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1.
Unusual late deglacial/early interglacial sedimentation rates following the last two glaciations have been detected from three cores on the eastern Reykjanes Ridge. Morphologic, textural, and mineralogic evidence pinpoint bottom currents as the process responsible for these unusual depositional rates, which rise at first to 10–40 times the regional mean, and then subside slightly to values still 2–5 times the norm. A portion of the lutite excesses in the interglacial sediment of these cores is accounted for by input of fine detritus eroded from subaerially exposed and isostatically uplifted Icelandic basalts. For the most part, however, the extraordinary depositional rates late in the deglacial periods are caused by erosion and redistribution of previously deposited glacialage sediments. We infer that reinitiation of overflow of deep water into the Atlantic due to deglacial removal of the Norwegian Sea ice cover triggered a strong bottom-current flow that dislodged sediment from exposed sites. The deglacial intensification of the deep circulation thus left a major sedimentological imprint on the sediments of the eastern Reykjanes Ridge. 相似文献
2.
Martyn S. Stoker Daniel Praeg Berit Oline Hjelstuen Jan Sverre Laberg Tove Nielsen Pat M. Shannon 《Marine and Petroleum Geology》2005,22(9-10):977
A regional correlation of Neogene stratigraphy has been attempted along and across the NW European Atlantic continental margin, between Mid-Norway and SW Ireland. Two unconformity-bounded successions are recognised. These are referred to as the lower and upper Neogene successions, and have been dated as Miocene–early Pliocene and early Pliocene–Holocene, respectively, in age. Their development is interpreted to reflect plate-wide, tectonically driven changes in the sedimentary, oceanographic and latterly climatic evolution of the NE Atlantic region. The lower Neogene succession mainly preserves a record of deep-water sedimentation that indicates an expansion of contourite sediment drifts above submarine unconformities, within this succession, on both sides of the eastern Greenland–Scotland Ridge from the mid-Miocene. This is interpreted to record enhanced deep-water exchange through the Faroe Conduit (deepest part of the Southern Gateway), and can be linked to compressive inversion of the Wyville–Thomson Ridge Complex. Thus, a pervasive, interconnected Arctic–North Atlantic deep-water circulation system is a Neogene phenomenon. The upper Neogene succession records a regional change, at about 4 Ma, in the patterns of contourite sedimentation (submarine erosion, new depocentres) coeval with the onset of rapid seaward-progradation of the continental margin by up to 100 km. This build-out of the shelf and slope is inferred to record a marked increase in sediment supply in response to uplift and tilting of the continental margin. Associated changes in deep-water circulation may be part of an Atlantic-wide reorganisation of ocean bottom currents. Glacial sediments form a major component of the prograding shelf margin (shelf-slope) sediment wedges, but stratigraphic data indicate that the onset of progradation pre-dates significant high-latitude glaciation by at least 1 Ma, and expansive Northern Hemisphere glaciation by at least 3 Ma. 相似文献
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4.
Rockall Trough may represent the oldest part of the Atlantic Ocean north of latitude 51°N. Stratigraphical evaluation of this hypothesis has been hindered by the lack of dateable samples of the Trough's sedimentary succession. A recent programme of dredging, aimed at sampling the sediment prism along the Trough's upper continental slope, yielded five hauls of in-situ carbonate rocks. Microfaunal analysis of these rocks suggests that the deposition of the marginal sediment prism spans the Late Cretaceous—Recent interval, tending to confirm that Rockall Trough was already in being in Late Cretaceous times. 相似文献
5.
Kristin C. Burkholder M. Susan Lozier 《Deep Sea Research Part I: Oceanographic Research Papers》2011,58(12):1196-1204
The role of Mediterranean Overflow Water (MOW) in creating subsurface salinity anomalies within the Rockall Trough, a gateway to high latitude areas of deep convection, has been examined closely in recent years. Eulerian investigations of high latitude property fields have suggested that these subsurface anomalies are likely the result of variability in the zonal extent of the eastern limb of the subpolar gyre: when expanded into the eastern North Atlantic, the gyre is presumed to limit the extent to which MOW is able to penetrate northward to subpolar latitudes. However, though the depth of the subsurface salinity anomalies in the Rockall Trough supports the hypothesis that the intermittent presence of MOW is involved in creating the anomalies, MOW pathways to the Rockall Trough have not yet been established. Here, Lagrangian trajectories from floats released in the eastern North Atlantic between 1996 and 1997 and synthetic trajectories launched within an eddy-resolving ocean general circulation model are used to demonstrate that two main density neutral transport pathways lead to the Rockall Trough. One pathway involves the transport of relatively fresh waters as part of the North Atlantic Current and the other involves the transport of relatively salty waters from the eastern reaches of the subtropical North Atlantic. The results from this study indicate that changes in these pathways over time can explain the subsurface salinity variability in the Rockall Trough. 相似文献
6.
J. Makris A. Ginzburg P.M. Shannon A.W.B. Jacob C.J. Bean U. Vogt 《Marine and Petroleum Geology》1991,8(4)
A 700 km wide-angle reflection/refraction profile carried out in the central North Atlantic west of Ireland crossed the Erris Trough, Rockall Trough and Rockall Bank, and terminated in the western Hatton-Rockall Basin. The results reveal the presence of a number of sedimentary basins separated by basement highs. The Rockall Trough, with a sedimentary pile up to 5 km thick, is underlain by thinned continental crust 8–10 km thick. Some major fault block structures are identified, especially on the eastern margin of the Rockall Trough and in the adjacent Erris Trough. The Hatton-Rockall Basin is underlain by westward-thinning continental crust 22–10 km thick. Sedimentary strata are up to 5 km thick. The strata in the Rockall Trough and Hatton-Rockall Basin probably range in age from Late Palaeozoic to Cenozoic. However, the basins have different sedimentation histories and differ in structural style. The geometry of the crust and sediments suggests that the Rockall Trough originated by pure shear crustal stretching, associated with rift deposits and Cenozoic thermal sag strata. In contrast, the development of the Erris Trough, located on unthinned continental crust, was facilitated by shallow, brittle extension with little deep crustal attenuation. A two-layered crust occurs throughout the region. The lower crustal velocity in the Hatton-Rockall Basin is higher than that in the Rockall Trough. The velocity structure shows no indication of crustal underplating by upper mantle material in the region. 相似文献
7.
Gavin M. Elliott Patrick M. Shannon Peter D.W. HaughtonLena K. Øvrebø 《Marine and Petroleum Geology》2010
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
8.
The northwest Hatton Bank margin is an ideal locality to demonstrate the interaction between bottom currents and slope configuration in controlling the distribution and morphology of bottom current deposits. The slope area investigated is isolated from any major terrigenous sediment supply and at present is influenced by the Deep Northern Boundary Current (DNBC). Swath bathymetry and high resolution acoustic data allow us to evaluate both local and regional controls on slope sedimentation and the possible mechanisms for bottom-current velocity variability across a slope setting within the NW European continental margin. The slope exhibits sculpting by bottom currents that flow in a predominantly southwest to northeast direction, and is only locally modified by slope failures. Positive relief features such as the Endymion Spur play an important role in constraining and accelerating bottom-current flow and, consequently, in redistributing sediment along the margin. We demonstrate that the size, morphology and distribution of bottom-current deposits along the slope vary as a function of the interaction between bottom currents, regional slope orientation and local seafloor topography. 相似文献
9.
通过对东海外陆架-陆坡-冲绳海槽表层沉积物粒度和黏土矿物组成的分析, 探讨了3个不同沉积单元底质沉积物成因及物质来源。结果表明, 东海外陆架→陆坡→冲绳海槽, 沉积物粒度整体上逐渐变细。东海外陆架沉积物具有明显海滩砂或者破浪带砂的特征, 为晚更新世冰消期海侵的产物, 属于“残留沉积”。海槽西坡沉积物粒度参数具有重力流沉积特征, 反映了沉积物由外陆架向海槽以滑塌或重力流的搬运方式。海槽槽底细颗粒沉积物悬移组分占主导的特点体现了半深海沉积特征, 其主要来源于台湾和中国大陆长江、闽江等入海河流。物质来源一方面反映了黑潮对于细颗粒台湾源物质的输运的巨大作用; 另一方面体现了来源于中国大陆源细颗粒物质存在以近底侧向搬运、“冬贮夏输”悬浮搬运等多种输运机制。 相似文献
10.
A regional study of the Eocene succession in the UK sector of the Rockall Plateau has yielded new insights into the early opening history of the NE Atlantic continental margin. Data acquired from British Geological Survey borehole 94/3, on the Rockall High, provides a high-resolution record of post-rift, Early to Mid-Eocene, subaqueous fan-delta development and sporadic volcanic activity, represented by pillow lavas, tuffs and subaerial lavas. This sequence correlates with the East Rockall Wedge, which is one of several prograding sediment wedges identified across the Rockall Plateau whose development was largely terminated in the mid-Lutetian. Linking the biostratigraphical data with the magnetic anomaly pattern in the adjacent ocean basin indicates that this switch-off in fan-delta sedimentation and volcanism was coincident with the change from a segmented/transform margin to a continuously spreading margin during chron C21. However, late-stage easterly prograding sediment wedges developed on the Hatton High during late Mid- to Late Eocene times; these can only have been sourced from the Hatton High, which was developing as an anticline during this interval. This deformation occurred in response to Mid- to Late Eocene compression along the ocean margin, possibly associated with the reorganisation to oblique spreading in the Iceland Basin, which culminated at the end of the Eocene with the formation of the North Hatton Anticline, and the deformation (tilting) of these wedges. A series of intra-Eocene unconformities, of which the mid-Lutetian unconformity is the best example, has been traced from the Rockall Plateau to the Faroe-Shetland region and onto the Greenland conjugate margin bordering the early ocean basin. Whilst there appears to be some correlation with 3rd order changes in eustatic sea level, it is clear from this study that tectonomagmatic processes related to changes in spreading directions between Greenland and Eurasia, and/or mantle thermal perturbations cannot be discounted. 相似文献
11.
Quaternary sediments at the southwest end of the Faeroe-Shetland Channel are preserved as a basin plain succession of variable fill geometry and lithology. In high-resolution seismic profiles the basin plain succession is characterised by laterally discontinuous and transparent, mounded lensoid bodies interbedded with acoustically well-layered sediments which display drape and onlapping reflection configurations. The lensoid bodies comprise an up to 50 m thick amalgamated package of mass-flow deposits consisting primarily of debris-flow diamictons. They represent resedimented glacigenic deposits derived from the West Shetland Shelf. Accumulation of these packages was episodic and related to specific rapid phases of downslope resedimentation, most probably concomitant with ice-marginal deposition on the West Shetland Slope. The acoustically well-layered sediments include glaciomarine hemipelagites and contourites. These indicate phases of reduced sediment supply from the adjacent shelf and slope areas, and probably represent the more pervasive “background” sedimentation in the basin. Although weak bottom-current activity may have prevailed throughout the glacial episodes, the onset of vigorous bottom-current circulation occurred at the changeover from a glacial to an interglacial regime. The debris flow packages form about 50% of the basin-plain sediments in this part of the Faeroe-Shetland Channel, thereby forming a significant component of the deep-water succession. 相似文献
12.
对南海西沙群岛附近陆坡、南海南部陆坡以及东沙群岛附近陆坡晚第四纪磁化率变化特征进行了综合对比,并初步分析了磁化率与碳酸钙百分含量之间的关系。就磁化率变化特征而言,西沙群岛附近陆坡和南海南部陆坡可归为一个单元,东沙群岛附近陆坡划为另一单元,其中,前一单元晚第四纪磁化率曲线的共性表现为深海氧同位素期次的奇数期向偶数期过渡时期以磁化率峰值为特点,偶数期向奇数期过渡时期以磁化率谷值为特点;后一单元晚第四纪磁化率曲线的共性表现为磁化率曲线大致平行于有孔虫氧同位素曲线。上述两个单元内的磁化率曲线共性都具有地层学意义,但这两个单元内部某些部位的晚第四纪磁化率变化具有地方局限性,不宜当作大范围地层对比的工具。南海陆坡磁化率曲线和碳酸钙百分含量曲线的关系存在镜像和平行两大基本类型,但这两种对应关系未必意味着磁化率和碳酸钙百分含量之间存在着直接的因果联系。南海陆坡磁化率变化的原因很可能与冰期旋回中南海陆源物质通量的变化密切相关,另外,局部海域磁化率还可能严重受沉积物早期成岩作用的影响。 相似文献
13.
Near-seabed (<50 m) sediments were studied throughout the Irish sector of the Rockall Trough (ca. 123,000 km2) based on a combined analysis of shallow seismic (3.5 kHz) and multibeam swath data acquired by the Irish National Seabed Survey and reprocessed here at higher resolution. The detailed identification of seven acoustic facies served to classify the Rockall Trough into six main sedimentary provinces, incorporating the well-known Feni Drift, Donegal-Barra Fan and Rockall Bank mass flow. In the northern part of the study area, extensive mass transport deposits from both flanks of the trough are the dominant depositional features. Debris flow deposits formed by ice streaming of the British-Irish ice sheet characterise most of the Donegal-Barra Fan, whereas turbidite deposition occurs towards the toe of the fan. On the western margin of the trough, the post-glacial Rockall Bank mass flow deposit displays a rough topography with no acoustic penetration. Several failure scarps are visible on the flank of the bank where the mass flow originated, and pass downslope into large sediment lobes and smaller debris flow deposits. Smaller-scale mass transport deposits were also discovered close to some canyons indenting the eastern slope. High seismic penetration characterises the Feni contourite drift deposit, and precise mapping of its geographical extent shows that it is considerably wider than previously reported. The sediment waves that drape this drift are interpreted as predominantly relict features, and their varied geometry suggests a complex oceanographic regime. In the deeper part of the trough, the data reveal novel evidence of the widespread occurrence (about 12,000 km2) of distinct seismic and backscatter signatures indicating the possible presence of fluid escape structures within fine-grained sediments of mixed contouritic, hemipelagic and turbiditic origin. Sediment overloading and increased pore pressure resulting from extensive mass wasting to the north of the area is a likely cause of dewatering-rooted fluid migration towards the seabed, but further investigations are required to confirm the nature and origin of such fluids in the Rockall Trough. 相似文献
14.
Jenny E. Ullgren Martin White 《Deep Sea Research Part I: Oceanographic Research Papers》2010,57(2):248-257
Mooring time series, CTD data, and Argo float data were analysed to investigate some aspects of the circulation at the southern entrance to the Rockall Trough during 2003–2004. The in situ data are used to describe the distribution at intermediate levels of Sub-Arctic Intermediate Water (SAIW) and Mediterranean Water (MW), as well as the temporal variability in the presence of these two water masses. Salty, MW-influenced water was found in the southeastern part of the study area, near the continental slope as far north as Porcupine Bank, consistent with earlier reports. Apart from the main tongue, the distribution of MW is patchy, and MW parcels were found not only adjacent to the slope but also offshore. Further north and west, water at intermediate depths was influenced by the fresher SAIW. Unlike in some earlier studies, SAIW did not extend as far east as the continental shelf. The year-long hydrographic and current time series from a mooring on the western slope of the entrance, at the southern end of Feni Ridge, showed pulses of SAIW influenced water throughout winter and spring. In late spring, the fresh pulses almost completely ceased; throughout summer only a few weak and shortlived fresh anomalies appeared. The weakening of the SAIW signal did not seem to be caused by winter convection, which did not extend to sufficient depth that winter. The relatively weak SAIW presence during most of the study period may be linked to the near neutral state of the NAO index. The warm and saline conditions observed at the southern entrance to Rockall Trough were in agreement with the rising temperatures and salinities found over large parts of the subpolar North Atlantic in the late 1990s and early 2000s. 相似文献
15.
Seismic analyses of Cenozoic contourite drift development in the Northern Norwegian Sea 总被引:1,自引:0,他引:1
Laberg Jan Sverre Dahlgren Torbjørn Vorren Tore O. Haflidason Haflidi Bryn Petter 《Marine Geophysical Researches》2001,22(5-6):401-416
Four drift accumulations have been identified on the continental margin of northern Norway; the Lofoten Drift, the Vesterålen Drift, the Nyk Drift and the Sklinnadjupet Drift. Based on seismic character these drifts were found to belong to two main groups; (1) mounded, elongated, upslope accretion drifts (Lofoten Drift, Vesterålen Drift and Nyk Drift), and (2) infilling drifts (Sklinnadjupet Drift). The drifts are located on the continental slope. Mainly surface and intermediate water circulation, contrary to many North Atlantic and Antarctic drifts that are related to bottom water circulation, and sediment availability have controlled their growth. Sediments were derived both from winnowing of the shelf and upper slope and from ice sheets when present on the shelf. The main source area was the Vøring margin. This explains the high maximum average sedimentation rate of the nearby Nyk (1.2 m/ka) and Sklinnadjupet (0.5 m/ka) Drifts compared with the distal Lofoten (0.036 m/ka) and Vesterålen (0.060 m/ka) Drifts. The high sedimentation rate of the Nyk Drift, deposited during the period between the late Saalian and the late Weichselian is of the same order of magnitude as previously reported for glacigenic slope sediments deposited during glacial maximum periods only. The Sklinnadjupet Drift is infilling a paleo-slide scar. The development of the infilling drift was possible due to the available accommodation space, a slide scar acting as a sediment trap. Based on the formation of diapirs originating from the Sklinnadjupet Drift sediments we infer these sediments to have a muddy composition with relatively high water content and low density, more easily liquefied and mobilised compared with the glacigenic diamictons. 相似文献
16.
The Adare Trough, located 100 km NE of Cape Adare, Antarctica, is the extinct third arm of a Tertiary spreading ridge that separated East from West Antarctica. We use seismic reflection data, tied to DSDP Site 274, to link our seismic stratigraphic interpretation to changes in ocean-bottom currents, Ross Sea ice cover, and regional tectonics through time. Two extended unconformities are observed in the seismic profiles. We suggest that the earliest hiatus (early Oligocene to Mid-Miocene) is related to low sediment supply from the adjacent Ross Shelf, comprised of small, isolated basins. The later hiatus (mid-Miocene to late Miocene) is likely caused by strong bottom currents sourced from the open-marine Ross Sea due to increased Antarctic glaciation induced by mid-Miocene cooling (from Mi-3). Further global cooling during the Pliocene, causing changes in global ocean circulation patterns, correlates with Adare Basin sediments and indicate the continuing but weakened influence of bottom currents. The contourite/turbidite pattern present in the Adare Trough seismic data is consistent with the 3-phase contourite growth system proposed for the Weddell Sea and Antarctic Peninsula. Multibeam bathymetry and seismic reflection profiles show ubiquitous volcanic cones and intrusions throughout the Adare Basin that we interpret to have formed from the Oligocene to the present. Seismic reflection profiles reveal trans-tensional/strike-slip faults that indicate oblique extension dominated Adare Trough tectonics at 32–15 Ma. Observed volcanism patterns and anomalously shallow basement depth in the Adare Trough area are most likely caused by mantle upwelling, an explanation supported by mantle density reconstructions, which show anomalously hot mantle beneath the Adare Trough area forming in the Late Tertiary. 相似文献
17.
Continental slope sedimentation adjacent to an ice margin I. Seismic facies of Labrador Slope 总被引:1,自引:0,他引:1
Reinhard Hesse 《Geo-Marine Letters》1992,12(4):189-199
Sleeve-gun, 3.5-kHz, and 12-kHz profiles from the Labrador Slope provide the basis for an analysis of sedimentary facies, processes, and evolution of a continental slope adjacent to an ice margin. The upper slope is deeply incised by numerous canyons reflecting headward canyon branching. The less rugged middle-slope topography has fewer canyons and large slide and slump scars followed downslope by debris-flow deposits. Echo character of seismic profiles reflects the difference in sediment types supplied from mud-dominated sources and sand-, gravel- and till-dominated sources. On the rise, debris-flow deposits are largely confined to canyons. Intercanyon areas are dominated by spill-over turbidites alternating with hemipelagic sediments, which on some of the southern to southwestern levees occur in sediment-wave fields formerly attributed to bottom-current activity. 相似文献
18.
浙江三门湾猫头深潭风暴快速沉积研究 总被引:2,自引:2,他引:2
三门湾频受热带风暴的影响,风暴期出现剧烈的滩冲、槽淤的泥沙交换,而在风暴后的正常天气条件下,则产生滩淤、槽冲的泥沙交换过程.通过猫头深潭(槽)9417号台风前后剖面水深重复测量、沉积特征及沉积物放射性同位素(210Pb,137Cs)测年等资料,揭示猫头深潭风暴快速沉积(骤淤).正常天气条件下风暴沉积产生再悬浮随潮运移,深潭水深得以恢复.在连续强热带风暴的影响下,又遇强的风暴增水,风暴沉积难以完全被冲刷,部分残留在深潭内,在猫头深潭沉积层中储存着风暴沉积的信息 相似文献
19.
Late Cenozoic seismic stratigraphy of the Vøring continental margin has been studied in detail, with emphasis on the geological development of the Naust Formation deposited during the last 3 million years. The Kai Formation (15–3 Ma) comprises mainly biogenic ooze deposited in the Møre and Vøring Basins. In Naust time, there was a marked increase in supply of sediments from the inner shelf areas and the western part of the Scandinavian mountain range, and glaciers expanded to the shelf and reached the shelf edge several times during the last 1.5–2 million years. During early to mid Naust time the shelf was widened by westerly prograding sediment units, but for a long period the shallowest part of the Helland-Hansen Arch (HHA) formed a barrier preventing glacially derived debris from being distributed farther west. West of the HHA, mainly stratified marine and glacimarine sediments were deposited. A substantial part of these sediments were transported by the north-flowing Norwegian Atlantic Current, which redistributed suspended particles from ice streams, rivers, coastal erosion and seabed winnowing. After burial of the crest of the HHA at c. 0.5 Ma, glacial debris and slide deposits were also deposited west of this high. In the north, massive units of glacial debris were distributed beyond the crest of the HHA, also in mid Naust time, thinning westwards and interfingering with fine-grained sediments on the lower slope. The Sklinnadjupet Slide, inferred to be c. 250,000 years old, corresponds in age with an earlier huge slide in the Storegga area. An elongated area of uneven seabed topography previously interpreted as diapirs (Vigrid diapirs) on the slope west of the HHA is shown to be formed by ooze eruption from the crest of the arch and submarine sliding. 相似文献
20.
Daniel Jean Stanley Harrison Sheng Douglas N. Lambert Peter A. Rona David W. McGrail J.Stacy Jenkyns 《Marine Geology》1981,40(3-4):215-235
Analyses of DSRV “Alvin” core samples on the Cape Hatteras margin indicate major textural and compositional changes at depths of about 1000 and well below 2500 m. The distribution patterns of petrologic parameters correlate well with water mass flow and suspended-sediment plumes measured on this margin by other workers. Our study also shows: (a) vigorous erosion and sediment transport at depths of less than 400 m resulting from the NE-trending Gulf Stream flow; (b) deposition, largely planktonic-rich sediment released from the Gulf Stream, on the upper- to mid-slope, to depths of about 800–1200 m; (c) winnowing, resuspension and deposition induced by periodically intensified slope currents on the mid-slope to uppermost rise, between about 1000 and 2500 m; and (d) prevailing deposition on the upper rise proper (below 2500 m), from transport by the SW-trending Western Boundary Undercurrent. Sediments moved by bottom currents have altered the composition and distribution patterns of material transported downslope by offshelf spillover; this mixing of gravity-emplaced and bottom-current-transported sediment obscures depositional boundaries. Moreover, reworking of the seafloor by benthic organisms alters physical properties and changes erodability of surficial sediments by bottom currents. Measurement of current flow above the seafloor and direct observation of the bottom are insufficient to delineate surficial sediment boundaries. Detailed petrologic analyses are needed to recognize the long-term signature of processes and define depositional provinces. 相似文献