Triassic Early Jurassic Times Research Articles

New evidence for a long Rhaetian from a Panthalassan succession (Wrangell Mountains, Alaska) and regional differences in carbon cycle perturbations at the Triassic-Jurassic transition

The end-Triassic mass extinction is one of the big five extinction events in Phanerozoic Earth history. It is linked with the emplacement of the Central Atlantic Magmatic Province and a host of interconnected environmental and climatic responses that caused profound deterioration of terrestrial and marine biospheres. Current understanding, however, is hampered by (i) a geographically limited set of localities and data; (ii) incomplete stratigraphic records caused by low relative sea-level in European sections during the Late Triassic and earliest Jurassic; and (iii) major discrepancies in the estimated duration of the latest Triassic Rhaetian that limit spatiotemporal evaluation of climatic and biotic responses locally and globally. Here, we investigate the Late Triassic–Early Jurassic time interval from a stratigraphically well-preserved sedimentary succession deposited in tropical oceanic Panthalassa. We present diverse new data from the lower McCarthy Formation exposed at Grotto Creek (Wrangell Mountains, southern Alaska), including ammonoid, bivalve, hydrozoan, and conodont biostratigraphy; organic carbon isotope (δ13Corg) stratigraphy; and CA-ID TIMS zircon U–Pb dates. These data are consistent with a Norian-Rhaetian Boundary (NRB) of ∼209 Ma, providing new evidence to support a long duration of the Rhaetian. They also constrain the Triassic-Jurassic boundary (TJB) to a ∼6 m interval in the section. Our TJB δ13Corg record from Grotto Creek, in conjunction with previous data, demonstrates consistent features that not only appear correlative on a global scale but also shows local heterogeneities compared to some Tethyan records. Notably, smaller excursions within a large negative carbon isotope excursion [NCIE] known from Tethyan localities are absent in Panthalassan records. This new comparative isotopic record becomes useful for (i) distinguishing regional overprinting of the global signal; (ii) raising questions about the ubiquity of smaller-scale NCIEs across the TJB; and (iii) highlighting the largely unresolved regional vs. global scale of some presumed carbon cycle perturbations. These paleontological and geochemical data establish the Grotto Creek section as an important Upper Triassic to Lower Jurassic succession due to its paleogeographic position and complete marine record. Our record represents the best documentation of the NRB and TJB intervals from Wrangellia, and likely the entire North American Cordillera.

The Sidi El Hemissi Triassic “ophites” (Souk Ahras, NE Algeria): petrology, geochemistry, and petrogenesis

The Sidi El Hemissi region, Souk-Ahras, is part of the Tellian Atlas where the Triassic sediments tectonically outcrop under the Tellian nappes of the Maghrebide chain. Mafic rocks, mainly gabbros and dolerites, called “ophites,” are interbedded as a ~ 200 × 30-m lenticular body within the Triassic gypsum-rich formation. These rocks show granular, microgranular, and microlithic textures and are composed of plagioclase, amphibole, pyroxene, and scarce olivine crystals. Albitization is the main alteration process, though chloritization, calcitization, and epidotization of ferromagnesian minerals can also be occasionally observed. The major, trace, and rare earth element studies show that these mafic rocks display relatively low P2O5 (less than 0.2 wt%) and moderate to low TiO2 contents (less than 2 wt%) and exhibit low-Ti continental tholeiitic basalt affinity. They are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) compared with high field strength elements (HFSE) and heavy rare earth elements (HREE). This, along with the observed weak Nb and Ce anomalies and the low-Ti contents, suggests an enriched mantle source for the generation of the magma, which likely underwent crustal contamination before emplacement within the Triassic sediments. The petrological and geochemical features of the Sidi El Hemissi ophites show many similarities with the basaltic rocks emplaced during the Late Triassic–Early Jurassic times, now cropping out in north-western Africa, south-western Europe, north-eastern, and south-eastern America. This large magmatic activity is believed to be related to the continental rifting associated with the early stages of the Pangea breakup.

Exhumation history of the West Kunlun Mountains, northwestern Tibet: Evidence for a long-lived, rejuvenated orogen

How and when the northwestern Tibetan Plateau originated and developed upon pre-existing crustal and topographic features is not well understood. To address this question, we present an integrated analysis of detrital zircon U–Pb and fission-track double dating of Cenozoic synorogenic sediments from the Kekeya and Sanju sections in the southwestern Tarim Basin. These data help establishing a new chronostratigraphic framework for the Sanju section and confirm a recent revision of the chronostratigraphy at Kekeya. Detrital zircon fission-track ages present prominent Triassic–Early Jurassic (∼250–170 Ma) and Early Cretaceous (∼130–100 Ma) static age peaks, and Paleocene–Early Miocene (∼60–21 Ma) to Eocene–Late Miocene (∼39–7 Ma) moving age peaks, representing source exhumation. Triassic–Early Jurassic static peak ages document unroofing of the Kunlun terrane, probably related to the subduction of Paleotethys oceanic lithosphere. In combination with the occurrence of synorogenic sediments on both flanks of the Kunlun terrane, these data suggest that an ancient West Kunlun range had emerged above sea level by Triassic–Early Jurassic times. Early Cretaceous fission-track peak ages are interpreted to document exhumation related to thrusting along the Tam Karaul fault, kinematically correlated to the Main Pamir thrust further west. Widespread Middle–Late Mesozoic crustal shortening and thickening likely enhanced the Early Mesozoic topography. Paleocene–Early Eocene fission-track peak ages are presumably partially reset. Limited regional exhumation indicates that the Early Cenozoic topographic and crustal pattern of the West Kunlun may be largely preserved from the Middle–Late Mesozoic. The Main Pamir–Tam Karaul thrust belt could be a first-order tectonic feature bounding the northwestern margin of the Middle–Late Mesozoic to Early Cenozoic Tibetan Plateau. Toward the Tarim basin, Late Oligocene–Early Miocene steady exhumation at a rate of ∼0.9 km/Myr is likely related to initial thrusting of the Tiklik fault and reactivation of the Tam Karaul thrust. Thrusting together with upper crustal shortening in the mountain front indicates basinward expansion of the West Kunlun orogen at this time. This episode of exhumation and uplift, associated with magmatism across western Tibet, is compatible with a double-sided lithospheric wedge model, primarily driven by breakoff of the Indian crustal slab. Accelerated exhumation of the mountain front at a rate of ∼1.1 km/Myr since ∼15 Ma supports active compressional deformation at the margins of the northwestern Tibetan Plateau. We thus propose that the West Kunlun Mountains are a long-lived topographic unit, dating back to Triassic–Early Jurassic times, and have experienced Middle–Late Mesozoic to Early Cenozoic rejuvenation and Late Oligocene–Miocene expansion.

Detrital zircon U–Pb geochronology, Hf isotopes and geochemistry constraints on crustal growth and Mesozoic tectonics of southeastern China

In situ U–Pb geochronological, Hf isotopic and REE geochemical analyses of detrital zircons from Mesozoic sandstones in central Jiangxi and east Hunan of the South China Block (SCB) are used to provide not only information about crustal evolution process, but more importantly new constraints on sedimentary provenances as well as Mesozoic basin evolutions in central SCB. A total of 417 concordant zircon analyses define five U–Pb age populations at ca. 2.6–2.3Ga, ca. 2.0–1.7Ga, ca. 1.0–0.7Ga, ca. 500–400Ma and ca. 300–200Ma. Integrated analyses of zircon U–Pb ages and Hf isotopes of detrital zircons reveal five episodes of juvenile continental crust growth: ca. 2.5Ga, ca. 1.7Ga, ca. 850Ma, ca. 440Ma and ca. 230Ma, with all but the ca. 2.5Ga episode likely represent that of the SCB.None of the three samples from T3–J1 strata showed strong ca. 1850Ma and ca. 230Ma peaks suggesting that the T3–J1 sediments probably sourced from the Yangtze Block. In contrast, stronger peaks of ca. 1850Ma and ca. 250–230Ma in post-J1 strata relative to that of T3–J1 strata suggest a dominantly local Cathaysian provenance. In addition, the distinct low εHf(T) values for zircons of ca. 430Ma from T3–J1 strata in comparison with higher εHf(T) values for that from post-J1 strata also support aforementioned viewpoint. Such sharp changes between the pre-J1 and post-J1 strata coincide with the remarkable change in regional palaeogeography from a broad shallow marine basin in the Late Triassic–Early Jurassic time to a basin-and-range-style province in the Middle Jurassic. The characteristics and time evolution of detrital zircons from the studied area are consistent with the flat-slab subduction model which involves the development of a broad sag basin during the T3–J1 time, and a rapid regional uplift in the Cathaysia Block caused by the reinitiating of normal subduction along the coastal region at ca. 190Ma.

Early Mesozoic Southern Mexico–Amazonian connection based on U–Pb ages from detrital zircons: The La Mora Paleo-River in the Mixteca Terrane and its paleogeographic and tectonic implications

The La Mora Formation is the oldest Mesozoic floodplain succession in the Mixteca Terrane of Southern Mexico. The presence of Amazonian detrital zircons in the La Mora Formation and in the overlying volcanic Diquiyú Unit indicates a major fluvial system that drained the Mixteca Terrane. The La Mora Paleo-River crossed the Oaxaquia microcontinent and the Mixteca Terrane prior to the breakup of Pangea, during Late Triassic–Early Jurassic time, when the Acatlán–Oaxaquia block was part of the northwestern portion of the Amazonian craton. Detrital zircons in the La Mora Formation have ages between 3307±31 and 210±12Ma, which suggest that they originated in Amazonia and Southern Mexico: 33.2% of the detrital zircons have ages that are found only in Amazonian sources; whereas 66.7% of the zircons may be associated with either the Amazonian craton, the Andean Basement, or Southern Mexico. We propose that the La Mora fluvial system drained the Amazon basin in a westward direction, with its mouth in central Pangea, and that it most likely fed the Tolimán submarine fan. The inferred location of the Acatlán–Oaxaquia tectonic block at the time of the La Mora fluvial system implies that the basement of Southern Mexico experienced a right lateral displacement of at least 2300km with respect to South America during the Mesozoic.

Cimmerian evolution of the Central Iranian basement: Evidence from metamorphic units of the Kashmar–Kerman Tectonic Zone

The Kashmar–Kerman Tectonic Zone (KKTZ) is an arcuate zone located between the Tabas and Yazd blocks in Central Iran. Detailed structural analysis and mapping of the metamorphic basement rocks of the KKTZ on the Boneh-Shurow, Tashk, Saghand and Sarkuh Metamorphic Complexes, as well as 40Ar/39Ar cooling ages on the Boneh-Shurow Metamorphic Complex indicate three deformation stages in the tectonometamorphic evolution of the KKTZ during the Cimmerian orogeny. The D1–1 event corresponds to continental accretion through the progressive formation of dextral shear zones, which is equivalent to an Early Cimmerian event during Late Triassic–Early Jurassic times. The D1–2 event is characterized by top-to-NE normal shear zones due to syn-collisional exhumation within the KKTZ and is considered to be a Mid-Cimmerian Middle Jurassic event at ca. 168Ma. The progressive regional compressional deformation is continued by the D2–1 and D2–2 events through formation of reverse shear zones and faults, with their SW movement. The progressive D2 event is considered to be Late Cimmerian and has taken place during Early Cretaceous. It is proposed that the mapped long-lived dextral shear zones within the KKTZ acted as block bounding faults between the Tabas and Yazd blocks in Central Iran.

Magnetic fabric and its significance in the sills and lava flows from Taimyr fold-belt, Arctic Siberia

Magnetic fabric research on the emplacement of the Taimyr fold-belt is still absent. Therefore, we have performed magnetic fabric studies on samples collected from 23 sites from mafic igneous rocks (75°N, 100°E) in South Taimyr. This study indicates a correlation between the anisotropy of magnetic susceptibility (AMS), magma flow and fold-related compression in the rocks. AMS measurements on 183 unheated and 122 heated samples reveal the magma flow directions. The magma flow direction is mainly parallel to the ESE–WNW and secondarily parallel to the SSW from the NNE in the mafic sills. The unheated and heated basaltic flows reveal a NE-trending flow vector. In some of the sills, AMS shows a tight clustering of the maximum K 1 axes close to the bedding pole, which is not thought to define the flow direction. Corresponding AMS measurements on igneous rocks allow us to infer the existence of magnetic fabrics of tectonic origin linked to the main folding episode that occurred subsequent to the Mid-Triassic magmatic event in latest Triassic–Early Jurassic times. In the sills, the distribution of most maximum K 1 axes is close to NS that corresponds to the maximum compressive stress or folding directions; contrarily, the minimum K 3 axes have an elongated distribution along the EW direction or parallel to the fold axis before exchanging K 1 and K 3 axes for inverse fabrics, but close to the NS stress direction after exchanging K 1 and K 3 axes for inverse fabrics. In the basaltic flows, the minimum K 3 axes almost parallel to the NS folding direction. The structural interpretation of all AMS data taken from the igneous bodies is in accordance with a NNW–SSE stress-related folding taking place around 198 Ma in the Taimyr Peninsula (Arctic Siberia). The relationship of susceptibility axes to bedding surfaces and magnetic foliation planes is a criterion that permits differentiation of normal magnetic fabrics from inverse fabrics in the igneous samples. Single-domain (SD) and small pseudo-single-domain (PSD) magnetite crystals in the sill samples are the main carriers of the AMS fabrics, implying the abundance of inverse magnetic fabrics. PSD and multidomain (MD) titanomagnetite (TM) grains as well as minor hematite are identified in the basalt flows, suggesting the occurrence of more normal magnetic fabrics in the basalts than in the sills.

Shallow burial dolomitisation of Middle–Upper Permian paleosols in an extensional tectonic context (SE Iberian Basin, Spain): Controls on temperature of precipitation and source of fluids

This work is focused on carbonate paleosols developed in three stratigraphic sections (Landete, Talayuelas and Henarejos) of the Middle–Late Permian Alcotas Formation in the SE Iberian Basin. The Alcotas Formation, of alluvial origin, was deposited in semi-connected half-grabens developed during the early stages of the Permian–Triassic rifting stage that affected the Iberian Basin. The studied sections were located in two of these half-grabens, the Henarejos section being much closer to the basin boundary fault than the other two sections. The mineralogy and texture of the carbonate precursor of paleosols in the three studied sections are not preserved because original carbonate is replaced by coarse crystals of dolomite and/or magnesite. Dolomite crystals are typically euhedral, displaying rhombohedral shapes and reddish luminescence, although in the Henarejos section dolomite displays non-planar boundaries and frequently saddle habit. Micas are deformed and adapted to dolomite crystals, which, in turn, are affected by stylolites, suggesting that dolomite precipitated before mechanical and chemical compaction. Carbon and oxygen isotopic compositions of dolomite from the three sections show different values (δ 13C VPDB mean values = − 6.7‰, − 5.5‰ and − 7.5‰; δ 18O VPDB mean values = − 4.0‰; –5.6‰ and − 8.2‰, at Landete, Talayuelas and Henarejos sections, respectively). The 87Sr/ 86Sr ratios are similar in the three sections yielding values between 0.71391 and 0.72213. The petrographic and geochemical features of dolomite in the three studied sections suggest precipitation from similar fluids and during shallow burial diagenesis. Assuming that the minimum temperature for dolomite precipitation in the Henarejos section was 60 °C (as suggested by the presence of non-planar saddle habit), and that the dolomitizing fluid had similar δ 18O values at the three localities, then dolomite in the Talayuelas and Landete sections precipitated at temperatures around 16 and 25 °C cooler, respectively. In addition, the δ 18O VSMOW values of the water from which dolomite precipitated would have ranged between − 0.3 and − 2.9‰. Dolomite is partially or totally replaced by non- to dark dull luminescent magnesite in the Landete and Talayuelas sections. Magnesite crystals are affected by stylolites, indicating that it precipitated before chemical compaction. The δ 13C mean values are − 6.5 and − 6.0‰ and the δ 18O VPDB mean values are − 6.7 and − 7.8‰, in the Landete and Talayuelas sections, respectively. The 87Sr/ 86Sr ratios of magnesite are similar in both sections yielding values between 0.71258 and 0.72508. This suggests that they probably precipitated from similar fluids during progressive burial and at higher temperatures than dolomites at the same sections. Assuming that magnesite precipitated from a fluid with similar δ 18O values in both sections, then it had to precipitate at a temperature around 8 °C higher in Talayuelas than in the Landete section. Dolomitisation and magnesite precipitation probably occurred via reflux of saline to hypersaline brines from the overlying Mid-Late Triassic Muschelkalk and/or Keuper facies. The temperatures inferred for dolomite precipitation, however, are too high for shallow burial if a normal geothermal gradient is applied. Thus, it can be inferred that saline fluids were heated as they flowed through the syn-sedimentary extensional faults that controlled Middle Permian to Middle Triassic sedimentation; consequently fluids would have been at higher temperatures near the Henarejos area, which was closer to the basin boundary fault than at the Talayuelas and Landete areas, which were situated further away. This contention is in agreement with recent studies which demonstrate that an important thermal event took place during Late Triassic–Early Jurassic times in the Iberian Peninsula.

Thermochronological constraints on two-stage extrusion of HP/UHP terranes in the Dabie–Sulu orogen, east-central China

Along the Qinling–Dabie–Sulu orogenic belt in China crops out the world's largest terrane composed of ultrahigh-pressure (UHP) metamorphic rocks. Differences in the timing and mechanisms of oceanic and continental subductions are assumed to be responsible for different ages of high-pressure (HP) and UHP slices in different parts of the belt. The western part of the Dabie orogen (western Dabie terrane) holds a key to understanding of the transition from oceanic to continental subduction. This paper reports geochronological results to test a two-stage tectonic model for the exhumation of HP/UHP rocks in western Dabie. This model involves two different stages and types of extrusion for exhumation of the HP/UHP rocks in east-central China. Mica Ar/Ar ages, ranging from 241 to 231 Ma, indicate a general middle Triassic cooling probably driven by early upward extrusion during the collision between the North and South China Blocks. Late Triassic–Early Jurassic cooling was associated with later eastward extrusion, ranging from 200 to 184 Ma. The second event is recorded also in mica in the region that was not affected by later deformation and magmatism. The lateral movement along lithosphere-scale faults resulted in the eastward extrusion of the HP–UHP metamorphic terrane, which was followed, in the Late Triassic–Early Jurassic time, by a major compressive event. These two extrusion events are correlative with the two stages of Triassic exhumation of the western Dabie HP–UHP rocks, respectively. Wintin the framework of the Qinling–Dabie–Sulu orogenic belt, it is suggested for western Dabie that the subduction/exhumation of blueschist-facies unit is related to the Mianlue suture, whereas the subduction/exhumation of HP/UHP eclogite-facies units is related to the Shangdan suture.

Petrascula iberica (DRAGASTAN & TRAPPE), Tersella genotii BARATTOLO & BIGOZZI, and the relationships of club-shaped dasycladalean algae during Late Triassic-Early Jurassic times

New observations on abundant material from the type locality of Petrascula iberica (DRAGASTAN & TRAPPE) GRANIER et al. 2003, reveal the presence of a reproductive structure (ampulla) located on the inner-middle part of each primary lateral. This fact suggests a new interpretation of the alga and the establishment of a new genus: Granieria BARATTOLO & ROMANO, n. gen. Also, the taxonomic status of the other club-shaped specimens from both the High and Middle Atlas of Morocco and the Central Apennines of Italy is reviewed.

Mesozoic tectonic evolution of the Southeast China Block: New insights from basin analysis

In order to better understand the Mesozoic tectonic evolution of Southeast China Block (SECB in short), this paper describes geological features of Mesozoic basins that are widely distributed in the SECB. The analyzed data are derived from a regional geological investigation on various Mesozoic basins and a recently compiled 1:1,500,000 geological map of Mesozoic–Cenozoic basins. Two types of basin are distinguished according to their tectonic settings, namely, the post-orogenic basin (Type I) and the intracontinental extensional basin (Type II); the latter includes the graben and the half-graben or faulted-depression basins. Our studies suggest that the formation of these basins connects with the evolution of geotectonics of the SECB. The post-orogenic basin (Type I) was formed in areas from the piedmont to the intraland during the interval from Late Triassic to Early Jurassic; and the formation of the intracontinental extensional basin (Type II) connects with an intracontinental crustal thinning setting in the Late Mesozoic. The graben basin was generated during the Middle Jurassic and is associated with a bimodal volcanic eruption; and the half-graben or faulted-depression basin, filled mainly by the rhyolite, tuff and sedimentary rocks during Early Cretaceous, is occupied by the Late Cretaceous–Paleogene red-colored terrestrial clastic rocks. We noticed that the modern outcrops of numerous granites and basins occur in a similar level, and the Mesozoic granitic bodies contact with the adjacent basins by large normal faults, suggesting that the modern landforms between granites and basins were yielded by the late crustal movement. The modern basin and range framework was settled down in the Cretaceous. Abundant sedimentary structures are found in the various basins, from that the deposited environments and paleo-currents are concluded; during the Late Triassic–Early Jurassic time, the source areas were situated to the north and northeast sides of the outcrop region. In this paper, we present the study results on one geological and geographical separating unit and two separating fault zones. The Wuyi orogenic belt is a Late Mesozoic paleo-geographically separating unit, the Ganjiang fault zone behaves as the western boundary of Early Cretaceous volcanic rocks, and the Zhenghe–Dapu fault zone separates the SE-China Coastal Late Mesozoic volcanic-sedimentary basins and the Wuyi orogenic belt. Finally, we discuss the geodynamic mechanisms forming various basins, proposing a three-stage model of the Mesozoic sedimentary evolution.

Two layer model of lithospheric compression and uplift/exhumation in an intracratonic setting: an example from the Cooper–Eromanga Basins, Australia

Seismic reflection profiles indicate the compressive nature of the structural style associated with the major uplift events in the Cooper–Eromanga Basins. Inversion geometries and reactivated features attest to a period of compression during Late Triassic–Early Jurassic times. In the Eromanga Basin, compressional structural styles associated with Late Cretaceous–Tertiary are apparent. Many of the Late Cretaceous–Tertiary structures coincide with exhumation highs in Late Cretaceous–Tertiary times. The two-layer lithospheric compression model is considered as the most complete explanation of both the uplift of areas subject to compression and crustal thickening, and of the regional uplift of areas not subject to any apparent Late Cretaceous–Tertiary compression. In the model, compression and thickening in the lower lithosphere is decoupled and laterally displaced from that in the upper crust. Thickening of the mantle lithosphere without thickening of the overlying crust can account for the initial subsidence then uplift of not inverted platform areas. The opening of the Tasman Sea and the Coral Seas can lead to stress transmission in the interior of the continent. These stresses are likely to generate uplift but cannot explain the distribution of uplift in areas not subject to compression.

Detrital record of Mesozoic shortening in the Yanshan belt, NE China: testing structural interpretations with basin analysis

ABSTRACTThe Yanshan fold‐thrust belt is an exposed portion of a major Mesozoic orogenic system that lies north of Beijing in northeast China. Structures and strata within the Yanshan record a complex history of thrust faulting characterized by multiple deformational events. Initially, Triassic thrusting led to the erosion of a thick sequence of Proterozoic and Palaeozoic sedimentary strata from northern reaches of the thrust belt; Triassic–Lower Jurassic strata that record this episode are deposited in a thin belt south of this zone of erosion. This was followed by postulated Late Jurassic emplacement of a major allochthon (the Chengde thrust plate), which is thought to have overridden structures and strata associated with the Triassic event and is cut by two younger thrusts (the Gubeikou and Chengde County thrusts). The Chengde allochthon is now expressed as a major east–west trending, thrust‐bounded synform (the Chengde synform), which has been interpreted as a folded klippe 20 km wide underlain by a single, north‐vergent thrust fault. Two sedimentary basins, defined on the basis of provenance, geochronology and palaeodispersal trends, developed within the Yanshan belt during Late Jurassic–Early Cretaceous time and are closely associated with the Chengde thrust and allied structures. Shouwangfen basin developed in the footwall of the Gubeikou thrust and records syntectonic unroofing of the hanging wall of that fault. Chengde basin developed in part atop Proterozoic strata interpreted as the upper plate of the Chengde allochthon and records unroofing of the adjacent Chengde County thrust. Both the Chengde County thrust and the Gubeikou thrust are younger than emplacement of the postulated Chengde allochthon, and structurally underlie it, yet neither Shouwangfen basin nor Chengde basin contain a detrital record of the erosion of this overlying structure. In addition, facies, palaeodispersal patterns and geochronology of Upper Jurassic strata that are cut by the Chengde thrust suggest only limited (ca. 5 km) displacement along this fault. We suggest that the units forming the Chengde synform are autochthonous, and that the synform is bounded by two limited‐displacement faults of opposing north and south vergence, rather than a single large north‐directed thrust. This conclusion implies that the Yanshan belt experienced far less Late Jurassic shortening than was previously thought, and has major implications for the Mesozoic evolution of the region. Specifically, we argue that the bulk of shortening and uplift in the Yanshan belt was accomplished during Triassic–Early Jurassic time, and that Late Jurassic structures modified and locally ponded sediments from a well‐developed southward drainage system developed atop this older orogen. Although Upper Jurassic strata are widespread throughout the Yanshan belt, it is clear that these strata developed within several discrete intermontane basins that are not correlable across the belt as a single entity. Thus, the Yanshan has no obvious associated foreland basin, and determining where the Mesozoic erosional products of this orogen ultimately lie is one of the more intriguing unresolved questions surrounding the palaeogeography of North China.

Burial/exhumation histories for the Cooper–Eromanga Basins and implications for hydrocarbon exploration, Eastern Australia

ABSTRACTThe Cooper–Eromanga Basins of South Australia and Queensland are not at their maximum burial depth due to Late Cretaceous–Tertiary, and Late Triassic–Early Jurassic exhumation. Apparent exhumation (maximum burial depth–present burial depth) for the Cooper Basin has been quantified using the compaction methodology. The results show that exhumation of the Cooper Basin for the majority of the wells is greater than the exhumation of the Eromanga Basin. Using the compaction methodology, apparent exhumation of Early to Middle Triassic age Arrabury and Tinchoo Formatios has been quantified. Both units yield similar results and do not support that the Arrabury/Tinchoo boundary represents the Cooper–Eromanga boundary. Hence, the Cooper Basin is believed to have reached its maximum burial depth in Late Triassic times. Sonic log data are not available for the units overlying the Late Cretaceous Winton Formation; thus, it is not possible to date exhumation beyond the Late Cretaceous–Tertiary using the compaction methodology. Tertiary sequences as are preserved are relatively thin and separated by marked unconformities and weathered surfaces; hence, exhumation rather than sedimentation dominated the Tertiary, and in exhumed areas, maximum burial depth was attained in Late Cretaceous times. The burial/exhumation history of representative wells was synthesized using sediment decompaction and establishing porosity/depth relations for the Cooper–Eromanga units. Considering the relative significance of the major periods of exhumation in the Cooper/Eromanga Basins, three broad types of burial/exhumation histories can be distinguished. Maximum burial depth of the Cooper Basin sequence was attained before the deposition of the Eromanga Basin sequence, i.e. Late Triassic–Early Jurassic times; maximum burial depth of the Cooper and Eromanga Basin sequences attained in Late Cretaceous times; and Cooper and Eromanga Basin sequences are currently at maximum burial‐depth. Incorporation of exhumation into burial history has major implications for hydrocarbon exploration.

Tectonic-sedimentary evolution of the western margin of the Mesozoic Vardar Ocean: evidence from the Pelagonian and Almopias zones, northern Greece

Abstract The Vardar Zone documents the Mesozoic-Early Cenozoic evolution of several small oceanic basins and a complex history of terrane assembly. Following a Hercynian phase of deformation and granitic intrusion within the Pelagonian Zone to the west, the Vardar Zone rifted in Permian-Triassic time, with the creation of an oceanic basin (Almopias Ocean) during the Late Triassic-Early Jurassic. During the Mid-Jurassic, this ocean subducted northeastwards beneath the Paikon Zone and the Serbo-Macedonian Zone, giving rise to arc volcanism and back-arc rifting. A second ocean basin, the Pindos Ocean, opened to the west of a Pelagonian microcontinent, also during Late Triassic-Early Jurassic time. During the Mid-Late Jurassic, ophiolites were emplaced northeastwards (in present co-ordinates) from the Pindos Ocean onto the Pelagonian microcontinent, forming the Pelagonian ophiolitic mélange within a flexural foredeep. This emplacement is dated at pre-Late Oxfordian-Early Kimmeridgian from the evidence of corals within neritic carbonates that depositionally overlie the emplaced ophiolitic rocks in several areas. Related greenschist- or amphibolite-facies metamorphism is attributed to deep burial following trench-margin collision and the attempted subduction of the Pelagonian continent. An inferred phase of NNW-SSE displacement, also of pre-latest Jurassic age, imparted a regionally persistent stretching lineation and related ductile fabric, apparently related to post-collisional strike-slip. The Pelagonian Zone and its emplaced ophiolitic rocks then underwent extensional exhumation during Late Jurassic-Early Cretaceous time. The western margin of the Vardar Zone experienced extensional (or transtensional) faulting, neritic carbonate and terrigenous clastic deposition, and intermediate-silicic magmatism during Late Jurassic-Early Cretaceous time. Oceanic crust (Meglenitsa Ophiolite) formed further east in the Vardar Zone during Late Jurassic-Early Cretaceous time, possibly above a subduction zone. A near-margin setting is suggested by the presence of a deep-water terrigenous cover, probably derived from the Paikon continental unit to the east. The Vardar Zone as a whole finally closed related to eastward subduction beneath Eurasia, culminating in collision with the Pelagonian microcontinent during latest Cretaceous-Eocene time, as recorded in foreland basin development, HP-LT metamorphism, ophiolite emplacement and large-scale westward thrusting. In contrast to models that suggest closure of the Vardar Ocean in the Mid-Late Jurassic, followed by reopening of a Cretaceous ocean, we believe that the Vardar Ocean remained partly open from Triassic to Late Cretaceous-Early Cenozoic time.

Plate tectonic evolution of the southern margin of Eurasia in the Mesozoic and Cenozoic

Thirteen time interval maps were constructed, which depict the Triassic to Neogene plate tectonic configuration, paleogeography and general lithofacies of the southern margin of Eurasia. The aim of this paper is to provide an outline of the geodynamic evolution and position of the major tectonic elements of the area within a global framework. The Hercynian Orogeny was completed by the collision of Gondwana and Laurussia, whereas the Tethys Ocean formed the embayment between the Eurasian and Gondwanian branches of Pangea. During Late Triassic–Early Jurassic times, several microplates were sutured to the Eurasian margin, closing the Paleotethys Ocean. A Jurassic–Cretaceous north-dipping subduction boundary was developed along this new continental margin south of the Pontides, Transcaucasus and Iranian plates. The subduction zone trench-pulling effect caused rifting, creating the back-arc basin of the Greater Caucasus–proto South Caspian Sea, which achieved its maximum width during the Late Cretaceous. In the western Tethys, separation of Eurasia from Gondwana resulted in the formation of the Ligurian–Penninic–Pieniny–Magura Ocean (Alpine Tethys) as an extension of Middle Atlantic system and a part of the Pangean breakup tectonic system. During Late Jurassic–Early Cretaceous times, the Outer Carpathian rift developed. The opening of the western Black Sea occurred by rifting and drifting of the western–central Pontides away from the Moesian and Scythian platforms of Eurasia during the Early Cretaceous–Cenomanian. The latest Cretaceous–Paleogene was the time of the closure of the Ligurian–Pieniny Ocean. Adria–Alcapa terranes continued their northward movement during Eocene–Early Miocene times. Their oblique collision with the North European plate led to the development of the accretionary wedge of the Outer Carpathians and its foreland basin. The formation of the West Carpathian thrusts was completed by the Miocene. The thrust front was still propagating eastwards in the eastern Carpathians. During the Late Cretaceous, the Lesser Caucasus, Sanandaj–Sirjan and Makran plates were sutured to the Iranian–Afghanistan plates in the Caucasus–Caspian Sea area. A north-dipping subduction zone jumped during Paleogene to the Scythian–Turan Platform. The Shatski terrane moved northward, closing the Greater Caucasus Basin and opening the eastern Black Sea. The South Caspian underwent reorganization during Oligocene–Neogene times. The southwestern part of the South Caspian Basin was reopened, while the northwestern part was gradually reduced in size. The collision of India and the Lut plate with Eurasia caused the deformation of Central Asia and created a system of NW–SE wrench faults. The remnants of Jurassic–Cretaceous back-arc systems, oceanic and attenuated crust, as well as Tertiary oceanic and attenuated crust were locked between adjacent continental plates and orogenic systems.

The Lennox Oil and Gas Field, Block 110/15, East Irish Sea

AbstractThe Lennox Field, located in blocks 110/15 and 110/14, was the second oil field to be developed in the East Irish Sea Basin. It contains 184 MMBBL of oil in-place within a 143 ft thick oil rim overlain by a large gas cap up to 750 ft thick. The GIIP is estimated to be 497 BCF. The field came on stream in February 1996, and it is now being developed with seven horizontal oil producers, including two multi-lateral wells and two crestal gas injectors. Production from the field can be divided into two distinct phases; the oil rim development phase, and the gas cap blow-down phase. The latter phase is currently anticipated to commence in 2004. The field structure consists of a roll-over anticline formed in the hanging wall of the Formby Point Fault during extensional faulting in Triassic-early Jurassic times, and later readjusted by contractional movements during Tertiary inversion. The oil and gas are trapped in the highly productive Triassic Ormskirk Sandstone Formation. The reservoir comprise high porosity aeolian and fluvial sandstones occurring at a shallow depth (c. 2500 ft) with a maximum gas column of 750 ft above an oil rim of 143 ft. The reservoir quality is principally controlled by primary depositional processes as no significant adverse diagenetic effects are observed. The hydrocarbon filling history was complex, with at least three phases of oil and gas generation. The field contains a light, saturated oil (45° API) with a GOR of 650 SCF/BBL. The crude contains high levels of H2S (0.1 mol%) and mercaptans (450 ppm), which are removed during processing at the Douglas complex. Water cut from the field is currently around 2-5%, and no free gas production has been observed to date. Gas production from Lennox is anticipated to start in 2004.

The Douglas Oil Fields, Block 110/13b, East Irish Sea

AbstractThe Douglas Field, on stream in February 1996, is the first oil field to be developed in the East Irish Sea Basin, with an estimated STOIIP of 202 MMBBL. The field structure consists of three tilted fault blocks formed during extensional faulting in Triassic-early Jurassic times, and later readjusted by contractional movements during Tertiary inversion. The oil is trapped in the Triassic Ormskirk Sandstone Formation, which comprises moderate to high porosity aeolian and fluvial sandstones. The reservoir depth is shallow (2140 ft) with a maximum oil column of 375 ft. The reservoir can be divided into several laterally extensive units based on vertical facies variations. The reservoir quality is principally controlled by primary depositional processes, and authigenic clay minerals are not important. However, bitumen is formed extensively in specific areas of the field causing significant permeability reduction. The hydrocarbon filling history of the field was complex, with the occurrence of at least two phases of oil generation and migration. The field contains a relatively 'dead' oil with a low GOR (170scf/bbl). Pressure maintenance is achieved through sea water injection, and to date ten production and six injection wells have been drilled. The crude is light (44° API) and contains high levels of H2S (0.5mol%) and mercaptans, which are removed during processing offshore.

Post Late Paleozoic tectonism in the southern Catalan Coastal Ranges (NE Spain), assessed by apatite fission track analysis

We report the first apatite fission-track thermochronologic data for 17 samples from the southern Catalan Coastal Ranges of NE Spain. Thermal histories of Carboniferous metasediments, Late Hercynian intrusions and Lower-Triassic Buntsandstein sediments from three tectonic blocks, Miramar, Prades and Priorat, are derived and interpreted within the geodynamic framework and tectonic evolution of the region. The apatite fission-track ages range from 198±24 to 38±5 Ma and mean fission-track lengths are all <13.3 μm. Samples throughout the study area underwent total track annealing during the Late Hercynian magmatic episode, followed by fast cooling prior to the deposition of Lower Triassic sediments. The Lower Triassic sediments and basement rocks underwent a temperature increase during a first Mesozoic rift phase in Middle Triassic–Early Jurassic times resulting in the complete or near complete annealing of the fission-tracks. During a second Mesozoic rifting stage, in Late Jurassic to Early Cretaceous time, differential tectonic block activity is observed in the three studied tectonic blocks. Subsequently, during Late Cretaceous a long-period of thermal stability, detected in all samples, is related to the post-rift episode. The onset of fast cooling registered in the apatite fission track system during Paleogene times is related to the Pyrenean orogeny. Compressional forces associated with the ongoing southern migration of the convergence forces at the Iberian plate boundaries caused unroofing of about 2–3 km of material of the Prades and northwestern flank of the Priorat block. Extensional collapse in Late Oligocene–Miocene related to the Western Mediterranean rifting triggered the denudation of about 2 km of material from the southeastern flank of the Miramar, Prades and Priorat blocks.

Palynological events during Late Triassic-Early Jurassic time in India

Recently acquired palynological data from Dubrajpur sediments show the presence of spore-pollen taxa diagnostic of the Late Triassic-Early Jurassic. The change in palynofloral composition is gradual and not abrupt. This palynosequence has been evaluated in terms of dominance/sub-dominance. Overall composition and First Appearance Datums of specific biomarker genera. viz.. Stereisporites, Foveosporites, Dictophyllidites, Nevesisporites. Lundbladispora, Enzonalasporites, Classopollis, Playfordiaspora. Striatopodocarpites, Arcuatipollenites, Infernopollenites, Straurosaccites, Minutosaccus, Podocarpidites, Araucariacites and Callialasporites. On the basis of the changing pattern of palynological characteristics seven palyno-events have been identified in the Late Triassic-Early Jurassic strata in Borehole RJNE-32, Rajmahal Basin, Bihar. The present palynoassemblages are compared with those published previously from the Late Triassic and Early Jurassic of India. The palynofloral transition from late Late Triassic to early Early Jurassic is reported in the Dubrajpur sequence.