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1.
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   

2.
A sediment slide complex has been mapped on the West African continental margin north of Dakar, Senegal. Four major slides covering approximately 44,300 km2 were delineated by seismic reflection profiles, 3.5 and 12 kHz echograms and piston cores. Although the slide areas have been altered by later erosion and deposition by turbidity flows, the major components of the slides — slide scar, zones of hummocky and blocky slide material and zones of debris flow — are recognizable. Cores containing flow folds with horizontal axial surfaces substantiate the echogram interpretations of debris flow. Morphology and depositional areas of the slides indicate that several major slide movements have occurred in each of the various slide areas. The triggering mechanism for these slides is perhaps earthquakes associated with the Cape Verde Islands, Cape Verde Plateau, and adjacent fracture zones.  相似文献   

3.
A Late Pleistocene submarine slide on the Bear Island Trough Mouth Fan   总被引：5，自引：2，他引：5  
A large submarine slide on the southern flank of the Bear Island Trough Mouth Fan, southwestern Barents Sea continental slope, has a run-out distance of about 400 km, a total volume of about 1100 km3, and is younger than 330 ka. Three seismic units, comprising mainly hemipelagic sediments has partly filled the slide scar. An increased sedimentation rate on the Bear Island Trough Mouth Fan from Late Pliocene time, probably in combination with abundant earthquakes, is the most likely cause of the slide. Based on these and previous studies, we suggest that large-scale slides were important sediment transport processes during Plio-Pleistocene.  相似文献   

4.
The western Svalbard continental margin contains thick sediment sequences with areas known to contain gas hydrates. Together with a dynamic tectonic environment, this makes the region prone to submarine slides. This paper presents results from geophysical mapping of the deepest part of the high Arctic environment, the Molloy Hole. The mapping includes multibeam bathymetry, acoustic backscatter and sub-bottom profiling. The geophysical data reveal seabed features indicative of sediment transport and larger-scale mass wasting. The large slide scar is here referred to as the Molloy Slide. It is located adjacent to the prominent Molloy Hole and Ridge system. The slide is estimated to have transported >65 km3 of sediments over the deep axial valley of the Molloy Ridge, and further into the Molloy Hole. A unique feature of this slide is that, although its run-out distance is relatively short (<5 km), it extends over an enormous vertical depth (>2,000 m) as a result of its position in a complex bathymetric setting. The slide was most likely triggered by seismic activity caused by seafloor spreading processes along the adjacent Molloy Ridge. However, gas-hydrate destabilization may also have played a role in the ensuing slide event.  相似文献   

5.
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   

6.
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   

7.
Reflection profiling in a region of anomalous topography and structure in the Bay of Bengal off Burma has revealed the presence of a large submarine slide (olistostrome) at the base of the continental slope off the Bassein River. The slide overlies a thick section of Bengal Deep-Sea Fan turbidites and has a complex internal structure consisting of two primary elements. The lower element is pervasively disturbed and is interpreted as a mudflow generated at the time of the slide which spread over a large area to as much as 35 km beyond the topographic toe. This mudflow poured into a distributary channel on the Bengal Fan and virtually filled it for 145 km along its length. The upper element comprises a series of relatively coherent blocks of stratified sediments (olistoliths) bounded by curved fault planes. The blocks have been transported as much as 55 km from the original Sunda Trench wall. Their dimensions, up to 360 m thick and 2.8 km between faults, are similar to olistoliths of the slide terrain in the Apennines. The blocks are blanketed by younger slope strata. The total area covered by the slide, including the mudflow, is almost 4,000 km2, and total volume of the slide is over 900 km3. Material of the slide consists of Bengal Fan turbidites offscraped above the Sunda Subduction zone and blanketed by rapidly deposited slope sediments from a western Irrawaddy River distributary (the Bassein River) during Late Quaternary glacial low sea level. This rapid loading, probably coupled with a large earthquake, triggered the slide.  相似文献   

8.
Seismic profiling with 3.5-kHz and GeoPulse in the Amazon submarine delta indicates that gas-charged sediments cover an area greater than 31,000 km2. Gas appears on seismic profiles as gas-brightening reflectors near the river mouth, where mud and sand are well stratified. In fine sediments of the distal portion of the system, gas turbidity zones predominate. Biogenic gas is generated during degradation of terrestrial and marine organic matter by bacteria. The depth of gas in sediment below the seabed depends in part on anaerobic methane oxidation and the base of the sulfate reduction zone and on stratigraphic traps.  相似文献   

9.
Continental shelf systems are highly dynamic sedimentary environments, where sediments from biogenic production as well as from terrigenous sources are redistributed in the shelf depositional system, and partly exported off the shelf to the slope and the deep sea. The Golfe d’Arguin (Mauritania, NW Africa) is dominated by such redistribution processes, involving clastic silt imported as dust from the Sahara desert and biogenic carbonates of marine origin. Indeed, surface-sediment grain size and mineralogy show a clear north–south partitioning of sediment type. Fine material is winnowed from the northern part of the gulf, and transported toward the southern part off the Banc d’Arguin, where coarse silt settles on the outer shelf and upper slope, at least down to 600 m water depth. Particles of the fine silt fraction, estimated in terms of eolian material collected aboard the research vessel, are thought to be exported further offshore as they correspond to grain sizes previously reported from adjacent deep-sea sediments. These findings suggest that the interpretation of dust records from the continental slope and rise off NW Africa must consider reworking and partitioning processes active on the Mauritanian shelf.  相似文献   

10.
Parallel laminated, graded, and homogeneous muds of turbidity current origin are the predominant facies in the non-fan slope-centered Ulleung marginal basin during the last glacial period. Dilute turbidity currents were probably generated from slumps, slides, and debris flows on the slope. A mid-slope core contains poorly sorted mud-clast muds of debris flow origin. During the period of 75,000 and 10,000 years BP, turbidity currents occurred approximately every 125 years, each depositing about 0.5 km3 of mud with an accumulation rate of up to 40 cm/103 years. The basin was largely suboxic with a rare incursion of bottom currents.  相似文献   

11.
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.  相似文献   

12.
The floor of the western equatorial Atlantic Ocean can be divided into several distinct provinces based on detailed characteristics of the bottom echos recorded with short-ping (< msec.) 3.5 and 12 kHz sound sources. Two major types of echos are recorded: (I) distinct echos; and (II) indistinct echos.Indistinct echos can be further sub-divided into (A) continuous prolonged echos; and (B) hyperbolic echos. Each class of echos contains two or more unique echo types. The regional distributions of the various echo types recorded from the continental rise, Amazon Cone, and abyssal plains reveal much information about sedimentary processes.In the western equatorial Atlantic, hyperbolic echos are recorded only from small, isolated portions of the continental rise. This contrasts with the continental rise of the western North Atlantic where previous investigators have shown that hyperbolic echos parallel bathymetric contours along the entire rise and thus reflect shaping of the rise by geostrophic contour currents (Heezen et al., 1966; Hollister, 1967). The fact that regions of hyperbolic echos show little or no relationship to bathymetric contours of the continental rise of the western equatorial Atlantic suggests that contour currents have been unimportant in shaping the rise in this region.The three most widespread echo types recorded from the continental rise, Amazon Cone, and abyssal plains reveal much information about terrigenous sediment dispersal and deposition in the western equatorial Atlantic. Comparison of the thicknesses and frequencies of coarse (silt- to gravel-size), bedded, terrigenous sediment in piston cores with the echo type recorded at each coring site shows a correlation between echo type and the relative amount of coarse, bedded sediment within the upper few meters of the sea floor. The regional distributions of these three echo types indicate that dispersal of coarse terrigenous sediment has been downslope across the continental rise and Amazon Cone to the abyssal plains via gravity-controlled sediment flows. The Amazon River is the major sediment source and most coarse sediment is deposited on the lower Amazon Cone and proximal portions of the Demerara abyssal plain.  相似文献   

13.
Abstract

The improvement of sensors such as various high‐resolution seismic and navigational systems and side‐scan sonar, of offshore shallow‐water drilling techniques, and of laboratory analyses has allowed the marine geologist to make more accurate identifications and maps of the distribution of numerous types of marine sediment instabilities, as well as to determine the mechanisms responsible for their occurrence. A large number of data on the continental shelf and upper continental slope off the modern delta of the Mississippi river have been compiled; these data will be used to document the major types of slope instabilities. The continental shelf and slope off the modern Mississippi river delta display various types of sediment instability. High rates of sedimentation (up to 80 m per century), weak, high‐water‐content clays, and differential weighting of clay sediments characterize this region. The major types of sediment instabilities that have been documented include (a) Peripheral slumping, with dimensions of slumps ranging from 200 to 1000 m; slumping often occurring in multiple stairstep arrangement; and downslope movement as high as 700 m per year. (b) Shallow diapiric intrusions, ranging in size from a few hundred meters to 2 km in diameter; vertical displacement ranging from 200 to 500 m; rate of sediment movement several meters per year; and intrusions caused by differential sediment loading, (c) Radial graben (tensional faulting), with widths from 50 to 500 m and lengths of several kilometers; both vertical and downslope lateral movements occurring; and downslope movements of surface material as much as 5 m per year common. (d) Circular collapse depressions, with diameters of depressions ranging from 50 to 500 m; topography of depression interiors, hummocky; and depressions possibly caused by dewatering or degassing of sediments under the influence of cyclic wave loading. (e) Surface mudflows, thick (often more than 35 m) masses of surface sediment flowage; often bounded by abrupt seaward slope; mudflows often extending laterally for distances in excess of 100 km; movement sporadic and lobate and rates of movement as much as several hundred meters per year; often being associated with extremely hummocky topography and mud volcanoes; and with extrusion of sediments the possible mechanism. (f) Shelf‐edge arcuate slumps, with large arcuate slumps displacing several hundred meters of sediment; slippage planes are commonly concave. Finally, (g) Various deep‐seated faults, with faults extending from deep horizons up to modern sediment surface; commonly being associated with abrupt scarps on the seafloor; numerous contemporaneous faults; and local slumping associated with fault scarps.  相似文献   

14.
The Baiyun submarine slide complex (BSSC) along the Pearl River Canyon of the northern South China Sea has been imaged by multibeam bathymetry and 2D/3D seismic data. By means of maximum likelihood classification with slope aspect and gradient as inputs, the BSSC is subdivided into four domains, denoted as slide area I, II, III and IV. Slide area I is surrounded by cliffs on three sides and has been intensely reshaped by turbidity currents generated by other kinds of mass movement outside the area; slide area II incorporates a shield volcano with a diameter of approximately 10 km and unconfined slides possibly resulting from the toe collapse of inter-canyon ridges; slide area III is dominated by repeated slides that mainly originated from cliffs constituting the eastern boundary of the BSSC; slide area IV is distinguished by a conical seamount with a diameter of 6.5 km and a height of 375 m, and two slides probably having a common source that are separated from each other by a suite of residual strata. The BSSC is interpreted to be composed of numerous slide events, which occurred in the period from 10.5 to 5.5 Ma BP. Six specific factors may have contributed to the development of the BSSC, i.e., gas hydrate dissociation, gas-bearing sediments, submarine volcanic activity, seismicity, sedimentation rate and seafloor geomorphology. A 2D conceptual geological model combining these factors is proposed as a plausible mechanism explaining the formation of the BSSC. However, the BSSC may also have been affected by the Dongsha event (10 Ma BP) as an overriding factor.  相似文献   

15.
The input of river-borne sediments to the New Zealand continental shelf has been calculated for all the major rivers and basins in New Zealand. South Island yields 284 ± 40 × 106 tonnes per year of sediment from a land area of 152 977 km2 and North Island yields 105 ± 9·4 × 106 tonnes per year from a land area of 114 621 km2. Particularly high discharges are noted off the west coast of South Island and east coast of North Island and result in higher offshore sedimentation there. The data are compatible with measured sedimentation rates on the New Zealand continental shelf. The specific sediment yield from South Island is amongst the highest previously recorded.  相似文献   

16.
In the Russian sector of the Gdansk Basin (Baltic Sea), high organic matter influx fuels microbial processes resulting in the formation of reduced sediments with elevated methane concentrations. Investigated areas of geoacoustic anomalies (～245 km2) were found to contain three distinct geomorphologic structures (pockmarks), with a total area of ～1 km2. Methane anomalies recorded in the water above one of these pockmarks were traced as high as 10 m above the bottom. In pockmark sediments, sulfate reduction and anaerobic oxidation of methane (AOM) occurred at high rates of 33 and 50 µmol dm?3 day?1, respectively. Integrated over 0–180 cm sediment depths, AOM exceeded methanogenesis almost tenfold. High AOM rates resulted from methane influx from deeper sediment layers. The δ13C signature of methane carbon (?78.1 to ?71.1‰) indicates the biogenic origin of pockmark methane. In pockmark sediments, up to 70% of reduced sulfur compounds was possibly produced via AOM.  相似文献   

17.
The transport pathways of fine sediments (fraction <2 μm) along the Brazilian continental shelf from Ceará to the Amazon River mouth were studied by means of clay mineral analyses. On the continental shelf southeast of the Amazon mouth, fluctuations in clay mineral compositions reflect simple mixing between the suspended load of the North Brazil Current and sediment from several smaller rivers. Previously, clay mineral variations west of the Amazon mouth have been explained by variable settling velocities of different grain-size classes within the <2 μm fraction or by selective coagulation of individual clay mineral groups. By contrast, our experiments with river bank samples show that selective coagulation does not occur in Amazon River sediments. A more appropriate explanation for observed variations in clay mineral composition off the Amazon mouth seems to be, similarly to that for the shelf between Ceará and the Amazon mouth, a mixing of Amazon sediments with suspended material of the North Brazil Current. This interpretation is supported by data on clay mineral composition east and south of the Amazon mouth, showing more affinity to sediments of the North Brazil Current than to the suspended load of the Amazon River. Additionally, relatively low sedimentation rates and low concentrations of fine-grained sediments on the shelf suggest that high riverine input by the Amazon River does not overprint the sediments of the North Brazil Current in this region. The strong North Brazil Current shunts the Amazon suspended load in a north-westerly direction along the north-eastern coast of South America. Hence, stronger sedimentation of Amazon sediments would occur only west of the river mouth.  相似文献   

18.
Agglutinated tubes of the polychaete Owenia fusiformis Delle Chiaje and their adjacent sediment were analyzed with respect to grain size, particle composition and tube structure to assess whether these parameters reflect a specific relationship between tube and adjacent sediment. Tubes and sediments were sampled southwest off Helgoland Island (southeastern North Sea) in water depth of 35–37 m. Particle selection was studied using settling velocity and component analysis. Both methods clearly showed that O. fusiformis actively selects coarse particles for tube building. The ability for size selection seems to be fully developed immediately after the transition from pelagic to benthic life, as considerable differences in the composition of tube and adjacent sediments occurred over the whole sampled sediment depth. In both tube and surface sediments quartz was the dominating component (>80%). In contrast, heavy minerals, the second frequent component of the surface sediments, were used less frequently for tube building. Biogenic components (e.g. shell material, echinoid fragments, foraminifera) occurring rarely in the sediment are nevertheless preferred as components for tube building. As underlying selection criteria a combination of size and the existence of at least one flattened surface that can be attached to the tube may be assumed.  相似文献   

19.
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   

20.
Seismic reflection profiles from the equatorial Atlantic off Africa between 12°N and 1°N reveal remarkably widespread deformation of oceanic sediments in an area lacking teleseismic activity. Uplift of abyssal plain sequences has occurred along the eastern extensions of large-offset fracture zones on the Mid-Atlantic Ridge. Between the major transforms pervasive deformation is imaged in the sedimentary column. Diapirism and faulting may have been associated with extensive fluid migration and sediment mobilization. The deformation occurred during the late Cenozoic and was probably related to reactivation of Cretaceous transforms.  相似文献   

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