末次盛冰期以来长江三角洲地区的沉积相和古地理

末次盛冰期低海平面期间,长江三角洲地区可划分为2个古地理单元：古河谷和古河间地。下切河谷底部侵蚀面和古河间地顶面构成了冰后期海侵沉积旋回的底界面,它相当于层序地层学中的层序界面。位于河口湾-浅海相中的最大海侵面将冰后期海侵沉积旋回分为其下的海侵层序和其上的海退层序。随着δ¹⁸O ₃期的海平面下降,长江开始下切,至δ¹⁸O ₂期低海面时形成巨大的下切河谷。冰后期海平面上升引发的海侵造成了长江古河谷系的充填和河床、河漫滩－河口湾和部分河口湾－浅海相的形成,尔后的进积产生了部分河口湾－浅海相及三角洲相等。溯源堆积是产生下部河流沉积单元的主要过程,其中河漫滩沉积中出现的潮汐层理和少量小个体有孔虫说明了海洋因素的影响,河口湾－浅海相泥质沉积主要形成于最大海侵之时,三角洲的进积则产生了具有多期河口坝的三角洲。古河间地表面的硬粘土层经历了沉积和成壤作用交替、持续成壤作用和早期成岩作用,它们大致分别对应于δ¹⁸O ₃期、δ¹⁸O ₂期和δ¹⁸O ₁期,硬粘土层中留下了这3种作用的烙印。长江三角洲古河间地的古土壤母质属河漫滩相。持续成壤阶段河流基面和地下水位均较低,年降雨量约为500～800 mm,相当于现今的温带地区,干湿周期变化明显,地下水升降频繁。所有这些表明,当时并非干旱气候。     

杭洲湾地区晚第四纪下切河谷内部相结构与生物气勘探

本文根据杭州湾沿海平原大量的钻井、静力触探井和分析化验等资料，研究了下切河谷（钱塘江和太湖下切河谷）充填物的沉积建造和沉积相，以及浅层生物气藏分布特征。研究表明，末次冰期以来，随着海平面变化，杭州湾地区下切河谷演化经历了深切、快速充填和埋藏三个阶段。末次冰盛期，海平面下降的幅度大，增加了河流梯度、加强了下切作用，本区形成了钱塘江和太湖下切河谷，随后在冰后期被充填和埋藏，下切河谷的两侧为暴露地表的古河间地。根据岩石学、沉积结构和沉积构造特征，本区下切河谷充填沉积物表现为向上变细的沉积层序，可以划分为4个沉积相类型，有河床滞留沉积物到部分曲流河沉积体系的边滩沉积、河漫滩-河口湾沉积、河口湾-浅海沉积和河口湾沙坝沉积。在河漫滩-河口湾相沉积期间，由于海平面上升、潮流体系、沉积物供给和可容空间条件适合一个潮流沙脊体系的发育，该相中砂质透镜体可能代表下切河谷内发育的潮流沙脊。对于河口湾-浅海沉积和河口湾沙坝沉积而言，由于沉积条件不再有利，没有形成沙脊沉积。所有的商业性生物气都存储在下切河谷内河漫滩-河口湾砂质透镜体中。     

中国河口三角洲地区晚第四纪下切河谷层序特征和形成*

应用800多口钻孔及文献资料,讨论了中国沿海滦河扇三角洲、长江三角洲和珠江三角洲及钱塘江河口湾4个地区的下切河谷体系,这些皆为丰沙河流形成的河口三角洲。这些河口三角洲地区的下切河谷为长形或扇形,长数十至数百千米,宽数十千米,深40～90 m。河口三角洲地区的下切河谷相序可分为4种类型,即FS-Ⅰ,FS-Ⅱ,FS-Ⅲ和FS-Ⅵ。可以将这4类相序自海向陆排成一个理想序列：FS-Ⅰ位于海岸线附近,FS-Ⅳ位于河口三角洲的顶部,显示海的影响逐渐减弱,陆相作用逐渐增强。下切河谷层序可分为海侵和海退序列。海侵序列的厚度占下切河谷层序的50％以上,体积占60％～70％。海侵序列是在海平面上升过程中,溯源堆积依次叠置而成的,其下部的河床相是在溯源堆积能到达、而涨潮流未能到达的下游河段产生的,往往不含海相微体化石和潮汐沉积构造。在海侵序列中未见区域上可对比的侵蚀面,表明冰后期海平面上升速率的变化、甚至小幅下降也未留下统一的侵蚀记录。下切河谷中的海退序列由河口湾充填及三角洲进积而成,其进程是各不相同的：长江古河口湾先被强潮河口湾相、后由三角洲相所充填,河口湾也经历了由强潮型向中潮型的转变;滦河扇三角洲和珠江三角洲,其古河口湾则被河流相和三角洲相所充填;钱塘江河口湾正被强潮河口湾相所充填。     

长江三角洲亚三角洲地层结构对比

依据对江苏省泰兴市黄桥镇钻孔和海门市钻孔、上海长兴岛钻孔地层岩心沉积特征的研究,讨论 了冰后期以来长江三角洲河口地区的层序地层。研究表明,沿古河谷纵剖面各河段地层中沉积相组合关系有明 显区别。在古河谷下端地区,下切河谷充填层序自下而上由河流相、河口湾相、浅海相和三角洲相组成,沉积 相组合最完整。在古河谷上端地区,由于河口湾在海侵发生时存在着涌潮作用,因此,河漫滩泥质沉积层顶部 的冲刷面被确定为最大海泛面,其上覆稳定的河口砂体;浅海相和河口湾相沉积在该区域变薄直至尖灭;层序 顶部由中潮坪突变为高潮坪,最终形成泛滥平原。这意味着,原先确定的自西向东的长江三角洲的几个亚三角 洲或河口砂体沉积,并非都具有向上变粗的三角洲层序。文中报道的３个钻孔,揭露了冰后期长江三角洲完整 的下切河谷充填层序。河口湾顶不具备三角洲层序的地区应称为滨海平原。     

江苏南通地区晚第四纪下切河谷沉积与环境演变

采用层序地层学基本原理,以海平面升降旋回为主线,根据钻井岩芯、古生物、测年和分析化验等资料,探讨了江苏南通地区晚第四纪地层层序、层序界面、沉积特征及沉积环境的演变。结果表明,研究区晚第四纪发育三期下切河谷,形成了三套沉积层序,自下而上三个层序的地质时代分别相当于晚第四纪早期、晚第四纪中期和晚第四纪晚期。由于后期河流的强烈下切破坏,早期沉积层序往往被剥蚀殆尽,仅残留下部的河床相粗粒沉积,造成不同期河床相的叠置;相对而言,晚第四纪晚期形成的下切河谷沉积层序以不同的沉积相组合被保存下来,自下而上划分为河床、河漫滩、河口湾、浅海和三角洲5种沉积相类型,表现为一个较完整的沉积层序。晚第四纪晚期下切河谷底界面,是末次冰期海面下降,河流下切形成的侵蚀面,与河间地古土壤层顶面的沉积间断面同属一个地史期的产物,一起构成区域不整合面,界面上下岩性突变,其上的冰后期地层属同一个海平面变化旋回,可互相对比,因而具有年代地层学意义。三期下切河谷层序的套叠结构表明,晚第四纪以来,研究区存在三次"低海面-海侵-高海面-海退"周期性海面变化。     

长江三角洲晚第四纪地层沉积特征与生物气成藏条件分析

对长江三角洲晚第四纪地层沉积特征进行了精细研究,探讨了研究区层序地层格架,在此基础上分析了浅层生物气成藏条件、特征和分布规律。研究表明,长江三角洲晚第四纪发育3期下切河谷,形成了3套沉积层序;因后期河流的强烈下切破坏,早期沉积层序往往被剥蚀殆尽,仅残留下部的河床相粗粒沉积,造成不同期河床相的叠置;相对而言,末次冰期以来形成的下切河谷沉积层序以相对完整的沉积相组合被保存下来。长江三角洲浅层天然气是未经运移的原生生物气,其主要富集于末次冰期以来的沉积层序内,气藏为自生自储同生型的岩性圈闭。河口湾—河漫滩和浅海相泥质沉积物既是气源岩,又是盖层,后者可作为良好的区域盖层;河口湾—河漫滩和河床相砂质沉积物为主要储集层。因此,研究区晚第四纪多期下切河谷沉积层序有利于浅层生物气藏的形成,特别是晚期下切河谷内河口湾—河漫滩相砂质透镜体以及河床相砂体可作为优先勘探目标。     

末次盛冰期以来长江南通段古河谷的地层特征

通过对南通段古河谷典型钻孔及剖面沉积物颗粒、微体生物化石、14C测年等的分析,对末次盛冰期以来的沉积地层进行了划分。该段古河谷末次冰期以来的地层,基本上反映了河床相—河漫滩相—浅海相—三角洲相的冰后期海侵沉积旋回。现代河床的底部,浅海相淤泥质亚黏土层已被侵蚀掉,沉积的是现代河床相砂层。     

钱塘江下切河谷充填物沉积序列和分布模式

以最新钻取的SE2孔沉积物为重点研究对象,对晚第四纪以来钱塘江下切河谷充填物的沉积特征和沉积相进行了精细研究,重建了研究区地层结构和层序地层格架,总结了强潮型钱塘江河口湾和下切河谷的沉积模式。钱塘江下切河谷充填物自下而上依次发育河床、河漫滩、古河口湾、近岸浅海和现代河口湾5种沉积相类型,表现为一个较完整的Ⅰ型层序,其内部层序界面、初始海泛面、最大海泛面、海侵和海退潮流侵蚀面、体系域内海侵面发育。钱塘江下切河谷充填物自海向陆可划分为海向段、近海段、近陆段和陆向段4段,各段沉积序列和海陆相互作用程度不同。在钱塘江下切河谷充填物中海陆过渡部位首次明确划分出了古河口湾相,并对其沉积特征和分布模式进行了初步探讨;其形成时间在9000 a BP左右,具有与现代河口湾不同的沉积特征,表现为中部为潮道砂体沉积,向陆渐变为受潮流影响的河流沉积,两侧被潮坪或盐沼沉积包围,沉积物在平面上自陆向海呈现粗-细-粗的分布模式。现代河口湾平面上自陆向海依次发育受潮流影响的河流沉积、粉砂质砂坎、潮道-潮流砂脊复合体和湾口泥质沉积区,沉积物呈现粗-细-粗-细的分布模式,与大多数河口湾常见的粗-细-粗的分布格局明显不同。     

天津—河北沿海钻孔地层中的末次盛冰期下切河谷

文章回顾了1965年李四光先生追寻末次冰期对环境影响的讲话,以及40余年来在天津-河北沿海钻孔地层中、末次盛冰期下切河谷的发现过程。在源到汇过程中,起码自晚更新世以来,研究区已经是古黄河沉积区,多处发现的末次盛冰期下切河谷底板多位于30~32 m深度,低于全新统底板一般在20 m的深度;与长江口地区钻孔末次盛冰期下切河谷深62 m相比,研究区下切河谷规模不如前者,也没有一个统一的大河口。末次盛冰期下切河谷最远地点,是距现代海岸约80 km的河北省孟村回族自治县县城北侧,即西汉黄河亚三角洲叶瓣顶部。众多钻孔见早全新世快速沉积,只能是黄河支流有这样充足的泥沙供给,在局部顶托了早全新世海侵作用发生。      

全新世长江三角洲的发育及其对相邻海岸沉积体系的影响

依据钻孔资料和已发表的文献,对全新世长江三角洲的形成和发育及其对相邻沉积体系的影响作了综合和概括.在末次冰期低海平面时,现今长江三角洲地区可分为下切河谷和两侧的河间地两个古地貌单元.冰后期海平面上升,下切河谷被淹,并转化为河口湾,海水随之扩展到两侧的古河间地.全新世最大海侵时形成以镇江-扬州为顶点的古河口湾.7000～7500年以来,当沉积速率超过海平面上升速度时,长江带来的物质大量沉积,河口湾被充填,并逐渐转变为潮汐平原和三角洲.河口湾被充填之后,长江带来的河流泥沙随之溢出河口湾,进入相邻的河口海岸地区,影响相邻沉积体系的形成和发育.长江泥沙向南进入钱塘江河口湾,在湾顶形成沙坎;向北输运,成为苏北南黄海潮成沙脊的重要物源,影响该潮成体系的形成和发展.舟山群岛海蚀平台上直接覆盖泥质沉积是以退积为主的河口湾向进积的三角洲环境转变的又一证据.长江输沙量在不断减少,而河口滩涂围垦力度在增加,这将导致相邻的河口海岸沉积体系增长速度减缓,出现侵蚀或侵蚀加剧.     

Stratigraphy and paleoenvironmental changes in the Yangtze Delta during the Late Quaternary

The Yangtze Delta area may be subdivided into paleo-valley and paleointerfluves of the last glaciation. The postglacial transgressive sedimentary cycle (PTSC) on the front zones of the paleointerfluves is composed of marsh-nearshore and barrier-lagoon, shallow marine and nearshore-tidal flat units, with a basement in stiff clay. The PTSC in back zones of the paleointerfluves consists of lacustrine-marsh deposits. The PTSC in the incised valley contains river channel, floodplain-estuary, estuarine-shallow marine and deltaic units, with an erosional surface at its bottom. The stiff clay and the erosional surface constitute the PTSC lower boundary.The stiff clay, consisting of a paleosol, experienced deposition alternating with pedogenesis during the falling of sea-level during δ¹⁸O stage 3; ongoing pedogenesis in the sea-level lowstand of δ¹⁸O stage 2; and early diagenesis after paleointerfluve inundation by the sea-level rise of δ¹⁸O stage 1. The climate during the paleosol formation was temperate with more than 500-mm a⁻¹ rainfall and frequently fluctuating groundwater.The Yangtze River incised its course during falling sea level of stage 3, and a huge incised valley was formed during the sea-level lowstand (stage 2). The filling of the incised valley took place during the postglacial sea-level rise, and delta formation occurred at a late stage of PTSC development.     

Sediment dynamics and hydrodynamics in the lower course of a river highly regulated by dams: the Ebro River

The lowest part of the Ebro River is a microtidal salt-wedge estuary. Penetration of the salt-wedge is largely controlled by the fluvial discharge and the morphology of the river bed, although sea level variations caused by tides and atmospheric conditions can also play significant roles. The concentration and distribution of suspended particulate matter in this part of the river and the fluvial sediment discharge are strongly influenced by the dynamics of the salt-wedge. Damming of the river has caused sediment to be trapped in reservoirs and has regulated the fluvial discharge. Intrusion of the salt wedge has thus also been regulated. At present, sediment discharge is between 1 and 1·5 × 10⁵ tons per year, which is less than 1% of the sediment that the Ebro River discharged into the sea before construction of the dams. This extreme reduction in sediment supply has allowed marine erosional processes to dominate in the delta.     

Discriminating between pore‐filling load and bed‐structure load: a new porosity‐based method,exemplified for the river Rhine

Sediments contained in the river bed do not necessarily contribute to morphological change. The finest part of the sediment mixture often fills the pores between the larger grains and can be removed without causing a drop in bed level. The discrimination between pore‐filling load and bed‐structure load, therefore, is of practical importance for morphological predictions. In this study, a new method is proposed to estimate the cut‐off grain size that forms the boundary between pore‐filling load and bed‐structure load. The method evaluates the pore structure of the river bed geometrically. Only detailed grain‐size distributions of the river bed are required as input to the method. A preliminary validation shows that the calculated porosity and cut‐off size values agree well with experimental data. Application of the new cut‐off size method to the river Rhine demonstrates that the estimated cut‐off size decreases in a downstream direction from about 2 to 0·05 mm, covariant with the downstream fining of bed sediments. Grain size fractions that are pore‐filling load in the upstream part of the river thus gradually become bed‐structure load in the downstream part. The estimated (mass) percentage of pore‐filling load in the river bed ranges from 0% in areas with a unimodal river bed, to about 22% in reaches with a bimodal sand‐gravel bed. The estimated bed porosity varies between 0·15 and 0·35, which is considerably less than the often‐used standard value of 0·40. The predicted cut‐off size between pore‐filling load and bed‐structure load (D_c,p) is fundamentally different from the cut‐off size between wash‐load and bed‐material load (D_c,w), irrespective of the method used to determine D_c,p or D_c,w. D_c,w values are in the order of 10^?1 mm and mainly dependent on the flow characteristics, whereas D_c,p values are generally much larger (about 10⁰ mm in gravel‐bed rivers) and dependent on the bed composition. Knowledge of D_c,w is important for the prediction of the total sediment transport in a river (including suspended fines that do not interact with the bed), whereas knowledge of D_c,p helps to improve morphological predictions, especially if spatial variations in D_c,p are taken into account. An alternative to using a spatially variable value of D_c,p in morphological models is to use a spatially variable bed porosity, which can also be predicted with the new method. In addition to the morphological benefits, the new method also has sedimentological applications. The possibility to determine quickly whether a sediment mixture is clast‐supported or matrix‐supported may help to better understand downstream fining trends, sediment entrainment thresholds and variations in hydraulic conductivity.     

Unravelling the conundrum of river response to rising sea‐level from laboratory to field. Part II. The Fly–Strickland River system,Papua New Guinea

The most recent deglaciation resulted in a global sea‐level rise of some 120 m over ca 12 000 years. A moving boundary numerical model is developed to predict the response of rivers to this rise. The model was motivated by experiments at small scale, which have identified two modes describing the transgression of a river mouth: (i) autoretreat without abandonment of the river delta (no sediment starvation at the topset–foreset break); and (ii) sediment‐starved autoretreat with abandonment of the delta. In the latter case, transgression is far more rapid, and its effects are felt much further upstream of the river mouth. A moving boundary numerical model that captures these features in experimental deltas is adapted to describe the response of the Fly–Strickland River system, Papua New Guinea. In the absence of better information, the model is applied to the case of sea‐level rise without local climate change in New Guinea. The model suggests that: (i) sea‐level rise has forced the river mouth to transgress over 700 km since the last glacial maximum; (ii) sediment‐starved autoretreat has forced enough bed aggradation to block a tributary with a low sediment load and create the present‐day Lake Murray; (iii) the resulting aggradation was sufficient to move the gravel–sand transition on the Strickland River upstream; (iv) the present‐day Fly Estuary may be, in part, a relict river valley drowned by sea‐level rise and partially filled by tidal effects; and (v) the Fly River is presently reforming its bankfull geometry and prograding into the Fly Estuary. A parametric study with the model indicates that sediment concentration during floods plays a key role in determining whether or not, and to what extent, transgression is expressed in terms of sediment‐starved autoretreat. A sufficiently high sediment concentration can prevent sediment‐starved autoretreat during the entire sea‐level cycle. This observation may explain why some present‐day river mouths are expressed in terms of deltas protruding into the sea, and others are wholly contained within embayments or estuaries in which water has invaded landward.     

南海北部新生代海陆变迁与不同盆地的沉积充填响应*

新生代以来南中国海的多幕旋回运动形成了其北部陆坡性质各异、演化有别的多个陆缘沉积盆地。依据各盆地新生界发育特征、主干地震剖面及钻井资料对南海北部的相对海平面变化与沉积环境进行系统分析,采用年代地层对比的方法探讨南海北部构造演化序列与海陆变迁规律的内在联系,再现了南海北部陆缘新生代的海陆变迁过程,从而建立了南海北部陆缘裂谷盆地、走滑拉分盆地和陆内裂谷盆地的构造—沉积充填一体化模式。新生代海平面整体呈上升趋势,古近纪各盆地以陆相河流、粗粒三角洲湖相沉积为主;而新近纪主要发育滨浅海及三角洲相,呈现出明显的早陆后海的规律。靠近陆地一侧的陆内裂谷盆地北部湾盆地海侵最晚,其古近系充填厚度明显大于新近系,以发育近源扇三角洲为特色;而靠近海域一侧的走滑拉分盆地(莺歌海盆地)则以新近纪海相沉积占优势;陆缘裂谷盆地(琼东南与珠江口盆地)古近纪陆相与新近纪海相相对均衡发育,发育大型三角洲与碳酸盐岩台地。不同盆地的沉积充填特征主要受构造运动与海侵规模控制,并由此奠定了不同盆地的资源前景。     

From outwash to coastal systems in the Portneuf–Forestville deltaic complex (Québec North Shore): Anatomy of a forced regressive deglacial sequence

Deglacial sequences typically include backstepping grounding zone wedges and prevailing glaciomarine depositional facies. However, in coastal domains, deglacial sequences are dominated by depositional systems ranging from turbiditic to fluvial facies. Such deglacial sequences are strongly impacted by glacio‐isostatic rebound, the rate and amplitude of which commonly outpaces those of post‐glacial eustatic sea‐level rise. This results in a sustained relative sea‐level fall covering the entire depositional time interval. This paper examines a Late Quaternary, forced regressive, deglacial sequence located on the North Shore of the St. Lawrence Estuary (Portneuf Peninsula, Québec, Canada) and aims to decipher the main controls that governed its stratigraphic architecture. The forced regressive deglacial sequence forms a thick (>100 m) and extensive (>100 km²) multiphased deltaic complex emplaced after the retreat of the Laurentide Ice Sheet margin from the study area ca 12 500 years ago. The sedimentary succession is composed of ice‐contact, glaciomarine, turbiditic, deltaic, fluvial and coastal depositional units. A four‐stage development is recognized: (i) an early ice‐contact stage (esker, glaciomarine mud and outwash fan); (ii) an in‐valley progradational stage (fjord head or moraine‐dammed lacustrine deltas) fed by glacigenics; (iii) an open‐coast deltaic progradation, when proglacial depositional systems expanded beyond the valley outlets and merged together; and (iv) a final stage of river entrenchment and shallow marine reworking that affected the previously emplaced deltaic complex. Most of the sedimentary volume (10 to 15 km³) was emplaced during the three‐first stages over a ca 2 kyr interval. In spite of sustained high rates of relative sea‐level fall (50 to 30 mm·year^?1), delta plain accretion occurred up to the end of the proglacial open‐coast progradational stage. River entrenchment only occurred later, after a significant decrease in the relative sea‐level fall rates (<30 mm·year^?1), and was concurrent with the formation and preservation of extensive coastal deposits (raised beaches, spit platform and barrier sands). The turnaround from delta plain accretion to river entrenchment and coastal erosion is interpreted to be a consequence of the retreat of the ice margin from the river drainage basins that led to the drastic drop of sediment supply and the abrupt decrease in progradation rates. The main internal stratigraphic discontinuity within the forced regressive deglacial sequence does not reflect changes in relative sea‐level variations.     

Radiocarbon dates from the East China sea and their geological implications

Radiocarbon analysis plays an important role in studying the Quaternary geologic history of the East China Sea. More than 200 ¹⁴C dates have been published in various Chinese publications. The continental shelf of the East China Sea is one of the few large continental shelves in the world. Many low-lying flats and deltaic plains lie along the coast making it a favorable site for sea-level studies. Radiocarbon data from Neolithic sites, chenier ramparts, peaty deposits, and submarine sediments converge to suggest that oscillations of sea level have occurred: they also suggest that the lowest glacial sea levels probably occurred between 22,000 and 19,000 yr B.P. Calculation of the volume of the Yangtze River Delta, together with ¹⁴C dates, indicates that more than 89% of the solid particles carried by the river were deposited in the delta. Due to the sedimentary load, the crust beneath the delta has subsided isostatically and tilted seaward. Marine shells provide many acceptable ¹⁴C dates, but because they are easily transported, most samples from the continental shelf cannot be directly related to the history of sea-level changes.     

Sea-level and tectonic control of middle to late Pleistocene turbidite systems in Santa Monica Basin, offshore California

Small turbidite systems offshore from southern California provide an opportunity to track sediment from river source through the turbidity‐current initiation process to ultimate deposition, and to evaluate the impact of changing sea level and tectonics. The Santa Monica Basin is almost a closed system for terrigenous sediment input, and is supplied principally from the Santa Clara River. The Hueneme fan is supplied directly by the river, whereas the smaller Mugu and Dume fans are nourished by southward longshore drift. This study of the Late Quaternary turbidite fill of the Santa Monica Basin uses a dense grid of high‐resolution seismic‐reflection profiles tied to new radiocarbon ages for Ocean Drilling Program (ODP) Site 1015 back to 32 ka. Over the last glacial cycle, sedimentation rates in the distal part of Santa Monica Basin averaged 2–3 mm yr^?1, with increases at times of extreme relative sea‐level lowstand. Coarser‐grained mid‐fan lobes prograded into the basin from the Hueneme, Mugu and Dume fans at times of rapid sea‐level fall. These pulses of coarse‐grained sediment resulted from river channel incision and delta cannibalization. During the extreme lowstand of the last glacial maximum, sediment delivery was concentrated on the Hueneme Fan, with mean depositional rates of up to 13 mm yr^?1 on the mid‐ and upper fan. During the marine isotope stage (MIS) 2 transgression, enhanced rates of sedimentation of > 4 mm yr^?1 occurred on the Mugu and Dume fans, as a result of distributary switching and southward littoral drift providing nourishment to these fan systems. Longer‐term sediment delivery to Santa Monica Basin was controlled by tectonics. Prior to MIS 10, the Anacapa ridge blocked the southward discharge of the Santa Clara River into the Santa Monica Basin. The pattern and distribution of turbidite sedimentation was strongly controlled by sea level through the rate of supply of coarse sediment and the style of initiation of turbidity currents. These two factors appear to have been more important than the absolute position of sea level.