Subaqueous liquefied and fluidized sediment flows and their deposits

A clear distinction must be made between liquefied and fluidized systems. In liquefied beds and flows, the solids settle downward through the fluid, displacing it upward, whereas, in fluidized beds, the fluid moves upward through the solids, which are temporarily suspended without net downward movement. Many recent references to fluidized sediment gravity flows refer, in fact, to flows of liquefied debris. Most uniformly liquefied beds of well-sorted sand- or gravel-sized sediment will resediment as simple two-layer systems. Liquefied flows can originate either by liquefaction followed by failure, as in many retrogressive flow slides, or by failure followed by liquefaction, as in the case of some slumps. Empirical and theoretical estimates of flow velocity, thickness, and travel distance suggest that natural laminar liquefied flows of fine-grained sand will generally resediment after moving a kilometre or less. Laminar flows of coarse-grained sand will resediment after moving only a few metres. Grain dispersive pressure is thought to be of little significance in the development or maintenance of liquefied flows. Many surficial submarine sand beds are apparently susceptible to liquefaction, including submarine canyon and continental rise deposits. Within submarine canyons and narrow fjords, steep slopes and channels promote the evolution of liquefied flows from slumps by liquefaction after failure and of high density turbidity currents from liquefied flows by the development of turbulence. Upon moving into the lower parts of submarine canyons or into proximal fan channels, liquefied flows will resediment and high density turbidity currents will tend to decline to flows transitional between liquefied flows and turbidity currents. The liquefied, coarser detritus within such transitional flows will be deposited while finer-grained debris will remain in suspension and continue downslope as dilute turbidity currents. Resedimentation of the liquefied portions of such flows may be responsible for the deposition of the A-subdivision of many turbidites and many thick, structureless ‘proximal turbidites’ or ‘fluxoturbidites’. Similar units can originate by liquefaction of the traction deposits of normal turbidity currents. Fluidized flows are probably uncommon, thin, and, where formed, originate through fluidization of the fine-grained tops of liquefied graded beds.     

Passage of debris flows and turbidity currents through a topographic constriction: seafloor erosion and deflection of flow pathways

Some of the Earth's largest submarine debris flows are found on the NW African margin. These debris flows are highly efficient, spreading hundreds of cubic kilometres of sediment over a wide area of the continental rise where slopes angles are often <1°. However, the processes by which these debris flows achieve such long run‐outs, affecting tens of thousands of square kilometres of seafloor, are poorly understood. The Saharan debris flow has a run‐out of ≈700 km, making it one of the longest debris flows on Earth. For its distal 450 km, it is underlain by a relatively thin and highly sheared basal volcaniclastic layer, which may have provided the low‐friction conditions that enabled its extraordinarily long run‐out. Between El Hierro Island and the Hijas Seamount on the continental rise, an ≈25‐ to 40‐km‐wide topographic gap is present, through which the Saharan debris flow and turbidites from the continental margin and flanks of the Canary Islands passed. Recently, the first deep‐towed sonar images have been obtained, showing dramatic erosional and depositional processes operating within this topographic `gap' or `constriction'. These images show evidence for the passage of the Saharan debris flow and highly erosive turbidity currents, including the largest comet marks reported from the deep ocean. Sonar data and a seismic reflection profile obtained 70 km to the east, upslope of the topographic `gap', indicate that seafloor sediments to a depth of ≈30 m have been eroded by the Saharan debris flow to form the basal volcaniclastic layer. Within the topographic `gap', the Saharan debris flow appears to have been deflected by a low (≈20 m) topographic ridge, whereas turbidity currents predating the debris flow appear to have overtopped the ridge. This evidence suggests that, as turbidity currents passed into the topographic constriction, they experienced flow acceleration and, as a result, became highly erosive. Such observations have implications for the mechanics of long run‐out debris flows and turbidity currents elsewhere in the deep sea, in particular how such large‐scale flows erode the substrate and interact with seafloor topography.     

Submarine fan and slope-apron deposition in a Cretaceous Forearc Basin: The Gürsökü Formation (Kavak–Samsun, N. Turkey)

The Late Cretaceous Gürsökü Formation represents the proximal fill of the Sinop–Samsun Forearc Basin that was probably initiated by extension during the Early Cretaceous. The succession records sedimentation in two contrasting depositional systems: a slope-apron flanking a faulted basin margin and coarse-grained submarine fans. The slope-apron deposits consist of thinly bedded turbiditic sandstones and mudstones, interbedded with non-channelized chaotic boulder beds and intraformational slump sheets representing a spectrum of processes ranging from debris flow to submarine slides. The submarine fan sediments are represented by conglomerates and sandstones interpreted as deposited from high density turbidity currents and non-cohesive debris flows. The occurrence of both slope apron and submarine fan depositional systems in the Gürsökü Formation may indicates that the region was a tectonically active basin margin during the Late Cretaceous.     

Evidence for carbonate platform failure during rapid sea‐level rise; ca 14 000 year old bioclastic flow deposits in the Lesser Antilles

Bioclastic flow deposits offshore from the Soufrière Hills volcano on Montserrat in the Lesser Antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr. The volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world, which occurred on Montserrat in 2003. These flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south‐west flank of Antigua or the east flank of Redonda; adjacent islands that are not volcanically active. The bioclastic flow deposits are relatively coarse‐grained and either ungraded or poorly graded, and were deposited by non‐cohesive debris flow and high density turbidity currents. The bioclastic deposit often comprises multiple sub‐units that cannot be correlated between core sites; some located just 2 km apart. Multiple sub‐units in the bioclastic deposit result from either flow reflection, stacking of multiple debris flow lobes, and/or multi‐stage collapse of the initial landslide. This study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka. Few large submarine landslides have been well dated, but the slope failures that have been dated are commonly associated with periods of rapid sea‐level change.     

藏南江孜盆地晚侏罗至早白垩世重力流沉积

西藏南部江孜盆地的沙拉岗矿区发育一套晚侏罗至早白垩世的碎屑流、滑动流与浊流等重力流沉积,主要由斜坡相碎屑岩夹硅质岩和灰岩构成。在这个斜坡背景中出现了上斜坡相,下斜坡相与海底扇相等3种类型的岩相组合。其中,上斜坡以各种规模的滑动沉积为特色,下斜坡以不同性质的碎屑流沉积为特征。海底扇具有完整的内扇、中扇与外扇组合,可划分出进积型和退积型两种序列类型,它们多半是由浊流形成的各种砂体组成的。     

Coarse-grained submarine fan and slope apron deposits in a Cretaceous back-arc basin, Antarctica

The Cretaceous of west James Ross Island, Antarctica represents the proximal fill of a late Mesozoic back-arc basin that was probably initiated by oblique extension during the early development of the Weddell Sea. The succession records sedimentation in two contrasting depositional systems: a laterally persistent slope apron flanking the faulted basin margin interrupted both spatially and temporally by coarse-grained submarine fans. Slope apron deposits are dominated by thinly interbedded turbiditic sandstones and mudstones (mudstone association), interspersed with non-channelized chaotic boulder beds, intraformational slump sheets and isolated exotic blocks representing a spectrum of mass-flow processes from debris flow to submarine gliding. Localized sand-rich sequences (sandstone-breccia association) represent sandy debris lobes at the mouths of active slope chutes. The submarine fan sediments (conglomerate association) are typified by coarse conglomerates and pebbly sandstones, interpreted as the deposits of high-density turbidity currents and non-cohesive debris flows. Three assemblages are recognized and are suggested to represent components of the inner channelled zone of coarse-grained submarine fans, from major fan channels through ephemeral, marginal channels or terraces to levee or interchannel environments. The occurrence of both slope apron and submarine fan depositional systems during the Early and Mid-Cretaceous is attributed to localized input of coarse arc-derived sediment along a tectonically active basin margin. Periods of extensive fan development were probably linked to regional tectonic uplift and rejuvenation of the arc source region; cyclicity within individual fan sequences is attributed to migration or switching of fan channels or canyons. Slope apron sedimentation was controlled largely by intrabasinal tectonics. Local unconformities and packets of amalgamated slide sheets and debris flow deposits probably reflect episodic movement on basin margin faults. Differential subsidence across the basin margin anchored the basin slope for at least 20 Myr and precluded basinward progradation of shallow marine environments.     

浊流沉积研究的新进展:鲍马序列、海底扇的重新审视

本文在总结前人对浊流沉积研究的基础上,分析前人对浊流与浊积岩、浊流沉积与浊流相模式的对应关系之间的认识,并对鲍马序列进行重新审视。在海底扇研究过程中,鲍马序列已经不能充分反映浊流沉积的全过程。鲍马序列所反应的沉积模式其实是由碎屑流、浊流、底流等多种形式流体组合和改造后的结果,海底扇沉积模式不能笼统归结为浊流沉积作用的结果。在完善重力流、底流等沉积作用的同时,建立一个与沉积作用相互联系的深海沉积系统,以对深海研究提供更好地指导和预测。     

Can liquefied debris flows deposit clean sand over large areas of sea floor? Field evidence from the Marnoso‐arenacea Formation,Italian Apennines

The Marnoso‐arenacea Formation in the Italian Apennines is the only ancient rock sequence where individual submarine sediment density flow deposits have been mapped out in detail for over 100 km. Bed correlations provide new insight into how submarine flows deposit sand, because bed architecture and sandstone shape provide an independent test of depositional process models. This test is important because it can be difficult or impossible to infer depositional process unambiguously from characteristics seen at just one outcrop, especially for massive clean‐sandstone intervals whose origin has been controversial. Beds have three different types of geometries (facies tracts) in downflow oriented transects. Facies tracts 1 and 2 contain clean graded and ungraded massive sandstone deposited incrementally by turbidity currents, and these intervals taper relatively gradually downflow. Mud‐rich sand deposited by cohesive debris flow occurs in the distal part of Facies tract 2. Facies tract 3 contains clean sandstone with a distinctive swirly fabric formed by patches of coarser and better‐sorted grains that most likely records pervasive liquefaction. This type of clean sandstone can extend for up to 30 km before pinching out relatively abruptly. This abrupt pinch out suggests that this clean sand was deposited by debris flow. In some beds there are downflow transitions from turbidite sandstone into clean debrite sandstone, suggesting that debris flows formed by transformation from high‐density turbidity currents. However, outsize clasts in one particular debrite are too large and dense to have been carried by an initial turbidity current, suggesting that this debris flow ran out for at least 15 km. Field data indicate that liquefied debris flows can sometimes deposit clean sand over large (10 to 30 km) expanses of sea floor, and that these clean debrite sand layers can terminate abruptly.     

碎屑流与浊流的流体性质及沉积特征研究进展

受浊流沉积模式(即鲍马序列和浊积扇模式)的驱动和浊积岩思维定势的影响,自1970s浊流与浊积岩的概念逐渐扩大,特别是通过"高密度浊流"术语的引入,以及将水下浊流与陆上河流的错误类比,使得一部分碎屑流与底流的沉积被认为是浊积岩。随着现代观测设备的应用以及详细的岩芯观察,碎屑流(特别是砂质碎屑流)和浊流被重新认识。浊流是一种具牛顿流变性质和紊乱状态的沉积物重力流,其沉积物支撑机制是湍流。碎屑流是一种具塑性流变性质和层流状态的沉积物重力流,其沉积物支撑机制主要是基质强度和颗粒间的摩擦强度。浊流沉积具特征的正粒序韵律结构,底部为突变接触而顶部为渐变接触;碎屑流沉积一般具上、下两层韵律结构,即下部发育具平行碎屑结构的层流段,上部发育具块状层理的"刚性"筏流段。但当碎屑流被周围流体整体稀释改造且改造不彻底时,强碎屑流可变为中—弱碎屑流,相应自下而上可形成逆—正粒序的沉积韵律结构,其中发育有呈漂浮状的石英颗粒和泥质撕裂屑等碎屑颗粒,明显区别于浊流沉积单一的正粒序韵律结构特征。碎屑流沉积顶、底部均为突变接触。浊流的沉积模式为简单的具平坦盆底的坡底模式,而碎屑流则为复杂的斜坡模式。     

Sedimentation of submarine fan deposits in the Pindos foreland basin,from late Eocene to early Oligocene,west Peloponnesus peninsula,SW Greece

Turbidite facies distribution and palaeocurrent analysis of submarine fan evolution in the Pindos foreland basin of west Peloponnesus peninsula (SW Greece) indicate that this part of the foreland was developed during Late Eocene to Early Oligocene in three linear sub‐basins (Tritea, Hrisovitsi and Finikounda). The basin fill conditions, with a multiple feeder system, which is characterized by axial transport of sediments and asymmetric stratigraphic thickness of the studied sediments, indicate that the Pindos Foreland Basin in this area was an underfilled foreland basin. Sediments are dominated by conglomerates, sandstones and mudstones. The flow types that controlled the depositional processes of the submarine fans were grain flows, debris flows and low‐ and high‐density turbidity currents. The sedimentary model that we propose for the depositional mechanisms and geometrical distribution of the turbidite units in the Tritea sub‐basin is a mixed sand‐mud submarine fan with a sequential interaction of progradation and retrogradation for the submarine fan development and shows a WNW main palaeocurrent direction. The Hrisovitsi sub‐basin turbidite system characterized by small‐scale channels was sediment starved, and the erosion during deposition was greater than the two other studied areas, indicating a more restricted basin topography with a NW main palaeocurrent direction. The Finikounda sub‐basin exhibits sand‐rich submarine fans, is characterized by the presence of distinct, small‐scale, thickening‐upward cycles and by the covering of a distal fan by a proximal fan. It was constructed under the simultaneous interaction of progradation and aggradation, where the main palaeocurrent direction was from NNW to SSE. Copyright © 2012 John Wiley & Sons, Ltd.     

The sequence of events around the epicentre of the 1929 Grand Banks earthquake: initiation of debris flows and turbidity current inferred from sidescan sonar

Continental slope sediment failures around the epicentre of the 1929 'Grand Banks' earthquake have been imaged with the SAR (Système Acoustique Remorqué) high-resolution, deep-towed sidescan sonar and sub-bottom profiler. The data are augmented by seismic reflection profiles, cores and observations from submersibles. Failure occurs only in water depths greater than about 650 m. Rotational, retrogressive slumps, on a variety of scales, appear to have been initiated on local steep areas of seabed above shallow (5–25 m) regional shear planes covering a large area of the failure zone. The slumps pass downslope into debris flows, which include blocky lemniscate bodies and intervening channels. Clear evidence of current erosion is found only in steep-sided valleys: we infer that debris flows passed through hydraulic jumps on these steep slopes and were transformed into turbidity currents which then evolved ignitively. Delayed retrogressive failure and transformation of debris flows into turbidity currents through hydraulic jumps provide a mechanism to produce a turbidity current with sustained flow over many hours.     

The tail of the Storegga Slide: insights from the geochemistry and sedimentology of the Norwegian Basin deposits

Deposits within the floor of the Norwegian Basin were sampled to characterize the deposition from the Storegga Slide, the largest known Holocene‐aged continental margin slope failure complex. A 29 to 67 cm thick veneer of variable‐coloured, finely layered Holocene sediment caps a homogeneous, extremely well‐sorted, poorly consolidated, very fine‐grained, grey‐coloured sediment section that is >20 m thick on the basin floor. This homogeneous unit is interpreted to represent the uppermost deposits generated by a gravity flow associated with the last major Storegga Slide event. Sediments analogous to the inferred source material of the slide deposits were collected from upslope on the Norwegian Margin. Sediments sampled within the basin are distinguishable from the purported source sediments, suggesting that size sorting has significantly altered this material along its flow path. Moreover, the very fine grain size (3·1 ± 0·3 μm) suggests that the >20 m thick homogeneous unit which was sampled settled from suspension after the turbulent flow was over. Although the turbulent phase of the gravity flow that moved material out into the basin may have been brief (days), significantly more time (years) is required for turbid sediments to settle and dewater and for the new sea floor to be colonized with a normal benthonic fauna. Pore water sulphate concentrations within the uppermost 20 m of the event deposit are higher than those normally found in sea water. Apparently the impact of microbial sulphate reduction over the last ca 8·1 cal ka bp since the re‐deposition of these sediments has not been adequate to regenerate a typical sulphate gradient of decreasing concentration with sub‐bottom depth. This observation suggests low rates of microbial reactions, which may be attributed to the refractory carbon composition in these re‐deposited sediments.     

Late Quaternary mass movement on the lower continental rise and abyssal plain off Western Sahara

The late Quaternary development of part of the lower continental rise off Western Sahara has been determined from an investigation of short (< 2 m) gravity cores collected from a deep-sea channel, the interchannel areas and an abyssal hill, between 30 and 33°N. Stratigraphic analysis is based on systematic variations in abundances of particular coccolith species and pelagic sediment types, referenced to the oxygen isotope time-scale. During the last 73 000 years deposition in the channel has included volcaniclastic sand/silt turbidites and minor marl turbidites as well as pelagic sediments. The interchannel area has fewer turbidites, and the sands present were probably deposited from turbidity currents which spilt over the channel sides. The last‘event’ to give rise to sands in the channel and interchannel area occurred about 45 000 years ago. Although the channel has been inactive as an area of turbidity current deposition for the last 20 000 years, sands were deposited elsewhere on the lower rise, indicating that turbidity current transport routes have varied in time. Turbidity current deposition on the abyssal plain and low-lying continental rise appears to be related to distinct sliding events involving transport of material from various sources. Thin marl turbidites are interbedded with pelagic sediments in the area of sediment drape. There is a strong correlation between these and the thick marl turbidites on the abyssal plain, suggesting that the same turbidity current‘events’, occurring about once every 25 000 years, gave rise to both sets of deposits. The thinner units probably represent deposition from the outer parts or tails of the large turbidity flows. The turbidites occur at glacial/interglacial transitions, suggesting that the slides that created them were triggered by mechanisms related to climatic change. Several volcaniclastic sand/silt units within the channel and in interchannel areas occupy mid-stage stratigraphic positions, perhaps indicating a different triggering mechanism for slides around volcanic islands. A debris flow deposit (debrite), between 30°N, 21°W and 31°N, 24°W, is related to the Saharan Sediment Slide, a major mass movement feature on the continental slope over 1000 km to the southeast. Stratigraphic correlations indicate that this slide produced a large turbidity current as well as a debris flow.     

Channelized debris‐flow deposits and their impact on turbidity currents: The Puchkirchen axial channel belt in the Austrian Molasse Basin

Deposits of submarine debris flows can build up substantial topography on the sea floor. The resulting sea floor morphology can strongly influence the pathways of and deposition from subsequent turbidity currents. Map views of sea floor morphology are available for parts of the modern sea floor and from high‐resolution seismic‐reflection data. However, these data sets usually lack lithological information. In contrast, outcrops provide cross‐sectional and lateral stratigraphic details of deep‐water strata with superb lithological control but provide little information on sea floor morphology. Here, a methodology is presented that extracts fundamental lithological information from sediment core and well logs with a novel calibration between core, well‐logs and seismic attributes within a large submarine axial channel belt in the Tertiary Molasse foreland basin, Austria. This channel belt was the course of multiple debris‐flow and turbidity current events, and the fill consists of interbedded layers deposited by both of these processes. Using the core‐well‐seismic calibration, three‐dimensional lithofacies proportion volumes were created. These volumes enable the interpretation of the three‐dimensional distribution of the important lithofacies and thus the investigation of sea floor morphology produced by debris‐flow events and its impact on succeeding turbidite deposition. These results show that the distribution of debris‐flow deposits follows a relatively regular pattern of levées and lobes. When subsequent high‐density turbidity currents encountered this mounded debris‐flow topography, they slowed and deposited a portion of their sandy high‐density loads just upstream of morphological highs. Understanding the depositional patterns of debris flows is key to understanding and predicting the location and character of associated sandstone accumulations. This detailed model of the filling style and the resulting stratigraphic architecture of a debris‐flow dominated deep‐marine depositional system can be used as an analogue for similar modern and ancient systems.     

Eocene fan delta-submarine fan deposition in the Wagwater Trough, east-central Jamaica

ABSTRACT The Wagwater Trough is a fault-bounded basin which cuts across east-central Jamaica. The basin formed during the late Palaeocene or early Eocene and the earliest sediments deposited in the trough were the Wagwater and Richmond formations of the Wagwater Group. These formations are composed of up to 7000 m of conglomerates, sandstones, and shales. Six facies have been recognized in the Wagwater Group: Facies I-unfossiliferous massive conglomerates; Facies II—channelized, non-marine conglomerates, sandstones, and shales; Facies III-interbedded, fossiliferous conglomerates and sandstones; Facies IV—fossiliferous muddy conglomerates; Facies V—channelized, marine conglomerates, sandstones, and shales; and Facies VI—thin-bedded sheet sandstones and shales. The Wagwater and Richmond formations are interpreted as fan delta-submarine fan deposits. Facies associations suggest that humid-region fan deltas prograded into the basin from the adjacent highlands and discharged very coarse sediments on to a steep submarine slope. At the coast waves reworked the braided-fluvial deposits of the subaerial fan delta into coarse sand and gravel beaches. Sediments deposited on the delta-front slope were frequently remobilized and moved downslope as slumps, debris flows, and turbidity currents. At the slope-basin break submarine fans were deposited. The submarine fans are characterized by coarse inner and mid-fan deposits which grade laterally into thin bedded turbidites of the outer fan and basin floor.     

湖南道县、宁远一带中泥盆统棋梓桥组中的台地前缘斜坡异地碳酸盐沉积

沿台地边缘斜坡沉积的古代碳酸盐再沉积物是最近十年来才引起沉积学者广泛注意的一种深水碳酸盐类型,包括碳酸盐重力流,重力滑动(滑塌)沉积及塌磊裙。重力流及由于重力作用引起的沉积物与浅水碳酸盐或深水原地碳酸盐沉积相比,有很大的差异。它们的主要岩石类型是异地碳酸盐,为一套在重力作用下顺坡而下流动的碳酸盐沉积物。前礁塌磊是台地边缘礁坠落到陡崖下的礁块堆积,也称为礁崖塌积。     

Transition from debris flow to hyperconcentrated flow in a submarine channel (the Cretaceous Cerro Toro Formation, southern Chile)

ABSTRACT It is important to understand the exact process whereby very large amounts of sediment are transported. This paper reports peculiar conglomerate beds reflecting the transition of submarine debris flows into hyperconcentrated flows, something that has been well documented only in subaerial debris-flow events until now. Voluminous debris flows generated along a Cretaceous submarine channel, southern Chile, transformed immediately into multiphase flows. Their deposits overlie fluted or grooved surfaces and comprise a lower division of clast-supported and imbricated pebble–cobble conglomerate with basal inverse grading and an upper division of clast- to matrix-supported, disorganized conglomerate with abundant intraformational clasts. The conglomerate beds suggest temporal succession of turbidity current, gravelly hyperconcentrated flow, and mud-rich debris flow phases. The multiphase flows resulted from progressive dilution of gravelly but cohesive debris flows that could hydroplane, in contrast to the flow transitions in subaerial environments, which involve mostly non-cohesive debris flows. This finding has significant implications for the definition, classification, and hazard assessment of submarine mass-movement processes and characterization of submarine reservoir rocks.     

Deep-sea volcaniclastic sedimentary systems: an example from La Fournaise volcano, Réunion Island, Indian Ocean

A volcaniclastic sedimentary fan extending to water depths of 4000 m is characterized using gravity cores, camera surveys, high-resolution sonar images, seismic records and bathymetry from the submarine portion of La Fournaise volcano, Réunion Island, a basaltic shield volcano in the SW Indian Ocean. Three main areas are identified from the study: (1) the proximal fan extending from 500 m water depth down to 2000 m water depth; (2) the outer fan extending from 2000 m water depth down to 3600 m water depth; (3) the basin extending beyond 3600 m water depth. Within these three main areas, seven distinct submarine environments are defined: the proximal fan is characterized by volcanic basement outcrops, sedimentary slides, deep-water deltas, debris-avalanche deposits, and eroded floor in the valley outlets; the outer fan is characterized by a discontinuous fine-grained sedimentary cover overlying coarse-grained turbidites or undifferentiated volcanic basement; the basin is characterized by hemipelagic muds and fine-grained turbidites interbedded with sandy and gravelly turbidite lobes. The evolution of the deep-sea volcaniclastic fan is strongly influenced by sector collapses, such as the one which occurred 0·0042 Ma ago. This collapse produced a minimum of 6 km³ of debris-avalanche deposit in the proximal area. The feeding regime of the deep-sea fan is ‘alluvial dominated’ before the occurrence of any sector collapse and ‘lava-dominated’ after the occurrence of a sector collapse. The main deep-water lava-fed delta is prograding among the blocks of the debris-avalanche deposits as a result of turbidity flows occurring on the delta slope. These turbidity flows are triggered routinely by wave-action, earthquakes and accumulation of new volcanic debris on top of the deltas. Both turbidity currents triggered on the deep-water delta slope, and those triggered by debris avalanche reworked volcaniclastic material as far as 100 km from the shore line.     

浊流和泥石流的异重流初期潜入点的实验研究

潜入点的水流泥沙条件是异重流的发生条件和持续条件,受到了国内外学者的广泛关注。初期潜入点Fr代表异重流的发生条件,而稳定潜入点Fr则代表异重流的持续条件。通过一系列的低浓度浊流和高浓度浊流及泥石流的异重流潜入点的实验研究,分析对比低浓度浊流和高浓度浊流及泥石流的异重流的关系,得到在均匀顺直水槽中的异重流初期的潜入点Fr规律。提出在一定的水槽宽度、泥沙浓度条件下,异重流的初期潜入点Fr与头部流速成正比。与其它文献不同研究条件的实验结果对比有很好的一致性,说明该研究结论有很好的可靠性。该研究结论的适用范围为初期潜入点,不适用于稳定潜入点和初期潜入点与稳定潜入点之间的过渡阶段。但高浓度浊流和泥石流的异重流初期潜入点Fr与其稳定潜入点Fr很接近,粘性泥石流的异重流更接近。     

The role of sedimentation rate and permeability in the slope stability of the formerly glaciated Norwegian continental margin: the Storegga slide model

Despite the gently dipping slopes (ca 1°), large-scale submarine slope failures have occurred on the mid-Norwegian continental margin (Storegga, Sklinnadjupet, Traenadjupet), suggesting the presence of special conditions predisposing to failure in this formerly glaciated margin. With a volume estimated between 2,400 and 3,200 km³ and an affected area of approximately 95,000 km², the Storegga slide represents one of the largest and best-studied submarine slides of Holocene age known worldwide. Finite element modeling of slope failure indicates that a large (6.5 < Ms < 7.0) seismic triggering mechanism would not be sufficient to cause failure at more than 110 m below the seabed as observed for the slip planes at Storegga (northern sidewall). This implies that other factors (e.g., liquefaction, strain softening, gas charging, rapid burial) are needed to explain the occurrence of the Storegga slide with a deep surface of failure. In this paper, we discuss the importance of the compaction effect of rapidly accumulated sediments in the slide area. During compaction, sediment grains reorganize themselves, thereby, expelling pore water. Consequently, depending on sedimentation rate and permeability, excess pore pressures might result beneath less permeable sediments. Our modeling and cross-checking illustrate how excess pore pressure generation due to high sedimentation rate could explain the development of layers of weakness, and thus, how such a large slide might have been initiated in deep sediments. Using the highest sedimentation rate estimated in the area (36 and 27 m/kyr between 16.2 and 15 kyr BP), 1D modeling shows excess pore pressure values of around 200 kPa at a depth of 100 m below the seafloor 15 kyr BP and 60 kPa at a depth of 100 m at the time of the slide (8 kyr BP). Excess pore pressure apparently drastically reduced the resistance of the sediment (incomplete consolidation). In addition, 2D modeling shows that permeability anisotropies can significantly affect the lateral extent of excess pore pressure dissipation, affecting, that way, normally consolidated sediments far from the excess pore pressure initiation area.