Triassic-Early Jurassic Research Articles

The Bentong–Raub Suture Zone

It is proposed that the Bentong–Raub Suture Zone represents a segment of the main Devonian to Middle Triassic Palaeo-Tethys ocean, and forms the boundary between the Gondwana-derived Sibumasu and Indochina terranes. Palaeo-Tethyan oceanic ribbon-bedded cherts preserved in the suture zone range in age from Middle Devonian to Middle Permian, and mélange includes chert and limestone clasts that range in age from Lower Carboniferous to Lower Permian. This indicates that the Palaeo-Tethys opened in the Devonian, when Indochina and other Chinese blocks separated from Gondwana, and closed in the Late Triassic (Peninsular Malaysia segment). The suture zone is the result of northwards subduction of the Palaeo-Tethys ocean beneath Indochina in the Late Palaeozoic and the Triassic collision of the Sibumasu terrane with, and the underthrusting of, Indochina. Tectonostratigraphic, palaeobiogeographic and palaeomagnetic data indicate that the Sibumasu Terrane separated from Gondwana in the late Sakmarian, and then drifted rapidly northwards during the Permian–Triassic. During the Permian subduction phase, the East Malaya volcano-plutonic arc, with I-Type granitoids and intermediate to acidic volcanism, was developed on the margin of Indochina. The main structural discontinuity in Peninsular Malaysia occurs between Palaeozoic and Triassic rocks, and orogenic deformation appears to have been initiated in the Upper Permian to Lower Triassic, when Sibumasu began to collide with Indochina. During the Early to Middle Triassic, A-Type subduction and crustal thickening generated the Main Range syn- to post-orogenic granites, which were emplaced in the Late Triassic–Early Jurassic. A foredeep basin developed on the depressed margin of Sibumasu in front of the uplifted accretionary complex in which the Semanggol “Formation” rocks accumulated. The suture zone is covered by a latest Triassic, Jurassic and Cretaceous, mainly continental, red bed overlap sequence.

Blueschist-bearing metamorphic core complexes in the Qiangtang block reveal deep crustal structure of northern Tibet

Research Article| January 01, 2000 Blueschist-bearing metamorphic core complexes in the Qiangtang block reveal deep crustal structure of northern Tibet Paul Kapp; Paul Kapp 1Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA Search for other works by this author on: GSW Google Scholar An Yin; An Yin 1Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA Search for other works by this author on: GSW Google Scholar Craig E. Manning; Craig E. Manning 1Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA Search for other works by this author on: GSW Google Scholar Mike Murphy; Mike Murphy 1Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA Search for other works by this author on: GSW Google Scholar T. Mark Harrison; T. Mark Harrison 1Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA Search for other works by this author on: GSW Google Scholar Matthew Spurlin; Matthew Spurlin 1Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA Search for other works by this author on: GSW Google Scholar Ding Lin; Ding Lin 2Institute of Geology, Academia Sinica, Beijing, People's Republic of China Search for other works by this author on: GSW Google Scholar Deng Xi-Guang; Deng Xi-Guang 2Institute of Geology, Academia Sinica, Beijing, People's Republic of China Search for other works by this author on: GSW Google Scholar Wu Cun-Ming Wu Cun-Ming 2Institute of Geology, Academia Sinica, Beijing, People's Republic of China Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (1): 19–22. https://doi.org/10.1130/0091-7613(2000)28<19:BMCCIT>2.0.CO;2 Article history received: 22 Apr 1999 rev-recd: 25 Aug 1999 accepted: 08 Sep 1999 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Paul Kapp, An Yin, Craig E. Manning, Mike Murphy, T. Mark Harrison, Matthew Spurlin, Ding Lin, Deng Xi-Guang, Wu Cun-Ming; Blueschist-bearing metamorphic core complexes in the Qiangtang block reveal deep crustal structure of northern Tibet. Geology 2000;; 28 (1): 19–22. doi: https://doi.org/10.1130/0091-7613(2000)28<19:BMCCIT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract A 500-km-long belt of metamorphic exposures in the Qiangtang block provides an opportunity to study the internal structure of northern Tibetan crust. Metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist-bearing melange and are bounded by Late Triassic–Early Jurassic, domal, low-angle normal faults. We propose that this melange was underplated to the Qiangtang block and was subsequently exhumed by detachment faulting; both the underplating and the exhumation occurred during early Mesozoic southward subduction of oceanic lithosphere along the Jinsha suture. This model predicts that the deeper crust of much of northern Tibet consists of accretionary melange, in contrast to the continental crystalline crust of southern Tibet, and may account for north-south variations of Cenozoic tectonism in Tibet. You do not currently have access to this article.

Compositional variations and provenance of Triassic sandstones from the Barents Shelf

ABSTRACT Triassic sandstones from a large area ranging from the Pechora Sea and Novaya Zemlya in the southeast to the western Barents Sea and Svalbard have been analyzed by petrographic, mineralogical, mineral-chemical, and Sm-Nd bulk-rock isotopic methods. The largest compositional variation is seen in the Lower Triassic deposits. Epidote-bearing, lithic-rich sandstones are characteristic of the eastern Barents Sea, Novaya Zemlya, and the Timan-Pechora Basin, and indicate a provenance from the Polar Urals. Quartzo-feldspathic sandstones in the Hammerfest Basin may have been derived from deeper crustal plutonic and metamorphic rocks of the Caledonides and Baltic margin, whereas the source of quartz-dominated sandstones on Spitsbergen was a paleoland in the northwest, in the vicinity of paleo-Greenland. Calculated Sm-Nd provenance ages for Lower Triassic deposits are in the range of 1500-1880 Ma for sandstones from Spitsbergen, the Hammerfest Basin, and the Finnmark Platform. Sandstone provenance ages for samples analyzed from the eastern Barents Sea are significantly younger, with a range of 600-1280 Ma and an average of 780 Ma. Sandstones in central shelf areas, i.e., the Nordkapp Basin-Bjarmeland Platform, are rich in both lithics and feldspar and have intermediate provenance ages (960-1630 Ma). The largest influence of older components occurs in samples from the Nordkapp Basin. This may be related to sediment supply both from the Caledonized Baltic margin and Uralides, and from local uplifted areas composed of Paleozoic sedimentary rocks. Middle and Upper Triassic sandstones display compositional variations at different times in different areas due to local changes in provenance. However, in the Upper Triassic there appears to be a convergence in sandstone compositions reflecting both more extensive mixing of components from the eastern and western provenance areas as well as changes to more evolved compositions of the erosional products derived from the paleo-Urals. There is also a question of increasing influence from possible paleolands in the northeast. The most important compositional change was the distinct increase in sandstone maturity that took place on a regional scale in the Arctic in the latest Triassic-Early Jurassic. This may in part have been caused by extensive reworking connected with a regional transgression and sea-level raise.

Falkland Plateau evolution and a mobile southernmost South America

Abstract Assessment of southwest Gondwana break-up and its implications for regional hydrocarbon prospectivity must now take into account the origin of the southeast margin of the Falkland Islands as a volcanic rifted continental margin, and of the floor of the major part of the Falkland Plateau Basin as elevated oceanic crust. A reconstruction of the Falkland Plateau against southern Africa shows a southward extensional widening of the Outeniqua Basin across the line of the Falkland-Agulhas Fracture Zone, changing from stretched continental to oceanic crustal structure. Such a model for Outeniqua Basin opening, and the independent westward and clockwise rotation of the Falkland Islands block, suggests that southernmost South America was also a collection of microplates moving independently within a generally extensional environment in the Late Triassic-Early Jurassic. This is incompatible with assumptions of a rigid southernmost South America over this time, and a dominant role for a continuous dextral strike-slip Gastre Fault.

Depositional history and stratigraphical evolution of the Sakamena group (Middle Karoo Supergroup) in the southern Morondava Basin, Madagascar

The Karoo Supergroup in Madagascar is subdivided into three lithostratigraphical units: the Late Carboniferous-Early Permian Sakoa Group; the Late Permian-Middle Triassic Sakamena Group; and the Triassic-Early Jurassic Isalo Group. The Sakamena Group is fairly well exposed in the southern Morondava Basin, where it is approximately 4000 m thick. The Sakamena Group is separated from the Sakoa Group by an angular unconformity. The Lower Sakamena Formation is characterised by two major facies associations: (1) interbedded muddy conglomerates and coarse sandstones; and (2) interbedded sandstones and mudstones, which were deposited in a rejuvenated rift setting by coarse-grained fluvial systems and debris flows on the rift margins. In the Vatambe area, facies represent fandelta deposition in a saline lake or tongue of the ocean. The Middle Sakamena Formation comprises three major facies: (1) laminated mudstones and sandstones; (2) sandstones; and (3) mudstones. The Middle Sakamena facies were deposited by low gradient meandering streams and in shallow lakes. The Upper Sakamena Formation was deposited in similar environments, except that it is comprised predominantly of red beds. The Isalo Group consists predominantly of coarse-grained sandstones (up to 6000 m thick). These sandstones were deposited by braided streams with the coarse detritus derived from a structural uplift in the east.

The Late Triassic-Early Jurassic active continental margin of western North America in northeastern Mexico

En el noreste de México se han reportado algunos afloramientos de rocas volcánicas Triásicas y Jurásicas, cuyas características petrológicas y geoquímicas demuestran una afinidad de arco que permite relacionarlas a una margen continental activa. Más elementos que apoyan este ambiente tectónico se localizan en los estados de Durango, Zacatecas, San Luis Potosí, Nuevo León y Tamaulipas, en donde una secuencia siliciclástica tipo flysch en la porción occidental y un bloque continental pre-mesozoico en la porción oriental subyacen a la secuencia volcanogénica. En el presente trabajo se propone que las rocas volcánicas Triásicas y Jurásicas, así como algunas capas rojas asociadas que se encuentran en el centro y noreste de México, están relacionadas genéticamente con una margen continental activa y solamente algunas de las secuencias de capas rojas expuestas en el extremo oriental de México estarían relacionadas a estructuras de rift, posiblemente asociadas a la partición de Pangea como se ha propuesto hasta ahora, o en nuestra opinión, relacionadas más bien a cuencas de trasarco, además de las secuencias relacionadas a procesos posteriores, como la transgresión marina del Calloviano-Oxfordiano, asociada a la evolución de la margen pasiva o la apertura del Golfo de México.

The Early Cretaceous Arperos oceanic basin (western Mexico). Geochemical evidence for an aseismic ridge formed near a spreading center — Comment

The Guerrero terrane of western Mexico is considered to be the largest tectonostratigraphic unit of the Cordilleran collage of North America (CentenoGarcia et al., 1993), but its origin and paleogeographic evolution remains a matter of much debate. Its paleotectonic setting has remained elusive for several reasons, the most important probably being its large size, composite nature, and lack of systematic biostratigraphic, paleogeographic, and paleomagnetic analysis over most of its outcrop area. Until recently the Guerrero terrane was considered a suite of marine Upper Jurassic–Lower Cretaceous sedimentary and volcanic rocks intruded by coeval plutons, and grouped in several subterranes for which no continental basement was ever found. Nonetheless, a continuously growing body of field geologic, and geochronologic work has definitely established the presence of basement rocks in the Guerrero terrane that are as old as Late Triassic–Early Jurassic and, moreover, there is now firm evidence for the existence of pre-Mesozoic sialic crust underlying a substantial area of the terrane. Therefore, the tectonostratigraphic and paleotectonic setting of the Guerrero terrane must be analysed separately for the Upper Triassic–Lower Jurassic basement and for its

Diachronous Rifting, Drifting, and Inversion on the Passive Margin of Central Eastern North America: An Analog for Other Passive Margins

Integration of new data with existing information indicates that the tectonic development of the passive margin of eastern North America between the Carolina Trough and Scotian Basin was considerably more complex than the classic two-stage, rift-drift model. First, the transition from rifting to drifting was diachronous. In the southeastern United States, the rift-drift transition occurred after the Late Triassic synrift deposition and before eastern North America magmatism in the earliest Jurassic (~200 Ma). In maritime Canada, the rift-drift transition occurred after eastern North America magmatic activity and synrift deposition in the Early Jurassic and before postrift deposition in the early Middle Jurassic (~185 Ma). Second, the deformational regime changed substantially after rifting on both the southern and northern segments of the margin. Generally, northwest-southeast postrift shortening replaced northwest-southeast synrift extension. Northeast-striking reverse faults formed, and many of the rift-basin boundary faults had reverse displacements. In the southeastern United States, the change in the deformational regime occurred in the Late Triassic-Early Jurassic during the rift-drift transition. Simultaneously, diabase sills and dikes, many striking nearly perpendicular to the trend of the rift basins, intruded the continental crust; and a massive wedge of volcanic or volcaniclastic rocks developed near the continent-ocean boundary. In maritime Canada, the change in the deformational regime occurred during or after the Early Jurassic and before or during the Early Cretaceous; that is, during the rift-drift transition or early stages of sea-floor spreading.

Genesis of the Sanbaqi deposit: a paleokarst-hosted uranium deposit in China

The Sanbaqi uranium deposit in Hunan Province, south China, is the largest of a group of paleokarst-hosted uranium deposits in Lower Carboniferous limestone. Mineralization is localized in cavities and fault-breccias formed by dissolution of carbonates. Four episodes of karst formation are recognized: late Triassic-early Jurassic, late Jurassic-early Cretaceous, Cretaceous-Tertiary and Recent. Field relations indicate that the main uranium mineralization is related to the second karst episode. This is supported by isotopic ages of two pitchblende samples at 129 Ma and 134 Ma, as indicated by their nearly concordant data points on concordia plot. These ages are in the time range of the early Yanshanian tectonic movements that affected southern China, and the faulting related to the movements likely triggered the mineralization process at the Sanbaqi deposit. Associated minerals include pyrite, millerite, ullmannite, niccolite, molybdenite, chalcopyrite, sphalerite, galena, calcite and dolomite. Fluid inclusion studies on calcite reveal that temperature of ore deposition was from 181° to 150 °C. The δ18O and δD values of the ore fluids range from 1.5 to 7.9 per mil and from −30.4 and −54.8 per mil, respectively. The mineralogical, fluid inclusion and isotopic data indicate that the minerlization took place in episodic pulses of hydrothermal fluids that were introduced along a set of ring faults. Mobilization and redeposition of earlier formed ore minerals in an open system added to the complexity of the paragenetic sequence. Younger episodes of mineralization occurred during the later karst events as suggested by the geological and additional pitchblende U-Pb isotopic data, during the Cretaceous-Tertiary late Yanshanian tectonic movements and recently. Finally, a comparison of the Sanbaqi uranium deposit with the uranium deposits hosted by solution collapse breccia pipes of the Colorado Plateau, USA, shows that they have many similarities.

The Penarth Group-Lias Group succession (Late Triassic-Early Jurassic) in the East Irish Sea Basin and neighbouring areas: a stratigraphical review

Abstract Small, widely separated outliers of latest Triassic and Early Jurassic rocks in and around the East Irish Sea are preserved in the hangingwall blocks of faults that were active during post-Pliensbachian-pre-Cenomanian times. The successions preserved in the outliers comprise the Penarth and Lias groups, in typical facies and with characteristic microfloras and macrofaunas, and are vestiges of a formerly extensive cover of those rocks to the northwest of their main outcrops in England. The Penarth Group, of Rhaetian (Late Triassic) age, comprises the Westbury Formation and the succeeding Lilstock Formation; the latter is represented largely, or entirely, by the Cotham Member. Lias Group successions preserved in the outliers include, above the basal (‘pre-planorbis’) beds, of latest Triassic age, Early Jurassic beds ranging in age from Hettangian ( planorbis Zone) to Pliensbachian ( spinatum Zone) and are considerably thicker than their correlatives in eastern England, indicating that subsidence was slower there than in western Britain in Hettangian, Sinemurian and Pliensbachian times.

Paleofloristic assemblages through geological time in Korea

Paleofloristic assemblages show various different characteristic features through geological time in Korea. In Korea, paleofloristic assemblages can be separated into the Carbopermian Pyeongan flora, Triassic-Jurassic Daedong flora, Upper Jurassic Myogok flora, Cretaceous Gyeongsang flora and Tertiary geoflora. The upper Paleozoic (Upper Carboniferous-Permian) Pyeongan flora is composed of six floral assemblages: (C 2 1 - C 3 1; C 3 2 - P 1 1; P 1 1; P 1 2; P 2 1 and P 2 2) according to floral compositions. The Triassic-Early Jurassic Daedong flora are characterized by their various gymno-spermous plant aspects. These assemblages indicate low/middle latitude with a swamp/lake lacustrine paleoenvironment according to composition. The Fililcinae is the most characteristic element in number of species and diversity. The ginkgoales and confierales are the sub-dominant elements. These plants are accompanied by non-marine Crustacea (Estheictes) and this aspect also indicate a shallow lake environment. The Upper Jurassic Myogok flora indicate the Tithonian (latest Jurassic) and probably earliest Cretaceous age by the palynomorphic character. These floral constituents are rather different from the lower Daedong and upper Gyengsang floras. The Cretaceous Gyeongsang flora represent the first appearance of the angiosperm plants in Korea. As mentioned, most of constitutants of the plant assemblages indicate lacustirne (swamp/lake) river deposits and a sub-tropical environment is suggested by some cycadeoides trunk fossils. During the Miocene, the eastern part of the Korean peninsular underwent local subsidence, and shallow non-marine sediments were deposited which contain warm/humid climatic plant fossils, coal-bearing formations and tuffaceous rocks. After the events, the marine environment invaded locally along the eastern coast of the peninsula up to the present.

SHRIMP constraints on sediment provenance and transport history in the Mesozoic Statfjord Formation, North Sea

Detrital zircons in sandstones of the Statfjord Formation (late Triassic-early Jurassic) in the northern North Sea have been dated with the sensitive high resolution ion microprobe (SHRIMP). These sandstones lack detailed biostratigraphic control, but display distinct variations in heavy mineral assemblages which facilitate a three-fold subdivision of the sequence. The lower and middle zones (heavy mineral Zones A and B) have similar characteristics, both displaying a marked upward decline in garnet abundance which has proved difficult to attribute unambiguously to change in either provenance or sedimentation history. The base of the highest zone (heavy mineral Zone C) is marked by changes in several heavy mineral indicators attributable to a shift in ultimate provenance compared with the underlying zones. The detrital zircon ages are a direct fingerprint of the sediment source terrains, and so resolve ambiguities in interpreting provenance from heavy mineral abundances alone. The age spectra indicate that both low-garnet and high-garnet parts of Zone A have the same ultimate provenance. This strongly suggests that garnet removal was not a function of changing provenance, but took place during the sedimentation process, probably by weathering during periods of prolonged alluvial storage. The source of Zone A is not represented in any presently exposed landmass: the required combination of Caledonian granitoids intruding or in immediate proximity to high-grade Archaean-sourced metasediments is likely to have been either to the west, in the northern part of the Shetland Platform, or to the north, in what is now the Norwegian Sea. Zircons in Zone C have a distinctly different age spectrum consistent with a source in the Western Gneiss Region of southern Scandinavia, where there is the required combination of Proterozoic protoliths and Caledonian reworking.

Petroleum Systems Characterization and Ages in the Neuquen Basin (Triassic-to-Tertiary), West-Central Andes, Argentina: ABSTRACT

The Neuquen Basin, implanted on the western margin of the South American Plate evolved from Late Triassic-Early Jurassic as a set of isolated troughs, some of them connected to the {open_quotes}Pacific Ocean,{close_quotes} to an intra-arc to back-arc marine setting in Early-Late Jurassic. At the present day, a 7-km-thick succession of clastics, carbonates, evaporates, and volcanic rocks is preserved in between the eastern side of the Andean folded belt and the South American hinterland, affected by gentle deformation. On the southeastern margin of the basin occurs a relatively complex structural trend, as a result of the inversion tectonics related to an E-W regional strike-slip, fault. Five source-rock intervals are documented, the oldest consists of dark shales accumulated in a lacustrine environment (L. Triassic-E. Jurassic) and the others contain Type I and II organic matter as a consequence of anoxic conditions associated with marine flooding events that took place during the Jurassic and Early Cretaceous. The presence of numerous reservoir levels (clastics, carbonates, and also sills) and many regional and local seal intervals (shales and evaporates) within the sedimentary pile, combined with a diverse structural style, allowed the oil and gas trapping. Hydrocarbons were generated in various kitchens working at different times,more » as result of the subsidence regime and geothermal gradient that diversely affected different regions of the Neuquen Basin.« less

Monzonite suites: the innermost Cordilleran plutonism of Patagonia

ABSTRACT:In Patagonia a Triassic-Early Jurassic Cordilleran interior magmatic belt preceded the widespread eruption of Middle Jurassic syn-extensional rhyolites. Two plutons (La Calandria and La Leona) represent the easternmost plutonic rocks of this belt, &gt; 750 km east of the present oceanic trench. They define a high-K calc-alkaline monzonite series in contrast with the main Andinotype arc magmatism of the Pacific margin: they are enriched in large ion lithophile elements (K, Rb, Ba, Sr and Th), LREE and P2O5and depleted in HREE and Y, with low FeO*/MgO ratio. The range of observed compositions (56-76% SiO2) resulted from high-level fractionation of plagioclase, hornbleńde, biotite, K-feldspar and accessories (sphene, apatite and zircon).Initial87Sr/86Sr ratios, average εNdtand mean depleted-mantle Nd model ages of the two plutons are 0·70487, -0·5 and 1050 Ma for La Calandria and 0·70509, -1·4 and 1125 Ma for La Leona, respectively. They are thus isotopically more primitive than the Middle Jurassic rhyolites, previously attributed to partial melting of Mesoproterozoic mafic lower crust. The preferred model for the origin of the monzonites is remelting of an amphibole- + garnet-bearing, plagioclase-poor, high-K mafic source (?underplating). This occurred in a distal sector of a dying oblique subduction regime, immediately preceding the extensional silicic volcanism.

Tectonic and stratigraphic evolution of the Mongolian People's Republic and its influence on hydrocarbon geology and potential

The stratigraphy of Mongolia can be divided into five ‘megasequences’, each characterized by a common tectonic and stratigraphic history. The earliest, megasequence 1 (Precambrian-Silurian), records the amalgamation of Precambrian Early Palaeozoic passive and active margin terranes and the resulting growth of a ‘Caledonian’ fold belt in western Mongolia. Megasequence 2 (Devonian-Permian) appears to be the most complex, with a variety of active margin collisions in southern and eastern Mongolia, and the continued deformation of the Caledonian fold belt in the west. A substantial fold belt, underpinned by a variety of granitic plutons, existed across Mongolia by the Triassic. There was widespread erosion and limited deposition in small foreland and intermontane basins: megasequence 3 (Triassic-Early Jurassic). Megasequence 4 (Mid-Jurassic-Cretaceous) records extension across the partially eroded fold belt, with deposition confined to a series of alluvial and lacustrine rift basins. These basins later experienced transpression, inversion and erosion followed by onlap and overstep, giving more widespread sedimentation from large post-rift alluvial systems during the Late Cretaceous and Tertiary (megasequence 5). Megasequences 1–3 are usually extensively folded. Megasequences 4 and 5 have been involved in transpressive inversion, and Mongolia is an area of present day compressive and transpressive tectonics, erosion and general uplift. The Late Jurassic-Early Cretaceous sequences (megasequence 4) are the most prospective for finding extractable hydrocarbons. Oil has been successfully produced from megasequence 4 alluvial and lacustrine rift sequences with rich, oil-prone lacustrine mudrocks which can form both source and seal facies. Interbedded sandstones, siltstones and mudstones form various combinations of reservoirs and seals. A large number of hydrocarbon trap types are yet to be tested. Past drilling has been concentrated around surface seeps and on simple structures that can be mapped at the surface, with usually only shallow tests (< 1200 m). Mongolia remains an underexplored frontier area and there can be no doubt that Mongolia contains yet to be developed and yet to be discovered petroleum.

Early Mesozoic (Late Triassic-Early Jurassic) Tethyan brachiopod biofacies: possible evolutionary intra-phylum niche replacement within the Brachiopoda

Distributions of brachiopods from low-latitude paleogeographic settings, primarily in the Tethyan Ocean of southern Europe, with additional data from North America allow some observations on the bathymetric distribution of early Mesozoic brachiopod orders. Norian and latest Triassic (Rhaetian) brachiopod biofacies are dominated in shallowest waters by short-looped terebratulids (Terebratulidina) while spire-bearing athyrids (Athyrida) are common components of deeper-water environments in the latest Triassic. In the late Early Jurassic (Pliensbachian), shallow-water brachiopod faunas are dominated by rhynchonellids, short-looped terebratulids are commoner in relatively deeper shelf waters, and spiriferids and long-looped terebratulids (Terebratellidina) are abundant in deeper-water shelf environments.Following the end-Triassic extinction event there appears to be niche-replacement in deep-water shelf environments of Late Triassic athyrids by spiriferids and long-looped terebratulids in the Early Jurassic. Rhynchonellids appear to have diversified into shallowest water environments; specialized short-looped terebratulids may have occupied deeper-water niches that resulted ultimately in the success of the enigmatic Pygopidae later in the Jurassic and Cretaceous.

Gondwanaland origin, dispersion, and accretion of East and Southeast Asian continental terranes

East and Southeast Asia is a complex assembly of allochthonous continental terranes, island arcs, accretionary complexes and small ocean basins. The boundaries between continental terranes are marked by major fault zones or by sutures recognized by the presence of ophiolites, mélanges and accretionary complexes. Stratigraphical, sedimentological, paleobiogeographical and paleomagnetic data suggest that all of the East and Southeast Asian continental terranes were derived directly or indirectly from the Iran-Himalaya-Australia margin of Gondwanaland. The evolution of the terranes is one of rifting from Gondwanaland, northwards drift and amalgamation/accretion to form present day East Asia. Three continental silvers were rifted from the northeast margin of Gondwanaland in the Silurian-Early Devonian (North China, South China, Indochina/East Malaya, Qamdo-Simao and Tarim terranes), Early-Middle Permian (Sibumasu, Lhasa and Qiangtang terranes) and Late Jurassic (West Burma terrane, Woyla terranes). The northwards drift of these terranes was effected by the opening and closing of three successive Tethys oceans, the Paleo-Tethys, Meso-Tethys and Ceno-Tethys. Terrane assembly took place between the Late Paleozoic and Cenozoic, but the precise timings of amalgamation and accretion are still contentious. Amalgamation of South China and Indochina/East Malaya occurred during the Early Carboniferous along the Song Ma Suture to form “Cathaysialand”. Cathaysialand, together with North China, formed a large continental region within the Paleotethys during the Late Carboniferous and Permian. Paleomagnetic data indicate that this continental region was in equatorial to low northern paleolatitudes which is consistent with the tropical Cathaysian flora developed on these terranes. The Tarim terrane (together with the Kunlun, Qaidam and Ala Shan terranes) accreted to Kazakhstan/Siberia in the Permian. This was followed by the suturing of Sibumasu and Qiangtang to Cathaysialand in the Late Permian-Early Triassic, largely closing the Paleo-Tethys. North and South China were amalgamated in the Late Triassic-Early Jurassic and finally welded to Laurasia around the same time. The Lhasa terrane accreted to the Sibumasu-Qiangtang terrane in the Late Jurassic and the Kurosegawa terrane of Japan, interpreted to be derived from Australian Gondwanaland, accreted to Japanese Eurasia, also in the Late Jurassic. The West Burma and Woyla terranes drifted northwards during the Late Jurassic and Early Cretaceous as the Ceno-Tethys opened and the Meso-Tethys was destroyed by subduction beneath Eurasia and were accreted to proto-Southeast Asia in the Early to Late Cretaceous. The Southwest Borneo and Semitau terranes amalgamated to each other and accreted to Indochina/East Malaya in the Late Cretaceous and the Hainanese terranes probably accreted to South China sometime in the Cretaceous.

Source-Rock Distribution Model of the Periadriatic Region

The Periadriatic area is a mosaic of geological provinces comprised of spatially and temporally similar tectonic-sedimentary cycles. Tectonic evolution progressed from a Triassic-Early Jurassic (Liassic) continental rifting stage on the northern edge of the African craton, through an Early Jurassic (Middle Liassic)-Late Cretaceous/Eocene oceanic rifting stage and passive margin formation, to a final continental collision and active margin deformation stage in the Late Cretaceous/Eocene to Holocene. This evolution occurred in a transtensional and transpressional framework resulting from the oblique separation and convergence of the African and European plates. Extensive shallow-water carbonate platform deposits covered large parts of the Periadriatic region in the Late Triassic. Platform breakup and development of a platform-to-basin carbonate shelf morphology began in the Late Triassic and extended through the Cretaceous. On the basis of this paleogeographic evolution, the regional geology of the Periadriatic region can be expressed in terms of three main Upper Triassic-Paleogene sedimentary sequences: the platform sequence (sequence A), the platform to basin sequence (sequence B), and the basin sequence (sequence C). These sequences developed during the initial rifting and subsequent passive-margin formation tectonic stages. Three distinct episodes of source-bed deposition can be identified. Middle-Upper Triassic to Lower Jurassic (Liassic) organic-rich, oil-prone and mature source beds generally are structurally controlled, and are closely associated with transtensional intraplatform basins formed in the early continental rifting stage. The principal Triassic source basins and most of the surface hydrocarbon indications and economically important oil fields of the Periadriatic region are associated with sequence B areas. The second episode of organic-rich rocks deposition includes both shallow and deeper water facies of Jurassic-Cretaceous age. Thin, immature, deep-water epioceanic organic-rich marls and shales are generally bathymetrically controlled. These units are related to basin-wide anoxic events in the Tethys ocean and its submerged continental margin, and are associated with sequence B areas. Middle-Upper Cretaceous shallow-water, oil-prone facies occur in isolated anoxic basins within carbonate platforms; these source sequences are generally thicker and are associated with sequence A areas. No major hydrocarbon accumulation can be directly attributed to the Jurassic-Cretaceous epioceanic and intraplatform source rock sequences. The third episode of source bed deposition characterizes the final active margin deformation stage, and is represented by Upper Tertiary organic-rich terrigenous units, mostly gas prone. These are essentially associated with turbiditic and flysch sequences of foredeep basins, and have generated the greater part of the commercial biogenic gases of the Periadriatic region.

Depth-Dependent Transformation of Kaolinite to Dickite In Sandstones of the Norwegian Continental Shelf

AbstractReplacement of kaolinite by dickite has been observed to occur with increasing depth of burial in sandstones from three different basins on the Norwegian continental shelf. In the Garn Formation (Middle Jurassic) of Haltenbanken, samples from 1.4-2-7 km below the sea floor (110°C) contain kaolinite, whereas deeper than 3.2 km (130°C) mainly dickite is present. In the Statfjord Formation (Late Triassic-Early Jurassic) from Gullfaks and Gullfaks Sør Fields, transformation of kaolinite to dickite occurs at ~3.1 km below the sea floor (120°C) From the Stø and Nordmela Formations (Lower to Middle Jurassic) to the Troms Area, kaolin polytypes have been identified in only two shallow and two deep samples, but the results are consistent with the transformation depth determined in two other areas studied. These occurrences are significant because they allow the temperature of the kaolinite/dickite transformation to be established with greater confidence than had been possible previously. Also the observation of this transformation in all three areas so far examined indicates that it may be a general and predictable feature of kaolinbearing sandstones worldwide and therefore a potentially reliable paleogeothermometer. In most cases, the kaolinite occurs as relatively large vermicular crystals, whereas dickite forms more euhedral, blockier crystals. This morphological difference, together with the nature of the structural difference in octahedral occupancy between the kaolinite and dickite, suggests that the transformation occurs by dissolution and reprecipitation, rather then in the solid state.

Structural and Sedimentological Controls on Diagenesis in the Ravenspurn North Gas Reservoir United Kingdom Southern North Sea

The Ravenspurn area is divided into two main northwest-southeast-trending fault blocks which are markedly different in terms of their diagenetic evolution and reservoir performance. The northeasterly B structure contained gas earlier and was unaffected by Middle to Late Jurassic illitization. The southwesterly A structure was uplifted later and received accumulated gas after reservoir quality was reduced by pervasive illitization. The deposition of allogenic clay and the formation of early quartz, nonferroan dolomite, and anhydrite reduced the reservoir quality of fluvial sheetflood. Burial diagenesis resulted initially in ferroan dolomite, kaolinite, and later quartz precipitation in available primary and secondary porosity. Stable-isotope and fluid-inclusion studies indicate that Ferroan dolomite and later quartz precipitated at about 100[degrees]C in the Triassic-Early Jurassic from reduced fluids derived partly from the Carboniferous basement. Gas accumulation took place first in the northeasterly B structure, which had early closure. Elsewhere diagenetic fluids evolved to a more alkaline state, and widespread illitization took place which particularly affected more permeable eolian facies. The illitization reduced the reservoir quality of the lower Leman Sandstone and contributed to diagenetic sealing (to the northwest) of the field. K-Ar dating indicates that peak illitization took place between 150 and 170 Ma (Middle-Late Jurassic). Subsequentmore » periods of uplift in the late Cimmerian and particularly during the early Tertiary-Miocene produced the final structure of Ravenspurn North and the spillage of gas into this structure. The combination of structural and diagenetic events explains the differences in reservoir quality and well performance of the two structural blocks in the field.« less