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Abstract

Multibeam bathymetric imagery and acoustic sub-bottom profiles are used to reveal distribution patterns of sub-surface sedimentation in Dove Basin (Scotia Sea). The goals of the study are to determine the imprint of the inflow of deep Antarctic water masses from the Weddell Sea into the Scotia Sea, to establish the factors driving the styles of contourite deposition and to discern the relative contribution of alongslope versus downslope processes to the construction of the uppermost late Quaternary sedimentary record in the basin. The most significant morpho-sedimentary features in Dove Basin are linked to contouritic processes and to mass movements. Plastered drifts on the flanks of the basin constitute the most common contouritic deposits. Basement-controlled drifts on top of structural elevations are common along the central ridge, the central basin plain and scattered along the basin flanks. Sheeted drifts occur on top of adjacent banks or are restricted to the deep basin. In contrast, mounded drifts are poorly represented in Dove basin. A laterally extensive contouritic channel runs along the central ridge. Contouritic channels are also identified in the upper parts of the lateral banks and slopes. Numerous slide scars along the upper parts of the slopes evolve downslope into semitransparent lens-shaped bodies, with occasional development of across-slope channels. Semitransparent lenses occur intercalated within stratified deposits in the slopes of the basin, in the central ridge and in the deepest abyssal plain. The spatial arrangement of contouritic morphologies points to the influence of the water column structure and the basin physiography. In the eastern sub-basin, two different fractions (lower and upper) of Weddell Sea Deep Water (WSDW) leave an imprint on contourite deposits owing to the sloping interface between the two fractions. Contouritic influence is more subdued in the western sub-basin, and limited to the imprint of the lower WSDW. The upper parts of the surrounding banks are under the influence of deep-reaching Circumpolar waters (i.e., Lower Circumpolar Deep Water), which develops both depositional and erosional morphologies. The cross-section V-shaped morphology of the basin and the common occurrence of structural highs drive the predominance of plastered and basement-controlled drifts in the sediment record. The frequent alternation between contourites and downslope gravity-flow deposits is likely due to different processes associated with over-steepening in the basin, such as basement-controlled steep slopes, deformed drifts atop basement elevations, and the development of thick contouritic piles. Dove Basin is an example of a basin without mounded, plastered or mixed hybrid drifts in the transition between the lower slope and the deep basin, because the upper boundary of the deepest water mass —the Weddell Sea Deep Water— flows shallower along the middle slope. This fact underlines the relevance of the position and depth of water masses in shaping the morphology of the feet of slopes along continental margins.

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... Bottom currents influence the distribution, composition and physical properties of contourites, and these in turn influence submarine slope stability (Bryn et al., 2005;Laberg et al., 2005;Laberg and Camerlenghi, 2008;Rebesco et al., 2014;Neves et al., 2016;Miramontes et al., 2018;Teixeira et al., 2019;Lobo et al., 2021;Rodrigues et al., 2021b). Gravitational stability depends on spatial and temporal variations in the clastic sedimentary processes of erosion, transport and deposition. ...
Chapter
Along-slope bottom currents and a series of secondary oceanographic processes interact at different scales to form sedimentary deposits referred to as contourite and mixed (turbidite-contourite) depositional systems. The recent proliferation of both academic and industry research on deep-marine sedimentation documents significant advances in the understanding of these systems, but most nonspecialists remain unaware of the features in question and how they form. Contourites and mixed depositional systems represent a major domain of continental margin and adjacent abyssal plain sedimentation in many of the world’s oceans. They also appear in Paleozoic, Mesozoic and Cenozoic stratigraphic sections. The growing interest in these systems has led to a refined but still evolving understanding of them. In addition to resolving their exact origins and evolutionary trajectories, research must also continue to ascertain their role in deep-sea ecosystems, geological hazards, environmental policy and economic development. Key gaps in understanding persist regarding their formation, their function in oceanographic systems and their evolution over time. This chapter summarizes current conceptual paradigms for contourite and mixed depositional systems, lists global geographic examples of these systems and discusses their identification and interpretation in terms of diagnostic features as they appear in 2D and 3D seismic datasets and at sedimentary facies scale. This chapter also considers the role that bottom currents play in shaping the seafloor and controlling the sedimentary stacking patterns of deepwater sedimentary successions. The growing interest in, and implications of, contourite and mixed depositional systems demonstrates that these systems represent significant deep-marine sedimentary environments. Combined efforts of researchers, industry partners and policy-makers can help advance understanding and responsible stewardship of deepwater depositional systems.
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Ona Basin, the westernmost oceanic basin in the southern Scotia Sea, is affected by the opposite flows of Circumpolar Deep Water (CDW) and Weddell Sea Deep Water (WSDW); thus, it represents a key location for exploring seafloor morphologies influenced by bottom currents. The present study aims to capture the spatial arrangement of recent subsurface contourite features, assuming a latitudinal influence of water masses and the interactions between along- and downslope processes, in order to contribute to the knowledge of regional deepwater flow pathways and to the sedimentary model of small sediment-starved oceanic basins. To this end, the investigation combines an interpretation of multibeam bathymetry and parametric echo sounder seismic data complemented with hydrological data. The distribution of morpho-sedimentary features in Ona Basin reveals two major domains. The southern margin of the basin can be regarded as a mixed/hybrid system containing abundant sediment drifts with channels and contourite moats and a lateral continuity interrupted by downslope morphologies. In contrast, the northern abyssal setting comprises relatively homogeneous large sheeted drifts with superimposed sediment waves, mounded drifts, and several scattered erosive features, likely reflecting the more distinct influence of deepwater contourite processes. Our work demonstrates that tectonic features in the southern basin control the interaction between deepwater along- and downslope processes, as the westward flow of the WSDW is deflected, channelized, and intensified along its westward route. In the northern region, the study indicates an overall clockwise rotation of the WSDW flow, with the spatial and vertical variability of CDW and WSDW affecting the distribution of bottom-current features around seamounts and/or structural highs. The results underscore the importance of sloping interphases in the water mass vertical structure, the degree of basin confinement, and the influence of local bathymetric elevations in sedimentation models of small sediment-starved oceanic basins.
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The aim of the present study was to characterise the morpho-sedimentary features and main stratigraphic stacking pattern off the Tierra del Fuego continental margin, the north-western sector of the Scotia Sea abyssal plain (Yaghan Basin) and the Malvinas/Falkland depression, based on single- and multi-channel seismic profiles. Distinct contourite features were identified within the sedimentary record from the Middle Miocene onwards. Each major drift developed in a water depth range coincident with a particular water mass, contourite terraces on top of some of these drifts being associated with interfaces between water masses. Two major palaeoceanographic changes were identified. One took place in the Middle Miocene with the onset of Antarctic Intermediate Water flow and the enhancement of Circumpolar Deep Water (CDW) flow, coevally with the onset of Weddell Sea Deep Water flow in the Scotia Sea. Another palaeoceanographic change occurred on the abyssal plain of the Yaghan Basin in the Late Miocene as a consequence of the onset of Southeast Pacific Deep Water flow and its complex interaction with the lower branch of the CDW. Interestingly, these two periods of change in bottom currents are coincident with regional tectonic episodes, as well as climate and Antarctic ice sheet oscillations. The results convincingly demonstrate that the identification of contourite features on the present-day seafloor and within the sedimentary record is the key for decoding the circulation of water masses in the past. Nevertheless, further detailed studies, especially the recovery of drill cores, are necessary to establish a more robust chronology of the evolutionary stages at the transition between the western Scotia Sea and the southern South Atlantic Ocean.
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Ona Basin is a small intra-oceanic basin located in the southwestern corner of the Scotia Sea. This region is crucial for an understanding of the early phases of opening of Drake Passage, since it may contain the oldest oceanic crust of the entire western Scotia Sea, where conflicting age differences from Eocene to Oligocene have been proposed to date. The precise timing of the gateway opening between the Pacific and Atlantic oceans, moreover, has significant paleoceanographic and global implications. Two sub-basins are identified in this region, the eastern and western Ona basins, separated by the submarine relief of the Ona High. A dense geophysical data set collected during the last two decades is analyzed here. The data include multichannel seismic reflection profiles, and magnetic and gravimetric data.
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The contourite paradigm was conceived a few decades ago, yet there remains a need to establish a sound connection between contourite deposits, basin evolution and oceanographic processes. Significant recent advances have been enabled by various factors, including the establishment of two IGCP projects and the realisation of several IODP expeditions. Contourites were first described in the Northern and Southern Atlantic Ocean, and since then, have been discovered in every major ocean basin and even in lakes. The 120 major contourite areas presently known are associated to myriad oceanographic processes in surface, intermediate and deep-water masses. The increasing recognition of these deposits is influencing palaeoclimatology & palaeoceanography, slope-stability/geological hazard assessment, and hydrocarbon exploration. Nevertheless, there is a pressing need for a better understanding of the sedimentological and oceanographic processes governing contourites, which involve dense bottom currents, tides, eddies, deep-sea storms, internal waves and tsunamis. Furthermore, in light of the latest knowledge on oceanographic processes and other governing factors (e.g. sediment supply and sea-level), existing facies models must now be revised. Persistent oceanographic processes significantly affect the seafloor, resulting in large-scale depositional and erosional features. Various classifications have been proposed to subdivide a continuous spectrum of partly overlapping features. Although much progress has been made in the large-scale, geophysically based recognition of these deposits, there remains a lack of unambiguous and commonly accepted diagnostic criteria for deciphering the small-scaled contourite facies and for distinguishing them from turbidite ones. Similarly, the study of sandy deposits generated or affected by bottom currents, which is still in its infancy, offers great research potential: these deposits might prove invaluable as future reservoir targets. Expectations for the forthcoming analysis of data from the IODP expedition 339 are high, as this work promises to tackle much of the aforementioned lack of knowledge. In the near future, geologists, oceanographers and benthic biologists will have to work in concert to achieve synergy in contourite research to demonstrate the importance of bottom currents in continental margin sedimentation and evolution.
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Geochemical characteristics of marine sediment from the southern Drake Passage were analyzed to reconstruct variations in sediment provenance and transport paths during the late Quaternary. The 5.95 m gravity core used in this study records paleoenvironmental changes during the last approximately 600 ka. Down-core variations in trace element, rare earth element, and Nd and Sr isotopic compositions reveal that sediment provenance varied according to glacial cycles. During glacial periods, detrital sediments in the southern Drake Passage were mostly derived from the nearby South Shetland Islands and shelf sediments. In contrast, interglacial sediments are composed of mixed sediments, derived from both West Antarctica and East Antarctica. The East Antarctic provenance of the interglacial sediments was inferred to be the Weddell Sea region. Sediment input from the Weddell Sea was reduced during glacial periods by extensive ice sheets and weakened current from the Weddell Sea. Sediment supply from the Weddell Sea increased during interglacial periods, especially those with higher warmth such as MIS 5, 9, and 11. This suggests that the influence of deep water from the Weddell Sea increases during interglacial periods and decreases during glacial periods, with the degree of influence increasing as interglacial intensity increases.
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The tectonic evolution of the transition zone from the Pacific Ocean to the Atlantic Ocean is closely linked with the destruction of the American–Antarctic continental bridge in the Scotia Sea. The western segment of the bridge combines the Terror, Pirie, and Bruce banks, as well as the Protector and Dove basins between them. Modeling—primarily based on original geological and geophysical materials—of linear magnetic anomalies and calculation of the floor kinematics in these basins have made it possible for the first time to reveal that the collapse of the western segment of the American–Antarctic continental bridge occurred 18–25 Ma ago via a two-stage separation of the Pirie Rise from the Bruce Rise with the formation of the Dove Basin and a two stage separation of the Terror Rise from the Pirie Rise with the formation of the Protector Basin.
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Extensive 3D seismic datasets acquired during exploration offshore southern Tanzania have revealed the complex architecture of two contrasting styles of hybrid turbidite-contourite deposits that formed in the Cretaceous (Albian-Early Campanian) and Paleogene (Paleocene-Oligocene). Both sequences are characterized by migrating channel-levee complexes, interpreted to record, and be diagnostic of, the synchronous interaction of eastward, downslope flowing turbidity currents and northerly, along-slope flowing contour currents. Flow stripping of the fine-grained suspended part of the turbulent flow by weak contour currents led to the formation of expanded levee-drifts on the northern (downstream) side of the channels, which prograded southwards (upstream), driving southwards migration of the turbidite channel axis. The difference in the architecture of the two successions is due to the variation in slope topography at the time turbidite activity commenced and the frequency of coarse clastic input into the basin. Cretaceous (Albian-Campanian) turbidite systems were strongly controlled by the position of pre-existing contourite drifts and moats. The contorted geometry of the system provided loci for the deposition of Cretaceous reservoirs comprising thick, amalgamated channel deposits with a high net-togross ratio (N:G) and good vertical connectivity, and intra-slope fans with lower N:G and poor vertical connectivity. Paleogene turbidite channels initially evolved on a smooth slope. Sustained southward channel migration produced Paleogene reservoirs comprising complexly laterally-connected sheets of channel and lobe deposits above a southwardyounging, diachronous compound unconformity. In both hybrid systems, contour current influence modified the geometry of the turbidite systems, resulting in temporal and spatial partitioning of the depocentres on the slope.
Article
On the basis of 2D multichannel and very-high-resolution seismic data and swath bathymetry, we report a sequence of giant mass-transport deposits (MTDs) in the Scan Basin (southern Scotia Sea, Antarctica). MTDs with a maximum thickness of c. 300 m extend up to 50 km from the Discovery and Bruce banks towards the Scan Basin. The headwall area consists of multiple U-shaped scars intercalated between volcanic edifices, up to 250 m high and 7 km wide, extending c. 14 km downslope from 1750 to 2900 m water depth. Seismic sections show that these giant MTDs are triggered by the intersection between diagenetic fronts related to silica transformation and vertical fluid-flow pipes linked to magmatic sills emplaced within the sedimentary sequence of the Scan Basin. This work supports that the diagenetic alteration of siliceous sediments is a possible cause of slope instability along world continental margins where bottom-simulating reflectors related to silica diagenesis are present at a regional scale.
Article
High resolution reflection seismic data, subbottom profiler and side scan sonar information, together with results from sediment core studies, have been used to study various glacially influenced morphodynamic features on the northern Faroese continental margin. On the shelf and upper slope a thick, buried turbate complex has been found, which we estimate to have been formed between ca 470,000 and 120,000 years BP. We interpret this turbate to be the result of episodic, extensive iceberg grounding during extreme glaciation within the period MIS 12–MIS 6 (Elster-Saale complex). Late MIS 6 is found to be the most likely age of the last episode of turbate formation as suggested by age/depth correlation with dated sediment cores from the nearby area. The origin of the iceberg turbate may be attributed to deglacial drift of deep-draft icebergs from icestream sources in the Arctic, East Greenland and/or northern Iceland. At the base of slope and adjacent deep-water area a complex of mud diapir features has been observed. Our data show that activation of these features are the result of density inversion processes caused by fine-grained, unconsolidated hemipelagic deposits being overlain by dense, glacigenic North Sea Fan (NSF) deposits. Due to sudden and fast accumulation of the fan sediments, a normal and regular compaction and dewatering of the underlying hemipelagic sediments could not occur; instead, sub-seabed sediment mobilisation took place resulting in enhanced diapir formation. A general intensification of the diapiric processes is likely related to the MIS 6 (Saalian) glacial period, as NSF-derived glacigenic debris flows reached the northern outlet of the Faroe-Shetland Channel. The most recent diapir activity is concluded to have occurred during the last (Weichselian) glaciation.
Article
The tectonic history of a margin dictates its general shape; however, its geomorphology is generally transformed by deep-sea sedimentary processes. The objective of this study is to show the influences of turbidity currents, contour currents and sediment mass failures on the geomorphology of the deep-water northwestern Atlantic margin (NWAM) between Blake Ridge and Hudson Trough, spanning about 32° of latitude and the shelf edge to the abyssal plain. This assessment is based on new multibeam echosounder data, global bathymetric models and sub-surface geophysical information. The deep-water NWAM is divided into four broad geomorphologic classifications based on their bathymetric shape: graded, above-grade, stepped and out-of-grade. These shapes were created as a function of the balance between sediment accumulation and removal that in turn were related to sedimentary processes and slope-accommodation. This descriptive method of classifying continental margins, while being non-interpretative, is more informative than the conventional continental shelf, slope and rise classification, and better facilitates interpretation concerning dominant sedimentary processes. Areas of the margin dominated by turbidity currents and slope by-pass developed graded slopes. If sediments did not by-pass the slope due to accommodation then an above grade or stepped slope resulted. Geostrophic currents created sedimentary bodies of a variety of forms and positions along the NWAM. Detached drifts form linear, above-grade slopes along their crests from the shelf edge to the deep basin. Plastered drifts formed stepped slope profiles. Sediment mass failure has had a variety of consequences on the margin morphology; large mass-failures created out-of-grade profiles, whereas smaller mass failures tended to remain on the slope and formed above-grade profiles at trough-mouth fans, or nearly graded profiles, such as offshore Cape Fear.
Conference Paper
The volume, area affected, and runout of submarine landslides can exceed those of terrestrial events by two orders of magnitude. The Storegga Slide off Norway affected an area the size of Scotland and moved enough sediment to bury the entire country to a depth of 80 m. Modern geophysics provides a clear picture of large landslides and what their source and depositional areas look like. From this, we can deduce the processes that operated during downslope transport. However, our understanding of many aspects of landslide processes is based on hypotheses that are difficult to test. Elevated pore pressures are essential for landslide initiation on low continental margin slopes, yet understanding of how high pressures are generated or how fluid migration affects slope stability is limited. Sediments may be pre-conditioned for failure by the processes that originally deposited them, e.g., through creation of weak layers, but the processes and parameters that might control this are largely unknown.
Article
Most of the West Antarctic continental margin has prograded during Neogene and Quaternary times, due largely to sediment delivery to the shelf break by ice sheets (Larter & Cunningham 1993; Nitsche et al. 2000; Cooper et al. 2008). Continental slope progradation is widely attributed to debris-flow deposition, but geophysical data that show the morphology of individual debris-flow deposits are rare. Morphologically, the continental slope can be divided into low seafloor gradient (<3°) trough-mouth fans (TMFs), developed at the mouths of some large palaeo-ice streams, and inter-fan areas with steeper slopes. We describe acoustic sub-bottom profiles and a sediment core from debris-flow deposits on Belgica Fan (Dowdeswell et al. 2008) and a sub-bottom profile of a debris-flow deposit on an inter-fan area on the Amundsen Sea continental slope (Fig. 1a).
Chapter
Generally, propagation of Antarctic waters in the bottom layer of the Atlantic Ocean is confined to depressions in the bottom topography. The general flow of these waters can be presented as follows (Fig. 3.1).
Article
The spatial distribution and temporal occurrence of mass transport deposits (MTDs) in the sedimentary infill of basins and submerged banks near the Scotia-Antarctic plate boundary allowed us to decode the evolution of the tectonic activity of the relevant structures in the region from the Oligocene to present-day. The 1020 MTDs identified in the available dataset of multichannel seismic reflection profiles in the region are subdivided according to the geographic and chronological distribution of these features. Their spatial distribution reveals a preferential location along the eastern margins of the eastern basins. This reflects local deformation due to the evolution of the Scotia-Antarctic transcurrent plate boundary and the impact of oceanic spreading along the East Scotia Ridge (ESR). The vertical distribution of the MTDs in the sedimentary record evidences intensified regional tectonic deformation from the middle Miocene to Quaternary. Intensified deformation started at about 15 Ma, when the ESR progressively replaces the West Scotia Ridge (WSR) as the main oceanic spreading center in the Scotia Sea. Coevally with the WSR demise at about 6.5 Ma, increased spreading rates of the ESR and numerous MTDs were formed. The high frequency of MTDs during the Pliocene, mainly along the western basins, is also related to greater tectonic activity due to uplift of the Shackleton Fracture Zone by tectonic inversion and extinction of the Antarctic-Phoenix Ridge and involved changes at Late Pliocene. The presence of MTDs in the southern Scotia Sea basins is a relevant indicator of the interplay between sedimentary instability and regional tectonics.
Article
Dove Basin, a small oceanic domain located within the southern Scotia Sea, evidences a complex tectonic evolution linked to the development of the Scotia Arc. The basin also straddles the junction between the main Southern Ocean water masses: the Antarctic Circumpolar Current (ACC), the Southeast Pacific Deep Water (SPDW) and the Weddell Sea Deep Water (WSDW). Analysis of multichannel seismic reflection profiles, together with swath bathymetry data, reveals the main structure and sediment distribution of the basin, allowing a reconstruction of the tectonostratigraphic evolution of the basin and assessment of the main bottom water flows that influenced its depositional development. Sediment dispersed in the basin was largely influenced by gravity-driven transport from adjacent continental margins, later modified by deep bottom currents. Sediments derived from melting icebergs and extensive ice-sheets also contributed to a fraction of the basin deposits. We identify four stages in the basin evolution which ―based on regional age assumptions― took place during the early Miocene, middle Miocene, late Miocene-early Pliocene, and late Pliocene-Quaternary. The onsets of the ACC flow in Dove Basin during the early Miocene, the WSDW flow during the middle Miocene, and the SPDW during the late Miocene were influenced by tectonic events that facilitated the opening of new oceanic gateways in the region. The analysis of Dove Basin reveals that tectonics is a primary factor influencing its sedimentary stacking patterns, the structural development of new oceanic gateways permitting the inception of deep-water flows that have since controlled the sedimentary processes.This article is protected by copyright. All rights reserved.
Article
The sedimentary record in the vicinity of the triple junction at the southern Drake Passage is analyzed in order to decode the palaeoceanographic evolution and the influence of tectonic events. The break-up of the last connection between South America and Antarctica led to the circulation of important oceanographic bottom flows, including the Antarctic Circumpolar Current (ACC) and the Weddell Sea Deep Water (WSDW). The Shackleton Fracture Zone (SFZ), a ridge crossing the central Drake Passage, has been proposed as a major barrier that constrained the free circulation of bottom flows in the area, but whose timing and importance is poorly established. Also, the South Scotia Ridge (SSR), a prominent relief composing the southern part of the Scotia Arc, has controlled oceanographic exchanges between the Weddell and Scotia seas, as bottom flows from the Weddell Sea to the Scotia Sea have been conducted across narrow gateways along the SSR. On the basis of a network of multichannel seismic profiles, we interpret the uplift dynamics of the SFZ in the southern Drake Passage and its influence on the evolution of the bottom-current circulation and by extension on contourite processes.
Article
The southern margin of the Scotia Sea hosts the convergent boundary between the Scotia and Antarctic plates where a number of small basins are sitated. Mass transport deposits (MTDs) within two of these small basins, Dove and Scan basins, reveal the importance of seismicity, slope instabilities and depositional processes in their growth patterns. Swath-bathymetry and very high-resolution seismic data show that there are over 200 MTDs in these basins in the last 100 ky record. MTD characterizations are determined on the basis of their regional distribution, shape, apparent size and depth. Their sedimentary and tectonic implications are discussed, as well as the evidence of different triggering mechanisms in this region, which is characterized at present by moderate-to-high magnitude, shallow to intermediate earthquakes. MTDs are more abundant in Dove Basin (with lenticular and wedge shapes), suggesting that this basin was affected by active tectonics to a greater degree than Scan Basin. This finding is significant in the overall evolutionary context of the Scotia Sea region and Scotia-Antarctic plate geodynamics. Nevertheless, other factors —volcanic activity, vigorous bottom-currents, and/or higher sedimentation rates — must also be considered for the generation of MTDs in the Scan Basin, where a variety of processes generated more diverse MTD morphologies. Paleoseismological estimations of the repeated occurrence of wedge shaped MTDs in contact with fault scarps point to potential sources of large magnitude (Mw ~ 7.2-7.3) paleoearthquakes in several sites, in agreement with the present high magnitudes of regional seismicity. This study shows MTDs to be appropriate as paleoearthquake indicators in active tectonic settings. The distribution of MTDs in the southern Scotia Sea has important implications for geodynamic and geohazard research. They may prove to be unmistakable stratigraphic markers for future basin analysis.
Article
The sedimentary construction of oceanic margins is most often carried out by the combined action of gravitational processes and processes related to bottom (contour) currents. One of the major difficulties encountered in the interpretation of seismic profiles crossing such margins is the differentiation of these two types of deposit, especially where they display very complicated imbricated geometries. The aim of this paper, therefore, is to derive criteria for the recognition of contourite vs. turbidite deposits, based on the analysis of many seismic profiles from both published and unpublished sources. The following features are the most diagnostic for the recognition of contourite drifts. At the scale of the basin, four different drift types can be distinguished according to the morphostructural context, their general morphology and the hydrodynamic conditions. These are: contourite-sheeted drifts (including abyssal sheets and slope-plastered sheets), elongate-mounded drifts (detached and separated types), channel-related drifts (including lateral and axial patch drifts and downstream contourite fans), and confined drifts trapped in small, tectonically active basins. At the scale of the drift, three features provide the best diagnostic criteria for recognising contourite deposits on seismic profiles: major discontinuities that can be traced across the whole drift and represent time lines corresponding to hydrological events, lenticular, convex-upward depositional units with a variable geometry, and a specific style of progradation–aggradation of these units that is influenced by interaction of the bottom current with Coriolis force and with the morphology. At the scale of depositional units, the seismofacies show a wide variety of reflector characteristics, many of which are very similar to those observed in turbidite series. Distinction between sediment wave seismofacies deposited by turbidity currents and bottom currents still remains ambiguous.
Article
The horizontal and vertical circulation of the Weddell Gyre is diagnosed using a box inverse model constructed with recent hydrographic sections and including mobile sea ice and eddy transports. The gyre is found to convey 42 ± 8 Sv (1 Sv = 106 m3 s-1) across the central Weddell Sea and to intensify to 54±15 Sv further offshore. This circulation injects 36±13 TW of heat from the Antarctic Circumpolar Current to the gyre, and exports 51 ± 23 mSv of freshwater, including 13 ± 1 mSv as sea ice to the mid-latitude Southern Ocean. The gyre's overturning circulation has an asymmetric double-cell structure, in which 13 ± 4 Sv of Circumpolar Deep Water (CDW) and relatively light Antarctic Bottom Water (AABW) are transformed into upper-ocean water masses by mid-gyre upwelling (at a rate of 2 ± 2 Sv) and into denser AABW by downwelling focussed at the western boundary (8 ± 2 Sv). The gyre circulation exhibits a substantial throughflow component, by which CDW and AABW enter the gyre from the Indian sector, undergo ventilation and densification within the gyre, and are exported to the South Atlantic across the gyre's northern rim. The relatively modest net production of AABW in the Weddell Gyre (6±2 Sv) suggests that the gyre's prominence in the closure of the lower limb of global oceanic overturning stems largely from the recycling and equatorward export of Indian-sourced AABW.
Article
Seabed data acquired from the southern Barents Sea continental margin offshore Norway reveal detailed morphology of large sandwave fields. Multibeam echosounder bathymetry and backscatter, shallow seismic, sediment samples and seabed video data collected by the MAREANO program have been used to describe and interpret the morphology, distribution and transport of the sandwaves. The bedforms lie on a slope dominated by relict glacial forms and muddy/sandy/gravelly sediments. Sandwave migration across small gravity mass failures of the glacial mud constrains the field initiation as early post glacial or later. The contour-parallel nature of the fields and crestlines normal to the bathymetry contours and the geostrophic Norwegian Atlantic Current (NwAC) demonstrate that the NNW-flowing oceanographic circulation is the primary driving current. The fields coincide with the depth range at which a transition between warm, saline and underlying cooler, less saline waters fluctuate across the seabed. Statistically rigorous measurements of height, width and various parameters of slope and symmetry confirm a tendency to downstream (NNW) sandwave migration but with significant exceptions. Anomalous bedform symmetry domains within the fields are tuned to meso-scale topography along (relict) glacial debris flow chutes, indicating current focusing. Upstream and upper slope-derived winnowed sand transport eroded from the glacial sediments is the supposed source. Sandwave flank slope values are comparable to the regional slope such that the gravitational vector would have a cumulative downslope migration affect unless balanced by upslope drivers. Perpendicular cross-cutting of stoss face 3-D ripples by linear (2-D) ripples in the sandwave troughs and lee faces is evidence for non-synchronous, episodic current variations. Though deep Ekman transport and internal wave action are unproven here, these could explain chute-related tuning of bedform symmetry through funneling in the debris flow chutes and favor sand recycling, thus contributing to long-term maintenance of the sandwave field.
Article
Results are reported from seven heat flow stations in small basins of the southern part of the central Scotia Sea (CSS), undertaken in order to determine basement ages. The basins are small, which makes magnetic anomaly-based ages ambiguous and preserves basin subsidence that may have been anomalous as a result of local factors. The fact that these small basins formed in a back-arc setting adds additional uncertainty to depth-based age estimates. The results confirm that basin extension commenced in the Eocene, and indirectly support a relatively young, backarc origin for the northern CSS, but do not affect previously published suggestions of the age of onset of the Antarctic Circumpolar Current.
Article
Swath-bathymetry data and 2D multichannel seismics reveal for the first time an up to ~60 km wide amphitheater-shaped slide scar on the eastern flank of the Jan Mayen Ridge, a micro-continent in the Norwegian–Greenland Sea. The scar opens southeastward where it continues as a narrower, topographically controlled translational area. It includes secondary scars, as well as channels and escarpments. Based on the identification of secondary scars, the slide is classified as a slide complex and the total volume of missing sediments was estimated at ~60 km3. From the overall shape of the scar, the upslope widening from a bottleneck- or channel-like bypass-area, the failure is inferred to have had a retrogressive development. The absence of ridges, slabs and sediment blocks indicates that the failed sediments have been evacuated entirely. The smaller channels were formed from single or repetitive smaller flows post-dating the large failure events.
Article
A minimum-complexity tectonic reconstruction, based on published and new basin opening models, depicts how the Scotia Sea grew by Cenozoic plate divergence, dismembering a Jurassic sheared margin of Gondwana. Part of the Jurassic–early Cretaceous ocean that accreted to this margin forms the core of the Central Scotia Plate, the arc plate above a trench at the eastern end of the Scotia Sea, which migrated east away from the Antarctic and South American plates. A sequence of extensional basins opened on the western edge of the Central Scotia Plate at 50–30 Ma, decoupled from the South American Plate to the northwest by slow motion on a long transform fault. Succeeding the basins, seafloor spreading started around 30 Ma on the West Scotia Ridge, which propagated northwards in the 23–17 Ma period and ceased to operate at 6 Ma. The circuits of plate motions inside and outside the Scotia Arc are joined via rotations that describe Antarctic–Central Scotia plate motion in Powell Basin until 20 Ma, and along the South Scotia Ridge thereafter. The modelled relative motion at the northern edge of the Scotia Sea is thus constrained only by the plate circuit, but nonetheless resembles that known coarsely from the geological record of Tierra del Fuego. A paleobathymetric interpretation of nine time slices in the model shows Drake Passage developing as an intermediate-depth oceanographic gateway at 50–30 Ma, with deep flow possible afterwards. Initially, this deep flow would have been made tortuous by numerous intermediate and shallow barriers. A frontal pattern resembling that in the modern Scotia Sea would have awaited the clearance of significant barriers by continuing seafloor spreading in the Scotia Sea at ~ 18.5 Ma, at Shag Rocks Passage, and after 10 Ma southeast of South Georgia.
Article
Bottom currents and their margin-shaping character became a central aspect in the research field of sediment dynamics and paleoceanography during the last decades due to their potential to form large contourite depositional systems (CDS), consisting of both erosive and depositional features.A major CDS at the northern Argentine continental margin was studied off the Rio de la Plata River by means of seismo- and hydro-acoustic methods including conventional and high-resolution seismic, parametric echosounder and single and swath bathymetry. Additionally, hydrographic data were considered allowing jointly interpretation of morphosedimentary features and the oceanographic framework, which is dominated by the presence of the dynamic and highly variable Brazil–Malvinas Confluence.We focus on three regional contouritic terraces identified on the slope in the vicinity of the Mar del Plata Canyon. The shallowest one, the La Plata Terrace (∼500 m), is located at the Brazil Current/Antarctic Intermediate Water interface characterized by its deep and distinct thermocline. In ∼1200 m water depth the Ewing Terrace correlates with the Antarctic Intermediate Water/Upper Circumpolar Deep Water interface. At the foot of the slope in ∼3500 m the Necochea Terrace marks the transition between Lower Circumpolar Deep Water and Antarctic Bottom Water during glacial times.Based on these correlations, a comprehensive conceptual model is proposed, in which the onset and evolution of contourite terraces is controlled by short- and long-term variations of water mass interfaces. We suggest that the terrace genesis is strongly connected to the turbulent current pattern typical for water mass interfaces. Furthermore, the erosive processes necessary for terrace formation are probably enhanced due to internal waves, which are generated along strong density gradients typical for water mass interfaces. The terraces widen through time due to locally focused, partly helical currents along the steep landward slopes and more tabular conditions seaward along the terrace surface.Considering this scheme of contourite terrace development, lateral variations of the morphosedimentary features off northern Argentina can be used to derive the evolution of the Brazil–Malvinas Confluence on geological time scales. We propose that the Brazil–Malvinas Confluence in modern times is located close to its southernmost position in the Quaternary, while its center was shifted northward during cold periods.
Article
Turbidity currents are energetic unsteady processes with velocities usually ranging from a few decimetres, toa few metres or a few tens of metres per second. This chapter illustrates the low-frequency contourite and turbidite alternations; high-frequency contourite and turbidite alternations; redistribution of gravity deposits by contour currents; and interaction of synchronous contour and turbidity currents. The most common process, the short-duration turbulent surge, is one of shortly waxing and then waning. Other turbulent flows such as long-duration turbidity currents sustained by retrogressive failures are less unsteady. Long-duration hyperpycnal turbidity currents generated at river mouths during floods are considered as quasi-steady flows. Contourite and turbidite alternation involves: (1) low frequency alternation, and (2) high-frequency alternations of contourites and turbidites. Sediment reworking and redistribution by contour-current processes are common in the deep-sea environment. Reworking and redistribution of gravity deposits by contour currents occur when the two processes alternate with time and when they are synchronous. Redistribution of turbiditic sediments by bottom currents are recognized mainly at the sedimentary-series scale, while intercalation between contourites and turbidites occurs at scales from the individual bed to the sedimentary series. True contour or turbidity current interaction occurs over large areas if the energy of both currents is balanced; and at the boundary between two depositional environments if the contour and gravity currents are unbalanced and act separately in adjacent areas.
Article
This chapter reviews the historical context and describes the different contourite facies recognized. The contourite facies models are outlined and the interpretation of contourite sequences is discussed. Some hybrid contourite facies are considered and criteria for the distinction between contourites and associated facies in deep-water systems are focused on. The problems and controversy surrounding the recognition of fossil contourites in the ancient records arealso described. The range of recognized contourite facies, including the facies groups, are tabulated. . Muddy contourites are homogeneous and appear as thick featureless units. Silty contourites have a larger silt-sized component and potential for revealing some internal structure. Gravel-rich contourites and gravel-bearing contourites are common in drifts at high latitudes as a result of input from ice-rafted material. The facies models for both muddy and sandy contourites are originally based on data from many examples of contourites. Composite contourite facies model showing grain-size variation through the standard mud-silt-sand contourite sequence, linked to variation in contour-current velocity is diagrammatically represented.
Article
This chapter provides a current state-of-the-art on the contourite systems with a major focus on the modern systems (drifts), the processes of deposition and the diagnostic sedimentological and seismic features. A short overview of the ancient contourite problem is also provided.Firstly, the history of contourites is briefly reminded since the pionner works during the sixties. The major stages in the advances of the knowledge of these deposits are underlined, and a consensus view is proposed on the definitions of contourites (“deep or shallow contourites”) and drifts, and on the large diversity of bottom currents that can be involved in contourite deposition.The second part of the chapter is then dedicated to the Ocean geostrophic circulation and the main characteristics of the surficial and the thermohaline circulation. It drives to the contour current definition and physical features. The sedimentary processes (erosion, transport and deposition) related to contour currents form the third part with an emphasis on the processes occurring in the nepheloïd layer and in the Benthic Boundary Layer as demonstrated by the results of the HEBBLE project.The two following parts concern the contourite deposits: facies, sequences, bedforms and geometry within drift sedimentary body. Examples of detailed studies of contourite core using combined physical methods are necessary to identify this deposit. Similarly a multi-scale approach is necessary to recognize drift construction from seismic lines only. Some sedimentological and seimic diagnostic features are proposed to identify Contourites.The last part point out how difficult is the recognition of ancient contourite. The lack of recent progress is due in part to diagenesis, outcrop discontinuities and tectonic deformation. Two types of ancient contourites, the bottom-current-reworked turbidites and the shallow-water ancient contourites, are presented in more details in order to highlight the debate that still surrounds their recognition and interpretation.
Article
The analyses of high-resolution seismic data, swath bathymetry data and sediment cores from the basin Svensksunddjupet in Isfjorden, Spitsbergen, reveal the deposits of repeated slope failures. Most of the slope failures occurred during three periods at c. 9650 cal. BP, 8350—8200 cal. BP and 3000 cal. BP. Suggested factors causing instability of the slopes include earthquake activity resulting from rapid isostatic uplift, excess pore pressure caused by relative sea-level fall and/or the presence of gas as well as climatic changes. Mass-transport deposits at the mouth of an incision at the southern slope of Svensksunddjupet are suggested to mainly relate to runoff from the land-based glacier Vardebreen. The increase of the mass-transport activity may indicate that Vardebreen either formed or increased in size at c. 7000 cal. BP.
Article
Turbidite, contourite and hemipelagic deposition are the main components of Wilkes Land continental rise sedimentation above the regional unconformity WL2. On the continental shelf, unconformity WL2 marks the start of shelf progradation, which is interpreted to correspond with the onset of glacial conditions in this segment of the east Antarctic margin. Unusually large (i.e. up to 900 m relief and 18 km between levee crests) channel-levee deposits, and high relief (up to 490 m) mounded contourite-style deposits develop above unconformity WLlb. Unconformity WLlb overlies unconformity WL2 and is interpreted to have formed under a fully continental glacial regime where ice streams reached the palaeo-continental shelf edge. Based on an analysis of multichannel seismic profiles and sediment cores, we differentiate three phases in the development of the sedimentary unit between WLlb and the present seafloor. From older to younger these are: Phase 1, dominated by turbidite deposition; Phase 2, dominated by turbidite and contourite deposition with significant mound building; and Phase 3, dominated by turbidite and contourite deposition without active mound building. We hypothesize that building of the mounds during Phase 2 corresponded with times of expansion of the Antarctic ice-sheet when vast amounts of sediment were eroded from the continent and continental shelf. The large amount of unsorted glacial sediment supplied to the outer shelf apparently travelled down the slope canyons and rise channels as turbidity current flows to feed the usually large continental rise channel-levee complexes. The suspended fines of the turbidity flows were then entrained in a palaeo-nepheloid layer and carried by the westward flowing palaeo-contour currents until their deposition in the mounds. During Phase 3, sediment supply to the continental rise, although important in volume and capable of turbidite and contour-current deposition, was insufficient to support further building of the mounds. We believe the decrease in sediment supply to the continental rise from Phase 2 to Phase 3 could be the result of a change on sediment depocentres, with most of the sediment supplied to the margin during Phase 3 being trapped on the continental shelf. We believe that ultimately these changes are related to the stage of glacial evolution of the continent.
Article
The Falkland Trough is a west-east bathymetric deep that separates the Falkland Plateau from the North Scotia Ridge in the western South Atlantic. It lies in the path of Circumpolar Deep Water flowing within the Antarctic Circumpolar Current (ACC), and Weddell Sea Deep Water flowing beneath the ACC east of Shag Rocks passage. Marine geophysical and sediment core data demonstrate the influence of ambient bottom currents on deposition in this area, and reveal two styles of contourite sedimentation: (1) deposition of glauconite-rich sandy contourites in exposed areas of the Falkland Plateau and Falkland Trough, where vigorous ACC bottom currents control sedimentation, and (2) deposition of biogenic sandy contourites, muddy contourites and hemipelagites (western Falkland Trough), and muddy diatom ooze (eastern Falkland Trough), in the form of two elongate sediment drifts, which have developed in the presence of more sluggish bottom currents. The drift sediments contain a depositional record of bottom current flow through the glacial cycle (southern-origin bottom water flow in the east, and probably ACC flow in the west); analyses of core data from the western Falkland Trough suggest a reduction in bottom current strength during the Last Glacial Maximum at present depths of > 2500 m below sea level.
Article
Multichannel seismic reflection data from the Cosmonaut Sea margin of East Antarctica have been interpreted in terms of depositional processes on the continental rise. A major sediment lens is present below the upper continental rise along the entire Cosmonaut Sea margin. The lens probably consists of sediments supplied from the shelf and slope, being constantly reworked by westward flowing bottom currents redepositing the sediments into a large-scale plastered drift deposit prior to the main glacigenic input along the margin. High-relief elongated and sometimes semicircular depositional structures are found on the upper continental rise, stratigraphically above the regional sediment lens, and were mainly deposited by the action of closely spaced turbidity currents. On the lower continental rise, large-scale sediment bodies extend perpendicular to the continental margin and were deposited as a result of down-slope turbidity transport and westward flowing bottom currents. The elongated sediment mounds on the upper and lower continental rise were deposited after initiation of glacigenic input to the slope and rise.