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1.
The Chandeleur Islands lie on the eastern side of the modern Mississippi River delta plain, near the edge of the St. Bernard Delta complex. Since abandonment approximately 2,000 years b.p., this delta complex has undergone subsidence and ravinement as the shoreline has transgressed across it. High-resolution seismic-reflection, sidescan-sonar, and bathymetry data show that seafloor erosion is influenced by locally variable shallow stratigraphy. The data reveal two general populations of shallow erosional depressions, either linear or subcircular in shape. Linear depressions occur primarily where sandy distributary-channel deposits are exposed on the seafloor. The subcircular pits are concentrated in areas where delta-front deposits crop out, and occasional seismic blanking indicates that gas is present. The difference in erosional patterns suggests that delta-front and distributary-channel deposits respond uniquely to wave and current energy expended on the inner shelf, particularly during stormy periods. Linear depressions may be the result of the sandy distributary-channel deposits eroding more readily by waves and coastal currents than the surrounding delta-front deposits. Pits may develop as gas discharge or liquefaction occurs within fine-grained delta-front deposits, causing seafloor collapse. These detailed observations suggest that ravinement of this inner shelf surface may be ongoing, is controlled by the underlying stratigraphy, and has varied morphologic expression.  相似文献   

2.
This paper examines the potential for remote classification of seafloor terrains using a combination of quantitative acoustic backscatter measurements and high resolution bathymetry derived from two classes of sonar systems currently used by the marine research community: multibeam echo-sounders and bathymetric sidescans sonar systems. The high-resolution bathymetry is important, not only to determine the topography of the area surveyed, but to provide accurate bottom slope corrections needed to convert the arrival angles of the seafloor echoes received by the sonars into true angles of incidence. An angular dependence of seafloor acoustic backscatter can then be derived for each region surveyed, making it possible to construct maps of acoustic backscattering strength in geographic coordinates over the areas of interest. Such maps, when combined with the high-resolution bathymetric maps normally compiled from the data output by the above sonar systems, could be very effective tools to quantify bottom types on a regional basis, and to develop automatic seafloor classification routines.  相似文献   

3.
Ongoing geological research into processes operating on the nearshore continental shelf and beyond is vital to our understanding of modern tsunami-generated sediment transport and deposition. This paper investigates the southern part of Sendai Bay, Japan, by means of high-resolution seismic surveys, vibracoring, bathymetric data assimilation, and radioisotope analysis of a core. For the first time, it was possible to identify an erosional surface in the shallow subsurface, formed by both seafloor erosion and associated offshore-directed sediment transport caused by the 2011 Tohoku-oki tsunami. The area of erosion and deposition extends at least 1,100 m offshore from the shoreline down to water depths of 16.7 m. The tsunami-generated sedimentological signature reaches up to 1.2 m below the present seafloor, whereas bathymetric changes due to storm-related reworking over a period of 3 years following the tsunami event have been limited to the upper ~0.3 m, despite the fact that the study area is located on an open shelf facing the Pacific Ocean. Tsunami-generated erosion surfaces may thus be preserved for extended periods of time, and may even enter the rock record, because the depth of tsunami erosion can exceed the depth of storm erosion. This finding is also important for interpretation of modern submarine strata, since erosion surfaces in shallow (depths less than ~1 m) seismic records from open coast shelves have generally been interpreted as storm-generated surfaces or transgressive ravinement surfaces.  相似文献   

4.
Seismic and bathymetric data from the Çanakkale Strait and its extensions onto the shelves of the Marmara and Aegean seas indicate that the strait was formed mainly by an erosional event. Four seismic units are observed on seismic profiles. The lower two of these (units 4 and 3) constitute the basement of a regionally widespread erosional unconformity (ravinement), which developed during marine isotope stage 2 (MIS 2). The two upper units (units 2 and 1), which overlie the ravinement surface, form a higher-order sequence. Sequence stratigraphic analysis indicates that units 2 and 1 deposited as lowstand and highstand systems tracts respectively, since the end of MIS 2. The transgressive systems tract is represented by a major erosional event which occurred throughout the Çanakkale sill area when the Mediterranean-Marmara Sea connection and, hence, the Çanakkale Strait was formed. The existence of the erosive ?arköy Canyon along the shelf edge of the southern Marmara Sea demonstrates that the flow direction causing the erosion was from south to north, thus proving that it was produced by Mediterranean water flowing over the sill into the Marmara Sea basin.  相似文献   

5.
Acoustic backscatter images of the seafloor obtained with sidescan sonar systems are displayed most often using a flat bottom assumption. Whenever this assumption is not valid, pixels are mapped incorrectly in the image frame, yielding distorted representations of the seafloor. Here, such distortions are corrected by using an appropriate representation of the relief, as measured by the sonar that collected the acoustic backscatter information. In addition, all spatial filtering operations required in the pixel relocation process take the sonar geometry into account. Examples of the process are provided by data collected in the Northeastern Pacific over Fieberling Guyot with the SeaMARC II bathymetric sidescan sonar system and the Sea Beam multibeam echo-sounder. The nearly complete (90%) Sea Beam bathymetry coverage of the Guyot serves as a reference to quantify the distortions found in the backscatter images and to evaluate the accuracy of the corrections performed with SeaMARC II bathymetry. As a byproduct, the processed SeaMARC II bathymetry and the Sea Beam bathymetry adapted to the SeaMARC II sonar geometry exhibit a 35m mean-square difference over the entire area surveyed.On leave at the Naval Research Laboratory, Code 7420, Washington D.C. 20375-5350.  相似文献   

6.
Side-scan sonar investigations in the eastern part of the macrotidal Bay of Seine have revealed the presence of numerous rippled scour depressions (RSDs) at water depths of 5–9 m. The sediments in these depressions consist essentially of coarse-grained shell hash derived from underlying Holocene sediments dated at roughly 6,500 years BP, and arranged in large wave-generated ripples. The shallow marine area where these features occur consists of a wave-generated ravinement surface produced during the marine flooding of the late Holocene transgression. It can be shown that, during the last 20 years at least, erosion of the muddy sand and sandy seabed has exposed underlying relict sediments. These consist of stiff clays, silts and a layer of shell debris which, when exposed, cover the bottom of large scour depressions which appear to be in equilibrium with the local hydrodynamic regime. Morphological and hydrodynamic data suggest that the RSDs are generated by strong cross-shore bottom currents flowing parallel to the features in the direction of the prevailing waves, and probably associated with storm-induced downwelling events.  相似文献   

7.
基于高分辨率的单道地震和多波束测深数据，识别并对比了东海陆架中部同一海区相距20余万年的层U14和层U2两期沙脊群，其中层U14期沙脊属于埋藏沙脊，位于东海海底以下90 m深处，推测属于距今320~200 ka的海侵体系域(TST)，沙脊顶界面是该期海侵的最大洪泛面(MFS)；层U2期沙脊位于东海陆架，属于衰退沙脊，系末次盛冰期(LGM)以来的TST，顶界面是LGM以来的MFS。尽管两期沙脊形成年代相距20余万年，地层层位相距近90 m，但是沙脊群总体走向一致，表明距今2×105 a以来东海陆架潮波基本格局稳定。从层U2期可识别出4个亚期沙脊，通过多波束海底地形图可识别出4组走向的沙脊，多亚期、多走向沙脊是LGM以来海平面阶梯状波动在海底地形演变过程中的响应证据。  相似文献   

8.
东海陆架两期沙脊的时空对比   总被引：3，自引：2，他引：1       下载免费PDF全文
基于高分辨率的单道地震和多波束测深数据,识别并对比了东海陆架中部同一海区相距20余万年的层U14和层U2两期沙脊群,其中层U14期沙脊属于埋藏沙脊,位于东海海底以下90m深处,推测属于距今320~200ka的海侵体系域(TST),沙脊顶界面是该期海侵的最大洪泛面(MFS);层U2期沙脊位于东海陆架,属于衰退沙脊,系末次盛冰期(LGM)以来的TST,顶界面是LGM以来的MFS。尽管两期沙脊形成年代相距20余万年,地层层位相距近90m,但是沙脊群总体走向一致,表明距今2×105/a以来东海陆架潮波基本格局稳定。从层U2期可识别出4个亚期沙脊,通过多波束海底地形图可识别出4组走向的沙脊,多亚期、多走向沙脊是LGM以来海平面阶梯状波动在海底地形演变过程中的响应证据。  相似文献   

9.
Seafloor geomorphology and surficial stratigraphy of the New Jersey middle continental shelf provide a detailed record of sea-level change during the last advance and retreat of the Laurentide ice sheet (120 kyr B.P. to Present). A NW–SE-oriented corridor on the middle shelf between water depths of 40 m (the mid-shelf “paleo-shore”) and 100 m (the Franklin “paleo-shore”) encompasses 500 line-km of 2D Huntec boomer profiles (500–3500 Hz), an embedded 4.6 km2 3D volume, and a 490 km2 swath bathymetry map. We use these data to develop a relative stratigraphy. Core samples from published studies also provide some chronological and sedimentological constraints on the upper <5 m of the stratigraphic succession.The following stratigraphic units and surfaces occur (from bottom to top): (1) “R”, a high-amplitude reflection that separates sediment >46.5 kyr old (by AMS 14C dating) from overlying sediment wedges; (2) the outer shelf wedge, a marine unit up to 50 m thick that onlaps “R”; (3) “Channels”, a reflection sub-parallel to the seafloor that incises “R”, and appears as a dendritic system of channels in map view; (4) “Channels” fill, the upper portion of which is sampled and known to represent deepening-upward marine sediments 12.3 kyr in age; (5) the “T” horizon, a seismically discontinuous surface that caps “Channels” fill; (6) oblique ridge deposits, coarse-grained shelly units comprised of km-scale, shallow shelf bedforms; and (7) ribbon-floored swales, bathymetric depressions parallel to modern shelf currents that truncate the oblique ridges and cut into surficial deposits.We interpret this succession of features in light of a global eustatic sea-level curve and the consequent migration of the coastline across the middle shelf during the last 120 kyr. The morphology of the New Jersey middle shelf shows a discrete sequence of stratigraphic elements, and reflects the pulsed episodicity of the last sea-level cycle. “R” is a complicated marine/non-marine erosional surface formed during the last regression, while the outer shelf wedge represents a shelf wedge emplaced during a minor glacial retreat before maximum Wisconsin lowstand (i.e., marine oxygen isotope stage 3.1). “Channels” is a widespread fluvial subarial erosion surface formed at the late Wisconsin glacial maximum 22 kyr B.P. The shoreline migrated back across the mid-shelf corridor non-uniformly during the period represented by “Channels” fill. Oblique ridges are relict features on the New Jersey middle shelf, while the ribbon-floored swales represent modern shelf erosion. There is no systematic relationship between modern seafloor morphology and the very shallowly buried stratigraphic succession.  相似文献   

10.
During RV SONNE cruise SO-79 to the eastern Pacific Ocean, two areas of about 65×80 km in the northern Peru Basin were surveyed with the acoustic mapping systems HYDROSWEEP (bathymetry), PARASOUND (3.5 kHz high-resolution seismic system), and a deep-towed side-scan sonar system. In addition, we sampled sediments using piston and box corers. The data show an unexpected variability of seafloor features: The bathymetry is characterized by an abyssal hill topography with predominately N-S ridges up to 300 m high, and scattered volcanic hills. Moreover, one 2000-m-high seamount was mapped. PARASOUND shows several distinct reflectors within the sediment cover, all of which are attributed to carbonate-rich strata. In the northern area, the uppermost prominent reflector is related to the Mid-Brunhes Event (0.45 Ma) in the sediment cores, while the lowermost represents acoustic basement. In the southern area, the seismic pattern reveals an upper opaque zone and a lower transparent zone. The base of the opaque zone is marked by a distinct reflector which corresponds to a huge carbonate peak (6–7 Ma) in the sediment cores. However, despite this general pattern, the PARASOUND records show a highly variable situation, with the distribution of sediment echo types strongly influenced by the seafloor topography. The side-scan sonar revealed the existence of numerous small volcanic cones up to 25 m high and nearly free of sediment. Additionally, the sonar records show a patchy (up to 800 m across) seafloor reflectiviti. We interpret this patchiness as a local lack of manganese nodule coverage. Volcanic cones and the most distinct nodule-free patches are usually on ridges. We interpret this variability as caused by winnowing and erosion, an interpretation that is supported by the occurrence of outcrops of Tertiary strata. This regional small-scale variability argues for a highly dynamic depositional history of the Peru Basin.  相似文献   

11.
长江三角洲地区晚第四纪地层及潜在环境问题   总被引：12，自引：0，他引：12  
根据钻孔的沉积相序将长江三角洲分为三角洲主体、三角洲南翼和北翼等３个基本的地层分区,鉴于两翼前缘和后缘的相序具有很大差异,因而两翼均划分出前缘和后缘２个亚区。地层结构的不同决定了与沉积层有关的潜在环境问题和灾害的分布,三角洲南翼和后缘为地面沉降发育区和潜在发育区,北翼的前缘为地下海水入侵的潜在发育区,三角洲主体是污染江水的潜在渗滤区,现今河口为底辟构造潜在发育区。  相似文献   

12.
依据地质钻探和海滩观测资料，分析了后江湾海岸在海进作用下，海岸形成海进地层层序。滨面遭受侵蚀并正在后退和变陡。晚更新统陆相杂色粘土层和砾砂层直接暴露于海底。在海域供沙不足的情况下，整个海湾的海滩被侵蚀后退，而海滩各岸段侵蚀程度存在差异。  相似文献   

13.
The EM12 multibeam echosounder can record acoustic backscatter information as well as high resolution bathymetry. The dataset presented, from the axis of the Mid-Atlantic Ridge at 45° N, was the first EM12 survey of a mid-ocean ridge. This paper presents methods for utilising the backscatter information. Data processing enables the production of a mosaic of acoustic backscatter, and visualisation techniques are investigated to provide initial qualitative views of the combined backscatter and bathymetry datasets. The co-registration of the backscatter and bathymetry data enables quantitative analysis of their relationships. Various sites of different geological type have been selected and their angular acoustic backscattering relationships estimated, including the effect on backscatter of incidence angle, its regional variability with bottom type and the influence of bottom slope. Incidence angles and bottom type are shown to affect backscatter to a similar degree, while slopes appear to contribute little. The geometry of hull-mounted systems, such as the EM12, is significantly different from that of conventional sidescan sonars, such as GLORIA, and the backscatter images from the two types differ in various respects. Because of the wide variations in incidence angle that are common with hull-mounted systems, and the importance of incidence angle in determining backscatter strength, it is vital to consider the effect of incidence angle during interpretation.  相似文献   

14.
High-resolution seismic reflection profiles and multibeam bathymetry data collected in 2006 and 2008 around Pantelleria Island show the widespread occurrence of contourite drifts and erosional elements ~30?km from the narrowest part (~145?km) of the Sicily Channel, where water masses from the Eastern Mediterranean flow towards the Western Mediterranean. The contourite drifts are rather small (up to 10?km long and 3.3?km wide), at water depths of ~250?C750?m. Most are elongated separated drifts with quite well-developed moats and crests, aligned roughly parallel to the regional bathymetric contours. Erosional elements include abraded surfaces, moats, scours and sub-circular depressions. In addition, a wide sector of the seafloor adjacent to a seamount located SW of Pantelleria Island is characterized by numerous biogenic build-ups colonized by deep-water corals (Madrepora oculata). The spatial distribution of sediment drifts, erosional features and biogenic build-ups suggests an origin from a north-westward-flowing bottom current, in this case the outflow of Levantine Intermediate Water and transitional Eastern Mediterranean Deep Water via the Sicily Channel. These findings for the Pantelleria offshore sector demonstrate that contourite processes are able to concentrate a high variety of closely spaced depositional and erosional features even in small areas (in this case, about 2,000?km2). This Pantelleria focusing can plausibly be related to a particular configuration of the prevailing bottom-current regime in complex interaction with an uneven bathymetry shaped mainly by tectonic and volcanic activity. The distribution of bottom currents seems to be strongly influenced by morphological features ranging from major seabed obstacles, such as the Pantelleria volcanic complex and the so-called southwest seamount, to smaller-scale escarpments and banks. This is consistent with previous findings for Mediterranean and other settings characterized by neotectonics and large topographic features.  相似文献   

15.
Two highly active mud volcanoes located in 990–1,265 m water depths were mapped on the northern Egyptian continental slope during the BIONIL expedition of R/V Meteor in October 2006. High-resolution swath bathymetry and backscatter imagery were acquired with an autonomous underwater vehicle (AUV)-mounted multibeam echosounder, operating at a frequency of 200 kHz. Data allowed for the construction of ~1 m pixel bathymetry and backscatter maps. The newly produced maps provide details of the seabed morphology and texture, and insights into the formation of the two mud volcanoes. They also contain key indicators on the distribution of seepage and its tectonic control. The acquisition of high-resolution seafloor bathymetry and acoustic imagery maps with an AUV-mounted multibeam echosounder fills the gap in spatial scale between conventional multibeam data collected from a surface vessel and in situ video observations made from a manned submersible or a remotely operating vehicle.  相似文献   

16.
Multibeam echosounders (MBES) have become a widely used acoustic remote sensing tool to map and study the seafloor, providing co-located bathymetry and seafloor backscatter. Although the uncertainty associated with MBES-derived bathymetric data has been studied extensively, the question of backscatter uncertainty has been addressed only minimally and hinders the quantitative use of MBES seafloor backscatter. This paper explores approaches to identifying uncertainty sources associated with MBES-derived backscatter measurements. The major sources of uncertainty are catalogued and the magnitudes of their relative contributions to the backscatter uncertainty budget are evaluated. These major uncertainty sources include seafloor insonified area (1–3 dB), absorption coefficient (up to >?6 dB), random fluctuations in echo level (5.5 dB for a Rayleigh distribution), and sonar calibration (device dependent). The magnitudes of these uncertainty sources vary based on how these effects are compensated for during data acquisition and processing. Various cases (no compensation, partial compensation and full compensation) for seafloor insonified area, transmission losses and random fluctuations were modeled to estimate their uncertainties in different scenarios. Uncertainty related to the seafloor insonified area can be reduced significantly by accounting for seafloor slope during backscatter processing while transmission losses can be constrained by collecting full water column absorption coefficient profiles (temperature and salinity profiles). To reduce random fluctuations to below 1 dB, at least 20 samples are recommended to be used while computing mean values. The estimation of uncertainty in backscatter measurements is constrained by the fact that not all instrumental components are characterized and documented sufficiently for commercially available MBES. Further involvement from manufacturers in providing this essential information is critically required.  相似文献   

17.
以多波束精确的水深数据为参照源,采用原始回波时间对多波束测深数据与其同源声纳数据进行匹配,从而获得高精度和高分辨率的海底影像数据,并避免了传统声纳图像处理过程中斜距改正所带来的几何形变。匹配结果采用光照图输出,并与三维水深图、原始声纳图像和CARIS处理后的声纳图像进行比较分析。该方法有效地提高了多波束数据的利用率,增强了对海底地形的探测分辨率。  相似文献   

18.
Based on multi-beam echo soundings and high-resolution single-channel seismic profiles, linear sand ridges in U14 and U2 on the East China Sea (ECS) shelf are identified and compared in detail. Linear sand ridges in U14 are buried sand ridges, which are 90 m below the seafloor. It is presumed that these buried sand ridges belong to the transgressive systems tract (TST) formed 320–200 ka ago and that their top interface is the maximal flooding surface (MFS). Linear sand ridges in U2 are regressive sand ridges. It is presumed that these buried sand ridges belong to the TST of the last glacial maximum (LGM) and that their top interface is the MFS of the LGM. Four sub-stage sand ridges of U2 are discerned from the high-resolution single-channel seismic profile and four strikes of regressive sand ridges are distinguished from the submarine topographic map based on the multi-beam echo soundings. These multi-stage and multi-strike linear sand ridges are the response of, and evidence for, the evolution of submarine topography with respect to sea-level fluctuations since the LGM. Although the difference in the age of formation between U14 and U2 is 200 ka and their sequences are 90 m apart, the general strikes of the sand ridges are similar. This indicates that the basic configuration of tidal waves on the ECS shelf has been stable for the last 200 ka. A basic evolutionary model of the strata of the ECS shelf is proposed, in which sea-level change is the controlling factor. During the sea-level change of about 100 ka, five to six strata are developed and the sand ridges develop in the TST. A similar story of the evolution of paleo-topography on the ECS shelf has been repeated during the last 300 ka.  相似文献   

19.
The innermost shelf off Sarasota, Florida was mapped using sidescan-sonar imagery, seismic-reflection profiles, surface sediment samples, and short cores to define the transition between an onshore siliciclastic sand province and an offshore carbonate province and to identify the processes controlling the distribution of these distinctive facies. The transition between these facies is abrupt and closely tied to the morphology of the inner shelf. A series of low-relief nearly shore-normal ridges characterize the inner shelf. Stratigraphically, the ridges are separated from the underlying Pleistocene and Tertiary carbonate strata by the Holocene ravinement surface. While surficial sediment is fine to very-fine siliciclastic sand on the southeastern sides of the ridges and shell hash covers their northwestern sides, the cores of these Holocene deposits are a mixture of both of these facies. Along the southeastern edges of the ridges the facies boundary coincides with the discontinuity that separates the ridge deposits from the underlying strata. The transition from siliciclastic to carbonate sediment on the northwestern sides of the ridges is equally abrupt, but it falls along the crests of the ridges rather than at their edges. Here the facies transition lies within the Holocene deposit, and appears to be the result of sediment reworking by modern processes. This facies distribution primarily appears to result from south-flowing currents generated during winter storms that winnow the fine siliciclastic sediment from the troughs and steeper northwestern sides of the ridges. A coarse shell lag is left armoring the steeper northwestern sides of the ridges, and the fine sediment is deposited on the gentler southeastern sides of the ridges. This pronounced partitioning of the surficial sediment appears to be the result of the siliciclastic sand being winnowed and transported by these currents while the carbonate shell hash falls below the threshold of sediment movement and is left as a lag. The resulting facies boundaries on this low-energy, sediment-starved inner continental shelf are of two origins which both are tied to the remarkably subtle ridge morphology. Along the southeastern sides of the ridges the facies boundary coincides with a stratigraphic discontinuity that separates Holocene from the older deposits while the transition along the northwestern sides of the ridges is within the Holocene deposit and is the result of sediment redistribution by modern processes.  相似文献   

20.
除平滑海底外，长江口主航道水下底床上还发育有沙波，沙纹、沙斑及形态规则的小型冲蚀凹坑、冲沟和疏浚沟痕等微地貌形态，基本上沿岸呈带状分布。根据声波反射原理，将航道内３０ｍ厚的地层划分为两层，并据此对长江口的现代沉积环境作了初步分析。  相似文献   

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