ABSTRACT Seaward‐dipping sequences (SDS) are typically observed in sediment‐driven progradation of slope deposits in post‐rift sequences, or along magma‐rich rifted margins, where they are primarily formed by subaerial lava flows and referred to as seaward dipping reflections (SDRs). However, new seismic reflection data from the sediment‐rich northwestern South China Sea (NW‐SCS) reveal the existence of SDS in sediment‐rich syn‐rift sequences, here referred to as syn‐rift sedimentary SDS. Based on a detailed tectono‐stratigraphic analysis, we describe how these syn‐rift sedimentary SDS differ from magmatic SDRs and propose a conceptual model for their formation. We show that the formation of these sequences is most likely controlled by a combination of high sediment supply rates, rapid subsidence and creation of horizontal accommodation space, first along large‐offset detachment faults and later, during breakup, by magmatic accretion. While inner magmatic SDRs typically form in subaerial conditions above continent‐dipping faults, we suggest that syn‐rift sedimentary SDS can derive from turbiditic flows in a relatively deep‐water environment above oceanward‐dipping detachment faults and/or interfinger with magmatic additions. Finally, we discuss the implications of syn‐rift sedimentary SDS for the interpretation of rifted margins in general.

ABSTRACT The search for natural hydrogen (H 2 ) in sedimentary basins is gaining increasing recognition due to its environmental friendliness. Therefore, as an alternative energy source, natural hydrogen could address the issue of environmental challenges and participate in the energy mix necessary for the energy transition. Despite ongoing research, many uncertainties remain in H 2 exploration, which is still at an early stage. In this work, we provide, for the first time, a numerical model of radiolytic natural H 2 generation from the fractured basement based on the Taranaki Basin example (New Zealand). This approach uses conventional hydrocarbon exploration techniques, with some adjustments to properly reproduce the subsurface natural H 2 behaviour. We calculated the potential radiolytic H 2 generation rate to be approximately 10.3 mg/g/Ma. This value was used as a constant rate input in the model. Potential reservoirs within the possible optimal H 2 preservation window (80°C–200°C) include the Tane formation sandstone, as well as the Taimana and Tikorangi carbonate formations. The model highlights that H 2 mass concentration in water is higher along the faults and in interbedded sand facies of the Rakopi and Wainui formations, implying that hydrogen in solution could migrate both by diffusion and advection along these paths. The density inversion of the seal and the underlying reservoirs began at 9.4 Ma and 6.8 Ma in the Witiora and Taranga boreholes respectively, due to the northwestward progradation of the Mohakatino Formation. This inversion indicates a period during which hydrogen‐saturated water could be trapped or experience delayed flow, potentially leading to the exsolution of supersaturated hydrogen into a gaseous phase. The Tane formation gas anomaly reported during the drilling could be due to the conversion of CO 2 into abiotic CH 4 via the Sabatier reaction at higher temperatures (> 200°C). Consequently, abiotic CH 4 could be an accurate proxy for depicting natural H 2 generation.

ABSTRACTSills are fundamental elements of volcanic plumbing systems emplaced in sedimentary basins. Even though sills are commonly considered simple, planar concordant igneous sheets, they are actually complex 3‐dimensional objects. Detailed knowledge of the 3D structure of sills and their host rocks is of primary relevance to better constrain the emplacement mechanisms and the impacts of sills on sedimentary basins. This study describes the results of 3‐dimensional geological mapping of a large (~14 × 9 km), well‐exposed Early Cretaceous dolerite sill in Central Spitsbergen, Svalbard, Arctic Norway, using a combination of digital outcrop modelling and field mapping. The sill was emplaced within Upper Palaeozoic sedimentary formations of Svalbard. It is made of distinct segments emplaced at different stratigraphic levels of the host rock stratigraphy. The mapping shows a clear stratigraphic control on the intrusion morphology. Sill segments emplaced at the boundary between two formations, which mark a strong lithological and rheological boundary, are straight and concordant. Conversely, segments emplaced within a more homogeneous formation exhibit more irregular, locally discordant shapes. The sill segments emplaced at distinct stratigraphic levels are connected by steeply dipping steps, which formed through dilatant shearing between the tips of the sill segments. The preferred NW‐SE orientation of the steps and the thinning of the sill towards the SE suggests a propagation direction of the magma towards the SE. Our study shows how 3‐dimensional knowledge of igneous intrusions is key for revealing their emplacement mechanisms.

Abstract The submarine Miocene Central Canyon and Pleistocene channel systems in the Qiongdongnan Basin constitute valuable sedimentary records that provide insight into the depositional processes and sediment routing from the hinterland to the deep sea. However, the primary source of sediment for the Pleistocene channel systems and the variation in relative sediment contributions since the Miocene from potential source terranes remain unknown. We have integrated new and published detrital zircon U–Pb ages and rare earth elements (REEs) from Pleistocene channel sands and late Miocene Central Canyon sands in the Qiongdongnan Basin to analyse the sediment routing system of these channel systems since the Miocene. Qualitative analyses of REEs, comparisons of detrital zircon age spectra, and multidimensional scaling plots suggest that the Red River is a significant source of sediment supply. The quantitative analysis of sediment mixing models indicates that the Pleistocene channel sands were mainly sourced from the Red River (62.8%–85.7%), followed by Central Vietnam rivers (4.8%–27.1%), with a minor amount derived from rivers in Hainan Island, Northern Vietnam and Southern Vietnam. Sand sediments, mainly from the Red River system, were deposited in the Yinggehai Basin, then transported and deposited again in the Qiongdongnan Basin. The relatively stable and major sediment supply from the Red River since the Miocene may have been driven by the uplift of the Tibetan Plateau. This study quantifies the relative provenance contributions to submarine channel systems in the Qiongdongnan Basin since the Miocene. It provides crucial geological implications for tectonic responses to channel migrations and the prediction of gas hydrates in sandy reservoirs.

Abstract Unravelling source‐to‐sink relationships of sediment in coastal regions can be particularly challenging due to a variety of transport directions and mixing within varying local environments in response to sea level fluctuations. Post‐glacial sea level rise in the Holocene has resulted in the flooding of former continental margins, locally leading to the separation of islands such as Rottnest in southwest Australia. Rottnest lies approximately 20 km offshore from the mouth of the Swan River, one of the largest permanent river systems across thousands of kilometres of west Australian coastline. In this contribution, we investigate the size, U–Pb age distribution and α‐dose values of detrital zircon grains within 13 sand samples collected from three upstream tributaries that drain the Archean Yilgarn Craton, the Swan River estuary, offshore waters surrounding Rottnest Island and modern beaches. We explore sediment derivation, storage and mixing on this passive margin. Carbonate–silicate sands of the region contain detrital zircon with Archean, Mesoproterozoic and Cambro‐Neoproterozoic age modes, reflecting regional crystalline basement. Eo‐ to Paleoarchean zircon grains, including a previously enigmatic >3500 Ma component, are traced from offshore into the estuary, and specifically the Avon River tributary. Detrital mixing models imply an overall fluvial contribution to the estuary and offshore systems of up to 50–65%. By contrast, modern beach samples are dominated by Swan Coastal Plain recycled sediment of up to 96%. The α‐dose values of the prominent 3300–3150 Ma age component suggest more efficient fluvial discharge in the Paleo‐Swan River than in more recent times. Modern estuary samples have lower average and progressively lower downstream zircon α‐dose values, consistent with prolonged chemical and physical reworking and loss of metamict grains with transport distance in the river. We conclude that fluvial drainage networks distribute a locally persistent catchment signal whilst coastal plains in tectonically quiescent settings appear characterized by sediment reprocessing and mixed provenance.

Abstract The Miocene sequence in the Roer Valley Rift System consists of alternating open‐to‐shallow marine, coastal and fluvio‐deltaic deposits. In this study, well logs, bio‐chronostratigraphy and seismostratigraphy are used to characterize major units and their bounding unconformities and to infer sediment dispersal patterns. Three major unconformities occur in the sequence: the early, middle and late Miocene unconformities (EMU, MMU and LMU). The EMU formed due to tectonic motions related to the Savian phase. After formation of the EMU, a broad depocentre developed in the south‐eastern part of the Roer Valley Graben (RVG). Sediment accumulation increased during this period and peaked in the middle Langhian, after which it diminished again to a low level during the late Serravallian. The decrease in sediment accumulation coincided with a period of tectonic subsidence along the major bounding fault zones (i.e. the Peel Boundary Fault System, the Feldbiss Fault System and the Veldhoven Fault System). The resulting transgression caused sediment starvation in the central RVG. Subsequently, global sea‐level fall during the early Tortonian caused large‐scale erosion, and formation of incised valleys on the highs adjacent to the RVG (Peel Block and Campine Block), as well as the south‐eastern RVG, forming the MMU. However, sedimentation continued during this period in the central part of the RVG where no erosional hiatus developed. From the Tortonian onwards, accumulation rates increased again. The depocentre shifted towards the north‐west and clinoforms developed in the RVG. During the latest Miocene, the depocentre was concentrated along the south‐western margin of the RVG. Meanwhile, the depositional environment of the entire RVRS gradually shallowed as the LMU was formed.

Abstract The shelf‐margin deltas are the primary ‘sources’ of interest in siliceous submarine fan source‐to‐sink systems. However, less has yet to be discovered about the roles and effects of the shallow water platform situated on continental slopes. Using the Miocene sediments of the Ledong‐Lingshui Sag in the western part of the Qiongdongnan Basin (QDNB), this paper studies the YC35 coarse‐grained submarine fan that developed in the Meishan Formation (Fm.). The material sources, depositional characteristics, and developmental mechanisms of this fan were investigated. The restored palaeogeomorphology, based on high‐resolution 3D seismic data, shows that a shallow water platform exists. According to geochemical palaeoenvironmental data, the shallow water platform was ideal for forming carbonate deposits in the Meishan Fm., owing to the warm and humid palaeoclimate and the shifting trend in palaeowater depth. Based on the combined source tracing of heavy minerals, coherence slices, and seismic profiles, the shallow water platform and the Ningyuan River source on Hainan Island contributed to the YC35 coarse‐grained submarine fan. Compared to submarine fans formed during different periods, coarse‐grained fan stands out due to its distinct sedimentary structure, rock composition, and microlithological characteristics. Aside from not following the usual Bowmar sequence, there is an excess of gravel and no clear bedding or lamination. A large number of rock fragments, mostly granite and sedimentary rock (carbonatite), make up the rock composition. According to these results, the shallow water platform significantly affects submarine fan material composition. In addition, the attribute slice based on the root mean square reveals that shallow water platforms moderate peripheral deposition. We attribute the development and proximal supply of the shallow water platform to diapirism and forced regression. Our research provides novel insights and comprehension into the investigation of submarine fan sedimentary systems.