障壁岛—潟湖沉积特征及模式:以美国普拉姆岛上更新统—全新统为例*

普拉姆岛(Plum island)是美国东北部缅因湾最大的障壁海岸,岛内向陆一侧为新英格兰地区最大潟湖和沼泽区,它们是晚第四纪末次冰期冰川作用和冰后期海岸作用的沉积响应。通过普拉姆岛研究区上更新统—全新统160个钻孔描述,识别出8种沉积物: 泥炭、冰川黏土、黏土、粉砂、细砂、中粗砂、砾、坠石。根据沉积物类型及其组合特征,结合沉积环境,共划分出8种沉积微相: 障壁沙丘、滨岸沙、水下临滨沙、河道、潮汐水道、潟湖、潮坪、沼泽。研究区在晚第四纪末次冰盛期(MIS2)被劳伦斯蒂德冰盖(Laurentide Ice Sheet)覆盖,发育冰川地貌,冰川泥覆盖在基岩之上,形成底层沉积; 冰后期(MIS1),冰盖消融,海平面发生变化,在冰川地貌鼓丘附近形成沙坝,最终沉积演化为障壁岛—潟湖环境,潟湖通过潮汐水道与广海相连通。     

Transgression of an estuarine channel and tidal flat complex: the Lower Triassic of Barles, Alpes de Haute Provence, France

The Lower Triassic succession of Barles, Alpes de Haute Provence, France, comprises an unconformable quartz arenite sand body of 90m thickness. The succession may be informally divided into (i) lower channellized cross-bedded member overlain by (ii) an upper fining upward member. The lower member comprises vertically stacked, subtidal channel units separated into five major sand bodies by thin developments of fine grained channel margin and shoal deposits. Subtidal channel fill deposits are dominated by varying scales of cross bedding. These scales vary systematically from the base to the top of the member, with large scale planar sets dominating the lowest channel sand body (sand body 1), medium scale planar and trough cross bedding characterizing sand bodies 2-4, the largest scale planar sets in the highest sand body (sand body 5). This upward change in cross bedding scale is concomitant with a decrease in both the relief of major channel sand body erosion surfaces, and the proportion of preserved interchannel shoal deposits. The succeeding fining upward member comprises small scale tidal channel units overlain by channel shoal and tidal flat deposits. Tidal flat sequences are characterized by parallel laminated, wave and current rippled sandstones separated by bioturbated, fine grained siltstones and mudstones. The vertical variation in facies of the Lower Triassic succession suggests two main periods of deposition. The lower member is considered to preserve successively more seaward components of a transgressive estuarine complex. The overlying upper member records the seaward progradation of tidal channel, shoal and tidal flat environments. The unconformity bounded nature of the lower member, combined with its systematic variation in facies, suggests it may represent an incised valley-estuarine fill developed in response to an early Triassic relative sea level fall and subsequent rise. Succeeding tidal channel and tidal flat deposits forming the upper fining upward member reflect a change in sediment supply and/or rate of relative sea level rise comparable with a progradational shoreline. It is unclear whether this final depositional episode represents a period of highstand progradation or a later lowstand shoreline system developed following a further period of relative sea level fall and rise.     

Morphologic and stratigraphic evolution of muddy ebb-tidal deltas along a subsiding coast: Barataria Bay, Mississippi River delta

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

Facies and sand-body geometry of the Queen City (Eocene) tide-dominated delta-margin embayment, NW Gulf of Mexico Basin

The Queen City Formation (Eocene) displays an array of tide-dominated coastal facies in the Tyler Basin of the northern Gulf of Mexico. This facies assemblage, which is atypical of the microtidal, wave-dominated, coastal depositional complexes that characterize the Cenozoic Gulf basin, reflects tidal amplification in a generally protected embayment on the east flank of a strongly prograded delta system. The shallow embayment was confined to the east by contemporaneous uplift and shoaling across the Sabine Uplift. Fluvial, barrier (including ebb tidal delta), heterolithic tidal, estuary-fill, and tidal point-bar facies are all found at outcrop. These facies were projected into the three-dimensional geometry of the tide-dominated depositional complex. Inlet, estuary, and distributary-fill sand bodies, which are linear and diporientated, dominate lithofacies maps. The Queen City facies assemblage in the Tyler Basin records a mixture of mesotidal to macrotidal environments that were interspersed in time and space with fluvial-dominated lobes, which periodically prograded eastward from the deltaic depocentre into the flanking embayment. Queen City deposition terminated with regional marine flooding and deposition of glauconitic, fossiliferous shelf sands and muds of the Weches Formation. Transgression is marked by a prominent ravinement surface that truncates underlying facies of the tide-dominated shore zone.     

Palaeoenvironmental control on sediment composition and provenance in the late Quaternary deltaic successions: a case study from the Po delta area (Northern Italy)

Accumulation of continental, deltaic and shallow‐marine sediments in the Po River coastal plain preserves a record of the Late Quaternary sea‐level changes and shoreline migrations. The palaeoenvironmental evolution of this area and the changes in composition and provenance of sediments have been investigated through integrated sedimentological, micropalaeontological (mainly foraminifers) and geochemical analyses of core S1, from the southern part of the Po River delta, within a chronological framework supported by radiocarbon dating and correlations with adjacent core sequences. Eleven lithofacies, grouped into five facies associations, and four palaeontological assemblages provide the basis to define the palaeoenvironmental reconstruction of this succession consisting, from the base to the top, of: (i) continental sediments accumulated during the Late Pleistocene; (ii) back‐barrier sediments marking the onset of Holocene sea‐level rise; (iii) transgressive sands deposited during the rapid landward migration of a barrier‐lagoon system; (iv) shallow‐marine and prodelta sediments with faunal associations indicating a gradual approach to the Po River mouth; and (v) sub‐recent delta front sands that form a considerable portion of the present coastal plain. Bulk chemical composition of sediments shows remarkable relationships with palaeoenvironments and locally improves facies characterizations. For example, they reveal carbonate leaching that emphasizes the occurrence of palaeosols in continental deposits or record enrichments in loss on ignition, S and Br, diagnostic of organic‐rich layers in back‐shore sediments. Selected geochemical elements (e.g. Mg and Ni) are particularly effective for the recognition of sediment provenances from the three main source areas observed in the subsurface deposits of the Po River coastal plain (e.g. Apenninic rivers, North Adriatic rivers and Po River). An Apenninic provenance is observed in continental and back‐barrier sediments. A North Adriatic provenance characterizes the transgressive sands and the shallow‐marine deposits; a significant Po River provenance is recorded in sediments related to the onset of the prodelta environment, confirmed by foraminiferal assemblages indicating remarkable increase in fluvial influxes. Copyright © 2006 John Wiley & Sons, Ltd.     

Recent and Pleistocene beach/dune sequences,western Australia

Wave-dominated sandy shores occur along much of the coast of Western Australia. Despite local variations there is a characteristic distribution of lithofacies (corresponding to different geomorphic zones). Five lithofacies are recognised: (1) trough-bedded sand/gravel; (2) laminated sand; (3) laminated/bubble sand; (4) laminated/disrupted sand; and (5) aeolian cross-stratified sand.The trough-bedded sand/gravel lithofacies is being deposited in the shallow shoreface below LWL. The laminated sand and laminated/bubble sand lithofacies are sands with gravel layers being deposited on the foreshore swash zone; extensive bubble (or vesicular) sand is common towards HWL especially in berms. The laminated/disrupted sand lithofacies is being deposited on the backshore between HWL and storm water levels and consists of horizontally layered to homogeneous sands with storm debris, especially wood, weed and floatable skeletons (e.g. Sepla and Spirula). The aeolian cross-stratified sand lithofacies is forming in beach ridge/dune areas and consists of fine sands with large-scale, generally landward-dipping forests; soils and rootlets are common.Recognition of these lithofacies within a sedimentary sequence enables reconstruction of gross shoreline conditions in terms of wave and eolian environments, tidal and storm heights, and palaeogeography. Each of these lithofacies with their characteristic features is recognised in Pleistocene sequences in Perth Basin. The Pleistocene sequences fit a model of coastal progradation with the trough-bedded sand/gravel lithofacies at the base and the aeolian sand lithofacies at the top. The value of such a stratigraphic sequence, however, extends beyond the Pleistocene.     

西秦岭直合隆地区三叠系深水水道沉积模式分析

西秦岭三叠系地层主要由深水沉积物组成。选取西秦岭直合隆地区出露较好的三组剖面进行研究,按重力流类型及静水条件下所发生的沉积现象,总结出六种岩石相类型,即滑塌岩相（F1）、碎屑流相（F2）、超高密度流相（F3）、高密度浊流相（F4）、低密度浊流相（F5）、深海泥岩相（F6）。通过对三处剖面的横向追踪对比,结合不同沉积环境下表现出特定的岩石相占比及砂体堆叠样式,共识别出限制性水道、弱限制性水道及水道天然堤三类沉积单元。限制性水道中砂体叠置关系复杂,以超高密度流相占主导地位,次为碎屑流相;弱限制性水道中砂体叠置较规整,以超高密度流相占主导地位,次为低密度浊流相;而水道天然堤表现为砂泥互层（总厚度约15 m）,主要为低密度浊流相和深海泥岩相。最后建立了研究区内深水水道沉积演化阶段模式图——早期限制性环境下水道较顺直,水道较窄,砂体叠置关系复杂;中期限制性环境相对早期有所减弱,水道弯曲度增大,水道变宽,出现溢岸沉积,砂体叠置关系复杂;晚期为弱限制性环境,弯曲水道两侧发育天然堤,砂体叠置规整。本次研究利用野外露头建立的深水水道沉积演化模式,在一定程度上还原其相关的重力流演化过程及发育期次,对全球其他类似研究具有一定的参考意义。     

全新世长江三角洲地区的海进海退层序

现代三角洲地区海进海退是经常发生的,引起的原因是多方面的。本文根据500多个钻孔资料的分析对比,着重讨论全新世长江三角洲海进海退层序的特点,发育过程和控制因素。     

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.     

Millennial/submillennial-scale sea-level fluctuations in western Mediterranean during the second highstand of MIS 5e

This paper investigates a series of small-scale, short-lived fluctuations of sea level registered in a prograding barrier spit that grew during the MIS 5e. This interglacial includes three highstands (Zazo et al., 2003) and we focus on the second highstand, of assumed duration ～10 ± 2 ka, given that U–Th ages do not provide more accurate data. Geometry and 3D architecture of beach facies, and thin-section petrography were used to investigate eight exposed offlapping subunits separated by seven conspicuous erosion surfaces, all interpreted as the result of repeated small-scale fluctuations of sea level.Each subunit records a relatively rapid rise of sea level that generated a gravelly shoreface with algal bioherms and a sandy uppermost shoreface and foreshore where most sand accumulated. A second range of still smaller-scaled oscillations of sea level has been deduced in this phase of sea-level fluctuation from lateral and vertical shifts of the foreshore-plunge-step-uppermost shoreface facies.Eventually, progradation with gently falling sea level took place and foreshore deposits underwent successive vadose cementation and subaerial dissolution, owing to relatively prolonged exposure. Later recovery of sea level re-established the highstand with sea level at approximately the same elevation, and there began deposition of a new subunit. The minimum sea-level variation (fall and subsequent rise) required to generate the observed features is 4 m. The time span available for the whole succession of events, and comparison with the Holocene prograding beach ridge complex in the nearby Roquetas (Almería) were used to calculate the periodicity of events. A millennial-suborbital time scale is suggested for fluctuations separating subunits and a decadal scale for the minor oscillations inside each subunit.     

鄂尔多斯盆地东缘大宁—吉县地区山西组页岩岩相与沉积相特征

以鄂尔多斯盆地东缘大宁—吉县地区二叠系山西组为研究对象,综合应用岩心、薄片及测井资料,开展山32亚段岩相类型、岩相组合和沉积相研究,研究结果表明:山32亚段包括粉砂质泥岩/页岩相、纹层状层理含粉砂泥岩/页岩相、透镜状层理粉砂质泥岩/页岩相、钙质页岩相、煤层、碳质页岩相、黑色页岩相、波状层理泥质粉砂岩相、交错层理中—细砂...     

Coastal Evolution Over the Past 3000 Years at Conrads Beach,Nova Scotia: the Influence of Local Sediment Supply on a Paraglacial Transgressive System

Many coastlines are retreating in response to sea level rise, compounded by glacial–isostatic subsidence in areas marginal to former ice sheets. The resulting barrier and estuarine deposits are dominated by transgressive stratigraphy. Where supplied primarily from relict glacial deposits, this “paraglacial” sediment input may rise and fall, increasing as a new source such as a drumlin headland is exposed to erosion but declining as the source becomes exhausted. Conrads Beach, on the Atlantic coast of Canada, has experienced a succession of barrier growth and reworking as sediment supply from several drumlin sources has varied over the past 3000 years. In the context of long-term regional transgression, there have been intervals of years to centuries characterized by local stability or progradation. Ground-penetrating radar profiles and refraction seismic data were used to image the facies architecture of Conrads Beach to depths of 6–8 and 10–24 m, respectively. Thirteen vibracores provided a record of lithofacies characteristics and geometry. Results show evidence of an estuarine basin at ~2800 years BP. As the outer coast retreated, erosion of drumlins provided multi-century sediment pulses to adjacent beaches and embayments. Locally increased sediment supply fed a prograding beach ridge complex from >600 to ~150 years BP and tidal channels feeding sediment to back-barrier flood delta deposits. This study documents the complexity of coastal adjustment to time- and source-varying sediment supply under long-term rising sea level. It expands and refines previous models, providing guidance required for effective management and hazard mitigation on transgressive paraglacial coasts.     

湘西北龙山、永顺地区龙马溪组潮控三角洲沉积的发现--志留纪“雪峰隆起”形成的新证据

通过露头沉积学与岩石学分析,在湘西北龙山、永顺地区下志留统龙马溪组中首次发现完整的潮控型三角洲沉积,也是华南地区龙马溪组中存在三角洲相的首次报道。识别出前三角洲、三角洲前缘和三角洲平原3个亚相及10个微相。前三角洲亚相以滨外泥页岩为主,并发育低密度浊积岩,主要见于龙马溪组底部和下部;三角洲前缘亚相包括席状砂、分流间湾、河口砂坝和远砂坝等;三角洲平原亚相包括分流河道、分流间湾、天然堤和决口扇等,主要见于龙马溪组中部、上部。通过分析剖面垂向序列及其指示的沉积环境变化,提出本区龙马溪组三角洲相碎屑岩是构造挤压背景下“雪峰隆起”开始形成的重要沉积响应。该三角洲沉积的发现,对于理解扬子东南缘早志留世构造-古地理转型的时限与过程具有特殊意义。     

Recent formations and their basal topography in and around Tokyo Bay,central Japan

This paper summarizes the subsurface geology of the recent (both $\text{Holocene}$ and latest $\text{Pleistocene}$ formations and the buried topography beneath them in and around Tokyo Bay, the type area of the late Quaternary in Japan. Buried abrasion platforms in the buried topography are classified into upper (ca. 0 to ?10 m high) and lower (ca. ?20 to ?40 m) platforms; upper and lower buried river terraces are also distinguished, and are correlated to the subaerial late Pleistocene terraces of Tc₁ and Tc₂, respectively. A buried valley system is elucidated, of which the trunk valley floor reaches ?70 m in Tokyo and emerges into a flat surface at the shelf edge in the entrance to Tokyo Bay. Approximate dates for these geomorphic surfaces are given. The height of sea level contemporaneous with the buried valley floor (ca. 20,000–15,000 yr BP) is estimated at about ?135 m. The recent formations are divided into two members, upper and lower, by a middle sand bed, in addition to the lowest buried valley floor gravel. The lower member, which is composed of brackish to marine deposits of complicated lithofacies, was accumulated in narrow drowned valleys during the early stage of the Yurakucho (Flandrian) transgression. The middle sand bed is the foreset bed of deltas, which was formed during a slight regression between ca. 11,000 and 10,000 yr BP. The upper member, which consists mainly of widespread homogeneous marine clay and deltaic sand, was accumulated in a wide bay and its embayments during the late stage of the Yurakucho transgression and the following stage of a relatively stable sea level.     

Sedimentology and palaeogeomorphology of four large valley systems incising delta plains, western Canada Foreland Basin: implications for mid-Cretaceous sea-level changes

The mid-Cenomanian Dunvegan Formation represents a delta complex deposited on a foreland basin ramp over about 2 my. The Dunvegan is divided into 10 transgressive–regressive allomembers, labelled J–A in ascending order, each defined by regional marine transgressive surfaces. Parasequences within allomembers show an aggradational to offlapping stacking pattern that reflects alternate generation and removal of accommodation. The upper surfaces of allomembers H–E are incised by extensive valley systems traceable for up to 320 km and over about 50 000 km². Valley depths range up to 41 m and can change significantly over short distances. However, the average depth of incision (mean 21 m) shows no systematic variation in longitudinal profiles and no evidence of headward shallowing. Valleys are typically 1–2 km wide, but locally widen to about 8 km. Widening is sometimes associated with confluence zones, but elsewhere it is not. Updip reaches of valleys are dominated by cross-bedded fluvial sandstone forming multistorey point-bar deposits. Sandstones contain widespread but uncommon paired carbonaceous drapes recognizable as tidal bundles. Inclined heterolithic stratification is locally well developed at the top of the valley fill. Downdip reaches of valleys, typically within 50 km of the lowstand shoreline, have a sandstone-dominated lower part and, locally, a mud-rich upper portion consisting of a variety of laminated heterolithic facies with a clear tidal signature. These heterolithic deposits may represent central basin, tidal flat, bayhead delta and point-bar environments. Valley filling took place mainly during the transgressive systems tract (TST) when tidally influenced environments migrated upvalley. Semi-diurnal tidal backwater effects extended at least 30 km landward of the regional maximum transgressive marine shoreline. The aggradational late TST and highstand systems tract (HST) includes deltaic and coastal plain deposits comprising lake and anastomosed river deposits that suggest a very low gradient (≈ 1:3000). Delta parasequences of the falling stage systems tract (FSST) offlap seaward and have no equivalent coastal plain deposits. The FSST has an average width of 60 km and an inferred gradient of 1:2500. The upper surfaces of the HST and FSST are extensively incised by valleys. The lowstand systems tract (LST) is subtly aggradational, lacks valleys and is characterized by large delta lobes fed by major distributaries. The width and inferred slope of the FSST, coupled with the thickness of aggradational TST and HST deposits on the coastal plain, suggest a vertical accommodation of about 35 m per transgressive event. About 11 m of this is attributed to isostatic subsidence resulting from water and sediment loads; the residual 24 m is attributed to eustatic rise. This sea-level change is of the same order of magnitude as the valley depths. The length of valleys, however, does not seem to be explicable solely in terms of downstream forcing by sea-level change, and an additional, upstream-forcing mechanism, possibly related to precipitation cycles in the Milankovitch band, might be inferred.     

The upper Hawkesbury River, New South Wales, Australia: a Holocene example of an estuarine bayhead delta

Holocene deposits of the Hawkesbury River estuary, located immediately north of Sydney on the New South Wales coast, record the complex interplay between sediment supply and relative sea-level rise within a deeply incised bedrock-confined valley system. The present day Hawkesbury River is interpreted as a wave-dominated estuarine complex, divisible into two broad facies zones: (i) an outer marine-dominated zone extending 6 km upstream from the estuary mouth that is characterized by a large, subtidal sandy flood-tidal delta. Ocean wave energy is partially dissipated by this flood-tidal delta, so that tidal level fluctuations are the predominant marine mechanism operating further landward; (ii) a river-dominated zone that is 103 km long and characterized by a well developed progradational bayhead delta that includes distributary channels, levees, and overbank deposits. This reach of the Hawkesbury River undergoes minor tidal level fluctuations and low fluvial runoff during baseflow conditions, but experiences strong flood flows during major runoff events. Fluvial deposits of the Hawkesbury River occur upstream of this zone. The focus of this paper is the Hawkesbury River bayhead delta. History of deposition within this delta over the last c. 12 ka is interpreted from six continuous cores located along the upper reaches of the Hawkesbury River. Detailed sedimentological analysis of facies, whole-core X-ray analysis of burrow traces and a chronostratigraphic framework derived from 10 C-14 dates reveal four stages of incised-valley infilling in the study area: (1) before 17 ka BP, a 0–1 m thick deposit of coarse-grained fluvial sand and silt was laid down under falling-to-lowstand sea level conditions; (2) from 17 to 6·5 ka BP, a 5–10 m thick deposit composed of fine-grained fluvial sand and silt, muddy bayhead delta and muddy central-basin deposits developed as the incised valley was flooded during eustatic sea-level rise; (3) during early highstand, between 6·5 and 3 ka BP, a 3–8 m thick bed of interbedded muddy central-basin deposits and sandy river flood deposits, formed in association with maximum flooding and progradation of sandy distributary mouth-bar deposits commenced; (4) since 3 ka BP, fluvial deposits have prograded toward the estuary mouth in distributary mouth-bar, interdistributary-bay and bayhead-delta plain environments to produce a 5–15 m thick progradational to aggradational bayhead-delta deposit. At the mouth of the Hawkesbury estuary subaqueous fluvial sands interfinger with and overlie marine sands. The Hawkesbury River bayhead-delta depositional succession provides an example of the potential for significant variation of facies within the estuarine to fluvial segment of incised-valley systems.     

Sedimentological and ichnological implications of rapid Holocene flooding of a gently sloping mud‐dominated incised valley – an example from the Red River (Gulf of Tonkin)

The Gulf of Tonkin coastline migrated at an average rate of ca 60 m year^?1 landward during Holocene sea‐level rise (20 to 8 ka). Due to a combination of rapid coastline migration and undersupply of sand, neither coastal barriers nor tidal sand bars developed at the mouth of the Red River incised valley. Only a 30 to 80 cm thick sandy interval formed at the base of full‐marine deposits. Thus, the river mouth represented a mud‐dominated open funnel‐shaped estuary during transgression. At the base of the valley fill, a thin fluvial lag deposit marks a period of lowered sea‐level when the river did not reach geomorphic equilibrium and was thus prone to erosion. The onset of base‐level rise is documented by non‐bioturbated to sparsely bioturbated mud that occasionally contains pyrite indicating short‐term seawater incursions. Siderite in overlying deposits points to low‐salinity estuarine conditions. The open funnel‐shaped river mouth favoured upstream incursion of seawater that varied inversely to the seasonal strongly fluctuating discharge: several centimetres to a few tens of centimetres thick intervals showing marine or freshwater dominance alternate, as indicated by bioturbational and physical sedimentary structures, and by the presence of Fe sulphides or siderite, respectively. Recurrent short‐term seawater incursions stressed the burrowing fauna. The degree of bioturbation increases upward corresponding to increasing marine influence. The uppermost estuarine sediments are completely bioturbated. The estuarine deposits aggraded on average rapidly, up to several metres kyr^?1. Siphonichnidal burrows produced by bivalves, however, document recurrent episodes of enhanced deposition (>0·5 m) and pronounced erosion (<1 m) that are otherwise not recorded. The slope of the incised valley affected the sedimentary facies. In steep valley segments, the marine transgressive surface (equivalent to the onset of full‐marine conditions) is accentuated by the Glossifungites ichnofacies, whereas in gently sloped valley segments the marine transgressive surface is gradational and bioturbated. Marine deposits are completely bioturbated.     

High‐resolution reconstruction of a coastal barrier system: impact of Holocene sea‐level change

This study presents a detailed reconstruction of the sedimentary effects of Holocene sea‐level rise on a modern coastal barrier system. Increasing concern over the evolution of coastal barrier systems due to future accelerated rates of sea‐level rise calls for a better understanding of coastal barrier response to sea‐level changes. The complex evolution and sequence stratigraphic framework of the investigated coastal barrier system is reconstructed using facies analysis, high‐resolution optically stimulated luminescence and radiocarbon dating. During the formation of the coastal barrier system starting 8 to 7 ka rapid relative sea‐level rise outpaced sediment accumulation. Not before rates of relative sea‐level rise had decreased to ca 2 mm yr^?1 did sediment accumulation outpace sea‐level rise. From ca 5·5 ka, rates of regionally averaged sediment accumulation increased to 4·3 mm yr^?1 and the back‐barrier basin was filled in. This increase in sediment accumulation resulted from retreat of the barrier island and probably also due to formation of a tidal inlet close to the study area. Continued transgression and shoreface retreat created a distinct hiatus and wave ravinement surface in the seaward part of the coastal barrier system before the barrier shoreline stabilized between 5·0 ka and 4·5 ka. Back‐barrier shoreline erosion due to sediment starvation in the back‐barrier basin was pronounced from 4·5 to 2·5 ka but, in the last 2·5 kyr, barrier sedimentation has kept up with and outpaced sea‐level. In the last 0·4 kyr the coastal barrier system has been prograding episodically. Sediment accumulation shows considerable variation, with periods of rapid sediment deposition and periods of non‐deposition or erosion resulting in a highly punctuated sediment record. The study demonstrates how core‐based facies interpretations supported by a high‐resolution chronology and a well‐documented sea‐level history allow identification of depositional environments, erosion surfaces and hiatuses within a very homogeneous stratigraphy, and allow a detailed temporal reconstruction of a coastal barrier system in relation to sea‐level rise and sediment supply.