Single-channel Seismic Profiles Research Articles

Geological hazards in the Oujiang estuary in the Zhejiang Province, China: type, distribution, and origin

Introduction: As our understanding of the ocean and its uses continues to advance, countries worldwide with ocean access are implementing new marine strategies. For example, understanding marine geological disasters can help develop the use of ocean resources and oceanic engineering.Methods: Therefore, this study used recent data from the Oujiang Estuary, a nearshore area in China, to determine its geological hazards.Results: The geological hazards were classified as active or restricted based on topography, geomorphology, shallow seismic profile, single-channel seismic profile, and geological drilling data. Active geological hazards primarily include shallow gas and active sand waves, whereas restrictive geological hazards include irregularly buried bedrock, erosion channels, steep submarine slopes, and buried paleochannels. We also evaluated the distribution characteristics and scope, such as the vertical distribution of shallow gas based on the seismic profiles, drilling rock facies, methane, and carbon dioxide contents in the top air, and the isotope values. We found that shallow gas was vertically distributed among multiple layers. The main gas-bearing layers were the clayey silt and sandy (silt) clay layers of the early Holocene and late Pleistocene strata. The shallow gas content was relatively low in the coarse sediment layer at the bottom of the late Pleistocene succession. Generally, the Holocene and late Pleistocene deposits do not contain gas, and the gas content in the middle and late Pleistocene strata (at greater depths) is relatively low.Discussion: The combined effects of the regional geological structure, sea level changes, modern hydrodynamics, and human activities have formed the geological environment of the Oujiang Estuary.

What could form a 35-km lineament of carbonate mounds on the ocean floor?

A multibeam survey carried out in November 2019 reveals a lineament of seafloor protrusions on the Santos Basin (SW Atlantic) continental slope. Shallow seismic data and seafloor composition suggest the lineament comprises a 35 km long arrangement of Cold-Water Coral (CWC) carbonate mounds, here named the Tupana Carbonate Ridge (TCR), defined in this contribution for the first time. Insights into genetic controlling factors of the TCR led to the analysis of single-channel seismic profiles that cross the lineament. Interpretation of deep seismic reflection data suggests a stratigraphic control provided by the lateral arrangement of coastal deposits formed during the protracted progradation that characterized a significant portion of Santos Basin's depositional history. Coastal sandstones encased in low-angle shelf-margin progradational wedges constitute a potential hydrocarbon reservoir supplied by the flow of fluids toward the surface through vertical structures. The ultimate source of fluids seems to be provided by halokinetic movements on a region depleted of salts known as the Albian Gap domain. This work questions the role of seafloor hydrodynamics in shaping the TCR morphology by eroding the carbonate mounds and forming elongated contourite systems.

Characteristics and distribution of geohazards since the middle miocene of the Xisha sea area, South China Sea

Geological hazards can cause significant harm to the construction and maintenance of reef infrastructure projects in the Xisha Sea area. This study uses high-resolution multichannel earthquake data, single-channel seismic profiles, and multi-beam survey data to identify and analyze the geological hazards in the Xisha Sea area since the Miocene. Based on the geophysical data interpretation, the destructive geological disaster factors that are active, such as active faults, shallow gas, diapers, landslides, multistage scarps (steps), scouring troughs, and canyons, as well as the restrictive geological disaster factors without activity ability, such as buried paleochannels, pockmarks, reefs, and undersea volcanoes, are identified and analyzed. This paper discusses the causes and hazards of geological hazards and, for the first time, draws a comprehensive plane layout of the geological hazards. The above analysis demonstrates that the scarps are mainly located around the atolls or platforms, and the slope of the southeast seabed topography is significantly higher than that of the northwest. There are seven medium and large landslides, mainly located around Yongxing Plateau and Yongle Plateau, caused by gravity and faulting. Shallow gas is mainly developed in the southern part of the North Reef and is indicated by diaper structures, faults, and gas chimneys. A series of shallow faults are developed in the study area, mainly steep normal faults. The scouring troughs are primarily distributed near the Yongxing Platform, Zhongjian North Platform, and Huaguang Platform. Submarine canyons are primarily located in the northern and southern parts of the Shidao Platform. Affected by multiple factors such as hydrodynamic conditions, the stability of sedimentary layers, and sediment supply, the scour degree varies, with the general depth ranging from several meters to several hundred meters. Underwater infrastructure in the study area should not be constructed in areas with active and destructive geological hazards. The results of this study can serve as a guide for further exploration in the Xisha area and disaster prevention and mitigation during construction activity in the area.

Subduction related faults and sedimentary basins: The Western Ionian Sea case

The Pliocene-Quaternary evolution of the western margin of the Calabrian Arc has been investigated by analysing the configuration and distribution of local sedimentary basins and their relationship with shallow and deep tectonic structures. Research focussed on the evolution of the more recent offshore sedimentary basins located to the east of Sicily, along the Alfeo Fault System, using multichannel seismic lines, high-resolution bathymetry and newly acquired single-channel seismic profiles. The chosen study area encompasses a crustal sector where external thrusts of the Calabrian Arc branch into the Malta Escarpment, and this zone has been interpreted as a Subduction Transform Edge Propagator (STEP) Fault. The Western Ionian Sea is bounded by regional faults that controlled the deposition of the Pliocene-Quaternary sequences and define the present-day bathymetry. In this work we identified four main seismic sequences and three main unconformities in the basins, to better understand the geometry and the depositional setting of the basins. In addition, lateral correlation and 3D reconstruction of the sedimentary basins allows us to establish the direct relationship between the Alfeo Fault System activity and the geometry, the thickness, the evolution and filling phases of the sedimentary basins in this area. Results presented show how the Alfeo Fault drove the development of separate basin depocenters in the early stages of tectonic activity, resulting in a single elongated basin in the latest phases. The inferred fault kinematics, the analysis of displacements along strike and sediment thickening suggest a link with the slab geometry, as occurs in other subduction systems that develop subduction related faults. The results and the method applied in this study can also be applied for the interpretation of similar systems with similar geological settings in other areas of the world.

High-Resolution Seismic Stratigraphy Offshore Haeundae Beach in Busan, South Korea

Choi, D.-L.; Shin, D.-H.; Jin, J.-Y.; Lee. Y.-K., and Kum, B.-C., 2020. High-resolution seismic stratigraphy offshore Haeundae beach in Busan, South Korea. In: Jung, H.-S.; Lee, S.; Ryu, J.-H., and Cui, T. (eds.), Advances in Geospatial Research of Coastal Environments. Journal of Coastal Research, Special Issue No. 102, pp. 187-193. Coconut Creek (Florida), ISSN 0749-0208.The stratigraphic architecture of the Haeundae shallow sea was evaluated using numerous high-resolution single-channel seismic profiles and drilled cores. Two major seismic units over the basement, the lower Unit II and upper Unit I, as well as two subunits (Units Ia and Ib) in the upper Unit I were identified in the study area. Unit II shows numerous chaotic diffraction hyperbolic seismic reflection patterns overlying high amplitude, and irregular reflection of the bedrock surface, corresponding well with the gravel-dominated deposits. Unit II is interpreted as nonmarine alluvial deposits in the Quaternary glaciations. Unit I is present over Unit II with two distinct subunits at different locations: Unit Ia, distributed inshore within ∼10 m depth, and Unit Ib, offshore beyond ∼13 m depth. Unit Ia, which is accumulated more in the western inshore section, exhibits an acoustically semi-transparent and sigmoidal clinoform wedge and is well correlated with sand-dominated deposits from core data. Unit Ib, which pinches out landward but gradually thickens seaward, is generally characterized by transparent seismic facies, likely made up of very fine sandy to muddy sediments from surface sediments. The depositional environments of Units Ia and Ib are categorized as shoreface settings and the landward innermost part of the Nakdong distal delta system during late Holocene sea-level highstand, respectively. Due to severe weather conditions associated with winter storms and summer typhoons as well as human interventions, the sediment morphology and its distribution are thought to have constantly been reactivated and modified.

Study of the types and characteristics of geological hazards in reefs of Nansha Waters, South China Sea

Reports of the 19th National Congress of the Communist Party of China identifies that the construction of islands and reefs in South China Sea (SCS) has been actively promoted. It is of great practical significance to study the types and characteristics of geological hazards in Nansha Islands and reefs, and it is the precondition and guarantees to carry out the construction of islands and reefs. At present, the research on marine geological hazards in China focuses on oil and gas enrichment areas such as Bohai bay, southern Yellow Sea and Northern of South China Sea, and there is little research on southern of South China Sea, especially Nansha Reefs. On the basis of collecting and sorting out previous research achievements, combining with existing geological and geophysical data and drilling data, his paper focuses on analysing two single-channel seismic profiles N01 and N02 collected by “Shiyan 2” of South China Sea institute of Chinese academy of sciences in Nansha Islands and reefs in 2013, and studies the types and characteristics of geological hazards in this area. The study shows that the whole Nansha block belongs to areas with weak seismic activity, most of the faults in the island reef area are weak and micro-active, and the volcanic activity in the island reef area is weak, and the coral reef growth rate is fast, which can be suitable for all kinds of engineering construction projects.

Pleistocene forced regressive deposits on the Korea Strait shelf influenced by tectonic and ocean currents

We examined the architecture of Pleistocene sequences of the Korea Strait (KS) margin controlled by ocean currents and compressional tectonic regimes, using a dense grid of high-resolution single-channel seismic profiles and sediment cores. Along with uplifted strata featuring an erosional and/or non-depositional flat seabed in the northern shelf of the study area, three Middle–Upper Pleistocene sequences are characterized by forced regressive deposits recorded at 100-ka intervals covering one postglacial sequence across the shelf. The three forced regressions together comprise an offlapping stacking pattern, where the degree of erosion of the upper surface of the older sequence is greater than that of the younger sequence. The tectonic tilt involved in landward uplift and seaward subsidence across the shelf basin causes differential erosional rates among the forced regression deposits. The Upper Pleistocene forced regression deposits are preserved as a set of 20-ka clinoform wedges with aggradational to progradational stacking patterns, leading to a falling stage systems tract (FSST) during the stepwise fall in sea level, followed by a shelf edge-perched lowstand systems tract (LST) wedge during the Last Glacial Maximum (LGM). The northeast-facing Tsushima Current (TC) passing through the KS played an important role in allowing the fine sediments originating from the Nakdong River to form the FSST clinoform wedges and the LST oblique prograding delta lobe along the shelf break.

Glacial-related morphology and sedimentary setting of a high-latitude lacustrine basin: The Lago Chepelmut (Tierra del Fuego, Argentina)

Lago Chepelmut is a relatively small lake in size, of ellipsoidal shape, located in the outer fold-and-thrust belt of the Fuegian Andes (southernmost South America). High-resolution single-channel seismic profiles, integrated with geological information in the surrounding area, have allowed to reconstruct for the first time a bathymetric map of the lake and the architecture, distribution and thickness of the sedimentary cover. Two main seismic units were identified in the seismic records: (i) a Lower Unit of glacial nature, likely associated to the Last Glacial Maximum (LGM), and irregularly distributed through the basin, and (ii) an Upper Unit of lacustrine origin which drapes the entire basin. Submerged moraine deposits within the lake were also found from seismic data, and correlated with moraine arcs widespread distributed in the surroundings of the basin. These morphologies represent the recessional deposits left by the Ewan glacier lobe, one of the easternmost fronts of the Tierra del Fuego glaciers during the LGM. The lacustrine sedimentary record shows that the lake level was not constant through the recent history of the lake. Moreover, data analysis has shown that there is also an important structural component that has conditioned the evolution of the basin, in addition to that linked to glacial activity.

Failure dynamics of landslide scars on the lower continental slope of the Tyrrhenian Calabrian margin: insights from an integrated morpho-bathymetric and seismic analysis

AbstractThe collection of high-resolution multibeam bathymetry off the tectonically controlled Tyrrhenian Calabrian margin (southern Tyrrhenian Sea) allowed us to recognize several mass-wasting processes, including shelf-indenting canyons and several landslide scars ranging over different spatial scales. In this paper, we aim to characterize two large submarine landslides (S1 and S2) affecting an area of c. 7 and 14 km2, respectively; both scars occur within water depths of 700–1000 m on slope gradients of 1.5−3°. S1 is interpreted as a disintegrative landslide, because most parts of the related landslide deposits were evacuated from the scar and are not recognizable on the present-day bathymetry, whereas the landslide deposits of S2 are well-preserved and mostly confined within the scar, indicating a different post-failure evolution. Based on the integration of multibeam bathymetry and single-channel seismic profiles, both the landslides are interpreted as translational failures, whereas their different post-failure behaviour has been associated with differences in material properties (inferred by headscarp morphology), depth of their failure plane and frontal confinement. We also suggest that thick contourite deposits recognized in the area may represent an important preconditioning factor for the development of these landslides, similarly to that observed in the nearby Capo Vaticano scar complex.

Characterization of overbanking features on the lower reach of the Gioia-Mesima canyon-channel system (southern Tyrrhenian Sea) through integration of morpho-stratigraphic data and physical modelling

The collection of high-resolution multibeam bathymetry, single-channel seismic profiles, TOBI side scan sonar data, and gravity cores allowed the characterization of the main morpho-sedimentary processes acting along the lower reach of the shelf-indenting Gioia-Mesima canyon-channel system (GMS) and the surrounding continental slope (southern Tyrrhenian Sea). This last area, developing across a depth range of 1000–1700 m, shows a complex morphology due to the interaction between downslope gravitative processes (mainly turbidite sheet flows) and abrupt changes in slope gradients related to tectonically-controlled scarps. Particularly, several erosive-depositional features (levee deposits, sediment undulations, channels) have been related to overflow processes from the northern flank of the GMS, although the lower reach of the GMS is characterized by strong entrenchment (canyon height ranging from 120 to 270 m) and low sinuosity. Morphological and seismic stratigraphy data indicate that the distribution and dimension of these features vary in response both to the proximity to the external levee of the GMS and to the topographic gradient of the lower continental slope. Particularly, we were able to discriminate between a gently sloping sector (on average 1.5°) dominated by sedimentary bypass of the turbidity currents and a steeper sector (about 3°), where the erosional capability of these currents seems to increase. Indeed, three channels, 4,3–6,5 km long and up to 20 m deep, incise this steeper sector, running parallel to each other at a distance of 1250–1500 m. To support the capability of overbanking flow in producing these channels, we used a physical model for the ignition of turbidity currents that provides realistic values for the ignitive state of the overbanking turbidity flows. More generally, the methodological approach used in this study may be useful to provide constraints on the genesis and evolution of erosive-depositional features in other tectonically-controlled margins, where sedimentary gravity flows interact with an uneven morphology.

Submarine Depositional Terraces at Salina Island (Southern Tyrrhenian Sea) and Implications on the Late-Quaternary Evolution of the Insular Shelf

The integrated analysis of high-resolution multibeam bathymetry and single-channel seismic profiles around Salina Island allowed us to characterize the stratigraphic architecture of the insular shelf. The shelf is formed by a gently-sloping erosive surface carved on the volcanic bedrock, mostly covered by sediments organized in a suite of terraced bodies, i.e. submarine depositional terraces. Based on their position on the shelf, depth range of their edge and inner geometry, different orders of terraces can be distinguished. The shallowest terrace (near-shore terrace) is a sedimentary prograding wedge, whose formation can be associated to the downward transport of sediments from the surf zone and shoreface during stormy conditions. According to the range depth of the terrace edge (i.e., 10–25 m, compatible with the estimated present-day, local storm-wave base level in the central and western Mediterranean), the formation of this wedge can be attributed to the present-day highstand. By assuming a similar genesis for the deeper terraces, mid-shelf terraces having the edge at depths of 40–50 m and 70–80 m can be attributed to the late and early stages of the Post-LGM transgression, respectively. Finally, the deepest terrace (shelf-edge terrace) has the edge at depths of 130–160 m, being thus referable to the lowstand occurred at ca. 20 ka. Based on the variability of edge depth in the different sectors, we also show how lowstand terraces can be used to provide insights on the recent vertical movements that affected Salina edifice in the last 20 ka, highlighting more generally their possible use for neo-tectonic studies elsewhere. Moreover, being these terraces associated to different paleo-sea levels, they can be used to constrain the relative age of the different erosive stages affecting shallow-water sectors.

Seismic stratigraphy of the Shatsky Rise sediment cap, northwest Pacific, and implications for pelagic sedimentation atop submarine plateaus

Pelagic sediments accumulated atop Shatsky Rise since Early Cretaceous time provide a valuable record of Pacific paleoceanography. The accepted sedimentation model invokes relatively continuous pelagic sediment accumulation punctuated by four regional hiatuses caused by changes in oceanic chemistry. That framework was proposed more than two decades in the past and based on low-quality single channel seismic profiles. It was developed at the center of Tamu Massif, the main topographic high of Shatsky Rise, and extended to other highs with uncertainty owing to sparse data. The availability of high-quality seismic data and additional coring by the Ocean Drilling Program and Integrated Ocean Drilling Program allows a re-examination. Seismic and core data indicate that sedimentation atop Shatsky Rise has been complex, with regional and local variability. Four major hiatuses were previously defined and tentatively correlated to nearby topographic highs. These breaks occurred approximately during the Aptian-Barremian and Cenomanian-Coniacian ages of the Cretaceous and during the Paleocene and Oligocene epochs of the Cenozoic. They are of variable length and tend to be of shorter duration at the summit of Tamu Massif, but of longer durations elsewhere. All produced erosional unconformities that may be related to mass wasting of the sediment cap edges. Each hiatus is approximately correlated with global falls of sea level, suggesting a connection, perhaps to intensified bottom currents at those times. Local variability is observed in the form of discontinuous layering, non-uniform sedimentation, thinning of the sediment caps at their edges, and local erosion near topographic highs. These variations are likely the result of shifting bottom currents, possibly resulting from interactions of internal tides with topography. Shatsky Rise sedimentation is inherently punctuated and non-uniform and the sedimentary record includes significant lateral variability. Other large oceanic plateaus are likely to exhibit similar sedimentary cap complexities.

The sedimentary succession of the last ~ 3.50 Myr in the western South Yellow Sea: Paleoenvironmental and tectonic implications

The South Yellow Sea (SYS) Basin, which is part of the West Pacific Continental Margin, began to form in the late Mesozoic to early Cenozoic as a result of regional tectonic activity in Asia. The pre-Middle Pleistocene sedimentary history of the area remains poorly understood or controversial, mainly due to a lack of long borehole cores penetrating pre-Quaternary strata. Borehole core CSDP-1 (300.10m long), the longest sediment core recovered thus far from the SYS, was drilled in 2013 in the western SYS. Recently, a reliable timescale for the core for the last ~3.50Myr was obtained by magnetostratigraphy (Liu et al., 2016a). Here we report the results of integrated analyses of the core, including sedimentary characteristics, downcore changes in environmental proxies (benthic foraminifera and ostracods), and accelerator mass spectrometry 14C and optically stimulated luminescence dates. In addition, we analyze 546km of single-channel seismic profiles and ~50km of a boomer seismic profile around the core site, and correlate the regional stratigraphy. The core sedimentary succession contains 17 depositional units (lithofacies associations) corresponding to three major seismic units, reflecting a variety of depositional environments during the last ~3.50Myr. Our results show that fluvial environments prevailed in the western SYS from ~3.50 to 1.66Ma, followed by alternations of tidal-flat to shallow-subtidal environments with fluvial environments from ~1.66 to 0.83Ma, and alternations of modern-like marine environments and fluvial environments from ~0.83Ma to the present-day in response to glacio-eustatic sea-level cycles. The depositional succession during the last ~3.50Myr was dominantly controlled by tectonism and glacio-eustatic sea-level cycles. The presumed first marine transgression since the Neogene in the western SYS at ~1.66Ma is interpreted to suggest that sea water could partially flood the Zhe-Min Uplift (ZMU) into the SYS due to tectonic subsidence, and further subsidence of the ZMU and the SYS shelf is inferred to have resulted in establishment of normal marine environments in the SYS during the highstands of glacio-eustatic sea-level cycles since ~0.83Ma. The average subsidence rates in the western SYS were ~120–165m/Myr during the last ~1.66Myr. The sedimentary succession comprises fifth- and fourth-order sequences, which are interpreted to have been driven by the 20-kyr or 41-kyr and by the 100-kyr glacio-eustatic sea-level cycles, respectively, since the late Pliocene. The sequences deposited since the Late Pleistocene are relatively complete, recording deposits from MIS 5 to MIS 1. However, the pre-Upper Pleistocene sequences show significant stratigraphic hiatuses, presumably caused by fluvial erosion during the glacial sea-level lowstands and subsequent transgressive erosion. This type of significant discontinuity in the sedimentary record can be considered to be diagnostic of shelf sequences controlled by glacio-eustatic sea-level cycles.

The role of shelf morphology and antecedent setting in the preservation of palaeo-shoreline (beachrock and aeolianite) sequences: the SE African shelf

On the SE African shelf, a submerged shoreline at a depth of 60 m is examined and its attributes compared between two shelf sectors with different morphologies, yet similar energy regimes. The aim is to assess the controls of antecedent conditioning on shoreline development and later preservation from transgressive ravinement. Using a combination of multibeam bathymetry and single-channel seismic profiles, the stratigraphy and morphology of the shoreline is investigated. Low-gradient bedrock examples reveal several distinctive seismic facies, including onlapping chaotic reflector packages which are interpreted as calcarenite rubble fields. These palaeo-shorelines possess planform equilibrium morphologies, including parabolic dunes and blowout forms along with relict shore platforms. They are strongly associated with incised valleys of last glacial maximum age which underlie the shoreline locations; these provide wide, back-barrier accommodation space during transgression. In contrast, palaeo-shorelines on the steeper-gradient shelf have a simpler stratigraphic arrangement. They are not as well preserved, are generally covered by thick drapes of sediment, and lack the elaborate planform morphologies of their lower-shelf gradient equivalents. Isolated incised valleys and the steep bedrock gradient limit accommodation space. The comparison indicates that antecedent bedrock slope and available accommodation are amongst the dominant controls on overstepping, and thus potential preservation, of palaeo-shorelines on the shelf. Lower-gradient shelves not only promote rapid shoreline translation but, together with wide, sandy back-barrier accommodation, also foster larger barrier volumes. In suitable climates such as in the Mediterranean and other sub-tropical areas, the ensuing shoreline stability promotes rapid and effective cementation of the barrier. In comparison, steep bedrock profiles with limited back-barrier accommodation have much lower preservation potential. Transgressive ravinement is more focussed on steep slopes, effectively removing more material during the ravinement process. The more dynamic environment may also reduce the effectiveness of diagenesis. The potential of beachrock and aeolianite palaeo-shorelines as submerged sea-level indicators may be optimal in low-gradient settings in Mediterranean to subtropical environments.

The Ventotene Volcanic Ridge: a newly explored complex in the central Tyrrhenian Sea (Italy)

New high-resolution geophysical data collected along the eastern margin of the Tyrrhenian back-arc basin, in the Pontine Islands area, reveal a ∼NW-SE elongated morphological high, the Ventotene Volcanic Ridge (VR), located on the northern edge of the Ventotene Basin. High-resolution multibeam bathymetry, combined with magnetic data, multi- and single-channel seismic profiles, and ROV dives, suggest that VR results from aggregation of a series of volcanic edifices. The summit of these volcanoes is flat and occurs at about 170 m water depth. Given their depths, we propose that flat morphologies were probably caused by surf erosion during Quaternary glacial sea level lowstands. Seismic stratigraphy together with magnetic data suggest that the volcanic activity in this area is older than 190–130 ka age and may be coeval with that of Ventotene Island (Middle Pleistocene). The submarine volcanoes, located 25 km north of Ventotene, are part of a ∼E-W regional volcanic alignment and extend the Pontine volcanism landward toward the Gaeta bay. Integration of structural data from multichannel seismic profiles in this sector of the eastern Tyrrhenian margin indicates that several normal and/or transtensional faults, striking WNW-ESE, NNW-SSE, and NE-SW, offset the basement and form alternating structural highs and depressions filled by thick, mostly undeformed, sedimentary units. Arc-related magmatism is widespread in the study area, where the VR is placed at the hangingwall of the west-directed Apennines subduction zone, which is undergoing tensional and transtensional tectonics. Bathymetric and topographic evidence shows that VR lies in between a major NE-SW trending escarpment east of Ponza and a NE-SW trending graben southwest of the Roccamonfina volcano, a NE-SW transfer zone that accommodate the extension along this segmented portion of the margin. This suggests that the interaction between NE-SW and NW-SE trending fault systems acts as a structural control on location of eruptive centers, given that main volcanic edifices develop along the NW-SE direction, compatible with the extensional setting of the Tyrrhenian basin.

Comparison of buried sand ridges and regressive sand ridges on the outer shelf of the East China Sea

Based on multi-beam echo soundings and high-resolution single-channel seismic profiles, linear sand ridges in U14 and U2 on the East China Sea (ECS) shelf are identified and compared in detail. Linear sand ridges in U14 are buried sand ridges, which are 90 m below the seafloor. It is presumed that these buried sand ridges belong to the transgressive systems tract (TST) formed 320–200 ka ago and that their top interface is the maximal flooding surface (MFS). Linear sand ridges in U2 are regressive sand ridges. It is presumed that these buried sand ridges belong to the TST of the last glacial maximum (LGM) and that their top interface is the MFS of the LGM. Four sub-stage sand ridges of U2 are discerned from the high-resolution single-channel seismic profile and four strikes of regressive sand ridges are distinguished from the submarine topographic map based on the multi-beam echo soundings. These multi-stage and multi-strike linear sand ridges are the response of, and evidence for, the evolution of submarine topography with respect to sea-level fluctuations since the LGM. Although the difference in the age of formation between U14 and U2 is 200 ka and their sequences are 90 m apart, the general strikes of the sand ridges are similar. This indicates that the basic configuration of tidal waves on the ECS shelf has been stable for the last 200 ka. A basic evolutionary model of the strata of the ECS shelf is proposed, in which sea-level change is the controlling factor. During the sea-level change of about 100 ka, five to six strata are developed and the sand ridges develop in the TST. A similar story of the evolution of paleo-topography on the ECS shelf has been repeated during the last 300 ka.

Interaction of down-slope and along-slope processes off Capo Vaticano (southern Tyrrhenian Sea, Italy), with particular reference to contourite-related landslides

The Late Quaternary architecture of the upper slope-shelf sector of the Calabro-Tyrrhenian continental margin off Capo Vaticano was reconstructed to decipher the interaction of along-slope and down-slope sedimentary processes, based on the analysis of very high-resolution single-channel seismic profiles and multibeam bathymetry. Both along-slope bottom currents and downslope gravity processes influenced the stratigraphic architecture of the margin during the last eustatic cycle, locally forming a mixed turbidite and contourite system. Along-slope processes formed elongated drifts located on the upper continental slope and outer shelf, while gravity-driven down-slope processes formed mass-transport deposits and turbidite systems with erosive channels, locally indenting the present-day shelf. Several slide events affected the elongated drifts and failure processes appear to be dominated by translational sliding that affect the upper 10–20m of the stratigraphic record, with glide plains developed within contourite deposits. The most striking feature is the Capo Vaticano slide complex, which displays a large spatial coverage (area of about 18km2), between ca −190m and −500m and is composed of several intersecting slide scars and overlapping deposits. These characteristics are peculiar for the Tyrrhenian continental margins, where slide events developed in open-slope areas are usually less complex and smaller in size. This would indicate that high sedimentation rates and occurrence of contourite deposits can represent a preconditioning factor for medium-large scale slope instability on the Tyrrhenian margins.

Spatio-temporal evolution of sediment waves developed on the Gulf of Valencia margin (NW Mediterranean) during the Plio-Quaternary

Several fields of large-scale sediment waves have been observed along the Gulf of Valencia continental margin (NW Mediterranean). The largest sediment waves develop on the continental slope, extending from 250 to 850m water depth, with wavelengths ranging between 500m and 1000m and wave heights from ~2m to ~50m. On the lower part of the slope, sediment waves are quasi-stationary “vertically accreting”, becoming up-slope migrating towards the mid- and upper part of the slope. A second group of sediment waves have developed over the outer continental shelf, with wavelengths of 400 to 800m and heights of 2 to 4m, also displaying an up-slope migrating pattern. Multi-channel seismic lines crossing the continental margin show that the sediment waves over the continental slope region have been continuously developed on the foreset region of the prograding margin clinoform. Several units of sediment waves have been identified in the sedimentary record, evolving in accordance with the margin progradation. Detailed analysis of single-channel (sparker) seismic profiles revealed the presence of several sediment depositional subunits over the outer continental shelf, some of them with successive development of sediment waves being truncated by erosive surfaces, likely related to Quaternary eustatic sea-level oscillations. These erosional surfaces can be followed downslope into paraconformable strata of the sediment waves on the continental slope, where constant bedform growth is observed, without being affected by sea level changes. Based on geophysical data, the thickness of the sediment waves mapped units show that the largest sediment waves (in wave ratio, length and height) develop where sediment deposition rates are the highest, coinciding with the upper part of the continental slope (foreset clinoforms), confined by the presence of structural highs. The development of these sediment waves has been previously explained by the interaction of internal waves over the continental slope. Because sediment waves are preserved in the sedimentary record since the Lower/Pliocene, internal waves activity could have been present in this part of the margin shortly after the Zanclean reflooding of the Mediterranean Basin, following the Messinian desiccation event ~5.6 My ago. Deep water hydrodynamic conditions were re-established at that time, modulating sediment transport and deposition over the continental slope and outer continental shelf.

Some interpretational aspects of time cross-sections based on seismic reflection data from the Sea of Okhotsk

The results of investigations in the Sea of Okhotsk by the seismic reflection method are considered with special attention paid to the acoustic anomalies. Two types of such anomalies are definable in the basin: local (“gas channel”) and areal (“turbid sequences”). It is shown that the acoustic anomalies distinguishable in the single channel seismic profiling (SCSP) records are characteristic of sediments with partial gas filling. Many local anomalies documented on the Sakhalin slope are spatially connected with zones of faulting. The simultaneous presence of acoustic anomalies and BSR in the upper part of the section serves as a reliable diagnostic feature of gas hydrates. The dispersion of the velocities in the wave packet characterizing the acoustic anomaly associated with the lower part of the sedimentary cover in the Kurile Basin points to the significant thickness of the gas-bearing sediments in the latter.

Quaternary active tectonic structures in the offshore Bajo Segura basin (SE Iberian Peninsula – Mediterranean Sea)

Abstract. The Bajo Segura fault zone (BSFZ) is the northern terminal splay of the Eastern Betic shear zone (EBSZ), a large left-lateral strike-slip fault system of sigmoid geometry stretching more than 450 km from Alicante to Almería. The BSFZ extends from the onshore Bajo Segura basin further into the Mediterranean Sea and shows a moderate instrumental seismic activity characterized by small earthquakes. Nevertheless, the zone was affected by large historical earthquakes of which the largest was the 1829 Torrevieja earthquake (IEMS98 X). The onshore area of the BSFZ is marked by active transpressive structures (faults and folds), whereas the offshore area has been scarcely explored from the tectonic point of view. During the EVENT-SHELF cruise, a total of 10 high-resolution single-channel seismic sparker profiles were obtained along and across the offshore Bajo Segura basin. Analysis of these profiles resulted in (a) the identification of 6 Quaternary seismo-stratigraphic units bounded by five horizons corresponding to regional erosional surfaces related to global sea level lowstands; and (b) the mapping of the active sub-seafloor structures and their correlation with those described onshore. Moreover, the results suggest that the Bajo Segura blind thrust fault or the Torrevieja left-lateral strike-slip fault, with prolongation offshore, could be considered as the source of the 1829 Torrevieja earthquake. These data improve our understanding of present deformation along the BSFZ and provide new insights into the seismic hazard in the area.