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.     

Shallow marine sand bar sequences: an example from the late Precambrian of North Norway

Five coarsening upward shallow marine sandstone sequences (2–10 m thick), are described from the late Precambrian of North Norway, where they occur in a laterally continuous and tectonically undeformed outcrop. The sequences consist of five facies with distinct assemblages of sedimentary structures and palaeocurrent patterns. Each facies is the product of alternate phases of sedimentation during relatively high- and low-energy periods. Facies 1 to 4 are interpreted as representing prograding, subtidal sand bars. Sand bar progradation occurred during the highest energy periods when unidirectional currents flowed to the northwest, depositing trough cross-bedded sandstones (facies 3 and 4) on the bar crests and flanks, and sheet sandstone beds (facies 1 and 2) in the offshore environments. Weaker northwesterly flowing currents continued during moderate energy fair weather periods. Low energy fair weather periods were dominated by wave processes, which formed largescale, low-angle, westerly inclined surfaces on the bar flanks (facies 4) and wave rippled sandstone beds (facies 2) and flat laminated siltstone layers (facies 1) in the offshore environments. One sand bar was dissected by channels and infilled by tabular cross-bedded sandstones (facies 5). Bipolar palaeocurrent evidence, with two modes separated into two laterally equivalent channel systems, suggests deposition by tidal currents in mutually evasive ebb and flood channels. The inferred processes of these sand bars are compared with those associated with modern storm-generated and tidal current generated linear sand ridges. Both are influenced by the interaction of relatively low and high energy conditions. The presence of the tidal channel facies, however, combined with the inferred strong bottom current regime, is more analogous to a tidal current hydraulic regime.     

Facies model of a fine‐grained,tide‐dominated delta: Lower Dir Abu Lifa Member (Eocene), Western Desert,Egypt

Existing facies models of tide‐dominated deltas largely omit fine‐grained, mud‐rich successions. Sedimentary facies and sequence stratigraphic analysis of the exceptionally well‐preserved Late Eocene Dir Abu Lifa Member (Western Desert, Egypt) aims to bridge this gap. The succession was deposited in a structurally controlled, shallow, macrotidal embayment and deposition was supplemented by fluvial processes but lacked wave influence. The succession contains two stacked, progradational parasequence sets bounded by regionally extensive flooding surfaces. Within this succession two main genetic elements are identified: non‐channelized tidal bars and tidal channels. Non‐channelized tidal bars comprise coarsening‐upward sandbodies, including large, downcurrent‐dipping accretion surfaces, sometimes capped by palaeosols indicating emergence. Tidal channels are preserved as single‐storey and multilateral bodies filled by: (i) laterally migrating, elongate tidal bars (inclined heterolithic strata, 5 to 25 m thick); (ii) forward‐facing lobate bars (sigmoidal heterolithic strata, up to 10 m thick); (iii) side bars displaying oblique to vertical accretion (4 to 7 m thick); or (iv) vertically‐accreting mud (1 to 4 m thick). Palaeocurrent data show that channels were swept by bidirectional tidal currents and typically were mutually evasive. Along‐strike variability defines a similar large‐scale architecture in both parasequence sets: a deeply scoured channel belt characterized by widespread inclined heterolithic strata is eroded from the parasequence‐set top, and flanked by stacked, non‐channelized tidal bars and smaller channelized bodies. The tide‐dominated delta is characterized by: (i) the regressive stratigraphic context; (ii) net‐progradational stratigraphic architecture within the succession; (iii) the absence of upward deepening trends and tidal ravinement surfaces; and (iv) architectural relations that demonstrate contemporaneous tidal distributary channel infill and tidal bar accretion at the delta front. The detailed facies analysis of this fine‐grained, tide‐dominated deltaic succession expands the range of depositional models available for the evaluation of ancient tidal successions, which are currently biased towards transgressive, valley‐confined estuarine and coarser grained deltaic depositional systems.     

海南岛洋浦湾沉积作用研究

本文对海南岛洋浦湾海域的洋浦湾、新英湾、洋浦深槽及拦门沙浅滩的沉积环境和沉积特征做了描述、讨论，对河流、海岸侵蚀和珊瑚礁生物的３种沉积物来源及数量做了分析计算，得出其总量为９×１０⁴ｍ^３／ａ。根据钻孔柱状样的¹⁴Ｃ及²¹⁰Ｐｂ分析，得出８０００ａＢ．Ｐ．以来沉积速率为０．１～０．２ｃｍ／ａ，近百年来沉积速率为１～２ｃｍ／ａ。     

Tidal currents and wind waves controlling sediment distribution in a subtidal point bar of the Venice Lagoon (Italy)

The present study aims to improve current understanding of the sedimentation of subtidal point bars, analyzing interaction between tidal currents and waves in shaping a submerged meander bend of the microtidal Venice Lagoon (Italy), and it is based on coupling of sedimentological studies, geophysical analyses and numerical modelling. The Venice Lagoon is characterized by an average depth of about 1·5 m over subtidal platforms and a mean tidal range of about 1·0 m. The morphodynamic evolution of the lagoon is strongly affected by intense seasonal windstorms, which promote the formation of wind waves triggering sediment resuspension and bottom erosion. The study channel is 70 to 100 m wide, it has a radius of curvature of about 260 m and cuts through a permanently submerged subtidal platform. Water depth ranges from 1·0 to 5·0 m below mean sea level on the subtidal platform and channel thalweg, respectively. Different from classical architectural models, the study point‐bar beds do not show sigmoidal geometries, but consist of horizontally‐bedded deposits abruptly overlying clinostratified beds. Sedimentation in the study bar is hypothesized to stem from the interaction between the in‐channel secondary helical flow, as for most meander bends, and wave winnowing of the subaqueous overbank areas. Laterally accreting point‐bar deposits point out that the curvature‐induced helical flow redistributed sediment from the channel thalweg to the bar top and contributed to the development of the ‘classical’ fining‐upward grain size trend. The marked truncation surface, separating clinostratified bar deposits from overlying horizontally‐bedded platform sediments is interpreted here as due to bar top wave‐winnowing, which also possibly promoted bank collapses. In the proposed model, sediments remobilized from bar top and subaqueous overbank areas were transported into the channel, forming peculiar ‘apron‐like’ accumulations, where sand accumulated through avalanching processes and mud settled down from suspension.     

长江口沉积物210Pb分布及沉积环境解释

在长江河口潮滩、分流河道和水下三角洲共获得18个柱样,进行沉积学分析和210Pb测定,并对其中6根柱样进行137Cs测定。经研究发现,长江口外在水深25～30m,122°30′N,31°00′E附近存在一个泥质沉积中心,沉积速率达2.0～6.3cm/yr。另外,在潮滩和涨潮槽也获得较高沉积速率,其中南汇和横沙岛潮滩沉积速率(1.03～1.94cm/yr)高于崇明东滩(0.51～0.76cm/yr),涨潮槽沉积速率也达0.86cm/yr。此外,在石洞口、南汇、九段沙潮滩和三角洲前缘有部分柱样未获沉积速率,推测为沉积环境不稳定或沉积速率过快所致。     

长江口沉积物~(210)Pb分布及沉积环境解释

在长江河口潮滩、分流河道和水下三角洲共获得18个柱样,进行沉积学分析和210Pb测定,并对其中6根柱样进行137Cs测定。经研究发现,长江口外在水深25～30m,122°30′N,31°00′E附近存在一个泥质沉积中心,沉积速率达2.0～6.3cm/yr。另外,在潮滩和涨潮槽也获得较高沉积速率,其中南汇和横沙岛潮滩沉积速率(1.03～1.94cm/yr)高于崇明东滩(0.51～0.76cm/yr),涨潮槽沉积速率也达0.86cm/yr。此外,在石洞口、南汇、九段沙潮滩和三角洲前缘有部分柱样未获沉积速率,推测为沉积环境不稳定或沉积速率过快所致。     

Formation of point bars through rising and falling flood stages: Evidence from bar morphology,sediment transport and bed shear stress

Flow processes and sediment transport in a channel bend and associated point bar have been studied in modern rivers, theoretical models and physical experiments: however, the relationship between flow process and point‐bar morphology has rarely been explained due to the complex nature of open channel flow. Plan‐view exposures of an ancient point‐bar complex, exposed at the top of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation, south‐central Utah, allowed reconstruction of bar morphology, sediment transport and bed shear stress, which were used to extrapolate flow processes. Studies of these outcrops show that compound point bars and scroll bars were probably formed during falling and rising flood stages, respectively. A simulation model of plan‐view channel form shows that channel dimensions, such as radius of curvature and sinuosity of the point‐bar complex, range between 205 m and 351 m and 1·04 and 1·22, respectively, throughout the evolution of the channel bend. Variations in strength of the helical flow were interpreted as the main control on facies architecture and bar morphology. Strong helical flow was related to the deposition of the scroll bars, while strength of helical flow is decreased for compound bars. The use of cross‐beds as a common palaeocurrent indicator was found to be inconsistent with mean flow directions and channel margin orientation.     

Deposition and preservation of fluvio-tidal shallow-marine sandstones: A re-evaluation of the Neoproterozoic Jura Quartzite (western Scotland)

The 2 to 5 km thick, sandstone-dominated (>90%) Jura Quartzite is an extreme example of a mature Neoproterozoic sandstone, previously interpreted as a tide-influenced shelf deposit and herein re-interpreted within a fluvio-tidal deltaic depositional model. Three issues are addressed: (i) evidence for the re-interpretation from tidal shelf to tidal delta; (ii) reasons for vertical facies uniformity; and (iii) sand supply mechanisms to form thick tidal-shelf sandstones. The predominant facies (compound cross-bedded, coarse-grained sandstones) represents the lower parts of metres to tens of metres high, transverse fluvio-tidal bedforms with superimposed smaller bedforms. Ubiquitous erosional surfaces, some with granule–pebble lags, record erosion of the upper parts of those bedforms. There was selective preservation of the higher energy, topographically-lower, parts of channel-bar systems. Strongly asymmetrical, bimodal, palaeocurrents are interpreted as due to associated selective preservation of fluvially-enhanced ebb tidal currents. Finer-grained facies are scarce, due largely to suspended sediment bypass. They record deposition in lower-energy environments, including channel mouth bars, between and down depositional-dip of higher energy fluvio-ebb tidal bars. The lack of wave-formed sedimentary structures and low continuity of mudstone and sandstone interbeds, support deposition in a non-shelf setting. Hence, a sand-rich, fluvial–tidal, current-dominated, largely sub-tidal, delta setting is proposed. This new interpretation avoids the problem of transporting large amounts of coarse sand to a shelf. Facies uniformity and vertical stacking are likely due to sediment oversupply and bypass rather than balanced sediment supply and subsidence rates. However, facies evidence of relative sea level changes is difficult to recognise, which is attributed to: (i) the areally extensive and polygenetic nature of the preserved facies, and (ii) a large stored sediment buffer that dampened response to relative sea-level and/or sediment supply changes. Consideration of preservation bias towards high-energy deposits may be more generally relevant, especially to thick Neoproterozoic and Lower Palaeozoic marine sandstones.     

Recent, arctic deltaic sedimentation: Olivier Islands, Mackenzie Delta, North-west Territories, Canada

Despite a low tidal range and relatively low wave conditions, the Mackenzie Delta is not prograding seaward but rather is undergoing transgressive shoreface erosion and drowning of distributary channel mouths. In the Olivier Islands region of the Mackenzie Delta the resultant morphology consists of a network of primary and secondary channels separated by vegetated islands. New ground is formed through channel infilling and landward-directed bar accretion. This sedimentation is characterized by seven sedimentary facies: (1) hard, cohesive silty clay at the base of primary channels which may be related to earlier, offshore deposition; (2) ripple laminated sand beds, believed to be channel-fill deposits; (3) ripple laminated sand and silt, interpreted as flood-stage subaqueous bar deposits; (4) ripple laminated or wavy bedded sand, silt and clay, representing the abandonment phase of channel-fill deposits and lateral subaqueous bar deposition from suspension settling; (5) a well sorted very fine sand bed, presumed to result from a single storm event; (6) parallel or wavy beds of rooted silt, sand and clay, interpreted as lower energy emergent bar deposits; and (7) parallel or wavy beds of rooted silt and clay, believed to represent present-day subaerial bar aggradation. The distribution of sedimentary facies can be interpreted in terms of the morphological evolution of the study area. Initial bar deposition of facies 3 and channel deposition of facies 2 was followed by lateral and upstream bar sedimentation of facies 3 and 4 which culminated with the deposition of the storm bed of facies 5. Facies 6 and 7 signify bar stabilization and abandonment. Patterned ground formed by thermal contraction and preserved in sediments as small, v-shaped sand wedges provides the most direct sedimentological indicator of the arctic climate. However, winter ice and permafrost also govern the stratigraphic development of interchannel and channel-mouth deposits. Ice cover confines flow at primary channel mouths, promoting the bypassing of sediments across the delta front during peak discharge in the spring. Permafrost minimizes consolidation subsidence and accommodation in the nearshore, further enhancing sediment bypass. Storms limit the seaward extent of bar development and promote a distinctive pattern of upstream and lateral island growth. The effects of these controls are reflected in the vertical distribution of facies in the Olivier Islands. The sedimentary succession differs markedly from that of a low-latitude delta.     

The morphodynamics and internal structure of intertidal fine-gravel dunes: Hills Flats,Severn Estuary,UK

In the macrotidal Severn estuary, UK, the dynamics of intertidal fine-gravel dunes were investigated. These dunes are migrating across a bedrock platform. Systematic observations were made of hydraulic climate, geometry, migration rates and internal sedimentary structures of the dunes. During spring tides, the ebb flow is dominant, dunes grow in height and have ebb orientated geometry with bedrock floors in the troughs. During neap tides, a weak flood flow may dominate. Dunes then are flood orientated or symmetrical. Neap dune heights decrease and the eroded sediment is stored in the dune troughs where the bedrock becomes blanketed by muddy gravel. During spring tides, instantaneous bed shear stresses reach 8 N m^− 2, sufficient to disrupt a 9 mm-gravel armour layer. However, a sustained bed shear stress of 4 N m^− 2 is required to initiate dune migration at which time the critical depth-mean velocity is 1 m s^− 1. Ebb and flood inequalities in the bed shear stress explain the changes in dune asymmetry and internal structures. During flood tides, the crests of the dunes reverse such that very mobile sedimentary ‘caps’ overlie a more stable dune ‘core’. Because ebb tides dominate, internal structures of the caps often are characterised by ebb orientated steep open-work foresets developed by strong tidal currents and some lower angle crossbeds deposited as weaker currents degrade foresets. The foresets forming the caps may be grouped into cosets (tidal bundles) and are separated from mud-infused cores of crossbeds that lie below, by reactivation and erosion surfaces blanketed by discontinuous mud drapes. The cores often exhibit distinctive muddy toe sets that define the spacing of tidal cosets.     

Flow,stress and sediment resuspension in a shallow tidal channel

In October of 2004, a 3-d observational program to measure flow and sediment resuspension within a coastal intertidal salt marsh was conducted in the North Inlet/Winyah Bay National Estuarine Research Reserve located near Georgetown, South Carolina. Current and acoustic backscatter profiles were obtained from a moored acoustic Doppler current profiler (ADCP) deployed in a shallow tidal channel during the spring phase of the tidal cycle under high discharge conditions. The channel serves as a conduit between Winyah Bay, a large brackish estuary, and North Inlet, a saline intertidal coastal salt marsh with little freshwater input. Salinity measurements indicate that the water column is vertically well mixed during flood, but becomes vertically stratified during early ebb. The stratification results from brackish (15 psu) Winyah Bay water entering North Inlet via the tidal channel, suggesting an exchange mechanism that permits North Inlet to receive a fraction of the poor water quality and high discharge flow from upland rivers. Although maximum flood currents exceed maximum ebb currents by 0.2 m s⁻¹, suspended sediment concentrations are highest during the latter ebb phase and persist for a longer fraction of the ebb cycle. Even though the channel is flood-dominated, the higher concentrations occurring over a longer fraction of the ebb phase indicate net particulate transport from Winyah Bay to North Inlet during spring tide accompanied by high discharge. Our evidence suggests that the higher concentrations during ebb result from increased bed friction caused by flow asymmetries and variations in water depth in which the highest stresses occur near the end of ebb near low water despite stronger maximum currents during flood.     

Coarse‐sand beach ridges at Cowley Beach,north‐eastern Australia: Their formative processes and potential as records of tropical cyclone history

Storm surges generated by tropical cyclones have been considered a primary process for building coarse‐sand beach ridges along the north‐eastern Queensland coast, Australia. This interpretation has led to the development of palaeotempestology based on the beach ridges. To better identify the sedimentary processes responsible for these ridges, a high‐resolution chronostratigraphic analysis of a series of ridges was carried out at Cowley Beach, Queensland, a meso‐tidal beach system with a >3 m tide range. Optically stimulated luminescence ages indicate that 10 ridges accreted seaward over the last 2500 to 2700 years. The ridge crests sit +3·5 to 5·1 m above Australian Height Datum (ca mean sea‐level). A ground‐penetrating radar profile shows two distinct radar facies, both of which are dissected by truncation surfaces. Hummocky structures in the upper facies indicate that the nucleus of the beach ridge forms as a berm at +2·5 m Australian Height Datum, equivalent to the fair‐weather swash limit during high tide. The lower facies comprises a sequence of seaward‐dipping reflections. Beach progradation thus occurs via fair‐weather‐wave accretion of sand, with erosion by storm waves resulting in a sporadic sedimentary record. The ridge deposits above the fair‐weather swash limit are primarily composed of coarse and medium sands with pumice gravels and are largely emplaced during surge events. Inundation of the ridges is more likely to occur in relation to a cyclone passing during high tide. The ridges may also include an aeolian component as cyclonic winds can transport beach sand inland, especially during low tide, and some layers above +2·5 m Australian Height Datum are finer than aeolian ripples found on the backshore. Coarse‐sand ridges at Cowley Beach are thus products of fair‐weather swash and cyclone inundation modulated by tides. Knowledge of this composite depositional process can better inform the development of robust palaeoenvironmental reconstructions from the ridges.     

Sediment transport and bedforms in a carbonate tidal inlet; Lee Stocking Island, Exumas, Bahamas

An active oolitic sand wave was monitored for a period of 37 days in order to address the relationship between the direction and strength of tidal currents and the resultant geometry, and amount and direction of migration of bedforms in carbonate sands. The study area is situated in a tidal channel near Lee Stocking Island (Exumas, Bahamas) containing an estimated 5.5 to 6 × 10⁵ m³ of mobile oolitic sand. Tidal ranges within the inlet are microtidal and the maximum current velocity at the studied site is 0.6 m s^?1. At least 300–400 m³ of mostly oolitic sand are formed within, or brought into, the channel area every year. The tidal inlet is subdivided into an ocean-orientated segment, in which sand waves are shaped by both flood and ebb tides, and a platform-orientated segment, where sand waves are mainly shaped by flood tides. The studied sand wave lies on the platformward flood-tide dominated segment in a water depth of 3.5.4.5 m. During the 37 days of observation, the oolitic and bioclastic sand wave migrated 4 m in the direction of the dominant flood current. The increments of migration were directly related to the strength of the tide. During each tidal cycle, bedforms formed depending on the strength of the tidal current, tidal range and their location on the sand wave. During flood tides, a steep lee and a gentle stoss side formed and current ripples and small dunes developed on the crest of the sand wave, while the trough developed only ripples. The average lee slope of the sand wave is 24.2°, and therefore steeper than typical siliciclastic sand waves. During ebb tides, portions of the crest are eroded creating a convex upward ebb stoss side, covered with climbing cuspate and linguoid ripples and composite dunes. The area between the ebb-lee side and the trough is covered with fan systems, sinuous ripples and dunes. The migration of all bedforms deviated to a variable degree from the main current direction, reflecting complex flow patterns in the tidal inlet. Small bedforms displayed the largest deviation, migrating at an angle of up to 90° and more to the dominant current direction during spring tides.     

Abandoned channel fill sequences in the tidal estuary of a small mountainous,dry‐summer river

This study proposes a modification of the current model for abandoned channel fill stratigraphy produced in unidirectional flow river reaches to incorporate seasonal tidal deposition. Evidence supporting this concept came from a study of two consecutive channel abandonment sequences in Ropers Slough of the lower Eel River Estuary in northern California. Aerial photographs showed that Ropers Slough was abandoned around 1943, reoccupied after the 1964 flood, and abandoned again in 1974 with fill continuing to the present. Planform geomorphic characteristics derived from these images were used in conjunction with sub‐centimetre resolution stratigraphic analyses to describe depositional processes and their resultant sedimentary deposits. Both abandonment sequences recorded quasi‐annual scale fluvial/tidal deposition couplets. In both cases, tidal deposits contained very little sand, were higher in organic and inorganic carbon content than the sandier, fluvially dominated deposits, and possessed millimetre‐scale horizontal laminations. The two abandonment fills differed significantly in terms of the temporal progression of channel narrowing and fluvial sediment deposition characteristics. Aerial photographic analysis showed that the first abandonment sequence led to a more rapid narrowing of Ropers Slough and produced deposits with a positive relationship between grain size/deposit thickness and discharge. The second abandonment resulted in a much slower narrowing of Ropers Slough and generally thinner fluvial deposits with no clear relationship between grain size/deposit thickness and discharge. The δ¹³C values and organic nitrogen to organic carbon ratios of deposits from the first phase overlapped with Eel River suspended sediment characteristics found for low flows (one to five times mean discharge), while those of the second phase were consistent with suspended sediment from higher flows (seven to ten times mean discharge). When considered together, the results indicate that the early fill sequence recorded a reach experiencing regular fluvial deposition through flow conditions during the wet season, while the latter fill sequence records a reach more disconnected from the main stem in terms of flow and sediment. The major factor affecting the difference in sedimentation between the two fill periods appears to have been the morphology of the upstream river bend in relation to the position of the bifurcation node. During the first fill period, the upstream entrance to Ropers Slough seems to have remained open, in part due to the placement of its entrance on the outside of the mainstem river bend, and despite stronger tidal effects caused by a larger tidal prism and closer proximity to the tidal inlet. By the second fill sequence, the upstream bend morphology had altered, placing the entrance to Ropers Slough on the inner bank of the mainstem bend, which resulted in more rapid plug bar formation. The role of tidal effects in the geomorphic trajectory of the two abandonment sequences is unclear, but appears to have been less important than local bifurcation geometry.     

珠江口的沉积作用和沉积相

珠江口为华南沿海的重要河口,是珠江水(系(西江、北江、东江、流溪河)的受水盆地。珠江流域每年有8000余万吨悬沙。大约3000万吨溶解质和少量底沙进入珠江河口区域,它们在迳流、潮流和其他海洋动力因素共同影响下,沉积作用甚为活跃。但是珠江口水域宽阔,地形复杂,各河口、河口湾或湾外区域的动力背景及泥沙运动不尽相同,因而其沉积作用和沉积特征也具有区域差异。     

Fluvial to tidal transition in proximal,mixed tide‐influenced and wave‐influenced deltaic deposits: Cretaceous lower Sego Sandstone,Utah, USA

Facies models for regressive, tide‐influenced deltaic systems are under‐represented in the literature compared with their fluvial‐dominated and wave‐dominated counterparts. Here, a facies model is presented of the mixed, tide‐influenced and wave‐influenced deltaic strata of the Sego Sandstone, which was deposited in the Western Interior Seaway of North America during the Late Cretaceous. Previous work on the Sego Sandstone has focused on the medial to distal parts of the outcrop belt where tides and waves interact. This study focuses on the proximal outcrop belt, in which fluvial and tidal processes interact. Five facies associations are recognized. Bioturbated mudstones (Facies Association 1) were deposited in an offshore environment and are gradationally overlain by hummocky cross‐stratified sandstones (Facies Association 2) deposited in a wave‐dominated lower shoreface environment. These facies associations are erosionally overlain by tide‐dominated cross‐bedded sandstones (Facies Association 4) interbedded with ripple cross‐laminated heterolithic sandstones (Facies Association 3) and channelized mudstones (Facies Association 5). Palaeocurrent directions derived from cross‐bedding indicate bidirectional currents which are flood‐dominated in the lower part of the studied interval and become increasingly ebb‐directed/fluvial‐directed upward. At the top of the succession, ebb‐dominated/fluvial‐dominated, high relief, narrow channel forms are present, which are interpreted as distributary channels. When distributary channels are abandoned they effectively become estuaries with landward sediment transport and fining trends. These estuaries have sandstones of Facies Association 4 at their mouth and fine landward through heterolithic sandstones of Facies Association 3 to channelized mudstones of Facies Association 5. Therefore, the complex distribution of relatively mud‐rich and sand‐rich deposits in the tide‐dominated part of the lower Sego Sandstone is attributed to the avulsion history of active fluvial distributaries, in response to a subtly expressed allogenic change in sediment supply and relative sea‐level controls and autocyclic delta lobe abandonment.     

磨刀门河口洪季波生流及其泄洪影响

磨刀门已由"径流型"向"径流-波浪型"河口转变,波浪已是该河口主要动力之一,但波浪对河口洪季水流及泄洪的影响缺少研究。在2-D潮流数学模型中添加随潮位实时变化的波浪辐射应力,建立波浪潮流耦合数学模型;波浪求解采用缓坡方程,背景水深由潮流模型实时提供,可通过比较考虑和未考虑波浪影响的河口流场来分析波浪对泄洪的影响。在年均常浪作用下,磨刀门河口洪季涨落潮阶段均有明显的波生环流结构。由于波浪作用方向向陆,波生流减弱了浅滩区的向海余流,增大了浅滩向陆余流;受浅滩向海余流减弱影响,河口动力自调整后形成归槽水流,促使深槽内向海余流增大。波浪有顶托河口泄洪之势,可改变滩槽泄洪分配比例;年均常浪的波高较小,其对潮流及泄洪的影响区域限制在浅水区,故对泄洪的负面影响有限。     

Of sand and mud: Sedimentological criteria for identifying the turbidity maximum zone in a tidally influenced river

The thickness and lateral distribution of sand and mud beds and bedsets on channel bars from the tidally influenced Fraser River, British Columbia, Canada, are quantitatively assessed. Fifty‐six vibracores totalling ca 114 m of vertical section are used to tabulate bed thicknesses. Statistical calculations are undertaken for nine channel bars ranging from the freshwater and tidal zone, to the sustained brackish water and tidal zone. The data reveal that thickness trends can be organized into three groups that broadly correspond to time‐averaged hydrodynamic and salinity conditions in the various distributary channels. Thick sand beds (up to 30 cm) and thin mud beds (up to 5 cm) characterize the freshwater tidal zone. The tidal and freshwater to brackish‐water transition zone comprises thin sands (up to 10 cm) and thicker muds (up to 19 cm), and the sustained brackish water tidal zone consists of thin muds (up to 6 cm) with relatively thicker sands (up to 25 cm). The results suggest that the locus of mud deposition occurs in the tidal freshwater to brackish‐water zone, probably reflecting mud flocculation and deposition at the turbidity maximum. Landward of the turbidity maximum, mud deposition is linked to tidal influence (tidal backwater effect and reverse eddy currents on channel margins) as mud beds thin in the landward direction. These results support the hypothesis that mud deposition is greatest at the turbidity maximum and decreases in both the seaward and landward direction. This study also showcases that mud‐bed thicknesses are greatest towards the turbidity maximum and thin in both the landward and seaward direction. In the rock record, the apex of mud deposition probably marks the position of the palaeo‐turbidity maximum.     

塔里木盆地塔中地区志留系柯坪塔格组下沥青砂岩段沉积相分析

塔中地区志留系柯坪塔格组下沥青砂岩段是塔里木盆地古生界碎屑岩层系油气主要富集层段之一。以钻井岩心观察描述为基础,结合测井、地震和分析测试资料进行综合分析,提出了塔中地区志留系下沥青砂岩段发育海侵背景下的滨岸-潮坪-辫状河三角洲沉积体系的观点。其中滨岸相沉积中主要发育上临滨和下临滨沉积微相,岩性以细砂岩、粉砂岩、含砾不等粒砂岩为主;潮坪沉积相中障壁砂坝、潮砂滩、潮汐水道、砂坪沉积微相发育,潮道沉积的岩性以砂岩、粗砂岩和含砾砂岩为主,发育楔形、板状和羽状交错层理,底部见侵蚀冲刷面和较多黄铁矿团块;辫状河三角洲中发育辫状河水下分流河道、分流间湾、前缘席状砂等沉积微相。海侵时期,下沥青砂岩段存在4期超覆砂体,从盆地中部向南部的塔中低隆逐期超覆,沉积微相在平面上也由此存在着复合超覆规律的演变,整体表现为滨岸相至潮坪相、辫状河三角洲沉积相沉积的演变。4期超覆沉积微相的平面分布在控制因素、展布范围和海侵方向上具有一定的继承性、相似性和规律性。