Triassic-Early Jurassic Research Articles

Application of neoichnological studies to behavioural and taphonomic interpretation of fossil bird-like tracks from lacustrine settings: The Late Triassic–Early Jurassic? Santo Domingo Formation, Argentina

The purpose of this study is to apply neoichnological observations to the behavioural and taphonomic interpretation of a Late Triassic–Early Jurassic track surface from the Santo Domingo Formation (Argentina) containing hundreds of bird-like tracks and trackways. In addition, the factors affecting the formation and preservation of bird tracks in lacustrine settings are particularly addressed. The 5.5 m 2 fossil track surface contains different types of trace fossils: the avian ichnotaxa Gruipeda dominguensis (the most abundant), bird-like tracks with elongated hallux impressions, small epichnial rounded pits, and invertebrate traces ( Skolithos, Taenidium). The modern environmental analogue chosen for the neoichnological studies was a coastal freshwater pond (the Bajo de los Huesos, Chubut, Argentina) seasonally occupied by sandpipers ( Calidris bairdii and Calidris fuscicollis; Charadriiformes). The comparison between the fossil succession and the modern example suggests that they share lithology and sedimentary structures and that sedimentary processes and local palaeoenvironment were fairly similar and do not bias ichnological comparisons. Field observations allowed to distinguish twenty one behaviours that produced distinct traces and four modern footprint types (1 to 4) related to specific substrate conditions. In particular, the preferential formation of bird tracks parallel to the waterline, also confirmed by studies on droppings and invertebrate fauna of the pond, and other associated sedimentary features (ripple marks, wrinkle marks, mud drape thickness) and trace fossils were important for recognition of the shoreline in the fossil example. These observations also allowed us to distinguish the adjacent deeper and shallower parts of the fossil pond and can be applicable to other similar case studies. Contrasting sediment properties and footprint types, some relationships and constraints on the formation and preservation of modern footprints are proposed; although these are very complex processes that will require further studies. Five of the behaviours recognised in the modern pond were inferred from the sixteen trackways distinguished on the fossil track surface, including walking, walking with a zig-zag path, short runs, probing, and landing with legs directed forward (possible trace of flight). The recognition of traces of flight (Volichnia), probing marks, and tracks showing morphology similar to modern shorebirds ( G. dominguensis), strongly suggest an avian affinity for the producers of the fossil tracks and, in consequence, the Santo Domingo track site would be younger than supposed.

Tectonic Oil Potential Relationships, Marmara Sea, NW Turkey

Gas discovery in the K. Marmara field in northern Marmara Sea proved the generation, migration, and accumulation of hydrocarbons in the offshore area in NW Turkey. Two principal tectonic events determined the petroleum potential of this region: subduction systems during Late Cretaceous time due to the obduction of the Istranca-Rhodope Massif southward onto the Karakaya Zone, which started rifting since Late Triassic-Early Jurassic, and the emplacement of the Ergene Wedge, distinguishable on the Thrace Basin's gravimetric map to the Northwest of the Marmara Sea in Miocene. Latest Miocene tectonic activities, which generated the North Anatolian Fault (NAF) and its splays in the Thrace Basin and in the Marmara Sea, improved the reservoir characteristics with the appearance of fractures and structures in the reservoir units. Evaluations and interpretations of seismic and aeromagnetic data indicate E–W oriented subduction zones at the central part of the Marmara Sea. The westward onshore extension of these zones continues toward the Sarkoy-Gelibolu-Saros Gulf area. While the prospective Eocene Sogucak reefoidal carbonates were deposited on the Late Cretaceous ophiolitic paleohighs, tested in Sarkoy-1, Tepekoy-1, Murefte-1, and Eriklice-1 onshore and Doluca-1 and Isiklar-1 offshore oil exploration wells, the Oligocene Osmancik sandstones, other prospective units, were sedimented in a deltaic environment. On the other hand, an important tectonic and structural feature, the Ergene Wedge, which is reflected by gravimetric map, occurs to the northwest of the study area during Miocene time. It widens toward the Greek boundary but narrows eastward. Due to tectonic activities, high temperatures, pressures, and strains should be accumulated in this zone, which generated necessary conditions for oil generation and migration in and around the Marmara Sea. The Eocene Sogucak (Mecidiye) and Oligocene Osmancik (Gazhanedere) formations constitute the main reservoir units, while the Eocene Hamitabat and Eocene-Oligocene Mezardere shales are accepted as the principal oil source rocks in the Thrace Basin. Geological and geophysical interpretations reveal that the Sogucak and Osmancik formation display interesting prospective trends in the Marmara Sea.

Oblique half-graben inversion of the Mesozoic Neuquén Rift in the Malargüe Fold and Thrust Belt, Mendoza, Argentina: New insights from analogue models

The Malargüe fold and thrust belt, located in the Andean mountains between 34°S and 36°30′S, formed in response to contraction during Cenozoic times. Its structural style and geometry was controlled by the Mesozoic rift system that formed the Neuquén basin in west-central Argentina. The rift architecture in the southern sector of this belt was previously interpreted in terms of the present N–S compressive structural trends, assuming the inversion of pre-existing normal faults with the same orientation. Here, we propose that the NW–SE-trending El Manzano–Liu Cullín lineament, located in the northern termination of the Sierra Azul, reflects the presence of a half-graben master fault in the subsurface. This hypothesis is supported by subsurface data, a balanced cross section, and it is tested using a series of scaled sandbox analogue models. We suggest that the lineament responds to a reactivated NW-trending half-graben fault, hidden by the mainly N–S-trending Andean structures. The proposed orientation is in agreement with the NE–SW extension developed in the Neuquén basin during the Triassic–Early Jurassic. The modeling of the inverted oblique half-graben reveals that the strikes of the main structures of inversion-related belts may often be independent of the orientation of the previously developed extensional system, providing a new perspective for their interpretation.

Ichnotaxonomy of bird-like footprints: an example from the Late Triassic-Early Jurassic of northwest Argentina

ABSTRACT The ichnotaxobases previously used to classify avian-like footprints, at the ichnogeneric, ichnospecific, and ichnofamily level, are varied and contrasting. Consequently, an agreement on the most adequate taxobases to use for classifying these vertebrate trace fossils is necessary. The authors follow an ichnotaxonomy treatment independent to the age, locality provenance, and possible tracemaker of the trace fossils. The ichnotaxobases used to classify tracks with avian affinities at ichnogeneric and ichnospecific levels are evaluated and a proposal is made for useful and appropriate ichnotaxonomic criteria, considering those currently in use. Previous criteria used to distinguish avian footprints from non-avian theropod or ornithischian tracks are discussed. These concepts are applied to the avian footprints from the upper part of the Late Triassic-Early Jurassic Santo Domingo Formation from La Rioja province, northwest Argentina, which has yielded a diverse assemblage of trace fossils. The most conspicuous avian footprint is Gruipeda dominguensis isp. nov. The ichnogenus Gruipeda Panin and Avram, 1962 is revised and an emendation of its diagnosis is suggested. Trisauropodiscus Ellenberger, 1972, from South Africa and Antarctichnus Covacevich and Lamperein, 1970 from Antarctica are considered as junior synonyms of Gruipeda. Three other morphotypes of avian footprints are left under open nomenclature: one is assigned as cf. Alaripeda isp., other as bird-like footprints type C, and the third bird-like footprint with elongated drag marks. These specimens could be related to avian origin, but the possibility of a case of convergence with birds is not discarded. The presence of tracks with a wide total divarication produced in ephemeral fluvial systems with shallow ponds and mudflats suggest that it is likely the attainment of a convergent avian-form feet to improve controlled movements.

Revision ofSemionotus bergeri Agassiz, 1833 (Upper Triassic, Germany), with comments on the taxonomic status ofSemionotus (Actinopterygii, Semionotiformes).

Semionotus bergeri Agassiz, 1833, from the Upper Triassic (Carnian) of southern Germany, the type species ofSemionotus, is here described in detail for the first time, including so far unstudied material in the Naturkunde-Museum Coburg, which allows a fairly complete reconstruction of this fish. The species is diagnosed by the presence of long frontals, about 4 times longer than wide, with antorbital lateral processes, 6–7 posterior infraorbitals, infraorbital at the posteroventral corner of the orbit notably larger than adjacent ones, infraorbitals at the posterior border of the orbit being as deep as long, and the middle element anteroventrally expanded, dorsal fin base about two times the anal fin base.S. elegans from the Late Triassic-Early Jurassic of the Newark Supergroup in North America, is so far the best know species ofSemionotus and is the only species of this genus that has been included in phylogentic analyses. However, the comparison ofS. bergeri with other proposed speciesof Semionotus, such asS. elegans, and other semionotids casts doubt on the monophyly of this genus as currently understood, since these species exhibit a mosaic distribution of characters.

Micropalaeontological evidence of brackish water conditions during deposition of the Knorringfjellet Formation, Late Triassic–Early Jurassic, Spitsbergen

The Knorringfjellet Formation is a succession of mudstones and sandstones, ranging in age from Norian to Toarcian in western and west-central Spitsbergen, and contains several unconformities with associated hiatuses. Its foraminiferal succession consists almost exclusively of agglutinated taxa, shows extremely low assemblage diversities and a dominance of small-sized species. These faunal features signalize restricted environmental conditions (in contrast to those of a normal marine shelf). Comparisons with ancient and modern analogues suggest that the main restricting factor was hyposaline conditions, to a lesser extent augmented by hypoxia in near-bottom waters. These conditions were caused by high fluvial influx creating gravity stratified water masses. The depositional area was part of an extensive but shallow shelf embayment, which had an open connection to the north to the polar ocean basin. Discontinuities recognized in the foraminiferal succession indicate depositional unconformities within the formation.

The Toarcian–Bathonian succession of the Antsiranana Basin (NW Madagascar): Facies analysis and tectono-sedimentary history in the development of the East Africa-Madagascar conjugate margins

The latest Early to Middle Jurassic succession of the Antsiranana Basin (NW Madagascar) records the complex transition from the continental rifting of Gondwana to the drifting of Madagascar-India from East Africa. The Madagascan Late Paleozoic–Mesozoic successions have been included in several paleogeographic and geodynamic models explaining the evolution of the Gondwana margins. Nevertheless, in some cases, as for the Toarcian–Bathonian deposits of the Antsiranana Basin, no significant stratigraphic revision has been carried out since the early 1970s. New field surveys allow reconsidering the stratigraphic and structural context and the palaeoenvironmental meaning of Toarcian–Bathonian successions occurring in different parts of the basin. These successions rest on the Triassic-Early Jurassic Isalo Sandstone which records pre-breakup rift events with a dominantly fluvial deposition. This situation is similar to other continental rift basins of Gondwana. After a regional Toarcian transgression the different portions of the Antsiranana Basin were characterized by significantly diversified and coeval depositional environments. The basin can be subdivided in a SW and NE part separated by a NW–SE trending structural high. In the SW part of the basin (Ampasindava sub-basin) the so-called “Jurassique paralique” [Rerat, J.C., 1964. Note sur les variations de faciès des sèries jurassiques du nord de Madagascar. Comptes Rendus Semaine gèologique, Tananarive, pp. 15–22] or “ Facies Mixtes de la Presqu’ile de Ampasindava” [Besairie, H., Collignon, M., 1972. Géologie de Madagascar; I. Les terrains sédimentaires. Annales Géologiques de Madagascar, 35, 1–463], a 1500 m thick prevalently terrigenous deposit, has been subdivided into four units. They document the long-lasting development of coastal–deltaic systems in a highly subsiding area. In the NE portion of the basin (Ankarana–Analamera sub-basin), a coeval mixed carbonate–terrigenous succession subdivided in five units for a total thickness of 500 m, was deposited during relative sea-level fluctuations in a ramp setting characterized by relatively lower subsidence. The stratigraphic-depositional evolution was dependant on the presence of NW-trending, actively growing highs which fed the south-western sub-basin. The clastic supply balanced the tectonically created accommodation space in this portion of the basin. The revised and extended paleogeographical reconstruction has been included into a breakup model of the East Africa-Madagascar rift during the opening of the Mozambique Channel.

Control of coupling among three major factors for formation of high-efficiency gas reservoir—A case study on the oolitic beach gas reservoir in Feixianguan Formation in the northeast Sichuan Basin

Through a case study of the high-efficiency gas reservoir in Feixianguan Formation in the northeast Sichuan Basin, quantitative and semi-quantitative analyses of key elements such as hydrocarbon generation, migration and accumulation, and reservoir evolution as well as their interplay in the critical moment of reservoir formation controlled by the energy field were carried out, by means of numerical modeling of the energy field. It was found that the climax time for Permian hydrocarbon generation was Late Triassic-Early Jurassic and accumulation of oil and gas has resulted in large-scale paleoreservoirs in paleostructural traps in Feixianguan Formation, a process facilitated by fractures connecting the sources. The paleoreservoirs have been turned into high-efficiency gas kitchens due to pyrolysis, which resulted from deep burial at a temperature of 170–210°C as induced by tremendously thick sedimentation in the foreland basin of Daba Mountain in Late Jurassic-Cretaceous period. Meanwhile, abundant acid gas like H2S produced from thermo-chemical sulfate reduction (TSR) at high temperatures leads to extensive dissolution of dolostone in the paleoreservoirs, which may in turn result in modification of the reservoirs and preservation of the reservoir rock porosity. The present distribution of gas reservoirs was ultimately determined in the processes of adjustment, cooling and decompression of the paleoreservoirs resulting from intense deformation in the front of Daba Mountain during the Himalayan orogeny.

Horizontal movements in the eastern Barents Sea constrained by numerical models and plate reconstructions

SUMMARY The eastern Barents Sea basins, west of Novaya Zemlya, were mildly inverted between Late Permian and Early Jurassic times, as indicated by mild folds in the basin sediments. Previous studies have suggested that the crustal part of Novaya Zemlya was thrust westward, but the magnitude of this compressive movement is not well known. Our aim is to provide an order-of-magnitude constraint on the amount of shortening associated with the displacement of Novaya Zemlya and inversion of the eastern Barents Sea basins by combining numerical models and plate reconstructions in an iterative process. We use a 2-D finite-element method to model inversion of a pre-defined basin. The total amount of shortening imposed on the models is first constrained by plate reconstructions for the Barents Sea region for the Late Palaeozoic to Early Mesozoic. We assume that the shortening is caused by westward movement of the Siberian plate, but the magnitude of this westward displacement in plate reconstructions is highly uncertain due to the allochthonous nature of the rocks of Novaya Zemlya and the scarcity of palaeomagnetic data in the region. Our models show that shortening localizes in the model basin and at Novaya Zemlya and that westward propagation of deformation is more efficient when the strength of the lower crust is reduced. Part of the movement of the Siberian plate could be accommodated by thrusts at Novaya Zemlya and perhaps in the domain of the Kara Sea at the western margin of the Siberian plate. By comparing the inversion obtained in the numerical models to the inferred inversion structures in the eastern Barents Sea basins we further constrain the amount of shortening that caused the inversion and therewith improve the plate reconstructions for the region. Our models indicate that the westward movement of Novaya Zemlya occurred in the Late Triassic–Early Jurassic (220–190 Ma) and was limited in magnitude to 100–200 km, which is considerably less than previous estimates (500–700 km).

Early Archean to Middle Jurassic Evolution of the Korean Peninsula and Its Correlation with Chinese Cratons: SHRIMP U‐Pb Zircon Age Constraints

U‐Pb zircon ages of tuffs and sandstones of the Daedong Supergroup (Bansong and Nampo groups) in the Korean Peninsula were determined using a sensitive high‐resolution ion microprobe (SHRIMP) in order to constrain their age of sedimentation and to unravel discrete geologic events as recorded in detrital zircons. The ages of four tuffaceous samples from the Bansong Group imply that the Daedong Supergroup formed at ca. 187–172 Ma in association with the Early‐Middle Jurassic orogeny. These data are in marked contrast with paleomagnetic arguments suggesting that the Bansong and Nampo groups are precollisional Early‐Middle Triassic deposits that are correlative with the North and South China blocks, respectively. Detrital zircons of the Daedong Supergroup define seven age components: (1) Early‐Middle Archean (3.64–2.97 Ga), (2) Late Archean–middle Early Proterozoic (2.63–2.33 Ga), (3) late Early Proterozoic (1.98–1.75 Ga), (4) Middle‐Late Proterozoic (1.2–0.6 Ga), (5) Devonian (400–355 Ma), (6) Early Permian (280–255 Ma), and (7) Middle Triassic–Early Jurassic (240–180 Ma). These age distributions, together with available geochronological data, suggest that crustal growth of the Korean Peninsula has continued since ca. 3.6 Ga and culminated at ca. 2.5 and 1.9–1.8 Ga. Major age populations of detrital zircons of the Bansong and Nampo groups are similar, except for the presence of Middle‐Late Proterozoic ages in the latter. Inasmuch as these ages are characteristic for the South China Block, the Gyeonggi massif, or at least the local source of the Nampo Group, is most likely a correlative of the South China Block.

The end Triassic mass extinction record of Williston Lake, British Columbia

Triassic–Jurassic boundary sections on the shore of Lake Williston, British Columbia record a spectrum of depositional environments from the western Canadian continental margin. A primarily deep-water, anoxic record of deposition, indicated by V/V + Ni, V/Cr, Th/U indices and pyrite framboid and facies data, is seen throughout the Late Triassic–Early Jurassic (Norian-Hettangian) interval in a section at Black Bear Ridge. Abundant Monotis bivalve populations in Norian strata are interpreted to have lived on the seabed during transient oxygenation events that punctuated the background anoxic conditions. Monotis went extinct around the Norian/Rhaetian boundary and the deep-water assemblages are subsequently dominated by thin-shelled pectinids. An intra-Rhaetian sea-level fall generated a major hiatus in the proximal Black Bear Ridge section whilst distinctive lowstand strata were developed in the more distal and more complete Ne Parle Pas Point and Pardonet Creek sections. These consist of shallow-water, phosphatic ooid sand facies that contain diverse bivalves. The majority of the infaunal bivalves (but not the epifaunal ones) disappeared at the acme of the regression. This is the end-Triassic mass extinction event and it occurred shortly before a terminal Triassic flooding event. This transgressive event is associated with a rapid negative shift of δ 13C org values interpreted to be the Initial Isotope Excursion of Hesselbo et al. [Hesselbo, S.P., Robinson, S.A., Surlyk, F., Piasecki, S., 2002. Terrestrial and marine extinction at the Triassic-Jurassic boundary synchronized with major carbon-cycle perturbation: a link to initiation of massive volcanism? Geology 30, 251–254]. The Rhaetian record of Williston Lake thus records an extinction of shallow-water infaunal bivalves during a lowstand prior to the development of a transgressive record that is associated with a major, negative C isotope.

Petrological and geochemical characteristics of metamorphic and igneous units from the allochthonous Madre de Dios Terrane, Southern Chile

The Denaro Complex, part of the Madre de Dios Terrane is composed of metamorphosed pillow basalts, metahyaloclastites, banded metalliferous and radiolarian metacherts, metapelites and redeposited calcareous metasandstones. The basaltic rocks show primary textures, minerals and structures. They are foliated especially in the vicinities of thrust faults, interpreted to have developed during the accretion of the terrane to the Gondwana margin. Composition of relic primary augite and chromite crystals plots into the MORB field of tectonic discriminant diagrams, as do the analyses of whole rock geochemistry, which indicates that these rocks are akin to volcanic rocks erupted along a constructive plate margin (N- and E-type MORBs), probably in a spreading axis-centered oceanic plateau or ridge. The metamorphic assemblages of pumpellyite–actinolite facies bear witness of metamorphism in a frontal accretionary wedge at elevated P and low T conditions, probably related to the Late Triassic–Early Jurassic Chonide event, which has been recognized elsewhere in the Patagonian Andes.

Tectonic evolution and paleogeography of the Mesozoic Pucará Basin, central Peru

The Pucará Basin of Peru is an elongate trough that subsided landward of a NNW-trending structural high during the Late Triassic–Early Jurassic. It formed as a postrift regional sag as the earlier Triassic fault-controlled Mitu rifts yoked together. The rift and transitional postrift basins were associated with a NW-striking sinistral shear zone that controlled isopachs and facies distributions and resulted in magmatism and mineralization along its trend. A distinct association of later dolomitization and MVT lead–zinc mineralization also occurs with these basin-forming shear zones. Although basaltic and andesitic extrusives are common, there is no evidence that the Pacific margin was a magmatic arc until the upper Pucará, and then only weakly developed in northern Peru. Except in the upper Pucará of northwest Peru, geochemical studies, including whole rock and trace element analyses, indicate that intercalations of volcanic material have intraplate rift affinities. The basin fill has a three-part stratigraphic subdivision, comprising lower and upper carbonate platforms with an intermediate phase of basin overdeepening and sediment starvation that resulted in a regional, organic-rich argillaceous drape. Stratigraphic accumulation was dominated by axial patterns of onlap and progradation, though facies characteristics show it was augmented by periodic flooding of the western basin margin high. Marine invertebrate fossils indicate normal marine salinities. The sedimentological interpretation is based on a SW–NE transect in the southern part of the Pucará Basin. The Chambará (Norian–Rhaetian) and Condorsinga (Toarcian) formations were constructed principally by shallow-water carbonate sedimentation in lagoon-like subtidal, intertidal, and supratidal paleoenvironments. The subtidal carbonate platform is dominated by oolitic grainstones with subordinate bioclastic packstones. Subordinate open-basin facies in the Chambará Formation consist principally of crinoidal packstones and bioclastic wackestones. In the intertidal and supratidal facies, evaporite pseudomorphs are common and generally associated with algal mats and widespread early diagenetic dolomitization. During the Chambará and Condorsinga, subsidence typically was balanced by carbonate production and shallow-water environments prevailed; the basin had the characteristics of an overfilled basin. Conversely, the intermediate late Rhaetian–Sinemurian stage of basin subsidence was marked by underfilled deep water conditions. This widespread transgressive inundation of the Pucará Basin, recorded in the argillaceous Aramachay stratigraphy, correlates with similar events in other Andean basins.

Basin evolution of the northern part of the Northeast German Basin — Insights from a 3D structural model

A 3D structural model for the entire southwestern Baltic Sea and the adjacent onshore areas was created with the purpose to analyse the structural framework and the sediment distribution in the area. The model was compiled with information from several geological time-isochore maps and digital depth maps from the area and consists of six post-Rotliegend successions: The Upper Permian Zechstein; Lower Triassic; Middle Triassic; Upper Triassic–Jurassic; Cretaceous and Cenozoic. This structural model was the basis for a 3D backstripping approach, considering salt flow as a consequence of spatially changing overburden load distribution, isostatic rebound and sedimentary compaction for each backstripping step in order to reconstruct the subsidence history in the region. This method allows determination of the amount of tectonic subsidence or uplifting as a consequence of the regional stress field acting on the basin and was followed by a correlation with periods of active salt movement. In general, the successions above the highly deformed Zechstein evaporites reveal a thickening trend towards the Glückstadt Graben, which also experienced the highest amount of tectonic subsidence during the Mesozoic and Cenozoic. Two periods of accelerating salt movement in the area has been correlated with the E–W directed extension during the Late Triassic–Early Jurassic and later by the Late Cretaceous–Early Cenozoic inversion, suggesting that the regional stress field plays a key role in halokinesis. The final part of this work dealt with a neotectonic forward modelling in an attempt to predict the future topography when the system is in a tectonic equilibrium. The result reveals that many of the salt structures in the region are still active and that future coastline will run with a WNW–ESE trend, arguing that the compressional stresses related to the Alpine collision are the prime factor for the present-day landscape evolution.

A new supercontinent self-destruct mechanism: evidence from the Late Triassic–Early Jurassic

We present a new conceptual model where supercontinents, by focusing subduction on narrow areas of the 670 km mantle discontinuity, trigger superplume events and initiate their own fragmentation. This supercontinent-triggered superplume mechanism for continental break-up is examined in light of the Mesozoic fragmentation of Pangaea–Gondwana. We summarize the evidence for a superplume event that occurred between 227 and 183 Ma ago, during the Late Triassic–Early Jurassic break-up of Pangaea–Gondwana. The evidence reviewed includes flood magmatism, kimberlite emplacement, plate reorganization and tectonism (including ophiolite obduction events), reversal rate frequency of the geomagnetic field, marine anoxia, deposition of carbon-rich sediments, including oil source rocks and coal, the carbon isotope record, major mass extinctions, and global sea levels. This Late Triassic–Early Jurassic superplume event was comparable in scale with those in the late Proterozoic ( c . 800 Ma) and during Cretaceous times ( c . 120–80 Ma). Similar to the mid-Cretaceous event, an extended phase of plume magmatism is implicated, with two oceanic and three continental large igneous provinces being emplaced.

Sm–Nd dating and rare-earth element tracing of calcite: Implications for fluid-flow events in the Bowen Basin, Australia

Late Permian coals and coal measures in the Bowen Basin, Australia, are extensively mineralised with carbonates as cleat fillings and veins. The calcites in coal samples exhibit large variations in rare-earth element (REE) patterns and Sm–Nd ratios with some superchondritic values. Samples enriched in heavy REE (HREE) relative to light REE (LREE) define an Sm–Nd isochron corresponding to an age of 235 ± 15 Ma, which is consistent with the timing of final contractional phase of the Hunter–Bowen orogeny. The corresponding initial εNd is + 1.2 combined with HREE-enriched patterns suggests significant fluid interaction with primitive source rocks, which are consistent with arc-related volcanogenic sediments in the basin. Calcites characterised by HREE depletion relative to LREE are not in Sm–Nd isotopic equilibrium with the former samples that is interpreted to be due to the effect of a later hydrothermal event during the Late Triassic–Early Jurassic. Vein calcites in mudrocks from the coal measures display more pronounced HREE depletion and significant positive Eu CN anomalies, which is related to the later hydrothermal event. REE distributions, together with the basin burial history, suggest that physico-chemical environments of fluid-flow events during the two separate major tectonic regimes were significantly different. This study shows that a combined application of Sm–Nd dating and REE geochemical tracing of authigenic carbonate minerals is very useful to constrain the evolution of multiple fluid-flow events in a sedimentary basin with a complex history.

The late Rhaetian transgression in southern Sweden: Regional (and global) recognition and relation to the Triassic–Jurassic boundary

Palynological investigations of Rhaetian outcrops and drillcores in combination with subsurface data from wells in Scania, southern Sweden, have revealed the presence of marine dinoflagellate cysts in sedimentary strata that were previously considered to have been deposited in a mainly terrestrial environment. Two distinct dinocyst events are identified. One older event where persistent, rare to common occurrences of Rhaetogonyaulax rhaetica and Lunnomidinium scaniense indicates deposition in shallow marine and marginal marine environments. This Lunnomidinium interval is preceded and succeeded by assemblages with rare to common R. rhaetica, and can be correlated with the mid to late Rhaetian R. rhaetica Zone. It is associated with spore/pollen assemblages of the mid Rhaetian Rhaetipollis– Limbosporites Zone. The younger event, the R. rhaetica maximum interval, is characterised by mass-occurrence of R. rhaetica, with less common or absent Dapcodinium priscum, in association with a dark grey to black mudstone/shale. The introduction of D. priscum in association with the over-whelming abundance of R. rhaetica allows correlation with the transition between the R. rhaetica Zone and the succeeding D. priscum Zone, and signals fully marine conditions. The R. rhaetica maximum interval is associated with spore/pollen assemblages of the late Rhaetian Ricciisporites– Polypodiisporites Zone, and is in Sweden succeeded both lithostratigraphically and palynostratigraphically by the T–J transition. The R. rhaetica maximum interval corresponds to a late Rhaetian maximum flooding event that took place in a shallow, gently sloping embayment that covered the Danish Basin area during the Late Triassic–Early Jurassic. This maximum flooding event can be recognised in late Rhaetian strata from different parts of the Northern Hemisphere. It is an important temporal constraint on the T–J boundary as it is associated with the disappearance of Triassic marine faunas, appears to coincide with the onset of the main pulse of CAMP volcanism, immediately precedes the initial carbon isotope excursion and the global sea-level drop that characterises the T–J transition.

Spiny Norman in the Garden of Eden? Dispersal and early biogeography of Placentalia

The persistent finding of clades endemic to the southern continents (Afrotheria and Xenarthra) near the base of the placental mammal tree has led molecular phylogeneticists to suggest an origin of Placentalia, the crown group of Eutheria, somewhere in the southern continents. Basal splits within the Placentalia have then been associated with vicariance due to the breakup of Gondwana. Southern-origin scenarios suffer from several problems. First, the place of origin of Placentalia cannot be reconstructed using phylogenetic reasoning without reference to outgroups. When available outgroups are considered, a Laurasian origin is most parsimonious. Second, a model of pure vicariance would require that basal placental splits occurred not with the breakup of Gondwana, but of Pangea in the Late Triassic—Early Jurassic. This event long preceded even the oldest molecular divergence estimates for the Placentalia and was coeval only with the earliest mammals in the fossil record. Third, a problem with the number of dispersal events that would be required emerges under different southern-origin scenarios. In considering the geographic distribution of the major placental clades at their first appearance (mostly Early Cenozoic), it becomes clear that a Laurasian center of origin would require fewer dispersal events. Southern-origin models would require at least twice the number of dispersal events in comparison with a model of Laurasian origins. This number of required dispersal events increases if extinct groups of placental mammals are also considered. Results are similar assuming a morphology-based phylogeny. These facts, along with earlier findings speaking against a major placental radiation deep in the Cretaceous without leaving fossil evidence, suggest an origin of Placentalia somewhere in Laurasia with few supraordinal splits occurring before the last 5–10 million years of the Cretaceous.

Mesozoic-Cenozoic Deformation of the Central Structure Belt in the Turpan-Hami Basin, Northwest China: Tectonic Evolution of an Intracontinental Basin, Central Asia

The central structure belt (CSB) in the Turpan-Hami Basin is composed of the Huoyanshan structure and the Qiketai structure, formed in the Late Triassic—Early Jurassic, and is characterized by extensional tectonics. The thickness of strata in the hanging wall of the fault is clearly larger than that in the footwall, and a deposition center evolved in the Taibei sag where the hanging wall of the fault is located. In the Late Jurassic, collision between the Lhasa block and the Eurasia continent resulted in the transformation of the Turpan-Hami Basin from an extensional into a compressional structure, and consequently in the tectonic inversion of the CSB from an extensional normal fault in the earlier stage to a compressive thrust fault in the later stage. Tertiary collision between the Indian and Eurasian plates occurred around 55 Ma, and this Himalayan orogenic event played a profound role in shaping the Tianshan area. However, the effect of the collision on this area was delayed and culminated approximately in Late Oligocene—Early Miocene time. The CSB was strongly deformed and thrust upward as a result of this tectonic event.

Detrital Zircon Data from the Eastern Mixteca Terrane, Southern Mexico: Evidence for an Ordovician—Mississippian Continental Rise and a Permo-Triassic Clastic Wedge Adjacent to Oaxaquia

The eastern part of the Mixteca terrane of southern Mexico is underlain by the Petlalcingo Group (part of the Acatlán Complex), and has been interpreted as either a Lower Paleozoic passive margin, or a trench/forearc sequence deposited in either the Iapetus or Rheic oceans. The group, from bottom to top, consists of: (1) the Magdalena Migmatite protolith (metapsammites, metapelites, calsilicates, and marbles), which grades up into (2) the meta-psammitic Chazumba Formation; overthrust by (3) the Cosoltepec Formation (phyllites and quartzites with minor mafic meta-volcanic horizons). The group is unconformably overlain by the Pennsylvanian—Middle Permian Tecomate Formation, which is overthrust by the ∼288 Ma Totoltepec pluton and unconformably overlain by Middle Jurassic rocks. In contrast to previous inferences that the protoliths of the units (1) to (3) were early Paleozoic in age, detrital zircon LA-ICPMS ages combined with published data constrain depositional ages as follows: (i) Magdalena Migmatite protolith: post-303 Ma-pre-171 Ma (Permian—Early Jurassic); (ii) Chazumba Formation: post—239 Ma—pre-174 Ma (Middle Triassic—Early Jurassic); and (iii) Cosoltepec Formation: post—455 Ma—pre-310 Ma (uppermost Ordovician-Mississippian). Given the different ages and depositional environments of the Cosoltepec Formation versus the Chazumba Formation and Magdalena protolith, we recommend redefining the Chazumba and Magdalena as lithodemes grouped in the Petlalcingo Suite and excluding the Cosoltepec Formation. Detrital zircons in all three units show a population peak at ∼850-1200 Ma, suggesting derivation from the adjacent ∼1 Ga Oaxacan Complex. A ∼470-640 Ma peak is limited to the Cosoltepec Formation whose source may be found in ∼470 Ma plutons in the Acatlan Complex, beneath the Yucatan Peninsula, and in the Brasiliano orogens of South America. The inferred turbiditic protolith of the Chazumba Formation and Magdalena protolith suggests that it represents a clastic wedge deposited in front of S-verging Permo-Triassic thrusts on the western margin of Pangea. The mainly oceanic affinity of the basalts in the Cosoltepec Formation suggests deposition of sedimentary protoliths in a continental rise fringing Oaxaquia. These data are more consistent with deposition of the Cosoltepec Formation in the Rheic Ocean than in the Iapetus Ocean.