弯曲海底峡谷中浊流的三维流动及沉积的初步研究

海底浊流的运动及其沉积,是目前浊流研究的热点之一。根据经过验证的基于雷诺平均纳维尔-斯托克斯方程及浮力项修正 k-ε 湍流模型的三维数值计算模型模拟了海底弯曲圆弧形峡谷内的浊流的流动和沉积,结果表明:（1）浊流在运动过程中由于对环境水体的夹带厚度不断增加,浊流厚度一般会超过峡谷深度,溢出峡谷,使浊流产生密度和动量损失;（2）浊流到达弯道部分后,由于离心力的作用会产生剥离,溢出更多的浊流至漫滩区域。浊流剥离的最大处为弯道顶点外岸下游处,其过量密度可达入流的37.5%;（3）对于模拟的横剖面为圆弧型的峡谷内的浊流来说,弯道顶点处的二次流在底部形成一个顺时针的循环圈,靠近峡谷底部从外岸指向内岸;（4）在峡谷中间及弯道顶点内岸下游处形成沉积,在弯道顶点外岸下游处形成侵蚀。这些特征对根据浊流的沉积观察推测其形成环境及油气储层的调查等方面有一定的参考意义。     

海底峡谷科学深潜考察研究现状

海底峡谷是大陆边缘重要的海底地貌形态,也是沉积物从浅海向深海搬运的重要通道和生物多样性热点,对海底科学研究、海底资源开发利用及海底基础设施建设与安全运营具有重要意义。载人深潜和以遥控潜器(ROV)和自主潜器(AUV)为代表的无人深潜技术为人类探索现代海底峡谷提供了重要途径。海底峡谷的深潜科学考察始于1940年代后期,最初为潜水员下潜考察,1960年代开始各种载人和无人潜器逐步应用于海底峡谷的科学探索。载人潜器能将科学家带至海底进行实地观察和精准取样。无人潜器的优点是:成本低、效率高,无人员安全之忧,水下作业时间长,且可以到达人类难以企及的极端海域。深潜技术在海底峡谷地学研究中的应用主要包括海底地形地貌和底质调查,各种侵蚀和沉积底形及其成因分析,块体搬运和流体动力学过程研究,冷泉、冷水珊瑚等生物群落分布及海底生物侵蚀作用研究。欧美发达国家过去70余年在海底峡谷科学深潜考察研究方面所积累的经验和成果对于我国正在兴起的深海深潜科学考察具有重要的启示和借鉴意义。     

南海北部珠江口外峡谷体系沉积特征、演化及其控制因素

深水海底峡谷内部的粗粒碎屑沉积物不仅可以作为良好的油气储层,也可以较为完整地记录海洋地质环境变迁的相关信息,是目前海洋地质领域研究的热点.为揭示南海北部珠江口外峡谷体系沉积演化过程及其控制因素,利用多波束测深和高分辨率二维多道地震数据,对珠江口外峡谷体系地形特征、沉积充填特征、形成发育过程和控制因素进行研究.研究发现珠江口外峡谷呈三段式发育：上段为NW-SE走向,宽度超过30 km,侵蚀强度不大,横截面为不规则形态;中段为E-W走向,宽度开始变窄（25～30 km）,横截面呈U型;下段为NW-SE走向,宽度达到最大（25～45 km）,横截面呈U型,中段和下段以沉积作用为主.珠江口外峡谷体系沉积演化主要分为3个阶段：早期阶段（23～15.5 Ma）,水道-海底扇阶段（15.5～11.6 Ma）和峡谷-海底扇/块体流阶段（11.6～0 Ma）.揭示了该峡谷珠江口外峡谷体系的发育和演化主要受构造运动、海平面变化和沉积物供给的控制作用,通过以上分析,将对南海北部海洋灾害地质、深水沉积体系研究及油气资源勘探有重要的指导意义.     

海底CORK观测30年:发展、应用与展望

过去50年深海钻探计划(DSDP)、大洋钻探计划(ODP)及综合大洋钻探计划(IODP)等国际间综合深海钻探计划的实施,使我们对地球和海洋的认识取得了显著进步。海底井控观测装置(CORK)的研发和应用是上述深海钻探计划带给我们的最宝贵财富之一。目前地球科学和海洋科学已由过去的间断性观测模式升级为现在的连续性原位观测模式,海底CORK观测系统为地球科学、海洋科学与生命科学领域的科学家对海底洋壳复杂而又相互关联的深部过程进行数秒至数十年时间尺度的研究提供了新手段和新机遇。通过对海底CORK观测系统的发展演变、在ODP和IODP航次中的使用以及在此过程中获得的科学经验和教训进行总结,对CORK系统如何应用于我国洋壳地质学、水文学、微生物学及生物地球化学过程研究提出看法。     

马尼拉海沟1.4 ka B.P.以来浊流事件沉积及其成因机制*

浊流是远距离沉积物运输的一种重要方式,海底浊流广泛存在于海底峡谷或海沟。马尼拉海沟位于南海东北部,是一条正在活动的板块汇聚边界。独特的地理位置(亚热带—热带)和气候条件(台风频发),使得马尼拉海沟浊流频发,然而,现今对马尼拉海沟的浊流研究甚少。本研究通过对马尼拉海沟北部水深3747 m处重力柱岩心(GEO6)进行高精度的粒度及沉积学特征分析,探讨马尼拉海沟浊流沉积规律。GEO6岩心底部细颗粒沉积物中浮游有孔虫的¹⁴C的AMS年龄为1405 a B.P.。高精度的粒度分析(0.25 cm)和沉积学特征显示: GEO6岩心记录有至少11次浊流沉积(T1-T11),且这些浊流都有明显的底部粒度最粗(砂质粉砂或砂)、向上粒度逐渐变细的正粒序特征,只有T8沉积体为反粒序特征,可能为异重流沉积。结合区域地质资料,本研究认为1.4 ka B.P.以来,研究区频繁的台风带来了大量陆源松散沉积物堆积在马尼拉海沟上游(高屏峡谷),不稳定的构造环境及地震频发导致这些松散沉积物垮塌并向下游马尼拉海沟输送,在海沟内形成频繁发育的浊流沉积体。     

马尼拉海沟1.4 ka B.P.以来浊流事件沉积及其成因机制*

浊流是远距离沉积物运输的一种重要方式,海底浊流广泛存在于海底峡谷或海沟。马尼拉海沟位于南海东北部,是一条正在活动的板块汇聚边界。独特的地理位置(亚热带—热带)和气候条件(台风频发),使得马尼拉海沟浊流频发,然而,现今对马尼拉海沟的浊流研究甚少。本研究通过对马尼拉海沟北部水深3747 m处重力柱岩心(GEO6)进行高精度的粒度及沉积学特征分析,探讨马尼拉海沟浊流沉积规律。GEO6岩心底部细颗粒沉积物中浮游有孔虫的¹⁴C的AMS年龄为1405 a B.P.。高精度的粒度分析(0.25 cm)和沉积学特征显示: GEO6岩心记录有至少11次浊流沉积(T1-T11),且这些浊流都有明显的底部粒度最粗(砂质粉砂或砂)、向上粒度逐渐变细的正粒序特征,只有T8沉积体为反粒序特征,可能为异重流沉积。结合区域地质资料,本研究认为1.4 ka B.P.以来,研究区频繁的台风带来了大量陆源松散沉积物堆积在马尼拉海沟上游(高屏峡谷),不稳定的构造环境及地震频发导致这些松散沉积物垮塌并向下游马尼拉海沟输送,在海沟内形成频繁发育的浊流沉积体。     

南海西沙海域东岛北峡谷体系形态、演化及其成因机制

海底峡谷是全球大陆边缘分布较广泛的地貌单元,是地形地貌、深水沉积和海洋地质灾害领域研究的重要内容。本研究基于高分辨率多波束测深和二维地震资料,以南海西沙东部海域为研究区,深入剖析了东岛北海底峡谷体系。东岛北海底峡谷分布在水深1000～3150 m范围,长度137 km,下切深度70～400 m,表现出西高东低的地形特征,总体由一条主峡谷和4条分支峡谷构成。峡谷上游有东岛西北部Ⅰ区和永兴海台东部Ⅱ区的沉积物供给,峡谷中游加入了东岛东北部Ⅲ区供给的沉积物。3个物源区的沉积物供应以线状的峡谷、水道和面状的块体流沉积类型为主。主峡谷北坡周缘分布有大量的海底麻坑,侧壁呈阶梯状不断后撤垮塌,因重力驱动作用和水流侵蚀,使峡谷壁外缘发育呈不规则小型“枝杈”状水道;主峡谷南岸因浊流作用,发育沉积物波。NE走向主断裂,控制着主干峡谷NE方向延伸,而峡谷南岸分布海山和海丘地形、岩浆底辟,影响主峡谷各分段的转向;同时峡谷和周缘下部地层发育的断层,控制峡谷侧壁向谷底呈阶梯状下降。     

琼东南盆地陵水区中央峡谷水道沉积数值模拟

深水峡谷是当今海洋油气勘探领域的研究热点。南海西北部琼东南盆地的中央峡谷已部署了多口深水钻井来勘探沉积充填的岩性油气藏,然而,由于这些探井数量相对有限、井间距离大以及地震资料分辨率较低,峡谷内砂体的叠复关系、边界接触关系以及沉积演化等问题尚不清楚,严重制约着储层空间分布与储层物性的精细刻画。对琼东南盆地陵水区中央峡谷开展了沉积数值模拟研究,根据研究区具体地质背景建立了地质模型和数学模型,通过流体动力学软件ANSYS FLUENT正演模拟了多个沉积期次和多套砂组在不同初始条件(如物源和入流速度)下的浊流沉积几何形态,包括砂体平面分布特征和隔夹层分布特征。模拟结果表明:峡谷平直段内,浊流受底床摩擦力的持续影响,流速降低且湍流强度减弱,使得较粗颗粒可沉降于底床,细颗粒可随浊流头部涡流悬浮;峡谷狭窄段内,浊流头部的湍流较强,侵蚀峡谷壁并使峡谷走向发生偏移,悬浮颗粒受离心力作用形成溢岸沉积;峡谷内砂体展布具有垂向分异性,砂体内部泥岩以夹层为主,厚度一般较小且横向连续性差。与现有地震、钻井资料的对比分析显示本次数值模拟结果具有有效性。本研究成果揭示了中央峡谷不同沉积期次和不同砂组的沉积水动力学过程,进而预测了砂体的空间展布特征,为储层物性预测提供了坚实支撑。     

序言

海洋科学是地球系统科学的前沿之一,十九大报告中提出了"坚持陆海统筹,加快建设海洋强国,以及21世纪海上丝绸之路"等战略目标.近半个世纪以来,海洋科学尤其是深海海底地球科学,推动了整个地球系统科学的发展,并加速了多海洋学科的交叉.中国的海底科学在几十年来获得了极大的发展,科考船的足迹遍布边缘海、三大洋和南北极,在大陆边缘...     

冲绳海槽中全新世的浊流沉积及其控制因素

浊流沉积是陆源物质向深海搬运的主要方式,其发生对于海底扇、峡谷以及深海油气藏的形成具有重要意义。对冲绳海槽3个柱状样岩芯的研究,均发现了发生于7300~7500cal.a B.P.的浊流层。这3个浊流层S1,S2和S3的发生伴随着粒度、粘土矿物以及log(Ti/Ca)的明显变化。浊流沉积层相比正常沉积层具有粗粒组分峰值较高以及下覆的不整合构造,粒度上呈现由粗到细的正粒序构造。伴随着粒度的变化,log(Ti/Ca)也发生明显增大。A7孔位的S2浊流层的底部以及Oki01孔位的S3浊流层的上部含有火山灰层,火山玻璃含量约为80%左右。火山灰层的粒度众数约24~32μm的单峰分布。通过对比其他古气候、古环境指标,发现7300~7500 cal.a B.P.的浊流事件可能与活跃的火山地震作用以及冬季风明显增强有关。     

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.     

Storm-induced turbidity currents on a sediment-starved shelf: Insight from direct monitoring and repeat seabed mapping of upslope migrating bedforms

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.     

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.     

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.     

Occurrence of submarine canyons,sediment waves and mass movements along the northern continental slope of the South China Sea

In this study, we reveal a series of newly discovered submarine canyons, sediment waves, and mass movements on a flat and smooth seafloor using high-resolution, multi-beam bathymetry and shallow seismic surveys along the northern slope of the South China Sea. We also describe their geomorphology and seismic stratigraphy characteristics in detail. These canyons display U-shaped cross sections and are roughly elongated in the NNW–SSE direction; they are typically 8–25 km long, 1.2–7 km wide, and form incisions up to 175 m into Pliocene–Quaternary slope deposits at water depths of 400–1000 m. Slide complexes and the sediment wave field are oriented in the NE–SW direction and cover areas of approximately 1790 and 926 \(\hbox {km}^{2}\), respectively. Debris/turbidity flows are present within these canyons and along their lower slopes. Detailed analysis of seismic facies indicates the presence of six seismic facies, in which Cenozoic strata located above the acoustic basement in the study area can be roughly subdivided into three sequences (1–3), which are separated by regional unconformities (Tg, T4, and T3). By combining these data with the regional geological setting and the results of previous studies, we are able to determine the genetic mechanisms used to create these canyons, sediment wave field, and mass movements. For example, frontally confined slide complexes could have been influenced by high sedimentation rates and high pore pressures. A series of very large subaqueous sediment waves, which record wavelengths of 1.4–2 km and wave heights of 30–50 m, were likely produced by interactions between internal solitary waves and along-slope bottom (contour) currents. Canyons were likely initially created by landslides and then widened laterally by the processes of downcutting, headward erosion, and active bottom currents and debris/turbidity flows on canyon floors. We therefore propose a three-dimensional model to describe the development of these mass movements, the sediment wave field, and canyons. The four stages of this model include a stable stage, followed by the failure of the slope, and subsequent formations of the sediment wave field and canyons.     

The influence of turbidity currents and contour currents on the distribution of deep‐water sediment waves offshore eastern Canada

Sediment waves are commonly observed on the sea floor and often vary in morphology and geometry according to factors such as seabed slope, density and discharge of turbidity currents, and the presence of persistent contour currents. This paper documents the morphology, internal geometry and distribution of deep‐water (4000 to 5000 m) bedforms observed on the sea floor offshore eastern Canada using high‐resolution multibeam bathymetry data and seismic stratigraphy. The bedforms have wavelengths of >1 km but fundamentally vary in terms of morphology and internal stratigraphy, and are distinguished into three main types. The first type, characterized by their long‐wavelength crescentic shape, is interpreted as net‐erosional cyclic steps. These cyclic steps were formed by turbidity currents flowing through canyons and overtopping and breaching levées. The second type, characterized by their linear shape and presence on levées, is interpreted as net‐depositional cyclic steps. These upslope migrating bedforms are strongly aggradational, indicating high sediment deposition from turbidity currents. The third type, characterized by their obliqueness to canyons, is observed on an open slope and is interpreted as antidunes. These antidunes were formed by the deflection of the upper dilute, low‐density parts of turbidity currents by contour currents. The modelling of the behaviour of these different types of turbidity currents reveals that fast‐flowing flows form cyclic steps while their upper parts overspill and are entrained westward by contour currents. The interaction between turbidity currents and contour currents results in flow thickening and reduced sediment concentration, which leads to lower flow velocities. Lower velocities, in turn, allow the formation of antidunes instead of cyclic steps because the densiometric Froude number (Fr′) decreases. Therefore, this study shows that both net‐erosional and net‐depositional cyclic steps are distributed along channels where turbidity currents prevail whereas antidunes form on open slopes, in a mixed turbidite/contourite system. This study provides insights into the influence of turbidity currents versus contour currents on the morphology, geometry and distribution of bedforms in a mixed turbidite–contourite system.     

Tsunamis as geological agents

When a tsunami wave series approaches and interacts with a coast, the consequent passage shorewards of great volumes of water and their invasion of the land, especially within bays and up river valleys, results in the disturbance of existing sediment and the removal seawards of land debris and coastal and shallow‐water marine sediments. Tsunami action builds up sequences of peculiar sediments in shallow water; it at least assists in the formation and maintenance of submarine canyons and, through them, produces turbidity currents of a particularly powerful kind. Tsunami action may explain many puzzling sedimentary phenomena, for example, sudden and drastic changes in near‐shore bathymetry; the formation of chaotic sediments such as some paraconglomerates and edgewise conglomerates. It offers solutions to problems arising from the study of turbiditic sequences, both modern and ancient.     

Sealers Bay submarine fan complex, Oligocene, southern New Zealand

The Oligocene Balleny Group of Chalky Island, southwestern Fiordland, comprises a typical continental margin sequence 900 m in thickness. Thin nearshore traction deposited sediments at the base are overlain by submarine canyon and fan lithofacies that were deposited by the full range of subaqueous mass-transport processes. A steep-walled channel within Balleny Group is interpreted as a fossil proximal fan-channel. The sedimentary fill of the channel is texturally similar to sediments moving by slump-creep in Recent submarine canyons and fan-valleys. The field data presented indicate (1) that a small canyon complex at Sealers Bay was initially cut by subaqueous debris-flows derived from an adjacent cliffed continental coast; (2) that transport within the upper parts of the canyon and fanchannel complex was primarily by inertia-flow and slump-creep; and (3) that these more proximal types of mass-transport gave way gradationally and successively to fluxoturbidity and turbidity currents at locations further down-slope, with consequent deposition of sediment in more distal fan-channel and fan-surface environments as fluxoturbidites and turbidites, with lesser contributions from inertia-flows.     

Sea level controls on the textural characteristics and depositional architecture of the Hueneme and associated submarine fan systems, Santa Monica Basin, California

Hueneme and Dume submarine fans in Santa Monica Basin consist of sandy channel and muddy levee facies on the upper fan, lenticular sand sheets on the middle fan, and thinly bedded turbidite and hemipelagic facies elsewhere. Fifteen widely correlatable key seismic reflections in high-resolution airgun and deep-towed boomer profiles subdivide the fan and basin deposits into time-slices that show different thickness and seismic-facies distributions, inferred to result from changes in Quaternary sea level and sediment supply. At times of low sea level, highly efficient turbidity currents generated by hyperpycnal flows or sediment failures at river deltas carry sand well out onto the middle-fan area. Thick, muddy flows formed rapidly prograding high levees mainly on the western (right-hand) side of three valleys that fed Hueneme fan at different times; the most recently active of the lowstand fan valleys, Hueneme fan valley, now heads in Hueneme Canyon. At times of high sea level, fans receive sand from submarine canyons that intercept littoral-drift cells and mixed sediment from earthquake-triggered slumps. Turbidity currents are confined to ‘underfit’ talweg channels in fan valleys and to steep, small, basin-margin fans like Dume fan. Mud is effectively separated from sand at high sea level and moves basinward across the shelf in plumes and in storm-generated lutite flows, contributing to a basin-floor blanket that is locally thicker than contemporary fan deposits and that onlaps older fans at the basin margin. The infilling of Santa Monica Basin has involved both fan and basin-floor aggradation accompanied by landward and basinward facies shifts. Progradation was restricted to the downslope growth of high muddy levees and the periodic basinward advance of the toe of the steeper and sandier Dume fan. Although the region is tectonically active, major sedimentation changes can be related to eustatic sea-level changes. The primary controls on facies shifts and fan growth appear to be an interplay of texture of source sediment, the efficiency with which turbidity currents transport sand, and the effects of delta distributary switching, all of which reflect sea-level changes.