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1.
Two sites in the eastern Fram Strait, the Vestnesa Ridge and the Yermak Plateau, have been surveyed and sampled providing a depositional record over the last glacial‐interglacial cycle. The Fram Strait is the only deep‐water connection from the Arctic Ocean to the North Atlantic and contains a marine sediment record of both high latitude thermohaline flow and ice sheet interaction. On the Vestnesa Ridge, the western Svalbard margin, a sediment drift was identified in 1226 m of water. Gravity and multicores from the crest of the drift recovered turbidites and contourites. 14C dating indicates an age range of 8287 to 26 900 years BP (Early Holocene to Late Weichselian). The Yermak Plateau is characterized by slope sediments in 961 m of water. Gravity and multicores recovered contourites and hemipelagites. 14C ages were between 8615 and 46 437 years BP (Early Holocene to mid‐Weichselian). Downcore dinoflagellate cyst analyses from both sites provide a record of changing surface water conditions since the mid‐Weichselian, suggesting variable sea ice extent, productivity and polynyas present even during the Last Glacial Maximum. Four layers of ice‐rafted debris were also identified and correlated within the cores. These events occurred ca at 9, 24 to 25, 26 to 27 and 43 ka, asynchronous with Heinrich layers in the wider north‐east Atlantic and here interpreted as reflecting instability in the Svalbard/Barents Ice sheet and the northward advection of warm Atlantic water during the Late Weichselian. The activity of the ancestral West Spitsbergen Current is interpreted using mean sortable silt records from the cores. On the Vestnesa Ridge drift the modern mass accumulation rate, calculated using excess 210Pb, is 0·076 g cm?2 year?1. On the Yermak Plateau slope the modern mass accumulation rate is 0·053 g cm?2 year?1. 相似文献
2.
The Gulf of Cadiz is swept by the strong saline Mediterranean Outflow Water (MOW). On the Algarve Margin (South Portugal), this current has constructed fine-grained contourite drifts. This margin is dissected by the Portimao Canyon and three short channels that only incise the upper slope, and are absent on a terrace located at mid-slope depths along the Algarve Margin. High-resolution seismic profiles and sediment cores highlight the original architecture of the sedimentary deposits on this terrace. Coarse-grained lenticular chaotic bodies formed during major relative sea-level lowstands are intercalated within the drift. The lobate shape and sandy nature of the lenticular chaotic bodies and their location at the mouths of the three short channels suggest they are gravity-generated deposits that are perched on the middle continental slope.In the Gulf of Cadiz, the interaction between contour current and gravity processes is strongly controlled by climatic variations and relative sea-level changes during the late Quaternary. During cold periods when sea-level was low, erosion intensified on the continental shelf and the deepest part of MOW was active. Sediment was transported downslope through the channels and deposited on sedimentary lobes perched on the mid-slope terrace. During warm periods when relative sea-level was high, the supply of sediment from the shelf was shut off and the shallowest part of MOW was more active. Contourite drifts fill the channels and bury the sandy lobes. 相似文献
3.
Adrian Gilli Flavio S. Anselmetti Daniel Ariztegui† Milan Beres† Judith A. McKenzie Vera Markgraf‡ 《Sedimentology》2005,52(1):1-23
The results of a seismic stratigraphic analysis of a closed lake basin, Lago Cardiel, in southernmost South America are reported. Very few high-resolution, continental records spanning the Late Quaternary have been obtained from this region. Seismic sequence stratigraphic analysis allows a reconstruction of lake level variations. Two major hiatuses of unknown age occurred during the early evolution of the basin with the deposition of an alluvial fan in a restricted area in the intervening time period. Following the development of a relatively shallow lake during the late Pleistocene and a short desiccation pulse around 11 220 14C yr BP, a transgression of over 135 m occurred at the beginning of the Holocene. The transgression was associated with the formation of beach ridges preserved in the lake stratigraphy on the floor of the modern Lago Cardiel at four different elevations. The preservation of largely unreworked beach ridges indicates a stepwise rise in the lake level. There is no seismic evidence of a major lowering of the lake below modern level during the entire Holocene. Deposition since the mid-Holocene is marked by strong lateral differences in sediment accumulation with a depocentre slightly to the north of the basin midpoint and a pronounced mounded distribution. Seismic reflection geometries, as well as sedimentological characteristics indicate a lacustrine contourite drift covering an area of 80–100 km2. As Lago Cardiel is under the influence of westerly winds, these most likely drove lake circulation. The identification of drowned beach ridges and of contourite drifts illustrates that high-resolution seismic stratigraphy is not only a powerful tool in reconstructing past lake level elevations for closed lake basins, but it can also provide information about the rate of lake level changes and the presence and strength of lake currents. 相似文献
4.
D. Van Rooij V. A. I. Huvenne D. Blamart J.-P. Henriet A. Wheeler H. de Haas 《International Journal of Earth Sciences》2009,98(4):849-863
The genesis and evolution of cold-water coral banks along the Northeastern Atlantic margin is known to be influenced by several
factors, among which the palaeotopography and nature of the coral settling surface, the presence of bottom currents and sediment
supply. In this paper, a case study is presented of the Enya mound cluster, located in the southernmost tip of the Belgica
mound province, west of Ireland. Below this mound cluster, seismic stratigraphy revealed a yet unmapped local unconformity
RD1b, being part of a composite erosion event (RD1). As such, from the Late Miocene to Late Pliocene, at least two erosional
events have incised the margin, ending with the final RD1a “moundbase” unconformity, acting as a base for the Enya mounds.
During the Quaternary the mounds became outgrown and were covered by a mounded contourite drift. In addition, they are closely
associated with a cluster of seabed pockmarks. The bottom current regime which became active since the Middle Pleistocene
has certainly influenced the fate of this mound cluster. The occurrence of the pockmarks seems to be related to relatively
recent fluid migration processes, however leaving an open question if any previous seepage phases were involved in the growth
or initiation of the coral banks. 相似文献
5.
First record of contourites from Lower Devonian Liptrap Formation in southeast Australia 总被引：2，自引：0，他引：2
Typical contourite deposits associated with submarine turbidite fan deposits are recognized for the first time from the Lower Devonian Liptrap Formation at Cape Liatrap, Victoria in southeast Australia. The contourites are well integrated within the turbidite fan deposits and are characterized by thin (5-8 cm), lenticular, well-sorted coarse-grained siltstones to fine-grained sandstones with current-ripples and cross beddings. The palaeocurrent directions of the turbidite fan and contourites are perpendicular to each other, with the former directed generally westward while the latter varying from 165° to 190° southward. In view of the facies types and architecture, we suggest that the turbidite fan was developed at the base of a westward inclined palaeo-slope, at the front of which the contourites were deposited as a result of southward flowing deep-sea contour (geostrophic) currents. The depositional setting inter- preted for the Liptrap Formation thus may provide a provisional model for the Lower Devonian conti- nental slope and abyssal basin environment in the southeastern part of the Melbourne Trough. 相似文献
6.
The sandy channel-lobe depositional systems in the Gulf of Cadiz: Gravity processes forced by contour current processes 总被引：1，自引：0，他引：1
The sedimentation in the Gulf of Cadiz (NE Atlantic Ocean) is significantly controlled by the Mediterranean Outflow Water (MOW). Along its pathway onto the continental slope, the MOW is canalized by contourite channels, some of them feeding gravity sandy channel-lobe depositional systems firstly recognized in previous study [Habgood et al., 2003. Deep-water sediment wave fields, bottom current sand channels and gravity flow channel-lobe systems: Gulf of Cadiz, NE Atlantic. Sedimentology 50(3), 483-510.].Using very high resolution acoustic data and cores, a detailed characterization and a new evolution pattern of these channel-lobe depositional systems is established. Complex internal geometry of the lobes shows several depositional units revealing a polyphase evolution of these systems, with a general progradation punctuated by retrogradation and avulsion phases. A gravity origin controlled by contouritic processes and climatic changes is demonstrated for the feeding and the evolution of these sandy channel-lobe depositional systems. Climate oscillations, via the MOW variations, act as a major forcing of the activity of the channel-lobe depositional systems during the Late Quaternary. 相似文献
7.
A. Maldonado F. Bohoyo J. Galindo-Zaldívar J. Hernández-Molina A. Jabaloy F. J. Lobo J. Rodríguez-Fernández E. Suriñach J. T. Vázquez 《Marine Geophysical Researches》2006,27(2):83-107
The distribution of seismic units in deposits of the basins near the Antarctic–Scotia plate boundary is described based on
the analysis of multichannel seismic reflection profiles. Five main seismic units are identified. The units are bounded by
high-amplitude continuous reflectors, named a to d from top to bottom. The two older units are of different age and seismic
facies in each basin and were generally deposited during active rifting and seafloor spreading. The three youngest units (3
to 1) exhibit, in contrast, rather similar seismic facies and can be correlated at a regional scale. The deposits are types
of contourite drift that resulted from the interplay between the northeastward flow of Weddell Sea Bottom Water (WSBW) and
the complex bathymetry in the northern Weddell Sea, and from the influence of the Antarctic Circumpolar Current and the WSBW
in the Scotia Sea. A major paleoceanographic event was recorded by Reflector c, during the Middle Miocene, which represents
the connection between the Scotia Sea and the Weddell Sea after the opening of Jane Basin. Unit 3 (tentatively dated ∼Middle
to Late Miocene) shows the initial incursions of the WSBW into the Scotia Sea, which influenced a northward progradational
pattern, in contrast to the underlying deposits. The age attributed to Reflector b is coincident with the end of spreading
at the West Scotia Ridge (∼6.4 Ma). Unit 2 (dated ∼Late Miocene to Early Pliocene) includes abundant high-energy, sheeted
deposits in the northern Weddell Sea, which may reflect a higher production of WSBW as a result of the advance of the West
Antarctic ice-sheet onto the continental shelf. Reflector a represents the last major regional paleoceanographic change. The
timing of this event (∼3.5–3.8 Ma) coincides with the end of spreading at the Phoenix–Antarctic Ridge, but may be also correlated
with global events such as initiation of the permanent Northern Hemisphere ice-sheet and a major sea level drop. Unit 1 (dated
∼Late Pliocene to Recent) is characterized by abundant chaotic, high-energy sheeted deposits, in addition to a variety of
contourites, which suggest intensified deep-water production. Units 1 and 2 show, in addition, a cyclic pattern, more abundant
wavy deposits and the development of internal unconformities, all of which attest to alternating periods of increased bottom
current energy. 相似文献
8.
A widespread (3200 km2), thick (10 cm to > 100 cm) sand deposit exists on the continental shelf and upper slope offshore of the Pennell Coast, Antarctica. The sand body occurs at water depths between 200 and 1200 m. The mean grain size of the sand varies from 3·3 phi to 1·2 phi, and the composition is dominantly volcanic. The only source for this volcanic material is Cape Adare, a volcanic headland. Sands are transported up to 70 km from Cape Adare by a westward‐flowing circumpolar boundary current that impinges on the upper slope and shelf. Radiocarbon dates from the sand and from the glacial unit beneath it indicate that the deposit formed within the past 9000 years. The occurrence of this sand sheet demonstrates the ability of contour currents to assemble extensive sand bodies on the shelf and upper slope and the relatively rapid rate at which these deposits may form. 相似文献
9.
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
10.
中国油气勘探的一个新领域——深水牵引流沉积 总被引：4，自引：0，他引：4
深水牵引流沉积主要分为等深流沉积和内波内潮汐沉积两类。前者可形成巨大的堆积体——等深积岩丘,其规模可与海底扇相比拟。现代海洋大陆坡及陆隆地带等深积岩丘非常发育;古代地层记录中也有等深积岩丘,如已开采数十年的阿拉伯克拉通白垩系等深积岩丘油田。后者则不仅可形成分选极好的砂级沉积物,而且可形成规模巨大的“沉积物波”,并成群成带地出现。海洋中沉积物波的迁移可形成巨大的沉积体。在世界各大洋深盆海底,均有大范围分布的现代大型沉积物波。中国地层记录中的大型沉积物波,首先在塔里木盆地塔中地区中一上奥陶统中被识别出来,并有良好油气显示。认为中国的广大地区发育有多个时代的海相深水沉积,深水牵引流沉积储集层是中国21世纪油气勘探具巨大潜力和现实可行性的新领域。其油气勘探首选地区是塔里木盆地、扬子地台南缘和鄂尔多斯西缘。 相似文献