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Ammonoid discoveries in the Antimonio Formation, Sonora, Mexico: new constraints on the Triassic–Jurassic boundary
Journal of South American Earth Sciences 13 (2000) 491±497
www.elsevier.nl/locate/jsames
Ammonoid discoveries in the Antimonio Formation, Sonora, Mexico: new constraints on the Triassic±Jurassic boundary C.M. GonzaÂlez-LeoÂn a,*, G.D. Stanley Jr. b, D.G. Taylor c a
EstacioÂn Regional del Noroeste, Instituto de GeologõÂa, Universidad Nacional AutoÂnoma de, MeÂxico, Apartado Postal 1039, Hermosillo, Sonora, 83000 Mexico b Department of Geology, University of Montana, Missoula, MT 59812, USA c Department of Geology, Portland State University, P.O. Box 751, Portland, OR 97207, USA Received 30 September 1998; revised 31 December 1999; accepted 31 January 2000
Abstract The Triassic±Jurassic systemic boundary was recently reported in the middle part of the Antimonio Formation, northwestern Sonora, where ®ve informal sedimentary packages were delineated and characteristic ammonoid faunas were used to establish age control within the succession. The boundary was suggested to lie within the middle part of the 24 m-thick package 4, in relatively unfossiliferous and organicrich, laminated clay-silt mudstone. Despite the absence of diagnostic Hettangian fossils above the postulated boundary interval, its existence was predicted on characteristic uppermost Triassic Crickmayi Zone Choristoceras ammonoids occurring below in package 3 and upper Hettangian to lower Sinemurian (Badouxia Zone) ammonoids found above in package 5. Recent ®eld investigations yielded new ammonoids of the uppermost Triassic Crickmayi Zone, which are described herein. They are assigned to Choristoceras cf. C. nobile Mojsisovics and Rhabdoceras cf. R. suessi Hauer. These characteristic ammonoids occur within the middle and top of package 4. Their discovery along with other stratigraphic evidence necessitates a revision of the boundary and recognition of a previously unrealized unconformity at the Triassic± Jurassic boundary in Sonora. A revised sea-level curve is necessary to account for these new stratigraphic and paleontological ®ndings. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Ammonoid discoveries; Antimonio Formation; Triassic±Jurassic boundary
1. Introduction The stratigraphic succession of the Antimonio Formation in northwestern Sonora encompasses the Triassic±Jurassic boundary. The section, containing associated rock types and biostratigraphically characteristic fossils of the Triassic± Jurassic boundary interval were recently described by GonzaÂlez-LeoÂn et al. (1996), and is well exposed in the northeastern foothills of the Sierra del Alamo, in the middle part of the Antimonio Formation. Near the boundary at this locality (Fig. 1) the section has been divided into ®ve informal packages (Fig. 2). The age of these packages is controlled by the succession of both latest Triassic and Early Jurassic ammonoids. GonzaÂlez-LeoÂn et al. (1996) proposed this section as a possible stratotype for the T±J boundary. In that earlier report we recognized no evidence of unconformities and, in the presumed conformable sequence, we postulated that * Corresponding author. Tel: 152-62-17-5019; fax: 152-62-17-5340. E-mail address: [email protected] (C.M. GonzaÂlez-LeoÂn).
the stratigraphic succession between the Triassic and Jurassic faunas might be without major breaks in deposition. On this basis, GonzaÂlez-LeoÂn et al. (1996) proposed that the Triassic±Jurassic boundary should fall within an unfossiliferous interval in package 4. We also proposed a possible sea-level curve for the whole section and discussed its correlation with other Triassic±Jurassic sections in the Queen Charlotte Islands, Nevada, Chile, and Peru. A preliminary abstract reporting the new ammonoid ®ndings has been published (GonzaÂlez-LeoÂn, 1997), and the purpose of this paper is to formally revise important stratigraphic relationships reported in the earlier paper (GonzaÂlez-LeoÂn et al., 1996). 2. Previous ®ndings The well-exposed sedimentary succession and fossils of the middle part of the Antimonio Formation stratotype yielded details of the Triassic±Jurassic boundary succession (GonzaÂlez-LeoÂn et al., 1996). Utilizing a sequence-stratigraphic approach, it was divided into ®ve sedimentary
0895-9811/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0895-981 1(00)00038-9
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Fig. 1. Geologic map of the middle part of the Antimonio Formation in Sierra del Alamo, northwestern Sonora (inset map). Lines 1 and 2 coincide, respectively, with measured sections 1 and 2 in Fig. 3.
rock packages (Fig. 2). Package 1 has a thickness variation of 45±80 m and consists of biostromal limestone interbedded with argillite and mudstone, calcareous sandstone, dolomitic limestone, and sandy limestone. Ammonoids found in its middle and upper parts indicate the Norian Columbianus and, or Cordilleranus Zones. Package 2, which sharply overlies package 1, is 17 m thick and consists of calcareous siltstone. It yielded ammonoids referable to the Amoenum Zone. Package 3, is a 2±8 m-thick lenticular and sandy limestone which sharply overlies package 2. It yielded uppermost Triassic Choristoceras ammonoids of the Crickmayi Zone. Package 4 gradationally overlies package 3. It is a 25 m thick unit composed of sparsely fossiliferous calcareous mudstone and micaceous siltstone with some organic laminations. Package 5 is 60 m thick and consists of interbedded siltstone, mudstone, and limestone, which yielded abundant Jurassic ammonoids belonging to the Early Jurassic Canadensis Zone. 3. New stratigraphic results Recent ®eld work carried out in the area in 1996 provided important new data on the section in question. As shown in Fig. 3, a new pro®le (section 2), 35 m thick, was measured through package 4 at a location about 650 m south of the previously measured pro®le (section 1). The lowermost 9 m of the new pro®le consist of siltstone and mudstone with
calcareous nodules overlain by a lenticular bed of mediumto ®ne-grained sandstone 1 m thick. Its uppermost 25 m consist of mudstone with calcareous nodules and subordinate siltstone. We also discovered that the contact between packages 4 and 5 is sharp, and without gradation. The mudstone/siltstone characteristic of the upper part of package 4 changes abruptly into the rock types of package 5. Occurrences of the latest Triassic ammonoid Choristoceras were ®rst reported and illustrated from package 3 (GonzaÂlez-LeoÂn et al., 1996), along with another ammonoid, Rhacophyllites (Fig. 2), and Early Jurassic Canadensis Zone ammonoids were reported from the basal part of package 5. Although no earliest Jurassic fossils (e.g. Planorbis, Liasicus Zones) had been found, these relationships prompted GonzaÂlez-LeoÂn et al. (1996) to predict that the Triassic±Jurassic boundary might fall within package 4, an interval then believed to be paleontologically barren. The newly discovered ammonoids Choristoceras and Rhabdoceras described in this paper were recovered from this interval at the positions indicated in Fig. 3. Their discovery helps elucidate more clearly the true nature of the system boundary in the Antimonio Formation and provides the revisions presented below. A single, poorly preserved specimen of the distinctive, coiled, heteromorph ammonoid Choristoceras was found in a limestone clast within a channel lag at the base of a lenticular sandstone bed in the lower part of package 4 at section 2 (Fig. 3, asterisk). The ammonoids at the top of package 4 consisted of small but abundant, ®nely-ribbed ammonoids preserved as external molds and steinkerns in light-colored, ®nely laminated, micaceous clay and siltstone 1.2±2 m below the contact between packages 4 and 5 (Fig. 3, section 1). They are referred to the coiled genus Choristoceras and the uncoiled genus Rhabdoceras that co-occurs at the top of package 4. The specimens of Rhabdoceras are assigned to Rhabdoceras cf. R. suessi Hauer. This species is known to range from the Cordilleranus to Crickmayi Zones of the latest Triassic (Tozer, 1994). Tan or yellowish iron oxides coat most of the fossils, and they are believed to be a product of oxidation of iron sul®de which most likely replaced the original shell material in a diagenetic environment favoring reducing conditions. Some ®ne laminae in which fossils are concentrated are dark with probable organic layers, while other laminae appear to be layers of reddish iron oxide. Aside from ammonoids, other fossils are exceedingly rare. They include the biconvex bivalve Chlamys sp. (McRoberts, 1997) and small questionable bone fragments. The Choristoceras at this horizon are all assigned to Choristoceras cf. C. nobile Mojsisovics (Fig. 4), an Alpine (Tethyan) species ®rst known from the Zlambach beds of Austria (Hauer, 1865). As reviewed by Tozer (1994), this species is characteristic of the Crickmayi Zone (see description below) and has become known from various Cordilleran sites in North and South America. Based on the lithologic and faunal characteristics, the
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Fig. 2. Revised view of Upper Triassic to Jurassic stratigraphy in the upper part of the lower member and lower part of the upper member of the Antimonio Formation in the Sierra del Alamo. This section is divided into 5 informal packages as indicated showing characteristic ammonoid occurrences. To left is the revised sea-level curve. The new ammonoid occurrences marked p are reported herein.
depositional environment of package 4 is interpreted to be a fairly deep, somewhat organic-rich, marine setting in which some iron sul®des formed. The characteristic ®ne, millimeter-scale laminae in the mudstone and siltstone, coupled with the complete well-preserved, random orientations among straight Rhabdoceras shells, the organic laminae, iron-oxide minerals, absence of bioturbation and trace fossils, and rarity of benthic fossils, together indicate sluggish or stagnant bottom currents and dysaerobic bottom conditions in the sedimentary basin. 4. Systematic paleontology All specimens (designated ERNO numbers) are housed in the EstacioÂn Regional del Noroeste, Instituto de GeologIÂa, Universidad Nacional AutoÂnoma de MeÂxico, Hermosillo. Family Rhabdoceratidae (Tozer, 1979) Genus Rhabdoceras (Hauer, 1860) Rhabdoceras cf. R. suessi Hauer (Fig. 5, ®gs. 8a,b±10)
(Tozer, 1994; p. 264±265, pl. 144, p. 264±265, ®gs. 1±8, pl. 117, ®gs. 9±16). Complete synonomy listÐsee Tozer (1994). Type species. Rhabdoceras suessi Hauer Discussion. All specimens are straight conchs (4 to 30 mm in length) and are incomplete, as is the case in most of the other known occurrences of R. suessi. The seven specimens at hand show ribs of a shape and spacing similar to Hauer's species. Other North American material is well illustrated and described by Tozer (1994, p. 264± 265). Rhabdoceras suessi is the only species of Rhabdoceras recognized among North American ammonoids (Tozer, 1994). Occurrences. According to Tozer (1994), R. suessi is an uppermost Triassic Tethyan ammonoid and is well documented from the Cordilleranus, Amoenum and Crickmayi Zones in western Canada, from the Pardonet Formation in northeast British Columbia, the Tyaughton Group in southern British Columbia, the Sutton member of the Parsons Bay Formation, Vancouver Island, the
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Fig. 4. Choristoceras cf. C. nobile Mojsisovics. A young individual on the bedding surface showing characteristic coiling with part of the living chamber faintly preserved. ERNO-1913b, X 6.
Fig. 3. Measured sections of package 4 showing characteristic ammonoid occurrences. This ®gure shows the location of the newly discovered latest Triassic ammonoids (marked p) near the top and near the middle part of package 4. sections 1 and 2 were measured along sections 1 and 2 of Fig. 1.
Lewes River Group, Yukon, and from the Sandilands Formation, Queen Charlotte Islands. In the United States, it is known from several localities in western Nevada including those in the Gabbs Formation of the Gabbs Valley Range (Laws, 1982) and those in the Clan Alpine Range. It is also known from the Taylorsville region of northern California (Silberling and Tozer, 1968). Material. Seven specimens were exposed on ®ve rock samples: ERNO-1962, 1970 and ERNO-1934, 1936, and 1937. Specimens consist of external molds and steinkerns and all come from the top of package 4, section 1, Antimonio Formation, Sierra del Alamo, Sonora. Family Choristoceratidae (Hyatt, 1900) Genus Choristoceras (Hauer, 1865) Type species. Choristoceras marshi (Hauer, 1865) The genus Choristoceras is a distinctive heteromorph ammonoid, which is characterized by criocone (detached) coiling. It has nodes on the ventral region of the conch. A variety of species have been discussed and illustrated by Tozer (1980, 1994) for localities in western North America, especially in western Canada.
Choristoceras cf. C. nobile (Mojsisovics, 1893) (Fig. 4 and Fig. 5, ®gs. 1±7 and 8c) Choristoceras nobile (Mojsisovics, 1893; p. 547, pl. 133, ®gs. 21; pl. 134, ®gs. 9±14) (Tozer, 1980, pl. 63, ®gs. 8a, b) Choristoceras cf. nobile Mojsisovics (Prinz, 1985, p. 168, pl. 1, ®g. 6), (Tozer, 1994, p.268, pl. 148, ®gs. 1±10). Description. Mostly evolute criocones including two partial living chambers. The height of the outer whorl ranges from 4± 10 mm (mean 6 mm). Height of the inner whorl ranges from 1.5±4.1 (mean 2.5 mm). Coiled whole conch diameters preserved among ®ve of the most complete specimens range from 12±26 mm. The prominent radial ribs number 30±33 per revolution. The specimens, including juveniles, are mostly external molds and steinkerns, and neither original shell material nor sutures are preserved. Juvenile whorls are entirely evolute and not in contact. However in more mature specimens, later whorls are in direct contact with succeeding ones. A few specimens bear nodes on their ventrolateral sides, and partial living chambers. No apertures are preserved. Discussion. In size, manner of coiling, and other characteristics, the Sonoran specimens match closely the type specimen of Mojsisovics (also illustrated by Tozer, 1980, pl. 63, ®g. 8a, b) and also specimens reported from Peru and the Queen Charlotte Islands. A Choristoceras fauna, represented by several species, is known from the Gabbs Formation, west central Nevada, but the material has not been published. However, C. nobile is unknown from the Gabbs Formation. The Sonoran material although slightly smaller in conch diameter, also compares with some Nevada
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Fig. 5. Characteristic ammonoids from near the top of section 1 (Fig. 3). Dark color of some specimens due to iron oxide coatings. 1±7, Choristoceras cf. C. nobile Mojsisovics. Figs. 1 and 2. part and counterpart, ERNO-1963b,a, X 2. Fig. 3. partial conch, ERNO-1935. Fig. 4. ERNO-1913a, X3. Fig. 5. ERNO-1938, X 3. Fig. 6. ERNO-1965, X 2. Fig. 7. ERNO-1964a, X 3. Fig. 8. Surface view showing Rhabdoceras cf. R. suessi (A and B) and Choristoceras cf. C. nobile (C), ERNO-1937, X 3. Figs. 9 and 10. Rhabdoceras cf. R. suessi Hauer. Fig. 9. ERNO-1936, X 3. Fig. 10. ERNO-1962, X 2.
specimens ®rst illustrated in an unpublished doctoral thesis by Muller (1930, pl. 17, ®gs. 1±4), and informally described as ªChoristoceras luningiº. Our material also bears some resemblance to both ªC. luningiº and ªChoristoceras sp. 2º of R. A. Laws (unpublished MS thesis, 1978, pl. 1, ®g. 5). Further study of these specimens would help clarify their relationships with the Sonoran material. Occurrences. C. nobile is also known from the uppermost Triassic, Zlambach beds, Salzkammergut, Austria; Rhaetian, Pucara Group, northern Peru (Prinz, 1985); Crickmayi
Zone, Sandilands Formation, Queen Charlotte Islands, British Columbia (Tozer, 1994). The Sonoran specimens may constitute the ®rst formal description of this species in America outside of Canada and Peru. Material. Twelve specimens, including ®ve nearly complete conchs and seven partial conchs, were collected during two series of investigations: ERNO-1960; 1961; 1963a,b; 1964a,b; 1965; 1966a,b; ERNO-1913; 1934; 1935; 1937; 1938 and 1939. All of them were collected from section 1 at top of package 4 (Fig. 3).
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5. New conclusions on the Triassic±Jurassic boundary With the new ammonoid discoveries in package 4, the contact between packages 4 and 5 and the true nature of the Triassic±Jurassic boundary in the Antimonio Formation is now better resolved. The contact is an unconformity characterized by an erosional relief of 10 m over a lateral distance of less than one kilometer. However, the low angle of erosional relief makes the unconformity dif®cult to detect physically. We also note local stratigraphic thickness variations in packages 3 and 4. They thicken from the north to the south (Fig. 3), but package 4 is perhaps re¯ecting the regional nature of the erosion, which preceded the deposition of package 5. Package 3 consists of distinctive shell beds containing a wide variety of marine invertebrates as well as angular and rounded intraclasts, phosphate pebbles and tooth and bone fragments. This package is dated as Upper Triassic (Crickmayi Zone) based on the occurrence of conch fragments of Choristoceras sp. However, one specimen of the hydrozoan Heterastridium (Amoenum Zone or older) was also found in package 3. Package 3 was interpreted as either a tempestite or lag deposit but either way, it is regarded as regressive in nature, while package 4 is a transgressive unit (GonzaÂlezLeoÂn et al., 1996). Together, packages 3 and 4 are interpreted as an unconformity-bounded sequence (Fig. 2), and employing the terminology of sequence stratigraphy, package 3 can be envisioned as the lower portion of a transgressive systems tract while package 4 represents the highstand systems tract. The erosional unconformity between packages 4 and 5 represents a regressive event and thus constitutes a sequence boundary (Fig. 2). The ammonoids Choristoceras and Rhabdoceras, discovered within package 4 document a previously unsuspected upward extension of Upper Triassic (Rhaetian) strata in this section, and show that the Triassic±Jurassic boundary does not fall within package 4 as we previously thought (GonzaÂlez-LeoÂn et al., 1996). This boundary must be moved upward to the contact between packages 4 and 5 (Fig. 2), which represents a time gap that omits most of the Hettangian and perhaps some of the latest Triassic as well. The base of package 5 in both sections can be recognized by the presence of a 2±2.4 m thick, coarse-grained to pebbly sandstone. This sandstone grades in its uppermost 50 cm into a calcareous sandstone with common Sunrisites sunrisensis (illustrated by GonzaÂlez-LeoÂn et al., 1996) of the upper Hettangian. The distinctive Crickmayi Zone ammonoids in this sequence permit comparison with other sections such as those in western Canada and Peru (Prinz, 1985; Tozer, 1994). Most notable, however, are occurrences in the Gabbs Valley Range of western Nevada where the Triassic±Jurassic boundary has been recognized (Taylor et al., 1983). Specimens of Choristoceras from the Gabbs Formation in Nevada have been illustrated in some unpublished theses (Muller, 1930; Laws, 1978), but detailed published
systematic study is needed before valid comparisons with the Sonora specimens are possible. For example, Laws (1978) lists several species of Choristoceras, which have yet to be identi®ed. Also of great interest is the thick, perhaps unbroken succession of the Upper Triassic Rhaetian to Lower Jurassic in the Queen Charlotte Islands (Tipper et al., 1994). The boundary here can be ascertained with fair con®dence using conodonts, Radiolaria and a succession of several species of Choristoceras. At Kennecott Point in the Queen Charlottes, Choristoceras nobile is not the highest occurring ammonoid among the Crickmayi Zone fauna and is succeeded higher in the section by Choristoceras rhaeticum. Because C. rhaeticum has not been found in the Sonora section, some of the uppermost Triassic strata along with the lowermost Jurassic may be missing in the Antimonio section. The thinness of Rhaetian strata in the Antimonio, compared with the thicker interval in Nevada, has already been pointed out, and the unconformity at the boundary in the Antimonio most likely is attributed to the post-Triassic regression discussed by GonzaÂlez-LeoÂn et al. (1996). The new stratigraphic relationships and ammonoids found in the sections measured at Sierra del Alamo (Fig. 3) makes it necessary to reevaluate the previously reported sea-level curve for this section (Fig. 2). Most notably, an erosional unconformity must be placed between package 4 and package 5. In our previous work (GonzaÂlez-LeoÂn et al., 1996), we noted that although they were not identical, there was however a similarity in the overall trends of the transgressive-regressive curves of Nevada and Sonora through latestmost Triassic and earliestmost Jurassic time. This similarity was mostly expressed by the presence in the Gabbs and Sunrise Formations (in the Gabbs Valley section of Nevada) of a sharp regression at the top of strata representing the Amoenum Zone, followed by a transgressive phase near the end of the Rhaetian, and a regression from early Hettangian to earliest Sinemurian time (GonzaÂlezLeoÂn et al., 1996, p. 425). Current research underway on the Triassic±Jurassic boundary in New York Canyon, Nevada by A. Hallam and C.A. McRoberts certainly will have an effect on our interpretation of the T/J boundary in Sonora. We also considered that the Triassic±Jurassic boundary in both regions was located within continuous sedimentary succession: within package 4 in Sonora and within the sedimentary succession close to the boundary between the Gabbs and Sunrise Formations in Nevada. For Sonora we ®nd that there is a sharp regression at the top of the Amoenum Zone, a transgressive phase during Rhaetian time and a regression during latest Triassic or more likely, during earliest Jurassic time, which makes it very similar to the Nevada section. Acknowledgements We greatly appreciate reviews of this manuscript made by C.A. McRoberts, N.J. Silberling, M.R. Sandy, and S.G.
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Lucas. Support and collaboration for Stanley and Taylor was possible under a CONACyT project (3934-T) awarded to Carlos GonzaÂlez-LeoÂn as well as a U.S. NSF International grant (INT 9603171). Stanley acknowledges partial ®eld support through a grant from the National Geographic Society. References GonzaÂlez-LeoÂn, C., Taylor, D.G., Stanley Jr., G.D., 1996. The Antimonio Formation in Sonora, Mexico, and the Triassic±Jurassic boundary. Canadian Journal of Earth Sciences 33, 418±428. GonzaÂlez-LeoÂn, C., 1997. The Triassic±Jurassic boundary in the Antimonio Formation from new evidences. In: GonzaÂlez-LeoÂn, C.M., Stanley, G.D., Jr. (Eds.), US±Mexico cooperative research: International workshop on the geology of Sonora memoir. Estacion Regional del Noroeste, Instituto de GeologIÂa, Universidad Nacional AutoÂnoma de MeÂxico Publicaciones Ocasionales, 1, pp. 33±36. Hauer, F. von, 1860. NachtraÈge zur Kenntnis der Cephalopoden-Fauna der HallstaÈtter Schichten. Sitzungberichte der Akademie der Wissenschaften in Wien (I) 41, 113±150 Pl. 1±5. Hauer, F. von, 1865. Choristoceras, eine neue Cephalopodensippe aus den Koessener Schichten. Sitzungberichte der Akademie der Wissenschaften in Wien (I) 52, 654±660 Pl. 1. Hyatt, A., 1900. Cephalopoda. In: von Zittel, K.A. (Ed.). 1st English ed.. Textbook of Palaeontology, Vol. 1. C.R. Eastman, London, pp. 502± 593 (®gs. 1049±1235). Laws, R.A., 1978. Paleoecology of Late Triassic faunas from Mineral County, Nevada and Shasta County, California. MS thesis, University of California, Berkeley, 146 pp., Pl. 4.
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Laws, R.A., 1982. Late Triassic depositional environments and molluscan associations from west-central Nevada. Palaeogeography, Palaeoclimatology, Palaeoecology 37, 131±148. McRoberts, C.A., 1997. Late Triassic (Norian-Rhaetian) bivalves from the Antimonio Formation, northwest Sonora, Mexico. Revista Mexicana de Ciencias GeoloÂgicas 14, 53±63. Mojsisovics, E., 1893. Das Gebirge um Hallstatt, Theil I: Die Cephalopoden der HallstaÈtter Kalke. Abhandlungen der geologischen Reichsanstalt Wien 6, 1±835. Muller, S.W., 1930. Triassic of Gabb's Valley Range, Nevada. Doctoral Dissertation, Stanford University, Stanford, California, 83 pp. Prinz, P., 1985. Stratigraphie und ammonitenfauna der Pucara-Gruppe (obertrias-unterjura) von nord-Peru. Palaeontographica Abteilung A 188, 153±197. Silberling, N.J., Tozer, E.T., 1968. Biostratigraphic classi®cation of the marine Triassic in North America. Geological Society of America Special Paper 110, 63 pp. Taylor, D.G., Smith, P.L., Laws, R.A., Guex, J., 1983. The stratigraphy and biofacies trends of the Lower Mesozoic Gabbs and Sunrise Formations, west-central Nevada. Canadian Journal of Earth Sciences 20, 1598± 1608. Tipper, H.W., Carter, E.S., Orchard, M.J., Tozer, E.T., 1994. The Triassic± Jurassic (T±J) boundary in the Queen Charlotte Islands, ammonites, conodonts, and radiolarians. Geobios 17, 485±492. Tozer, E.T., 1979. Latest Triassic ammonoid faunas and biochronology, Western Canada. Current Research, Geological Survey of Canada, Paper 79-1B, pp. 127±135. Tozer, E.T., 1980. Latest Triassic (upper Norian) ammonoid and Monotis faunas and correlations. Rivista Italiana di Paleontologia e Stratigra®a 85, 843±876. Tozer, E.T., 1994. Canadian Triassic ammonoid faunas. Geological Survey of Canada, Bulletin 467, 1±663.
www.elsevier.nl/locate/jsames
Ammonoid discoveries in the Antimonio Formation, Sonora, Mexico: new constraints on the Triassic±Jurassic boundary C.M. GonzaÂlez-LeoÂn a,*, G.D. Stanley Jr. b, D.G. Taylor c a
EstacioÂn Regional del Noroeste, Instituto de GeologõÂa, Universidad Nacional AutoÂnoma de, MeÂxico, Apartado Postal 1039, Hermosillo, Sonora, 83000 Mexico b Department of Geology, University of Montana, Missoula, MT 59812, USA c Department of Geology, Portland State University, P.O. Box 751, Portland, OR 97207, USA Received 30 September 1998; revised 31 December 1999; accepted 31 January 2000
Abstract The Triassic±Jurassic systemic boundary was recently reported in the middle part of the Antimonio Formation, northwestern Sonora, where ®ve informal sedimentary packages were delineated and characteristic ammonoid faunas were used to establish age control within the succession. The boundary was suggested to lie within the middle part of the 24 m-thick package 4, in relatively unfossiliferous and organicrich, laminated clay-silt mudstone. Despite the absence of diagnostic Hettangian fossils above the postulated boundary interval, its existence was predicted on characteristic uppermost Triassic Crickmayi Zone Choristoceras ammonoids occurring below in package 3 and upper Hettangian to lower Sinemurian (Badouxia Zone) ammonoids found above in package 5. Recent ®eld investigations yielded new ammonoids of the uppermost Triassic Crickmayi Zone, which are described herein. They are assigned to Choristoceras cf. C. nobile Mojsisovics and Rhabdoceras cf. R. suessi Hauer. These characteristic ammonoids occur within the middle and top of package 4. Their discovery along with other stratigraphic evidence necessitates a revision of the boundary and recognition of a previously unrealized unconformity at the Triassic± Jurassic boundary in Sonora. A revised sea-level curve is necessary to account for these new stratigraphic and paleontological ®ndings. q 2000 Elsevier Science Ltd. All rights reserved. Keywords: Ammonoid discoveries; Antimonio Formation; Triassic±Jurassic boundary
1. Introduction The stratigraphic succession of the Antimonio Formation in northwestern Sonora encompasses the Triassic±Jurassic boundary. The section, containing associated rock types and biostratigraphically characteristic fossils of the Triassic± Jurassic boundary interval were recently described by GonzaÂlez-LeoÂn et al. (1996), and is well exposed in the northeastern foothills of the Sierra del Alamo, in the middle part of the Antimonio Formation. Near the boundary at this locality (Fig. 1) the section has been divided into ®ve informal packages (Fig. 2). The age of these packages is controlled by the succession of both latest Triassic and Early Jurassic ammonoids. GonzaÂlez-LeoÂn et al. (1996) proposed this section as a possible stratotype for the T±J boundary. In that earlier report we recognized no evidence of unconformities and, in the presumed conformable sequence, we postulated that * Corresponding author. Tel: 152-62-17-5019; fax: 152-62-17-5340. E-mail address: [email protected] (C.M. GonzaÂlez-LeoÂn).
the stratigraphic succession between the Triassic and Jurassic faunas might be without major breaks in deposition. On this basis, GonzaÂlez-LeoÂn et al. (1996) proposed that the Triassic±Jurassic boundary should fall within an unfossiliferous interval in package 4. We also proposed a possible sea-level curve for the whole section and discussed its correlation with other Triassic±Jurassic sections in the Queen Charlotte Islands, Nevada, Chile, and Peru. A preliminary abstract reporting the new ammonoid ®ndings has been published (GonzaÂlez-LeoÂn, 1997), and the purpose of this paper is to formally revise important stratigraphic relationships reported in the earlier paper (GonzaÂlez-LeoÂn et al., 1996). 2. Previous ®ndings The well-exposed sedimentary succession and fossils of the middle part of the Antimonio Formation stratotype yielded details of the Triassic±Jurassic boundary succession (GonzaÂlez-LeoÂn et al., 1996). Utilizing a sequence-stratigraphic approach, it was divided into ®ve sedimentary
0895-9811/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved. PII: S 0895-981 1(00)00038-9
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Fig. 1. Geologic map of the middle part of the Antimonio Formation in Sierra del Alamo, northwestern Sonora (inset map). Lines 1 and 2 coincide, respectively, with measured sections 1 and 2 in Fig. 3.
rock packages (Fig. 2). Package 1 has a thickness variation of 45±80 m and consists of biostromal limestone interbedded with argillite and mudstone, calcareous sandstone, dolomitic limestone, and sandy limestone. Ammonoids found in its middle and upper parts indicate the Norian Columbianus and, or Cordilleranus Zones. Package 2, which sharply overlies package 1, is 17 m thick and consists of calcareous siltstone. It yielded ammonoids referable to the Amoenum Zone. Package 3, is a 2±8 m-thick lenticular and sandy limestone which sharply overlies package 2. It yielded uppermost Triassic Choristoceras ammonoids of the Crickmayi Zone. Package 4 gradationally overlies package 3. It is a 25 m thick unit composed of sparsely fossiliferous calcareous mudstone and micaceous siltstone with some organic laminations. Package 5 is 60 m thick and consists of interbedded siltstone, mudstone, and limestone, which yielded abundant Jurassic ammonoids belonging to the Early Jurassic Canadensis Zone. 3. New stratigraphic results Recent ®eld work carried out in the area in 1996 provided important new data on the section in question. As shown in Fig. 3, a new pro®le (section 2), 35 m thick, was measured through package 4 at a location about 650 m south of the previously measured pro®le (section 1). The lowermost 9 m of the new pro®le consist of siltstone and mudstone with
calcareous nodules overlain by a lenticular bed of mediumto ®ne-grained sandstone 1 m thick. Its uppermost 25 m consist of mudstone with calcareous nodules and subordinate siltstone. We also discovered that the contact between packages 4 and 5 is sharp, and without gradation. The mudstone/siltstone characteristic of the upper part of package 4 changes abruptly into the rock types of package 5. Occurrences of the latest Triassic ammonoid Choristoceras were ®rst reported and illustrated from package 3 (GonzaÂlez-LeoÂn et al., 1996), along with another ammonoid, Rhacophyllites (Fig. 2), and Early Jurassic Canadensis Zone ammonoids were reported from the basal part of package 5. Although no earliest Jurassic fossils (e.g. Planorbis, Liasicus Zones) had been found, these relationships prompted GonzaÂlez-LeoÂn et al. (1996) to predict that the Triassic±Jurassic boundary might fall within package 4, an interval then believed to be paleontologically barren. The newly discovered ammonoids Choristoceras and Rhabdoceras described in this paper were recovered from this interval at the positions indicated in Fig. 3. Their discovery helps elucidate more clearly the true nature of the system boundary in the Antimonio Formation and provides the revisions presented below. A single, poorly preserved specimen of the distinctive, coiled, heteromorph ammonoid Choristoceras was found in a limestone clast within a channel lag at the base of a lenticular sandstone bed in the lower part of package 4 at section 2 (Fig. 3, asterisk). The ammonoids at the top of package 4 consisted of small but abundant, ®nely-ribbed ammonoids preserved as external molds and steinkerns in light-colored, ®nely laminated, micaceous clay and siltstone 1.2±2 m below the contact between packages 4 and 5 (Fig. 3, section 1). They are referred to the coiled genus Choristoceras and the uncoiled genus Rhabdoceras that co-occurs at the top of package 4. The specimens of Rhabdoceras are assigned to Rhabdoceras cf. R. suessi Hauer. This species is known to range from the Cordilleranus to Crickmayi Zones of the latest Triassic (Tozer, 1994). Tan or yellowish iron oxides coat most of the fossils, and they are believed to be a product of oxidation of iron sul®de which most likely replaced the original shell material in a diagenetic environment favoring reducing conditions. Some ®ne laminae in which fossils are concentrated are dark with probable organic layers, while other laminae appear to be layers of reddish iron oxide. Aside from ammonoids, other fossils are exceedingly rare. They include the biconvex bivalve Chlamys sp. (McRoberts, 1997) and small questionable bone fragments. The Choristoceras at this horizon are all assigned to Choristoceras cf. C. nobile Mojsisovics (Fig. 4), an Alpine (Tethyan) species ®rst known from the Zlambach beds of Austria (Hauer, 1865). As reviewed by Tozer (1994), this species is characteristic of the Crickmayi Zone (see description below) and has become known from various Cordilleran sites in North and South America. Based on the lithologic and faunal characteristics, the
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Fig. 2. Revised view of Upper Triassic to Jurassic stratigraphy in the upper part of the lower member and lower part of the upper member of the Antimonio Formation in the Sierra del Alamo. This section is divided into 5 informal packages as indicated showing characteristic ammonoid occurrences. To left is the revised sea-level curve. The new ammonoid occurrences marked p are reported herein.
depositional environment of package 4 is interpreted to be a fairly deep, somewhat organic-rich, marine setting in which some iron sul®des formed. The characteristic ®ne, millimeter-scale laminae in the mudstone and siltstone, coupled with the complete well-preserved, random orientations among straight Rhabdoceras shells, the organic laminae, iron-oxide minerals, absence of bioturbation and trace fossils, and rarity of benthic fossils, together indicate sluggish or stagnant bottom currents and dysaerobic bottom conditions in the sedimentary basin. 4. Systematic paleontology All specimens (designated ERNO numbers) are housed in the EstacioÂn Regional del Noroeste, Instituto de GeologIÂa, Universidad Nacional AutoÂnoma de MeÂxico, Hermosillo. Family Rhabdoceratidae (Tozer, 1979) Genus Rhabdoceras (Hauer, 1860) Rhabdoceras cf. R. suessi Hauer (Fig. 5, ®gs. 8a,b±10)
(Tozer, 1994; p. 264±265, pl. 144, p. 264±265, ®gs. 1±8, pl. 117, ®gs. 9±16). Complete synonomy listÐsee Tozer (1994). Type species. Rhabdoceras suessi Hauer Discussion. All specimens are straight conchs (4 to 30 mm in length) and are incomplete, as is the case in most of the other known occurrences of R. suessi. The seven specimens at hand show ribs of a shape and spacing similar to Hauer's species. Other North American material is well illustrated and described by Tozer (1994, p. 264± 265). Rhabdoceras suessi is the only species of Rhabdoceras recognized among North American ammonoids (Tozer, 1994). Occurrences. According to Tozer (1994), R. suessi is an uppermost Triassic Tethyan ammonoid and is well documented from the Cordilleranus, Amoenum and Crickmayi Zones in western Canada, from the Pardonet Formation in northeast British Columbia, the Tyaughton Group in southern British Columbia, the Sutton member of the Parsons Bay Formation, Vancouver Island, the
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Fig. 4. Choristoceras cf. C. nobile Mojsisovics. A young individual on the bedding surface showing characteristic coiling with part of the living chamber faintly preserved. ERNO-1913b, X 6.
Fig. 3. Measured sections of package 4 showing characteristic ammonoid occurrences. This ®gure shows the location of the newly discovered latest Triassic ammonoids (marked p) near the top and near the middle part of package 4. sections 1 and 2 were measured along sections 1 and 2 of Fig. 1.
Lewes River Group, Yukon, and from the Sandilands Formation, Queen Charlotte Islands. In the United States, it is known from several localities in western Nevada including those in the Gabbs Formation of the Gabbs Valley Range (Laws, 1982) and those in the Clan Alpine Range. It is also known from the Taylorsville region of northern California (Silberling and Tozer, 1968). Material. Seven specimens were exposed on ®ve rock samples: ERNO-1962, 1970 and ERNO-1934, 1936, and 1937. Specimens consist of external molds and steinkerns and all come from the top of package 4, section 1, Antimonio Formation, Sierra del Alamo, Sonora. Family Choristoceratidae (Hyatt, 1900) Genus Choristoceras (Hauer, 1865) Type species. Choristoceras marshi (Hauer, 1865) The genus Choristoceras is a distinctive heteromorph ammonoid, which is characterized by criocone (detached) coiling. It has nodes on the ventral region of the conch. A variety of species have been discussed and illustrated by Tozer (1980, 1994) for localities in western North America, especially in western Canada.
Choristoceras cf. C. nobile (Mojsisovics, 1893) (Fig. 4 and Fig. 5, ®gs. 1±7 and 8c) Choristoceras nobile (Mojsisovics, 1893; p. 547, pl. 133, ®gs. 21; pl. 134, ®gs. 9±14) (Tozer, 1980, pl. 63, ®gs. 8a, b) Choristoceras cf. nobile Mojsisovics (Prinz, 1985, p. 168, pl. 1, ®g. 6), (Tozer, 1994, p.268, pl. 148, ®gs. 1±10). Description. Mostly evolute criocones including two partial living chambers. The height of the outer whorl ranges from 4± 10 mm (mean 6 mm). Height of the inner whorl ranges from 1.5±4.1 (mean 2.5 mm). Coiled whole conch diameters preserved among ®ve of the most complete specimens range from 12±26 mm. The prominent radial ribs number 30±33 per revolution. The specimens, including juveniles, are mostly external molds and steinkerns, and neither original shell material nor sutures are preserved. Juvenile whorls are entirely evolute and not in contact. However in more mature specimens, later whorls are in direct contact with succeeding ones. A few specimens bear nodes on their ventrolateral sides, and partial living chambers. No apertures are preserved. Discussion. In size, manner of coiling, and other characteristics, the Sonoran specimens match closely the type specimen of Mojsisovics (also illustrated by Tozer, 1980, pl. 63, ®g. 8a, b) and also specimens reported from Peru and the Queen Charlotte Islands. A Choristoceras fauna, represented by several species, is known from the Gabbs Formation, west central Nevada, but the material has not been published. However, C. nobile is unknown from the Gabbs Formation. The Sonoran material although slightly smaller in conch diameter, also compares with some Nevada
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Fig. 5. Characteristic ammonoids from near the top of section 1 (Fig. 3). Dark color of some specimens due to iron oxide coatings. 1±7, Choristoceras cf. C. nobile Mojsisovics. Figs. 1 and 2. part and counterpart, ERNO-1963b,a, X 2. Fig. 3. partial conch, ERNO-1935. Fig. 4. ERNO-1913a, X3. Fig. 5. ERNO-1938, X 3. Fig. 6. ERNO-1965, X 2. Fig. 7. ERNO-1964a, X 3. Fig. 8. Surface view showing Rhabdoceras cf. R. suessi (A and B) and Choristoceras cf. C. nobile (C), ERNO-1937, X 3. Figs. 9 and 10. Rhabdoceras cf. R. suessi Hauer. Fig. 9. ERNO-1936, X 3. Fig. 10. ERNO-1962, X 2.
specimens ®rst illustrated in an unpublished doctoral thesis by Muller (1930, pl. 17, ®gs. 1±4), and informally described as ªChoristoceras luningiº. Our material also bears some resemblance to both ªC. luningiº and ªChoristoceras sp. 2º of R. A. Laws (unpublished MS thesis, 1978, pl. 1, ®g. 5). Further study of these specimens would help clarify their relationships with the Sonoran material. Occurrences. C. nobile is also known from the uppermost Triassic, Zlambach beds, Salzkammergut, Austria; Rhaetian, Pucara Group, northern Peru (Prinz, 1985); Crickmayi
Zone, Sandilands Formation, Queen Charlotte Islands, British Columbia (Tozer, 1994). The Sonoran specimens may constitute the ®rst formal description of this species in America outside of Canada and Peru. Material. Twelve specimens, including ®ve nearly complete conchs and seven partial conchs, were collected during two series of investigations: ERNO-1960; 1961; 1963a,b; 1964a,b; 1965; 1966a,b; ERNO-1913; 1934; 1935; 1937; 1938 and 1939. All of them were collected from section 1 at top of package 4 (Fig. 3).
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5. New conclusions on the Triassic±Jurassic boundary With the new ammonoid discoveries in package 4, the contact between packages 4 and 5 and the true nature of the Triassic±Jurassic boundary in the Antimonio Formation is now better resolved. The contact is an unconformity characterized by an erosional relief of 10 m over a lateral distance of less than one kilometer. However, the low angle of erosional relief makes the unconformity dif®cult to detect physically. We also note local stratigraphic thickness variations in packages 3 and 4. They thicken from the north to the south (Fig. 3), but package 4 is perhaps re¯ecting the regional nature of the erosion, which preceded the deposition of package 5. Package 3 consists of distinctive shell beds containing a wide variety of marine invertebrates as well as angular and rounded intraclasts, phosphate pebbles and tooth and bone fragments. This package is dated as Upper Triassic (Crickmayi Zone) based on the occurrence of conch fragments of Choristoceras sp. However, one specimen of the hydrozoan Heterastridium (Amoenum Zone or older) was also found in package 3. Package 3 was interpreted as either a tempestite or lag deposit but either way, it is regarded as regressive in nature, while package 4 is a transgressive unit (GonzaÂlezLeoÂn et al., 1996). Together, packages 3 and 4 are interpreted as an unconformity-bounded sequence (Fig. 2), and employing the terminology of sequence stratigraphy, package 3 can be envisioned as the lower portion of a transgressive systems tract while package 4 represents the highstand systems tract. The erosional unconformity between packages 4 and 5 represents a regressive event and thus constitutes a sequence boundary (Fig. 2). The ammonoids Choristoceras and Rhabdoceras, discovered within package 4 document a previously unsuspected upward extension of Upper Triassic (Rhaetian) strata in this section, and show that the Triassic±Jurassic boundary does not fall within package 4 as we previously thought (GonzaÂlez-LeoÂn et al., 1996). This boundary must be moved upward to the contact between packages 4 and 5 (Fig. 2), which represents a time gap that omits most of the Hettangian and perhaps some of the latest Triassic as well. The base of package 5 in both sections can be recognized by the presence of a 2±2.4 m thick, coarse-grained to pebbly sandstone. This sandstone grades in its uppermost 50 cm into a calcareous sandstone with common Sunrisites sunrisensis (illustrated by GonzaÂlez-LeoÂn et al., 1996) of the upper Hettangian. The distinctive Crickmayi Zone ammonoids in this sequence permit comparison with other sections such as those in western Canada and Peru (Prinz, 1985; Tozer, 1994). Most notable, however, are occurrences in the Gabbs Valley Range of western Nevada where the Triassic±Jurassic boundary has been recognized (Taylor et al., 1983). Specimens of Choristoceras from the Gabbs Formation in Nevada have been illustrated in some unpublished theses (Muller, 1930; Laws, 1978), but detailed published
systematic study is needed before valid comparisons with the Sonora specimens are possible. For example, Laws (1978) lists several species of Choristoceras, which have yet to be identi®ed. Also of great interest is the thick, perhaps unbroken succession of the Upper Triassic Rhaetian to Lower Jurassic in the Queen Charlotte Islands (Tipper et al., 1994). The boundary here can be ascertained with fair con®dence using conodonts, Radiolaria and a succession of several species of Choristoceras. At Kennecott Point in the Queen Charlottes, Choristoceras nobile is not the highest occurring ammonoid among the Crickmayi Zone fauna and is succeeded higher in the section by Choristoceras rhaeticum. Because C. rhaeticum has not been found in the Sonora section, some of the uppermost Triassic strata along with the lowermost Jurassic may be missing in the Antimonio section. The thinness of Rhaetian strata in the Antimonio, compared with the thicker interval in Nevada, has already been pointed out, and the unconformity at the boundary in the Antimonio most likely is attributed to the post-Triassic regression discussed by GonzaÂlez-LeoÂn et al. (1996). The new stratigraphic relationships and ammonoids found in the sections measured at Sierra del Alamo (Fig. 3) makes it necessary to reevaluate the previously reported sea-level curve for this section (Fig. 2). Most notably, an erosional unconformity must be placed between package 4 and package 5. In our previous work (GonzaÂlez-LeoÂn et al., 1996), we noted that although they were not identical, there was however a similarity in the overall trends of the transgressive-regressive curves of Nevada and Sonora through latestmost Triassic and earliestmost Jurassic time. This similarity was mostly expressed by the presence in the Gabbs and Sunrise Formations (in the Gabbs Valley section of Nevada) of a sharp regression at the top of strata representing the Amoenum Zone, followed by a transgressive phase near the end of the Rhaetian, and a regression from early Hettangian to earliest Sinemurian time (GonzaÂlezLeoÂn et al., 1996, p. 425). Current research underway on the Triassic±Jurassic boundary in New York Canyon, Nevada by A. Hallam and C.A. McRoberts certainly will have an effect on our interpretation of the T/J boundary in Sonora. We also considered that the Triassic±Jurassic boundary in both regions was located within continuous sedimentary succession: within package 4 in Sonora and within the sedimentary succession close to the boundary between the Gabbs and Sunrise Formations in Nevada. For Sonora we ®nd that there is a sharp regression at the top of the Amoenum Zone, a transgressive phase during Rhaetian time and a regression during latest Triassic or more likely, during earliest Jurassic time, which makes it very similar to the Nevada section. Acknowledgements We greatly appreciate reviews of this manuscript made by C.A. McRoberts, N.J. Silberling, M.R. Sandy, and S.G.
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Lucas. Support and collaboration for Stanley and Taylor was possible under a CONACyT project (3934-T) awarded to Carlos GonzaÂlez-LeoÂn as well as a U.S. NSF International grant (INT 9603171). Stanley acknowledges partial ®eld support through a grant from the National Geographic Society. References GonzaÂlez-LeoÂn, C., Taylor, D.G., Stanley Jr., G.D., 1996. The Antimonio Formation in Sonora, Mexico, and the Triassic±Jurassic boundary. Canadian Journal of Earth Sciences 33, 418±428. GonzaÂlez-LeoÂn, C., 1997. The Triassic±Jurassic boundary in the Antimonio Formation from new evidences. In: GonzaÂlez-LeoÂn, C.M., Stanley, G.D., Jr. (Eds.), US±Mexico cooperative research: International workshop on the geology of Sonora memoir. Estacion Regional del Noroeste, Instituto de GeologIÂa, Universidad Nacional AutoÂnoma de MeÂxico Publicaciones Ocasionales, 1, pp. 33±36. Hauer, F. von, 1860. NachtraÈge zur Kenntnis der Cephalopoden-Fauna der HallstaÈtter Schichten. Sitzungberichte der Akademie der Wissenschaften in Wien (I) 41, 113±150 Pl. 1±5. Hauer, F. von, 1865. Choristoceras, eine neue Cephalopodensippe aus den Koessener Schichten. Sitzungberichte der Akademie der Wissenschaften in Wien (I) 52, 654±660 Pl. 1. Hyatt, A., 1900. Cephalopoda. In: von Zittel, K.A. (Ed.). 1st English ed.. Textbook of Palaeontology, Vol. 1. C.R. Eastman, London, pp. 502± 593 (®gs. 1049±1235). Laws, R.A., 1978. Paleoecology of Late Triassic faunas from Mineral County, Nevada and Shasta County, California. MS thesis, University of California, Berkeley, 146 pp., Pl. 4.
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Laws, R.A., 1982. Late Triassic depositional environments and molluscan associations from west-central Nevada. Palaeogeography, Palaeoclimatology, Palaeoecology 37, 131±148. McRoberts, C.A., 1997. Late Triassic (Norian-Rhaetian) bivalves from the Antimonio Formation, northwest Sonora, Mexico. Revista Mexicana de Ciencias GeoloÂgicas 14, 53±63. Mojsisovics, E., 1893. Das Gebirge um Hallstatt, Theil I: Die Cephalopoden der HallstaÈtter Kalke. Abhandlungen der geologischen Reichsanstalt Wien 6, 1±835. Muller, S.W., 1930. Triassic of Gabb's Valley Range, Nevada. Doctoral Dissertation, Stanford University, Stanford, California, 83 pp. Prinz, P., 1985. Stratigraphie und ammonitenfauna der Pucara-Gruppe (obertrias-unterjura) von nord-Peru. Palaeontographica Abteilung A 188, 153±197. Silberling, N.J., Tozer, E.T., 1968. Biostratigraphic classi®cation of the marine Triassic in North America. Geological Society of America Special Paper 110, 63 pp. Taylor, D.G., Smith, P.L., Laws, R.A., Guex, J., 1983. The stratigraphy and biofacies trends of the Lower Mesozoic Gabbs and Sunrise Formations, west-central Nevada. Canadian Journal of Earth Sciences 20, 1598± 1608. Tipper, H.W., Carter, E.S., Orchard, M.J., Tozer, E.T., 1994. The Triassic± Jurassic (T±J) boundary in the Queen Charlotte Islands, ammonites, conodonts, and radiolarians. Geobios 17, 485±492. Tozer, E.T., 1979. Latest Triassic ammonoid faunas and biochronology, Western Canada. Current Research, Geological Survey of Canada, Paper 79-1B, pp. 127±135. Tozer, E.T., 1980. Latest Triassic (upper Norian) ammonoid and Monotis faunas and correlations. Rivista Italiana di Paleontologia e Stratigra®a 85, 843±876. Tozer, E.T., 1994. Canadian Triassic ammonoid faunas. Geological Survey of Canada, Bulletin 467, 1±663.