scispace - formally typeset
Search or ask a question

Showing papers in "Journal of the Geological Society in 2019"


Journal ArticleDOI
TL;DR: Tethyan ophiolites are highly diverse in their crustal-mantle structures and compositions, pointing to major differences in their melt sources and tectonic settings of magmatic construction.
Abstract: Tethyan ophiolites have played a major role in the development of the ophiolite concept during the past 200 years, and contributed significantly to the formulations of the suprasubduction-zone (SSZ) ophiolite paradigm and the plate-tectonics theory. Their melt evolution and magmatic construction witnessed the dispersal of the supercontinent Pangaea via continental rifting and seafloor spreading, opening and closure of multiple seaways in an eastward widening, latitudinal ocean basin (Neotethys), and deep mantle recycling processes through subduction-zone tectonics and plume activities. Neotethyan ophiolites are highly diverse in their crustal–mantle structures and compositions, pointing to major differences in their melt sources and tectonic settings of magmatic construction. The papers in this thematic set demonstrate these differences in the Neotethyan ophiolite record through new structural, geochemical, isotopic and geochronological data and interpretations. The Jurassic Western Alpine Ophiolites formed in a narrow basin (Alpine Tethys) that developed between Europe and North Africa–Adria–Iberia. Their peridotites represent exhumed, continental lithospheric mantle. The Triassic, Jurassic and Cretaceous ophiolites east of Adria formed in different Neotethyan seaways, and their SSZ magmatic construction involved multiple episodes of melting, depletion and refertilization of subduction-modified oceanic mantle. The Late Cretaceous peri-Arabian and peri-Indian ophiolites, which are discontinuously exposed along a c. 9000 km long belt from SW Anatolia to SE Tibet, formed above a Trans-Tethyan subduction–accretion system within Southern Neotethys. Diachronous collisions of Arabia and India with this intraoceanic subduction–accretion system in the latest Mesozoic and the Paleogene (respectively) resulted in diachronous ophiolite emplacement in the Mediterranean and Tibetan–Himalayan orogenic belts. Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

52 citations


Journal ArticleDOI
TL;DR: In this paper, a detailed overview of the Mazon creek Lagerstatte is presented, including the palaeoenvironmental conditions, organisms present and the complex taphonomic processes involved in fossil formation.
Abstract: One of the best records of late Paleozoic ecosystems, the Mazon Creek Lagerstatte is world famous for its striking flora and fauna preserved within siderite concretions. Distinct from other late Carboniferous concretionary Lagerstatten because of the remarkable fidelity of soft tissues and pigments that are frequently preserved, the Mazon Creek has seen a revival in investigations during the last 10 years using modern palaeontological techniques. However, many of these modern investigations build upon a literature that incorrectly interprets the palaeoenvironment of the Mazon Creek and the separate biotas: there is a lack of evidence to support a distinct freshwater fauna. Here, we present a detailed overview of the Mazon Creek Lagerstatte, including the palaeoenvironmental conditions, organisms present and the complex taphonomic processes involved in fossil formation. Investigation into the formation of siderite concretions and the complex taphonomic processes controlling soft-bodied preservation are still continuing but are reviewed in detail.

43 citations


Journal ArticleDOI
TL;DR: In this paper, a regional synthesis of the structural architecture and tectonic evolution of the Western Alpine Ophiolites (WAO), exposed in NW Italy, is presented.
Abstract: We present a regional synthesis of the structural architecture and tectonic evolution of the Western Alpine Ophiolites (WAO), exposed in NW Italy. The WAO represent the remnants of Alpine Tethys (Ligurian–Piedmont Ocean) that opened between Europe and Adria, and developed in four stages from the Middle Jurassic to Early Cretaceous. Emplacement of gabbroic intrusions into the extending lithospheric mantle of Europe–Adria marked the main magmatic event (Stage 1). Coalescent shear zones in the fossil upper mantle form lithospheric-scale detachment faults, which led to the exhumation of upper mantle peridotites and gabbros on the seafloor, and extensive serpentinization (Stage 2). Detachment faults, and serpentinized peridotites–gabbros in their footwalls, represent preserved fossil oceanic core complexes within the WAO. Emplacement of ophiolitic breccias and basaltic lava flows marked the syn-extensional phase (Stage 3). Radiolarian chert and limestone were deposited unconformably on this syn-extensional volcanic–sedimentary sequence, marking the post-extensional phase (Stage 4). Magmatic ages of gabbroic intrusions and mafic–felsic dykes, and depositional ages of post-extensional sequences in the WAO constrain the timing of the opening of the Ligurian–Piedmont Ocean to the Middle Jurassic (c. 170–168 Ma), followed by a tectonic quiescence stage in the Late Jurassic–Early Cretaceous. Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection9 available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

41 citations


Journal ArticleDOI
TL;DR: In this article, a combined determination of zircon U-Pb ages and Hf isotopes of Mesozoic to Paleogene intrusive and volcanic rocks from southern Tibet to Myanmar to characterize the two parallel magmatic belts that have previously been considered separately.
Abstract: Before the India–Asia collision, Neotethyan subduction gave rise to an Andean-type convergent margin on the southern margin of Asia. To investigate the spatial and temporal distribution of the subduction-related magmatism, we undertook a combined determination of zircon U–Pb ages and Hf isotopes of Mesozoic to Paleogene intrusive and volcanic rocks from southern Tibet to Myanmar to characterize the two parallel magmatic belts that have previously been considered separately. One belt extends from the Gangdese Batholith in the Southern Lhasa sub-terrane to the Lohit Batholith, the Sodon Pluton and the Popa–Loimye Arc in the West Burma Block, and the other from the Central Lhasa Plutonic Belt to the Bomi–Chayu Batholith, the Dianxi Batholith and the Shan Scarps Batholith in central Myanmar. The Gangdese belt, as the main arc component, consists typically of I-type granitoids that contain magmatic zircons showing positive eHf(t) values. In contrast, the Central Lhasa Plutonic Belt belt is dominated by S-type granites in which most zircons show negative eHf(t) values suggesting the involvement of older continental crust in their petrogenesis. The distinct geochemical characteristics indicate not only distinct tectonic settings of their genesis but also the diverse nature of the crust forming the exotic continental ribbons amalgamated to Asia. Supplementary material: Details of sample locations and analytical results are available at: https://doi.org/10.6084/m9.figshare.c.4311485

39 citations


Journal ArticleDOI
TL;DR: In this article, the authors used the Strangman9s Cove (Devon) outcrop of the Mercia Mudstone Group in the UK to study the vegetation changes and palaeoclimate trends during the mid-Carnian Pluvial Episode (CPE).
Abstract: The generally arid Late Triassic climate was interrupted by a wet phase during the mid-Carnian termed the Carnian Pluvial Episode (CPE). Quantitative palynological data from the Mercia Mudstone Group in the Wessex Basin (UK) reveal vegetation changes and palaeoclimate trends. Palynostratigraphy and bulk organic carbon isotope data allow correlation to other Carnian successions. The palynostratigraphy indicates that the Dunscombe Mudstone is Julian and the lowest part of the overlying Branscombe Mudstone Formation is Tuvalian. The Aulisporites acme characterizing the CPE in Tethyan successions and the Germanic Basin is missing in the UK. The quantitative palynological record suggests the predominance of xerophyte floral elements with a few horizons of increased hygrophytes. A humidity signal is not seen owing to the dry climate in central Pangea. Also, the signal might be masked by the overrepresentation of xerophyte regional pollen and the predominance of xerophyte hinterland flora. The bias towards regional pollen rain is enhanced by the potential increase in continental runoff related to seasonally humid conditions and differences in pollen production rates and transport mechanisms. The vegetation of British CPE successions suggests a more complex climate history during the Carnian, indicating that the CPE is not recognized by the same changes everywhere. Supplementary material: A detailed lithological log of the Strangman9s Cove (Devon) outcrop with the description of the Mercia Mudstone Group lithostratigraphical units, a description of the laboratory techniques, seven photoplates with selected spores and pollen grains, a list of all identified palynomorphs, Excel sheets with the palynological and palynofacies counts, bulk organic carbon isotope ratios and total organic carbon values of the Strangman9s Cove outcrop are available at https://doi.org/10.6084/m9.figshare.c.4138085

37 citations


Journal ArticleDOI
TL;DR: In this article, the effect of a gradual increase of the mantle fO2 on the mobilization of C is discussed along with the main variables affecting its transport by subduction into the mantle.
Abstract: Over recent decades, many experimental studies have focused on the effect of CO2 on phase equilibria and melting behaviour of synthetic eclogites and peridotites as a function of pressure and temperature. These studies have been of fundamental importance to understanding the origin of carbonated magmas varying in composition from carbonatitic to kimberlitic. The occurrence of diamonds in natural rocks is further evidence of the presence of (reduced) carbon in the Earth9s interior. The oxygenation of the Earth9s interior (i.e. its redox state) through time has strongly influenced the speciation of carbon from the mantle to mantle-derived magmas and, in turn, to the volcanic gases released to the atmosphere. This paper explains how the knowledge of the oxygen fugacity recorded by mantle rocks and determined through the use of appropriate oxy-thermobarometers allows modelling of the speciation of carbon in the mantle, its mobilization in the asthenospheric mantle by redox partial melting, and its sequestration and storage during subduction by redox freezing processes. The effect of a gradual increase of the mantle fO2 on the mobilization of C is here discussed along with the main variables affecting its transport by subduction into the mantle.

35 citations


Journal ArticleDOI
TL;DR: The lower Cambrian Lagerstatte of Sirius Passet, Nansen Land, North Greenland, is one of the oldest of the Phanerozoic, exceptionally preserved biotas as discussed by the authors.
Abstract: The lower Cambrian Lagerstatte of Sirius Passet, Nansen Land, North Greenland, is one of the oldest of the Phanerozoic, exceptionally preserved biotas. The Lagerstatte evidences the escalation of numbers of new body plans and life modes that formed the basis for a modern, functionally tiered ecosystem. The fauna is dominated by predators – infaunal, benthic and pelagic – and the presence of abundant nekton, including large sweep-net feeders, suggests an ecosystem rich in nutrients. Recent discoveries have helped reconstruct digestive systems and their contents; muscle fibres; and visual and nervous systems, for a number of taxa. New collections have confirmed the complex combination of taphonomic pathways associated with the biota and its potentially substantial biodiversity. These complex animal-based communities within the Buen Formation were associated with microbial matgrounds, now preserved in black mudstones deposited below storm-wave base that provide insight to the shift from late Neoproterozoic (Ediacaran) to Cambrian substrates and communities. Moreover, the encasing sediment holds important data on the palaeoenvironment and the water-column chemistry, suggesting that these animal-based communities developed in conditions with very low oxygen concentrations.

34 citations


Journal ArticleDOI
TL;DR: In this article, the authors present a comprehensive high-precision zircon U-Pb geochronology dataset for the major intrusive complexes and several volcanic ash layers and integrate this with a high-resolution stratigraphic framework of Middle Triassic volcano-sedimentary successions.
Abstract: Middle Triassic magmatism in the Southern Alps (northern Italy) consists of widespread volcanoclastic deposits, basaltic lava flows and intrusive complexes. Despite their importance in understanding the geodynamic evolution of the westernmost Tethys, the timing of magmatic activity and the links between the different igneous products remain poorly understood. We present a comprehensive high-precision zircon U–Pb geochronology dataset for the major intrusive complexes and several volcanic ash layers and integrate this with a high-resolution stratigraphic framework of Middle Triassic volcano-sedimentary successions. The main interval of Middle Triassic magmatism lasted at least 5.07 ± 0.06 myr. Magmatic activity started with silicic eruptions between 242.653 ± 0.036 and 238.646 ± 0.037 Ma, followed by a Supplementary material: Isotope dilution thermal ionization mass spectrometry U–Pb and laser ablation inductively coupled plasma mass spectrometry trace element data tables, sample coordinates, supplementary geochemical data, cathodoluminescence images of isotope dilution thermal ionization mass spectrometry dated zircons and supplementary field documentation are available at https://doi.org/10.6084/m9.figshare.c.4287506

33 citations


Journal ArticleDOI
TL;DR: The Yazidaban ophiolitic melange, located in the westernmost region of the Qimantagh-Xiangride suture between the Northern and Central Kunlun Orogenic belts, consists predominantly of serpentinite, basalt, diabase and andesite.
Abstract: The East Kunlun Orogenic Belt of the northern Tibetan Plateau can be divided by the Qimantagh–Xiangride, Aqikekulehu–Kunzhong and Muztagh–Buqingshan sutures into, from north to south, the Northern Qimantagh, Central Kunlun and Southern Kunlun belts. The Yazidaban ophiolitic melange, located in the westernmost region of the Qimantagh–Xiangride suture between the Northern Qimantagh and Central Kunlun belts, consists predominantly of serpentinite, basalt, diabase and andesite. The serpentinites are characterized by low total rare earth element (ΣREE) concentrations and depletion of mid-REEs, showing an ophiolitic ultramafic affinity. The basalts have low ΣREE concentrations, a slight enrichment in light REEs, depletion of large ion lithophile elements and insignificant fractionation of high field strength elements, which are attributed to an incompatible element-enriched mid-ocean ridge basalt. Both diabases and andesites are characterized by high ΣREE, strong enrichment in light REEs, depletion of Nb, Ta, P and Ti, and significant fractionation of high field strength elements. These geochemical characteristics indicate that the diabase and andesite were mostly generated in a subduction-related setting. Igneous zircons from the diabase yielded a 206Pb/238U age of 421.5 ± 2.2 Ma (MSWD = 0.44), representing the formation time of the subduction-related rocks. Together with the regional geology, these constraints on the ultramafic rock, basalt, andesite and diabase suggest that the Yazidaban ophiolitic melange represents the remnants of oceanic crust from a back-arc basin and the associated subduction-related magmatic rocks. Supplementary material: Details of U–Th–Pb laser ablation inductively coupled plasma mass spectrometry data of zircons from the diabase, and major and trace element compositions for the samples from the Qimantagh melange, East Kunlun Orogen are available at https://doi.org/10.6084/m9.figshare.c.4283219

32 citations


Journal ArticleDOI
TL;DR: In this article, the carbon balance of regional aquifers is a powerful tool to quantify the diffuse degassing of deep inorganic carbon sources because the method integrates the CO2 flux over large areas.
Abstract: Tectonically active regions are often characterized by large amounts of carbon dioxide degassing, and estimation of the total CO2 discharged to the atmosphere from tectonic structures, hydrothermal systems and inactive volcanic areas is crucial for the definition of present-day global Earth degassing The carbon balance of regional aquifers is a powerful tool to quantify the diffuse degassing of deep inorganic carbon sources because the method integrates the CO2 flux over large areas Its application to peninsular Italy shows that the region is characterized by specific CO2 fluxes higher than the baseline determined for the geothermal regions of the world, and that the amount of endogenous CO2 discharged through diffuse regional degassing (c 21 × 1011 mol a−1) is the major component of the geological CO2 budget of Italy, definitely prevailing over the CO2 discharged by Italian active volcanoes and volcanoes with hydrothermal activity Furthermore, the positive correlation between geothermal heat and deep CO2 dissolved in the groundwater of central Italy suggests that (1) the geothermal heat is transported into the aquifers by the same hot CO2-rich fluids causing the Italian CO2 anomaly and (2) the advective heat flow is the dominant form of heat transfer of the region Supplementary material: The location, flow rate, extent of the hydrogeological basin, chemical and isotopic analyses of the 160 springs considered in this study, and the results of the carbon mass balance are reported in a table available at https://doiorg/106084/m9figsharec4237025

30 citations


Journal ArticleDOI
TL;DR: In this article, the role of COH fluids in the stability of hydrous and carbonate minerals is discussed comparing experimental results with thermodynamic models, together with the measurement of experimental COH fluid composition in terms of volatiles and dissolved solutes.
Abstract: The valence of carbon is governed by the oxidation state of the host system The subducted oceanic lithosphere contains considerable amounts of iron so that Fe 3+ /Fe 2+ equilibria in mineral assemblages are able to buffer the f O 2 and the valence of carbon Alternatively, carbon itself can be a carrier of redox budget when transferred from the slab to the mantle, prompting the oxidation of the sub-arc mantle Also, the oxidation of sedimentary carbonaceous matter to CO 2 in the slab could consume the available redox budget Therefore, the correct use of intensive and extensive variables to define the slab-to-mantle redox budget by C-bearing fluids is of primary importance when considering different fluid/rock ratios Fluid-mediated processes at the slab-mantle interface can be investigated also experimentally The presence of CO 2 (or CH 4 at highly reduced conditions) in aqueous COH fluids in peridotitic systems affects the positions of carbonation/decarbonation reactions and of the solidus Some methods to produce and analyse COH fluid-saturated experiments in model systems are introduced, together with the measurement of experimental COH fluids composition in terms of volatiles and dissolved solutes The role of COH fluids in the stability of hydrous and carbonate minerals is discussed comparing experimental results with thermodynamic models

Journal ArticleDOI
TL;DR: In this paper, an analysis of the early Billefjorden Trough, an exceptionally well-exposed Carboniferous half-graben located in Svalbard, Arctic Norway, allows detailed reconstruction of fault development and basin geometry.
Abstract: Analysis of the sedimentary architecture of the early Billefjorden Trough, an exceptionally well-exposed Carboniferous half-graben located in Svalbard, Arctic Norway, allows detailed reconstruction of fault development and basin geometry. Sedimentary facies distribution reveals facies variability in an early synrift fill, when an array of meso-scale intra-basinal faults segmented the basin into rotated blocks with differential subsidence. Growth of these faults into larger systems impacted the distribution of accommodation space and the drainage pattern expressed in fluvial and deltaic sandstone-dominated units. An axially back-stepping character of the early synrift facies associations (Stages 1–3) is reflected in aggrading homogenous facies types of continental depositional Stage 1, which evolved into back-stepping facies belts of deltaic to nearshore depositional environment, Stages 2 and 3, due to the opening of a connection to the sea. The drainage pattern and accommodation distribution in the early synrift deposits reflect a relatively symmetrical basin that predates a later prominent half-graben configuration of the basin. Supplementary material: Figures presenting lithostratigraphic columns of the well data (Supplementary material 1), location of logs in Ragnardalen (Supplementary material 2) and geological map used in Figure 2b (Supplementary material 3) are available at https://doi.org/10.6084/m9.figshare.c.4293077

Journal ArticleDOI
TL;DR: In this paper, the authors report geological and zircon-rutile-titanite U-Pb data for rocks from the presumed Jurassic ophiolite obtained to verify whether these are real ophiliitic units and to understand their relations either to the Late Cretaceous Zagros-Bitlis ophiolaites in western Iran-southern Anatolia or to the Sevan-Akera (northern Armenia) and Izmir-Ankara (southern Pontides) complexes.
Abstract: The Khoy complex in NW Iran has been widely regarded to consist of both Jurassic and Cretaceous ophiolites, but whereas the western Late Cretaceous ophiolite is unequivocal the nature of the eastern Jurassic body has been unclear. Field observations show that the presumed eastern meta-ophiolite has no similarities to an ophiolite sequence. Here, we report geological and zircon–rutile–titanite U–Pb data for rocks from the presumed Jurassic ophiolite obtained to verify whether these are real ophiolitic units and to understand their relations either to the Late Cretaceous Zagros–Bitlis ophiolites in western Iran–southern Anatolia or to the Sevan–Akera (northern Armenia) and Izmir–Ankara (southern Pontides) complexes. The new U–Pb ages show that the ‘presumed’ ophiolite is in fact a collage of Ediacaran to Cambrian (c. 606–517 Ma) and Jurassic (c. 160 Ma) meta-igneous rocks, similar to ages obtained for igneous rocks of the Sanandaj–Sirjan Zone, which has been interpreted as a Jurassic continental rift. The Jurassic igneous rocks contain abundant Ediacaran, Ordovician–Silurian and Carboniferous–Permian inherited zircons, further suggesting involvement of pre-existing continental crust. Our results indicate that Jurassic continental rifting provided a lithospheric weakness along which a new subduction zone formed in Late Cretaceous times.

Journal ArticleDOI
TL;DR: In this paper, Li et al. reported new biostratigraphic and carbon stable isotope data from Hanwang and Jushui, northwestern Sichuan Basin, showing that the lithological change is Late Carnian to Early Norian in age.
Abstract: The northwestern Sichuan Basin (South China) was a portion of eastern Tethys where, during the Late Triassic, a sharp lithological transition from oolitic–bioclastic limestones and sponge reef mounds to dark grey terrigenous clays, siltstones and sandstones is visible in several localities. The timing and significance of this major facies transition is still unclear. Here we report new biostratigraphic and carbon stable isotope data from Hanwang and Jushui, northwestern Sichuan Basin. Sporomorphs, ammonoids, conodonts and halobiid bivalves show that the lithological change is Late Carnian to Early Norian in age. This amended age determination facilitates recalibration of the magnetostratigraphy in the area allowing correlation between the Late Triassic of the Sichuan Basin and the Astrochronostratigraphic Polarity Time Scale. A carbon stable isotopic perturbation across the Carnian–Norian Boundary (CNB) is missing in our sections, or it is concealed because of the mixed organic matter sources. Our findings pinpoint the position of the CNB to a short stratigraphic interval of c. 12 m thickness in the Sichuan Basin. The studied sections greatly extend the palaeogeographical documentation of the CNB and provide novel information on biostratigraphy and chemostratigraphy that should be considered in defining the best position of the Norian Global Stratotype Section and Point.

Journal ArticleDOI
TL;DR: In this paper, the Red Crag Formation was used to estimate how much time is preserved at outcrop scale and found that every outcrop provides only a vanishingly small window onto unanchored weeks to months within the 600-800'kyr of "Crag-time".
Abstract: It is widely understood that Earth9s stratigraphic record is an incomplete record of time, but the implications that this has for interpreting sedimentary outcrop have received little attention. Here we consider how time is preserved at outcrop using the Neogene–Quaternary Red Crag Formation, England. The Red Crag Formation hosts sedimentological and ichnological proxies that can be used to assess the time taken to accumulate outcrop expressions of strata, as ancient depositional environments fluctuated between states of deposition, erosion and stasis. We use these to estimate how much time is preserved at outcrop scale and find that every outcrop provides only a vanishingly small window onto unanchored weeks to months within the 600–800 kyr of ‘Crag-time’. Much of the apparently missing time may be accounted for by the parts of the formation at subcrop, rather than outcrop: stratigraphic time has not been lost, but is hidden. The time-completeness of the Red Crag Formation at outcrop appears analogous to that recorded in much older rock units, implying that direct comparison between strata of all ages is valid and that perceived stratigraphic incompleteness is an inconsequential barrier to viewing the outcrop sedimentary-stratigraphic record as a truthful chronicle of Earth history. Supplementary material: Further details of the regional geology and specific information on outcrops are available at https://doi.org/10.6084/m9.figshare.c.4561001

Journal ArticleDOI
TL;DR: In this article, the authors present the background needed to carry out theoretical geochemical modelling of fluids and fluid-rock interactions from surficial conditions into the upper mantle, and a description of the general criteria for predicting equilibrium and non-equilibrium chemical reactions is followed by a summary of how the thermodynamic activities of species are related to measurable concentrations through standard states and activity coefficients.
Abstract: Carbon is subducted to depths where metamorphism liberates water-bearing fluids. The C-bearing fluids facilitate partial melting of the upper mantle, generating magmas that may erupt as arc volcanics. Degassing of the magmas releases CO2 and other volatile species to the atmosphere. Over geological time, this process contributes to the composition of the atmosphere and planetary habitability. Here I summarize the background needed to carry out theoretical geochemical modelling of fluids and fluid–rock interactions from surficial conditions into the upper mantle. A description of the general criteria for predicting equilibrium and non-equilibrium chemical reactions is followed by a summary of how the thermodynamic activities of species are related to measurable concentrations through standard states and activity coefficients. Specific examples at ambient conditions involving dilute water are detailed. The concept of aqueous speciation and how it can be calculated arises from this discussion. Next, I discuss how to calculate standard Gibbs free energies and aqueous activity coefficients at elevated temperatures and pressures. The revised Helgeson–Kirkham–Flowers equations of state are summarized and the revised predictive correlations for the estimation of equation of state coefficients in the Deep Earth Water (DEW) model are presented. Finally, the DEW model is applied to the solubility and speciation of aqueous aluminium.

Journal ArticleDOI
TL;DR: In this paper, detrital zircon U-Pb age and Lu-Hf isotopes, compiled with published data, provide constraints on the basin provenance and reveal the dynamic tectonic evolution of Mesoproterozoic northern Australia.
Abstract: The upper Beetaloo Sub-basin of the McArthur Basin, Northern Territory, Australia, records over 500 myr of tectonic history of the North Australian Craton from c. 1.45 to 0.9 Ga. The basin sequences include shallow-water clastic sedimentary rocks that preserve the oldest global commercial hydrocarbon reserves. New detrital zircon U–Pb age and Lu–Hf isotopes, compiled with published data, provide constraints on the basin provenance and reveal the dynamic tectonic evolution of Mesoproterozoic northern Australia. Data from the oldest formation examined, the c. 1.4 Ga Bessie Creek Sandstone, suggest provenance from (present-day) eastern sources (e.g. the Mount Isa Province and the palinspastically adjacent Curnamona and Georgetown provinces) with considerable spatial heterogeneity. These eastern source regions are interpreted as uplifted rift-shoulder highs, formed by contemporaneous extension between Proterozoic Australia and Laurentia. Progressively younger formations (the Velkerri Formation, the Moroak Sandstone and the Kyalla Formation) demonstrate a rapid swamping of the basin by detritus from southerly sources (e.g. the Arunta Region) that occurred at c. 1.4–1.3 Ga. This is particularly characterized by the up-section reduction of c. 1.6–1.5 Ga detrital zircon grains. This change in provenance is interpreted to relate to closure of an ocean basin during the period 1.35–1.25 Ga, which resulted in uplift of the southern margin of the North Australia Craton. Three ungrouped latest Mesoproterozoic to Neoproterozoic sedimentary units, the lower and upper Jamison sandstone and the Hayfield mudstone, were deposited after the emplacement of the Warakurna Large Igneous Province and are sourced from the Musgrave Province. Detrital zircon U–Pb and Hf isotope affinities between the lower and upper Jamison sandstone and the Hayfield mudstone and the latest Mesoproterozoic to early Neoproterozoic successions along the eastern margin of the North China Craton suggest that they share a similar provenance. This supports correlations between the Mesoproterozoic of the North China Yanshan Basin and the greater McArthur Basin. Supplementary material: Sample descriptions, details of data and concordia plots are available at https://doi.org/10.6084/m9.figshare.c.4444790

Journal ArticleDOI
TL;DR: In this article, the carbon isotope (δ13C) geochemistry of carbonates from sections in southwestern China and northern Oman was studied, showing a progressive positive shift from 1.4 to 2.8 in the early to middle Julian 1 substage, followed by a negative shift of c. 3.8 to −0.3 in the Julian 2 substage.
Abstract: The Carnian Humid Episode is an interval of prominent climatic changes in the Late Triassic. We studied the carbon isotope (δ13C) geochemistry of carbonates from sections in southwestern China and northern Oman. δ13C records from the Yongyue section (western Guizhou, South China) show a progressive positive shift from 1.4 to 2.8‰ in the early to middle Julian 1 substage. This positive trend is followed by a swift negative shift of c. 4.2‰ from 2.8 to −1.4‰ in the Julian 2 substage. δ13C from the Wadi Mayhah section (northern Oman) shows a positive shift from 2.2 to 2.8‰ in the Julian 1 substage, followed by a negative shift of c. 3.2‰ from 2.8 to −0.3‰ in the Julian 2 substage. The δ13C records from the two study sections generally correlate well with each other as well as with published records, pointing to a considerable input of isotopically light carbon starting in the late Julian 1 substage. Such a large amount of light carbon probably derived from direct degassing and the sediment–sill contact metamorphism of the Panthalassan Wrangellia Large Igneous Province and contemporary Tethyan volcanism. The voluminous volcanogenic greenhouse gases probably contributed to the warming pulse in the middle Carnian. Thus the dry–wet climatic transition during the Carnian Humid Episode is best interpreted as a warm climate-driven intensification of the activities of the atmospheric circulation and hydrological cycle.

Journal ArticleDOI
TL;DR: The Middle Jurassic Rattray Volcanic Province is located at the triple junction of the North Sea continental rift system and was previously thought to be sourced from three large central volcanoes: the Glenn, Fisher Bank and Ivanhoe volcanic centres.
Abstract: The Middle Jurassic Rattray Volcanic Province is located at the triple junction of the North Sea continental rift system. It has previously been thought to be sourced from three large central volcanoes: the Glenn, Fisher Bank and Ivanhoe volcanic centres. Re-interpretation using 3D seismic and well data shows that no volcanic centres are present and the Rattray Volcanics were instead sourced in fissure eruptions from linear vents, including the Buchan–Glenn Fissure System, a c. 25 km long zone of WSW–ENE-striking linear fissure vents and associated small volcanic edifices across the Buchan–Glenn Horst. The orientation of the fissures is broadly parallel to the Highland Boundary Fault, which intersects the Rattray Volcanics at the Buchan–Glenn Fissure System, implying that Mid-Jurassic magmatism exploited pre-existing crustal structural anisotropies established during the Caledonian Orogeny. The lack of large intrusive complexes beneath the Rattray Volcanics indicates that the pre-Middle Jurassic sedimentary sequences (e.g. the Devonian–Carboniferous Old Red Sandstone Group, the Permian Rotliegend and Zechstein groups and the Triassic Skagerrak Formation) extend further than previously supposed and therefore the presence of possible subvolcanic reservoir and source rock units within the triple junction of the Central North Sea may have been overlooked.

Journal ArticleDOI
TL;DR: The Spence Shale Member of the Langston Formation is a Cambrian (Miaolingian: Wuliuan) Lagerstatte in northeastern Utah and southeastern Idaho as discussed by the authors.
Abstract: The Spence Shale Member of the Langston Formation is a Cambrian (Miaolingian: Wuliuan) Lagerstatte in northeastern Utah and southeastern Idaho. It is older than the more well-known Wheeler and Marjum Lagerstatten from western Utah, and the Burgess Shale from Canada. The Spence Shale shares several species with these younger deposits, yet it also contains a remarkable number of unique species. Because of its relatively broad geographical distribution, and the variety of palaeoenvironments and taphonomy, the fossil composition and likelihood of recovering weakly skeletonized (or soft-bodied) taxa varies across localities. The Spence Shale is widely acknowledged not only for its soft-bodied taxa, but also for its abundant trilobites and hyoliths. Recent discoveries from the Spence Shale include problematic taxa and provide insights about the nature of palaeoenvironmental and taphonomic variation between different localities. Supplementary material: A generic presence–absence matrix of the Spence Shale fauna and a list of the Spence Shale localities are available at: https://doi.org/10.6084/m9.figshare.c.4423145

Journal ArticleDOI
TL;DR: In this article, the authors focused on the deep-water chert-mudstone strata of the Gufeng Formation and its transition to the Yinping Formation at Chaohu.
Abstract: The Capitanian (Guadalupian) witnessed one of the major crises of the Phanerozoic and, like many other extinctions, it coincided with the eruption of a large igneous province, in this case the Emeishan Traps of SW China. However, the timing and causal relationships of this event are in dispute. This study concentrates on the deep-water chert–mudstone strata of the Gufeng Formation and its transition to the Yinping Formation at Chaohu. Zircons from tuffs in the uppermost Gufeng Formation yield a U–Pb age of 261.6 ± 1.6 Ma, and comparison with sections around Emeishan suggests that the tuffs appeared in the Jinogondolella altudaensis conodont zone and persisted to the Jinogondolella xuanhanensis zone. This coincides with the Emeishan eruptions, and suggests that the tuffs probably derived from this province. Mineralogical and geochemical characteristics also show the tuffs are of acid volcanogenic origin and have a geochemical fingerprint of the Emeishan large igneous province. Our dating shows that a crisis amongst radiolarian and a subsequent productivity decline occurred during the middle Capitanian, prior to the Guadalupian–Lopingian boundary. The Emeishan eruptions began immediately before this, indicating a likely causal relationship between these events. Major regression and marine anoxia/euxinia are two other important extinction-relevant environmental changes that occurred during this critical interval.

Journal ArticleDOI
TL;DR: In this article, the authors investigate the sedimentary system located at their toe across magma-poor rifted margins and show that mega fault-scarps are preferentially located at rift domain boundaries, implying a topographic escarpment that juxtaposes rift domains of different crustal thickness.
Abstract: Thinning of the continental crust during rifting is accommodated by a number of major faults, only a few of which produce long-lived mega fault-scarps. In this paper, we investigate mega fault-scarps and the sedimentary system located at their toe across magma-poor rifted margins. Our approach combines observations from subsurface examples along present-day margins and field analysis of fossil examples exposed in the Alpine Tethys margins. Whereas present-day examples of rifted margins imaged by seismic techniques provide details about the architecture of the top basement and the relationship between faults and sediments, outcrops give access to the sedimentary features of rocks related to mega fault-scarps. Our study shows that (1) mega fault-scarps are preferentially located at rift domain boundaries, implying a topographic escarpment that juxtaposes rift domains of different crustal thickness, and (2) mega fault-scarps are long-lived local sources for syn- and post-tectonic breccia, respectively produced during and after fault activity.

Journal ArticleDOI
TL;DR: In this article, the authors focus on the Late Cretaceous, non-ophiolitic Sabzevar magmatic rocks, and present both precise U-Pb ages and a tectonomagmatic scenario for the genesis of these igneous rocks.
Abstract: Despite detailed geological studies on the Late Cretaceous Sabzevar ophiolites, the non-ophiolitic, subduction-related igneous rocks of NE Iran are still enigmatic. This paper focuses on the Late Cretaceous, non-ophiolitic Sabzevar magmatic rocks, and presents both precise U–Pb ages and a tectonomagmatic scenario for the genesis of these igneous rocks. A thick sequence of Late Cretaceous acidic volcanic rocks associated with sandstones, shales and pelagic limestones dominates the northern parts of the Lut block. This sequence is intruded by a series of shallow mafic to felsic intrusions. New zircon U–Pb results on the intrusive and extrusive rocks show ages of 75–101 Ma. These ages show a pulsed magmatism in NE Iran, which lasted for c. 25 myr. The whole-rock geochemistry of the igneous rocks shows suprasubduction-zone-related geochemical signatures, represented by negative anomalies in high field strength elements and enrichment in large ion lithophile elements. The initial 87Sr/86Sr ratios and eNd(t) values of the extrusive rocks range from 0.70411 to 0.70628 and from +5.9 to +7.4 respectively, and for intrusive rocks are in the range of 0.70423–0.70579 and +5.8 to +7.2. High eNd(t) values for these rocks confirm that their melts were derived from a depleted-mantle source. Both geochemical and isotopic data indicate that the genesis of these rocks was related to the partial melting of mid-ocean ridge basalt-type slab or depleted-mantle wedge sources during the northward movement of the Sabzevar oceanic slab beneath the southern edge of Eurasia (Turan block) in Late Cretaceous time.

Journal ArticleDOI
TL;DR: The Aladag Ophiolite as discussed by the authors is one of the largest Neotethyan ophiolites in Turkey and represents Cretaceous oceanic lithosphere derived from the Inner Tauride seaway.
Abstract: The Aladag Ophiolite (also known as the Pozanti-Karsanti Ophiolite) is one of the largest Neotethyan ophiolites in Turkey and represents Cretaceous oceanic lithosphere derived from the Inner Tauride seaway of Neotethys. Its peridotites consist of spinel harzburgites, dunites and chromitites. The harzburgites display a depleted mineral and whole-rock geochemistry, indicating high degrees of partial melting. They contain Pd- and Pt-rich interstitial sulphides, which crystallized from S-saturated melts during melt–rock interactions. Their spoon-shaped rare earth element patterns indicate metasomatism by fluids, derived from a subducting slab, which were enriched in light rare earth elements. The chromitites in both the mantle and mantle–crust transition zone rock units show similar geochemical compositions, typical of high-Cr chromitites. They also show similar chondrite-normalized platinum group element patterns characterized by Os, Ir, Ru and Rh enrichments relative to Pt and Pd. The parental melts of the Aladag Ophiolite chromitites were S-undersaturated and boninitic in composition. The presence of microdiamond, moissanite, zircon and needle-shaped clinopyroxene exsolutions in chromite (chr) grains suggests that their formation might have originally involved high-temperature and high-pressure conditions within the mantle transition zone before their participation in further melting and the formation of oceanic crust at shallow depths in a forearc tectonic setting during the initiation of intra-oceanic subduction within the Inner Tauride seaway. Supplementary material: Sampling and analytical methods, representative microprobe analyses and whole-rock major and trace element compositions are available at https://doi.org/10.6084/m9.figshare.c.4153631 Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

Journal ArticleDOI
TL;DR: In this article, the authors show that the total estimated thickness of sediment during Cretaceous is likely an overestimate as the sedimentary fill consists of significant quantities of igneous material which was emplaced 10-55 Ma years after deposition.
Abstract: The identification of extensive intrusive igneous complexes both in subsurface data and in field studies has resulted in quantification of the volumes of igneous material. Despite this research there is still little connection established between the amount of igneous material intruded into a basin and its effect on subsequent basin evolution in terms of burial and loading. To understand how additional igneous material may influence basin evolution we present work from the Faroe-Shetland Basin (FSB) utilising subsurface data. This study highlights that the total estimated thickness of sediment during Cretaceous is likely an overestimate as the sedimentary fill consists of significant quantities of igneous material which was emplaced 10-55 Ma years after deposition. Previously this additional igneous material has not been accounted for in estimates of sedimentation rates and the burial history of the FSB. Importantly petroleum system modelling to understand generation and expulsion of hydrocarbons benefits from correct estimates of basin fill. The overthickening of basins by igneous material will affect the timing of hydrocarbon generation and subsequently the proper evaluation of exploration targets. In order to fully understand basin evolution the volumes of igneous material and when this material was emplaced must be acknowledged and considered.

Journal ArticleDOI
TL;DR: In this article, a basin-scale study on the Schilfsandstein, the type-example of the mid-Carnian episode, applied sedimentological, palynological and palaeobotanical proxies of the palaeoclimate to a large dataset of cored wells and outcrops.
Abstract: Recently intensified research on the mid-Carnian episode stimulated discussions about the mid-Carnian climate and a supposed humid climate shift. This basin-scale study on the Schilfsandstein, the type-example of the mid-Carnian episode, applied sedimentological, palynological and palaeobotanical proxies of the palaeoclimate to a large dataset of cored wells and outcrops. The results demonstrate the primary control of circum-Tethyan eustatic cycles on the Central European Basin where transgressions contributed to basin-scale facies shifts. The palaeoclimate proxies point to a uniform arid to semi-arid Carnian climate with low chemical weathering and high evaporation. Consequently, transgressions into the Central European Basin led to increased evaporation forcing the hydrological cycle. The increased runoff from source areas resulted in high-groundwater stages on lowlands characterized by hydromorphic palaeosols and intrazonal vegetation with hygrophytic elements. During lowstands, reduced evaporation and runoff led to increased drainage and desiccation of lowlands characterized by formation of vertisols, calcisols and gypsisols and zonal vegetation with xerophytic elements. The proposed model of sea-level control on the hydrological cycle integrates coeval and subsequent occurrences of wet and dry lowlands, hydromorphic and well-drained palaeosols, and intrazonal and zonal vegetations. Thus, the Schilfsandstein does not provide arguments for a humid mid-Carnian episode. Supplementary material: Datasets of Palynomorph Eco Group (PEG) and Macroplant Eco Group (MEG) analyses are available at https://doi.org/10.6084/m9.figshare.c.4182593

Journal ArticleDOI
TL;DR: In this paper, the Naver nappe experienced relatively fast heating (c.c. 50°C/myr−1) and relatively slow cooling (15 −20°C /myr −1), with peak temperatures occurring at c.c. 425/Ma.
Abstract: The Northern Highlands Terrane of Scotland hosts several thrust nappes that were deformed and metamorphosed during the Silurian Scandian orogeny. Quantitative petrological analysis of metamorphic assemblages indicates that the hinterland-positioned Naver nappe experienced decompression heating from 8–9 kbar and 600°C to 6–7 kbar and 700°C. Monazite–xenotime thermometry and geochronology delineate a detailed temperature–time history for the Naver nappe. Monazite often exhibits compositional zoning, which is used to establish multiple temperature–time points in several samples. These data indicate that the Naver nappe experienced relatively fast heating (c. 50°C myr−1) and relatively slow cooling (15–20°C myr−1), with peak temperatures occurring at c. 425 Ma. This temperature–time evolution is compatible with the early Emsian (407–403 Ma) deposition of unmetamorphosed conglomerates that rest on high-grade metamorphic rocks in the Naver nappe, but requires an acceleration in the cooling rate to 40–50°C myr−1 at 420–410 Ma. Geochronological constraints from this study and previous work suggest that deformation and metamorphism in the hinterland of the Scandian orogen in northern mainland Scotland are younger than the c. 430 Ma deformation in the foreland-positioned Moine thrust zone. We postulate that heat from pervasive granitic intrusions in the Naver nappe weakened the crust, allowing deformation to retreat to the hinterland of the orogen. Supplementary material: A description of our analytical methods, all U–Pb-trace element data, additional figures explaining our petrological analysis and other relevant data are available at: https://doi.org/10.6084/m9.figshare.c.4458041

Journal ArticleDOI
TL;DR: In this article, the authors identify at least four horizons of enhanced anoxia (anoxic horizons; AHs) in the uppermost Eifelian-Middle Frasnian mudrock dominated strata of the Mackenzie Valley and Peel area of NW Canada.
Abstract: At least four horizons of enhanced anoxia (anoxic horizons; AHs) are recognized in the uppermost Eifelian–Middle Frasnian mudrock-dominated strata of the Mackenzie Valley and Peel area of NW Canada. Aluminium-normalized Mo and U logs in two cored sections reveal AH-I at the Eifelian–Givetian boundary, AH-II in basal Frasnian, and AH-III and AH-IV bundled in the Middle Frasnian interval. These four horizons are characterized by attenuated siliciclastic components. Spectral gamma-ray K + Th and U are the best tools to trace these horizons in wells and outcrops. AHs are biostratigraphically correlated with ‘black-shale events’ in several basins of the world. Depositional environment is depicted as a stratified basin where the water-column chemocline defined co-sedimentation of anoxic mudrocks in topographic lows and oxic grey shales and carbonate banks on seafloor elevations. Based on inductively coupled plasma elemental data from 1687 samples, siliciclastic-lean basinal mudrock units that host AHs are strongly enriched in Mo (median EFMo ∼ 97–172 EFMo/EFU ≈ (3–3.5) × SW, where EFMo and EFU are respectively Al-normalized Mo and U in enrichment factor notation and SW is average present-day seawater value) compared with siliciclastic-rich units (median EFMo ∼ 17–37) and show strong EFU/EFMo covariation (r ≈ 0.8 in Canol Formation and Bluefish Member). Supported by a lack of geological evidence for an oceanographic barrier, this enrichment indicates unrestricted water exchange with Panthalassa. At the same time, development of oligotrophy is indicated by a lack of P enrichment and weak to non-existent enrichment in Zn and Cu. These features are reconciled through a model by earlier workers that involves a global shift to a warm greenhouse mode with slowed oceanic convection, expanded oxygen minimum zones and a failure of nutrient resupply from the upwelling. The onset of mass degassing in continental large igneous provinces represents a potential trigger for this mid-Devonian shift. Devonian black-shale events in this scenario represent genuine oceanic anoxic events marking hothouse episodes in their nascent form. Supplementary material: Details of methods, analytical protocols and data scatterplots, stratigraphic cross-sections showing traceability of anoxic horizons, and inductively coupled plasma elemental and Rock-Eval 6 data used in this study are available at: https://doi.org/10.6084/m9.figshare.c.4212428

Journal ArticleDOI
TL;DR: In this article, the authors proposed a revised position of the Falkland Islands microcontinent constrained by the presence of a mega-decollement, controlling the Gondwanide Orogen, described south of the islands and underneath South Africa and the Outeniqua Basin.
Abstract: The early stages of transform margin formation are associated with crustal fragmentation and block rotation. The restricted size of the resultant microcontinental blocks precludes palaeogeographical reconstructions and reliable estimations of the amount of rotation they can undergo. An example considered here is the Falkland Plateau. This is located adjacent to the Agulhas–Falkland Fracture Zone and its westernmost province is the Falkland Islands microcontinent. The position of the plateau and the islands prior to Gondwana break-up remains contentious. This study integrates seismic reflection and gravity data to propose a revised position of the Falkland Islands microcontinent constrained by (1) the presence of a mega-decollement, controlling the Gondwanide Orogen, described north of the Falkland Islands and underneath South Africa and the Outeniqua Basin, and (2) the similar architecture of fault networks mapped north of the islands and in the northernmost Outeniqua Basin. This revised position requires a re-evaluation of the timing and rate of rotation of the Falkland Islands microcontinent and affects the expected crustal architecture adjacent to the islands. Our model yields rotation rates between 5.5° and 8° Ma−1 and two potential times for rotation, and predicts more unstretched crust beneath the basin east of the Falkland Islands than previous models.

Journal ArticleDOI
TL;DR: In this paper, the authors measured conodont size variation patterns during the Early Triassic and found that these size changes are primarily controlled by rapidly changing and episodic environmental conditions and ecological recovery in the end-Permian mass extinction.
Abstract: Eleven conodont zones are established for the Lower Triassic of the Motianling section in the Nanpanjiang Basin, South China. Detailed size measurements on 2244 P1 conodont elements demonstrate dynamic size variation patterns during the Early Triassic that are calibrated to the Early Triassic timescale and conodont zonation. Correlation of conodont size variation with various environmental indicators shows that these size changes are primarily controlled by rapidly changing and episodic environmental conditions and ecological recovery in the aftermath of the end-Permian mass extinction. Decreasing size trends occur in the early Griesbachian, late Griesbachian and at the Smithian–Spathian boundary. Episodes of size increase (rebounds) occur in the middle Griesbachian, Induan–Olenekian boundary and middle–late Spathian. The observed temporal size variations indicate that conodont animals were significantly affected by rapidly changing environmental conditions (mainly temperature and redox conditions) during the Early Triassic and recurrent size increases (rebounds) occurred during periods of environmental and climatic amelioration (climatic cooling and oxygenation), demonstrating a complicated recovery history of marine ecosystems in the aftermath of the largest Phanerozoic biotic crisis. Supplementary material: Data for all species are available at https://doi.org/10.6084/m9.figshare.c.4528616