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1.
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   

2.
Long-range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor. Seven acoustic facies that arise from differing sediment or rock types have been mapped over the low-relief Saharan continental rise and Madeira abyssal plain. These have been calibrated with sampling, profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data. The mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise/abysaal plain transition over 1000 km from the nearest continental slope.  相似文献   

3.
A nearly continuous zone of sediment waves is present on the lower continental rise off western Morocco which parallels the regional bathymetric trends. The individual sediment waves within the zone migrate upslope with time and, in general, also trend parallel to the regional bathymetric contours. These observations suggest that geostrophic contour currents are responsible for the formation of sediment waves. Physical oceanographic measurements and sea-floor photographs indicate only a very weak bottom circulation in this region. This suggests either that strong bottom currents are not essential for the formation of sediment waves or that relatively stronger bottom currents flowed along the continental margin of Morocco in the recent past. Turbidity flows may also influence the distribution of these sediment waves.  相似文献   

4.
A sediment study suggests that Washington and Norfolk canyons off the Mid-Atlantic States are not inactive, but have served periodically since the Late Pleistocene as conduits of sediment originating on the adjacent shelf and upper slope. Large quantities of sand occur in the canyon heads as thin beds and laminae, and on the continental slope as mixtures of sand (to >40%), silt and clay that are extensively reworked by burrowing organisms. Sandy turbidites occur in the canyons on the rise. Basinward dispersal, from the outer shelf and uppermost slope, is recorded by heavy mineral suites and bioclastic components, primarily foraminifera of shallow marine origin, in the lower slope and upper continental rise canyon cores. The down-axis movement of material, presumably episodic, in the Holocene to recent results from offshelf spillover into canyon heads, failure on the steep walls bordering canyons on the slope, and resuspension by bottom currents.  相似文献   

5.
We present a series of experiments that investigate the morphology of sediment deposits within sinuous submarine channels of different sinuosity (S = 1.14–1.94) and planform (symmetric and asymmetric bends), generated by bedload-dominated turbidity current flows. Flows were generated by releasing dense saline gravity currents over a mobile sediment bed through pre-formed sinuous channels. Flows had a basal-outwards helicity and produced a characteristic bed morphology with point bars downstream of the bend apex at the inside of bends and scour at the outside of bends. An increasing loss of fluid through overspill with increasing channel sinuosity results in a decreasing magnitude of cross-stream velocity downstream, a decreasing amount of erosion and deposition, and decreasing transverse slopes of in-channel deposits. Basal fluid from within the channel is transported over the outer-levee at bends, implying that proximal outer-bend levee deposits will have similar sediment composition to that within the channel. More deposition of coarse material might be expected on levees and in overbank regions close to higher amplitude bends. No simple relationship was observed between superelevation and sinuosity, probably due to changes in the relative influences of downstream velocity and bend curvature on centrifugal force and inertial run-up. In the channel with the tightest initial bend curvature, overspill fluid from Bend 1 re-entered the channel at Bend 2, dominating flow characteristics and disrupting the basal-outwards helicity observed in the other channels. Higher sinuosity channels and those with shallow regional and levee slopes are thus more likely to have a higher proportion of anomalous flow and sedimentation patterns due to the influence of overspill fluid re-entry into the channel. The results of this investigation are combined with published observations to enable the synthesis of a new model for sedimentation in sinuous submarine channels.  相似文献   

6.
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7.
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   

8.
西南极海沉积硒的地球化学状态   总被引：1，自引：0，他引：1  
根据1984年11月—1985年4月首次南大洋考察所取得的样品和资料,通过萃取液萃取并利用荧光素法测定不同形态硒,对硒在南大洋沉积物中的地球化学状态进行了研究。结果表明,西南大洋表层沉积物的总硒含量为278－1168×（10-9）,其与粘土粒级有较密切的关系。硒含量的地理分布表现为海湾＞半深海＞外陆架;沉积物中硒含量不同,但其地球化学形态分配比基本相同,分别为可交换态占15％,有机结合态占36％,无机盐态占4％,晶质氧化物态占9％,矿物晶格态占37％;沉积物剖面中间隙水硒的含量变化与铁一致,在铁氧化物还原带达到峰值;可交换态硒的含量随深度的增加而升高,而晶质氧化物态的含量则随深度而降低;有机质的早期成岩分解是沉积硒参与再循环的主要来源;计算得出沉积柱表面逸入上覆海水的硒通量为3．5ng／（cm2·a）,沉积通量为13．4ng／（cm2·a）。  相似文献   

9.
Abstract

Eight types of reflections are interpreted from 3,800 km of 3.5 kHz profiles taken over a 25,000 km2 area of the upper continental slope and shelf in the northeastern Gulf of Mexico off Panama City, Florida. The corresponding sediments in five of the reflection types were sampled in 77 piston cores from which data were obtained on in situ acoustic velocities (V), bulk densities (gr), sediment texture (mean grain size = Mz), CaCO3 content (C), sedimentary structures, and gross sediment composition. A distinct bottom echo with numerous subbottom reflectors (Type I) is observed in deeper areas where terrigenous clay or lutite (Mgi = 9.9 to, gr = 1.4 g/cc, porosity (P) = 74 percent, C = 28 percent, and V (upper 2 m) = 1,435 m/s) predominates. Type I reflection grades upslope into Type IV, which shows a distinct bottom echo with fewer subbottom reflectors, and the corresponding sediment is a foraminiferal silty clay (mz = 9.4 to, gr = 1.43 g/cc, P = 73 percent, V = 1,447 m/s, and C = 37 percent). The uppermost slope gives indistinct, semiprolonged bottom echoes with faint subbottoms (Type VI) where calcareous silt (Mz = 6.6 to, gr = 1.57 g/cc, P = 65 percent, C = 70 percent, and V = 1,482 m/s) is the main sediment type. The shelf sediments (gr = 1.66 g/cc, P = 58 percent, V = sl1,530 m/ s), varying from coarse silt (Mz = 5.3 to) to very coarse sand (Mz = ‐0.3 to) and 25 to 100 percent carbonate, show indistinct, semiprolonged bottom echoes with intermittent or mushy subbottoms (Type VII). Prolonged echoes with no subbottoms (Type VIII) are observed in areas where algal sands of variable grain size (Mz ‐ ‐0.9 to 2.7 to, gr = 1.66 g/cc, P = 59 percent, V = 1,530 to 1,690 m/s) occur.

The major trends in reflection types (loss in depth of penetration, loss in number of reflectors, and prolongation of initial bottom reflections) follow gradients of sedimentary and physical properties of the sediments, which are increases in mean grain size, bulk density, in situ acoustic velocity, CaCO3 content, and decrease in porosity. Increases in the reflection coefficient and attenuation of the sound energy in the shelf sediments are probably important factors in the observed decrease in the depth of penetration of the sound energy in the shelf sediments.  相似文献   

10.
A section of the continental rise of northeastern South America northeast of the Orinoco delta contains physiographic features built by the interaction of southward-flowing North Atlantic Deep Water and turbidity currents generated in the Orinoco region during the last Pleistocene glacials. A sedimentary outer ridge of low relief (Demerara Outer Ridge) trends northeast along the rise and a field of westward-migrating sediment waves trending north-northwest is superimposed on the outer ridge. The sediment waves have a maximum amplitude and wavelength of 20 m and 4 km, respectively. Seismic profiler records indicate that the outer ridge was probably built during the Pleistocene. A major turbidity-current pathway adjacent to the outer ridge on the north supplied sediment to the southward-flowing North Atlantic Deep Water which then deposited this sediment down-stream on the outer ridge and formed the sediment waves. Piston cores from the outer ridge contain numerous silt—sand beds and appear to be contourites. The cores consist primarily of gray hemipelagic clay of a Late Wisconsin age and have high (>10 cm1000 yrs) sedimentation rates. In contrast, cores from the continental rise north of the turbidite channel are brown clays with relatively low sedimentation rates (3.0 cm1000 yrs) and do not contain silt—sand contourites.  相似文献   

11.
The South China Sea (SCS) shows favorable conditions for gas hydrate accumulation and exploration prospects. Bottom simulating reflectors (BSRs) are widely distributed in the SCS. Using seismic and sequence stratigraphy, the spatial distribution of BSRs has been determined in three sequences deposited since the Late Miocene. The features of gas hydrate accumulations in northern SCS were systematically analyzed by an integrated analysis of gas source conditions, migration pathways, heat flow values, occurrence characteristics, and depositional conditions (including depositional facies, rates of deposition, sand content, and lithological features) as well as some depositional bodies (structural slopes, slump blocks, and sediment waves). This research shows that particular geological controls are important for the presence of BSRs in the SCS, not so much the basic thermodynamic controls such as temperature, pressure and a gas source. Based on this, a typical depositional accumulation model has been established. This model summarizes the distribution of each depositional system in the continental shelf, continental slope, and continental rise, and also shows the typical elements of gas hydrate accumulations. BSRs appear to commonly occur more in slope-break zones, deep-water gravity flows, and contourites. The gas hydrate-bearing sediments in the Shenhu drilling area mostly contain silt or clay, with a silt content of about 70%. In the continental shelf, BSRs are laterally continuous, and the key to gas hydrate formation and accumulation lies in gas transportation and migration conditions. In the continental slope, a majority of the BSRs are associated with zones of steep and rough relief with long-term alternation of uplift and subsidence. Rapid sediment unloading can provide a favorable sedimentary reservoir for gas hydrates. In the continental rise, BSRs occur in the sediments of submarine fans, turbidity currents.  相似文献   

12.
潮流场对渤、黄、东海陆架底质分布的控制作用   总被引：10，自引：0，他引：10  
运用二维潮流数学模型，模拟了渤、黄、东海陆架的M2潮汐、潮流。结果表明，渤、黄、东海陆架的潮流有强弱之分以及往复流和旋转汉之别。在此基础上，计算了8种粒径沙的湖平均悬移输沙率、潮平均推移输沙以及相应的输沙率散度。根据输沙率散度的正负，划分了海底冲刷区与淤积区。根据不同粒径泥沙输沙率散度的相对大小，确定出海底的主要底质类型为砂质沉积、粉砂质泥沉积和以粉砂为主的混合沉积。计算结果表明，海底3种主要底负类型的分布格局与海底的冲淤格局以及与输沙率矢量的发散和聚合状况基本一致。在渤、黄、东海陆架，沙脊主要在强往复流区形成，沙席主要在强或较强的旋转流区形成，泥质沉积主要在弱潮流区形成。砂质沉积、泥质沉积以及混合沉积这3种主要底质类型并非孤立存在，而是受渤、黄、东海陆架潮流场控制而形成的一个完整的潮流沉积体系。渤、黄、东海陆架的砂质沉积与泥质沉积并非残留沉积，而是潮流沉积。在没有冷涡的情况下，黄、东海陆架的典型泥质沉积在弱潮流环境中同样可以形成，因此，认为冷涡并非黄、东海陆架典型泥质沉积形成的必要条件。  相似文献   

13.
《Journal of Sea Research》2010,63(4):276-285
A decrease in the areas covered by seagrasses within the Ria de Aveiro, Portugal, has been observed over the past five decades, resulting in a corresponding increase of the areas of uncovered sediment supporting the growth of sparse macroalgae populations only. Presently, several macroalgae (Ulva spp., Gracilaria sp.) and one seagrass species (Nanozostera noltii (Hornem.) Toml. & Posl.) comprise the submerged aquatic vegetation (SAV) adapted to this shallow, high-energy environment, characterised by fast tidal currents and turbid waters and in which large areas of the bed are exposed during low tide. This study shows that there is a strong inter-relation between the SAV and the surface sediment in intertidal areas. The sediment covered by N. noltii was finer (median grain size 95 µm) and had a high percentage of organic matter (mean value 7.6%), compared with the sediment colonised by macroalgae (median grain size 239 µm; mean organic content 3.2%). The concentrations of both total nitrogen and phosphorous were significantly greater (P < 0.001) in surface sediments covered by N. noltii. Thus, sediments within N. noltii appear to act as a large reservoir of N and P by accumulating greater concentrations of fine sediment particles (silt and clay) and organic matter when compared with the coarser sediment covered with macroalgae only. Hence, the reduction in the area covered by seagrasses will likely result in a gradual loss of nutrients and fine sediment from the Ria de Aveiro channels.  相似文献   

14.
Sand waves occur in eastern Long Island Sound with heights up to 4 m and lengths to 100 m. The waves do not form if either more than 10% mud or 12% coarse sand is present in the sediment. Mud suppresses wave formation by increasing the cohesion of the sediment. Sand-wave shape is independent of the water depth, d, provided the sand-wave height, H, is smaller than 0.86 d1.19. Both symmetric and asymmetric wave forms are present. Observation of the migration of sand waves by repeated bathymetric surveys indicates a net sand flux greater than 0.01 cm3 cm?1 sec?1 in the direction faced by the steep slopes of the waves (i.e. westward, into the Sound). Under this sand flux, waves more than 30 cm high will not be measurably altered by a reversal of the semidiurnal tidal current.  相似文献   

15.
In the Castle Creek study area, a vertically dipping, 2.5 km-thick succession of basin-floor to base-of-slope Neoproterozoic rocks are superbly exposed. In part of that outcrop, inner-bend (point-bar) deposits of sharp-based, laterally accreting sinuous channels are exposed, of which one is described in detail (Isaac Channel unit 2.2—IC2.2). IC2.2 is up to 13 m thick and extends laterally for at least 400 m. Lateral-accretion deposits, or simply lateral accretion deposits (LADs), are inclined at 7–12° toward the channel base and are about 120–140 m long. Grain size changes little obliquely upward along an individual LAD, or vertically upward through the channel-fill. LADs consist of two repeating and interstratified kinds: coarse-grained LADs consisting of strata up to granule conglomerate, and fine-grained LADs composed of thin- to medium-bedded finer-grained turbidites. In the lower part of the channel-fill, strata consist only of amalgamated coarse-grained LADs composed of decimetre-thick beds composed of very coarse sandstone/granule conglomerate that grade upward to medium sandstone. Tractional sedimentary structures are absent and fine-grained strata, specifically mudstone, occur only as isolated patches of intraclast breccia. In the upper part of the channel-fill, however, LADs consist of a rhythmic interfingering of coarse- and fine-grained LADs. Coarse-grained LADs consist of 2–3 bed-thick packages that are separated and then pinch-out rapidly into fine-grained LADs. Close to their up-dip pinch-out these coarse strata consist commonly of poorly sorted, ungraded very coarse sandstone/granule conglomerate overlain abruptly by planar-laminated or medium-scale (dune) cross-stratified, medium-grained sandstone. Fine-grained LADs are composed of mudstone interbedded with thin- and medium-bedded Tbcd and Tcd turbidites that obliquely downward and become truncated as the super- and subjacent coarse-grained LADs amalgamate.The rhythmic intercalation of coarse- and fine-grained LADs is interpreted to be related to temporal changes in the nature of sediment deposition along the point-bar of a deep-marine sinuous channel. Following failure along the cut-bank margin (outer bend), deposition of coarse-grained sediment on the point-bar (inner bend) occurred in order to re-establish an equilibrium channel geometry, and thereby equilibrium sediment transport conditions (i.e. sediment bypass). Once equilibrium was re-established deposition of finer, thinner-bedded strata of the succeeding fine LAD resumed. These strata represent deposition from the dilute tail region of flows that for the most part had already transited that particular channel bend and transported the bulk of its coarse sediment further down-dip. This history of alternating coarse and fine-grained sedimentation was repeated several times in the channel bend as it migrated laterally. Moreover, in coarse LADs, the restricted occurrence of tractional sedimentary structures close to their up-dip pinch-out suggests that although suspension deposition may have dominated over much of the lateral accretion surface, it was succeeded, at least on the upper part of the lateral accretion surface, by sediment reworking and bed-load transport, possibly related to elevated turbulent stresses caused by mixing along the sharp density interface in a strongly stratified turbulent flow.Although seemingly similar to LADs reported from fluvial point-bars, deep-marine LADs of the Windermere exhibit many important differences. Some of these differences are likely related to the differences in the mode of sand (and coarser) sediment transport in deep-marine versus non-marine environments, specifically, suspension versus bed load, respectively. In addition, fundamental differences in the flow structure between subaqueous suspension currents and open-channel flows most probably exert an additional first-order control contributing to these differences.  相似文献   

16.
Multibeam bathymetry, high (sleeve airguns) and very high resolution (parametric system-TOPAS-) seismic records were used to define the morphosedimentary features and investigate the depositional architecture of the Cantabrian continental margin. The outer shelf (down to 180–245 m water depth) displays an intensively eroded seafloor surface that truncates consolidated ancient folded and fractured deposits. Recent deposits are only locally present as lowstand shelf-margin deposits and a transparent drape with bedforms. The continental slope is affected by sedimentary processes that have combined to create the morphosedimentary features seen today. The upper (down to 2000 m water depth) and lower (down to 3700–4600 m water depth) slopes are mostly subject to different types of slope failures, such as slides, mass-transport deposits (a mix of slumping and mass-flows), and turbidity currents. The upper slope is also subject to the action of bottom currents (the Mediterranean Water — MW) that interact with the Le Danois Bank favouring the reworking of the sediment and the sculpting of a contourite system. The continental rise is a bypass region of debris flows and turbidity currents where a complex channel-lobe transition zone (CLTZ) of the Cap Ferret Fan develops.The recent architecture depositional model is complex and results from the remaining structural template and the great variability of interconnected sedimentary systems and processes. This margin can be considered as starved due to the great sediment evacuation over a relatively steep entire depositional profile. Sediment is eroded mostly from the Cantabrian and also the Pyrenees mountains (source) and transported by small stream/river mountains to the sea. It bypasses the continental shelf and when sediment arrives at the slope it is transported through a major submarine drainage system (large submarine valleys and mass-movement processes) down to the continental rise and adjacent Biscay Abyssal Plain (sink). Factors controlling this architecture are tectonism and sediment source/dispersal, which are closely interrelated, whereas sea-level changes and oceanography have played a minor role (on a long-term scale).  相似文献   

17.
A set of multi-channel seismic profiles (∼15,000 km) is used to study the depositional evolution of the Cosmonaut Sea margin of East Antarctica. We recognize a regional sediment wedge, below the upper parts of the continental rise, herein termed the Cosmonaut Sea Wedge. The wedge is situated stratigraphically below the inferred glaciomarine section and extends for at least 1,200 km along the continental margin with a width that ranges from 80 to about 250 km. The morphology of the wedge and its associated depositional features indicate a complex depositional history, where the deep marine depositional sites were influenced by both downslope and alongslope processes. This interaction resulted in the formation of several proximal depocentres, which at their distal northern end are flanked by elongated mounded drifts and contourite sheets. The internal stratification of the mounded drift deposits indicates that westward flowing bottom currents reworked the marginal deposits. The action of these currents together with sea-level changes is considered to have controlled the growth of the wedge. We interpret the Cosmonaut Sea Wedge as a composite feature comprising several bottom current reworked fan systems. The wide spectrum of depositional geometries in the stratigraphic column reflects dramatic variations in sediment supply from the continental margin as well as varying interaction between downslope and alongslope processes.  相似文献   

18.
A factor analysis of 180 bottom sediment samples from the east-central Bering Sea continental shelf identifies five factors that account for 95% of the variation in the 17 whole ø size classes that were used as variables. Factor I represents coarse sediments that have been bypassed in areas of active water circulation. Factors II and III represent fine and very fine sands that have been hydraulically sorted, reworked, and mixed. Factor IV represents coarse to medium silt that has been segregated from areas of relatively high energy. Factor V represents both the production of sediments finer than medium silt and deposition within the lowest-energy environment in this area.Modern and palimpsest sediments are areally prevalent over this section of the shelf. Relict sediments occur in only a few small areas. The dispersal of sediments is affected by surface and tidal currents as well as wave action. Ice rafting is not an important geological agent. Data from the eastcentral Bering Sea shelf indicate that sediments on subarctic continental shelves are not necessarily characterized by an abundance of rocky sediments or gravel.  相似文献   

19.
Multichannel seismic reflection profiles from the continental rise west of the Antarctic Peninsula between 63° and 69°S show the growth of eight very large mound-shaped sedimentary bodies. MCS profiles and long-range side-scan sonar (GLORIA) images show the sea floor between mounds is traversed by channels originating in a dendritic pattern near the base of the continental slope. The mounds are interpreted as sediment drifts, constructed mainly from the fine-grained components of turbidity currents originating on the continental slope, entrained in a nepheloid layer within the ambient southwesterly bottom currents and redeposited downcurrent.  相似文献   

20.
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