Middle Jurassic Age Research Articles

Mafic VMS Mineralization in the Mesozoic Metavolcanic Rocks of the Evros Ophiolite, Xylagani Area, Greece

The sulfide mineralization at Xylagani is hosted in metamorphosed mafic massive and pillow lava. It has an Early–Middle Jurassic age and belongs to the Makri unit, which represents the upper crustal section of the Evros ophiolite in the Circum Rhodope Belt, Northern Greece. The protolith of the host rock is basalt that has a boninitic-to-low-Ti tholeiitic composition and was formed in an intra-oceanic supra-subduction zone within a juvenile forearc-to-volcanic arc setting. The volcanic rocks were subjected to ocean-floor metamorphism at very low-grade prehnite–pumpellyite facies and low-grade greenschist facies at temperatures of up to 360 °C and pressures between 1 and 4 kbar. The mineralization shows typical features of a stratabound–stratiform deposit and occurs as silicified lenses and layers with disseminated and massive sulfides and gold. Based on host rock composition, geotectonic setting, and base metal content, the mineralization at Xylagani is classified as a Cu-rich mafic volcanic-associated deposit, i.e., Cyprus-type VMS (volcanogenic massive sulfide). The mineralization consists of pyrite, chalcopyrite, gold, pyrrhotite, sphalerite, galena, and tennantite-(Zn). It was formed at a subseafloor setting where hydrothermal fluids circulated through the host volcanic rocks, resulting in a pervasive alteration (silicification and chloritization) and the development of a replacement VMS deposit. The very low-to-low-grade orogenic metamorphism and related deformation during the Alpine collision in the Middle Jurassic to Early Cretaceous periods remobilized the mineralization and formed milky quartz veins with rare sulfides, crosscutting the metavolcanic rocks.

Palynology of the Middle Jurassic strata from the Alborz Ranges, northwestern Iran

Palynomorphs are used for palynostratigraphy, and to derive paleoecological and paleobiogeographical inferences from the Middle Jurassic Shemshak Formation, Alborz Ranges, northwestern Iran. The rock unit contains diverse, reasonably preserved palynofloras dominated by miospores and dinoflagellate cysts. Vertical distribution of miospores allows for the introduction of Klukisporites variegatus -Striatella spp.- Contignisporites burgeri assemblage zone in the host strata. The presence of such key miospore species as Striatella jurassica, S. patenii, S. seebergensis, and Contignisporites burgeri indicates a Middle Jurassic (late Bajocian–Bathonian) age. Based on the stratigraphic distribution of dinoflagellate cysts, encountered Cribroperidinium crispum Total Range Biozone (late Bajocian); Dichadogonyaulax sellwoodii Interval Biozone (Bathonian–early Callovian), and Subzone “a” of the D. sellwoodii Zone (early–middle Bathonian) are identified. Miospores dominating the examined assemblages are assigned, in ascending order of abundance, to Pterophyta, Coniferophyta, Pteridospermophyta, Lycophyta, Ginkgophyta, Cycadophyta, and Sphenophyta. Such parental vegetation apparently flourished under a wet, warm-to-warm-temperate climate. The co-occurrence of such index warm-moderate water dinoflagellate cysts as Pareodinia halosa, Ctenidodinium continuum, and Pareodinia ceratophora supports this overview. The assemblages though contain both Eurasian and Gondwanan elements but bear closer similarity to those from the former, thus suggesting paleoproximity to the northeastern margin of the Neotethys Ocean during the Middle Jurassic.

Introduction of Ammonites from the Dalichai Formation in Polour Section Central Alborz, Iran

The studies done on Dalichai Formation in Central Alborz, South Polour Formation of an 80 m thick that are systematically sampled at intervals of 2 meters and selected the 40 rock samples for the preparation of thin sections considered and in addition to lithology; characteristics of the stone, and index examples of Ammonites were also studied. Dalichai formation lithology in the study area is rich in terms of alternating gray sandstone and marl with Ammonites Fauna. Based on the study of fossil fauna, the oldest identified ammonite in the studied section is Procerites (Early Bathonian), and the youngest is Reineckeia sp. (Late Callovian). The distribution of ammonites indicates that the eastern Alborz region had marine connections with northwestern Europe and the Mediterranean marginal areas during the Middle Jurassic period, and the study area was located north of the Tethys Ocean. Based on its stratigraphic position of the middle Jurassic Age Formation, Dalichai Ammonites with peer review study sections and microfossils Bathonian-Callovian age Dalichai is accurate. Lar Formation covers this formation with a gradual contact and has been parallel to the Shemshak Formation on an erosional unconformity. Through the examination of stratigraphy and the correlation of formations in the studied sequence, it was determined that the lower part of the Dalichai Formation is equivalent to other formations in Iran, including the Parvdeh Formation in the Shotori Mountains, the Kashfrood Formation in the Kopeh Dagh, and the lower part of the Baghamshah Formation in the Tabas Basin. The variations in thickness of the Dalichai Formation in the eastern Alborz region indicate relative subsidence of the sedimentary basin, a delay in sea-level rise, and the exit of sediments from seawater. These changes are indicative of the Middle Cimmerian tectonic event, which is observed as an erosional unconformity at the lower boundary of the Dalichai Formation.

Detrital zircon provenance links between outcrop and subsurface, Rocas Verdes and Magallanes-Austral Basins, Patagonia

Abstract Analyzing how provenance signatures of tectonically complex ancient sedimentary basins vary between the outcrop and the subsurface provides a more complete sediment provenance history. The 500+-km-long outcrop belt of the Rocas Verdes and the Magallanes-Austral Basins in southern Patagonia and its subsurface to the east allow us to constrain relationships between longitudinal and transverse sediment sources feeding the basin. New detrital zircon U-Pb ages (n = 2558) from nine subsurface core samples from the Springhill, Piedra Clavada, and La Anita Formations along a 140 km E-W seismic reflection transect (50°S, 71.5°W–69.5°W) reveal the provenance of the basin's eastern subsurface extent and the influence that tectonic activity can have on sediment capture. Detrital zircon ages (n = 1068) were also collected from four outcrop Springhill Formation samples 140 km NW from the transect near Lago San Martín. Maximum depositional ages (MDAs) from the Springhill Formation cores range from 159 to 157 Ma, which are older than correlative outcrop ages of 151–148 Ma. MDAs from the Piedra Clavada and La Anita Formations range from 93.7 to 91.5 Ma and from 79.3 to 78.3 Ma, respectively, and overlap in age or are slightly younger than outcrop ages. The unimodal Late Jurassic age distribution from the Springhill Formation suggests that it almost exclusively was sourced by recycling of the underlying El Quemado Complex. Early–Middle Jurassic ages in the Early Cretaceous Piedra Clavada Formation samples suggest the basin was either sourcing from the North Patagonian Massif to the N-NE and from the Deseado Massif to the NE or recycling exhumed volcaniclastic sequences in the northern margin of the basin. By the Late Cretaceous, the basin fill was more locally sourced, as suggested by the muted abundance of Early–Middle Jurassic ages in La Anita Formation samples and a second mode of Late Jurassic–Early Cretaceous ages locally derived from recycling of the exhumed El Quemado Complex from the Patagonian fold-and-thrust belt. We suggest that the latitudinal and local transverse drainages that sourced the Magallanes-Austral Basin outcrop belt also fed the eastern, subsurface extent of the basin. Results of this work highlight the importance of using several types of provenance methods and a three-dimensional approach to study the erosional history of sedimentary basins.

Discovery of the Callovian Oceanic Anoxic Event in the Qiangtang Basin, eastern Tethys: Insights from in situ calcite U[sbnd]Pb dating

Discovery of the Callovian Oceanic Anoxic Event in the Qiangtang Basin, eastern Tethys: Insights from in situ calcite U[sbnd]Pb dating

Sedimentary Expressions of the Early Jurassic Jenkyns Event in an Inland Lacustrine System in the Yin'gen–Ejinaqi Basin, North China

Abstract The Jenkyns Event, more widely known as the Toarcian Oceanic Anoxic Event (T‐OAE), is marked by globally distributed negative carbon‐isotope excursions, widespread oxygen depletion, and large‐scale organic carbon burial, which indicate major climate/environmental perturbations in Earth's surface systems during the Early Jurassic. Although extensive research has been conducted in European continental settings, particularly in the western peri‐Tethys regions, the impacts of this event beyond Europe remains largely unexplored. Here, a multiapproach study including investigations into the spore‐pollen assemblages, pyrite framboids, clay minerals, total organic carbon (TOC) levels, and organic carbon isotope (δ13Corg) levels in a lacustrine borehole section (MED1) from the Yin'gen–Ejinaqi Basin, North China, provides evidence of the occurrence of the Jenkyns Event and its extensive sedimentary responses in the eastern Tethys terrestrial systems. Two distinct spore‐pollen assemblages have been identified in MED1 (drilling depth: 982.4 m to 1267.5 m), with the Cycadopites–Protopinus–Osmundacidites assemblage in the lower part (1267.5 m to 1132.9 m) indicating a middle Early Jurassic age and the Classopollis assemblage in the upper part (1132.9 m to 985.7 m) suggesting a Toarcian age. Framboidal pyrite data suggest more anoxic conditions during the deposition of black mudstone and shale intercalations in the lower part of the Classopollis assemblage (1132.9 m to 1066.9 m), which combined with organic carbon enrichment and negative δ13Corg excursions, are considered the paleoenvironmental response to the Jenkyns Event in the study area. Furthermore, the evolution of vegetation groups changed from plant groups characterized by bisaccate and cycad pollen, as well as fern spores, to vegetation groups represented by Cheirolepidiaceae pollen across the Jenkyns Event, as evidenced by spore‐pollen data, together with the clay mineral assemblage change characterized by a notable increase in illite at the expense of kaolinite, suggests that while a subtropical‐temperate climate persisted, a change toward warmer and drier conditions most likely occurred in the early Toarcian in the study area. In contrast to the humidification evidenced in many coastal settings, this aridification trend in the Yin'gen–Ejinaqi Basin aligns with the conditions in many inland areas. It is hypothesized that the underlying cause of these divergent changes may be linked to certain patterns of spatially variable water availability on land, potentially driven by extremified hydrological conditions.

Age and tectonic setting of Mesozoic extension constrained by the first volcanic events in the Eastern Cordillera and Middle Magdalena Valley, Colombia

Before the rise of the Northern Andes in Cenozoic time, Triassic to Jurassic extensional basins in northwestern South America accommodated predominantly continental strata partly intercalated with volcanic rocks. Coeval plutonism is attributed to a magmatic arc related to the subduction of the Farallon plate beneath South America. The basins later became involved in the Andean orogeny and are now partially exposed in the Eastern Cordillera and Middle Magdalena Valley of Colombia. We have employed (U/Pb) geochronology on zircons from Triassic-Jurassic felsic to intermediate volcanic and volcaniclastic rocks. Most of the ten samples have a substantial proportion of detrital zircons, but only three had no Mesozoic grains. The Mesozoic ages obtained range from ca. 201 Ma to ca. 177 Ma and overlap with published crystallization ages (K/Ar; Ar/Ar; U/Pb) from plutonic bodies. Volcanics from the Jordán and Girón formations are latest Triassic to Early Jurassic and synchronous with major plutonic activity. These ages constrain the early evolution of the extensional basins that formed from about the Triassic-Jurassic transition in an intra-arc position and facilitated the preservation of sediment and arc-derived volcanics. Middle Jurassic ages from the Noreán Fm. are synchronous with sparse plutonism west of the Middle Magdalena Valley. At this time, the magmatic arc had migrated westward, while intrusive activity in the Eastern Cordillera ceased. A geochemical rift signature only appears in scarce Early Cretaceous mafic intrusions that resumed magmatic activity in the Eastern Cordillera. This magmatism, now in a back-arc position, coincides with maximum subsidence of the large Cretaceous basin that extended across the older intra-arc rift basins. Extension and lithospheric thinning ceased by the end of the Early Cretaceous.Graphical abstract

Middle Jurassic to Early Cretaceous orogenesis in the Klamath Mountains Province (Northern California–southern Oregon, USA) occurred by tectonic switching: Insights from detrital zircon U-Pb geochronology of the Condrey Mountain schist

The Klamath Mountains Province of Northern California and southern Oregon, USA, consists of generally east-dipping terranes assembled via Paleozoic to Mesozoic subduction along the western margin of North America. The Klamath Mountains Province more than doubled in mass from Middle Jurassic to Early Cretaceous time, due to alternating episodes of extension (e.g., rifting and formation of the Josephine ophiolite) and shortening (e.g., Siskiyou and Nevadan events). However, the tectonic mechanisms driving this profound Mesozoic growth of the Klamath Mountains Province are poorly understood. In this paper, we show that formation of the Condrey Mountain schist (CMS) of the central Klamath Mountains Province spanned this critical time period and use the archive contained within the CMS as a key to deciphering the Mesozoic tectonics of the Klamath Mountains Province. Igneous samples from the outer CMS subunit yield U-Pb zircon ages of ca. 175–170 Ma, which reflect volcanic protolith eruptive timing. One detrital sample from the same subunit contains abundant (~54% of zircon grains analyzed) Middle Jurassic ages with Paleozoic and Proterozoic grains comprising the remainder and yields a maximum depositional age (MDA) of ca. 170 Ma. These ages, in the context of lithologic and thermochronologic relations, suggest that outer CMS protoliths accumulated in an outboard rift basin and subsequently underthrust the Klamath Mountains Province during the Late Jurassic Nevadan orogeny. Five samples of the chiefly metasedimentary inner CMS yield MDAs ranging from 160 Ma to 130 Ma, with younger ages corresponding to deeper structural levels. Such inverted age zonation is common in subduction complexes and, considering existing K-Ar ages, suggests that the inner CMS was assembled by progressive underplating over a >10 m.y. timespan. Despite this age zonation, age spectra derived from structurally shallow and deep portions of the inner CMS closely overlap those derived from the oldest section of the Franciscan subduction complex (South Fork Mountain schist). These relations suggest that the inner CMS is a composite of South Fork Mountain schist slices that were sequentially underplated beneath the Klamath Mountains Province. The age, inboard position, and structural position (i.e., the CMS resides directly beneath Jurassic arc assemblages with no intervening mantle) of the CMS suggest that these rocks were emplaced during one or more previously unrecognized episodes of shallow-angle subduction restricted to the Klamath Mountains Province. Furthermore, emplacement of the deepest portions of the CMS corresponds with the ca. 136 Ma termination of magmatism in the Klamath Mountains Province, which we relate to the disruption of asthenospheric flow during slab shallowing. The timing of shallow-angle subduction shortly precedes that of the westward translation of the Klamath Mountains Province relative to correlative rocks in the northern Sierra Nevada Range, which suggests that subduction dynamics were responsible for relocating the Klamath Mountains Province from the arc to the forearc. In aggregate, the above relations require at least three distinct phases of extension and/or rifting, each followed by an episode of shallow-angle underthrusting. The dynamic upper-plate deformation envisioned here is best interpreted in the context of tectonic switching, whereby slab steepening and trench retreat alternate with slab shallowing due to recurrent subduction of buoyant oceanic features.

Перспективи нафтогазоносності Північно-Яворівської площі Більче-Волицької зони Передкарпатського прогину (за результатами комплексних геофізичних і геохімічних досліджень)

A complex of geophysical and geochemical studies was carried out in the Bilche-Volytsia zone on the Northern Yavoriv area with the aim of detailing the geological structure of Mesozoic and Neogene deposits in the context of identifying oil and gas promising traps of various types and preparing them for deep drilling. To solve the problem, the following studies were applied: seismic surveying by the common depth point method, electrical surveying by the method of sounding the formation of the field in the near zone, geochemical surveying by the method of gas discharge surveying. The main resulting material of the performed geophysical and geochemical studies on the Northern Yavoriv area is a structural-tectonic model, seismic and seismogeological sections, maps of longitudinal resistances and geoelectrical sections, maps of geochemical parameters, mapping maps by types of research. As a result of conducting seismic, electrical, and geochemical studies, the geological structure of the Northern Yavoriv area was detailed. Structural plans by stratigraphic levels are presented: Pre-Neogene surface, Tyrassian of the Badenian age suite, the top of the Lower Dashava subsuite of Sarmatian age, the top of Upper Dashava subsuites of Sarmatian age. The Pre-Neogene surface is represented by deposits of the Upper Cretaceous and the Upper and Middle Jurassic ages. Along this horizon, there is a dip from the northeast to the southwest. The structural-tectonic model is complicated by thrusts, longitudinal shifts, normal and strike-slip faults, which divide the research area into separate blocks. According to the structural constructions on the roof of the Tyrassian suite, against the background of the general rise of the horizon to the northeast in this part, a structural nose can be observed, which has a southeast extension. A syncline can be traced along the Horodok flexure in the southwest along the horizons of the Lower Dashava subsuite and the roof of the Upper Dashava subsuite, within which there is a general coincidence of the structural plans of the mentioned horizons. The difference in structural plans is observed in the area where the horizons adjoin the plane of the Horodok flexure. The Northern Yavoriv structure of the semi-anticlinal type is mapped on the reflection seismic horizon of the roof of the Upper Dashava subsuite (VD-10). The structural and tectonic model of this horizon of the Northern Yavoriv area borders the Svydnytsia gas field through a syncline bend. According to the completed structural constructions along this horizon, as well as a complex comparison of methods, this area may represent a certain interest in terms of gas. It is recommended to drill well No. 1 on the Northern Yavoriv structure in order to search for Upper Dashava deposits.

Middle to Late Jurassic pelagites and gravity mass flow deposits of a displaced Neotethyan margin: Microfacies and biostratigraphical studies in Northeast Hungary

Microfacies, depositional age and sedimentary environment were characterised for two Jurassic successions, which were deposited on the Adriatic microcontinental margin of the Neotethys Ocean. Investigations were carried out mostly on cores drilled in the Mesozoic basement of the eastern part of the M?tra Mountains (Recsk area) and the westernmost part of the B?kk Mountains, NE Hungary. This area represents the continuation of the Inner Dinaric nappesystem, and was displaced along the Mid?Hungarian Shear Zone during the Late Oligocene to Early Miocene. The pre?Cenozoic basement of the area is characterised by three juxtaposed units: the lowermost Recsk Succession, the Tarna Olistostrome and the topmost Darn?hegy M?lange nappe. The Recsk Succession is made up of Upper Triassic, cherty carbonates of pelagic basin development that are overlain by pelagic limestones of Early to early Middle Jurassic age. The carbonate sedimentation changed gradually into shale?domi nated one during the late Bathonian to the early Callovian. In the Bajocian to early Callovian interval the Recsk area was located at the toe of a coeval carbo nate platform, which provided gravitational mass flows reaching the investigated area. The external margin of this platform drowned and got covered by the pelagic shale in the late Bajocian. The Tarna Olistostrome is built up by a Tithonian pelagic mixed carbonatic and siliciclastic succession with breccia/olistostrome horizons. The clasts derived from the Upper Permian? Lower Jurassic succession of a distal Adriatic margin. The Darn?hegy M?lange is a typical sub?ophiolitic m?lange comprising scrapped off blocks and slices from the lower plate and gravitationally redeposited / tectonically sheared blocks from the overriding ophiolite nappe. The age of the m?lange is Callovian?Oxfordian. These inferences may serve as a base for new geodynamic evaluations of the studied region.

Formation of Brahmani Natural Arch, Eastern India: A proposed geoheritage site of India

Natural arch is one of the most fascinating geomorphological features formed by erosional process and many of them are formed within gently dipping sandstone. A natural arch (Location: 21°54′32.20″ N, 83°43′46.20″ E, RL: ∼370 m) of such type is present in the rocks of Upper Kamthi Formation of Middle Jurassic age (160–184 Ma), Upper Gondwana Group, Ib River Coalfield within dense Chhengapahar Reserve Forest near Brahmani Village, Sundargarh District, Odisha. This oval shaped natural arch with a maximum length and height of ∼30 m at the base and ∼12 m respectively. The alcove of the natural arch has maximum height and width of ∼7 m and ∼15 m respectively, is opened along NE-SW direction on the top of the hill. Flesh red colored, ripple topped, trough to planar cross stratified, coarse to gritty, texturally and mineralogically immature, ferruginous sandstone of Upper Kamthi Formation forms the hill. Kaolinisation of feldspathic members makes the sandstone loose and porous. Two orthogonal set of faults-joints (130°/60°W and 40°/60°E) initiated the formation of natural arch while the porosity of the immature, arkosic sandstone accelerate the process through subaerial weathering where surface/rain water percolation–temperature fluctuation played effective roles. Therefore, the formation of natural arch is a result of structurally triggered weathering and erosion. Efforts have already been taken to preserve this place as national geo-heritage site by Geological Survey of India, Government of India.

Sulfide Pb-Zn mineralization in the Tianshuihai terrane, northwest Tibetan plateau: A case study of the Huoshaoyun Pb-Zn deposit

The Tianshuihai terrane, located in the northwest of the Tibetan plateau, is a newly known metallogenic district of Pb-Zn deposits in China, with a prospective resource of Pb and Zn over 30 Mt. Lead and Zn mineralization in the terrane comprises two types of non-sulfide and sulfide. However, the genesis of sulfide Pb-Zn mineralization and its relationship to regional geological evolution remain enigmatic. In this paper, a detailed mineralogical and geochemical study of the Huoshaoyun Pb-Zn deposit combine with a synthesized review of the evolution of the Tianshuihai terrane provide comprehensive constraints on the genesis of sulfide Pb-Zn mineralization and what geological events of which it may have been part. Economic orebodies in the Huoshaoyun deposit are hosted by a sequence of unmetamorphosed, flat-lying carbonate complex of Middle Jurassic age. Ore minerals consist predominantly of smithsonite and cerussite with variable sphalerite and galena. The sulfide minerals are texturally early and were replaced by non-sulfide minerals. Sulfur isotope compositions of galena are remarkably inhomogeneous, range from −18.52 to 25.40 ‰. Conversely, lead isotope compositions of galena display fairly uniform 206Pb/204Pb (18.485–18.785), 207Pb/204Pb (15.640–15.764), and 208Pb/204Pb (38.781–39.303) ratios. We thus propose that sulfide Pb-Zn mineralization in the Huoshaoyun deposit belongs to Mississippi Valley (MVT) - type. The metals were most probably sourced from the regional strata, e.g., the Permian Shenxianwan, Triassic Keleqinghe, and Jurassic Longshan formations. Sulfide Pb-Zn mineralization in the Tianshuihai terrane likely occurred sometime between the Oligocene to Early Miocene in a paleo-forland basin, coincided with the widespread potassic and ultra-potassic magmatic activities, a marked change in tectonic regime from compressional to extensional, an elevated heat flow in the Tibetan Plateau, and a great MVT-type Pb-Zn mineralization event in the Tethys tectonic domain. This study raises awareness about the presence of MVT-type Pb-Zn deposits in the Tianshuihai terrane and the great potential for Pb-Zn exploration in this region.

Age assignment of dolomite in palaeo-reservoirsof the Qiangtang Basin: New evidence from palaeontology and carbonate in situ U-Pb dating

Age assignment of dolomite in palaeo-reservoirsof the Qiangtang Basin: New evidence from palaeontology and carbonate in situ U-Pb dating

Cretaceous radiolarians from the Siquisique Ophiolite (northern Venezuela)

Cretaceous radiolarians from the Siquisique Ophiolite (northern Venezuela)

A new giant theropod dinosaur track from the Middle Jurassic of the Cleveland Basin, Yorkshire, UK

A new specimen of a rare large theropod dinosaur print of Middle Jurassic age is described from the Long Nab Member of the Scalby Formation, Cleveland Basin, Yorkshire. This is only the sixth specimen of this type recorded from the Cleveland Basin since they were first discovered in 1934. The present specimen is included in the same, but slightly modified morphotype Bxviii as some of the previous ones, since it shows additional features including an elongated metapodium. The specimen is assigned to the ichnogenus Megalosauripus , and was possibly made by a Megalosaurus -like theropod. The elongated metapodium may be the result of resting or crouching behaviour.

Suprasubduction ophiolite (SSZ) components in a middle to lower upper Jurassic Hallstatt Mélange in the Northern Calcareous Alps (Raucherschober/Schafkogel area)

Suprasubduction ophiolite (SSZ) components in a middle to lower upper Jurassic Hallstatt Mélange in the Northern Calcareous Alps (Raucherschober/Schafkogel area)

Meta-bauxite deposit in the Tavşanlı Zone, NW Turkey: A new locality for gem-quality diaspore formation

The tectonic units of Sakarya Zone (Pontides) and Tavşanlı Zone (Anatolides) are exposed in the Mihalıççık area of central Anatolia. A Cretaceous accretionary complex that forms the İzmir-Ankara-Erzincan suture zone separates the units. Within this region, the Tavşanlı Zone is made up of a coherent series and tectonically overlying late Cretaceous mélange, which has undergone blueschist facies metamorphism. Coherent series consist of strongly retrogressed schists and conformably overlying platform-type thick marbles, known as the İnönü marbles.The İnönü marbles contain meta-bauxite lenses in the lower levels, which can be classified as karstic bauxites deposited in the karst holes within the original limestone. Considering the general stratigraphy of the Tavşanlı Zone, a middle Jurassic age can be envisaged for the bauxite formation. Geochemical data suggest that the bauxites were largely derived from redeposited argillic sediments, with minor and variable inputs from the weathering of intermediate to acidic rock. The mineral assemblage of the meta-bauxite consists of diaspore, chloritoid, muscovite, paragonite, Al-spinel (hercynite), magnetite, hematite and ilmenite. In addition, apatite, rutile, monazite, xenotime and zircon occur as accessory minerals. Limonite, goethite, akaganeite and kaolinite also formed during the latest stage of alteration. P-T conditions for the meta-bauxites are estimated as 20–27 kbar and 330–470 °C that are indicative of formation under high-P / low-T blueschist facies conditions.The meta-bauxites are cut by veins of diaspore, muscovite-paragonite, goethite, calcite and ankerite, and include gem-quality diaspore crystals up to 6 cm long. Textural evidence indicates that the diaspore formation is related to the extraction of aluminium from the meta-bauxite body and migration of Al-rich fluids into the shear zones during the last stage of the high-P metamorphism. During exhumation of the Tavşanlı Zone, these veins were brecciated by brittle deformation and the fractured minerals were cemented by granoblastic calcite and ankerite during the latest stage of the vein formation.

U–Pb age constraints on the Jurassic succession and paleoflora of Mount Flora, Antarctic Peninsula

The spectacular fossil plant assemblage preserved in the non-marine Mount Flora Formation of the northern Antarctic Peninsula represents the diverse Jurassic flora that once covered the Gondwanan continents at high paleolatitudes. The depositional facies of the formation plays a key role in the tectonic interpretations and basin evolution models that attempt to reconcile large igneous province magmatism, continental break up, and magmatic arc development throughout the Jurassic and Cretaceous. Limited U–Pb in situ geochronology reported from the Mount Flora Formation and adjacent units lack the necessary resolution to overcome ambiguous correlations and biostratigraphic discrepancies. We present two high-precision U–Pb zircon ages (CA-ID-TIMS method) from a distinct tuffaceous interval of the Mount Flora Formation at Hope Bay, which document a terminal Middle Jurassic age (Callovian Stage) for the formation and its paleoflora. In excess of 1400 new collected fossil plant specimens exhibit a highly diverse Jurassic plant association that dominated the Antarctic Peninsula nearly 17 million years after its disappearance from northern Patagonia. This suggests similar paleoecological conditions were established diachronously throughout basins of southern Gondwana, possibly facilitating floral migrations in response to local climate change. The depositional facies of the Mount Flora Formation, its age proximity to the marine Nordenskjöld Formation in the Antarctic Larsen Basin and its coincidence with a regional unconformity in the northern Patagonia point out to a complex interplay among magmatic arc development, tectonic extension and continental break up that dominated the geologic and paleoenvironmental evolution of southern Gondwana near the end of the Middle Jurassic.

Arctic Alaska Basin, Hanna Trough and Beaufortian Rifted Margin Composite Tectono-Sedimentary Elements

Abstract The Arctic Alaska region includes three composite tectono-sedimentary elements (CTSEs): (1) the Arctic Alaska Basin (AAB), (2) the Hanna Trough (HT) and (3) the Beaufortian Rifted Margin (BRM) CTSEs. These CTSEs comprise Mississippian–Lower Cretaceous (Neocomian) strata beneath much of the Alaska North Slope, the Chukchi Sea and westernmost North Slope, and the Beaufort Sea, respectively. These sedimentary successions rest on Devonian and older sedimentary and metasedimentary rocks, considered economic basement, and are overlain by Cretaceous–Cenozoic syn- and post-tectonic strata deposited in the foreland of the Chukotka and Brooks Range orogens and in the Amerasia Basin. (1) The Mississippian–Neocomian AAB CTSE includes two TSEs: (a) the Ellesmerian Platform TSE, comprising mainly shelf strata of Mississippian–Middle Jurassic age, and including a relatively undeformed domain in the north and a fold-and-thrust domain in the south; and (b) the Beaufortian Rift-Shoulder TSE, which includes Middle Jurassic–Neocomian deposits related to rift-shoulder uplift. (2) The HT CTSE includes four TSEs: (a) the Ellesmerian Synrift TSE, which comprises Late Devonian(?)–Middle Mississippian growth strata deposited in graben and half-graben during intracontinental rifting; (b) the Ellesmerian–Beaufortian Sag-Basin TSE, which comprises Middle Mississippian–Upper Triassic strata deposited in a sag basin following cessation of rifting; (c) the Beaufortian Synrift TSE, comprising Jurassic–Neocomian graben-fill deposits related to rifting in the Amerasia and North Chukchi basins; and (d) the Beaufortian Rift-Shoulder TSE, which comprises Jurassic–Neocomian strata related to rifting and deposited outside rift basins. (3) The BRM CTSE includes two TSEs: (a) the Beaufortian Synrift TSE, comprising Middle Jurassic–Neocomian synrift strata deposited on attenuated continental crust associated with opening of the Amerasia Basin; and (b) the Ellesmerian Platform TSE, comprising mainly shelf strata of Mississippian–Middle Jurassic age that lie beneath Beaufortian synrift strata. The AAB, HT and BRM CTSEs contain oil-prone source rocks in Triassic, Jurassic and Cretaceous strata, and proven reservoir rocks spanning Mississippian–Lower Cretaceous strata. A structurally high-standing area in the northern AAB CTSE, northern HT CTSE and southernmost BRM CTSE lies in the oil window, whereas all other areas lie in the gas window. Known hydrocarbon accumulations in the three CTSEs total more than 30 Bboe, and yet-to-find estimates suggest that a similar volume remains to be discovered.

New stegosaurs from the Middle Jurassic Lower Member of the Shaximiao Formation of Chongqing, China

ABSTRACT Stegosaurs are a major clade of ornithischian dinosaurs, yet because of their fragmentary fossil record, their interrelationships and early evolution are poorly understood. Here, we describe a new stegosaur, Bashanosaurus primitivus, gen. et sp. nov., and some other indeterminate stegosaur materials. We provide new U-Pb detrital zircon ages for horizons bounding the holotypic quarry in the Lower Member of the Shaximiao Formation, Yunyang, Chongqing Municipality, China, which indicate a Middle Jurassic (Bajocian) age. Bashanosaurus represents the earliest record of stegosaurs in Asia and one of the earliest records of this clade in the world. The dorsal vertebrae, scapula, coracoid, femur, and plates of Bashanosaurus primitivus possess several unique characters among Stegosauria, including the elevation of the parapophyses of dorsal vertebrae on stalks at the base of the transverse processes, a flared distal end of the scapula, and a small acromial process. Among armored dinosaurs (thyreophorans), these features are reminiscent of the basally branching taxon Scelidosaurus, indicating that Bashanosaurus possesses a unique mosaic of plesiomorphic thyreophoran and derived stegosaur features. Phylogenetic analysis shows that Bashanosaurus primitivus is the earliest-diverging stegosaur, along with Chungkingosaurus, from the Upper Member of the Shaximiao Formation, consistent with the early age of the taxon.