Jurassic Sedimentary Rocks Research Articles

Mesozoic evolution of large-scale drainage systems in the Indochina Block: evidence from palaeomagnetic and U–Pb geochronological constraints

The collage of the Indochina Block to the Eurasian continent during the Mesozoic resulted in major changes in source-to-sink systems of East Asia, whilst the sedimentation in the Khorat–Simao Basin of the Indochina Block transited from marine to continental environments. Previous studies uncovered the existence of transcontinental river systems in the East Asia mainland during the Cretaceous to Early Cenozoic, although there have been few studies on pre-Cretaceous drainage systems. To determine the impact on the evolution of palaeo-drainage systems triggered by tectonic movement of the Indochina Block, we performed palaeomagnetic and U–Pb geochronologic analyses on the Late Triassic to Early Cretaceous sedimentary rocks in the Simao Basin. Palaeomagnetic data indicate stable Jurassic tectonic environments between the Indochina and Songpan–Ganzi–South China blocks, following their Late Triassic collision. Based on detrital zircon U–Pb dating, the Late Triassic age spectra consist mainly of Phanerozoic components, while the Jurassic to Cretaceous age spectra display more Precambrian age peaks. The contribution of potential provenance to the Simao Basin changed during the Early Cretaceous, suggesting several sedimentary provenance transitions. Also, we believe that large continental-scale drainage systems flowing through the North Qiangtang, Sichuan, Simao and Khorat Basins were sourced from the Songpan–Ganzi Block since the end of the Late Jurassic. Supplementary material: The findings of a quantitative statistical study (Fig. S1; Table S5), the location of the Late Triassic to Early Cretaceous samples from the Simao Basin (Table S1), palaeomagnetic poles for Eurasia, South China, Indochina, North Qiangtang and Tarim (Table S2), the result of quantitative analysis for the Late Triassic and Middle Jurassic sedimentary rocks from the Simao and Sichuan basins (Table S3), and results of zircon U–Pb dating (Table S4) are available at https://doi.org/10.6084/m9.figshare.c.6862918 Thematic collection: This article is part of the Mesozoic and Cenozoic tectonics, landscape and climate change collection available at: https://www.lyellcollection.org/topic/collections/mesozoic-and-cenozoic-tectonics-landscape-and-climate-change

Depositional environment and paleoclimate of the Middle Jurassic sedimentary rocks in Matruh Basin, Northwestern Desert, Egypt

Depositional environment and paleoclimate of the Middle Jurassic sedimentary rocks in Matruh Basin, Northwestern Desert, Egypt

Pre-collisional architecture of the European distal margin: Inferences from the high-pressure continental units of central Corsica (France)

Abstract The Lower Units of Alpine Corsica, France, are fragments of continental crust strongly deformed and metamorphosed under high-pressure metamorphic conditions. Three slices of Lower Units are well exposed in the area between the Asco and Tavignano valleys, Central Corsica. Despite their complex structural setting, they provide the opportunity for a reconstruction of the pristine stratigraphic setting of the Lower Units. In our reconstruction, these units consist of a Paleozoic basement topped by Triassic to Early Jurassic sedimentary rocks unconformably covered by Middle to Late Eocene foredeep deposits. However, the three units exposed in the study area display strong differences mainly in the thickness of the Mesozoic sequence. These differences are here interpreted as acquired during the first stage of the rifting process in a setting controlled by normal faults. During the collision-related tectonics and the accretion of the Lower Units to the Alpine orogenic wedge, these normal faults were probably reactivated with a reverse kinematics. The stratigraphic logs of the Lower Units strictly resemble those of the Pre-Piedmont Units from Western Alps. This similarity indicates a common origin of the Lower Units and the Pre-Piedmont Units from the same domain (i.e., the European distal continental margin).

Chloride accessible porosity fractions across the Jurassic sedimentary rocks of northern Switzerland

The Jurassic clay-rich sedimentary sequence in northern Switzerland was investigated in the context of nuclear waste disposal. Tracer profiles across this sequence play an important role when assessing transport processes and long-term evolution of the environment. Negatively charged clay-mineral surfaces cause a depletion of anions in the porewater close to the surfaces compared to free porewater further away. The chloride accessible porosity fraction (fCl) is an important parameter to scale Cl bulk porewater concentrations from aqueous extraction to free porewater concentrations, which are required to establish anion profiles used for transport modelling of anionic tracers. A vast set of fCl data from advective displacement (AD), squeezing (SQ) and through-diffusion (DI) experiments allowed for method comparison and yielded insight into controlling factors of fCl across the sedimentary sequence. Two observations were made: (1) fCl varies strongly depending on lithology and partly on ionic strength; (2) at high ionic strength, the three methods yield consistent results, but at low ionic strength, fCl from DI are lower than those from AD and SQ. The observations are discussed in terms of influencing factors, including structural charge, ionic strength, rock texture, pore sizes and the fact that the different methods sample different parts of the pore space. The stronger variation of fCl from DI experiments with the ionic strength of the experimental porewater is in agreement with theoretical concepts. The volume fraction of small pores (r < 2 nm) inversely correlates with fCl; it accounts for ∼50% of the anion depleted pore volume. Two empirical approaches were tested to derive functions for fCl valid for the lithologically diverse sequence: the clay-content model uses a constant fCl at a clay-mineral content >30% and extrapolates to a fCl of 1 at zero clay content. The formation model applies formation- or member-specific average values of experimental fCl for each unit. Both approaches work reasonably well, with the formation model producing less apparent outliers of free porewater concentrations than the clay-content model. Still some scatter remains, particularly in heterogeneous and low-porosity formations. This work results in a comprehensive and coherent fCl dataset for all studied Jurassic formations and serves as a basis to interpret the profiles of anions with regard to transport across the entire low-permeability sequence.

Late Paleozoic-Jurassic tectonic evolution of the eastern Deseado Massif in central-southern Patagonia

Previous tectonic studies have indicated that the peri-cratonic lithosphere, located away from continental margins, is sensitive to far-field stresses propagating from active plate margins, which induce variable deformation. In order to gain a better understanding of potential intraplate tectonic events associated with the geodynamic evolution of the active margin of southwestern Gondwana, we conducted a tectono-sedimentary study of the Permian-Jurassic volcano-sedimentary record in the Deseado Massif, located in southern Patagonia. Our multidisciplinary analysis includes detailed geological mapping of an area of approximately 150 km2, structural analysis, geoelectric tomography, 2D seismic data, new geochronological dating, petrographic studies, and stratigraphic loggings of the volcano-sedimentary basin record. This comprehensive data set has allowed us to establish the tectonic, sedimentary, and magmatic evolution of the eastern Deseado Massif. Specifically, we have identified major normal faults associated with the syn-extensional deposition of late Permian and Jurassic sedimentary and volcanic rocks, as well as the Late Triassic emplacement of intermediate and felsic intrusive bodies. Additionally, interspersed large-scale shortening events were recognized, which induced positive tectonic inversion events in the region, recording contrasting stress fields during the analyzed lapse. Based on this, six major intraplate tectonomagmatic events were defined: (i) a potential post-Devonian pre-late Permian exhumation of the Neoproterozoic-early Paleozoic igneous-metamorphic basement, which we tentatively link to the Gondwanide orogeny; (ii) intraplate extension in the Late Permian (255 ± 4 Ma) related to the deposition of the Dos Hermanos Member of the La Golondrina Formation; (iii) Late Triassic (231 ± 3 Ma) intrusion of andesitic bodies, tentatively linked to the inland migration of arc magmatism associated with the South Gondwana flat slab; (iv) subsequent Late Triassic positive tectonic inversion of Permian extensional faults caused by a large-scale contractional event linked to the South Gondwana flat slab; (v) the extension-related emplacement and deposition of Early-Middle Jurassic (176 ± 3 Ma; 172 ± 4 Ma) sedimentary (lacustrine and fan deltas-related deposits), pyroclastic rocks (ignimbrites and ash tuffs), and lavas (lava domes and dykes) related to the Chon Aike silicic large igneous province; and (vi) poorly-constrained post-Middle Jurassic positive tectonic inversion of Jurassic faults. Therefore, we suggest that the geological events preserved in the Deseado Massif provide a key deformational record of the distal effects associated with ancient geodynamic processes that occurred along the southwestern active margin of Gondwana.

Structural and tectonic evolution between Indo-Myanmar ranges and central Myanmar basin: Insights from the Kabaw Fault

The Kabaw Fault is a north-south trending arcuate suture zone formed between the accretionary prism of the Indo-Myanmar Ranges (IMR), and the Central Myanmar Basin (CMB). The lineament of the Kabaw Fault is defined as a stratigraphic contact zone between flysch sediments of the IMR and molassic sediments of the CMB along the eastern base of the IMR. The accretionary prism of IMR was initially formed as a single wedge (pro-wedge) during Triassic, however, the forceful subduction of the Indian Plate resulted in the formation of an open wedge (double-sided wedge) during the Cretaceous. The double-sided wedge of the accretionary prism of IMR is revealed by the structural deformation pattern of the molassic sediments and the Triassic flysch exposures in the fore-arc basin of CMB, however, the eastern part of the wedge is buried under the fore-arc basin. At the eastern base of IMR, the ophiolite occurrences are close to the Kabaw Fault except in the Kabaw Valley where the ophiolites are associated with the Jurassic sedimentary rock units. The Kabaw Valley is generally occurred as a north-south elongated lowland basin, positioned along the western part of the Kabaw Fault. The east-dipping geometry of the Kabaw Fault is manifested by the evidence of east-verging synclinal folds of the fore-arc basin in the CMB, and the east-verging folding is due to slip up on the slope of the retro-wedge. The dextral strike-slip movement of the Kabaw Fault is displayed by the shifting of mud volcanoes in the Pyay to Kalaywa region and the two-phase folding and deformation within the Chindwin sub-basin and the Minbu-Salin sub-basin. The present study proves not only the Kabaw Fault as an east-dipping reverse fault with structural evidence, but also reveals the double-sided wedge system of the IMR accretionary prism.

Geochemical fingerprint—inter-basin chemostratigraphic correlation allowed for a more detailed stratigraphic subdivision of the Pliensbachian–Bathonian strata from the Carpathian Foredeep basement (Ukraine)

In this study, we combined the geochemical characteristics based on Rock-Eval pyrolysis and gas chromatography – mass spectrometry (GC–MS) results with the facies analysis. These surveys were conducted within grey to black claystones/mudstones intervals of the Podil’tsi and Kokhanivka formations, related to the Pliensbachian and Toarcian–Bathonian ages, respectively. The geochemical results revealed that the Podil’tsi Formation contains mixed marine/terrigenous, early-mature to mature organic matter. The deposition of this formation took place in dysoxic redox conditions of a sulphate-poor marine palaeoenvironment, with oxygen scarcity within the photic zone, as documented by green- and brown-pigmented Chlorobiaceae. Oleanane is present within the samples from the Podil’tsi Formation, which is uncommon within Lower Jurassic sedimentary rocks. The Kokhanivka Formation contains mostly early-mature, terrestrial organic matter, deposited in suboxic conditions of a sulphate-poor, fluvial–deltaic palaeoenvironment. The absence of aliphatic diterpenoids within the Middle Jurassic strata points to the low significance of conifers in the sediment supply area at this time. All of the Podil’tsi and most of the Kokhanivka formations are characterised by poor hydrocarbon potential. Only the middle part of the Kokhanivka Formation, built by brown, organic-rich claystones, shows fair-to-good hydrocarbon potential. Based on our results, a chemostratigraphic correlation of the Toarcian–Bathonian strata from the Carpathian Foredeep with the same strata from the neighbouring Polish Basin was performed. The juxtaposition of the geochemical and facies results suggests that the interval of brown organic-rich claystones, from the middle part of the Kokhanivka Formation can be related to the Middle–Upper Aalenian Age.

Application of geophysical prospecting methods ERT and MASW in the landslide of Daofu County, China

Natural disasters such as debris flow caused by earthquakes seriously threaten the local infrastructure and economy, as well as the lives of people in the area. As the material source of debris flow, it has significance to accurately and effectively study the underground structure of the landslide to prevent debris flow disasters. A landslide has a complex structural system, and its underground characteristics play an important role in its stability. The early identification of fracture surfaces and unstable bodies, and assessment of potential hazards are essential for prevention and protection. The research object of this paper is a landslide that occurred in Yige Village, Xianshui Town, Daofu County, which is on the Xianshui River Earthquake Zone, an area subject to frequent earthquakes. In western Sichuan, the frequent occurrence of landslides has caused considerable economic losses. Developing methods for efficient and accurate risk assessment is a top priority. The Daofu landslide is a typical example of a landslide directly threatening the road below and forming a debris flow channel. The lithology is composed of Jurassic sedimentary rocks, such as marl and clay, covered by limestone. In this study, we combined traditional methods (drilling and field investigation) with two geophysical techniques, multichannel analysis of surface waves (MASW) and electrical resistivity tomography (ERT) to effectively determine the electrical characteristics, velocity characteristics and spatial structure of the landslide. It is found that the buried depth of the sliding surface of the landslide is about 16–20 m. The sliding body above the sliding surface forms a low velocity and low resistivity Quaternary cover. The rock mass below the sliding surface is Triassic Zhuwo Formation sandstone and slate with high velocity and high resistivity. According to comprehensive analysis, the landslide lacks sufficient stability under rainstorm. Our study shows that the use of MASW and ERT can quickly and effectively characterize the subsurface of landslides to assess landslide risk and prevent debris flow hazards.

Geochemistry and clay mineral studies of Jurassic sedimentary rocks from the Spiti region, Himachal Pradesh, North India

An attempt is made in the present study to unravel the provenance, paleoweathering and paleoclimatic conditions of the Jurassic (Spiti Formation) black shales and sandstones from the Spiti region, Tethys Himalaya, using multi proxy approach. The sandstones are subarkose in composition and texturally poorly sorted, subrounded to subangular in shape with moderate spheriocity. The range of the chemical index of alteration (CIA) is 55–90, recorded in the black shales strongly suggests moderate to strong chemical weathering conditions in the source area, which in turn reflect fluctuating climatic conditions prevailing during the deposition of these sediments in Jurassic period in the Spiti region. Geochemical studies reveal that shales are enriched in felsic elements (high SiO2, Al2O3, K2O) and depleted in mafic components (Fe2O3 and MgO).The various geochemical discriminant plots and elemental ratios (SiO2/Al2O3, K2O/Al2O3, Al2O3/TiO2, K2O/Na2O, etc.) indicate the rocks to be the product of weathering of felsic rocks. The paleoclimate in the source area seems to be mostly semi-humid. The plot of the samples on the A-CN-K ternary diagram indicates a granitic weathering trend. The X-ray Diffraction studies show that the prominent clay minerals in the Spiti shales are illite, smectite, chlorite, kaolinite and vermiculite along with quartz, muscovite, alkali feldspar, calcite and phosphatic phase.When plotted on the tectonic discrimination diagram, the samples indicate passive margin tectonic setting.

Degree of Pyritization (DOP) and Indicator of Anoxicity (IA) in Jurassic sedimentary rocks from Bulgaria

The Degree of Pyritization (DOP) and the Indicator of Anoxicity (IA) were used as tools to assess the degree of redox conditions in Lower and Middle Jurassic strata from Bulgaria. Normally-oxygenated (with DOP values &lt; 0.45 and IA values &lt; 0.4), oxygen-restricted (0.45 &lt; DOP &lt; 0.7; 0.4 &lt; IA &lt; 0.5), and oxygen-depleted (DOP &gt; 0.7 and IA &gt; 0.5) bottom-water conditions were recognized on a selection of samples, mainly from the Ozirovo Formation (upper Pliensbachian–Toarcian), but also from the Etropole Formation (lower Bajocian). DOP and IA yielded the highest values (&gt; 0.9 for DOP and &gt; 0.5 for IA) from samples of the lower Toarcian Tenuicostatum and Falciferum ammonite zones, and thus, denoted declined oxygen levels and enrichment of reactive Fe at the sea floor. Beyond this stratigraphic level, both parameters indicated for prevailing normally-oxygenated to oxygen-restricted setting, with limitedly imposed environments beneath stratified water-column.

Depositional conditions for Lower–Middle Jurassic sedimentary rocks from Northern Bulgaria inferred from whole-rock elemental composition

The Lower–Middle Jurassic sedimentary rocks from Northern Bulgaria represent important stratigraphic markers and a potential source for oil and gas accumulation. Here, we present preliminary major and trace element results for 22 sedimentary rock samples from the Bachiishte, Ozirovo, and Etropole Formations collected from the Balkanides and the Moesian Platform in Central-northern and Northwestern Bulgaria. The three studied formations have largely similar geochemical characteristics. The application of established geochemical proxies and comparison with recent and ancient sediments suggests that the samples were deposited in a shallow, high-energy environment with no water restriction. Eroding mafic to intermediate lithologies provided the sediment into the submerging Moesian Platform. Most redox-sensitive geochemical parameters suggest an overall oxygenated environment during sediment deposition, although specific samples show evidence for periodically dysoxic or anoxic water column conditions.

A new subsurface record of the Pliensbachian–Toarcian, Lower Jurassic, of Yorkshire

Here, we describe the upper Pliensbachian to middle Toarcian stratigraphy of the Dove's Nest borehole, which was drilled near Whitby, North Yorkshire, in 2013. The core represents a single, continuous vertical section through unweathered, immature Lower Jurassic sedimentary rocks. The thickness of the Lias Group formations in the Dove's Nest core is approximately the same as that exposed along the North Yorkshire coast between Hawsker Bottoms and Whitby. The studied succession consists of epeiric-neritic sediments and comprises cross-laminated very fine sandstones, (oolitic) ironstones, and argillaceous mudstones. Dark argillaceous mudstone is the dominant lithology. These sediments were deposited in the Cleveland Basin, a more subsident area of an epeiric sea, the Laurasian Sea. We present a set of geochemical data that includes organic carbon isotope ratios ( δ 13 C org ) and total organic carbon (TOC). The δ 13 C org record contains a negative excursion across the Pliensbachian–Toarcian boundary and another in the lower Toarcian that corresponds to the Toarcian Oceanic Anoxic Event (T-OAE). Below the T-OAE negative excursion, δ 13 C org values are less 13 C-depleted than above it. We find no evidence of a long-term δ 13 C org positive excursion. TOC values below the T-OAE negative excursion are lower than above it. Sedimentary evidence suggests that, during much of the Pliensbachian–Toarcian interval, the seafloor of the Cleveland Basin was above storm wave-base and that storm-driven bottom currents were responsible for much sediment erosion, transport, and redeposition during the interval of oceanic anoxia. The abrupt shifts observed in the δ 13 C org record (lower Toarcian) are likely to reflect the impact of erosion by storms on the morphology of the δ 13 C record of the T-OAE. Supplementary material: stratigraphic and geochemical data [organic carbon isotope ratios ( δ 13 C org ) and total organic carbon (TOC)] are available at https://doi.org/10.6084/m9.figshare.c.6154436

Burial and exhumation history of Jurassic sedimentary rocks in the southern margin of the Junggar Basin: Implications for the growth of the northern Tianshan Mountains

• Apatite fission-track dating yields 147–50 Ma for the Jurassic sedimentary rocks in the southern margin of Junggar Basin. • The Jurassic sedimentary rocks experienced two cooling stages at 150–110 Ma and the Late Miocene (20–10 Ma). • The exhumation of Jurassic sedimentary rocks was associated with the northward expansion of the Tianshan Mountains. • The two cooling stages had a wide range of regional dynamic responses in northwestern China. Constraining the burial and exhumation evolution of sedimentary basins is crucial for understanding the deformation histories of basin–mountain transition zones. The thick Jurassic sedimentary successions on the southern margin of the Junggar Basin are an essential archive of the uplift and deformation of the Tianshan Mountains. Apatite fission-track and vitrinite reflectance analyses were performed on the Jurassic sedimentary rocks bordering the northern Tianshan Mountains, providing direct insight into the burial and exhumation of the southern margin of the Junggar Basin. Our apatite fission-track data indicate four main cooling stages during 150–125 Ma; 120–105 Ma; 100–80 Ma; and 75–45 Ma. Thermal-history modeling results demonstrate that the Jurassic sedimentary rocks were exhumed in the northern Tianshan Mountains, indicating a history of probable Early to Middle Jurassic sedimentary burial followed by slow cooling commencing at approximately 150–110 Ma. Then, this region undergone gradual deplanation between the Late Jurassic–Cretaceous and the Miocene and rapid cooling since the Late Miocene (20–10 Ma). These two stages of cooling and exhumation are interpreted to reflect the accretion of the Lhasa Block and the India-Eurasia collision, respectively. Furthermore, the thermal history of the Jurassic sedimentary rocks is consistent with the sedimentation pattern and depositional environments in the southern margin of the Junggar Basin, providing complementary evidence for the time of exhumation and implying that denudation migrated into the Junggar Basin concurrent with uplift of the northern Tianshan Mountains.

ESTIMATING 3D FOREST STRUCTURE USING LIDAR DATA FOR RISK ASSESSMENT OF SLOPE COLLAPSE CONSIDERING ROOT SYSTEM DEVELOPMENT

Abstract. In order to predict slope failure, it is useful to be able to grasp the state of root system development of trees by remote sensing techniques. Since there is a positive correlation between tree diameter and root system development, a field survey to verify how predictable the number of trees and breast height diameter can be from LiDAR data was conducted in the Abukuma Mountains Granite Area and the Yamizo Mountains Jurassic Sedimentary Rock Area. It was carried out for the logging site. As a result, it was found that the relative tree density can be determined and that the laser transmittance and the sum of cross-sectional areas of the trees are inversely correlated.

Detrital zircon in an active sedimentary recycling system: Challenging the ‘source‐to‐sink’ approach to zircon‐based provenance analysis

ABSTRACTThe age and properties of detrital zircon in a sediment reflect properties of the rocks in which the zircon crystallized, and not necessarily the immediate precursor of the host sediment. Where clastic sediments are recycled, these properties are preserved so that U‐Pb ages and Lu‐Hf data on zircon grains will no longer give information on the routing of detritus. Because the crustal evolution and sedimentary recycling history of Southern Africa is well‐established, Holocene sediments in the region provide a good test example for detrital zircon geochronology applied to sands or sandstones with a complex recycling history. Data from 16 samples of unconsolidated sand, including dunes from the southern part of the Kalahari Basin, beach sands at the Atlantic coast, and isolated inland dune occurrences, yield ranges of early Mesozoic to late Archaean ages. Uranium–lead and lutetium–hafnium isotope data provide no evidence of direct derivation of zircon from protosources in crystalline bedrock. The complex distribution patterns can be decomposed into seven ‘provenance components’ that have previously been encountered in Palaeoproterozoic to Jurassic sedimentary rocks in the region. These components occur in non‐random combinations, in proportions that suggest mixing of recycled material from different sedimentary precursors. Taking pre‐Cenozoic geology, Cretaceous to Pleistocene drainage history of southern Africa and Holocene wind and current patterns into account, it is possible to work out a consistent model for transport of detritus from sedimentary precursors into the basin, followed by aeolian remobilization within the basin itself. The detrital zircon distributions can thus be interpreted from prior knowledge of the basin filling history, regional geology, geomorphology, drainage evolution and wind pattern. This amounts to a line of reasoning from ‘sink back to (intermediate) ‘source’, which is the opposite of the ‘(proto)source‐to‐sink’ paradigm commonly invoked in detrital zircon studies.

Wildfire activity enhanced during phases of maximum orbital eccentricity and precessional forcing in the Early Jurassic

Fire regimes are changing due to both anthropogenic climatic drivers and vegetation management challenges, making it difficult to determine how climate alone might influence wildfire activity. Earth has been subject to natural-background climate variability throughout its past due to variations in Earth’s orbital parameters (Milkankovitch cycles), which provides an opportunity to assess climate-only driven variations in wildfire. Here we present a 350,000 yr long record of fossil charcoal from mid-latitude (~35°N) Jurassic sedimentary rocks. These results are coupled to estimates of variations in the hydrological cycle using clay mineral, palynofacies and elemental analyses, and lithological and biogeochemical signatures. We show that fire activity strongly increased during extreme seasonal contrast (monsoonal climate), which has been linked to maximal precessional forcing (boreal summer in perihelion) (21,000 yr cycles), and we hypothesize that long eccentricity modulation further enhances precession-forced fire activity.

Elemental composition of dispersed vitrinite in marine Jurassic source rocks of the Vulcan Sub-basin, Australia: Implications for vitrinite reflectance suppression

The vast majority of petroleum exploration wells in the offshore basins of Western Australia bottom in Jurassic sedimentary rocks for which vitrinite reflectance results commonly appear to be anomalously low. Light element electron probe microanalysis (EPMA) of dispersed vitrinites in five samples of marine, Jurassic, potential source rocks from three petroleum exploration wells drilled in the Vulcan Sub-basin, Timor Sea, enables better understandings of this problem. The elemental abundance of carbon, oxygen, nitrogen, sulphur, aluminium and silicon were measured, and hydrogen content was calculated by difference. The EPMA method was validated by comparison of microprobe results for vitrinite in an Australian, Permian, medium volatile bituminous coal, with those determined by conventional elemental analysis according to the Australian ‘standard method’. Results on the Vulcan Sub-basin samples show that hydrogen contents of the Jurassic vitrinites are anomalously high, plotting as ‘type II’ ‘oil prone’ organic matter, on a van Krevelen-type diagram (atomic H/C vs O/C), a region normally associated with liptinite macerals. Implications of the EPMA results were investigated using fluorescence alteration of multiple macerals (FAMM) analyses, a laser-based technique that enables determination of the degree of vitrinite reflectance suppression. The elemental analyses, together with associated optical microscopy, confirm the occurrence of vitrinite having suppressed reflectance in marine Jurassic rocks of the Vulcan Sub-basin, which has important consequences for petroleum exploration if using vitrinite reflectance data in modelling thermal maturation and petroleum generation. In addition, the perhydrous vitrinites themselves may have considerably more petroleum generation potential than previously thought.

Paleomagnetism of the Middle Jurassic Sedimentary Rocks of the Elgon Formation of the Ulban Terrane

Paleomagnetism of the Middle Jurassic Sedimentary Rocks of the Elgon Formation of the Ulban Terrane

Triassic–Jurassic Accretionary History and Tectonic Origin of Stikinia From U‐Pb Geochronology and Lu‐Hf Isotope Analysis, British Columbia

AbstractThe timing of assembly and tectonic origins of terranes in the northern Cordillera of Alaska, British Columbia, and the Pacific Northwest are debated. Stikinia, a long‐lived arc terrane, has an enigmatic regional Mesozoic accretionary history and its tectonic origins remain unconstrained. Zircon U‐Pb geochronology and Lu‐Hf isotopic data on Triassic–Jurassic sedimentary and igneous rocks from central Stikinia shed light on the terrane‐scale effects of a latest Triassic–Early Jurassic collision between Stikinia and pericratonic Yukon‐Tanana terrane. Main age peaks from central Stikinia are 250–160 Ma, reflecting ongoing Mesozoic arc‐related igneous activity within Stikinia. Comparison of isotopic evolution and unconformity development between central Stikinia and northern Stikinia (Whitehorse trough) provide new constraints on regional latest Triassic–earliest Jurassic deformation. We attribute the shortening‐related deformation to variable along‐strike interactions during end‐on collision with the Yukon‐Tanana terrane, with significant crustal thickening at the northern apex of Stikinia that did not persist farther south. A small pre‐Devonian zircon population is significant, as the oldest exposed rocks in Stikinia are Early Devonian. Pre‐Devonian age peaks differ from those of the northern Yukon‐Tanana terrane, but resemble zircons from southern Wrangellia. These zircons are likely multi‐cyclic, derived from crust that originated in the Arctic region near the northern end of the Caledonide orogeny. We suggest that Stikinia was an independent crustal block prior to latest Triassic onset of collision with Yukon‐Tanana terrane. The ongoing, end‐on collision in turn promoted oroclinal assembly of the peri‐Laurentian terranes.

Reappraisal of the Mesozoic tectonic transition from the Paleo-Tethyan to Paleo-Pacific domains in South China

Abstract The southeastern (SE) South China Block was mainly influenced by the Paleo-Tethyan and Paleo-Pacific dynamic domains during the Mesozoic. The initial timing of the tectonic transition between these two domains in the SE South China Block still remains debated. The transition would affect the nature of the lithosphere and material provenance of sediments, and, therefore, igneous and sedimentary rocks in the area could record such dynamic processes. In this study, published geochronological and geochemical data of the Triassic and Jurassic igneous rocks and detrital zircon data of contemporaneous sedimentary rocks in the SE South China Block were compiled, aiming to provide constraints on the tectonic transition via tracing the spatial-temporal variations in the nature of the lithosphere and sedimentary provenance signals. The compiled results suggest that the magmatic intensity and volume decreased significantly from the Late Triassic to Early–Middle Jurassic, with an obvious magmatic quiescence between them, and increased from the Early–Middle Jurassic to Late Jurassic. The εNd(t) and zircon εHf(t) values of mafic rocks, granitoids, and shoshonitic rocks remarkably increased from the Late Triassic to Early–Middle Jurassic, indicative of variations in the lithospheric mantle and continental crust. Such variations suggest that the initial tectonic transition occurred at the earliest Early Jurassic. Based on the southward paleocurrents from Early Jurassic sandstone, E-W–trending extension of Early–Middle Jurassic mafic and shoshonitic rocks, and similar sedimentary provenances of Late Triassic and Early–Middle Jurassic sedimentary rocks, these features imply that the SE South China Block was not immediately influenced by the Paleo-Pacific domain during the Early–Middle Jurassic. However, from the Early–Middle Jurassic to Late Jurassic and Early Cretaceous, the spatial distribution, geochemical signatures, magmatic intensity, and magmatic volume of igneous rocks and provenance of sedimentary rocks exhibit obvious variations, and the regional fold hinge direction changed from E-W–trending to NE-trending, suggesting significant effects from Paleo-Pacific subduction on the SE South China Block. Thus, the Mesozoic tectonic transition from the Paleo-Tethyan to the Paleo-Pacific dynamic domain in the SE South China Block likely occurred during the Early–Middle Jurassic.