion of this part from the Trachyceras generic zone and consideration as an independent canadensis Zone, which overlies the regoledanus Zone. By the presence of the Trachyceras genus and other ammonoids, the upper part of the canadensis Subzone was ascribed to the Trachyceras aon Zone, which is interpreted as an independent complex zone. (2) The lower boundary of the Carnian Stage, fol lowing the principle of priority, is accepted in the base ment of the aon Zone and is drawn in the Stuores Wiesen section at the level of 78.8 m higher than the basement of the San Cassiano Formation by the first appearance of the Trachyceras genus, which is a tradi tional biomarker of the lower parts of the Carnian Stage. (3) The finding of the Trachyceras genus in the omkutchanicum Zone of the lower reaches of the Lena River, in particular, the finding of the form which is similar to the representatives of the aon Zone by the amount of bulges, allows for the first time direct corre lation of the omkutchanicum and aon zones and pale ontological substantiation of the position of the lower boundary of the Carnian Stage in the Boreal Realm. (4) By the presence of the Sirenites ovinus Tozer species, the armiger Zone of northeastern Asia is com parable with the nanseni Zone of British Columbia and Arctic Canada and, via the latter, with the upper zone of the Lower Carnian, viz., Alpine austriacum Zone. The boundary of the Lower and Upper Carnian substages in northeastern Asia approximately coin Vol. 22
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cides with those of the armiger and pentastichus zones. ACKNOWLEDGMENTS This work is supported by the Programs of Funda mental Research of the Russian Academy of Sciences nos. 23 and 28. Reviewers N.Yu. Bragin, V.A. Zakharov REFERENCES Balini, M., Lucas, S.G., Jenks, J.F., et al., Triassic ammonoid biostratigraphy: an overview, in The Triassic Timescale, Lucas, S.G., Ed., Geol. Soc. London. Spec. Publ., 2010, vol. 334, pp. 221–262. Böhm, J., Über die obertriadische fauna der Bäreninsel, Kgl. Sven. Vet.Akad. Handl., 1903, vol. 37, no. 3, pp. 1–76. Bragin, N.Yu., Konstantinov, A.G., and Sobolev, E.S., Upper Triassic stratigraphy and paleobiogeography of Kotel’nyi Island (New Siberian Islands), Stratigr. Geol. Correl., 2012, vol. 20, no. 6, pp. 541–566. Broglio Loriga, C., Cirilli, S., De Zanche, V., et al., The Prati di Stuores/Stuores Wiesen section (Dolomites, Italy): a candidate Global Stratotype Section and point for the base of the Carnian Stage, Riv. Ital. Paleontol. Stratigr., 1999, vol. 105, no. 1, Tav. 1–10, pp. 37–78. Bychkov, Yu.M., New Late Triassic of trachyceratides in the northeastern part of the USSR, Kolyma, 1973, no. 10, pp. 35–38. Bychkov, Yu.M., About subdivision of the Carnian Protra chyceras seimkanense zone, in Bio i litostratigrafiya triasa Sibiri (Triassic Bio and Lithostratigraphy of Siberia), Mos cow: Nauka, 1982, pp. 71–74. Bychkov, Yu.M. and Alabusheva, A.V., Rare and new cerati des of the northeastern part of the USSR, Kolyma, 1982, no. 8, pp. 28–30. Bychkov, Yu.M. and Polubotko, I.V., Upper Triassic, in Obshchaya shkala triasovoi sistemy SSSR. Ob’’yasnitel’naya zapiska (The General Stratigraphic Scale of Triassic System of the USSR. Explanatory Note), Leningrad: Izd. VSEGEI, 1984, pp. 63–104. Bychkov, Yu.M., Pozdnetriasovye trakhitseratidy i sirenitidy verkhov’ev Yany Okhotskoi (Late Triassic Trachyceratides and Sirenitides of the Upper Yana near Okhotsk), Magadan: SVNTs DVO RAN, 1995 [in Russian]. Dagys, A.S., Arkhipov, Yu.V., and Bychkov, Yu.M., Strati grafiya triasovoi sistemy SeveroVostochnoi Azii (Stratigra phy of Triassic System in Northeastern Asia), Moscow: Nauka, 1979 [in Russian]. Dagys, A.S. and Tozer, E.T., Correlation of Triassic of North Canada and Siberia, Geol. Geofiz., 1989, no. 6, pp. 3–10. Dagys, A.S. and Konstantinov, A.G., A new zonal scheme of Boreal Ladinian, Albertiana, 1992, no. 10, pp. 17–21. Dagys, A., Weitschat, W., Konstantinov, A., et al., Evolution of the boreal marine biota and biostratigraphy at the Mid dle/Upper Triassic boundary, Mitt. Geol.Paläontol. Inst. Univ., Hamburg, 1993, vol. 75, pp. 193–209.
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Translated by I. Melekestseva
Vol. 22
No. 2
2014
