Southern Continental Margin Research Articles

New species of Laetmonice (Aphroditidae, Annelida) from bathyal and abyssal depths around Australia

Research voyages on board RV ‘Investigator’ between 2015 and 2022 sampled benthic communities of Australia’s Eastern and Southern continental margins from the slope down to abyssal depths (463–5000 m) as well as the seamounts off the Indian Ocean Australian Territories (IOT) that include Christmas Island and Cocos (Keeling) Islands. Over 500 specimens of the annelid family Aphroditidae were collected during the voyages. Some of the most common aphroditids collected during these voyages belonged to the large-bodied members of the genus Laetmonice. We used fragments of COI and 16S genes to investigate the diversity of the genus. Phylogenetic studies revealed the presence of at least nine distinct mitochondrial genetic lineages within Laetmonice, four of which have been described here as Laetmonice hutchingsae sp. nov., L. murrayae sp. nov., L. mensahaedorum sp. nov. and L. paxtonae sp. nov. The new taxa are clearly structured by their bathymetric distributions. These results argue against eurybathic distribution within the genus Laetmonice and demonstrate the existence of genetically divergent taxa isolated by bathymetric environmental gradients.

Magmatism along the Nansha Trough on the southern continental margin of the South China Sea: Recent evidence from along-strike seismic profile

The Nansha Trough (NT) is part of the southern continental margin boundary of the South China Sea (SCS). It has undergone complex tectonic superposition and evolutionary processes involving the subduction demise of the Proto-SCS and subsequent spreading of the SCS. This study provides the first systematic identification and analysis of igneous bodies and seamounts along the NT, based on a multi-channel seismic profile (NDL1) recently acquired along it. The seamounts within the trough are of magmatic origin and the carbonate build-ups observed at the summits of some seamounts exhibit a substantial thickness. Igneous bodies within the trough are consistently associated with high P-wave anomalies. Furthermore, at the eastern and western sides, there are distinct gravity-magnetic-anomaly patterns. On the eastern side, Yinqing Seamount, Nanle Hill and volcanic mounds show high gravity and strong negative magnetic anomalies. In contrast, on the western side, Jinghong Seamount, Yangshu Hill and intrusive bodies show less pronounced magnetic anomalies. This difference may be related to differences in magmatic periods. Unlike the extensive post-spreading magmatism in the SCS's northern margin and deep basin, the most widespread magmatic activity in the NT occurred at ca. 16 Ma before decreasing during the Miocene. This decrease may be closely related to subduction cessation in the Proto-SCS and the collision between the Nansha Block and Borneo. The identification and analysis of NT igneous bodies and their evolutionary processes help delineate the southern boundary of magmatism at the SCS margin. They also provide crucial information for constraining the magmatic processes of Proto-SCS subduction termination and SCS spreading evolution.

S-wave velocity structure and Vp/Vs ratios beneath Liyuexi Trough, Nansha Block, the South China Sea: Implications for crustal lithology and tectonic nature of the southern continental margin

By processing the vertical component data of OBS2019–2, a wide angle seismic line situated adjacent to the Liyuexi Trough in the Nansha Block of the South China Sea (SCS), we obtained a crustal model of P-wave velocity structure beneath the line and acquired information about the deep crustal structure in the Liyuexi Trough region. However, the lithology remains to be fully constrained. This paper presents the results of processing the horizontal component data of OBS2019–2 to obtain the S-wave velocity structure model and Vp/Vs ratios. The results show that the Vp/Vs ratios of sedimentary layer range from 1.90 to 4.60, while the corresponding S-wave velocities range from 0.4 to 1.8 km/s. As for the crust, the Vp/Vs ratios fall between 1.70 and 1.89, while the S-wave velocities range between 2.9 and 4.2 km/s. The lithological compositions of oceanic crust from top to bottom may consist of basalt, diabase, and gabbro. In contrast, the lithologies of continental crust transition from felsic to mafic downwards, including andesite, granodiorite, granite, greenschist, and anorthosite. The rock compositions of the Continent-Ocean Transition (COT) region are intermediate between the continental and oceanic crusts, exhibiting transitional features. The rock composition of the high-velocity layer (HVL) in the lower crust may consist of amphibolite, and the presence of magma intrusions and underplating in this region could be associated with the Zhongnan-Liyue Fault. Compared to the Dongsha Block, the lithologies of the Liyue Block and Zhongsha Block are more similar, indicating a higher possibility of conjugation between the Liyue Block and the Zhongsha Block.

The crustal structure on the southern continental margin of the Gulf of Mexico, as inferred by inverse modeling of gravity data

In order to infer the crustal structure in the southern continental margin of the Gulf of Mexico, we performed inverse modeling of isostatic residual gravity data, to obtain a three-dimensional model of density contrast in a region that includes the Los Tuxtlas Volcanic Field, the Veracruz basin and partially the Salina del Istmo basin. As a result of the modeling, the calculated gravity anomaly fits well with the observed data. The model suggests that the Los Tuxtlas Volcanic Field evolved on the verge of a dense body, whose roof is 15 km deep and has a thickness of 10 km, it extends towards the north into the Gulf of Mexico, and it is interpreted as oceanic crust. The southern rim of this body possibly represents the location of the Western Main Transformation Fault, related to the opening of the Gulf of Mexico during the Jurassic, although this inference is speculative. On the inferred basement, possibly constituted by Cretaceous carbonates, the model shows a large sedimentary basin that accommodates Cenozoic siliciclastic sediments with a thickness of up to 10 km, has geometric characteristics related to some known tectonic features, such as the Buried Tectonic Front, in addition to a syncline-type structure, with basal surface at 6 km depth, which accommodates the Western Homocline and the eastern edge of the Tlacotalpan Syncline. The model shows deformed strata that thin towards the tectonic front of the Sierra Zongolica, surrounding the Alto de Anegada and the Los Tuxtlas Volcanic Field, as well as some features related to the Coatzacoalcos Reentrant.

Identifying Submarine Engineering Geologic Hazards in a Potential Gas Hydrate Target Area on the Southern Continental Margin of the South China Sea

The southern continental margin-slope area of the South China Sea is a complex passive continental margin with diverse tectonic structures and movements. This area is rich in gas hydrate resources and is also an area with a high incidence of potential geological hazards. Identifying and understanding the potential submarine geological hazards in this area is very important for disaster prevention and management during the future exploration and development of marine resources. In this paper, five types of potentially hazardous geological bodies are identified in the research area through high-precision two-dimensional seismic processing and interpretation, including submarine mounds, pockmarks, mass transport deposits, submarine collapses and faults. At the same time, the seismic reflection characteristics and the changes in its morphology and surrounding strata are described. In addition to the causes of geological hazards in this region and their influence on exploration and development, the research prospects of geological hazards in this region are also suggested. Special tectonic and sedimentary conditions, fluid activities and hydrate decomposition may be the conditions for geological hazards in this region, which pose a significant threat to the exploration and development of seabed resources and marine engineering construction in this region. Not only does our conclusion provide useful data for the development and utilization of gas hydrate, but it also presents theoretical suggestions for reducing geological hazards in the development process.

Provenance and paleogeographic implications for the Cenozoic sedimentary cover of the Ghomaride Complex (Internal Rif Belt), Morocco

The Cenozoic sedimentary cover from the Ghomaride Complex (Internal Rif Belt) has been studied in the Tetuan area (N Morocco) where a suite of sedimentary successions from shallow-marine to deep-marine environments crops out. For that purpose stratigraphic relations and petrological and geochemical signatures have been analyzed. Sandstone suites of the overall succession are heterogeneous and testify a multi-source area, in response of accretionary processes of the Ghomaride-Maláguide units and the exhumation of the lower units of the Internal Rif Zone (e.g. Sebtide-Alpujárride Complex). Pre-orogenic and Syn-orogenic (according to the eoalpine phase) deposits have been identified consisting in two depositional sequences: lower Paleocene and Cuisian-Bartonian, and upper Oligocene-upper Aquitanian and lower Burdigalian, respectively. Pre-orogenic deposits are mainly intra-arenite and hybrid arenites made of a minor amount of siliciclastic detritus but with abundance of intrabasinal carbonate grains. The syn-orogenic sandstone suites are quartzolithic, having abundance of low-grade metamorphic and sedimentary lithic fragments. Sedimentary lithic fragments are derived from the Mesozoic successions of the Ghomaride-Maláguide Complex while metamorphic detritus is related to an unknow Internal Rif Zone basement that was exhumed starting from the late Oligocene and mainly early Miocene. Modal analyses of sandstone suites for the extrabasinal grains mainly indicate lower rank metamorphic and sedimentary source terranes of a recycled orogen. Major and trace elements coupled to the mineralogical composition of the mudrock samples indicate a provenance from felsic source area(s) with a minor but not negligible contribution from mafic rocks mainly in the syn-orogenic suites. Geochemical analyses (Al–Ti–Zr ternary plot) indicate minor reworking and recycling processes before the final deposition through prolonged processes of sedimentary transportation. The trends evident in both CIA and CIA’ diagrams indicate source areas characterized by moderate weathering in non-steady-state conditions with a weak change of weathering condition from the pre-orogenic to the syn-orogenic cycle. Deposition during the Paleocene and Eocene, took place in the southern continental margin of the Ghomaride-Malaguide domain as a carbonate ramp. Contrarily, sedimentation in the late Oligocene-late Aquitanian took place in wedge-top basins within the Ghomaride-Malaguide domain. These changes occurred during the Burdigalian, when back arc basins were developed in the Internal Betic-Rif Zone. The Cenozoic reconstructed record was contemporaneous of the structuring of the Circum-Mediterranean chains and the Ghomaride-Maláguide Complex played a key role in the geodynamic evolution of the Rif Cordillera, representing a key tectonic element of the western Mesomediterranean domains.

ГЕОДИНАМІКА

Purpose. The objective of this study is the investigation of the Silurian-Lower Devonian (Lochkovian) carbonate-clay sedimentary complex of the southwestern slope of the East-European platform. Its formation was the result of a single cycle of geodynamic and sedimentary events in the lithological record of the southern continental margin of Baltica. Methodology. The study is based on well-logs correlation, lithological and geochemical investigations of core samples, and petrographic thin sections analysis. The obtained results were used to establish patterns of changes in the material composition of the studied strata in time and space in order to determine the basin development dynamics of the continental margin of the Eastern European platform southwestern slope in the Silurian-Early Devonian. Results. It is established that the formation of the carbonate-clay sequence represents a single sedimentary cycle and was the result of a complex of geodynamic, depositional and paleoclimatic events that took place on the Baltica southern continental margin. The Silurian period was characterized by intensive development of benthic organisms and reef structure formation in the proximal part of the basin and clay-carbonate muds enriched with dispersed organic matter in its distal part. In the Early Devonian, carbonate biogenic sedimentation continued throughout the basin. The carbonate maximum content (80–98%) proves the existence of the reef constructions in the basin sedimentary record. Lower values of carbonate content are characteristic of marls (40–55%) and biodetritic limestones (56–75%), which make up the main part of the Silurian sequence. There are no reef constructions in the section of the Lochkovian stage of Devonian, and the calcium carbonate content in the rocks varies from 45 to 83%. The content of CaCO3 in mudstones and calcareous mudstones of the Silurian varies from 5 to 15%. Originality. The applied complex of lithological, geochemical, geological-geophysical and paleogeographic investigations allowed studying the sedimentary basin dynamics in the Silurian-Early Devonian with the assessment of the possibility of participation of these strata in oil and gas hydrocarbons generation. Practical significance. The obtained results show that the carbonate-clay complex of sedimentary rocks of the Silurian and Lower Devonian (Lochkovian) of the studied region can be considered as a separate petroleum system, including source rocks, reservoir rocks and possible conventional and unconventional oil and gas accumulations.

Newly discovered late Devonian and early Carboniferous ophiolite fragments in the Diyanmiao mélange in southeastern Inner Mongolia: Implications for the late Paleozoic tectonic evolution of the southeastern Central Asian Orogenic Belt

The Late Paleozoic tectonic evolution of the southeastern Central Asian Orogenic Belt (CAOB) has long been controversial, and one of the most contentious issues has been whether there was a collisional orogenic event during the Middle Paleozoic. Here we report the discovery of a Late Devonian (373.5 ± 6.8 Ma) ophiolite fragment in the Diyanmiao mélange of southeastern Inner Mongolia. This fragment, coupled with previously reported Carboniferous ophiolites in the same region, confirm the former presence of a Late Devonian ocean basin that evolved into the Carboniferous basin. The geochemical characteristics of the Devonian and Carboniferous mafic rocks in the ophiolites are consistent with evolutionary trends from mid-ocean ridge (MOR)-like to supra-subduction zone (SSZ) compositions. The presence of a Late Devonian-Carboniferous trench-arc-basin system in the region, which is represented by the Daqing-Diyanmiao and Bayan'aobao-Hongger ophiolite belts, the Baolidao arc, and the Erenhot-Hegenshan Oceanic Basin (EHB), confirms the presence of a late Devonian-Carboniferous orogenic belt in the southeastern CAOB. The western segment of the Baolidao arc was developed on continental basement (the Xilin Gol complex), whereas the eastern segment was formed on oceanic basement. This tectonic configuration is similar to that of the modern Aleutian arc, Kamchatka-Kuril Islands and Japan-Ryukyu arc system in the northern and western Pacific Ocean. During the early Permian the oceanic basin represented by the Erenhot-Hegenshan basin closed, and the Baolidao arc was accreted onto the southern continental margin of the Southern Mongolian continental block.

A global magnetotelluric graphite type locality: Multi-decade, multi-scale studies of the Eyre Peninsula anomaly

A major electrically conducting crustal structure has been imaged since the 1980 s using widely-spaced long-period magnetotelluric (MT) and geomagnetic depth sounding (GDS) sites across the Archean to Paleoproterozoic Eyre Peninsula, South Australia. The structure extends from the southern continental margin, trending SW-NE for at least 300 km. To constrain depth and lateral extent of the anomaly a transect of 39 broadband MT sites were collected with ∼ 2 km separation across lower Eyre Peninsula. Two-dimensional inversion images a low-resistivity conductor at depth 2–10 km, 20 km wide and dipping ∼ 10° to the east, spatially correlated with a topographic high. The conductor is bound on the eastern margin by the transpressional Kalinjala Shear Zone along the Paleoproterozoic Donington Suite, and on the western margin by the Archean Sleaford Complex. The low resistivity structure, with conductance of 20,000 S occurs in Paleoproterozoic carbonaceous marine sediments of the Darke Peak and Cleve groups that were thickened to amphibolite-grade metamorphic depths of at least 15 km and 550 ⁰C during the Kimban Orogeny (1730–1690 Ma), converting carbon to flake graphite in metapelites. Tight folding and exhumation of amphibolite-grade metapelites during the Kimban Orogeny, and dextral strain along the Kalinjala Shear Zone resulted in a laterally continuous electrical crustal conductor which outcrops as economic-grade graphite deposits adjacent to Kalinjala Shear Zone mylonites. In addition, a ∼ 40 km wide lower crustal conductor of integrated conductance 2000 S is imaged west from the Kalinjala Shear Zone. This lower crustal conductor may also be graphitic and represents deeply buried metapelites that have not been exhumed. We argue that the Paleoproterozoic Eyre Peninsula Anomaly results from graphitisation of sediments with significantly enhanced amount of organic carbon compared to any other time in Earth’s history, during the ∼ 2 Ga Lomagundi–Jatuli Event, which is associated with the great oxidation event.

Late Triassic dacites from Well NK-1 in the Nansha Block: Constraints on the Mesozoic tectonic evolution of the southern South China Sea margin

Identifying whether and when the southern continental margin of the South China Sea (SCS) was part of the South China Block (SCB) is important for Mesozoic–Cenozoic paleogeographic reconstructions of Southeast Asia. In this paper, we present geochronological and geochemical data for dacites from Well NK-1 in the Nansha Block (Dangerous Grounds) of the southern SCS margin to investigate their petrogenesis and tectonic affinity. Zircon UPb and 40Ar/39Ar datings show that these dacites formed during the Late Triassic (218–217 Ma). The dacites have high total alkali (K2O + Na2O = 7.39–8.42 wt%) and relatively low CaO contents (1.87–3.38 wt%), and show geochemical affinities with A-type granites, with high Zr saturation temperatures (806–855 °C), FeOTot/MgO ratios (5.22–7.48), Zr + Nb + Ce + Y contents (530–680 ppm) and 10,000 × Ga/Al values (2.40–2.94), and relatively low A/CNK values (0.86–1.04). The primitive magma for the dacites was likely generated by partial melting of ancient crustal materials. Late Triassic dacites in the Nansha Block and Triassic A-type granites in the SCB formed in an extensional setting at the continental margin of the SCB. This setting was probably related to the Paleo-Pacific plate subduction that restarted prior to the Early Triassic. Therefore, the southern SCS margin was part of the SCB continental margin during the early Mesozoic, which provides more detailed insights into the paleogeography of SE Asia since the Mesozoic.

Types, characteristics, distribution, and genesis of pockmarks in the South China Sea: insights from high-resolution multibeam bathymetric and multichannel seismic data

ABSTRACT Ubiquitous pockmarks in continental margins are important to understand the tectonic and sedimentary history and flow activity of their host margins. The study of an extensive field of pockmarks in the South China Sea (SCS) is important both scientifically and economically, for the exploration of oil, gas, and gas hydrate resources and the prevention and control of marine geological disasters. We report our new findings on the characteristics, distribution, and genesis of pockmarks in the SCS based on interpretation of high-resolution multibeam bathymetric data and multichannel seismic profiles collected in the past decade. Three pockmark fields (total area 572,824 km2), are recognized in the northern, western, and southern continental margins of the SCS. The pockmarks are highly variable in shape: circular, elliptical, crescent-shaped, elongated or forming a chain in plan view and U-, V- or W-shaped in cross-section. Pockmarks are normally 10–200 m in diameter, with maximum diameters/axial lengths of 7.3 km for circular, 5.5 km for elliptical, 8.6 km for crescent-shaped, 9.5 km for elongated, and 30 km for chain-type pockmarks. Pockmarks are normally less than 35 m deep, with a maximum depth of 341 m at the southwestern SCS margin. These pockmarks might have originated from methane seepages or from pore fluids from deeper sources, which migrated vertically along gas chimneys, mud diapirs, mud volcanoes, faults or buried channels. In addition to possible structural controls, the pockmark morphologies are affected by bottom currents.

The Devonian-Carboniferous boundary in the United States

The Devonian-Carboniferous boundary is documented in marine strata from the Appalachian Basin, Michigan Basin, Illinois Basin, Forest City Basin, Williston Basin, Tobosa Basin, southern continental margin, western platform, and several sub-basins of the Antler Foreland in the USA based on conodonts, brachiopods, miospores, carbon isotope data, and magnetic susceptibility data. The boundary is narrowly constrained and conformable in the Illinois Basin and Oklahoma; less well constrained in the Appalachian Basin, Antler Foreland, western platform, and the Alberta Platform, and often developed on an unconformity.

An Integration of Magnetic, Gravity and Seismic Data in Evaluating the Algoa Basin in the Eastern Cape Province of South Africa for Stratigraphic and Structural Geodynamics

The Algoa Basin is a hydrocarbon potential basin which lies off the southern continental margin of South Africa, along the coastal region of Port Elizabeth area with both onshore and offshore components. Despite the continued consensus on the basin’s hydrocarbon potential, both its structural and stratigraphic geodynamics are still not well documented. In this study, gravity data constrained by borehole data was modelled to reveal onshore subsurface geology whereas the seismic sections were used for offshore. In addition, magnetic data was used to delineate magnetic zones and lineaments. The onshore magnetic results show the localised occurrence of large-scale geologic bodies, with high and positive anomalies forming bounding rims coincidental with the edges of the basin. The average depths to the top of the shallowest and deepest magnetic sources were estimated from the power spectrum to be approximately 0.4–0.6 km and 9.5 km, respectively. The onshore Bouguer gravity map is characterised by a gravity low zone over the basin’s sediment where it attains its lowest value over the depocentre. 2¾ D gravity modelling results along five profiles show that the basin comprises of two to three ‘“V” shaped’ sub-basins of shallow crustal depth with sediment thickness of up to 4.6 km. The models also revealed discontinuity in sediment units, confirming the diachronous nature of the Uitenhage Group. Analyses of offshore seven seismic sections show that the stratigraphic sequence consists of diachronous geological units, which are characterised by displacements, forming a series of minor and major faults. Complementary to onshore gravity models, seismic sections also shows relatively shallow vertical extension to the Paleozoic crustal basement between 1.3 and 5.3 km.

Sediment distribution pattern of the South Caspian Sea: possible hydroclimatic implications

The south Caspian subbasin extends from the foot of the Alborz Mountains to the deepest part of Caspian Sea. The lithological background and tectonic setting of the study area primarily control the deposition of thick sedimentary sequences of the basin, while hydroclimatic processes exert secondary impacts on the sediment distribution pattern. To evaluate past hydroclimatic changes, short sediment cores were retrieved from the continental shelf and upper slope of the south Caspian Sea and were treated for basic sedimentological properties and mineralogy. The results show that the sediments are composed of terrigenous, biogenic, and chemical components. The chemical component has a significant signature in the internal structure of the sediment, which reflects degradation of organic material due to a succession of aerobic and anaerobic conditions. Aerobic conditions reflect the extreme climatic conditions that lead to formation of well-oxygenated deep water in lowstands and extremely cold winters, when dense oxygenated water is allowed to touch the bottom sediments. The sediment distribution pattern demonstrates three distinctive areas in the southern continental margin with different organic matter and carbonate content. Fluvial inputs and wave hydrodynamics control the sediment properties and their distribution pattern in the upper shelf, while in deeper parts, the dynamics of water stratification and its temporal and long-term variations, relating to climate and water level changes, determine sediment composition. Overall, the sedimentary distribution pattern is a result of their sources in the catchment basin and the hydroclimate of the Caspian Sea.

Origin and geodynamic significance of the Siuna Serpentinite Mélange, Northeast Nicaragua: Insights from the large-scale structure, petrology and geochemistry of the ultramafic blocks

The Siuna Serpentinite Mélange (SSM) is located in the boundary zone between continental Chortis block and oceanic Mesquito Composite Terrane in NE Nicaragua. In this paper, we present new large-scale structure, meso-scale kinematic analysis, 40Ar39Ar geochronology, mineral chemistry, in situ trace element composition of clinopyroxene, and bulk rock geochemical data for representative ultramafic rocks preserved as blocks in the SSM. Structurally, the mélange is a > 2.5 km-thick tectonic unit characterized by a disrupted block-in-matrix fabric with occurrence of exotic blocks, which have igneous and sedimentary protoliths and were variably metamorphosed to high-pressure (P) conditions. There are also blocks of ultramafic rocks, which consist mainly of harzburgite and clinopyroxene (Cpx)-rich harzburgite, with minor lherzolite and pyroxenite, and rare dunite and chromitite. In these rocks, relics of bastite and mesh textures of lizardite are replaced by antigorite blades and overprinted by antigorite schistosity.The presented data give rise to argue the following points. (1) The serpentinite matrix of the mélange resulted from hydration, ductile deformation and metamorphism at high-P conditions of abyssal and supra-subduction zone (SSZ) mantle peridotites in a subduction channel. (2) The geochemical characteristics of the ultramafic protoliths can be explained by at least two-stages of melting and refertilization processes: the first-stage occurred in a mid-ocean ridge (MOR) environment and produced Cpx-rich harzburgite and the second-stage took place in an SSZ setting and resulted in harzburgite, dunite and chromitite. (3) The second-stage magmatic processes produced melts that range in composition between island-arc tholeiites (IAT) and boninites, similar to arc-like protoliths of some mafic blocks in the mélange, which establishes a genetic link between blocks and matrix. (4) The presence of ultramafic and mafic blocks in the SSM, derived from both an island-arc upper plate and a MOR-like lower plate sources, suggests its position in an intra-oceanic subduction zone during the late Jurassic-early Cretaceous. (5) By the Hauterivian, the SSM became exhumed and tectonically emplaced onto the Chortis block as a consequence of the collision between the intra-oceanic arc and the southern continental margin of North America.

Hydrochemistry of thermal waters in crystalline rocks of the Sikhote-Alin ridge, Far East of Russia

Low-enthalpy thermal waters (30-50°C) of HCO3-Na and HCO3-SO4-Na types with nitrogen as a dominant associated gas discharge on the southern continental margin of the Russian Far East and traditionally are of great importance for recreation and balneology facilities. All studied thermal waters have meteoric origin and relate to Palaeocene granite intrusions. Based on the chemical characteristics and isotopic composition, thermal waters are divided onto “immature” thermal waters with a short residence time (3H=1.3±0.2TU), and “mature” thermal waters (3H=0.3±0.2TU) originated as a result of a longer evolution history (more than 60 years). The chemical composition of waters is largely determined by water-rock interactions, residence time and the local geothermal gradient.

Nitrogen isotopic composition of sediments from the eastern Tethys during Oceanic Anoxic Event 2

Low nitrogen isotopic compositions (δ15N) have been reported in deposits from the proto-North Atlantic and western Tethys during Oceanic Anoxic Event 2 (OAE2) at the Cenomanian-Turonian (C-T) transition. It has been proposed that the low δ15N values may have resulted from enhancement of N2 fixation and/or incomplete assimilation of the upwelled ammonium due to expansion of anoxic/euxinic conditions. However, few δ15N measurements are available for the sediments from the eastern Tethys and the Pacific and therefore the global significance of the N cycle and its environmental implications remains to be tested. Here, we present δ15N data from OAE2 deposits in the Gongzha section in the Tingri area of southern Tibet, which was located on the southern margin of the eastern Tethys during the mid-Cretaceous. The δ15N data reveal a large perturbation of the N cycle during OAE2. The pronounced negative shift in δ15N prior to OAE2, as well as the persistent low δ15N values during OAE2, imply the significant contribution of N2 fixation. The changes in the marine N cycle suggest that the expansion of an anoxic environment may also have occurred in the southern continental margin areas of eastern Tethys. The coincidence of the marine faunal turnover and the perturbation of the N cycle suggests that the expansion of anoxic water masses may have played an important role in the extinction of marine organisms across the C-T boundary in eastern Tethys.

Palaeogeography of tropical seasonal coastal wetlands in northern Britain during the early Mississippian Romer's Gap

ABSTRACTThe lower Mississippian Ballagan Formation of northern Britain is one of only two successions worldwide to yield the earliest known tetrapods with terrestrial capability following the end-Devonian mass extinction event. Studies of the sedimentary environments and habitats in which these beasts lived have been an integral part of a major research project into how, why and under what circumstances this profound step in the evolution of life on Earth occurred. Here, a new palaeogeographic map is constructed from outcrop data integrated with new and archived borehole material. The map shows the extent of a very low-relief coastal wetland developed along the tropical southern continental margin of Laurussia. Coastal floodplains in the Midland Valley and Tweed basins were separated from the marginal marine seaway of the Northumberland–Solway Basin to the south by an archipelago of more elevated areas. A complex mosaic of sedimentary environments was juxtaposed, and included fresh and brackish to saline and hypersaline lakes, a diverse suite of floodplain palaeosols and a persistent fluvial system in the east of the region. The strongly seasonal climate led to the formation of evaporite deposits alternating with flooding events, both meteoric and marine. Storm surges drove marine floods from the SW into both the western Midland Valley and Northumberland–Solway Basin; marine water also flooded into the Tweed Basin and Tayside in the east. The Ballagan Formation is a rare example in the geological record of a tropical, seasonal coastal wetland that contains abundant, small-scale evaporite deposits. The diverse sedimentary environments and palaeosol types indicate a network of different terrestrial and aquatic habitats in which the tetrapods lived.

Petroleum geological characteristics of two basin belts in southern continental margin in South China Sea

Based on the seismic, drilling, cores and outcrops data, the formation of basins, source rocks and hydrocarbon accumulations in the southern South China Sea (SSCS) were systematically analyzed to reveal the petroleum geological features of continental margin basins and summarize the distribution rule of oil and gas in the SSCS. The results show that the South China Sea (SCS) has experienced three tectonic stages, namely, the formation and development of proto-SCS, the subduction of proto-SCS and development of neo-SCS, the rapid subsidence and shrinking of SCS. The tectonic evolution of proto-SCS and neo-SCS controlled the regional tectonic pattern of continental margin in the SSCS, forming southern and northern basin belts, and also dominated the formation of basins, source rocks and hydrocarbon accumulation characteristics. The source rocks were mainly Miocene coal source rocks in the southern basin belt, with different thermal evolution degree, and the near-shore source rock was chiefly oil-generating while the off-shore source rock was mainly gas-generating. Compressive folding structural belts and reefs were the favorable belts. Within the northern basin belt, the source rocks were gas-prone and dominated by the Eocene to Oligocene, with high thermal evolution degree. Reefs and faulted blocks were the major accumulation areas. Our study demonstrates that the continental marginal basins of SSCS still have great exploration potential and is an important strategic area of oil and gas exploration and development. The southern basin belt focuses on oil and gas exploration and the northern basin belt focuses on gas exploration.

Cretaceous geological evolution of the Pontides

Abstract The Pontides forming the southern continental margin of the Black Sea consist of the Strandja, İstanbul and Sakarya zones. The Zonguldak-Ulus Basin, located in the NE part of the İstanbul Zone, has traditionally been viewed as opening during the Barremian and deepening until the Albian under the control of normal faults. New outcrop data indicate that the southern and eastern parts of this basin facing towards the Intra-Pontide Ocean in the south were already open during the Berriasian or earlier. Uplift and erosion of the Zonguldak-Ulus Basin during the Cenomanian is attributed to collision of the İstanbul and the Sakarya zones along the Intra-Pontide Suture. The Sinop Basin in the Sakarya Zone opened during Hauterivian–Barremian time. Sedimentation in this basin continued in a deepening environment until the development of the Pontide Magmatic Belt during the Turonian. The contact between the İstanbul and the Sakarya zones is represented by a shear zone that consists of siliciclastic distal turbidites, debris-flow deposits and radiolarian cherts imbricated with Middle Jurassic–Lower Cretaceous magmatic arc fragments. This shear zone is interpreted as being the eastern continuation of the Intra-Pontide Suture, separating the İstanbul and the Sakarya zones. The Western Black Sea Basin to the north of the Pontides possibly opened in two stages. In the first stage, coeval with the opening of the Zonguldak-Ulus Basin, the rifting was a wide-rift style and caused thinning of the continental crust. During the Turonian–Santonian, the Pontide Magmatic Belt started to develop as an extensional arc, and caused break-up of the already thinned crust and the start of oceanic spreading in the Western Black Sea Basin.