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1.
Detailed sedimentological and stratigraphic analyses were carried out on seven Kullenberg cores collected across the Brazilian continental margin during the French cruises Byblos and Apsara III, in order to highlight the factors controlling the sediment flux distribution in the Southern Brazil Basin during the late Quaternary. On the continental slope and upper continental rise above 3000 m depth, sediment fluxes are important and highly variable (4·2–14·2 g cm?2 10?3 yr). The sediments show a pelagic or turbiditic character, depending on the width of the shelf and proximity of canyons. The material is characterized by high kaolinite contents, and originates from the coastal rivers draining the South American continent north of Rio de Janeiro. On the middle continental rise between 3000 and 4000 m depth, sediment fluxes are the lowest observed in the area (0·9 g cm?2 10?3 yr), because terrigenous input is trapped at shallower depths on the São Paulo Plateau. Pelagic settling is the dominant process. In the deep domains, below 4000 m depth, contouritic accumulations are developed on the path of the northwards moving Antarctic bottom water (AABW) currents. The deposits consist of fine-grained silty-clayey muds with very low carbonate contents. The sediment fluxes (1·45 g cm?2 10?3 yr) are higher than on the middle continental rise, as a consequence of fine-grained terrigenous supply derived from higher latitudes (Argentine Basin and Southern Ocean), and transported in the basin through the Vema Channel by the AABW currents. This material is characterized by high smectite and chlorite contents. These data reveal large sediment flux variations which are linked to distinct depth-related domains. Such a distribution is the consequence of the presence of two available sources of terrigenous sediments: (1) the Brazilian continental areas with a downslope material transport and a sediment distribution controlled by the morphology of the margin, and (2) the Argentine Basin with an alongslope material transport by deep-sea currents which dominate the sedimentation in the abyssal domains.  相似文献   

2.
We describe here a sequence of soft sediment deformation (SSD) structures at Dive Agar beach near Srivardhan in the west coast of India. The ~120-cm-thick sediment package is represented by a basal undeformed sand (layer A) sharply cut by ~30-cm-thick intermixed beach sand and terrigenous sand (layer B1) followed by complex load structures and convolutions (8?C15?cm) within a coarse sandy layer (B2). The layer B2 is scoured by terrigenous sand (layer C1) which is capped with a silty mud layer (C2). The entire sequence (B2?CC1?CC2) is intruded by sand dykes originating from the lower layer B1. This sediment package is further overlain by a heavy mineral reach marine sand (layer D) with liquefactions long axes inclined southward as a result of forceful long-shore drift. The profile ends up with coarse-grained, poorly sorted sand including angular clasts of terrigenous outwash deposits indicating return of distal inundations. Intense deformation (liquefaction) is restricted to the heavy mineral-rich marine and the intermixed sands (layers B2 and D), whereas the terrigenous sand layers show scoured bases with oscillatory and pebbly tops. The presence of complex load structures injecting into the underlying layers, the top-truncated sand dykes, macro-thrust faults, scouring, and inclusion of coral fragments can explain it as a record of tsunami in the west coast. Occurrence of un-decayed consumer plastic material within the deformed layers suggests it as one of the most recent tsunami events (i.e., 2004 IOT), the only reported event after 1945 in the west coast. Alternative marine and terrigenous sands are characteristic of tsunami run-up and backwash deposits, while the dimensions of SSDs may be related to the <2?m magnitude (height) of the 2004 IOT at Dive Agar.  相似文献   

3.
The Bosphorus Strait accommodates two‐way flow between the Aegean and Black Seas. The Aegean (Mediterranean) inflow has speeds of 5 to 15 cm sec?1 in the strait and a salinity contrast of ～12‰ to 16‰ with the Black Sea surface waters on the shelf. An anastomosed channel network crosses the shelf and in water deeper than 70 m is characterized by first‐order channels 5 to 10 m deep, local lateral accretion bedding, muddy in‐channel barforms, and a variety of sediment waves both on channel floors and bar crests, crevasse channels entering the overbank area and levée/overbank deposits which are radiocarbon‐dated in cores to be younger than ～7·5 to 8·0 ka. This channel network accommodates the saline density current formed by the Mediterranean inflow. The density contrast between the density underflow and the ambient water mass is ～0·01 g cm?3, similar to the density contrast ascribed to low‐concentration turbidity currents in the deep sea. Channel‐floor deposits are sandy to gravelly with local shell concentrations. Low‐relief bedforms on the channel floor have relatively straight crests, upflow‐dipping cross‐stratification, heights 1 to 1·5 m and wavelengths 85 to 155 m. Bankfull flows are subcritical, so these probably are not antidunes. Bar tops are ornamented locally with mudwaves having heights 1 to 2 m and wavelengths ～20 to 100 m; these are potentially antidunes formed under shallow overbank flows. Towards the shelf edge, the degree of channel bifurcation increases dramatically and bar tops are dissected locally by secondary channels, some of which terminate in hanging valleys. Conical mounds on the shelf (possibly mud volcanoes or sites of fluid seepage) interact with the channel network by promoting accretion of muddy streamlined macroforms in their lee. This channel network may be one of the largest and most accessible natural laboratories on Earth for the study of continuously flowing density currents. Although the driver is salinity contrast, the underflow transports sufficient sediment to form levée wedges and large streamlined barforms, and presumably transports sediment into deep water.  相似文献   

4.
Evolution and mechanics of a Miocene tidal sandwave   总被引：3，自引：0，他引：3  
A remarkable exposure of Miocene marine molasse in western Switzerland records the evolution of a tidal sandwave over a period of approximately 2 1/2 months. The sandwave is composed of tidal ‘bundles’ in which a sandwave reactivation stage and full vortex stage can be recognized for the dominant flow (ebb tide) and a rippled flood apron overlain by high water drape for the reversed flow. Bundle thicknesses vary systematically through neap–spring cycles, with a periodicity of 27 demonstrating the semi-diurnal lunar control of sedimentation. Waves were an additional component, especially when superimposed on flood tides, producing near-symmetrical combined-flow ripple marks in the flood apron. Tidal current velocities are estimated using critical shear velocities for entrainment, the ripple-dune transition and the dune-plane bed transition. Using appropriate estimates of roughness lengths and a logarithmic velocity law, maximum tidal speeds at 1 m above the bed were approximately 0·6 m sec?1 for ebbs and up to 0·5 m sec?1 for floods. The enhancement by waves of bed shear stress (τwc/τ of approximately 2 for 1 m high waves) under flood currents implies flood tidal velocities closer to 0·2–0·3 m sec?1. Peak instantaneous bedload sediment transport rates using a modified Bagnold equation are nearly 5 times greater under ebb tides than floods. The average net sediment transport rate at springs (0·04 kg m?1 sec?1) is over 10 times greater than at neaps (0·002 kg m?1 sec?1). Comparison with transport rates in modern tidal environments suggests that the marine molasse of Switzerland was deposited under spatially confined and relatively swift tidal flows not dissimilar to those of the present Dutch tidal estuaries.  相似文献   

5.
The settling behaviour of particulate suspensions and their deposits has been documented using a series of settling tube experiments. Suspensions comprised saline solution and noncohesive glass‐ballotini sand of particle size 35·5 μm < d < 250 μm and volume fractions, φs, up to 0·6 and cohesive kaolinite clay of particle size d < 35·5 μm and volume fractions, φm, up to 0·15. Five texturally distinct deposits were found, associated with different settling regimes: (I) clean, graded sand beds produced by incremental deposition under unhindered or hindered settling conditions; (II) partially graded, clean sand beds with an ungraded base and a graded top, produced by incremental deposition under hindered settling conditions; (III) graded muddy sands produced by compaction with significant particle sorting by elutriation; (IV) ungraded clean sand produced by compaction and (V) ungraded muddy sand produced by compaction. A transition from particle size segregation (regime I) to suppressed size segregation (regime II or III) to virtually no size segregation (IV or V) occurred as sediment concentration was increased. In noncohesive particulate suspensions, segregation was initially suppressed at φs ～ 0·2 and entirely inhibited at φs ≥ 0·6. In noncohesive and cohesive mixtures with low sand concentrations (φs < 0·2), particle segregation was initially suppressed at φm ～ 0·07 and entirely suppressed at φm ≥ 0·13. The experimental results have a number of implications for the depositional dynamics of submarine sediment gravity flows and other particulate flows that carry sand and mud; because the influence of moving flow is ignored in these experiments, the results will only be applicable to flows in which settling processes, in the depositional boundary, dominate over shear‐flow processes, as might be the case for rapidly decelerating currents with high suspended load fallout rates. The ‘abrupt’ change in settling regimes between regime I and V, over a relatively small change in mud concentration (<5% by volume), favours the development of either mud‐poor, graded sandy deposits or mud‐rich, ungraded sandy deposits. This may explain the bimodality in sediment texture (clean ‘turbidite’ or muddy ‘debrite’ sand or sandstone) found in some turbidite systems. Furthermore, it supports the notion that distal ‘linked’ debrites could form because of a relatively small increase in the mud concentration of turbidity currents, perhaps associated with erosion of a muddy sea floor. Ungraded, clean sand deposits were formed by noncohesive suspensions with concentrations 0·2 ≤ φs ≤ 0·4. Hydrodynamic sorting is interpreted as being suppressed in this case by relatively high bed aggradation rates which could also occur in association with sustained, stratified turbidity currents or noncohesive debris flows with relatively high near‐bed sediment concentrations.  相似文献   

6.
The 2004 tsunami deposits and probable paleotsunami deposits were studied at the southern Kho Khao Island, on Andaman Sea coast of Thailand. The 2004 tsunami laid down about 8?cm of fining upward medium sand and locally about 40?cm of massive coarse sand with common mud clasts. The sediments were characterized by the presence of marine foraminiferal assemblage; however, already after 5?years many of carbonate foraminiferal tests were partly or completely dissolved. The probable paleotsunami deposits form layer about 1?m thick. It consists of massive very coarse sand with common big shells and mud clasts. Its composition suggests a marine origin and the presence of mud clasts, and similarity to the 2004 tsunami deposits suggests that the layer was left by paleotsunami, which took place probably during the late Holocene, even though two shells within the layer gave 14C ages of 40,000?years or more.  相似文献   

7.
This study analyses the three‐dimensional geometry of sedimentary features recorded on the modern sea floor and in the shallow subsurface of a shelf to upper slope region offshore Australia that is characterized by a pronounced internal wave regime. The data interpreted comprise an extensive, >12 500 km2 industrial three‐dimensional seismic‐reflection survey that images the northern part of the Browse Basin, Australian North West Shelf. The most prominent seismic–morphological features on the modern sea floor are submarine terrace escarpments, fault‐scarps and incised channels, as well as restricted areas of seismic distortion interpreted as mass wasting deposits. Besides these kilometre‐scale sea floor irregularities, smaller bedforms were discovered also, including a multitude of sediment waves with a lateral extent of several kilometres and heights up to 10 m. These sedimentological features generally occur in extensive fields in water depths below 250 m mostly at the foot of submerged terraces, along the scarps of modern faults and along the shelf break between the outer shelf and the upper continental rise. Additional bedforms that characterize the more planar regions of the outer shelf are elongate, north‐west/south‐east oriented furrows and ridges. The formation of both sediment waves and furrow‐ridge systems requires flow velocities between 0·3 m sec?1 and 1·5 m sec?1, which could be generated by oceanic currents, gravity currents or internal waves. In the studied setting, these velocities can be best explained as being generated by bottom currents induced by internal waves, an interpretation that is discussed against oceanographic background data and modelling results. In addition to the documentation of three‐dimensional seismic–geomorphological features of the modern sea floor, it was also possible to map kilometre‐scale buried sediment wave fields in the seismic volume down to ca 500 ms two‐way‐time below the present sea floor, indicating the general potential for the preservation of such bedforms in the sedimentary record.  相似文献   

8.
Methane produced in anoxic organic-rich sediments of Cape Lookout Bight, North Carolina, enters the water column via two seasonally dependent mechanisms: diffusion and bubble ebullition. Diffusive transport measured in situ with benthic chambers averages 49 and 163 μmol · m ?2 · hr ?1 during November–May and June–October respectively. High summer sediment methane production causes saturation concentrations and formation of bubbles near the sediment-water interface. Subsequent bubble ebullition is triggered by low-tide hydrostatic pressure release. June–October sediment-water gas fluxes at the surface average 411 ml (377 ml STP: 16.8 mmol) · m?2 per low tide. Bubbling maintains open bubble tubes which apparently enhance diffusive transport. When tubes are present, apparent sediment diffusivities are 1.2–3.1-fold higher than theoretical molecular values reaching a peak value of 5.2 × 10?5 cm2 · sec?1. Dissolution of 15% of the rising bubble flux containing 86% methane supplies 170μmol · m?2 · hr?1 of methane to the bight water column during summer months; the remainder is lost to the troposphere. Bottom water methane concentration increases observed during bubbling can be predicted using a 5–15 μm stagnant boundary layer dissolution model. Advective transport to surrounding waters is the major dissolved methane sink: aerobic oxidation and diffusive atmospheric evasion losses are minor within the bight.  相似文献   

9.
台湾峡谷HD133和HD77柱状样的沉积构成和发育背景   总被引：1，自引：0，他引：1  
分别对南海东北部台湾峡谷内水深3 280 m的HD133和峡谷外水深3 378 m的HD77重力活塞柱状样进行了沉积物粒度、古生物和碳酸钙含量分析,利用AMS14C同位素测年和沉积速率初步认定是属于MIS3a以来的沉积。按沉积物粒度和碳酸钙含量可将两支柱状样划分为3套沉积层段:上部层段1和下部层段3均以粉砂质黏土为主,夹薄层粉砂,深水底栖有孔虫含量高,碳酸钙低于10%,代表受重力流作用较弱的正常深海沉积;中部层段2发育一套以中-细粒为主的厚砂层,含大量浅水底栖有孔虫,碳酸钙含量可高达60%,AMS14C测年出现倒置现象,表明主要为浅水重力流沉积。柱状样的沉积构成响应同期海平面变化,特别表现在深水砂层沉积的两大控制因素:在时间上,低海平面时期大量浅水和陆源碎屑物质直接输送到陆坡之下的深水区,形成富砂的层段2;在空间上,峡谷水道是重力流的物质输送通道,地形优势使得重力流携带物优先在水道中发生沉积,造成HD133柱的含砂量明显高于HD77柱状样。  相似文献   

10.
The monitoring of turbidity currents enables accurate internal structure and timing of these flows to be understood. Without monitoring, triggers of turbidity currents often remain hypothetical and are inferred from sedimentary structures of deposits and their age. In this study, the bottom currents within 20 m of the seabed in one of the Pointe-des-Monts (Gulf of St. Lawrence, eastern Canada) submarine canyons were monitored for two consecutive years using Acoustic Doppler Current Profilers. In addition, multibeam bathymetric surveys were carried out during deployment of the Acoustic Doppler Current Profilers and recovery operations. These new surveys, along with previous multibeam surveys carried out over the last decade, revealed that crescentic bedforms have migrated upslope by about 20 to 40 m since 2007, despite the limited supply of sediment on the shelf or river inflow in the region. During the winter of 2017, two turbidity currents with velocities reaching 0·5 m sec−1 and 2·0 m sec−1, respectively, were recorded and were responsible for the rapid (<1 min) upstream migration of crescentic bedforms measured between the autumn surveys of 2016 and 2017. The 200 kg (in water) mooring was also displaced 10 m down-canyon, up the stoss side of a bedform, suggesting that a dense basal layer could be driving the flow during the first minute of the event. Two other weaker turbidity currents with speeds <0·5 m sec−1 occurred, but did not lead to any significant change on the seabed. These four turbidity currents coincided with strong and sustained wind speed >60 km h−1 and higher than normal wave heights. Repeat seabed mapping suggests that the turbidity currents cannot be attributed to a canyon-wall slope failure. Rather, sustained windstorms triggered turbidity currents either by remobilizing limited volumes of sediment on the shelf or by resuspending sediment in the canyon head. Turbidity currents can thus be triggered when the sediment volume available is limited, likely by eroding and incorporating canyon thalweg sediment in the flow, thereby igniting the flow. This process appears to be particularly important for the generation of turbidity currents capable of eroding the lee side of upslope migrating bedforms in sediment-starved environments and might have wider implications for the activity of submarine canyons worldwide. In addition, this study suggests that a large external trigger (in this case storms) is required to initiate turbidity currents in sediment-starved environments, which contrasts with supply-dominated environments where turbidity currents are sometimes recorded without a clear triggering mechanism.  相似文献   

11.
中国的潮滩   总被引：31，自引：1，他引：31  
王颖  朱大奎 《第四纪研究》1990,10(4):291-300
本文将中国潮滩分为平原型和港湾型二类。以渤海湾和江苏海岸为例,讨论了平原潮滩的环境条件、沉积动力过程及沉积相,就浙江、福建具代表性的三个港湾,叙述了港湾潮滩的特征,还讨论了沉积物供应量及潮汐作用对潮滩发育的影响。  相似文献   

12.
A 4·7 km2 field of sediment waves occurs in front of the Slims River delta in Kluane Lake, the largest lake in the Yukon Territory. Slims River heads in the Kaskawulsh Glacier, part of the St Elias Ice Field and discharges up to 400 m3 s?1 of water with suspended sediment concentrations of up to 7 g l?1. The 19 km long sandur of Slims River was created in the past 400 years since Kaskawulsh Glacier advanced and dammed the lake and the sandur has advanced into Kluane Lake at an average rate of 48 m a?1. However, this rate is decreasing as flow is diverted from Slims River because of the retreat of the Kaskawulsh Glacier. The sandur and a road constructed on the delta remove coarse‐grained sediment, so the river delivers dominantly mud to the lake. Inflow during summer generates quasi‐continuous turbidity currents with velocities up to 0·6 m s?1. The front of the delta consists of a plane surface sloping lakeward at 0·0188 (1·08°). A field of sediment waves averaging 130 m in length and 2·3 m in amplitude has developed on this surface. Slopes on the waves vary from ?0·067 (?3·83°, i.e. sloping in the opposite direction to the regional slope) to 0·135 (7·69°). The internal structure of the sediment waves, as documented by seismic profiling, shows that sedimentation on the stoss portion of the wave averages 2·7 times that on the lee portion. Rates of sediment accumulation in the wave field are about 0·3 m a?1, so these lacustrine waves have formed in a much shorter period of time (less than 200 years) and are advancing upslope towards the delta much more quickly (1 to 2 m a?1) than typical marine sediment waves. These waves formed on the flat surface of the lake floor, apparently in the absence of pre‐existing forms, and they are altered and destroyed as the wave field advances and the characteristics of the turbidity currents change.  相似文献   

13.
Shingled Quaternary debris flow lenses on the north-east Newfoundland Slope   总被引：1，自引：0，他引：1  
Debris flow deposits are the principal component of Quaternary continental slope sediments between the north-east Newfoundland Shelf and central Orphan Basin. In seismic profiles, these deposits occur as shingled, elongate, acoustically transparent lenses with their long axes orientated downslope. Deposits of individual flows form positive mounds on the sea floor; subsequent flows were diverted by the pre-existing topography into bathymetric lows between older debris flow deposits. These deposits show a large variation in the area of sea floor covered by individual flows (about 60–1000 km2), average thickness of deposits (9–37 m) and volume of sediment displaced (1–27 km3). The ratio of average thickness to a measure of deposit diameter, termed the aspect ratio, has a threefold variation from 0·0006 to 0·0021. Very low depositional slopes and low aspect ratios suggest relatively low viscosities, probably due to inmixing of water during downslope transport. Stratified sediments form three distinct horizons and are locally interbedded with the debris flow deposits. These are mainly hemipelagic deposits. The slope and rise to the west of the Orphan Basin are constructional in character. The apparent absence of upper slope erosional features and the abundance of debris flow deposits on the slope suggest that the supply of sediment to the continental slope occurred predominantly during times of maximum extent of Quaternary glacial ice. The ice sheet grounding line during several glacial maxima must have been situated at or near the present shelf break, supplying vast amounts of sediment directly to the upper slope. Oversteepening and subsequent slope failures fed material into deeper water.  相似文献   

14.
The Barataria barrier coast formed between two major distributaries of the Mississippi River delta: the Plaquemines deltaic headland to the east and the Lafourche deltaic headland to the west. Rapid relative sea‐level rise (1·03 cm year?1) and other erosional processes within Barataria Bay have led to substantial increases in the area of open water (> 775 km2 since 1956) and the attendant bay tidal prism. Historically, the increase in tidal discharge at inlets has produced larger channel cross‐sections and prograding ebb‐tidal deltas. For example, the ebb delta at Barataria Pass has built seaward > 2·2 km since the 1880s. Shoreline erosion and an increasing bay tidal prism also facilitated the formation of new inlets. Four major lithofacies characterize the Barataria coast ebb‐tidal deltas and associated sedimentary environments. These include a proximal delta facies composed of massive to laminated, fine grey‐brown to pale yellow sand and a distal delta facies consisting of thinly laminated, grey to pale yellow sand and silty sand with mud layers. The higher energy proximal delta deposits contain a greater percentage of sand (75–100%) compared with the distal delta sediments (60–80%). Associated sedimentary units include a nearshore facies consisting of horizontally laminated, fine to very fine grey sand with mud layers and an offshore facies that is composed of grey to dark grey, laminated sandy silt to silty clay. All facies coarsen upwards except the offshore facies, which fines upwards. An evolutionary model is presented for the stratigraphic development of the ebb‐tidal deltas in a regime of increasing tidal energy resulting from coastal land loss and tidal prism growth. Ebb‐tidal delta facies prograde over nearshore sediments, which interfinger with offshore facies. The seaward decrease in tidal current velocity of the ebb discharge produces a gradational contact between proximal and distal tidal delta facies. As the tidal discharge increases and the inlet grows in dimensions, the proximal and distal tidal delta facies prograde seawards. Owing to the relatively low gradient of the inner continental shelf, the ebb‐tidal delta lithosome is presently no more than 5 m thick and is generally only 2–3 m in thickness. The ebb delta sediment is sourced from deepening of the inlet and the associated channels and from the longshore sediment transport system. The final stage in the model envisages erosion and segmentation of the barrier chain, leading to a decrease in tidal discharge through the former major inlets. This process ultimately results in fine‐grained sedimentation seaward of the inlets and the encasement of the ebb‐tidal delta lithosome in mud. The ebb‐tidal deltas along the Barataria coast are distinguished from most other ebb deltas along sand‐rich coasts by their muddy content and lack of large‐scale stratification produced by channel cut‐and‐fills and bar migration.  相似文献   

15.
The chemical and isotopic composition of pore fluids is presented for five deep-rooted mud volcanoes aligned on a transect across the Gulf of Cadiz continental margin at water depths between 350 and 3860 m. Generally decreasing interstitial Li concentrations and 87Sr/86Sr ratios with increasing distance from shore are attributed to systematically changing fluid sources across the continental margin. Although highest Li concentrations at the near-shore mud volcanoes coincide with high salinities derived from dissolution of halite and late-stage evaporites, clayey, terrigenous sediments are identified as the ultimate Li source to all pore fluids investigated. Light δ7Li values, partly close to those of hydrothermal vent fluids (δ7Li: +11.9‰), indicate that Li has been mobilized during high-temperature fluid/sediment or fluid/rock interactions in the deep sub-surface. Intense leaching of terrigenous clay has led to radiogenic 87Sr/86Sr ratios (∼0.7106) in pore fluids of the near-shore mud volcanoes. In contrast, non-radiogenic 87Sr/86Sr ratios (∼0.7075) at the distal locations are attributed to admixing of a basement-derived fluid component, carrying an isotopic signature from interaction with the basaltic crust. This inference is substantiated by temperature constraints from Li isotope equilibrium calculations suggesting exchange processes at particularly high temperatures (>200 °C) for the least radiogenic pore fluids of the most distal location.Advective pore fluids in the off-shore reaches of the Gulf of Cadiz are influenced by successive exchange processes with both oceanic crust and terrigenous, fine-grained sediments, resulting in a chemical and isotopic signature similar to that of fluids in near-shore ridge flank hydrothermal systems. This suggests that deep-rooted mud volcanoes in the Gulf of Cadiz represent a fluid pathway intermediate between mid-ocean ridge hydrothermal vent and shallow, marginal cold seep. Due to the thicker sediment coverage and slower fluid advection rates, the overall geochemical signature is shifted towards the sediment-diagenetic signal compared to ridge flank hydrothermal environments.  相似文献   

16.
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17.
Glacimarine sediment deposited in the fjord adjacent to Muir Glacier in south-eastern Alaska consists of rhythmically laminated muds, stratified sandy mud, sand and gravelly mud facies. Cyclicity is recorded by gravelly mud facies deposited during winter by ice-rafting, black mud laminae formed by spring plankton blooms and variations in tidal rhythmite thickness and texture produced by the interaction of meltwater discharges and tidal currents in the macrotidal fjord. Regular cyclicity in laminae thickness is tested statistically by Fourier transform and can be attributed to a lunar tidal cycle control in the five cores collected up to 6 km from the sediment source. Cores close to the source can have additional laminae as a result of discharge fluctuations, and distal cores may lack full cycles because of variability in the plume path and attenuation with distance. Cyclic variations in sediment texture are recorded in magnetic susceptibility (MS) profiles of the cores. High MS values are produced by turbidite sand beds or by stratified sandy mud deposited by overflow plumes during peak summer meltwater discharge. Low values reflect muddy intervals deposited during periods of low meltwater discharge, such as during autumn and winter. Sediment accumulation rates measured by 210Pb dating range from 82 cm year–1, 2 km from the sediment source at the head of the fjord, to 16 cm year–1, 6 km away. These rates are within the same range as average sediment accumulation rates determined from cyclic seasonal markers within the cores. These data show that, with careful documentation, annual cycles of glacimarine sediment accumulation can be detected within marine cores. Cores collected from the distal portion of the basin were deposited during the transition of Muir Glacier from a tidewater terminus ending in deep water to a terrestrial glacier with an ice-contact delta deposited in front of the terminus. This transition is recorded by a coarsening-upward sedimentary sequence formed by turbidite sands originating from the prograding delta above fine-grained, laminated basin fill deposited by turbid overflow plumes.  相似文献   

18.
High-resolution seismic boomer profiles, with a vertical resolution of less than 1 m, together with piston cores and previous side-scan sonar data, are used to describe late Quaternary sedimentation on the Var deep-sea fan. Chronological control is provided by foram biostratigraphy and radiocarbon dating in cores, and is extended over the fan by seismic correlation. Regional erosional events correspond to the oxygen isotopic stage 2 and 6 glacial maxima. Cores and seismic data define a widespread surface sand layer that is correlated with prodelta failure in 1979 and subsequent submarine cable breaks. Numerical modelling constrains the character of this 1979 turbidity current. It originated from a relatively small slide on the upper prodelta that put sufficient material in suspension to form an accelerating turbidity current which eroded sand from the Var Canyon. The turbidity current was only 30 m thick on the Upper Valley, but experienced significant flow expansion in the Middle Valley to thicknesses of more than 120 m, where it spilled over the eastern Var Sedimentary Ridge at a velocity of about 2·5 m s?1. Other Holocene turbidity currents (with a recurrence interval of 1000 years) were somewhat muddier and thicker, but also deposited sand on the levees of the Middle Valley, and are inferred to have had a similar slide-related origin. Late Pleistocene turbidity currents deposited thick mud beds on the Var Sedimentary Ridge. The presence of sediment waves and the mean cross-flow slope inferred from levee asymmetry indicates that some of these flows were many hundreds of metres thick and flowed at velocities of about 0·35 m s?1. This contrast with Holocene turbidites suggests that a slide origin is unlikely. Estimated times for deposition of thick mud beds on the levees are many days to weeks. The Late Pleistocene flows may therefore result from hyperpycnal flow of glacial outwash in the Var River. The variation in the Late Pleistocene to Holocene turbidite sedimentation is controlled more by variations in sediment supply than by sea-level change.  相似文献   

19.
The chemical and isotopic composition of pore fluids is presented for five deep-rooted mud volcanoes aligned on a transect across the Gulf of Cadiz continental margin at water depths between 350 and 3860 m. Generally decreasing interstitial Li concentrations and 87Sr/86Sr ratios with increasing distance from shore are attributed to systematically changing fluid sources across the continental margin. Although highest Li concentrations at the near-shore mud volcanoes coincide with high salinities derived from dissolution of halite and late-stage evaporites, clayey, terrigenous sediments are identified as the ultimate Li source to all pore fluids investigated. Light δ7Li values, partly close to those of hydrothermal vent fluids (δ7Li: +11.9‰), indicate that Li has been mobilized during high-temperature fluid/sediment or fluid/rock interactions in the deep sub-surface. Intense leaching of terrigenous clay has led to radiogenic 87Sr/86Sr ratios (0.7106) in pore fluids of the near-shore mud volcanoes. In contrast, non-radiogenic 87Sr/86Sr ratios (0.7075) at the distal locations are attributed to admixing of a basement-derived fluid component, carrying an isotopic signature from interaction with the basaltic crust. This inference is substantiated by temperature constraints from Li isotope equilibrium calculations suggesting exchange processes at particularly high temperatures (>200 °C) for the least radiogenic pore fluids of the most distal location.Advective pore fluids in the off-shore reaches of the Gulf of Cadiz are influenced by successive exchange processes with both oceanic crust and terrigenous, fine-grained sediments, resulting in a chemical and isotopic signature similar to that of fluids in near-shore ridge flank hydrothermal systems. This suggests that deep-rooted mud volcanoes in the Gulf of Cadiz represent a fluid pathway intermediate between mid-ocean ridge hydrothermal vent and shallow, marginal cold seep. Due to the thicker sediment coverage and slower fluid advection rates, the overall geochemical signature is shifted towards the sediment-diagenetic signal compared to ridge flank hydrothermal environments.  相似文献   

20.
Abstract Core BAP96‐CP, sampled from the deepest part of the Bay of La Paz, Gulf of California, has been analysed sedimentologically taking into account regional climate and oceanography. Laminated sediments at the bottom of the bay are essentially not bioturbated by benthic fauna. A subanoxic condition (O2 < 0·2 mL L?1) inhibits the proliferation of benthic fauna. Within the bay, the relative abundances of terrigenous and biogenic inputs change periodically. The terrigenous input is greater than the biogenic input and apparently experiences larger fluctuations. The terrigenous input dominates in dark laminae, whereas the biogenic input mostly occurs in light laminae. Thus, it is assumed that, down the core, the alternation of dark and light laminae represents cycles in the extent of dilution of the biogenic input by terrigenous input. The terrigenous input into the Bay of La Paz is mostly regulated by pluvial runoff. Thus, its temporal fluctuation follows the periods shown by the regional pluvial regime, particularly the 11·2 year period. This is equal to the frequency of sunspot cycles.  相似文献   

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