High-resolution Seismic Reflection Data Research Articles

Cenozoic basement-involved rifting of the northern South China Sea margin

The northern South China Sea (SCS) margin developed on a thinning crust comprising basement-involved structures inherited from the post-orogenic extension of the Cathaysia Block. While, the pre-rift structural configuration and its influence on the geometry and development of the Cenozoic rift basins are ambiguous. Based on 3D high-resolution seismic reflection data in the western Pearl River Mouth Basin (PRMB) and onshore fieldwork, we delineated the intrabasement boudinage structure which is constituted by the low-angle E–W-striking Indosinian fold-thrust belts and the steeply dipping NE-striking Yanshanian thrusts. It is concluded that the pre-rift structure played a key role in controlling the architecture and two-phase early rifting of the northern SCS margin. (1) During the Early Eocene syn-rift phase, the Indosinian thrusts were rejuvenated first and controlled the formation of a series of ENE-striking grabens. (2) During the Late Eocene syn-rift phase, both the Indosinian and Yanshanian thrusts were reactivated. The Indosinian thrusts continued to maintain normal faulting and even evolved into the detachment faults. The Yanshanian thrusts were inverted into the NE-striking dextral strike-slip faults, which controlled the formation of the pull-apart basin group en échelon and even the SCS opening. The lithospheric thinning mechanism of the SCS continental margin experienced a major change from the Early Eocene pure shear to the Late Eocene simple shear.

High resolution seismic reflection PP and PS imaging of the bedrock surface below glacial deposits in Marsta, Sweden

Multi-component high resolution seismic reflection data were acquired at the Uppsala University field test site in Marsta, Sweden, in March 2019 with the aim of obtaining an improved understanding of the subsurface structure in the area. An advantage of the site is that a number of boreholes have been drilled there for both hydrogeological and geophysical purposes, allowing surface geophysics to be compared with downhole information. The presence of a low velocity layer above the water table generates significant processing challenges due to trapped waves. A further complication with the data is that the upper part of this layer was frozen at the time of the survey, resulting in the sediments just below the surface having a significantly higher velocity than those below. By analyzing and processing both the vertical and radial component data it was possible to build a velocity model that is consistent with the observed data. A strong PS converted reflection allows the bedrock to be imaged on the radial component stacks to much higher resolution than on the vertical component stacks. Both common conversion point binning and pre-stack depth migration were used to process the radial component data. We confirmed that our processing strategy was effective by generating synthetic data that were processed in a similar manner as the real data. The PS images indicate a step in the bedrock of about 2 m, with depth to it increasing from about 15 m to 17 m close to one of the boreholes. This step is not observed on the PP stacks, due to their lower resolution.

A Case Study Using Seismic Reflection and Well Logs to Reduce and Quantify Uncertainty During a Hydrogeological Assessment

Building a representative three-dimensional conceptual model of the hydrostratigraphic units is the critical first step when undertaking a hydrogeological assessment. The construction of such conceptual model requires integrating all geological data available for reducing the uncertainty of the conceptual model. In addition, the different types of data collected do not have the same level of uncertainty, and this has to be considered during the modeling. This manuscript presents a geostatistical workflow developed to integrate high resolution seismic reflection data with geological well markers and well logs. The proposed workflow allows reducing and quantifying the uncertainty when developing the conceptual model of hydrostratigraphic units and estimating the spatial distribution of hydraulic conductivity needed to simulate groundwater flow. The study involves the field investigation and numerical modeling for understanding the hydraulic connection between a local sand and gravel esker aquifer overlying a fractured bedrock regional aquifer. Seismic reflection data recorded in 2019 and 2021 significantly modify the conceptual model originally envisioned at site by discovering unknown deep esker extensions. Seismic data also help reducing the spatial uncertainty by delimiting the lateral extents, thickness, and depth of the esker aquifer between existing boreholes. Thickness maps of major hydrostratigraphic units are computed by first interpolating the data with higher uncertainty, which are then locally deformed by successive kriging with external drift to honor data with lower uncertainty. Well logs are then combined with seismic reflection data to better represent the spatial distribution of hydrofacies within the esker aquifer, and identify subunits in the bedrock aquifer. These modifications in the conceptual model of hydrostratigraphic units and the better understanding of the vertical heterogeneity of the hydraulic conductivity significantly impact the simulation of groundwater flow, both regionally and locally at site.

Paleolimnological reconstructions for Lake Arakhley (Central Transbaikalia, Russia) inferred from high-resolution reflection seismic data

Abstract. High-resolution seismic data (65 km profiles) have been used to study the sedimentary infill of Ivan-Arakhley lake system (Lakes Arakhley, Ivan and Shakshinskoe). It found that full limnological cycle bears only sediment cover of Lake Arakhley. Total sediment infill of Lake Arakhley is 6-7 m, and it was be divided into three sequences. The uppermost ~2.5 m of sediment cover was presented by a normal lacustrine filling. Lacustrine sediments began to form ca. 21 cal. ka BP. The lake was almost dried during the regression occurred between ca. 3 and 2.6 cal. ka BP.

The onset of seafloor spreading at the northeastern continent-ocean boundary of the South China Sea

The transition from continental to oceanic crust and the initial formation of typical oceanic crust at passive rifted margin are still poorly understood. In this study, deep crustal structures in the transitional domain of the northeastern South China Sea (SCS) margin are investigated and studied according to newly-acquired high-resolution reflection seismic data and gravity modeling. Three strong reflectors (R1, R2 and R3) are imaged in the narrow transitional domain. About 1 s in the two-way travel time (TWTT) beneath the top basement, R1 shows a similar morphology to the top basement, and quite likely represents a partial serpentinization interface. The presence of R1 suggests a tectonically dominated extensional stage prior to the final lithospheric rupture of the SCS. R2 is from magmatic intrusions or plutons (sills or dikes) from decompression melting. R3 is observed at ~2.5–2.2 s TWTT beneath the top basement but shoals seawards. R3 likely resulted from magmatic relicts in the paleo-melt transfer channels and/or from frozen molten materials in a shear zone of the lithospheric and asthenospheric mantle. The occurrence of R3 indicates the onset of a robust seafloor spreading system. The gradual seaward increase in magmatic activity highlights that, towards the final lithospheric breakup, tectonic extension was overtaken by magmatism. Increased magmatism and partial serpentinization formed a narrow hybrid transitional crust. Nevertheless, the breakup was not an instantaneous event. The last-stage transient magmatism occurred in a very narrow zone and does not rule out that the margin was predominantly amagmatic during the rifting and breakup. Based on reflection seismic and gravity modeling, we propose that the onset of typical seafloor spreading occurred at about 30 Ma around the study area, and the continent-ocean boundary (COB) should be located further seawards in the northeastern SCS margin.

Seismic reconstruction of seafloor sediment deformation during volcanic debris avalanche emplacement offshore Sakar, Papua New Guinea

Volcanic island sector collapses have produced some of the most voluminous mass movements on Earth and have the potential to trigger devastating tsunamis. In the marine environment, landslide deposits offshore the flanks of volcanic islands often consist of a mixture of volcanic material and incorporated seafloor sediments. The interaction of the initial volcanic failure and the substrate can be highly complex and have an impact on both the total landslide deposit volume and its emplacement velocity, which are important parameters during tsunami generation and need to be correctly assessed in numerical landslide-tsunami simulations. Here, we present a 2D seismic analysis of two previously unknown, overlapping volcanic landslide deposits north-west of the island of Sakar (Papua New Guinea) in the Bismarck Sea. The deposits are separated by a package of well-stratified sediment. Despite both originating from the same source, with the same broad movement direction, and having similar deposit volumes (~15.5–26 km3), the interaction of these landslides with the seafloor is markedly different. High-resolution seismic reflection data show that the lower, older deposit comprises a proximal, chaotic, volcanic debris avalanche component and a distal, frontally confined component of deformed pre-existing well-bedded seafloor sediment. We infer that deformation of the seafloor sediment unit was caused by interaction of the initial volcanic debris avalanche with the substrate. The deformed sediment unit shows various compressional structures, including thrusting and folding, over a downslope distance of more than 20 km, generating >27% of shortening over a 5 km distance at the deposit's toe. The volume of the deformed sediments is almost the same as the driving debris avalanche deposit. In contrast, the upper, younger landslide deposit does not show evidence for substrate incorporation or deformation. Instead, the landslide is a structurally simpler deposit, formed by a debris avalanche that spread freely along the contemporaneous seafloor (i.e., the top boundary of the intervening sediment unit that now separates this younger landslide from the older deposit). Our observations show that the physical characteristics of the substrate on which a landslide is emplaced control the amount of seafloor incorporation, the potential for secondary seafloor failure, and the total landslide runout far more than the nature of the original slide material or other characteristics of the source region. Our results indicate the importance of accounting for substrate interaction when evaluating submarine landslide deposits, which is often only evident from internal imaging rather than surface morphological features. If substrate incorporation or deformation is extensive, then treating landslide deposits as a single entity substantially overestimates the volume of the initial failure, which is much more important for tsunami generation than secondary sediment failure.

Footprints of palaeocurrents in sedimentary sequences of the Cenozoic across the Maurice Ewing Bank

High-resolution seismic reflection data across the Maurice Ewing Bank, the easternmost section of the Falkland/Malvinas Plateau in the SW South Atlantic, is integrated with information from Deep Sea Drilling Project Leg 36, Sites 327, 329, and 330 and Leg 71 Site 511. Five seismic units were identified ranging in age from the middle Jurassic to Quaternary and are interpreted with respect to the evolution of the oceanic circulation in the Atlantic sector of the Southern Ocean in response to tectonic and climatic events. Sedimentary sequences of late Cretaceous and early Paleogene include little and restricted evidence of current activity, attributable to shallow/intermediate-depth connections between the developing South Atlantic and the Southern Ocean. In contrast, sedimentary sequences of the late Eocene/Oligocene and Neogene reveal a history of strong current-related erosion and deposition. These features are identified in specific water-depth ranges and interpreted to document proto-Upper and -Lower Circumpolar Deep Waters to have shaped the bank since the Oligocene. The Maurice Ewing Bank bathymetric high thus has been acting as a barrier for the deep and bottom water masses flowing within the Antarctic Circumpolar Current since its establishment at the Eocene-Oligocene boundary. This study provides evidence for a multi-layered ocean already in the Paleocene/early Eocene.

Development and Interplay of Two Orthogonal Fault Systems in the NW Qaidam Basin, Northern Tibetan Plateau

How fold and thrust belts (FTBs) evolve over time and space in a transpressional regime remains poorly understood. Based on high-resolution 3D seismic reflection data and remote sensing images, we herein present a detailed structural analysis of the Cenozoic faults in the NW margin of the Qaidam Basin that is bounded to the north by the left-reverse Altyn Tagh fault system. Two sets of orthogonal, basement-involved faults with contrasting geometries, kinematics, and temporal development are identified. One set consists of generally E-W-striking, N-dipping, reverse faults with a component of sinistral shear. They are parallel or subparallel to the Altyn Tagh fault system, led to southward tilting of the basement, and formed a local unconformity between the middle Miocene Shangyoushashan formation and underlying strata. They developed in an out-of-sequence order, and were mostly active during 43.8–15.3 Ma but in relatively tectonic quiescence with limited weak reactivation since then. The second set is mainly composed of the NNW-striking reverse faults with dextral shear components. They are approximately perpendicular to the Altyn Tagh fault system, and intensively active since ∼15.3 Ma, much later than the initiation of the E-W-striking faults. Together with published results, we ascribe the development of these two sets of orthogonal faults as the transition from transpression to left lateral slip on the central segment of the Altyn Tagh fault system. The two fault sets interplayed with each other in two ways: 1) the older E-W-striking faults were offset by younger NNW-striking faults, and 2) the younger NNW-striking faults curved to link with the preexisting E-W-striking faults. Our findings reveal that transpressional-dominated FTBs evolve in a more complicated way than the contractional-dominated ones, and more site-based case studies are needed to reveal the underlying primary principles.

Offshore/onshore correlation of the North-Anatolian fault deformations in the Western Sea of Marmara

Newly collected high-resolution seismic reflection and magnetic data collected offshore of the Ganos Fault (NW Turkey) in the western Sea of Marmara, along the submerged portion of the North-Anatolian Fault, highlight the presence of a complex fault pattern. Based on these data, we propose a new tectonic model for this key region, connecting the onshore fault pattern to the transtensional basins of the deepest Sea of Marmara. According to this new model, three main faults control the deformation pattern of this region, fault SF1, a tectonic element that shapes the Tekirdag Basin, and fault SF2, which although not affecting the sea floor, is structurally related to SF1. Another fault we called SF3, connects the offshore to the Ganos fault onshore creating a transtensional pattern nearby the shelf. All such kinematic indicators witness an incipient activity, and a seismogenic potential that should be determined by a detailed mapping of both the surficial expression of fault at the seafloor, and the deformation pattern observed in the post-glacial sedimentary sequence. The active fault pattern detected nearby the coastal areas and their structural relationships with the Tekirdag basin show that it could be interpreted as a negative flower structure, which accommodates strain partitioning. This observation is crucial to seismic hazard assessments in this region that has been the site of destructive earthquakes in the recent and remote past.

Geologic controls on paleodrainage incision and morphology during sea level lowstands on the Cascadia shelf in Oregon, USA

The lowstand extensions of two fluvial systems from the Cascadia subduction margin are investigated using high-resolution seismic reflection data to determine the relative controls on paleodrainage morphology. We document distinct differences between the two systems, located ~40 km apart, attributed to underlying structure and lithology, shelf gradient, and drainage basin characteristics. A cross-shelf valley exists for only one of the drainage systems, the Umpqua River. The outer shelf part of the valley is a syncline, which formed a natural structural low for lowstand drainage, while the inner shelf part of the valley exhibits incision and is associated with a mid-shelf convex change is slope. The other drainage system, the Siuslaw River, did not develop an across-shelf valley and is only denoted in the seismic data by an erosional subaerial unconformity. Structural trends are perpendicular to cross-shelf drainage, supporting a more anastomosing stream morphology for the paleo-Siuslaw. The shelf edge for this system was not exposed for a long period of time during the last lowstand, preventing substantial outer shelf incision. Differences in fluvial elevation, discharge, and hinterland geology, also played a role in the cross-shelf morphologies of the paleo-Umpqua and paleo-Siuslaw. These results highlight how controls on lowstand paleodrainage morphology can change over short distances on an active margin.

Along-strike variations in protothrust zone characteristics at the Nankai Trough subduction margin

AbstractSignificant along-strike changes in the protothrust zone at the toe of the Nankai Trough accretionary prism were imaged in new high-resolution seismic reflection data. The width of the protothrust zone varies greatly along strike; two spatially discrete segments have a wide protothrust zone (∼3.3–7.8 km, ∼50–110 protothrusts), and two segments have almost no protothrust zone (∼0.5–2.8 km, <20 protothrusts). The widest protothrust zone occurs in the region with the widest and thickest sediment wedge and subducting turbidite package, both of which are influenced by basement topography. The trench wedge size and lithology, the lithology of the subducting section, and the basement topography all influence the rate of consolidation in the trench wedge, which we hypothesize is an important control over the presence and width of the protothrust zone. We conclude that protothrusts are fractures that form from shear surfaces in deformation band clusters as the trench fill sediment is consolidated. Strain localization occurs at sites with a high density of protothrusts, which become the probable locations of future frontal thrust propagation. The frontal thrust may propagate forward with a lower buildup of strain where it is adjacent to a wide protothrust zone than at areas with a narrow or no protothrust zone. This is reflected in the accretionary prism geometry, where wide protothrust zones occur adjacent to fault-propagation folds with shallow prism toe surface slopes.

Response of a wave-dominated coastline and delta to antecedent conditioning and fluctuating rates of postglacial sea-level rise

In the context of rising sea levels, wave-dominated coasts and deltas are among the most threatened of coastal landforms. The study of submerged deltas, in tandem with other drowned wave-dominated shoreline sequences, can shed light on the long-term coastal behaviour of these threatened shorelines when subject to high-end scenarios of relative sea-level rise. Using ultra high-resolution seismic reflection data and gravity core samples, this paper investigates the post-glacial stratigraphic development and architecture of the shelf off southern Mozambique. We document several well-preserved beachrock/aeolianite palaeo-shoreline complexes from water depths of −100, −75 and −60 m, interpreted as representing the positions of former sea-level slowstands or stillstands. These overlie the regional subaerial unconformity associated with the last glacial maximum and are linked to two phases of deltaic deposition. Pro-deltaic deposits on the outer shelf were linked with the period of sea-level stability attributed to the −100 m shoreline, while proximal delta front deposits on the inner- to mid-shelf are linked to a slowstand evident in the local sea level record at −40 m. Well-developed lower delta plain facies overlie both shoreline and delta front facies and show evidence of strong channelisation during normal regression of the delta. These are overlain across the shelf by isolated and stranded pieces of the now relict shoreface, trapped to seaward by the shoreline complexes. The more proximal parts of the shelf are in turn overlain by fluvial-derived muddy facies of the current prodeltas of the area.The preservation of shoreline and delta deposits was enhanced by intervening rapid increments in sea-level rise associated with meltwater pulses 1A (−100 m) and 1B,(−60 m) though the preservation of the −75 shoreline feature, in spite of modest rates of Older Dryas base-level change, points toward other factors such as rapid cementation as dominant in this instance. The region's high sediment supply exerted a primary control on shelf preservation by dampening the erosive extent of wave-ravinement. Variations in antecedent gradient are responsible for much of the documented along-shelf and down-dip depositional variability: low-gradient settings promoted the development of broad coastal plains and strongly wave-dominated deltaic facies, while steeper areas of the shelf favoured the development of coastal landforms in relatively narrow complexes fronted by deep-water deltaic deposits. Steeper areas of the shelf also tend be characterised by an embayed along-coast profile that locally mitigates wave-energy, reinforcing the influence of inherited gradient.Once preserved, drowned coastal barriers introduced an additional geological control as post-glacial sediment became trapped between these features on the outer shelf. The dispersal of modern sediments is strongly influenced by the hydrodynamic regime Delagoa Bight, under which fluvial muds are transported northeastward along the shelf by a persistent inner-mid shelf counter-current. A considerable proportion of the modern sediment budget is lost off-shelf where the shelf narrows.This study shows that low antecedent gradients foster gentle coastal profiles predisposed to delta overstepping. Though local topographic knickpoints can aid in preserving the delta during submergence, they do not buffer the delta from the general effects of inundation which include drowning of the delta top. Once overstepped, the former delta can be overlain by distal deltaic sediments associated with the higher sea level and associated fluvial and oceanographic forcing quite different from earlier phases of delta construction.

Depositional architecture and evolution of basin-floor fan systems since the Late Miocene in the Northwest Sub-Basin, South China Sea

The sediment budget of the Northwest Sub-basin, South China Sea since the Late Miocene (11.6 Ma, average thickness > 1000 m) accounts for more than two-thirds of the total infill since the initial ocean spreading of the sub-basin (32 Ma). The sediment sources and architectural pattern of these deposits, however, are poorly known. Using high-resolution 2D reflection seismic data with age constraint from IODP boreholes, we have documented two interdigitating basin-floor fan systems that developed since the Late Miocene. These were fed by two of the largest deep-water canyon systems worldwide, from the west (the Central Canyon/Xisha Trough) and the northeast (the Pearl River Canyon), as well as from smaller headless canyons and gullies across the surrounding slopes. Based on careful analysis of seismic facies, their geometry and occurrence, we identify the principal deep-water architectural elements, including the multi-scale channels, channel-levee complexes, lobes, sheets and drapes, mass-transport deposits, volcanic intrusions, turbidity-current sediment-wave fields, and a contourite drift/terrace.Tentative reconstructions show that the development of these Late Miocene-Quaternary basin-floor fan systems was dominated by changes of sediment supply. The Xisha fan reached its largest extent during the Late Miocene, while the Pearl River fan was most active during the Late Miocene to Quaternary. During the Late Miocene, both the conduits of the Central Canyon and the Pearl River Canyon were active with abundant sediment supply, generating the two incipient fan systems. Sediment supply from the west via the Central Canyon persisted throughout the Late Miocene, being coarser-grained than that of the Pearl River fan. With the demise of the Central Canyon during the Pliocene and consequent sharp decrease in sediment supply, the Xisha fan size reduced significantly. By contrast, supply of mud-rich sediments from the Pearl River and northern slope increased through the Pliocene and into the Quaternary, leading to the modern sedimentary pattern of interdigitating basin-floor fans. Insights into the evolution of sediment supply and fan development through time derived in this study contribute to a better understanding of how source to sink systems feed marginal oceanic basins such as the South China Sea.

Pre-glacial and post-glacial history of the Scandinavian Ice Sheet in NW Russia – Evidence from Lake Ladoga

Lacustrine sedimentary records are valuable archives of regional paleoenvironmental and paleoclimatic changes. High-resolution seismic reflection data collected in 2013, in conjunction with data from sediment core Co1309, provide a detailed reconstruction of the preglacial and postglacial environmental and sedimentological history of Lake Ladoga (NW Russia) during the late Quaternary. The exceptionally good quality of the seismic data reveals an up to 90-m-thick sedimentary infill in undisturbed places, containing four distinct sequences documented in unprecedented detail. The deepest sequence S1 most likely represents the bedrock of the lacustrine basin, which was reshaped during the subsequent glaciations. The overlying sequence S2 contains large-scale mounds of variable dimension, shape and internal architecture, which suggest a complex history of formation and deformation. We interpret these mounds as a moraine-mound complex resulting from thrusting and bulldozing processes of pre-existing, subglacial sediments during the overall retreat of the Scandinavian Ice Sheet following the Last Glacial Maximum. The finely-layered sequence S3 coincides with varved sediments in core Co1309 that mark the Scandinavian Ice Sheet deglaciation in the Ladoga basin. The youngest sequence S4, characterised by a semi-transparent facies in the acoustic data, coincides with lacustrine sediments in core Co1309 that were deposited during the Holocene. Our results also document major thoroughgoing fault systems in the northwestern part of the lake basin, along with unconformities related to lake-level fluctuations, notably in the eastern-, western- and southernmost regions. By coupling hydroacoustic and sedimentological data, we are able to pinpoint lateral variations and modifications of sedimentary inputs through time during the evolution of the Ladoga basin, along with unravelling the paleoenvironmental history of Lake Ladoga since the pre-LGM.

Seismic geomorphology of three types of deepwater fans and their relationship with slope morphology: Qiongdongnan Basin, northern South China Sea

Analyses of high-resolution seismic reflection, drilling, and well-log data were combined to identify and investigate deepwater fans developed in the eastern part of the Qiongdongnan Basin, South China Sea, during the Middle Miocene and their relationship to slope morphology. We identified three types of deepwater fans that developed on the northern continental margin of the South China Sea during the Middle Miocene: (1) those consisting of small canyons and depositional lobes; (2) those consisting of large canyons and depositional lobes; and (3) those consisting of distributary channels without depositional lobes. Sourced from deltaic sediments deposited on the northern continental shelf margin, the lithologies of the sediments in all fans are dominated by siltstone. The scale of development and the internal features are mainly controlled by the slope topography. The deepwater fans consisting of small canyons and depositional lobes developed on slopes with low gradient, caused by the continuous progradation of the shelf-edge towards the basin centre due to sediment transport in high-speed flows. The canyon heads developed in this setting are located below the shelf-break; these canyons are narrower, shallower, and shorter than those of the deepwater fans consisting of large canyons. Thus, the scale of the depositional lobes deposited by gravity flow through such canyons is smaller than that of deepwater fans with large depositional lobes. Deepwater fans consisting of large canyons and depositional lobes developed on steep narrow slopes, caused by rising shelf-edge trajectories due to the equilibrium between the sediment supply and subsidence rates. The canyon heads developed in this setting are located upon the shelf-break and extend toward the toe of the slope, incising the shelf-break; these canyons are wider, deeper, and longer than those of the deepwater fans consisting of small canyons. The scale of the depositional lobes deposited by gravity flows transported through such canyons is correspondingly larger than that of deepwater fans with small depositional lobes. Deepwater fans consisting of distributary channels without depositional lobes developed on gentler and wider slopes than slopes where small canyons and depositional lobes formed, owing to the low sediment supply. The deepwater distributary channels have a long transportation distance and wider distribution area, enabling the formation of large-scale deepwater fans. Depositional models for all three types of deepwater fans were postulated.

Ice sheet and palaeoclimate controls on drainage network evolution: an example from Dogger Bank, North Sea

Abstract. Submerged landscapes on continental shelves archive drainage networks formed during periods of sea-level lowstand. The evolution of these postglacial drainage networks also reveals how past climate changes affected the landscape. Ice-marginal and paraglacial drainage networks on low-relief topography are susceptible to reorganisation of water supply, forced by ice-marginal rearrangement, precipitation and temperature variations, and marine inundation. A rare geological archive of climate-driven landscape evolution during the transition from ice-marginal (ca. 23 ka) to a fully submerged marine environment (ca. 8 ka) is preserved at Dogger Bank, in the southern North Sea. In this study, our analysis of high-resolution seismic reflection and cone penetration test data reveal a channel network over a 1330 km2 area that incised glacial and proglacial lake-fill sediments. The channel network sits below coastal and shallow marine sediments and is therefore interpreted to represent a terrestrial drainage network. When mapped out, the channel form morphology reveals two distinct sets. The first set comprises two low-sinuosity, wide (>400 m) channels that contain macroforms of braid and side bars. These channels are interpreted to have originated as proglacial rivers, which drained the ice-sheet margin to the north. The second set of channels (75–200 m wide, with one larger, ∼400 m wide) has higher sinuosity and forms a subdendritic network of tributaries to the proglacial channels. The timing of channel formation lacks chronostratigraphic control. However, the proglacial rivers must have formed as the ice sheet was still on Dogger Bank, before 23 ka, to supply meltwater to the rivers. Ice-sheet retreat from Dogger Bank led to reorganisation of meltwater drainage and abandonment of the proglacial rivers. Palaeoclimate simulations show a cold and dry period at Dogger Bank between 23 and 17 ka. After 17 ka, precipitation increased, and drainage of precipitation formed the second set of channels. The second set of rivers remained active until marine transgression of Dogger Bank at ca. 8.5–8 ka. Overall, this study provides a detailed insight into the evolution of river networks across Dogger Bank and highlights the interplay between external (climate) and internal (local) forcings in drainage network evolution.

Geophysical characterization of late-Quaternary glaciofluvial complex and glacial stratigraphy in the Satakunta sandstone area, Köyliö, southwest Finland

AbstractWe acquired high-resolution reflection seismic data using a broadband digital-based landstreamer system to characterize a depression related to the Mesoproterozoic (Jothnian) Satakunta sandstone basin in the Köyliö study area, southwest Finland. This ca. 800-m-wide depression is infilled with up to 100-m-thick (late) Quaternary interlobate glaciofluvial complex sediments. The seismic images clearly reveal details of the glaciofluvial complex, sandstone depression topography, and brittle structures related to the formation of the sandstone basin by oblique transtension. Additionally, we identified the setting of a diabase laccolith within the sandstone and the geometry and position of the steep sandstone contact. The esker core does not follow the sandstone-Svecofennian basement rock contact or lean to it. The esker core is at a depth of 50–60 m on the flank of the depression. The seismic data image the esker core and other architectural elements of the esker. We highlight the potential of the digital-based landstreamer in the research of complex Quaternary sediments in major bedrock depressions including the characterization of the underlying bedrock properties. We also discuss the possible tunnel valley origin of the sandstone depression.

An innovative seismic and statistical approach to understand 3D magmatic structures and ore deposits in the western Bushveld Complex, South Africa

In this paper, we demonstrate the effectiveness of a novel approach to identify and quantify structural features that disrupt ore-bearing horizons using 3D seismic attributes and a horizon-surface processing technique (difference-of-two-surfaces) on high-resolution 3D reflection seismic data from the Lonmin Platinum Mine in the Bushveld Complex, South Africa. We demonstrate that seismic attribute analysis is an effective tool to enhance the intelligibility of the UG-2, a PGE-enriched chromitite layer, as well as potholes (slump structures), faults, iron-rich ultramafic pegmatite bodies (IRUPs) and other subtle geological features. Furthermore, the horizon-surface processing technique is able to quantify the dimensions and geometry of potholes. In total, we identify 66 potholes and show that they are probably randomly distributed in 2D on the UG-2 horizon with some extent of clustering. Roughly two classes of potholes are identifiable by size. The larger potholes are clustered around a major normal fault (the Marikana Fault) and its branches in the area. Using a first-order pothole structure/geometry statistical analysis, we demonstrate that: (1) the diameters and surface areas of the potholes are strongly related to their depths; and (2) the volume of the potholes is substantially log-normally distributed. We interpret these findings to implicate a coherent and pervasive pothole formation mechanism. Furthermore, we show that statistically-derived pothole structure/geometry knowledge can be used to constrain the validity of several hypotheses. Our findings indicate the most probable pothole formation hypothesis is a widespread extensional regime in the Bushveld Complex magma chamber. Therefore, our study provides important, large-scaled and highly data-driven insights with an approach that can be applied to a wide range of magmatic structures. Furthermore, relationships identified between various pothole characteristics, in addition to their academic value, also allows for predictive modelling that can guide mine planning and resource extraction.

The role of large-scale mass wasting processes in changing the sediment dispersal pattern in the deep-water Central Canyon of the northwestern South China Sea

Large-scale mass transport complex (MTC) (usually covering hundreds to thousands of km2 with a volume of tens to hundreds of km3) resulting from slope failures, can change the overall sediment dispersal pattern in deep-water canyon. However, the causal mechanisms remain enigmatic. We investigate the large-scale MTCs in the Central Canyon that are imaged in 3D by high-resolution seismic reflection data from the northwestern South China Sea. Our results indicate that a total volume of 435.3 km3 of sediments related to large-scale mass wasting processes were dumped into the Central Canyon from its side direction between 5.7 and 4.2 Ma. This led to adjustments of the geomorphology of the thalweg slope, which changed from a simply stepped slope (2 steps separated by 1 ramp) into a complexly stepped slope (3 steps separated by 2 ramps). The adjustments are argued to be caused by the thickness variations of MTCs1-4 that also ponded the axial turbidity flows at their up-dip directions. And then, the adjusted thalweg slope controlled the formation of a multi-stage submarine fan, which is proved to be an important hydrocarbon exploration target within the Central Canyon. Besides the first step in the canyon where sediments were mainly transported downslope due to the initially large velocities of turbidity flows, the following ramps were also considered as sediment bypass zones due to the further accelerated flows. Whereas, the subsequent steps became sediment deposition dominated zones as turbidity flows decelerated abruptly. Slope overstepping resulting from high sediment supply that was controlled by the East Asia summer monsoon, together with seismicity caused by the structural inversion of the Red River Fault were responsible for the occurrences of the large-scale mass wasting processes. We highlight how the large-scale mass wasting processes change the sediment dispersal pattern in the deep-water canyon for the first time. This study is of great significant to identify the locations of hydrocarbon reservoirs and evaluate the spatial variability in depositional reservoir qualities within the deep-water canyon.

Deep-ocean paleo-seafloor erosion in the northwestern Pacific identified by high-resolution seismic images

The deposition and diagenesis of pelagic marine sediments together play important roles in global geochemical cycles by returning elements, such as silica, from the hydrosphere to the geosphere and stably sequestering them within deep-ocean sedimentary sections around the world. Here, we present new, high-resolution (vertical resolution 2–5 m) seismic reflection data from the northwestern Pacific that we interpret to indicate a period of erosional activity that has returned silica to the hydrosphere and thus represents a departure from uniform sequestration by pelagic sediment deposition. The interpreted erosional features are observed as a ~ 200- to 300-m-wide horizontal variation in reflection character within the ~20-m interval overlying the regional chert/porcellanite layer. This reflection character variation is not consistent with uniform pelagic sedimentation, and it has not previously been noted in the northwestern Pacific, where most seismic data are lower-resolution. Erosion of seafloor sediments may have been caused by bottom-current activity, but the geometry of the observed features suggests that fluid-expulsion erosion from diagenesis of primarily siliceous sediment played some role in driving this mass transfer. This period of erosional activity occurred after this portion of the Pacific plate left the equatorial zone of high biological productivity ~75 Ma, and ended between approximately 35 and 55 Ma. Regardless of the details of the process, erosion of deep-ocean seafloor sediments represents a return of silica and other chemicals from the sediment to the bottom water and, if pervasive in the oceans, these processes should be accounted for in models of global geochemical cycles.