Structure and morphology of the west Reykjanes basin and the southeast Greenland continental margin

The continental-shelf morphology is dominated by glacial erosion and deposition. Erosion is prominent on the near-shore shelf and deposition along the outer shelf edge. The continental slope is characterized by delta-shaped progradations (glaciomarine-sediment fans) seaward of the shelf channels. Canyons cross the continental slope only in the region southeast of Cape Farewell. The continental rise is incised by a number of submarine canyons. Broad sediment ridges on the upper continental rise are probably canyon-eroded remains of extensive Plio-Pleistocene fans. A mid-ocean channel which crosses the continental rise is possibly related to the axis of maximum depth of Denmark Strait. Despite the presence of strong bottom currents, there is no indication of depositional sediment drifts along the continental margin of Greenland between Cape Farewell and Denmark Strait. This may be a function of high current velocity or low sediment load.Sea floor older than 60 m.y. B.P. is present just seaward of the Greenland continental margin implying either downwarped continental material or an early rift formed prior to the separation of Greenland from the European plate. A left lateral offset of anomalies 20 and 21 at 65°N indicates a major fracture zone related to the Greenland continental margin offset nearby.     

Seismic and sequence stratigraphy of the central western continental margin of India: late-Quaternary evolution

Seismic and sequence stratigraphic architecture of the central western continental margin of India (between Coondapur and south of Mangalore) has been investigated with shallow seismic data. Seismic stratigraphic analysis defined nine seismic units, that are configured in a major type-1 depositional sequence possibly related to fourth-order eustatic sea-level changes, comprising regressive, lowstand, transgressive and highstand systems tracts. The late-Quaternary evolution of the continental margin took place under the influence of an asymmetric relative fourth-order sea-level cycle punctuated by higher frequency cycles. These cycles of minor order were characterised by rapid sea-level rises and gradual sea-level falls that generated depositional sequences spanning different time scales. During the regressive periods, dipping strata were developed, while erosional surfaces and incised valleys were formed during the lowstands of sea level. Terraces, v-shaped depressions, lagoon-like structures observed on the outer continental shelf are the result of the transgressive period. In the study area we have recognised a complex erosional surface that records a long time span during the relative sea-level fall (regressive period) and the following sea-level lowstand and has been reworked during the last transgression. We also infer that sedimentation processes changed from siliciclastic sedimentation to carbonate sedimentation and again to siliciclastic sedimentation, marking an important phase in the late-Quaternary evolution of the western continental shelf of India. We attribute this to an abrupt climate change at the end of the oxygen isotope stage 2, between the Last Glacial Maximum and the Bølling-Allerod event (14?000 yr BP). This sensitive climate change (warming) favoured the formation of reefs at various depths on the shelf, besides the development of Fifty Fathom Flat, a carbonate platform on the outer shelf off Bombay developed prior to 8300 yr BP. The highstand systems tracts were deposited after the sea level reached its present position.     

Surficial sediments on the continental margin off the west coast of South Africa

A combination of climatic and oceanic factors has resulted in slow sedimentation rates on the continental shelf off the west coast of South Africa since Tertiary times. This has enabled a study to be made of the residual Late Tertiary, relict Pleistocene and Holocene sediments.Sediments on the continental shelf form rough belts parallel to the coast. Most of the coarse sediment is confined to the littoral zone and Holocene mud is concentrated at the base of a rocky nearshore platform. A veneer of Quaternary quartzitic sand seawards of the Recent mud belt wedges out onto a Tertiary erosion surface on the mid shelf. Residual glauconite and phosphorite sands derived by erosion during Tertiary sea-level fluctuations cover large parts of the mid shelf in the south. Most of the slope and parts of the outer shelf in the north are draped by Recent foraminiferal and coccolithophorid debris.     

Morphosedimentary features and recent depositional architectural model of the Cantabrian continental margin

Multibeam bathymetry, high (sleeve airguns) and very high resolution (parametric system-TOPAS-) seismic records were used to define the morphosedimentary features and investigate the depositional architecture of the Cantabrian continental margin. The outer shelf (down to 180–245 m water depth) displays an intensively eroded seafloor surface that truncates consolidated ancient folded and fractured deposits. Recent deposits are only locally present as lowstand shelf-margin deposits and a transparent drape with bedforms. The continental slope is affected by sedimentary processes that have combined to create the morphosedimentary features seen today. The upper (down to 2000 m water depth) and lower (down to 3700–4600 m water depth) slopes are mostly subject to different types of slope failures, such as slides, mass-transport deposits (a mix of slumping and mass-flows), and turbidity currents. The upper slope is also subject to the action of bottom currents (the Mediterranean Water — MW) that interact with the Le Danois Bank favouring the reworking of the sediment and the sculpting of a contourite system. The continental rise is a bypass region of debris flows and turbidity currents where a complex channel-lobe transition zone (CLTZ) of the Cap Ferret Fan develops.The recent architecture depositional model is complex and results from the remaining structural template and the great variability of interconnected sedimentary systems and processes. This margin can be considered as starved due to the great sediment evacuation over a relatively steep entire depositional profile. Sediment is eroded mostly from the Cantabrian and also the Pyrenees mountains (source) and transported by small stream/river mountains to the sea. It bypasses the continental shelf and when sediment arrives at the slope it is transported through a major submarine drainage system (large submarine valleys and mass-movement processes) down to the continental rise and adjacent Biscay Abyssal Plain (sink). Factors controlling this architecture are tectonism and sediment source/dispersal, which are closely interrelated, whereas sea-level changes and oceanography have played a minor role (on a long-term scale).     

On the origin and flow behavior of submarine slides on deep-sea fans along the Norwegian–Barents Sea continental margin

 Debris lobes with characteristic lengths, widths, and thickness of 30–200 km, 2–10 km, and 10–50 m, respectively, represent the main building blocks of deep-sea fans along the Norwegian–Barents Sea continental margin. Their formation is closely related to the input of clay-rich sediments to the upper continental slope by glaciers during periods of maximum ice advance. It is likely that slide release was a consequence of an instability arising from high sedimentation rates on the upper continental slope. The flow behavior of the debris lobes can be described by a Bingham flow model. Received: 17 November 1995 / Revision received: 24 June 1996     

Sedimentation processes on the continental rise of northeastern South America

A section of the continental rise of northeastern South America northeast of the Orinoco delta contains physiographic features built by the interaction of southward-flowing North Atlantic Deep Water and turbidity currents generated in the Orinoco region during the last Pleistocene glacials. A sedimentary outer ridge of low relief (Demerara Outer Ridge) trends northeast along the rise and a field of westward-migrating sediment waves trending north-northwest is superimposed on the outer ridge. The sediment waves have a maximum amplitude and wavelength of 20 m and 4 km, respectively. Seismic profiler records indicate that the outer ridge was probably built during the Pleistocene. A major turbidity-current pathway adjacent to the outer ridge on the north supplied sediment to the southward-flowing North Atlantic Deep Water which then deposited this sediment down-stream on the outer ridge and formed the sediment waves. Piston cores from the outer ridge contain numerous silt—sand beds and appear to be contourites. The cores consist primarily of gray hemipelagic clay of a Late Wisconsin age and have high ($\text{>}\text{10}\text{cm}\text{1000}\text{yrs}$) sedimentation rates. In contrast, cores from the continental rise north of the turbidite channel are brown clays with relatively low sedimentation rates ($\text{3.0}\text{cm}\text{1000}\text{yrs}$) and do not contain silt—sand contourites.     

Echo character of the East Brazilian continental margin and its relationship to sedimentary processes

The nature and regional distributions of various types of bottom echoes recorded on 3.5-kHz echograms from the East Brazilian continental margin (8–30°S) provide valuable information about sedimentary processes which have been active on a regional scale. The ten types of echoes observed fall into two major classes: distinct and indistinct. Indistinct echoes have two sub-classes; prolonged and hyperbolic. A qualitative correlation is observed between three types of distinct and indistinct-prolonged echoes and the relative abundance of coarse, bedded sediment (silt, sand, gravel) in piston cores. Regions returning distinct echoes with continuous parallel sub-bottoms contain little or no coarse sediment; regions returning indistinct very prolonged echoes with no sub-bottoms contain very high concentrations of coarse sediment; and regions returning indistinct semiprolonged echoes with intermittent sub-bottoms contain moderate or intermediate amounts of coarse sediment. Thus the regional distributions of these three echo types reflect the dispersal of coarse terrigenous sediment throughout the region. High concentrations of coarse sediment are restricted to relatively small areas which are generally proximal to large deep-sea channels, whereas very low concentrations occur in distal regions such as the lowermost continental rise and adjacent abyssal plain. Moderate concentrations of coarse sediment occur throughout most of the continental rise. Five of the six types of hyperbolic echoes observed are reflected from erosional/depositional bed forms. Although some of these bed forms (especially on the upper continental rise) have probably been produced by gravity-controlled mass flows (turbidity currents, slumps, etc.) the fact that the most extensive and widespread regions of hyperbolic echoes occur in distal regions beneath the present axis of flow of the Antarctic Bottom Water suggests that most of these bed forms are the result of sediment reworking by the contour-following bottom currents of this water mass.     

Late Pleistocene to Holocene sedimentation and hydrocarbon seeps on the continental shelf of a steep,tectonically active margin,southern California,USA

Small, steep, uplifting coastal watersheds are prolific sediment producers that contribute significantly to the global marine sediment budget. This study illustrates how sedimentation evolves in one such system where the continental shelf is largely sediment-starved, with most terrestrial sediment bypassing the shelf in favor of deposition in deeper basins. The Santa Barbara–Ventura coast of southern California, USA, is considered a classic area for the study of active tectonics and of Tertiary and Quaternary climatic evolution, interpretations of which depend upon an understanding of sedimentation patterns. High-resolution seismic-reflection data over >570 km² of this shelf show that sediment production is concentrated in a few drainage basins, with the Ventura and Santa Clara River deltas containing most of the upper Pleistocene to Holocene sediment on the shelf. Away from those deltas, the major factor controlling shelf sedimentation is the interaction of wave energy with coastline geometry. Depocenters containing sediment 5–20 m thick exist opposite broad coastal embayments, whereas relict material (bedrock below a regional unconformity) is exposed at the sea floor in areas of the shelf opposite coastal headlands. Locally, natural hydrocarbon seeps interact with sediment deposition either to produce elevated tar-and-sediment mounds or as gas plumes that hinder sediment settling. As much as 80% of fluvial sediment delivered by the Ventura and Santa Clara Rivers is transported off the shelf (some into the Santa Barbara Basin and some into the Santa Monica Basin via Hueneme Canyon), leaving a shelf with relatively little recent sediment accumulation. Understanding factors that control large-scale sediment dispersal along a rapidly uplifting coast that produces substantial quantities of sediment has implications for interpreting the ancient stratigraphic record of active and transform continental margins, and for inferring the distribution of hydrocarbon resources in relict shelf deposits.     

Slope failures along the western continental margin of India: a consequence of gas-hydrate dissociation, rapid sedimentation rate, and seismic activity?

Along the western continental margin of India (WCMI), several bottom simulating reflectors were identified on analogue single-channel seismic records, some of these located in areas where slumping and mass wasting were observed. The causes, consequences and degree of geographic variation of these geomorphic processes are assessed in terms of possible gas-hydrate dissociation during Pleistocene sea-level changes, high sedimentation resulting in underconsolidation, and seismotectonic activity prevailing along the WCMI margin. One consequence of possible gas-hydrate dissociation along the continental slope could be sediment failure and mass transport down the slope. By contrast, in the flat deep-sea areas, gas-hydrate dissociation may have led to gas seepage and the development of pockmarks at the seafloor.     

Evidence for current-controlled sedimentation along the southern Mozambique continental margin since Early Miocene times

Major plastered drift sequences were imaged using high-resolution multichannel seismics during R/V Meteor cruises M63/1 and M75/3 south of the Mozambique Channel along the continental margin of Mozambique off the Limpopo River. Detailed seismic-stratigraphic analyses enabled the reconstruction of the onset and development of the modern, discontinuous, eddy-dominated Mozambique Current. Major drift sequences can first be identified during the Early Miocene. Consistent with earlier findings, a progressive northward shift of the depocenter indicates that, on a geological timescale, a steady but variable Mozambique Current existed from this time onward. It can furthermore be shown that, during the Early/Middle Miocene, a coast-parallel current was established off the Limpopo River as part of a lee eddy system driven by the Mozambique Current. Modern sedimentation is controlled by the interplay between slope morphology and the lee eddy system, resulting in upwelling of Antarctic Intermediate Water. Drift accumulations at larger depths are related to the reworking of sediment by deep-reaching eddies that migrate southward, forming the Mozambique Current and eventually merging with the Agulhas Current.     

Magnetic anomalies over the Central Levant continental margin

The magnetic field over the central Levant continental margin, off northern Israel and southern Lebanon, and the adjacent Levant Basin has two distinct trends. Mount Carmel and its offshore continuation (Carmel Nose), which are the surface expression of a large subbottom structure that extends from the land area across the continental shelf to the continental slope, form a dividing zone between the two magnetic trends. South of the Carmel structure the magnetic field trends east-west, while north of the Carmel structure it trends northeast and north-northeast.Several pronounced magnetic anomalies exist mainly north of the Carmel structure, the majority of which trend north-northeast and northeast, parallel and sub-parallel to the trend of the magnetic field in this area. Some also trend northwest, perpendicular to the trend of the magnetic field. In several cases the magnetic anomalies indicate large lithological elements which continue from land to sea.Gravity and seismic refraction data show that the two magnetic domains north and south of the Carmel structure are associated with areas of different crustal structure. South of the Carmel structure the continetal-oceanic crustal transition zone is located beyond the continental margin at the base of the continental slope, while north of the Carmel structure it is located under the continental shelf, near the shore. On land, there are also differences in the structure of the crust north and south of the Carmel structure, the crust being much thinner north of the structure than south of it.We suggest that some of the large magnetic anomalies off the Central Levant were formed during the rifting phase of the eastern Mediterranean.     

Particle flux across the mid-European continental margin

Results are presented from particle flux studies using sediment trap and current meter moorings along a transect at the European continental margin at 49°N within the EU-funded Ocean Margin Exchange (OMEX) project. Two moorings were placed, at the mid- and outer slope in water depths of 1500 and 3660 m, with traps at 600 and 1050 m and at 580, 1440 and 3220 m, respectively. Residual currents at the mid-slope follow the slope contour, whereas seasonal off-slope flow was registered at the outer slope. At 600 m on the slope fluxes are similar to those in the abyssal North Atlantic. The flux of all components (bulk dry weight, particulate organic and inorganic carbon, lithogenic matter and opal) increased with water depth. Highest fluxes were recorded at 1440 m at the outer slope, where off-slope residual currents mediate particle export. The injection of biogenic and lithogenic particles below the depth of winter mixing results in the export of particles from shallower waters. Calculated lateral fluxes of particulate organic carbon exceed the primary flux by over a factor of 2 at 1440 m on the outer slope. Estimated lateral fluxes of suspended particulate matter in the water column and intermediate nepheloid layers at the outer slope are potentially large compared to sinking fluxes measured by sediment traps. A comparison is made of particle flux at three continental margin sites and two sites in the adjacent open North Atlantic, from which it is seen that bulk and organic matter flux increases exponentially with proximity to the shelf break. The percentage contribution of particulate organic carbon to biogenic fluxes increases from a mean of 5.7% in the abyssal N. Atlantic to 13.9% at the continental margins.     

An integrated thermomechanical model for transform continental margin evolution

 A 2D numerical thermal model for transform continental margin evolution is presented that calculates thermally driven uplift and subsidence profiles across the margin, for any margin segment assuming both regional and local isostasy. Lateral variations in the magnitude of continental uplift along the transform are predicted. For a margin with a length of 900 km, with a spreading rate of 1 cm yr^-1, maximum continental uplift of 1300–1400 m is calculated, assuming local isostasy. Using a regional isostatic approximation, maximum uplift is reduced substantially to 335–470 m, and the exact magnitude, location, and timing of the maximum effect depends strongly on the assumption of a coupled or decoupled continent–ocean boundary. The length of time a margin point experiences continent–ocean shearing prior to ridge passing is also shown to be very significant. Received: 23 February 1995 / Revision received: 17 August 1995     

The continental margin of Uruguay: Crustal architecture and segmentation

The Uruguayan continental margin comprises three sedimentary basins: the Punta del Este, Pelotas and Oriental del Plata basins, the genesis of which is related to the break-up of Gondwana and the opening of the Atlantic Ocean. Herein the continental margin of Uruguay is studied on the basis of 2D multichannel reflection seismic data, as well as gravity and magnetic surveys. As is typical of South Atlantic margins, the Uruguayan continental margin is of the volcanic rifted type. Large wedges of seaward-dipping reflectors (SDRs) are clearly recognizable in seismic sections. SDRs, flat-lying basalt flows, and a high-velocity lower crust (HVLC) form part of the transitional crust. The SDR sequence (subdivided into two wedges) has a maximum width of 85 km and is not continuous parallel to the margin, but is interrupted at the central portion of the Uruguayan margin. The oceanic crust is highly dissected by faults, which affect post-rift sediments. A depocenter over oceanic crust is reported (deepwater Pelotas Basin), and volcanic cones are observed in a few sections. The structure of continental crust-SDRs-flat flows-oceanic crust is reflected in the magnetic anomaly map. The positive free-air gravity anomaly is related to the shelf-break, while the most prominent positive magnetic anomaly is undoubtedly correlated to the landward edge of the SDR sequence. Given the attenuation, interruption and/or sinistral displacement of several features (most notably SDR sequence, magnetic anomalies and depocenters), we recognize a system of NW-SE trending transfer faults, here named Río de la Plata Transfer System (RPTS). Two tectono-structural segments separated by the RPTS can therefore be recognized in the Uruguayan continental margin: Segment I to the south and Segment II to the north.     

Synthesis of the crustal structure of the transform continental margin off Ghana, northern Gulf of Guinea

 Results of a detailed geophysical transect across the transform continental margin off Ghana, at the eastern end of the Romanche Fracture Zone in the Equatorial Atlantic, are presented. Seismic refraction, single-channel seismic reflection, gravity, and magnetic data were collected, and seismic, gravity, and magnetic models along the transect are shown. The 6- to 11-km-wide ocean–continent transition (OCT) is characterized by a high-velocity, high-density, high-magnetization crustal zone. The models show no evidence for any underplating of the continental crust adjacent to the margin but minor melting and intrusion of the continental crust may have occurred in the vicinity of the OCT. Received: 6 February 1995/Revision received: 24 July 1995     

Comparison of marine gas hydrates in sediments of an active and passive continental margin

Two sites of the Deep Sea Drilling Project in contrasting geologic settings provide a basis for comparison of the geochemical conditions associated with marine gas hydrates in continental margin sediments. Site 533 is located at 3191 m water depth on a spit-like extension of the continental rise on a passive margin in the Atlantic Ocean. Site 568, at 2031 m water depth, is in upper slope sediment of an active accretionary margin in the Pacific Ocean. Both sites are characterized by high rates of sedimentation, and the organic carbon contents of these sediments generally exceed 0.5%. Anomalous seismic reflections that transgress sedimentary structures and parallel the seafloor, suggested the presence of gas hydrates at both sites, and, during coring, small samples of gas hydrate were recovered at subbottom depths of 238m (Site 533) and 404 m (Site 568). The principal gaseous components of the gas hydrates wer methane, ethane, and CO₂. Residual methane in sediments at both sites usually exceeded 10 mll^?1 of wet sediment. Carbon isotopic compositions of methane, CO₂, and ΣCO₂ followed parallel trends with depth, suggesting that methane formed mainly as a result of biological reduction of oxidized carbon. Salinity of pore waters decreased with depth, a likely result of gas hydrate formation. These geochemical characteristics define some of the conditions associated with the occurrence of gas hydrates formed by in situ processes in continental margin sediments.     

Gas-hydrate stability thickness map along the Indian continental margin

The gas-hydrate stability thickness (GHST) map along the Indian continental margin is prepared from available bathymetry, sea-bottom temperature and geothermal gradient data. The bottom-simulating reflector (BSR) often marks the base of gas-hydrate stability zone. The prior information about the stability thickness in a particular area will help in identifying BSR on seismic data. The map is also useful to the exploration scientists to set a depth window within which proxies for gas-hydrate can be looked into. A GHST map was initially prepared in 1998 based on the-then available data. A lot of new data has been generated by various organizations under the Indian National Gas Hydrate Programs for the advancement of exploration and exploitation activities. By incorporating the new data from the published and available documents, we have modified the GHST map along the Indian margin. Besides filling the data gap, the new map shows the gas-hydrate stability zone in the Andaman offshore. In addition, we show maps of sea-bottom temperature, sediment thickness, geothermal gradient and heat flow to provide a bird’s eye view of these parameters along the continental margin of India.     

Seismic stratigraphy of the central Scotian Rise: A record of continental margin glaciation

3.5-k Hz profiles from low channel levees on the Scotian Rise show transparent Holocene acoustic facies overlying stratified glacial facies, dated by Carbon-14 in cores. Corresponding acoustic facies in seismic records are correlated with glaciations by counting back from the present sea floor using sedimentation rates from Carbon-14 dating and biostratigraphy from wells as a guide. The regional thickness and character of the seismic units correlate with the duration and intensity of glacial periods inferred from the global isotopic record. Changes in glacial supply to the continental margin are interpreted using this chronology, which shows stage 12 as the first widespread erosional glacial event.     

The bathymetry of the North Victoria Land continental margin

Abstract

The very unique continental margin of North Victoria Land, Antarctica, is characterized by complex bathymetry, reflecting control by glacial, tectonic, and marine processes. The abnormally shallow shelf can be divided into a deep, rugged, glacially dominated inner shelf and a smoother, shallower outer shelf, which is dominated by marine and glacial marine processes. Deep u‐shaped glacial troughs incise the shelf, while relict v‐shaped canyons incise the upper slope. Trending northwest‐southeast along the eastern edge of the area lies a rugged chain of seamounts representing the southern extension of the Balleny Fracture Zone. The continental slope is dominated by strong contour currents and gravity processes.     

Late Cenozoic sea-level changes and the onset of glaciation: impact on continental slope progradation off eastern Canada

Late Cenozoic sedimentation from four varied sites on the continental slopes off southeastern Canada has been analysed using high-resolution airgun multichannel seismic profiles, supplemented with some single channel data. Biostratigraphic ties are available to exploratory wells at three of the sites. Uniform, slow accumulation of hemipelagic sediments was locally terminated by the late Miocene sea-level lowering, which is also reflected in changes in foraminiferan faunas on the continental shelf. Data are very limited for the early Pliocene but suggest a return to slow hemipelagic sedimentation. At the beginning of the late Pliocene, there was a change in sedimentation style marked by a several-fold increase in accumulation rates and cutting of slope valleys. This late Pliocene cutting of slope valleys corresponds to the onset of late Cenozoic growth of the Laurentian Fan and the initiation of turbidite sedimentation on the Sohm Abyssal Plain. Although it corresponds to a time of sea-level lowering, the contrast with the late Miocene lowstand indicates that there must also have been a change in sediment delivery to the coastline, perhaps as a result of increased rainfall or development of valley glaciers. High sedimentation rates continued into the early Pleistocene, but the extent of slope dissection by gullies increased. Gully-cutting episodes alternated with sediment-draping episodes. Throughout the southeastern Canadian continental margin, there was a change in sedimentation style in the middle Pleistocene that resulted from extensive ice sheets crossing the continental shelf and delivering coarse sediment directly to the continental slope.