Michael J. Orchard

Carbon isotope anomaly and other geochemical changes at the Triassic-Jurassic boundary from a marine section in Hungary

Most mass extinctions are linked with carbon isotope excursions, implying that biotic crises are coupled with changes in the global carbon cycle. The isotopic evolution during the end-Triassic extinction is far less documented than that for the other major Phanerozoic extinctions. Here we report a sharp and short-lived 23.5‰ excursion in carbon isotope values for carbonate (d 13 Ccarb) corresponding to a 22‰ excursion in the isotopic composition of marine organic matter (d 13 Corg) and other geochemical

Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from south China

Four volcanic-ash beds bracket the Early-Middle Triassic boundary, as defined by conodont biostratigraphy, in a stratigraphic section in south China. High-precision U-Pb dates of single zircons allow us to place the Early to Middle Triassic (Olenekian-Anisian) boundary at 247.2 Ma. Magnetic-reversal stratigraphy allows global correlation. The new dates constrain the Early Triassic interval characterized by delayed biotic recovery and carbon-cycle instability to ∼5 m.y. This time constraint must be considered in any model for the end-Permian extinction and subsequent recovery.

CONODONTS OF THE LOWERMOST TRIASSIC OF SPITI, AND NEW ZONATION BASED ON NEOGONDOLELLA SUCCESSIONS

Conodonts from the lowermost Triassic Otoceras woodwardi beds and adjacent strata of Spiti are described and compared with Permian-Triassic (P-T) boundary bed faunas from elsewhere. A new pelagic zonation based on Neogondolella is introduced: the interval characterized by N. carinata-N. taylorae is subdivided into three parts based on successive first appearances of N. meishanensis , N. krystyni Orchard n. sp. and N. discreta Orchard and Krystyn n. sp., the nominal species of three successive zones. The development of these Griesbachian species involves a progressive morphological change in the configuration of the axial part (blade-carina-cusp) of the pectiniform elements. The pelagic conodont zonation is intercalibrated with the parallel zonation based on species of Hindeodus and Isarcicella , and with ammonoid faunas from Spiti, other Himalayan localities, and the Arctic. The meishanensis Zone embraces the parvus Zone and part of the overlying staeschei Zone. Strata containing O. woodwardi in Spiti carry the indices to the staeschei and krystyni zones. The Neogondolella conodont fauna associated with Otoceras differs from that of the latest Permian Changshing Limestone of China, but resembles that from the P-T boundary transition beds at Meishan, where a meishanensis Zone of restricted scope occurs beneath the parvus datum. The faunal change which introduces the characteristic Neogondolella species of the N. carinata-N. taylorae fauna occurs at the base of the P-T boundary transition beds at Meishan, the proposed boundary stratotype. Slightly above this level, the disappearance of most Neogondolella species and the introduction of new Hindeodus species coincides with a change in conodont biofacies rather than an extinction event. In the Spiti sections, the N. carinata-N. taylorae fauna, associated at first with H. parvus (as in Selong, Tibet), persists through the entire Griesbachian. Indices of the three Neogondolella zones are also recognized in the Salt Range and the Canadian Arctic. Four new conodont species are described: Neogondolella discreta, N. kazi, N. krystyni , and N. nassichuki .

Geology of the western flank of the Coast Mountains between Cape Fanshaw and Taku Inlet, southeastern Alaska

The western flank of the Coast Mountains batholith between Cape Fanshaw and Taku Inlet is underlain primarily by Jura-Cretaceous strata of the Gravina belt; pre-Permian(?), Permian, and Triassic strata of the Taku terrane; and mid-Proterozoic(?) to upper Paleozoic rocks of continental margin affinity. The continental margin rocks include mid-Proterozoic(?) to lower Paleozoic(?) quartzite and marble of the Tracy Arm assemblage; mid-Paleozoic metavolcanic and subordinate metasedimentary rocks of the Endicott Arm assemblage; and upper Paleozoic quartz-rich metaturbidites and metaconglomerate of the Port Houghton assemblage. We suggest that these three assemblages are correlative with components of the Yukon-Tanana terrane, which underlies a large region of Yukon and eastern Alaska. Rocks of the Gravina belt, Taku terrane, and Yukon-Tanana terrane are juxtaposed along west-vergent thrust faults of mid-Cretaceous age and are internally deformed and disrupted along latest Cretaceous to early Eocene dip-slip and possibly strike-slip shear zones. These rocks and structures, together with mid-Cretaceous to Eocene plutons of the Coast Mountains batholith, separate rocks of the Alexander-Wrangellia terrane to the west from rocks of the Stikine terrane to the east. Mid-Cretaceous thrust faults in the area belong to a system of thrusts that form the inboard margin of the Alexander-Wrangellia terrane from central Alaska to northwestern Washington. The continental margin rocks in the northern Coast Mountains may be part of the in situ North American margin that has been overthrust by the Stikine and adjacent terranes. Alternatively, these rocks may have (1) rifted from and then returned to the North American margin, (2) moved >800 km along left-lateral or right-lateral faults from elsewhere along the North American margin, or (3) originated near a continent other than North America.

TAXONOMY AND CORRELATION OF LOWER TRIASSIC (SPATHIAN) SEGMINATE CONODONTS FROM OMAN AND REVISION OF SOME SPECIES OF NEOSPATHODUS

Exceptionally well preserved Lower Triassic conodonts from Oman include an array of Neospathodus species, many of which are new. Those from the upper Lower Triassic, or Spathian, are described in conjunction with a restudy of conodont type material from Chios, Greece, and an assessment of contemporary collections from Pakistan and western North America. The taxonomic scope of three key species from Chios, Neospathodus homeri, N. triangularis, and N. gondolelloides, is revised. Seven new species are described from Oman: N. abruptus, N. brevissimus, N. brochus, N. crassatus, N. curtatus, N. pusillus, and N. symmetricus; and one new species, N. clinatus, is described from Pakistan. The allied Icriospathodus collinsoni is also described from Oman. The occurrence and range of Neospathodus species are presented in the context of the ammonoid succession in the Spathian of North America. Both N. homeri and N. triangularis, as revised, have shorter ranges and are more age diagnostic than previously thought. Neospathodus gondolelloides is a distinct taxon, and not synonymous with Chiosella timorensis. Five informal faunal divisions are identified based on Neospathodus and allied species. In ascending stratigraphic order, these are typified by Icriospathodus collinsoni, Neospathodus homeri, N. triangularis, N. symmetricus, and N. gondolelloides. Oman collections represent three of these faunas, which occur also in the Columbites through Prohungarites/Subcolumbites ammonoid beds of western U.S.A.

Impact of differential tectonic subsidence on isolated carbonate-platform evolution: Triassic of the Nanpanjiang Basin, south China

The Nanpanjiang Basin of south China contains four exceptionally well-exposed, isolated Triassic carbonate platforms. Detailed mapping of two-dimensional transects and description of stratigraphic sections allow the reconstruction of facies architecture, sequence stratigraphy, and evolution of the platforms. Biostratigraphy, magnetic-susceptibility profiles, and volcanic-ash horizons allow chronostratigraphic correlation and, thus, a basinwide evaluation of mechanisms controlling platform evolution. A comparison of platform architecture demonstrates that southerly platforms have substantially greater thickness, backstepping geometry, pinnacle development, and earlier drowning that resulted from greater tectonic subsidence proximal to a probable convergent margin along the southern perimeter of the basin. Felsic volcanics thicken southward and contributed to the termination of the southernmost platform, indicating the development of a volcanic arc along the southern margin of the South China tectonic block. The northernmost isolated platform had greater longevity and lesser accumulation and lacks backstepping and pinnacle phases of development. Basin-margin intertonguing relationships, or lack thereof, demonstrate that earlier siliciclastic influx into the basin to the south and concurrent starved-basin conditions to the north impacted the evolution of platform-margin geometries. Comparative analysis of platform evolution shows that the timing and rates of tectonic subsidence controlled the timing of platform termination by drowning, backstep geometries, pinnacle development, and overall platform thickness. The timing of siliciclastic basin fill dictated differences in platform-margin geometries such as slope angle, relief above basin floor, and the presence or absence of basinward platform progradation. Despite the dramatic differences in platform architecture, eustatic sea level fluctuations imparted a basinwide sequence-stratigraphic signal.

Remnants of Paleozoic cover on the Archean Canadian Shield: Limestone xenoliths from kimberlite in the central Slave craton

Paleozoic limestone xenoliths have been recovered from kimberlite pipes that intrude the Archean Canadian Shield. Xenoliths from the Jericho pipe in the central Slave craton are commonly fossiliferous and contain a diverse macrofauna and microfauna, including conodonts. Most of the 21 conodont collections that have been recovered are Middle Devonian in age (late Eifelian to early Givetian, and early to middle Givetian). In general, the fauna is indicative of deposition in shallow- and open-marine environments; a few collections are indicative of slightly restricted shoal-shelf seas. A few low-diversity conodont suites could be pre-Middle Devonian. The nearest similar Paleozoic rocks are more than 400 km to the southwest of the Jericho pipe in the foothills of the Mackenzie Mountains. The Jericho limestone xenoliths provide the only solid evidence that shallow Middle Devonian seas inundated the now-exposed Slave craton of the Canadian Shield.

Lower Triassic conodonts from the Canadian Arctic, their intercalibration with ammonoid‐based stages and a comparison with other North American Olenekian faunas

Conodont faunas are described from the type sections of Lower Triassic stages in the Canadian Arctic. The collections come largely from ammonoid-bearing strata of the Strigatus (upper Griesbachian), Candidus (lower Dienerian), Romunduri and Tardus (lower and upper Smithian), and Subrobustus (upper Spathian) zones. These demonstrate that diverse late Griesbachian conodont faunas included the first species of Borinella, Neospathodus and Scythogondolella, accompanied by several species of Neogondolella. Dienerian faunas are dominated by Neospathodus, but explosive radiation led to several Smithian Scythogondolella species and diverse cosmopolitan “neospathodid” species of Conservatella, Discretella and Neospathodus. Smithian Paullella (nomen novum) and Wapitiodus are recorded here for the first time in the Arctic collections, and emphasize the correlation with successions in the Western Canada Sedimentary Basin (WCSB) and in the western USA. Conodonts from the Romunduri Zone differ in the three regions: Conservatella, Discretella and Paullella are rare in the Arctic, are more common in WCSB and are most common in the USA; Scythogondolella species are common in the Arctic, less so in WCSB and are virtually absent in the USA. Conodonts from the Tardus Zone, both in the Arctic and elsewhere, contain cosmopolitan Borinella buurensis, Neospathodus waageni, Scythogondolella mosheri and Scythogondolella milleri. Spathian conodont faunas from the Subrobustus Zone are dominated by Neogondolella species, which also occur in the WCSB associated with Triassospathodus. These two genera have an inverse relationship: Triassospathodus dominates the USA successions, and is virtually absent in the Arctic. Scythogondolella ellesmerensis sp. nov. and Scythogondolella lachrymiformis sp. nov. are described.

The Triassic-Jurassic (T-J) boundary in Queen Charlotte Islands, British Columbia defined by ammonites, conodonts and radiolarians

Abstract In the Queen Charlotte Islands, on the Pacific shore of Birtish Columbia, Canada, Triassic-Jurassic boundary beds are present in several well-exposed sections of the Sandilands Formation. Triassic and Jurassic ammonoids, Triassic conodonts and Triassic and Jurassic radiolarians indicate the position of the boundary to within a few metres. The section may be essentially continuous across the boundary. The evidence suggests more or less simultaneous extinction of ammonoid, conodont and radiolarian faunas at the T-J boundary.

Deep-sea record of impact apparently unrelated to mass extinction in the Late Triassic

The 34-million-year (My) interval of the Late Triassic is marked by the formation of several large impact structures on Earth. Late Triassic impact events have been considered a factor in biotic extinction events in the Late Triassic (e.g., end-Triassic extinction event), but this scenario remains controversial because of a lack of stratigraphic records of ejecta deposits. Here, we report evidence for an impact event (platinum group elements anomaly with nickel-rich magnetite and microspherules) from the middle Norian (Upper Triassic) deep-sea sediment in Japan. This includes anomalously high abundances of iridium, up to 41.5 parts per billion (ppb), in the ejecta deposit, which suggests that the iridium-enriched ejecta layers of the Late Triassic may be found on a global scale. The ejecta deposit is constrained by microfossils that suggest correlation with the 215.5-Mya, 100-km-wide Manicouagan impact crater in Canada. Our analysis of radiolarians shows no evidence of a mass extinction event across the impact event horizon, and no contemporaneous faunal turnover is seen in other marine planktons. However, such an event has been reported among marine faunas and terrestrial tetrapods and floras in North America. We, therefore, suggest that the Manicouagan impact triggered the extinction of terrestrial and marine organisms near the impact site but not within the pelagic marine realm.