The sediment budget and dynamics of a delta‐canyon‐lobe system over the Anthropocene timescale: The Rhone River delta,Lake Geneva (Switzerland/France)

Deltas are important coastal sediment accumulation zones in both marine and lacustrine settings. However, currents derived from tides, waves or rivers can transfer that sediment into distal, deep environments, connecting terrestrial and deep marine depozones. The sediment transfer system of the Rhone River in Lake Geneva is composed of a sublacustrine delta, a deeply incised canyon and a distal lobe, which resembles, at a smaller scale, deep‐sea fan systems fed by high discharge rivers. From the comparison of two bathymetric datasets, collected in 1891 and 2014, a sediment budget was calculated for eastern Lake Geneva, based on which sediment distribution patterns were defined. During the past 125 years, sediment deposition occurred mostly in three high sedimentation rate areas: the proximal delta front, the canyon‐levée system and the distal lobe. Mean sedimentation rates in these areas vary from 0·0246 m year^?1 (distal lobe) to 0·0737 m year^?1 (delta front). Although the delta front–levées–distal lobe complex only comprises 17·0% of the analysed area, it stored 74·9% of the total deposited sediment. Results show that 52·5% of the total sediment stored in this complex was transported toward distal locations through the sublacustrine canyon. Namely, the canyon–levée complex stored 15·9% of the total sediment, while 36·6% was deposited in the distal lobe. The results thus show that in deltaic systems where density currents can occur regularly, a significant proportion of riverine sediment input may be transferred to the canyon‐lobe systems leading to important distal sediment accumulation zones.     

坦桑尼亚滨海盆地陆坡峡谷沉积特征及其控制因素*

东非陆缘深水盆地具有巨大油气资源潜力,但对陆坡峡谷沉积特征研究较少,制约有利储集层预测。本研究利用三维地震资料,对东非坦桑尼亚滨海盆地陆坡峡谷开展精细研究。结果表明:(1)研究区陆坡发育多条大型海底峡谷;上陆坡处,坡度较陡,峡谷内以侵蚀作用为主,沉积物主要局限在褶皱推覆带的翼部;褶皱推覆带之外的下陆坡区,坡度变缓,峡谷末端发育席状砂质沉积及砂泥混杂的碎屑流沉积,同时在峡谷北侧发育向北延伸的泥质漂积体;在陆坡边缘,发育海底滑塌,形成块体搬运沉积。(2)峡谷沉积受陆源物质供给、褶皱推覆带、北大西洋底流以及陆坡边界断层等因素控制。受东非裂谷海域分支活动影响,研究区陆架窄、陆坡陡,陆源物质可迅速通过陆架,进入陆坡峡谷:与河流相连的峡谷,物源充足、规模较大,有沉积物发育而没有与河流直接相连的峡谷物源有限、规模较小,峡谷内无明显沉积;褶皱推覆带通过改变海底地形来控制峡谷内沉积分布,褶皱翼部发育沉积,核部则以侵蚀为主;褶皱推覆带外,北大西洋底流与峡谷末端重力流发生交互作用,细粒物质被搬运至峡谷北岸形成漂积体;陆坡边缘断层活跃,峡谷被断层切割,形成断崖,并引发海底滑塌,陆坡处不发育水道及朵体沉积,陆源物质通过峡谷被搬运至更深的深海盆地内。     

鄱阳湖浅水三角洲沉积体系三维定量正演模拟

以鄱阳湖现代浅水三角洲沉积体系为例, 应用三维正演地层模拟软件Sedsim, 在参考前人研究的基础上, 首次将湖盆底部地形、湖(海)平面变化、沉积物注入量及注入方式、气候、沉积物供给速率等动力要素结合在一起, 对该浅水三角洲沉积体系的形成过程及1200年以来的演化进行定量正演模拟, 并采用历史和野外数据对鄱阳湖现代浅水三角洲沉积模型进行约束和校正.模拟结果表明, 鄱阳湖浅水三角洲沉积体系的发育是湖盆地形、湖平面变化、物源供给等多因素作用的综合结果.在该三角洲沉积体系中, 由于水体较浅、沉积底形坡度平坦且基准面变化频繁, 三角洲前缘发育的砂体基本上以席状砂为主, 并主要分布于湖区敞流通道附近.湖平面之上的三角洲平原河道发育与改道的现象主要受湖平面变化速率的影响, 即基准面缓慢上升期间和基准面快速下降期间, 河道发育的现象较明显.该模拟结果不仅能够对大型浅水三角洲的内部特征及形成过程有着更直观的认识, 而且也为今后研究不同地区相似的三角洲沉积体系的形成过程提供了可借鉴的分析模型与理论依据.      

现代乌伦古湖滨岸沉积环境与沉积体系分布及其控制因素

通过现场实地踏勘、拍照、开挖探槽、利用卫星图解译等方法,对乌伦古湖环布伦托海区域和吉力湖北部乌伦古河现代三角洲地区的湖泊滨岸沉积环境和沉积体系进行了现代沉积调查。研究表明乌伦古湖滨岸沉积环境可以划分为基岩型湖岸、砾质湖岸、砂质湖岸、泥质湖岸等4种类型,发育山前基岩型湖岸、侵蚀基岩型湖岸、砾质冲积扇-扇三角洲、砾质辫状河三角洲、砾质滩坝、砂质滩坝、砂质三角洲、风成沙丘和泥质沼泽等9种滨岸沉积体系。山前基岩湖岸分布在布伦托海的北部,主要发育小型塌积扇、倒石锥和狭窄的湖滩。侵蚀型基岩湖岸位于布伦托海西岸和东北角地区,发育湖滩宽20~40 m。砾质冲积扇-扇三角洲沉积体系分布在布伦托海西北部25.8 km狭长区域,表现为一系列冲积扇-扇三角洲体系在山前形成裙边状展布的辫状平原,顺流向长5~15 km。砾质辫状河三角洲体系发育在布伦托海西部,砾质滩坝发育在砾质三角洲前缘,沉积物一般为中砾和粗砾,泥质含量低。现代乌伦古河三角洲位于吉力湖北部,沙丘广泛分布在布伦托海东部的三角洲平原。砂质滩坝发育在布伦托海东岸南部地区,滩坝带宽30~100 m,发育大量障碍痕、冰划痕。泥质沼泽占据湖岸总长度29.22 km,沼泽地带植物繁茂,水动力微弱,泥质和有机质含量高。根据卫星照片推测乌伦古湖水位可能发生过3次较大的下降,现代乌伦古河三角洲可能经过了4个发育阶段,但目前缺乏地质年代学证据。构造格局控制了湖泊边界的地形地貌特征,平行构造线走向容易形成规模较大的沉积体系,垂直构造走向形成的沉积体系规模较小。寒旱地区湖泊周缘入湖河流较少,具有季节性和暂时性特点,洪水泥石流、塌积扇等重力沉积体系比较发育。湖泊封冻是寒旱区湖泊区别于温暖地区湖泊的重要特征。在相同气候背景下,源汇地区的高差和河流的流程、流量决定了沉积物的供给总量和沉积体系的特征。湖盆边界形态影响沿岸流的发育,也影响湖泊风动力方向和强度。乌伦古湖滨岸沉积体系的多样性对研究古代湖泊滨岸沉积体系具有重要的启发,开展湖泊滨岸沉积环境和沉积体系调查对完善陆相湖盆沉积体系模式,对发现新的储层类型,对重建湖泊古地理环境具有重要的意义。     

An ancient submarine canyon in the Oligocene-Miocene of the western Niger Delta

ABSTRACT
An anomalous sequence of thick fossiliferous marine shale of late Oligocene-early Miocene age, here termed the Opuama Shale Member, occurs within the paralic Agbada Formation in the subsurface of the western Niger Delta. The Opuama Shale fills a deep palaeochannel which was cut into Eocene-early Oligocene paralic beds. Planktonic and benthic foraminifera suggest that deposition in the channel began in the late Oligocene-early Miocene at outer shelf-slope depths, and that by the late early Miocene the channel had filled to shallow neritic depths. The channel, which is believed to be of submarine origin, is termed the Opuama canyon. The Opuama canyon originated during the pronounced early-Middle Oligocene drop in sea-level, and was cut by turbidity currents. It was filled, during the major late Oligocene-early Miocene rise of sea-level.
The location of the Opuama canyon in the western re-entrant of the Niger Delta supports Burke's (1972) conclusion that the western and eastern (where ancient submarine canyon fills are known) re-entrants of the delta are potential sites for ancient submarine canyons. Burke based his conclusion on the observation that the eastern and western re-entrants of the modern delta are areas where opposing longshore drifts converge and generate turbidity currents which cut submarine canyons.     

Seismic stratigraphy and development of Avon canyon in Benin (Dahomey) basin, southwestern Nigeria

Interpretation of a grid of high resolution seismic profiles from the offshore eastern part of the Benin (Dahomey) basin in southwestern Nigeria area permitted the identification of cyclic events of cut and fill associated with the Avon canyon. Seismic stratigraphic analysis was carried out to evaluate the canyon morphology, origin and evolution. At least three generations of ancient submarine canyons and a newly formed submarine canyon have been identified. Seismic reflection parameters of the ancient canyons are characterized by transparent to slightly transparent, continuous to slightly discontinuous, high to moderate amplitude and parallel to sub-parallel reflections. Locally, high amplitude and chaotic reflections were observed. The reflection configurations consist of regular oblique, chaotic oblique, progradational and parallel to sub-parallel types. These seismic reflection characteristics are probably due to variable sedimentation processes within the canyons, which were affected by mass wasting. Canyon morphological features include step-wise and spoon-shaped wall development, deep valley incision, a V-shaped valley, similar orientation in the southeast direction, and simple to complex erosion features in the axial floor. The canyons have a composite origin, caused partly by lowering of the sea level probably associated with the formation of the Antarctic Ice Sheet about 30 Ma ago and partly by complex sedimentary processes. Regional correlation with geological ages using the reflectors show that the canyons cut through the Cretaceous and lower Tertiary sediments while the sedimentary infill of the canyon is predominantly Miocene and younger. Gravity-driven depositional processes, downward excavation by down slope sediment flows, mass wasting from the canyon walls and variation in terrigenous sediment supply have played significant roles in maintaining the canyons. These canyons were probably conduits for sediment transport to deep-waters in the Gulf of Guinea during their period of formation.     

Turbidite deposition on the glacially influenced,canyon‐dominated Southwest Grand Banks Slope,Canada

The deeply dissected Southwest Grand Banks Slope offshore the Grand Banks of Newfoundland was investigated using multiple data sets in order to determine how canyons and intercanyon ridges developed and what sedimentary processes acted on glacially influenced slopes. The canyons are a product of Quaternary ice‐related processes that operated along the margin, such as ice stream outwash and proglacial plume fallout. Three types of canyon are defined based on their dimensions, axial sedimentary processes and the location of the canyon head. There are canyons formed by glacial outwash with aggradational and erosional floors, and canyons formed on the slope by retrogressive failure. The steep, narrow intercanyon ridges that separate the canyons are composite morphological features formed by a complex history of sediment aggradation and degradation. Ridge aggradation occurred as a result of mid to late Quaternary background sedimentation (proglacial plume fallout and hemipelagic settling) and turbidite deposition. Intercanyon ridge degradation was caused mainly by sediment removal due to local slump failures and erosive sediment gravity flows. Levée‐like deposits are present as little as 15 km from the shelf break. At 30 km from the shelf, turbidity currents spilled over a 400 m high ridge and reconfined in a canyon formed by retrogressive failure, where a thalweg channel was developed. These observations imply that turbidity currents evolved rapidly in this slope‐proximal environment and attained flow depths of hundreds of metres over distances of a few tens of kilometres, suggesting turbulent subglacial outwash from tunnel valleys as the principal turbidity current‐generating mechanism.     

鄂尔多斯盆地西峰油田长8油组辫状河三角洲沉积特征与层序演化

对鄂尔多斯盆地西峰油田长8油组辫状河三角洲沉积特征与层序演化的研究表明,在长8组油组沉积时,湖盆分别经历了强烈下降和缓慢下沉两个不同的沉积阶段,湖盆层序的演化明显控制着辫状河三角洲沉积体系及有利相带的发育和分布.在长8油组沉积早期,湖盆缓慢下沉,沉积物供给充分,辫状河三角洲形成强进积层序,其分流作用及侧向迁移能力强,区内平原亚相覆盖面广,有利砂层厚度大,分布稳定,预测其将成为区内进一步油气勘探的重要目的层;在长8油组沉积晚期,湖盆强烈下降,沉积速率减小,以退积层序发育为特征,退积过程中,辫状河三角洲平原亚相向湖岸退缩,导致西峰油田主要赋存于辫状河三角洲前缘水下分流河道之中.     

湖缘峡谷及其含油性

在陆相断陷湖盆陡坡带,控盆断裂的剧烈活动导致在相对较短时期内形成盆地的快速沉降,与湖盆边缘地形形成巨大高差。沿此边缘,经过早期阵发性洪水及河流的强烈下蚀,并不断向源侵蚀,在古断面上形成宽达数km,深达上km的深切谷,称之为湖缘峡谷。东营凹陷北部陡坡带的Y921峡谷即为湖缘峡谷,其现今形态为长7000m、宽3500m、深3500m、剖面呈“U”型的特征。古构造及古生物分析认为,在古新世至渐新世时期,陈南断层剧烈活动,使断层两侧形成较大的高差,在干旱、阵发性洪水侵蚀下逐渐形成Y921峡谷,并在其底部形成侵蚀面。孢粉分析认为,仅沙四上段沉积时期就有近1000m的高差。后期,随湖平面上升,峡谷逐渐充填了古近系沙河街组四段、三段和部分二段地层,主要为辫状河粗碎屑砂砾岩、扇三角洲相交错层理砂岩和滨浅湖相泥岩、粉砂岩。这种峡谷的特点是短、宽、深,其充填模式主要为辫状河-扇三角洲-湖泊相沉积体系。由于峡谷前缘为深湖相泥岩作为生油岩,陈南断层和峡谷内的冲刷面作为油气运移通道,峡谷内砂砾岩作为储层,因此在峡谷内形成了多种非构造油气藏类型。     

Sedimentary dynamics and evolutionary history of a Late Carboniferous Gondwanic lake in north-western Argentina

Lacustrine deposits are well represented in the lower part of the Late Carboniferous Agua Colorada Formation in the north-west Sierra de Narváez, Catamarca Province, Argentina. Lake Narváez was one of the several water bodies formed in the region immediately after the Gondwana glaciation. The lacustrine transport system has been divided into three distinct zones: delta, shallow lake and deep lake. Delta progradation proceeded from the ESE. Coarse-grained delta plain and turbidite delta front deposits suggest that the delta was formed close to the headwaters (‘short-headed stream delta’type). During periods of high discharge, river mouths acted as bypass zones and fine and very fine sands were transported further into the lake by underflow currents. The clastic material supplied by the deltaic system was partially reworked by wave action. Sands accumulated in unstable conditions at the upper delta front as a consequence of delta progradation. As a result of the addition of clastics in the steep delta front, turbidity currents were formed, spreading their load along the lower delta slope. Deep lacustrine deposits are typically stacked, forming two different kinds of progradational turbidite lobe sequences. Type I lobes were formed in a basinal setting and were probably detached from their feeder systems as a result of sediment-bypassing in a shallow lake during periods of low lake level. These turbidite lobes are replaced upwards by type II lobes, which were formed on the delta slope during periods of lake level rise that allowed the onset of delta progradation. The presence of highly deformed sandstone bodies suggests rapid depositional rates in a high slope setting, whereas the occurrence of hummocky cross-stratified sandstones indicates wave reworking of the sands initially emplaced by turbidity currents. Therefore, the inner part of type II lobes was formed above storm wave base. The depositional history of Lake Narváez can be traced through four evolutionary stages: lake transgression, formation of type I lobes, formation of type II lobes and delta progradation. Tectonic activity was probably important at the early stage of lake evolution, but the subsequent depositional history was mainly controlled by fluctuations of lake level.     

Late Proterozoic Patsy Springs Canyon,Adelaide Geosyncline: submarine or subaerial origin?

A significant aspect of Late Proterozoic sedimentation in the Adelaide Geosyncline, South Australia, is the presence of kilometre-deep erosional incisions which have been termed canyons. These structures were formerly described to be of submarine origin, cut and filled in an inferred basin-slope setting by subaqueous processes. Subsequent detailed research, particularly on a specific incision known as Patsy Springs Canyon, indicates that sedimentary structures within some of the canyon-filling sediments are indicative of deposition above fair weather wave base. In addition, an unusual carbonate unit, which is observed to veneer upper portions of canyon shoulders and to contribute to carbonate breccias interbedded with canyon-fill, has a stable isotope signature which may imply a non-marine origin. The presence of the carbonate veneer, where it is in situ, suggests that at least upper portions of the canyons could have been emergent during the canyon-filling phase. Considering these observations, and combining them with regional stratigraphical relationships, an alternative model for canyon genesis is proposed involving subaerial erosion and subsequent filling by coastal onlap. Such a model requires base-level changes of the order of 1 km, in order to account for observed canyon cutting and filling. Vertical movements associated with halokinesis, or thermally-induced uplift of the order of 1 km, could have resulted in the observed erosional events. Alternatively, a Messinian-style evaporitic lowering of base-level is currently receiving serious attention. With present knowledge this mechanism most satisfactorily explains all observations.     

Interplay between river discharge and topography of the basin floor in a hyperpycnal lacustrine delta

Basin‐floor topography influences the flow path of hyperpycnal plumes and delta morphology during progradation of the Red River delta in Lake Texoma, USA. The Red River discharge is typically a hyperpycnal plume due to elevated total dissolved solids. Because the river plume is a bottom‐hugging hyperpycnal flow, lake bathymetry and topography strongly influence deposition and subsequent delta morphology. In addition to elevated total dissolved solid concentrations compared with Lake Texoma water, the density contrast of the Red River outflow is increased by high suspended‐sediment concentrations during high‐discharge events. Steep lateral slopes in the Lake Texoma basin deflect hyperpycnal river plumes and, subsequently, change the delta progradation direction before the delta reaches the opposite bank of the lake. Analysis of multi‐temporal aerial and satellite images indicates that the hyperpycnal delta follows the steepest lake‐bottom gradients, corresponding to the pre‐impoundment river thalweg (i.e. bypassing shallow parts of the lake). An analytical model for the hyperpycnal‐plume trajectory indicates plume deflection during low‐discharge or high‐discharge events, towards the deepest part of the basin. The magnitude of plume deflection is a function of river discharge and basin‐margin gradients. Plume deflection can vary between 10° and 80° from the channel axis towards the old river thalweg. The high deflection appears in the case of maximum basin side gradients of 12·8° and in conditions of low river discharge. During low‐discharge periods, the Red River delta builds a lobate shape with multiple terminal distributary channels whereas, during high‐discharge periods the Red River delta builds an elongate shape with a single large distributary channel. The elongate morphology of the delta is formed through the development of a single distributary channel and abandonment of the other distributaries. Therefore, the lobate shaped delta is expected to be preserved in the rock record.     

A large-scale meandering submarine canyon: outcrop example from the late Proterozoic Adelaide Geosyncline, South Australia

The late Proterozoic Adelaide Geosyncline, along with overlying Cambrian strata, comprises a thick sequence of sediments and sparse volcanics which accumulated in a major rift and passive margin setting. During late syn-rift or early post-rift phases, large volumes of terrigenous and carbonate sediments of the late Proterozoic Umberatana and Wilpena Groups and Cambrian Hawker Group filled the rift. Submarine canyon development was related to at least four of these depositional cycles, the most notable of which resulted in incision and subsequent filling of the major (several kilometres in width and up to 1.5 km deep) submarine canyons by the Wonoka Formation. The Wonoka Formation canyons are not obviously fault controlled. They are interpreted to have been eroded by turbidity currents during a relative low-stand of sea-level. They were subsequently filled by a fining-upwards suite of sediments which reflects subsequent relative rise of sea-level and carbonate platform development. Ultimately the canyon complex was buried by north-westerly progradation of overlying fluvial and slope sequences (Billy Springs Beds and possibly correlative upper Pound Subgroup). It is considered likely that more distal elements of this prograding clastic wedge provided the necessary material for canyon erosion, prior to canyon filling and ultimate burial by what may have been elements of the same depositional cycle. It is considered possible that the series of isolated outcrops of canyon cross-sections within the Wonoka Formation are sections of a single canyon thalweg developed within a considerably broader zone of slope degradation. If this interpretation is correct, then the gorge-like Patsy Springs Canyon lies in more proximal regions of the basin-slope, whereas 40 km to the north-east the lower slope is cut by the Fortress Hill Canyon Complex. Palaeocurrent analyses of channel-fill turbidites within the canyons imply that the Fortress Hill Complex is in fact the outcropping western edge of a sinuous, incised canyon thalweg. The Wonoka Formation canyons, containing basal sedimentary breccias but only minor conglomerates, are considered typical of passive margin canyon development. They are contrasted with the generally highly conglomeratic channel-fills observed in outcropping Tertiary and Cretaceous examples of active margin canyons and upper fan valleys.     

Würmian deglaciation of western Lake Geneva (Switzerland) based on seismic stratigraphy

Western Lake Geneva (le Petit-Lac) was filled during the Quaternary over a major erosion surface truncating the cemented, folded and thrusted Tertiary sediments of the foreland Alpine basin. The carving of the lake occurred during Quaternary glaciations with ice originating from the Rhone valley catchment basin flowing in two branches oriented SW and NE over the Swiss Plateau. Lake Geneva is situated on the South-Western branch of this paleo ice-cap.For the first time, a dense grid of high-resolution seismic profiles (airgun 5-inch³, airgun 1-inch³ and echosounder) has imaged the whole Quaternary sequence, providing a paleoenvironmental interpretation and a detailed reconstruction of the Rhone glacier retreat stages during glacial events that led to the formation of western Lake Geneva.The Quaternary sequence filling up the bedrock valley is exceptionally thick with up to 220 m of deposits and consists of glacial, glacio-lacustrine and lacustrine sediments. Fourteen seismic units have been defined (units U1–U14). Unit U1 represents the remnants of glacial deposits older than the last glacial cycle, preserved in the deepest part of the lake and in secondary bedrock valleys. Unit U2 represents gravel and sands deposited by meltwater circulation at the bottom of the glacial valley. Unit U3 is a thick, stratified unit marking the beginning of the deglaciation, when the Rhone glacier became thinner and buoyant and allowed the formation of a subglacial lake. Younger glacial units (units U4, U5, U7, U9, U11) are acoustically chaotic sediments deposited subglacially under the water table (undermelt tills), while the glacier was thinning. These glacial units are bounded by synform erosion surfaces corresponding to readvances of the glacier.The transition from a glacial to a glacio-lacustrine environment started with the appearance of a marginal esker-fan system (unit U6). Esker formation was followed by a small advance–retreat cycle leading to the deposition of unit U7. Then, the ice front receded and stratified sediments were deposited in a glacio-lacustrine environment (units U8, U10 and U12). This retreat was punctuated by two readvances – Coppet (unit U9) and Nyon (unit U11) – producing large push moraines and proglacial debris flows. Finally, a lacustrine environment with a characteristic lake current pattern and mass movement deposits took place (units U13 and U14).Except for unit U1, the sedimentary sequence records the Würmian deglaciation in a fjord-like environment occupied by a tidewater glacier with a steep, calving ice front. The presence of an esker-fan system reveals the importance of subglacial meltwater flow in continental deglaciation. Push-moraines and erosion surfaces below the glacier indicate at least 5 readvances during the deglaciation thus revealing that oscillations of ice front are the key process in deglaciation of perialpine fjord-lakes. The dating of these continental glacier fluctuations would allow correlation with oceanic and ice records and help to understand the climatic mechanisms between oceans and continents.     

Small landslide types and controls in glacial deposits: Lower Skagit river drainage,northern cascade range,Washington

Observations of 167 small, shallow landslides spanning a 22-year period on extensively logged slopes of Quaternary terraces in the lower Skagit and Baker Valleys, Washington, shows that there is a relationship between the common slope failures in this area and the slope angle, stratigraphy, and logging practices. Landslide frequency increases upvalley, as do mean annual precipitation and the frequency of perched water tables. Debris slides are most common, occur on steep slopes (>50%) composed of sand and gravel, and are most abundant in areas previously logged by the clear—cut method. Debris flows occur on shallower slopes (>30%) where the stratigraphy leads to perched water tables. Debris flows larger than 600 m² in area appear to be unrelated to logging practices. Slump flows, described here for the first time, occur on similar slope angles and stratigraphic situations as debris flows. They differ mainly by the presence of semiconsolidated material, usually till, at the slide head. Where till is breached—commonly along road cuts—water infiltration is increased, saturating underlying fine-grained deposits, which then fail by debris flowage. Secondary slumping of till happens when the slope steepens during debris flow failure. Small landslides surrounding Lake Shannon may contribute up to 80% of the total particulate matter yield to the fluvial system at present, increasing lake sedimentation by a rate of 5 mm/yr.     

Measurements of density underflows from Walensee,Switzerland*

Speed and direction of bottom currents induced by density underflow of two sediment-laden rivers were measured by oceanographic current meters in the Walensee (= Lake of Walenstadt), Switzerland. The apparently shooting flow of currents (up to 30 cm/s in this study) is suggested as an explanation for laminations in turbidite sequences. The current speed apparently stabilizes on slopes around 2°; this angle seems to correspond to the critical slope where the flow of the measured currents becomes steady. Current direction is controlled by bottom topography and direction of river inflow. Reversal of current direction observed at two sites is probably due to the underflow-induced backward motion of the overlying lake water. Underflow activity in Walensee is correlative with density peaks of the river water input. The currents are compared to Lake Mead (Southwestern U.S.) underflows and sporadic currents in some submarine canyons.     

江西省鄱阳湖信江决口三角洲沉积特征及模式

利用实地考察、浅钻孔、探槽及卫星影像等方法,对鄱阳湖信江决口三角洲进行了系统研究。该决口三角洲是洪水冲破信江西岸天然堤流入附近的河漫湖泊形成的,其平面形态为鸟足状,总体延伸方向与主河道近垂直。在横切剖面上整个决口三角洲沉积体呈透镜状覆盖在早期的湿地或河漫湖泊沉积之上。鄱阳湖信江决口三角洲可分为3个亚相:决口三角洲平原、决口三角洲前缘和决口前三角洲。决口三角洲平原主要发育决口河道、决口天然堤、废弃决口河道和湿地,其中决口河道又可以进一步划分为决口水道、边滩、汊口滩;决口三角洲前缘主要发育河口坝、支流间湾。河漫湖泊规模小、深度浅、波浪能量弱,对河口坝和决口河道砂体的改造较弱,不易形成连片分布的席状砂;决口前三角洲不发育。决口三角洲沉积受河流作用和河漫湖泊作用的共同控制,其演化可以划分为主河道天然堤生长阶段、决口初期阶段、进积分汊阶段及分汊—废弃阶段4个阶段。     

The effect of the Messinian Deep Stage on karst development around the Mediterranean Sea. Examples from Southern France

It is difficult to explain the position and behaviour of the main karst springs of southern France without calling on a drop in the water table below those encountered at the lowest levels of Pleistocene glacio-eustatic fluctuations. The principal karst features around the Mediterranean are probably inherited from the Messinian period (“Salinity crisis”) when sea level dropped dramatically due to the closing of the Straight of Gibraltar and desiccation of the Mediterranean Sea. Important deep karst systems were formed because the regional ground water dropped and the main valleys were entrenched as canyons. Sea level rise during the Pliocene caused sedimentation in the Messinian canyons and water, under a low hydraulic head, entered the upper cave levels.

The powerful submarine spring of Port-Miou is located south of Marseille in a drowned canyon of the Calanques massif. The main water flow comes from a vertical shaft that extends to a depth of more than 147 m bsl. The close shelf margin comprises a submarine karst plateau cut by a deep canyon whose bottom reaches 1,000 m bsl. The canyon ends upstream in a pocket valley without relation to any important continental valley. This canyon was probably excavated by the underground paleoriver of Port-Miou during the Messinian Salinity Crisis. Currently, seawater mixes with karst water at depth. The crisis also affected inland karst aquifers. The famous spring of Fontaine de Vaucluse was explored by a ROV (remote observation vehicle) to a depth of 308 m, 224 m below current sea level. Flutes observed on the wall of the shaft indicate the spring was formerly an air-filled shaft connected to a deep underground river flowing towards a deep valley. Outcroppings and seismic data confirm the presence of deep paleo-valleys filled with Pliocene sediments in the current Rhône and Durance valleys. In the Ardèche, several vauclusian springs may also be related to the Messinian Rhône canyon, located at about 200 m below present sea level. A Pliocene base level rise resulted in horizontal dry cave levels. In the hinterland of Gulf of Lion, the Cévennes karst margin was drained toward the hydrologic window opened by the Messinian erosional surface on the continental shelf.     

松辽盆地北部双城地区扶余油层层序地层格架与沉积体系展布

通过多口岩心精细观察,结合135口探井资料分析,识别出各种沉积微相标志,确定了松辽盆地北部双城地区扶余油层主要为曲流河-浅水三角洲沉积体系,进一步划分为5种亚相、11种微相。以高分辨率层序地层学理论为指导,通过岩-电结合、井-震结合等技术手段,识别层序界面,将其详细划分为2个长期基准面半旋回、7个中期基准面旋回,建立等时层序地层格架。以中期基准面旋回为编图单元,结合地震属性综合分析,编制了不同时期岩相古地理图,分析其演化规律,建立沉积模式,研究表明：研究区扶余油层主要经历了2个大的演化阶段：Q3-sq1-Q4-sq1时期,由浅水三角洲平原沉积逐渐向曲流河沉积转换;Q4-sq2-Q4-sq3时期,由曲流河沉积向浅水三角洲平原、前缘沉积转换,反映水体先变浅又逐渐变深的一次湖退－湖侵沉积过程,为近期油气勘探提供重要的指导意义。     

Distinctive erosional and depositional structures formed at a canyon mouth: A lower Pleistocene deep‐water succession in the Kazusa forearc basin on the Boso Peninsula,Japan

The canyon mouth is an important component of submarine‐fan systems and is thought to play a significant role in the transformation of turbidity currents. However, the depositional and erosional structures that characterize canyon mouths have received less attention than other components of submarine‐fan systems. This study investigates the facies organization and geometry of turbidites that are interpreted to have developed at a canyon mouth in the early Pleistocene Kazusa forearc basin on the Boso Peninsula, Japan. The canyon‐mouth deposits have the following distinctive features: (i) The turbidite succession is thinner than both the canyon‐fill and submarine‐fan successions and is represented by amalgamation of sandstones and pebbly sandstones as a result of bypassing of turbidity currents. (ii) Sandstone beds and bedsets show an overall lenticular geometry and are commonly overlain by mud drapes, which are massive and contain fewer bioturbation structures than do the hemipelagic muddy deposits. (iii) The mud drapes have a microstructure characterized by aggregates of clay particles, which show features similar to those of fluid‐mud deposits, and are interpreted to represent deposition from fluid mud developed from turbidity current clouds. (iv) Large‐scale erosional surfaces are infilled with thick‐bedded to very thick‐bedded turbidites, which show lithofacies quite similar to those of the surrounding deposits, and are considered to be equivalent to scours. (v) Concave‐up erosional surfaces, some of which face in the upslope direction, are overlain by backset bedding, which is associated with many mud clasts. (vi) Tractional structures, some of which are equivalent to coarse‐grained sediment waves, were also developed, and were overlain locally by mud drapes, in association with mud drape‐filled scours, cut and fill structures and backset bedding. The combination of these outcrop‐scale erosional and depositional structures, together with the microstructure of the mud drapes, can be used to identify canyon‐mouth deposits in ancient deep‐water successions.