Swath Mapping Research Articles

Identification and Analysis of Airborne Laser Swath Mapping Data in a Novel Feature Space

Several novel features are examined to determine their effectiveness in separating buildings from trees in airborne laser swath mapping (ALSM) data. New one- and two-dimensional distance measures are created to quantify the separability of the classes using the different features. Several features involving the intensity of the laser returns were found to be highly effective at separating the classes. The new distance measure provides insight into what makes a good/bad feature when discriminating between classes. It also lays the groundwork for future classification of ALSM data by providing a systematic method of ranking features to be used for classification.

Active shortening of the Cascadia forearc and implications for seismic hazards of the Puget Lowland

Margin‐parallel shortening of the Cascadia forearc is a consequence of oblique subduction of the Juan de Fuca plate beneath North America. Strike‐slip, thrust, and oblique crustal faults beneath the densely populated Puget Lowland accommodate much of this north‐south compression, resulting in large crustal earthquakes. To better understand this forearc deformation and improve earthquake hazard assessment, we here use seismic reflection surveys, coastal exposures of Pleistocene strata, potential‐field data, and airborne laser swath mapping to document and interpret a significant structural boundary near the City of Tacoma. This boundary is a complex structural zone characterized by two distinct segments. The northwest trending, eastern segment, extending from Tacoma to Carr Inlet, is formed by the broad (∼11.5 km), southwest dipping (∼11°–20°) Rosedale monocline. This monocline raises Crescent Formation basement about 2.5 km, resulting in a moderate gravity gradient. We interpret the Rosedale monocline as a fault‐bend fold, forming above a deep thrust fault. Within the Rosedale monocline, inferred Quaternary strata thin northward and form a growth triangle that is 4.1 to 6.6 km wide at its base, suggesting ∼2–3 mm/yr of slip on the underlying thrust. The western section of the >40‐km‐long, north dipping Tacoma fault, extending from Hood Canal to Carr Inlet, forms the western segment of the Tacoma basin margin. Structural relief on this portion of the basin margin may be several kilometers, resulting in steep gravity and aeromagnetic anomalies. Quaternary structural relief along the Tacoma fault is as much as 350–400 m, indicating a minimum slip rate of about 0.2 mm/yr. The inferred eastern section of the Tacoma fault (east of Carr Inlet) crosses the southern part of the Seattle uplift, has variable geometry along strike, and diminished structural relief. The Tacoma fault is regarded as a north dipping backthrust to the Seattle fault, so that slip on a master thrust fault at depth could result in movement on the Seattle fault, the Tacoma fault, or both.

Evidence for Late Holocene Earthquakes on the Utsalady Point Fault, Northern Puget Lowland, Washington

Trenches across the Utsalady Point fault in the northern Puget Lowland of Washington reveal evidence of at least one and probably two late Holocene earthquakes. The “Teeka” and “Duffers” trenches were located along a 1.4-km-long, 1- to 4-m-high, northwest-trending, southwest-facing, topographic scarp recognized from Airborne Laser Swath Mapping. Glaciomarine drift exposed in the trenches reveals evidence of about 95 to 150 cm of vertical and 200 to 220 cm of left-lateral slip in the Teeka trench. Radiocarbon ages from a buried soil A horizon and overlying slope colluvium along with the historical record of earthquakes suggest that this faulting occurred 100 to 400 calendar years b.p. (a.d. 1550 to 1850). In the Duffers trench, 370 to 450 cm of vertical separation is accommodated by faulting (∼210 cm) and folding (∼160 to 240 cm), with probable but undetermined amounts of lateral slip. Stratigraphic relations and radiocarbon ages from buried soil, colluvium, and fissure fill in the hanging wall suggest the deformation at Duffers is most likely from two earthquakes that occurred between 100 to 500 and 1100 to 2200 calendar years b.p., but deformation during a single earthquake is also possible. For the two-earthquake hypothesis, deformation at Teeka trench in the first event involved folding but not faulting. Regional relations suggest that the earthquake(s) were M ≥ ∼6.7 and that offshore rupture may have produced tsunamis. Based on this investigation and related recent studies, the maximum recurrence interval for large ground-rupturing crustal-fault earthquakes in the Puget Lowland is about 400 to 600 years or less.

National center for airborne laser mapping proposed

Researchers from universities, U.S. government agencies, U.S. national laboratories, and private industry met in the spring to learn about the current capabilities of Airborne Laser Swath Mapping (ALSM), share their experiences in using the technology for a wide variety of research applications, outline research that would be made possible by research‐grade ALSM data, and discuss the proposed operation and management of the brand new National Center for Airborne Laser Mapping (NCALM).The workshop successfully identified a community of researchers with common interests in the advancement and use of ALSM—a community which strongly supports the immediate establishment of the NCALM.

Hydrothermal recharge and discharge across 50 km guided by seamounts on a young ridge flank.

Hydrothermal circulation within the sea floor, through lithosphere older than one million years (Myr), is responsible for 30% of the energy released from plate cooling, and for 70% of the global heat flow anomaly (the difference between observed thermal output and that predicted by conductive cooling models). Hydrothermal fluids remove significant amounts of heat from the oceanic lithosphere for plates typically up to about 65 Myr old. But in view of the relatively impermeable sediments that cover most ridge flanks, it has been difficult to explain how these fluids transport heat from the crust to the ocean. Here we present results of swath mapping, heat flow, geochemistry and seismic surveys from the young eastern flank of the Juan de Fuca ridge, which show that isolated basement outcrops penetrating through thick sediments guide hydrothermal discharge and recharge between sites separated by more than 50 km. Our analyses reveal distinct thermal patterns at the sea floor adjacent to recharging and discharging outcrops. We find that such a circulation through basement outcrops can be sustained in a setting of pressure differences and crustal properties as reported in independent observations and modelling studies.

Swath mapping the base of the arctic ice canopy

Satellite data have demonstrated that the areal extent of the Arctic ice canopy has decreased over the past few decades, however, whether sea ice thickness in the Arctic Ocean has substantially decreased remains controversial because of limited information available for the base of the ice canopy. Studies of sea ice thickness to date have involved analysis of ice draft data acquired by single-beam ice profiling sonars on US and British Navy submarines or single-beam moored sonars. During 1998 and 1999, a seafloor-mapping sonar mounted on the hull of the US Navy nuclear submarine USS Hawkbill serendipitously acquired sonar returns from the base of the arctic ice canopy despite design features intended to minimize such returns. Standard topographic processing of the data revealed coherent signals collected between the outgoing sonar pulse and the first returned bottom echoes. Existing processing software was modified to produce swath maps of the information collected from the beginning of ping transmission until the seafloor echoes were detected. These images demonstrate that the recorded water-column signals are different on the port and starboard sides of the system and that individual features can be traced from one side of the submarine to the other. Further software modification allowed us to produce the first wide-swath (2–6 km) sidescan images of the base of the Arctic ice canopy. Although the resultant maps cannot resolve the debate regarding changes in Arctic sea ice thickness, the approach will have far-reaching impact on the design of future systems, whether based on submarines or unmanned vehicles. Our analyses demonstrate that system performance is regulated by the ratio between sonar operating depth and height of the sonar above the seafloor. Where the ratio approaches 1, echoes from the seabed interfere with echoes from the ice canopy, preventing the creation of ice maps. When the sonar is operated below 135 m, signal attenuation results in ice maps with little useful information. However, where the sonar operational depth is less than 135 m and the total water depth ranges between 500 and 1200 m, our approach produces swaths up to 6 km wide exhibiting detailed lineations similar to those observed in higher frequency upward-looking sidescan sonar data.

Mapping the Seabed and Habitats in National Marine Sanctuaries—Examples from the East, Gulf and West Coasts

The National Marine Sanctuary System requires seabed and habitat maps to serve as a basis for managing sanctuary resources and for conducting research. NOAA, the agency that manages the sanctuaries, and the USGS have conducted mapping projects in three sanctuaries (Stellwagen Bank NMS, Flower Garden Banks NMS, and Channel Islands NMS) with an emphasis on collaboration of geologists and biologists from the two agencies and from academic institutions. Mapping of seabed habitats is a developing field that requires the integration of geologic and biologic studies and the use of swath imaging techniques such as multibeam and sidescan sonar. Major products of swath mapping are shaded-relief topographic imagery which shows seabed features in great detail, and backscatter imagery which provides an indication of the types of materials that constitute the seabed. Sea floor images provide an excellent basis for conducting the groundtruthing studies (using video, photo, and sampling techniques) that are required to collect the data necessary for making meaningful interpretative maps of the seabed. The compilation of interpretive maps showing seabed environments and habitats also requires the development of a sea floor classification system that will be a basis for comparing, managing, and researching characteristic areas of the seabed. Seabed maps of the sanctuaries are proving useful for management and research decisions that address commercial and recreational fishing, habitat disturbance, engineering projects, tourism, and cultural resources.

海底拡大系下の非対称電気伝導度構造

A comparative study of deep mantle structures beneath spreading centres revealed asymmetry of lithosphere and mantle. Multidisciplinary marine expeditions were conducted at two survey sites : one around a super-fast spreading ridge in the East Pacific Rise, and the other around a slow and stable ridge segment of the continental breakup region, the Gulf of Aden. At the former site, a Mantle ELectromagnetic and Tomography experiment (MELT) was conducted with a large number of seismometers and electromagnetometers deployed at the seafloor to image the shape and the geometry of partial melt bodies below the fast spreading centre. At the latter, a seafloor magnetotelluric array was deployed to obtain a twodimensional electrical conductivity structure beneath the continental breakup region, and the structure was compared with precise swath mapping and sea surface gravimetry. At both sites, the deep electrical conductivity structures showed remarkable asymmetry with respect to the ridge crest. Partial melt bodies that correspond to the seafloor-producing factory at the surface could not be observed right below the ridge crest, but they significantly shifted by several tens of kilometers from the ridge crest. The asymmetry also appeared in bathymetry, gravity and seismic structures. This implies that the mid-ocean ridges are created at the contacts of different oceanic mantles rather than right above the symmetric partial melt bodies.

Hydrothermal heat flux through aged oceanic crust: where does the heat escape?

Recent publications suggest that most of the fluid flow in the upper oceanic crust is channelized through small volumes of rock and vented into the ocean. This implies that at flanks of generally thinly sedimented mid-ocean ridges, focused discharge at the seafloor should be concentrated most likely at outcrops, high-angle normal faults or seamounts. These vents should be associated with a significant heat flow signature. However, only few observations worldwide support this assumption up to now. On our quest for focused fluid exchange between young oceanic crust and the ocean we surveyed a 720 km long and 40–90 km wide off-axis portion of seafloor intersecting the East Pacific Rise near 14°14′S. A wealth of geophysical methods including high-resolution swath mapping bathymetry, single channel seismics, sediment echo sounding, magnetics and heat flow determinations were used. Heat flow data in the tectonic corridor cover crustal ages of 0.3–9.3 Ma. With respect to the conductive plate cooling model the data show the well-known pattern of low values close to the ridge, associated with vigorous hydrothermal circulation of cold seawater through the young upper crust, and a fast recovery to almost lithospheric conductive cooling values at a surprisingly young crustal age of 9.3 Ma. Although the sediment cover is fairly thin, measurements with a 3.6 m violin bow type heat probe were possible almost everywhere within the investigated area. A detailed survey between two large seamounts at 4.5 Ma revealed localized extremely high values of up to 618 mW/m 2 (275% of the expected heat flow) at the foot of the seamount. This is interpreted as a clear indication of focused discharge of hydrothermal fluid. If we, however, relate heat flow normalized by the expected conductive heat loss to the character of igneous basement, heat flow is highest in areas with an almost flat and sedimented basement, and lowest within ∼10–20 km of seamounts and other rough basement relief. We therefore hypothesize that the large number of seamounts covering the ocean floors governs a major amount of convective heat loss of aging oceanic lithosphere.

Morphology and origin of the Challenger Deep in the Southern Mariana Trench

A high resolution bathymetry survey reveals the detailed morphology of the Southern Mariana Trench. Distinct right stepping, N80°E trending en echelon deeps were found on the trench bottom within the Challenger Deep. Horst and graben structures were revealed on the outer swell of the Southern Mariana Trench. These structures, as well as slope‐failure on the inner and outer slopes of the trench are similar to features observed in other deep sea trenches. We propose here that the Southern Mariana Trench is a transform fault based on swath mapping, morphological analysis and tectonic interpretation. The en echelon deeps formed in a right‐lateral strike‐slip stress regime related to oblique Pacific/Caroline Plate subduction under the Southern Mariana Trench combined with the Mariana Trough backarc spreading. We confirm that the world's deepest point lies in the western portion of the Challenger Deep, among the en echelon deeps.

High-Resolution Topography along Surface Rupture of the 16 October 1999 Hector Mine, California, Earthquake (Mw 7.1) from Airborne Laser Swath Mapping

In order to document surface rupture associated with the Hector Mine earthquake, in particular, the area of maximum slip and the deformed surface of Lavic Lake playa, we acquired high-resolution data using relatively new topographic-mapping methods. We performed a raster-laser scan of the main surface breaks along the entire rupture zone, as well as along an unruptured portion of the Bullion fault. The image of the ground surface produced by this method is highly detailed, comparable to that obtained when geologists make particularly detailed site maps for geomorphic or paleoseismic studies. In this case, however, for the first time after a surface-rupturing earthquake, the detailed mapping is along the entire fault zone rather than being confined to selected sites. These data are geodetically referenced, using the Global Positioning System, thus enabling more accurate mapping of the rupture traces. In addition, digital photographs taken along the same flight lines can be overlaid onto the precise topographic data, improving terrain visualization. We demonstrate the potential of these techniques for measuring fault-slip vectors.

Airborne laser swath mapping shines new light on Earth's topography

Airborne Laser Swath Mapping (ALSM) now makes it possible to delineate the physical features of the surface of Earth on spatial scales as fine as a few decimeters horizontally and a few centimeters vertically As a result, Earth scientists may finally be able to answer long‐standing scientific questions about such surficial processes as erosion, faulting, volcanism, and plate motion. Information on surface relief, drainage patterns, and vegetation, critical to understanding the evolution of ecologically sensitive areas, can be collected over hundreds of square kilometers in a period of days. Information collected by ALSM can also be used to address a wide variety of engineering issues, such as developing and managing natural resources7semi; mitigating the impacts of such natural disasters as floods, hurricanes, tornados, landslides, and sinkholes; and building and maintaining transportation infrastructure.

Deformational styles of the eastern Mediterranean Ridge and surroundings from combined swath mapping and seismic reflection profiling

Recent swath mapping and seismic reflection profiling across the eastern(Levantine) branch of the Mediterranean Ridge (MR), in the eastern Mediterranean Sea, illustrate a strong variability of the deformational styles that characterize this precollisional accretionary prism. Along a north–south cross-section of the MR, a structural analysis, based on surface mapping, combined with vertical seismic reflection data, reveals two main structural domains. A southern Outer Domain consists of a series of three disconnected distinct fold belts. Folding affects a sedimentary cover which includes an approximately 2-km thick Pliocene and Quaternary wedge in this domain, resting on fairly thick Messinian evaporitic sequences that act as probable decollement layers. The MR Inner Domain includes three regions showing evidences of strong internal deformations and of numerous probable mud cones and mud flows, but no seismic evidence of significant underlying Messinian evaporites can be detected. The inner sub-regions are thrusting northwards over an area made of faulted, and locally uplifted, acoustic basement blocks that constitute the southernmost extension of the Crete continental margin, acting as a continental buttress for the MR. This innermost domain is itself structurally disconnected from the Crete continental margin by the en échelon Pliny troughs system that shows evidence of left lateral displacement. Altogether, the different structural and sedimentary cover patterns reveal a strong contrast between both MR Inner and Outer Domains, and important lateral variations within the Inner MR itself. Strike-slip faulting seams to characterizes both areas, and large-scale mud accumulations potentially exist in the northern one. Our results support a model of imbricated accretionary prisms, including, at least, two stages: (1) a pre-Messinian stage during which the Inner MR probably developed in response to northward subduction of the African lithosphere beneath southern Europe, and (2) a Messinian to post-Messinian period, during which the kinematics of the Aegean–Anatolian microplates and the presence of thick Messinian deposits became prevalent and which led to the creation of the Outer MR folded wedge piling against the previous one. In our interpretation, the occurrence of thick Messinian evaporites in this area induces important local and regional modifications of the structural pattern. Strike-slip activities reflect partitioning related to oblique subduction, and likely lateral escape of the sedimentary cover in the whole studied area.

Le canyon de Capbreton : nouvelles approches morphostructurales et morphosédimentaires. Premiers résultats de la campagne Itsas

A recent cruise of the Suroit using new technologies for acquisition and processing of acoustic data (very high-resolution seismic and swath mapping) associated to an accurate satellite positioning, allowed producing a new bathymetric map of the upper part of the canyon. It shows the meandering shape of the canyon and superposed polyphase terrace. Such a morphology is due to activity of gravity flows that occurred as soon as the Upper Eocene. Very high-resolution seismic data allowed to confirm the influence of old tectonic structures on the location and the general shape of the canyon and to show that neotectonics that affects Plio-Quaternary deposits corresponds to recurrent activity of old tectonic structures.

Sediment facies and pathways of sand transport about a large deep water headland, Cape Rodney, New Zealand

Cape Rodney is a large headland that protrudes 3–4 km into deep water in the Hauraki Gulf and separates the Mangawhai‐Pakiri and Omaha littoral cells. Detailed swath mapping of seabed sediments around Cape Rodney was carried out using by side‐scan sonar and ground‐truthed by SCUBA, grab sampling, and video. Despite the barrier imposed by the headland two pathways of sand transport around the headland, separated by the topographic high of Leigh Reef, have been identified. One lies close to the headland, where sand from the beach and nearshore of the Mangawhai‐Pakiri embayment is driven by waves and currents along a 500‐m‐wide pathway in c. 20–25 m depth around the headland to the vicinity of Leigh Harbour. The other lies in 50 m water‐depth seawards of Leigh Reef. Here fine sand, sourced from the nearshore of the Mangawhai‐Pakiri embayment and driven offshore from the tip of the headland, is transported back and forth by tidal currents in 50 m water depth on the floor of the Jellicoe Channel. The sand bodies along both these pathways are thin and so sand leakage from the Mangawhai‐Pakiri embayment is thought to be small. Transport at these depths is dependent on both tide and wave generated currents and episodic occurring during storm events. The sediment facies associated with little sand transport about a headland in deep water is one of thin and discontinuous and patchy sand cover between rocky areas and over coarser megarippled substrate. Ocean swell, tidally driven phase eddies that spin up on both sides of the headland, and bathymetry all play a role in shaping those facies.

Benefits of swath mapping for the identification of marine habitats in the New Caledonia Economic Zone

The ZoNéCo programme is devoted to the evaluation of the marine resources of the Economic Zone of New Caledonia. The results are essentially dependent on the quality of the seafloor mapping. From 1993 to 1996, four geological and geophysical surveys using the EM12 DUAL multibeam echosounder provided swath-mapping and acoustic imagery data of the seafloor of selected sites on the northern and southern parts of the Norfolk ridge, the Loyalty basin, around the Loyalty islands and in the westernmost part of the Economic Zone of New Caledonia. The accuracy of these documents shows the morphology of the seafloor in detail and allows rocky substratum to be differentiated from muddy bottom. It allows favorable emplacements of future exploratory fishing surveys to be determined. The benefits of swath mapping are illustrated by the Halipro 2 deep sea trawling cruise (1996) which used the swath mapping data of ZoNéCo 1 cruise (1993) on the southern prolongation of the New Caledonia mainland and Loyalty Islands.

How continents break up: Insights from Papua New Guinea

The Woodlark Basin in the western Pacific forms a continuous system of active continental rifting evolving to well‐developed seafloor spreading. Thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin. Magnetic isochrons indicate a single Euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins. In agreement with rigid plate tectonic models, continental rifting initiated geologically synchronously (at ∼6 Ma) along the length of the protomargins within a deforming plate boundary zone. Strain localization and seafloor spreading, however, developed in a time transgressive fashion from east to west within this zone of deformation. Spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the Solomon and Coral Seas. The transition to spreading occurred after a rather uniform degree of continental extension: 200±40 km. Both early and late stage rifting involved high‐ and low‐angle normal faults. We identify distinct styles in the transition from rifting to spreading which we refer to as nucleation, propagation, and stalling. These breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures. Continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating. The initial spreading segments achieved much of their length at nucleation (within rift basins separated along strike by accommodation zones), with subsequent lengthening by spreading propagation into rifting continental crust. This early propagation, and the subsequent development of transform faults between initially nontransform spreading segment offsets, produced rift and spreading segmentation boundaries that are not simply correlated. The spreading centers nucleated approximately orthogonal in strike to the opening direction but, as the protomargins were oblique to this direction, nucleation jumps occurred in order to maintain the new spreading centers within the protomargins. Thus stepwise spreading nucleation in order to remain within a rheologically weak zone, rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis, is the dominant form of the rifting‐to‐spreading transition in the Woodlark Basin.

Opposing gradients of permanent strain in the aseismic zone and elastic strain across the seismogenic zone of the Kodiak shelf and slope, Alaska

An industry‐acquired seismic reflection line from the Aleutian Arc to the Alaska Trench was prestack depth‐migrated and further constrained with wide‐angle ocean bottom hydrophone seismic velocity data and swath mapping. From the seismic reflection image, individual thrust slices, the backstop, and the forearc basin were resolved sufficiently well to balance the section. Tectonic shortening and age estimates provided rates of permanent contraction in five discrete segments over ∼150 km during the past 3 Myr. About 70% of the plate convergence is measured as contraction across the ∼40‐km‐wide Neogene accretionary prism which composes less than 10% of the forearc. Permanent contraction is minimal above the 1964 Alaska earthquake aftershock zone indicating dominently elastic strain. Since elastic strain is poorly recorded in the geological record, it was approximated from the historical record of the 1964 Alaska earthquake. The 1964 coseismic vertical deformation displays an inverse pattern to the permanent strain, consistent with the earthquake cycle concept. Elastic deformation is dominant beneath the shelf and upper slope, rapid permanent deformation is concentrated beneath the lower slope, and a transition between them occurs beneath the middle slope.

Acoustic backscatter of the 1995 flood deposit on the Eel shelf

Acoustic swath mapping and sediment box coring conducted on the continental shelf near the mouth of the Eel River revealed regional variations in acoustic backscatter that can be related to the shelf sedimentology. The acoustic-backscatter variations observed on the shelf were unusually narrow compared to the response of similar sediment types documented in other areas. However, the acoustic data revealed four principal bottom types on the shelf that can be related to sedimentologic differences observed in cores. The four areas are: (1) low acoustic backscatter associated with the nearshore-sand facies and the prodelta terraces of the Eel and Mad rivers, composed of fine sands and coarse silts with low porosity; (2) high acoustic backscatter associated with fine silts characterized by high porosity and deposited by the 1995 flood of the Eel River; (3) intermediate acoustic backscatter in the outer-shelf muds, where clayey silts are accumulating and the 1995 flood apparently had limited direct effect; and (4) intermediate acoustic backscatter near the fringes of the 1995 flood deposits and in areas where the flood sediments were more disrupted by post-depositional processes. The highest acoustic backscatter was identified in areas where the 1995 flood sediments remained relatively intact and near the shelf surface into the summer of 1995. Cores collected from these areas contained wavy or lenticular bedding. The rapid deposition of the high-porosity muddy layers results in better preservation of incorporated ripple forms than in areas less directly impacted by the flood deposit. The high-porosity muddy layers allow acoustic penetration into the sediments and result in greater acoustic backscatter from incorporated roughness elements.

Mudwave activity and current-controlled sedimentation in Powell Basin, northern Weddell Sea, Antarctica

An area of mudwave development has been identified from the northwestern Powell Basin, northern Weddell Sea, Antarctica. The wave-field is located near the base of the continental slope in water depths of 2800–3100 m, along the pathway of Antarctic Bottom Water (AABW) flow from the Weddell Sea. The waves are presently active, migrating up a low angle (0.6°) slope to the northwest. They are typically 20–50 m in amplitude and 2–4 km in wavelength, and cover an area greater than 1000 km 2. Swath mapping and high-resolution multi-channel seismic reflection profiling across the area show that the waves developed in association with basin floor channels and strongly current-influenced sediments. Acoustically, the sediments can be divided into three units. The uppermost unit, C, is 0.5 s twt (two-way time) thick with highly continuous, parallel reflectors displaying well-developed waveforms mimicking those at the seabed and, towards the southeast, a basin floor channel. Unit B is up to 0.8 s twt thick, with strong and continuous subhorizontal reflectors. In the mid-upper section of B are developed buried sinusoidal wave-forms and a base of slope depression or `moat' and a sediment drift which is infilled and buried, the reflector pattern suggesting sediment supply under current control alongslope, interpreted as contouritic sediments. A lower transparent unit, A, is 1–1.5 s twt thick, with occasional reflectors. The original construction of the waves may have been by turbidity currents predominantly supplied from the basin floor channels. Present-day deposition across the waves is maintained by fine-grained sediment as a result of the lateral transfer of distal turbidites from the basin floor channels by bottom currents. It is suggested that the initiation of current-influenced sedimentation was closely linked to the onset of AABW flow during the Early Miocene, following the separation of the South Orkney Microcontinent and the opening of the Powell Basin during the Late Oligocene.