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1

Climate change at the Triassic/Jurassic boundary in the northwestern Tethyan realm, inferred from sections in the Tatra Mountains (Slovakia)

Michalik J., Biron A., Lintnerova O., Gotz A., Ruckwied K.

Acta Geologica Polonica

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2010

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Vol. 60, no. 4

535-548

EN

Sedimentological, palynological, clay mineralogical and carbon isotope studies were carried out on the Triassic/Jurassic (T/J) boundary interval in the NW Tethyan Realm. The analyses are based on two sections in the Slovakian Tatra Mountains (Western Carpathians): the Siroky Zl'ab section in the Med'odoly Valley and the Furkaska section above the Juranova Valley. Clay mineralogical analysis suggests an increasing intensity of chemical weathering in the hinterland due to increasing humidity. The palynological data do not allow the inference of a major T/J boundary mass extinction event. The observed striking increase in spores points instead to sudden climatic change, interpreted as a result of distant volcanic activity associated with the onset of rifting of Pangea. The [delta^13]C[org] excursion across the T/J boundary follows the globally documented perturbation of the carbon cycle during this period. it may be used for a more precise regional and global correlation.

2

The Triassic/Jurassic boundary in three contrasting facies in Hungary

Pálfy J., Barbacka M., Császár G., Demeny A., Görög Á., Haas J., Götz A., Oravecz-Scheffer A., Ozsvárt P., Piros O., Ruckwied K., Szeitz P., Zajzon N.

Volumina Jurassica

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2006

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Vol. 4, no. 1

292-293

EN

Using an integrated stratigraphic approach, the Triassic/Jurassic (T/J) boundary has been studied in two different tectonostratigraphic units and three contrasting facies types in Hungary. In the northeasternmost part of the Transdanubian Range unit (part of the Alcapa terrane), the Csövár section has been intensively studied recently. Here a marine carbonate succession represents slope to basinal facies, deposited in an intraplatform basin near the margin of a Late Triassic Dachstein-type platform system. The T/J boundary is defined on the basis of ammonoid, radiolarian, conodont, and foraminiferan faunas. Paleontological data not only contribute to a biostratigraphic subdivision, but are also interpreted in the context of end-Triassic extinction and earliest Jurassic recovery of different fossil groups. A significant negative carbon isotope excursion is recorded in both carbonate and organic matter in the boundary interval. In other parts of the Transdanubian Range, a Late Triassic carbonate platform is preserved as the thick Dachstein Limestone Formation. In the Gerecse Mts. and the Tata horst, a T/J unconformity truncates the topmost Rhaetian part of the Dachstein Fm. and separates it from the overlying Hettangian strata, deposited in a deepening marine environment. Earliest Hettangian deposits and fossils are absent. A potentially more complete succession occurs farther to the west, in the Bakony Mts. Here the shallow marine carbonate deposition continues into the Hettangian, represented by the Kardosrét Limestone Fm. that overlies the Norian-Rhaetian Dachstein Fm. The lithostratigraphic boundary corresponds to the T/J boundary that marks a significant break in platform development. A disconformity and small hiatus is assumed but its duration has not yet been satisfactorily determined. A surface section on Körishegy and core material from boreholes Zt-62 (near Zirc) and Süt-28 (near Sümeg) have been investigated. The T/J boundary is marked by a sharp disappearance of Triasina hantkeni and changes in the accompanying foraminiferan and dasycladacean algal assemblage. A preliminary stable isotope study of the cores has failed to identify a negative δ ¹ ³C excursion, suggesting a hiatus at the boundary. The Mecsek Mts. in southern Hungary is part of the Tisza unit (or Tisza terrane). Here the T/J boundary falls within the coal-bearing, terrestrial to marginal marine, locally more than 1000 m thick Mecsek Formation but its precise placement has been proved difficult. Palynology offers the best potential for biostratigraphic subdivision. New palynological and paleobotanical studies are underway, aimed at a more precise palynostratigraphy and a reconstruction of climate and vegetation history in the boundary interval. In summary, the T/J boundary sections in Hungary occur in terrestrial, shallow marine (carbonate platform), and deeper marine (slope to basinal) facies. Their study provide new data towards a better understanding of the biotic and environmental changes at this critical interval of Earth history.

3

The Triassic/Jurassic boundary beds of the Karwendel Syncline (Austria) - initial report of a new GSSP candidate for the base of the Jurassic

von Hillebrandt A., Krystyn L., Kuerschner W.

Volumina Jurassica

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2006

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Vol. 4, no. 1

287-288

EN

The northwestern part of the Northern Calcareous Alps (NCA) is famous for its fully marine and complete Triassic/Jurassic transition beds formed in an intraplatform through of the western Tethys called as Eiberg Basin (Krystyn et al. 2005). A classic boundary section (Kendlbachgraben) with a relatively rich Rhaetian invertebrate fauna is known since 150 years but with the handicap of a 15 m gap between the last Triassic (Choristoceras marshi) and the first Jurassic ammonite (Psiloceras calliphyllum). A wealth of macro- and microfaunistic information has since been added (Golebiowski 1990) favouring a TJB directly on top of the Koessen Formation (Eiberg Member), just above the disappearance of many Triassic fossil groups (ammonoids, conodonts, brachiopods). New palynological and isotopic research in the area (Kuerschner et al. in press) now points to a boundary located 6 m higher within the Tiefengraben Member (or Grenzmergel) of the lower Kendlbach Formation where along with a distinct δ ¹ ³C shift the appearance of Cerebropollenites thiergatii marks the onset of Jurassic palynomorphs (Fig. 1). Another drastic negative C isotope excursion occurs at the top of the Koessen Formation concomitant with the disappearance of Triassic macrofauna, and both isotopic events can be recognized in boundary sections in England, Greenland and Nevada. The Triassic/Jurassic boundary (TJB) beds of the Karwendel Syncline (northern Tyrol) are well exposed at many places, easily correlatable by palynology (Fig. 1) and show a thicker (about 20 m), lithologically different, marl-dominated Tiefengraben Member with reddish clays ("Schattwald beds" auct.) at the base. They are richer and more diverse in micro- and macrofauna, and they contain an ammonite horizon around 7 m above the Koessen top with a new psiloceratid ammonite unknown from Europe and the Tethys realm. Based on its less intended suture line, the involute conch (umbilical width 40%) and juvenile tubercles (Knötchenstadium) the species is close to the South American earliest psiloceratid P. tilmanni but differs in a subtriangular cross-section. The new "Liassic" ammonite layer corresponds closely to the turnover in the Forminifera and to the onset of "Jurassic" ostracods (e.g. Cytherelloidea pulchella) as well as palynomorphs, with C. thiergatii as a first order correlation tool to marginal marine and continental basins. Bivalves in the basal Grenzmergel may bridge the present macrofaunal gap down to the Koessen Formation top but have still to be evaluated; a nannoplankton analysis is already under way. The investigated sections provide an important new insight in the nature of the physical and biological changes occurring around the boundary with rapidly changing palynomorph associations that point to several short-termed climatic oscillations. In which way they have affected the environmental conditions across the TJB and have controlled the stratigraphic ranges of biomarkers will be an important issue of the ongoing studies. Irrespective of any extinction scenario we see the fossil and geochemical record of the Karwendel Syncline as a major improvement of our knowledge of this time interval and as justified reason to introduce there a section as GSSP candidate for the Triassic/Jurassic boundary.

4

The birth of Jurassic: from the sea or from the sky? Evidences from the Northern Apennines (Italy)

Fazzuoli M., Orti L.

Volumina Jurassica

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2006

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Vol. 4, no. 1

279-279

EN

At the Triassic/Jurassic (T/J) boundary one of the most important mass extinctions occurred. The inferred causes for this extinction are numerous: eustatic variations; climatic changes; extraterrestrial impact events; strong volcanic episodes etc. One of the diagnostic evidences of a bolide impact in the stratigraphic records is the occurrence of PDF structures in quartz grains ("shocked quartz"). According to Bice et al. (1992), shocked metamorphosed quartz grains has been found in the Il Fiume Gorge, near Corfino (Tuscany) within three closely spaced shaly beds, at the boundary between the Rhaetic (Upper Norian) Calcare a Rhaetavicula and the inferred Lower Jurassic (Hettangian) Calcare Massiccio, suggesting "that multiple impacts occurred in latest Triassic, one of which coincided with a locally, and perhaps globally, significant extinction at the T/J boundary". According to our observations at the same stratigraphic section cited by Bice et al. (1992) the following results can be summarized: - The upper part of the Calcare a Rhaetavicula consists of dark grey bioclastic wackestone beds alternating with bioclastic packstone beds (storm layers) and with marly shales. - The top interval of this formation, 2 m thick, consists of 8-30 cm thick beds of dark grey oolitic grainstone, interlayered with three cm thick levels of marls; another 30 cm thick marly level tops this transition interval. In these marly levels the shocked quartz grains were found. - Above a sharp boundary, the overlying lithologies belong to the Calcare Massiccio (massive limestone). The lower interval, 90 cm thick, is slightly bedded and it is represented by barren mudstone; upwards the rock is massive, cliff forming, and also consists of barren mudstone, more or less dolomitized, up to about 30 m, where bioclastic and intraclastic grainstone/packstones occur. In this level abundant Rhaetic benthic foraminifers are present, such as Gandinella apenninica and Aulotortus sinuosus. - At the regional scale, a Rhaetic fossil assemblage (with Triasina hantkeni etc.) has been found also in other stratigraphic sections of the Calcare Massiccio in the same area (Fazzuoli et al. 1988). In conclusion, in the upper part of the Calcare a Rhaetavicula a lowering of the sea level occurred and the depositional environment evolved from a mid ramp to a high energy inner ramp and then to a lagoon, typical of the Calcare Massiccio. During this lowstand, according to the occurrence of shocked quartz, one or more impact events took place. However, considering the occurrence of Triassic fossils well above the levels with the shocked quartz, it is possible to affirm that the acclaimed extra-terrestrial event occurred within the Rhaetic, not at the Triassic/Jurassic boundary.

5

The age of the Triassic/Jurassic boundary: new data and their implications for the extinction and recovery

Pálfy J., Mundil R.

Volumina Jurassica

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2006

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Vol. 4, no. 1

294-294

EN

Providing accurate and precise radio-isotopic ages for the Triassic/Jurassic (T/J) boundary is a goal for time scale calibration but it also bears on the timing and causality of the end-Triassic mass extinction and subsequent recovery. The currently used estimate of the T/J boundary age hinges on a multi-grain zircon U-Pb age from a marine T/J boundary section in the Queen Charlotte Islands, Canada (199.6±0.4 Ma, Pálfy et al. 2000), and ⁴⁰Ar/³⁹Ar ages of volcanism of the Central Atlantic Magmatic Province (CAMP) cluster around 199.9 Ma (Knight et al. 2004), supporting a hypothesis that implicates this Large Igneous Province in triggering severe environmental changes and the biotic extinction. However, the apparent synchrony of the end-Triassic extinction and CAMP volcanism requires scrutiny for at least two reasons. Multi-grain zircon U-Pb analyses are prone to leave slight Pb loss undetected, hence producing marginally younger ages. Comparison of ⁴⁰Ar/³⁹Ar and U-Pb dates is affected by a current revision of the decay constant of ⁴⁰K and the age of fluence monitors that may require recalculation of all Phanerozoic ⁴⁰Ar/³⁹Ar ages and their upward adjustment by ˜1% (e.g. Min et al. 2000). Here we present new single-crystal zircon U-Pb ages that provide new constraints on the T/J boundary. Applying the pretreatment of thermal annealing and chemical abrasion (Mattinson 2005) resulted in a coherent cluster of zircon ages with a preliminary ²⁰⁶Pb/²³⁸U age of 198.0±0.6 Ma from a volcanic tuff layer within Early Sinemurian sediments in the Mecsek Mts., southern Hungary. The same method yielded an age of 200.6±0.3 Ma for a volcanic ash layer in ammonite-bearing Middle Hettangian marine sediments at Puale Bay, Alaska. The Early Sinemurian age provides an upper limit for the earliest Jurassic recovery interval. The two new ages suggest that the T/J boundary might be older than previously thought, likely >201 Ma. Considering the systematic bias in existing ⁴⁰Ar/³⁹Ar ages for the CAMP, synchrony of volcanism and extinction remains a strong possibility but requires further tests. Significantly, a single-crystal 206Pb/238U age of 201.3±0.3 Ma has been reported from the North Mountain basalt, a CAMP flow in Nova Scotia, Canada (Schoene et al. 2006). Further dating effort is needed to re-analyze critically important volcanic ash layers from the T/J boundary sections of the Queen Charlotte Islands.

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Sedimentary features and palynofacies of the Triassic/Jurassic boundary interval (Csövár section, N Hungary)

Ruckwied K., Götz A., Haas J., Pálfy J.

Volumina Jurassica

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2006

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Vol. 4, no. 1

296-296

EN

The studied Csövár section is situated NE of Budapest (Hungary) and exposes a continuous, marine Triassic/Jurassic boundary section of slope to basinal facies. Late Triassic palaeogeographic reconstructions indicate that this area was located close to the offshore margin of the Dachstein carbonate platform system, which was segmented by intraplatform basins. A predominantly limestone succession is exposed in two outcrops: the Pokol-völgy quarry and the southern slope of the Vár-hegy. Based on ammonites and the last appearance of conodonts, the Triassic/Jurassic boundary can be drawn within the Csövár Formation. Facies analysis of the Rhaetian-Hettangian deposits reveals a long-term change in sea level, superimposed by short-term fluctuations. After a period of highstand platform progradation in the Late Norian, a significant sea-level fall occurred in the Early Rhaetian, exposing large parts of the platform. A renewed transgression led to the formation of smaller buildups fringing the higher parts of the previous foreslope that served as habitat of crinoids, representing the main source of carbonate turbidites. The higher part of the Rhaetian is characterized by proximal turbidites with intercalated lithoclastic debris flows. Distal turbidites and radiolarian basin facies become prevalent upsection, dominating in the earliest Hettangian. The next significant facies change in the Early Hettangian is marked by the appearance of redeposited oncoid-grapestone beds, indicating the end of the Rhaetian to earliest Hettangian sequence. Palynofacies of the sedimentary series exposed in the Pokol-völgy quarry is dominated by terrestrial components, reflecting a high supply from the hinterland. Numerous needle-shaped opaque particles, as well as a high amount of large translucent plant fragments within the phytoclast group, may point to the transport mechanism of sedimentary organic matter, strongly related to the occurrence and frequency of turbidites along the slope. The studied samples of the Pokol-völgy quarry yield a typical Upper Rhaetian palynomorph assemblage, characterized by a high amount of Circumpolles (Classopollis), Rhaetipollis germanicus, Ovalipollis pseudoalatus and numerous trilete spores. The marine fraction is marked by foraminiferal test linings and prasinophytes of the genus Pterospermella; acritarchs are very rare and dinoflagellate cysts are absent. This microplankton assemblage is characteristic of a permanently stratified basin. Palynofacies of the carbonates exposed in the upper part of the Vár-hegy section, dated as Lower Hettangian, is dominated by degraded organic matter, small equidimensional phytoclasts and foraminiferal test linings, pointing to a distal basinal setting. The integrated analysis of sedimentary and organic facies enables one to reconstruct the depositional environment and to detect major sedimentary processes. The dominance of turbidites, together with the preservation and composition of sedimentary organic matter, supports the complex basin topography.

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Prospects for correlation of the Triassic/Jurassic boundary between marine and non-marine facies using carbon-isotope stratigraphy: a case study from Xinjiang, northwest China

Hesselbo S., Deng S., Lu Y.

Volumina Jurassica

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2006

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Vol. 4, no. 1

286-287

EN

An apparently global pattern of carbon-isotope change in carbonates and organic matter across the T/J boundary has increasingly been recognized from marine sections, and may be useful as a basis for correlation. Evidence from other geological events indicates that this isotopic pattern should also have the power to resolve correlations between marine and non-marine sections where organic matter is preserved. In the present study we apply this method to the non-marine succession in the Junggar Basin, Xinjiang, northwest China. The Haojiagou Section there includes an excellent ˜700 m thick exposure of the Haojiagou Formation and lower member of the Badaowan Formation, from which fossils indicative of the T/J boundary have been described. However, different horizons have been suggested on the basis of different fossil groups. The T/J boundary has previously been placed at the lowest level in the section (at the base of Bed 25, Haojiagou Formation) on the basis of megaspores, and Bed 40, higher in the Haojiagou Formation, has been suggested to be Early Jurassic in age based on the occurrence of conchostracan taxa and the bivalve Ferganochonca. In contrast, on the basis of palynology, the T/J boundary has been placed higher in the section, at the base of Bed 45 (also the base of the Badaowan Formation), where there is an absence of pre-Jurassic index fossils such as Taeniaesporites, and occurrences of the post-Triassic index fossils, such as Cyathidites. Macrofossil plants suggests that the boundary lies between Beds 37 and 51 based on Hausmannia sp. in Bed 37, and Todites princeps and Clathopteris elegans in Bed 51 and higher. Carbon-isotope data have been obtained from both bulk sedimentary organic matter and from fossil wood fragments that were individually selected. Delta 13CWood carbon isotope values show a systematic stratigraphic trend from -21 per mil in Bed 25, to -26 per mil in Bed 52, although some samples around bed 25 also show very light isotopic values. Bulk organic isotope data show a similar overall trend, but show considerably less variance. Although the data are noisy, they suggest that the "main" negative isotope excursion of marine sections corresponds to the formational boundary and the "T/J boundary" recognized on the basis of plant macrofossils and microfossils, whilst the "initial" isotope excursion of marine sections may be expressed in wood values around beds 23-25. Depending on GSSP decisions either of these horizons may be regarded as the T/J boundary.

8

Palynomorph assemblages of Triassic/Jurassic boundary key sections of the NW Tethyan realm: evidence for climatic change

Ruckwied K., Götz A., Pálfy J., Michalík J.

Volumina Jurassica

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2006

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Vol. 4, no. 1

297-297

EN

The composition and diversity of Late Rhaetian/Hettangian microfloras of the NW Tethyan realm are investigated with respect to the Triassic/Jurassic mass extinction and the processes that may have caused this crisis. Key locations for detailed palynological studies are selected in the Tatra Mountains (Slovakia) and the Mecsek Mountains (Hungary). The Furkaska section (Tatra Mts., Slovakia) exposes a complete succession of a proximal marine setting. The Upper Triassic Fatra Formation is characterized by bioclastic limestones and fine-grained clastics overlain by dark claystones with intercalated sandstones of the lowermost Jurassic Kopieniec Formation. Based on geochemical data and microfacies analyses, the boundary interval is placed near the transition of the two formations. In the area of Pécs and Komló (Mecsek Mts., Hungary) outcrops and core material reveal a fluvial-lacustrine succession continued by paralic coal deposits. The Upper Triassic Karolinavölgy Sandstone Formation is built up by arcosic sandstones and siltstones overlain by the coal-bearing uppermost Triassic to Lower Liassic Mecsek Coal Formation. The palynomorph assemblages of both settings display typical Rhaetian/Liassic microfloras, dominated by bisaccate pollen grains, trilete spores and Circumpolles. Striking quantitative changes in the composition of the palynomorph assemblages are interpreted with respect to climatic changes within this period. Palynology has been proven as a powerful tool for terrestrial to marine correlation. The floral turnover on land can be directly read from changes in the palynomorph assemblages in the investigated marine and terrestrial sections.

9

Oyster and brachiopod stable isotopes across the Triassic/Jurassic boundary and in the Early Jurassic

Korte C., Hesselbo S., Jenkyns H., Dietl G., Dulai A., Vörös A.

Volumina Jurassica

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2006

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Vol. 4, no. 1

289-289

EN

Oxygen and carbon isotope values have been obtained from oysters for the Triassic/Jurassic boundary section at Lavernock Point (Wales), and from brachiopods and oysters for different Hettangian, Sinemurian and Pliensbachian localities of South Germany and Hungary. Low-Mg-calcite brachiopods and oysters are particularly suitable for such studies because this carbonate phase is the most resistant to diagenetic alteration. Nevertheless, all fossils have been screened by chemical and optical techniques (optical microscope, scanning electron microscopy, trace element analyses) to evaluate the isotope data for diagenetic change, and only samples with Mn content less than 250 ppm and Sr content more than 400 ppm, complemented by well preserved textures under SEM, were considered in this study. For the Triassic/Jurassic boundary (TJB) the carbon isotope values are at about 2.5‰ in the lower Langport Member, increase to 4.5‰ in the lowest Blue Lias and decrease subsequently to 1.5‰ just below the Planorbis Zone. The data remain low with variations between 1.5 and 2.5‰ up to the Liassicus Zone. These results correspond to the organic carbon isotope trend for the Triassic/Jurassic boundary section at St Audrie's Bay (Hesselbo et al. 2002). Oxygen isotope values increase from -0.5‰ in lower Langport Member to 0‰ at the base of the Blue Lias, decrease in the Blue Lias down to -1.5‰ just below the Planorbis Zone and change in parallel with the organic and inorganic carbon-isotope trends. The δ ¹ ⁸O values indicate decreasing seawater temperature with increasing δ ¹ ³C in the Langport Member and increasing water temperatures of about 6°C in the lower Blue Lias. The distinct warming trend occurred during the "main" TJB negative excursion. Carbon and oxygen isotope values from Hettangian, Sinemurian and Pliensbachian brachiopods and oysters, as well as from some complementary belemnites, show similar values and trends compared to the data compilation by Jenkyns et al. (2002). Carbon isotope values are between 1 and 2‰ in the Hettangian and Early Sinemurian followed by an increase of about 1‰ during the Sinemurian, a nearly 3‰ decrease in the Early Pliensbachian and higher δ ¹ ³C values (˜2.5‰) in the later Pliensbachian.

10

Latest Triassic - earliest Jurassic geodynamic evolution of the Southeastern Asia and mass extinction event

Golonka J., Krobicki M.

Volumina Jurassica

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2006

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Vol. 4, no. 1

281-282

EN

The most significant geodynamic process in the Southeastern Asia was convergent Late Triassic event which was the continuation of Indosinian Orogeny and induced consolidation of Chinese blocks. Also, Indochina and Indonesia were sutured to South China, and Qiangtang block approached the Eurasian margin. Siliciclastic shallow marine shale sedimentation covered the western part of South China Plate, while the central and eastern parts were uplifted. Siliciclastic sedimentation prevailed. The collisional Triassic events continued between North China and Mongolia. Volcanics and collisional granites were common in both parts of the newly formed plate as well as on the Siberian margin of the Mongol-Okhotsk Ocean. Continental clastics and volcanics prevailed in the area between Africa and North America near Triassic/Jurassic boundary. The earliest Jurassic was the time of complete assembly of eastern Pangea and of onset of the break-up of the supercontinent and separation of Gondwana and North America. The colour of these rocks changed from red to grey along the Triassic/Jurassic boundary due to increased wetness and global transition from oxic to anoxic conditions. In Indochina block the Upper Triassic rocks are deformed and include in the thrust system. The Indosinian Orogeny continued after its collisional phase. In Indochina late phase of Indosinian Orogeny lasted around 50 million years until earliest Jurassic. This phase included back-arc deposits, synorogenic rocks and postorogenic molasse. Several phases of the orogeny could be distinguished based on metamorphic, plutonic and structural events. Carbonate sedimentation predominated along the Neotethyan margins, between 35°N and 35°S latitudes. The northwestern Neotethys region consisted of numerous horst blocks capped by carbonate platforms with adjacent grabens filled with deeper-water black mudstone and organic-rich shale facies. Reef communities were rare at that time. The time around the Triassic/Jurassic boundary marked the very important extinction event. From the plate tectonic and palaeogeographic point of view the following events could have influenced the extinction of biotas: 1. The closure of Paleotethys and assembly of the Asian part of Pangea. This event caused the origin of new subduction zone along the Neotethys margin first in its western, later in the eastern part. Neotethys went from spreading to subducting phase. The new volcanic arcs were born along the subduction line. 2. The break-up of Pangea in the future Central Atlantic area and transition from rifting to drifting phase. 3. The very extensive volcanism and origin of Large Igneous Provinces type of basalts. This volcanism was related to the break-up of Pangea and caused rising amount of atmospheric CO2. 4. Sea level fluctuation. Drop of sea-level was related to the assembly of Pangea and was followed immediately by the rise caused by the Pangea break-up. It affected especially carbonate platforms and reefs. 5. Anoxia. It was related to the formation of restricted basins and nutrient oversupply caused perhaps by volcanic activity and sea-level fluctuations. All the above mentioned events are bound together by close relationships. Plate tectonic activity caused palaeogeographic and palaeoclimatic change, which triggered mass extinction.

11

Hettangian ammonite succession and analysis of the Triassic/Jurassic boundary sequence at Black Bear Ridge, Williston Lake, British Columbia

Hall R., Pitaru S., Henderson C.

Volumina Jurassica

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2006

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Vol. 4, no. 1

285-286

EN

The basal Fernie Formation at Black Bear Ridge consists of 22 m of concretionary, brown weathering siltstones containing abundant ammonites which prove the presence of lower, middle, and upper Hettangian strata. Based on over 300 specimens we have recognized 13 ammonite horizons which may be grouped into six local zones. Lower Hettangian faunas are dominated by strongly ribbed psiloceratids associated, in the upper part of their range, with small Kammerkarites; no smooth psiloceratids of the tilmannipacificum-planorbis group have been found. The Middle Hettangian is marked by the appearance of Kammerkarites hircinum associated with Kammerkarites spp., Discamphiceras, and Pleuroacanthites mulleri. Upper parts of the Middle Hettangian yield Sunrisites sunrisense, Saxoceras portlocki, Kammerkarites spp., Discamphiceras and Alsatites. First appearance of schlotheimiids, which marks the base of Upper Hettangian strata, is above the last occurrences of Sunrisites sunrisense. Faunas in higher strata are dominated by Laqueoceras nigroursus, which ranges up into beds containing Badouxia oregonensis; near the top of the section, Pseudaetomoceras doetzkirchneri appears. Lower Hettangian psiloceratid faunas from Black Bear Ridge can best be correlated with European zonations. Elsewhere, first occurrences of Kammerkarites usually mark the base of the Middle Hettangian, and they are not usually associated with psiloceratids as at Black Bear Ridge. However, strata with Kammerkarites in both Alaska and Queen Charlotte Islands have been placed in the Lower Hettangian. Sunrisites here is associated with both Saxoceras portlocki and Alsatites subliasicus, indicating its Middle Hettangian age, while in both Nevada and South America it occurs above intervals with Kammerkarites and Saxoceras, and is associated with Schlotheimia. At Black Bear Ridge the latter genus only appears above the last Sunrisites. In both Nevada and South America, Laqueoceras is also associated with Sunrisites, not succeeding it as here. Occurrences of Badouxia and Pseudaetomoceras clearly indicate correlation of the highest parts of the sequence with the Upper Hettangian. Correlation of the ammonite associations recorded at Black Bear Ridge with a regional zonation scheme proposed for North America is only partially successful due to differing local ranges of key taxa, and the absence of some genera (Euphyllites, Mullerites, Franziceras, Fergusonites, and Eolytoceras) in this section. Between the "Monotis beds", dense coquinas comprising the uppermost Pardonet Formation (Norian), and the first Jurassic ammonites in the base of the Fernie Formation, is a sparsely fossiliferous interval of flaggy-bedded, brown siltstones 2.3 m thick. Fragmented pectiniform and ramiform conodont elements of Norigondolella sp. and Epigondolella sp., obtained by us from two beds 1.5 and 1.7 m above the "Monotis beds", indicate a probable Rhaetian age for part of this interval. Positive carbon- and nitrogen-isotope excursions at the top of the "Monotis beds" have been correlated with the Norian/Rhaetian boundary (Sephton et al. 2002); they interpreted the Rhaetian/Hettangian (Triassic/Jurassic) boundary to occur 9.0 m higher in the section, though the anticipated negative carbon-isotope excursion was not found at this level. Our new ammonite data confirm that strata just 2.3 m above the top of the "Monotis beds" are already of Hettangian age.

12

Early Hettangian ammonites and radiolarians from the Queen Charlotte Islands (British Columbia, Canada) and their bearing on the definition of the Triassic/Jurassic boundary

Longridge L., Carter E., Smith P., Tipper H.

Volumina Jurassica

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2006

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Vol. 4, no. 1

290-290

EN

The extinction that marks the Triassic/Jurassic boundary (T/J boundary) is one of the so-called "big five" that punctuate the Phanerozoic. Two sequences spanning the boundary occur in the Queen Charlotte Islands (QCI) of British Columbia; one is located at Kennecott Point on Graham Island (section I), the other on the southeastern shore of Kunga Island (section III). A second section at Kennecott Point contains fossils of Early Hettangian age only (section II). Eleven ammonite taxa are present in the Lower Hettangian sections of the QCI. Ammonite faunas indicate an Early Hettangian age for the upper portion of section I and permit correlation of the upper portion of section II with the middle Lower Hettangian Minutum Zone. Section III contains moderately diverse Early Hettangian ammonites that allow correlation of the lower portion of the section with the Minutum and Pacificum zones and the upper portion with the upper Lower Hettangian Polymorphum Zone. Lower Hettangian radiolarians from the Canoptum merum Zone are present in all three sections; a few Upper Rhaetian holdovers from the Globolaxtorum tozeri Zone are also present in sections I and II. The T/J boundary radiolarian faunas correlate closely with those in the Inuyama area of Japan indicating that radiolarians were globally distributed at that time. Although ammonite preservation is poor, radiolarian preservation is excellent and the turnover combined with continuous deposition and lack of facies changes over this interval marks the most distinct boundary level currently recognized worldwide. Section I has also produced a carbon isotope curve which records a prolonged negative excursion spanning the T/J boundary while section III has provided a U-Pb date of 199.6±0.3 Ma which constrains the boundary. The latter section has already been proposed as a potential basal Hettangian Global Stratotype Section and Point (GSSP). Although all three QCI sections lack ammonites from the basal Hettangian Spelae ammonite Zone, sections I and III have both yielded earliest Hettangian radiolarian collections which contain Rhaetian holdovers suggesting an approximate correlation of the lower portion of these sections with the Spelae Zone. A section in New York Canyon, Nevada has also been proposed as a potential GSSP. This section provides a virtually complete ammonoid succession but lacks geochronology and microfossils. In essence, the sections at Kunga Island and New York Canyon are complementary. Close correlations between the two sections are possible using ammonite faunas as well as the negative carbon isotope anomalies which span the T/J boundary in Nevada and at Kennecott Point. We suggest the section from Nevada be designated as holostratotype (and datum) for the basal Hettangian and the QCI section be designated as a parastratotype to improve recognition of this interval.

13

Criterion for definition of the Triassic/Jurassic boundary

Lucas S., Guex J., Tanner L.

Volumina Jurassica

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2006

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Vol. 4, no. 1

291-291

EN

The criterion for definition of the Triassic/Jurassic boundary (TJB) should be a marker event of optimal global correlateability. Only an ammonite event meets this criterion; other potential marker events for definition of the TJB have less correlation potential. Since the 1960s, the LO of the ammonite Psiloceras (usually the species P. planorbis) has provided the working definition of the TJB. However, other criteria for boundary definition have been advocated, including a change in the bivalve fauna (LO of Agerchlamys), a sudden negative excursion of carbon isotopes and the LO of Psiloceras tilmanni, which precedes the LO of P. planorbis. Other criteria that can be advocated include the supposed TJB mass extinction, the HO (highest occurrence) of conodonts or a significant evolutionary turnover of radiolarians. Distinction of the Triassic and Jurassic systems in marine biostratigraphy has a long tradition rooted in ammonite biostratigraphy. This is because the ceratitedominated ammonite faunas of the Triassic virtually disappeared across the system boundary and were totally replaced by the smooth-shelled psiloceratids of the Early Jurassic. Because of the long history of study of this ammonite turnover, its details are extremely well documented on a global scale, especially in western North America, South America and Western Europe. This ammonite turnover thus provides wide-ranging correlations that are intensively studied, extensively published and documented. No other bio-event associated with the TJB can claim such investigation, and no bio-event is comparable to the ammonite turnover to provide a globally correlateable criterion for boundary definition. Using the LO of Psiloceras tilmanni as to define the TJB thus has these advantages: 1. it maintains longstanding tradition of placing the boundary so that all smooth-shelled psiloceratids are Jurassic; 2. it is a boundary above all bio-events traditionally considered Triassic (Fig. 1); 3. it provides an ammonite-based definition of broad correlation potential (P. tilmanni has a distribution from Nevada to Chile); 4. it places the boundary close to (just above) other marker events that can be used to identify the TJB in sections that lack ammonites (Fig. 1). The LO of P. tilmanni thus defines a TJB of optimal correlation potential.