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1.
Abstract

Large diapiric and nondiapiric masses of Jurassic salt and Tertiary shale underlie the northern Gulf of Mexico continental slope and adjacent outer continental shelf. These masses show evidence of being structurally active at present and in the very recent geologic past. Local steepening of the sea floor in response to the vertical growth of these structures is a serious concern to those involved in the site selection and the construction of future oil and gas production and transportation facilities in this frontier petroleum province.

The seabed of the northern Gulf slope is hummocky and consists of many hillocks, knolls, and ridges interspersed by topographic depressions and canyon systems. Topographic highs and lows relate respectively to vertical diapiric growth and to withdrawal of large volumes of salt and shale. Topographic highs vary considerably in shape and size, but all have very limited areas of nearly flat sea floor. Intraslope topographic lows consist of three principal types: (1) remnants of submarine canyons blocked by diapiric uplift that terminated active downslope sediment transport common during stages of low sea level; (2) closed depressions formed by subsidence in response to salt and shale withdrawal and flow into surrounding diapiric uplifts; and (3) small collapse basins formed by faulting in strata arched over structural crests of diapirs.

Distribution patterns of both diapiric features and sediment accumulations on the slope are the result of the complex relationship that exists between sediment loading and diapirism. Diapiric activity is proportional to the thickness of salt or underconsolidated shale available for mobilization, and to the sedimentary load distribution on these highly plastic deposits. Variations in overburden load, in turn, are dependent on rates, volumes, and bulk densities of depo‐sitional influx; proximity to sources of supply, erosion, and distribution of sediments; and topographic control of sediment accumulation. Sediment capture in diapirically controlled interdomal basins and canyon systems localizes overburden load, thus inducing further diapiric growth, and complex structural and stratigraphic patterns are induced throughout the continental slope region.

Drill cores in the slope province indicate that most of the slope sediments are fine‐grained muds; appreciable quantities of sand‐size sediment are present principally in canyon axes. Turbidite sand layers drilled on a topographic high adjacent to the Gyre Basin reflect uplift far above their original deposition level, and calculations yield rates of uplift that average 2 to 4 m per 100 years. Seismic reflection profiles provide considerable evidence of “fresh”; slumps and ero‐sional surfaces on the flanks of many topographic highs not yet blanketed by a veneer of young sediments. This evidence thus supports our conclusion that the present continental slope region of the northern Gulf of Mexico is undergoing active diapirism and consequent slope steepening. Because most of the sediment on the flanks of diapiric structures consists of underconsolidated muds, slumping will take place regularly in response to further diapiric movement.  相似文献   

2.
Abstract

The continental slope off the coast of Israel is riddled with numerous large slump scars at depths greater than 400 m. Recent scar slumps are situated in the steepest central portions of the continental slope (400–450 m depth, α=6°), frequently disfiguring older slump scars in its lower portions. The slumping materials were probably largely transported downslope in the form of density currents, and occasionally by sliding of large sediment chunks. Upslope retrogressive slumping phases progressively disfigure the shape of the slump scars until they totally disappear, causing net reduction of the thickness of the sedimentary column. To provide a basis for the quantitative analysis of slumping, laboratory vane tests, triaxial consolidated, undrained compression tests with pore‐pressure measurements, drained direct shear tests, and consolidation tests were performed oh undisturbed samples. Because the sediments consist of normally consolidated silty clays, the geotechnical properties measured on the core samples can be readily extrapolated for greater depths, assuming the sediments are homogeneous. Angles of internal friction measured by direct shearing under drained conditions are ?d =24°‐25°, designating the maximum possible angle of a stable infinite slope. These angles are appreciably higher than the steepest slopes in the investigated area, and a drained slumping mechanism is therefore considered unlikely. The slopes of the slump scar walls are about 20°; therefore, in the absence of active erosional, sedimentological, or tectonic agents, these walls have long‐term stability (drained shear). Undrained shear failure resulting in slope instability may be attributable to rapid changes in slope geometry (undercutting or oversteepening of the slope), fluctuations in pore pressure, or accelerations associated with earthquakes. Undrained shear‐strength parameters were determined by both laboratory consolidated‐un‐drained triaxial tests and by miniature vane shear tests. The angles of internal friction that were measured are ?cu =15°‐17°, and the cu/po values range between 0.22 and 0.75. An analysis of the force equilibrium within the sediments leads to the conclusion that horizontal earthquake‐induced accelerations, as little as 5–6% of gravity, are sufficient to cause slope failure in the steepest slope zone (400–450 m depth, α = 6°, cu /po =0.25). Collapse resulting from liquefaction is unlikely, as the sediments are normally consolidated silty clays with intermediate sensitivity, St =2–4.

The existence of slump scars in the lower portion of the continental slope, characterized by gentle slopes (α=1°‐3°) and sediments with high shear strength (c u /p o=0.30–0.50) is attributed to large horizontal accelerations(k=12–16% of gravity). Owing to the wide range of geotechnical properties of the sediments (cu /po = 0.20–0.75) and the inclination of the continental slope (α=1°‐6°), the same earthquake may generate a wide range of horizontal accelerations in different portions of the continental slope, and slumping may occur wherever the stability equilibrium is disrupted.  相似文献   

3.
Abstract

Numerous large sediment slides and slumps have been discovered and surveyed on the continental margins of Northwest Africa, Southwest Africa, Brazil (Amazon Cone), the Mediterranean, the Gulf of Mexico, and North America over the past 10 years. The mass movements are of two primary types: (1) translational slides, and (2) rotational slumps. Translational slides are characterized by a slide scar and a downslope zone of debris flows, after traveling in some areas for several hundreds of kilometers on slopes of less than 0.5°. Rotational slumps are bounded by steep scarps, but they do not involve large‐scale translation of sediments, although seismic records indicate disturbance in the down‐dropped block. Many of the slides and slumps have occurred in water depths greater than 2000 m on initial slopes of less than 1.5°. The largest slide so far discovered is off Spanish Sahara; in this case, the slide scar is 18,000 km2 in area, at least 600 km3 in volume of translated sediments. No apparent consistent relationship has yet been observed between the presence of the slides and the sedimentary environment in which they occurred. The slides off Southwest Africa and Spanish Sahara occurred in pelagic sediments rich in planktonic organic matter. In contrast, the slides off North America, Senegal‐Mauritania, and Brazil (Amazon Cone) occurred in sediments containing a high percentage of terrigenous material from nearby landmasses. Large sediment slides have also occurred in pelagic sediments on isolated oceanic rises such as the Madeira Rise (East‐Central Atlantic) and the Ontong‐Java Plateau (Pacific), where sedimentation rates are less than 2 cm/1000 years. The failure mechanism of the slides initiated near the shelf edge can probably be explained by sediment overloading during low glacio‐eustatic sea level, which allowed rivers to debouch sediments directly onto the outer shelf or upper slope. Possible mechanisms of failure of the deepwater slides and slumps include earthquakes, undercutting of the slope by bottom currents, and changes in porewater pressures induced as a direct or indirect result of glacio‐eustatic changes in sea level.  相似文献   

4.
Ranger Slide is a modest (12 km3) slide deposit of Pliocene and younger sediment on the continental slope in northern Sebastian Vizcaino Bay, Mexico. A limited survey using a deeply-towed instrument shows that hummocky terrain immediately downslope from the slide scar consists of large blocks of semiconsolidated sediment, some exceeding a kilometer in length and 107 m3 in volume. Most blocks have rotated, fallen apart, and/or deformed during movement. The form, structure, and processes related to emplacement of the blocks within the hummocky topographic zone of Ranger Slide may be common to many submarine slides on slopes involving semiconsolidated, terrigenous sediment.  相似文献   

5.
Two types of morphologic features in the head of Navarinsky Canyon are attributed to mass movement of near-surface sediment. A series of pull-aparts is located downslope of large sand waves. These pull-aparts, possibly induced by liquefaction, affect the upper 5 to 10 m of sandy sediment (water depths 350 to 600 m) on a 1o slope. A hummocky elongate mound of muddy sand (water depths 550 to 800 m) contains chaotic internal reflectors to a subbottom depth of 30 to 40 m and possibly is the product of a shallow slide. We speculate that Holocene seismicity is the likely triggering mechanism.  相似文献   

6.
High-resolution seismic reflection data recorded on the continental slope off the east coast of the United States have revealed instances of sediment mass movement (slumps) which appear to occur above clathrate accumulations. The slumping is believed to be related to the liberation of free gas by clathrate decomposition and consequent weakening of unconsolidated sediments above the clathrate. Pleistocene sea-level lowering and/or post-Pleistocene bottom water temperature increases may have had a significant role in this process.  相似文献   

7.
The continental margin off Uruguay and northern Argentina is characterized by high fluvial input by the de la Plata River and a complex oceanographic regime. Here we present first results from RV Meteor Cruise M78/3 of May?CJuly 2009, which overall aimed at investigating sediment transport processes from the coast to the deep sea by means of hydroacoustic and seismic mapping, as well as coring using conventional tools and the new MARUM seafloor drill rig (MeBo). Various mechanisms of sediment instabilities were identified based on geophysical and core data, documenting particularly the continental slope offshore Uruguay to be locus of submarine landsliding. Individual landslides are relatively small with volumes <2km3. Gravitational downslope sediment transport also occurs through the prominent Mar del Plata Canyon and several smaller canyons. The canyons originate at a midslope position, and the absence of buried upslope continuations strongly suggests upslope erosion as main process for canyon evolution. Many other morphological features (e.g., slope-parallel scarps with scour geometries) and abundant contourites in a 35-m-long MeBo core reveal that sediment transport and erosion are controlled predominantly by strong contour currents. Despite numerous landslide events, their geohazard potential is considered to be relatively small, because of their small volumes and their occurrence at relatively deep water depths of more than 1,500?m.  相似文献   

8.
Abstract

The continental slope off the coast of Israel is riddled with numerous large slump scars at depths greater than 400 m. Recent scar slumps are situated in the steepest central portions of the continental slope (400–450 m depth, α = 6°), frequently disfiguring older slump scars in its lower portions. The slumping materials were probably largely transported downslope in the form of density currents, and occasionally by sliding of large sediment chunks. Upslope retrogressive slumping phases progressively disfigure the shape of the slump scars until they totally disappear, causing net reduction of the thickness of the sedimentary column. To provide a basis for the quantitative analysis of slumping, laboratory vane tests, triaxial consolidated, undrained compression tests with pore‐pressure measurements, drained direct shear tests, and consolidation tests were performed oh undisturbed samples. Because the sediments consist of normally consolidated silty clays, the geotechnical properties measured on the core samples can be readily extrapolated for greater depths, assuming the sediments are homogeneous. Angles of internal friction measured by direct shearing under drained conditions are ?d =24°‐25°, designating the maximum possible angle of a stable infinite slope. These angles are appreciably higher than the steepest slopes in the investigated area, and a drained slumping mechanism is therefore considered unlikely. The slopes of the slump scar walls are about 20°; therefore, in the absence of active erosional, sedimentological, or tectonic agents, these walls have long‐term stability (drained shear). Undrained shear failure resulting in slope instability may be attributable to rapid changes in slope geometry (undercutting or oversteepening of the slope), fluctuations in pore pressure, or accelerations associated with earthquakes. Undrained shear‐strength parameters were determined by both laboratory consolidated‐un‐ drained triaxial tests and by miniature vane shear tests. The angles of internal friction that were measured are ?cu =15°‐17°, and the cu/p o values range between 0.22 and 0.75. An analysis of the force equilibrium within the sediments leads to the conclusion that horizontal earthquake‐induced accelerations, as little as 5–6% of gravity, are sufficient to cause slope failure in the steepest slope zone (400–450 m depth, α=6°, cu/p o=0.25). Collapse resulting from liquefaction is unlikely, as the sediments are normally consolidated silty clays with intermediate sensitivity, St =2–4.

The existence of slump scars in the lower portion of the continental slope, characterized by gentle slopes (α=1°‐3°) and sediments with high shear strength (cu/p o=0.30–0.50) is attributed to large horizontal accelerations (k= 12–16% of gravity). Owing to the wide range of geotechnical properties of the sediments (cu/p o= 0.20–0.75) and the inclination of the continental slope (α=1°‐6°), the same earthquake may generate a wide range of horizontal accelerations in different portions of the continental slope, and slumping may occur wherever the stability equilibrium is disrupted.  相似文献   

9.
High sedimentation rates on the subaqueous delta of the Mississippi River create localized sediment instabilities that result in downslope movement through well-defined chutes or gullies. A simplified kinematic wave equation model that neglects second-order effects such as diffusion treats the failure mechanism as a sedimentation and slope oversteepening process and sediment motion as a propagating kinematic wave. The model allows estimation of sedimentation rates necessary to initiate slope failures for a range of observed depths of basal shear planes. Model results indicate that slope oversteepening is a viable failure mechanism and a thin surface sediment layer may be moving downslope in a slow, continuous motion.  相似文献   

10.
Abstract

Submarine faults and slides or slumps of Quaternary age are potential environmental hazards on the outer continental shelf (OCS) of the northern Gulf of Alaska. Most faults that approach or reach the seafloor cut strata that may be equivalent in age to the upper Yakataga Formation (Pliocene‐Pleistocene). Along several faults, the seafloor is vertically offset from 5 to 20 m. A few faults appear to cut Holocene sediments, but none of these shows displacement at the seafloor. Submarine slides or slumps have been found in two places in the OCS region: (1) seaward of the Malaspina Glacier and Icy Bay, an area of 1200 km2 with a slope of less than 0.5°, and (2) across the entire span of the Copper river prodelta, an area of 1730 km2, having a slope of about 0.5°. Seismic profiles across these areas show disrupted reflectors and irregular topography commonly associated with submarine slides or slumps. Potential slide or slump areas have been delineated in areas of thick sediment accumulation and relatively steep slopes. These areas include (1) Kayak Trough, (2) parts of Hinchinbrook Entrance and Sea Valley, (3) parts of the outer shelf and upper slope between Kayak Island and Yakutat Bay, and (4) Bering Trough.  相似文献   

11.
Abstract

Vertical variations of geotechnical properties in the uppermost sediment layers characterize the main sedimentary processes acting on the construction and destruction of progressive‐type continental slopes. In the Gulf of Lions, the original thicknesses and distribution of the uppermost sedimentary layers of the continental slope and rise, which consist of Holocene muds overlying Pleistocene muds, have been greatly modified by erosion and several kinds of slope failure processes. Each process is typified through sets of geotechnical properties measured in the eroded or slumped sections and in the associated sediment accumulations.

In slump scars, the water‐rich Holocene muds lie on fine, overconsolidated, Pleistocene muds with high plasticity and low shear strength. In bottom current‐eroded slopes, where modern sedimentation is extremely reduced, the Pleistocene muds frequently outcrop and may sometimes be overlain by a very thin layer of Holocene muds. The Pleistocene muds of eroded slopes are overconsolidated and more silty and less plastic than the Pleistocene muds from slopes affected by slope failure, their shear strength being 10 times greater.

Deposits at the toe of slumps are very often formed by several superposed three‐layer units (triplets of interstratified Holocene, transitional, and Pleistocene layers) issued from retrogressive slumping occurring in the slump scars above their head area. The main body of each layer is then relatively undisturbed, showing the usual burial geotechnical gradients due to overburden pressure (i.e., decrease of water content and increase of unit weight and shear strength). At the toe of bottom current‐eroded slopes, a thick and homogeneous layer of Holocene muds overlies the Pleistocene muds; this Holocene layer has unappreciable burial depth gradient of its geotechnical parameters because of a high rate of modem and continuous deposition.  相似文献   

12.
The Rockall Bank Mass Flow (RBMF) is a large, multi-phase submarine slope failure and mass flow complex. It is located in an area where the Feni Drift impinges upon the eastern flank of the Rockall Bank in the NE Atlantic. A 6100 km2 region of slope failure scarps, extending over a wide water depth range and with individual scarps reaching up to 22 km long and 150 m high, lies upslope of a series of mass flow lobes that cover at least 18,000 km2 of the base of slope and floor of the Rockall Trough. The downslope lobe complex has a negative topographic relief along much of its northern boundary, being inset below the level of the undisplaced contourite drift at the base of slope. The southern margin is topographically more subtle but is marked by the sharp termination of sediment waves outside the lobe. Within the lobe complex the southern margin of the largest lobe shows a positive relief along its southern margin. The initial failure is suggested to have occurred along coherent layer-parallel detachment surfaces at depths of up to 100 m and this promoted initial downslope block sliding which in turn transformed into debris flows which moved out into the basin. The remains of a deep erosional moat linked to the onlapping contourite complex bisects the region of failed slope, and post-failure thermohaline currents have continued to modify the mass flow in this area. Differential sedimentation and erosion associated with the moat may have promoted slope instability. Following the major failure phase, continuous readjustments of the slope occurred and resulted in small-volume turbidites found in shallow gravity cores collected on the lobes. The short term trigger for the failure remains uncertain but earthquake events associated with a deep-seated tectonic lineament to the north of the mass flow may have been important. A Late Pleistocene age for the slope failure is likely. The RBMF is unusual in that it records large-scale collapse of a contourite body that impinged on a sediment-undersupplied slope system. Unlike many other large slope failure complexes along the NE Atlantic margin, the RBMF occurs in a region where there was little overloading by glacial sediment.  相似文献   

13.
Multibeam bathymetry, high resolution multi-channel, and very high resolution single-channel (3.5 kHz) seismic records were used to depict the complex geomorphology that defines the Galicia Bank region (Atlantic, NW Iberian Peninsula). This region (≈620–5,000 m water depth) is characterized by a great variety of features: structural features (scarps, highs, valleys, fold bulges), fluid dynamics-related features (structural undulations and collapse craters), mass-movement features (gullies, channels, mass-flow deposits, slope-lobe complexes, and mass-transport deposits), bottom-current features (moats, furrows, abraded surface, sediment waves, and drifts), (hemi)pelagic features, mixed features (abraded surfaces associated to mixed sediments) and bioconstructions. These features represent architectural elements of four sedimentary systems: slope apron, contouritic, current-controlled (hemi)pelagic, and (hemi)pelagic. These systems are a reflection of different sedimentary processes: downslope (mass transport, mass flows, turbidity flows), alongslope (bottom currents of Mediterranean Outflow Water, Labrador Sea Water, North Atlantic Deep Water, and Lower Deep Water), vertical settling, and the interplay between them. The architectural and sediment dynamic complexities, for their part, are conditioned by the morphostructural complexity of the region, whose structures (exposed scarps and highs) favor multiple submarine sediment sources, affect the type and evolution of the mass-movement processes, and interact with different water masses. This region and similar sedimentary environments far from the continental sediment sources, as seamounts, are ideal zones for carrying out submarine source-to-sink studies, and can represent areas subject to hazards, both geologic and oceanographic in origin.  相似文献   

14.
Abstract

Subbottom profiling (3.5 kHz) in the uppermost slope of the outer shelf of the northwestern Aegean Sea has shown downslope slumping and intense sediment deformation as well as a succession of microslumping within the Late Pleistocene delta sequences. Postdeposition bottom erosion by currents and the overconsolidated sediments (Cu/Po > 1) indicate the termination of deformations a long time ago. Deformation processes are estimated to have been active 18,000–10,000 B.P. Large‐scale active downslope deformations with clockwise sediment block rotation have affected the area as well. These longer‐period deformations are associated with regional neotectonics and older unconformities (i. e., Plio‐Pleistocene) as potential slide planes.

Geotechnical properties reflect mainly textural variations and locally (within sapropelic layers) high organic matter content. Relatively high values of shear strength were measured (5–29 kPa) with intermediate sensitivities (2–5), whereas relatively low values of water content (33–81%) were found along the five selective sediment cores.

Infinity slope stability analysis revealed that the slopes in the study area are most stable, although theoretical evaluations (Cu estimated from linear regression analysis) indicate relative instability for the potential glide plane at 20 m depth. The outer shelf is covered by compact relict sands, and their gentle and low‐angle (<0.2°) slopes are the most stable region of the investigated area.  相似文献   

15.
Geomorphological features (derived from 16,000 lkm of echo‐sounding and bathymetric data) and deep‐seated tectonic structures of the continental margin off NW India are presented. The shelf break over the entire region occurs between 80 to 154 m water depth, and adjacent to Saurashtra and Bombay High the depth and orientation of the shelf edge show marked variations. The boundary of the slope is shallower in the northern portion (about 1450 m in the vicinity of the Indus) than in the southern region (2900 m off Bombay).

The steep slope off the Gulf of Kachchh has relatively smooth physiography due to higher input of fluvial sediment and burial of structures. The gentler slope off Saurashtra and the Bombay High area has numerous complex features, the most prominent among them being benches at depths of 180–230 m (width 2–10 km) and 650–780 m and a series of bathymetric highs and lows. Slope breaks are also observed between 400 and 600 m off Bombay and between 560 and 960 m off Saurashtra. These features are surface manifestations of the anticlinal features extending along the shelf in this region. Unevenness (order of 100–300 m) due to slumping is also observed at the base of the slope.

Based on the correlation between tectonic structures of this area and these subphysiographic features, extension of the Saurashtra Anticline onto the slope, a new strike slip fault (the southern boundary fault of Narmada graben) and an along‐shelf anticlinal structure off Saurashtra are delineated.  相似文献   

16.
Abstract

As part of a National Oceanic and Atmospheric Administration (NOAA) program to understand bottom and nearbottom processes on the continental margin, the continental slope seaward of the coast of Delaware, just east of the Baltimore Canyon Trough, and northeast of Wilmington Canyon was studied in detail. With a suite of geophysical data, a 7.5 × 13.0‐km portion of the continental slope was surveyed and found to be composed of a large submarine slide, approximately 11 km 3 in volume. The slide varies from 50 to 300 m in thickness and is believed to be composed of Pleistocene Age sediments. The internal structure of the continental slope can be seen on the seismic reflection profiles, as well as the readily identifiable continuous slip surface. Pliocene to Cretaceous horizons comprise the continental margin with Pliocene to Eocene horizons truncated at the slip surface. Sediment failure occurred on the slope between the late Tertriary erosion surface, which shaped the continental margin, and the overlying Quaternary sediments. A mechanism suggested to have contributed to the sediment failure is a late Pleistocene lower stand of sea level. Creep of surficial sediments is believed to be active on the surface of the submarine slide, indicating present‐day instability.  相似文献   

17.
Abstract

The Hellenic Arc is located within one of world's most seismically active areas and has experienced extreme tectonism through Tertiary and Quaternary times. This activity controls the rates of uplift and subsidence and determines the sediments supply and depot centers. This paper discusses the various geological hazards detected in selected parts of the Hellenic Arc and examines the causative factors. The areas surveyed were the North Aegean Trough, the Kythera Ridge in the outer island arc, the Corfu/Kefalinia/Zante shelf/slope, and the Thermaicos, Corinth, Patras, Amvrakia, and Malliacos gulfs, as well as the Trichonis Lake. The potential geological hazards observed are (1) active faulting, (2) sediment instabilities, (3) gas‐charged sediments, (4) salt doming, (5) erosion, transport, and deposition of sediments, and (6) seismicity, volcanism, and tsunamis. The major types of sediment instabilities that have been documented on seabed include (1) surficial sediment creep in slopes ranging from 1 to 2° resulting infolding and faulting of the surficial sediments, (2) translational and rotational slides in slopes ranging from 2 to 40°, (3) debris flow, and (4) turbidity currents. Factors that contribute to slope instability in the Hellenic Arc are (1) sloping bottom, (2) thick accumulations of Plio‐Quatemary sediments, (3) present day high rates of sedimentation, (4) closely spaced active faults, (5) earthquakes, and (6) active diapirism. The contribution of long period waves to slope failure in these areas appears to be of minor importance, since the slope failures occur in depths of more than 150 m. Gas‐charged sediments and pockmarks have been observed in areas associated with deltaic, fjord‐like, and open sea environments. This gas is presumably formed by the decomposition of biogenic material. Numerous disasters that took place during historical times and greatly affected the coastal zone were caused by the above‐mentioned hazards. Damage to offshore installations are limited to cable failure.  相似文献   

18.
Multichannel seismic data from the eastern parts of the Riiser-Larsen Sea have been analyzed with a sequence stratigraphic approach. The data set covers a wide bathymetric range from the lower continental slope to the abyssal plain. Four different sequences (termed RLS-A to RLS-D, from deepest to shallowest) are recognized within the sedimentary section. The RLS-A sequence encompasses the inferred pre-glacial part of the deposits. Initial phases of ice sheet arrival at the eastern Riiser-Larsen Sea margin resulted in the deposition of multiple debris flow units and/or slumps on the upper part of the continental rise (RLS-B). The nature and distribution of these deposits indicate sediment supply from a line or a multi-point source. The subsequent stage of downslope sediment transport activity was dominated by turbidity currents, depositing mainly as distal turbidite sheets on the lower rise/abyssal plain (RLS-C). We attribute this to margin progradation and/or a more focussed sediment delivery to the continental shelf edge. As the accommodation space on the lower rise/abyssal plain declined and the base level was raised, the turbidite channels started to backstep and develop large channel–levee complexes on the upper parts of the continental rise (RLS-D). The deposition of various drift deposits on the lower rise/abyssal plain and along the western margin of the Gunnerus Ridge indicates that the RLS-D sequence is also associated with increased activity of contour currents. The drift deposits overlie a distinct regional unconformity which is considered to reflect a major paleoceanographic event, probably related to a Middle Miocene intensification of the Antarctic Circumpolar Current.  相似文献   

19.
Submarine slope processes on a fan delta,Howe Sound,British Columbia   总被引：1，自引：0，他引：1  
Modern side-scan sonar technology was used to resurvey the site of the Howe Sound submarine slide described by Terzaghi. Chutes, hummocky topography, and subparallel scarps indicative of submarine mass movement are widespread. Submarine slope processes appear to be far more important to the development of this coarsegrained fan delta than suspected.  相似文献   

20.
A detailed analysis of high-resolution seismic data (Chirp, 2–7 kHz) in the Ulleung Basin reveals spatial variation in echo characteristics and geometry of large-scale debris lobes. In the proximal part, the debris lobes are dominated by hummocky surface echoes and gradually transitional downslope to seafloor-tangent hyperbolic and smooth prolonged bottom echoes, reflecting progressive decrease in size, spacing, and relief of surface forms. The strongly convex-upward upper surface with steep blunt margins in the proximal part is gradually transitional downslope to slightly convex- upward or nearly flat form with less blunt margins. The gradual downslope decrease in both scale and spacing of surface forms and convexity of upper surface within each debris lobe is suggestive of progressive dilution in flow concentration within a debris flow, probably due to mixing of ambient water and removal of suspended sediment by deposition during downslope movement. Received: 3 February 1999 / Revision received: 27 April 1999  相似文献   

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