Tidal Delta Research Articles

Morpho-Dynamics Shoreline Offset at the Entrance of Qua Iboe River Estuary, South East Coast of Nigeria

The morpho-dynamic variability of shorelines was examined to ascertain the causes of shoreline offset around the entrance of Qua-Iboe River estuary, south-east coast of Nigeria. Shoreline mapping revealed a landward updrift shoreline offset displacement relative to the estuary mouth over a distance of 600 m off the former seaward limit. Daily beach profiles showed an averaged beach width of 200 m with a concave foreshore contiguous to the estuary mouth at the updrift side while the downdrift was characterised by a narrow beach width of 190 m and a convex foreshore. The updrift surf-zone was three times higher in surf-scaling parameters which caused higher rates of erosion and volumetric loss of sediment compared to the downdrift side which was characterised by accretion. The shoreline offset was attributed to the actions and forces of wind/wave, tidal and long-shore currents on the shoreline which were accentuated by storm surge incidence in 2011. A clockwise rotation of the ebb tidal channel in the delta at the estuary mouth estimated at a period of 9 years and a shift in the ebb tidal delta in the downdrift direction offered the best explanation to the updrift erosion phenomenon. Moreover, Estuary-Deltaic-Surf zone processes were noted as a system which modulates the morpho-dynamic variability of the shoreline. A programmed monitoring by appropriate government agencies of the changes and maintenance of the ebb tidal delta through periodic beach replenishment with sediment are recommended as sustainable shoreline protection strategies.

NATURAL DESIGN AND MANAGEMENT OF AN ESTUARY MOUTH SYSTEM

Estuaries often are the focus of both man-made infrastructure and valuable flora and fauna. Sand spits are common at the mouth of estuaries, and are one of the key features that influence hydrodynamics and geomorphology in estuaries. The Environment Agency has delivered a long term coastal management scheme, which has recently been designed and built, to manage the Dawlish Warren sand spit at the mouth of the Exe Estuary, in south Devon, UK. This scheme is designed to • Protect 3,000 properties and critical national infrastructure from flooding. • Restore 2km of internationally designated (SAC) dune and associated ecology continue protection to the SPA bird assemblages in the Exe Estuary. • Provide a flexible way forward through this century for the sand spit to be managed in a sustainable and balanced manner. The Exe Estuary is macrotidal with a range of 4m, and main tributary inflows of the order of 20-400m3/s. Dawlish Warren sand spit covers three quarters of the estuary mouth width, sheltering the estuary from the coastal wave and tide climate. Consequently, the inestuary extreme wave heights are limited to less than 1.1m, with storm surges of up to 1-2m. The estuary exhibits high tidal velocities through its mouth, with flood/ebb tidal deltas and offshore banks present. The sand spit has breached around 13 times over the last two centuries, prompting reactive construction of concrete, rock armour and

Millennial-scale crustal movements inferred from Holocene sedimentary succession of the Omoto plain, northern Sanriku coast, Northeast Japan: Relevance for modeling megathrust earthquake cycles

Crustal movements on a millennial timescale (103–104 years) are important data for assessing the deformation of subduction zones during megathrust earthquake cycles. Such data did not exist until recently for the Sanriku coast, near the source region of the 2011 Tohoku-oki earthquake. Since the 2011 earthquake, studies of Holocene sedimentary successions have documented subsidence on the central to southern segment of the Sanriku coast on a millennial timescale, but comparable data are lacking for the northern Sanriku coast. This study derived millennial-scale crustal movements from a well-dated Holocene sedimentary succession in the Omoto plain on the northern Sanriku coast. Sedimentary and fossil evidence record the following sequence of environmental changes influenced by sea-level changes: upward growth and landward progradation of a barrier and flood tidal delta from 10,000–9000 cal BP, an estuarine environment from 9000 to 8000 cal BP after growth and progradation of the barrier and flood tidal delta ceased, and a regressive delta and subsequent fluvial environment after 8000 cal BP. Thick deposit of delta to floodplain sediment since mid-Holocene is not apparent in the Omoto plain, whereas this feature typical of subsidence area is documented along the central Sanriku coast. Although the absolute direction of the vertical deformation trend could not be well constrained in the study area because of compaction effects, the evidence indicates that subsidence during the mid and late Holocene was either absent or not noticeable on the Omoto plain. This difference in deformation histories corresponds to a contrast in coseismic subsidence during the 2011 earthquake between the northern Sanriku coast (small subsidence) and the central to southern Sanriku coast (large subsidence). The results of this study suggest that the northern and central to southern parts of the Sanriku coast should be considered separately when assessing regional crustal deformation related to subduction.

A Positive Feedback Between Sediment Deposition and Tidal Prism May Affect the Morphodynamic Evolution of Tidal Deltas

AbstractTidal deltas are fragile systems whose morphology can be easily impacted by variations in water and sediment fluxes caused by natural and human processes. Here we explore the relation between tidal prism and sediment dynamics in tidal deltas using the recent evolution of the Yangtze River estuary, China, as an example. Using the numerical model Delft3D, we examine how changes in delta morphology can trigger variations in tidal signal, suspended and bed load transport, and how these could ultimately cause additional morphological changes. Our results show that a positive feedback between sediment deposition and tidal prism dominates the morphodynamic evolution of the delta. Accretion of the shoals in the delta front increases the dissipation of tides and decreases the tidal prism leading to weaker tidal flows. This reduction in tidal currents lowers the sediment flushing capacity of the system, promoting deposition on the shoals and tidal dissipation. This positive feedback potentially traps more sediment in the delta topset and possibly offsets the decrease in sediment load triggered by the construction of upstream dams, often present in tidal deltas.

Growing Forced Bars Determine Nonideal Estuary Planform

The planform of estuaries is often described with an ideal shape, which exponentially converges in landward direction. We show how growing topographically forced nonmigratory (i.e., anchored) bars determine the large‐scale estuary planform, which explains the deviations observed in the planform of natural estuaries filled with bars compared to the ideal planform. Experiments were conducted in a 20‐m long, 3‐m‐wide tilting flume, the Metronome. From a narrow, converging channel a self‐formed estuary developed characterized by multiple channels, braided bars, a meandering ebb channel, and an ebb delta. Bars hardly migrated due to the alternating current, but the bar width increased with increasing estuary width. At locations where the estuary width was narrow, major channel confluences were present, while the zones between the confluences were characterized by a higher braiding index, periodically migrating channels, and a relatively large estuary width. At the seaward boundary, confluences were forced in place by the presence of the ebb tidal delta. Between confluences, bars were topographically forced to be nonmigratory. Diversion of flow around forced midchannel bars caused bank erosion. This resulted in a planform shape with a quasiperiodic widening and narrowing at the scale of forced bars. Observations in natural systems show that major confluence locations can also be caused by inherited geology and human engineering, but otherwise the estuary outline is similarly affected by tidal bars. These observations provide a framework for understanding the evolution of tidal bar patterns and the planform shape of the estuary, which has wide implications for navigation, dredging, and ecology.

X‐Band Radar Mapping of Morphological Changes at a Dynamic Coastal Inlet

AbstractRemote sensing of the complex interactions between bathymetry, tidal flow, and waves in coastal inlets provides high‐resolution data sets that can be exploited to characterize the morphological variability of shallow ebb tidal deltas (ETD). Here we used observations from a mobile X‐band radar platform to determine optimal conditions during which radar‐derived shoal signatures best represent positions of underlying shoals. Significant increases in the spatial errors of radar shoal signatures were observed when offshore wave energy intensified. Consequently, the lowest spatial errors occurred when the radar shoal signatures were primarily a function of the tidal flow (i.e., low sea state). We used these findings to quantify shoal migration patterns at the New River Inlet, North Carolina. We found that the southwestern portion of the ETD had the largest morphological variability with typical shoreward migration rates of 2–3 m/day driven by incident wave energy. The migration rates and patterns estimated from X‐band observations were consistent with numerical modeling results and a previous video‐based New River remote sensing study. Our results confirm that X‐band radar can be used to quickly map shallow ETDs (e.g., 5 min of observations), allowing for rapid morphological assessments from shore or boat‐based platforms. The methods would prove particularly useful for initial assessment as well as continual monitoring of dynamic tidal inlets where large morphological responses can be rapidly assessed and used for geomorphological studies, and as decision aids for maritime or engineering operations.

A positive feedback between sediment deposition and tidal prism may affect the morphodynamic evolution of tidal deltas

A positive feedback between sediment deposition and tidal prism may affect the morphodynamic evolution of tidal deltas

Detecting ebb-tidal delta migration using Landsat imagery

Ebb-tidal deltas are important coastal sedimentary features that influence broad-scale coastal system behaviour. However, sediment transport within and around tidal deltas is complex, making it very difficult to understand the role they play within coastal sedimentary systems. Here we present details of a new method to examine ebb-tidal delta processes using long records of cloud-masked Landsat 7 and 8 satellite imagery. Our approach is similar to that adopted in nearshore process studies, in which time-averaged collections of digital photographs from terrestrial cameras are used to differentiate between broken and unbroken water based on image tone. Using the Google Earth Engine we are able to rapidly segment 100s of Landsat images into broken and unbroken water classes based on a threshold value. By then calculating the frequency of wave breaking at the Manukau ebb-tidal delta (METD) near Auckland, New Zealand, we can repeatedly map geomorphic features such as channels, linear bar formation, channel margin linear bars, swash bars, and the terminal lobe that marks the outer limit of wave breaking. Using a ~17-year long record of observations we observe >2 km of northward alongshore channel migration and the potential emergence of a new channel to the south. The approach is simple, quick and with the growing constellations of earth observation satellites, has easy application to a range of other coastal systems.

A base-level stratigraphic approach to determining Holocene subsidence of the Ganges–Meghna–Brahmaputra Delta plain

Relative sea level history, which is the result of the combined effects of land subsidence, sediment supply and absolute sea level history may be reconstructed from preserved sediment thicknesses. However, variations in the preserved sediment thicknesses between different sedimentary environments strongly limit the accuracy of this type of geological approach, particularly in fluvial channelized systems, such as delta plains. To address this, we apply three different and independent stratigraphic approaches to the case of the Ganges–Brahmaputra–Meghna Delta (GBMD). Our approach has been made possible by a broad dataset of geological archives we have collected, which includes more than 400 hand-drilled stratigraphic wells, 198 radiocarbon ages, and river seismic reflection data (255 km of high-resolution multichannel seismic images). The seaward gradient of accommodation and the limit of the subsiding delta plain are estimated, assuming that the delta is near or at the base-level, which is considered to be the relative sea-level. First, a statistical analysis of the variability of preserved sediment thicknesses is used to derive the average pattern of accommodation from the Holocene isopach. Secondly, the preserved sediment thicknesses are analyzed by geomorphotectonic domains to estimate an average pattern of accommodation. Thirdly, the burial history of the seismically imaged last glacial incision of the Brahmaputra River is reconstructed. Results suggest that the variability of preserved sediment thicknesses can be up to 35% in a delta plain between river channel and flood plain deposits for the same relative sea-level history. Taking this variability into consideration, the Holocene relative sea-level history of the GBMD and the most likely pattern of subsidence are determined. Results provide evidence of moderate Holocene subsidence over the delta, gently increasing seaward from <0.2 mm/yr landward of the Hinge Zone, which can be considered as the northern limit of the subsiding delta plain, to 2 ± 0.7 mm/yr in the middle fluvial delta to 4 ± 1.4 mm/yr in the lower tidal delta. This enables us to construct the first millennial-scale map of subsidence pattern on the GBMD in which uncertainties on subsidence rates are provided. This map may aid in evaluating the negative impact that human modification may have on subsidence and relative sea level in the GBMD, and thereby help to determine better sustainable coastal management practices for the GBMD and other large delta plains.

Genesis and growth of a carbonate Holocene wedge on the northern Little Bahama Bank

Abstract The study focuses on the sedimentary dynamics at the transition between the modern Bahamas carbonate platform and related adjacent slopes and deeper environments. A Holocene carbonate sedimentary wedge is developed at the edge of the carbonate platform extending along the northern Little Bahamas Bank (LBB; Bahamas). The transition area between the platform and the wedge shows superposed terraces. The Holocene wedge thickness varies from 0 to 35 m. Its surface is dominated by homogeneous, fine-grained, soft sediments. It lies on a Pleistocene substratum and fills depressions corresponding to lowstand erosional surfaces. On the uppermost slope, this wedge represents the main depocenter of fine-grained bank-derived sediments since the last bank flooding. Twelve gravity cores were sampled from this wedge and allow the identification of both the processes that can supply this wedge and the wedge evolution based on content analyses and 19 radiocarbon ages. At present, particle export from the LBB margins mostly occurs during episodes of winter cold fronts but also occurs during hurricanes. Evidence of hurricane impact in the sedimentary record is suggested by the recovery of two laminated sequences showing both decreasing sedimentation rates and varying velocities during platform export. The local morphology of the platform edge allows the better sorting and export of sediments according to the presence of tidal outlets and sediment sorting on the wedge. However, because no ooids from tidal deltas located on the platform edge were found in cores, tidal currents are probably not energetic enough to both mobilize and export sediment, even if tide–induced currents may help the export of fine-grained carbonate particles remobilized by either cold fronts or hurricanes on the platform and upper slope. The formation of the wedge started at 13.6 ± 3.5 kyr cal BP following the meltwater pulse 1 A. It was supplied by shallow environment-derived particles produced on four narrow terraces between 60 and 20 mbsl after meltwater pulses. Since the LGM (Last Glacial Maximum), terrace formation seems to correspond to periods of sea-level stillstands and wedge formation, whilst escarpments correspond to periods of high rates of rising sea-level due to meltwater pulses. The main period of wedge growth started around 6.5 ± 0.9 kyr cal BP, when sea level finally reached the platform margins. The maximum flooding period around 4 kyr cal BP is characterized by the highest contents of aragonite needles exported from the platform.

Development of tolerant rice varieties for stress-prone ecosystems in the coastal deltas of Indonesia

Large areas of lowlands within tidal deltas have been reclaimed for crop production in Indonesia. However, major production constraints such as floods, seawater intrusion from tidal rivers, and adverse soils remain a significant problem, resulting in low rice yields (2–3 t ha−1) and production systems that are prone to risk. This review describes the recent progress in technology development, to address multiple abiotic stresses through the genetic improvement of rice. During the last 15 years, rice breeding programs and germplasm evaluation systems for coastal deltas have been revitalized. Local landraces, modern varieties and elite stress-tolerant genotypes were screened and physiological response to the stress and agronomic performance of promising genotypes were characterized. These efforts resulted in the release of the first series of flood- and salinity-tolerant varieties for lowlands in Indonesia. On-farm trials with these varieties demonstrated yield advantages of up to 125%, over popular varieties currently used by farmers in the marginal lowlands. Some of these varieties made an impact; Ciherang-Sub1, a submergence-tolerant variety near-isogenic of Indonesia’s popular variety (Ciherang), occupied over 430,000 ha of rice lowlands within four years of its release in 2012. There are large “exploitable yield gaps” between the attainable farm yield and mean farm yield, and appropriate crop and natural resource management guidelines for the new rice varieties are being developed. These achievements highlight opportunities for sustainable development in unfavorable rice environments of coastal deltas and are relevant to other areas in South and Southeast Asia.

Recent morphology and sedimentary processes along the western slope of Great Bahama Bank (Bahamas)

AbstractCarbonate slopes and associated resedimented deposits have recently gained renewed interest because they represent volumetrically significant parts of carbonate platforms. Carbonate slopes are highly variable compositionally, architecturally and spatially due to a spectrum of sediment sources, resedimentation processes and controlling factors. Here, new high resolution acoustic data (including EM302 multi‐beam echo‐sounder and very high resolution seismic) and piston cores document highly diverse and complex morphological features along the north‐western slope of Great Bahama Bank. The recent morphology of the slope is the result of the interplay between depositional and erosive processes that vary through time and along strike. The different sedimentary processes are recorded as a Pleistocene lowstand surface, characterized by many erosional features and a Holocene sedimentary wedge along the upper to middle slope that partially covers the underlying Pleistocene surface. Sedimentary processes during the Holocene are dominated by density cascading flows, which export muddy aragonitic sediment from the platform top towards the slope. Sedimentation rates, however, vary along strike due to platform top morphology combined with the variable strength of the basinal current. Reefs and islands in the Bimini area block off‐bank sediment export, and shoals and tidal deltas from Cat Cay to the south reduce the density cascading processes. Numerous small and large slope failure scars show the instability of the steep slopes of Great Bahama Bank. Bottom currents dominate the lower slope and the basin. Striations and moats are the morphological expressions of current directions, while areas of non‐deposition document strong current and concomitant removal of off‐bank transported sediment along parts of the slope, while the Santaren Drift and the drift on the north‐western edge of Great Bahama Bank act as the depositional locus for the fine‐grained sediments transported in the current.

Flow dynamics in large tidal delta of the Northern Dvina River: 2D simulation

Abstract A numerical modelling of flow dynamics in a tidal river mouth of comprehensive morphology is assumed to be one of the most effective methods of both scientific research and civil engineering projects. Realistic results of simulations can be obtained only on the basis of field observations. This approach is realized for a 2D hydrodynamic model of the Northern Dvina River mouth area. The Northern Dvina delta has a very complicated distributary network and suffers from both spring snow-melt floods and autumn storm surges. The STREAM_2D software package based on the 2D shallow water equations was used for the model development. The model was calibrated and validated on the background of water level data at state gauges and special water discharges measurements in the essential delta branches during the semi-diurnal tidal cycles. Sensitivity tests were provided to evaluate the most significant reasons for model errors. It was discovered that the distribution of roughness coefficients amidst delta channels and floodplain does not affect the flow dynamics in the delta significantly. However, the tidal range variations over a neap-spring cycle and mean sea level changes along the delta marine edge are of major importance.

Environmental changes recorded in the Holocene sedimentary infill of a tropical estuary

Early-to late Holocene sedimentary infill of the Potengi-Jundiai Estuary (Brazil) was studied in a 31.5 m long sediment core. The borehole was drilled in a mangrove area within the central part of the flood tide delta. The sedimentary record, encompassing the last 10 kyr, was studied for sediment texture and for the n-alkanes composition of organic matter (OM). Sediments rich in OM and bivalve shells accumulated during the period of sea-level rise, mostly in an intertidal environment of mangrove forest, mudflat and subtidal estuarine flood delta. The characterization of n-alkanes (C10-C33) and lignin methoxyphenols by means of direct analytical pyrolysis (Py-GC/MS) technique allowed determining the sources of the extant OM. Downcore fluctuations were observed in n-alkanes and in several n-alkane based indices, such as long chain carbon preference index (CPI C24-C33), long to short chain ratio (L/S) and average chain length (ACL), as well as in the relative proportion of lignin derived compounds. The n-alkanes distribution points to changing sources of OM input to the estuarine sediments over most of the Holocene period. Major inputs of terrestrial vegetation alternated with periods of mixed input from aquatic macrophytes as the rate of sea-level rise changed over time. The n-alkanes distribution and the lignin markers were useful for reconstructing paleoenvironmental conditions, particularly during the early Holocene when the lower Potengi River valley (now an estuarine area) could accommodate terrestrial forest vegetation before switching to halophytic mangroves as sea levels rose.

Effects of mud supply on large-scale estuary morphology and development over centuries to millennia

Abstract. Alluvial river estuaries consist largely of sand but are typically flanked by mudflats and salt marshes. The analogy with meandering rivers that are kept narrower than braided rivers by cohesive floodplain formation raises the question of how large-scale estuarine morphology and the late Holocene development of estuaries are affected by cohesive sediment. In this study we combine sand and mud transport processes and study their interaction effects on morphologically modelled estuaries on centennial to millennial timescales. The numerical modelling package Delft3D was applied in 2-DH starting from an idealised convergent estuary. The mixed sediment was modelled with an active layer and storage module with fluxes predicted by the Partheniades–Krone relations for mud and Engelund–Hansen for sand. The model was subjected to a range of idealised boundary conditions of tidal range, river discharge, waves and mud input. The model results show that mud is predominantly stored in mudflats on the side of the estuary. Marine mud supply only influences the mouth of the estuary, whereas fluvial mud is distributed along the whole estuary. Coastal waves stir up mud and remove the tendency to form muddy coastlines and the formation of mudflats in the downstream part of the estuary. Widening continues in estuaries with only sand, while mud supply leads to a narrower constant width and reduced channel and bar dynamics. This self-confinement eventually leads to a dynamic equilibrium in which lateral channel migration and mudflat expansion are balanced on average. However, for higher mud concentrations, higher discharge and low tidal amplitude, the estuary narrows and fills to become a tidal delta.

Salinization and arsenic contamination of surface water in southwest Bangladesh

To identify the causes of salinization and arsenic contamination of surface water on an embanked island (i.e., polder) in the tidal delta plain of SW Bangladesh we collected and analyzed water samples in the dry (May) and wet (October) seasons in 2012–2013. Samples were collected from rice paddies (wet season), saltwater ponds used for brine shrimp aquaculture (dry season), freshwater ponds and tidal channels (both wet and dry season), and rainwater collectors. Continuous measurements of salinity from March 2012 to February 2013 show that tidal channel water increases from ~0.15 ppt in the wet season up to ~20 ppt in the dry season. On the polder, surface water exceeds the World Health Organization drinking water guideline of 10 μg As/L in 78% of shrimp ponds and 27% of rice paddies, raising concerns that produced shrimp and rice could have unsafe levels of As. Drinking water sources also often have unsafe As levels, with 83% of tubewell and 43% of freshwater pond samples having >10 μg As/L. Water compositions and field observations are consistent with shrimp pond water being sourced from tidal channels during the dry season, rather than the locally saline groundwater from tubewells. Irrigation water for rice paddies is also obtained from the tidal channels, but during the wet season when surface waters are fresh. Salts become concentrated in irrigation water through evaporation, with average salinity increasing from 0.43 ppt in the tidal channel source to 0.91 ppt in the rice paddies. Our observations suggest that the practice of seasonally alternating rice and shrimp farming in a field has a negligible effect on rice paddy water salinity. Also, shrimp ponds do not significantly affect the salinity of adjacent surface water bodies or subjacent groundwater because impermeable shallow surface deposits of silt and clay mostly isolate surface water bodies from each other and from the shallow groundwater aquifer. Bivariate plots of conservative element concentrations show that all surface water types lie on mixing lines between dry season tidal channel water and rainwater, i.e., all are related by varying degrees of salinization. High As concentrations in dry season tidal channel water and shrimp ponds likely result from groundwater exfiltration and upstream irrigation in the dry season. Arsenic is transferred from tidal channels to rice paddies through irrigation. Including groundwater samples from the same area (Ayers et al. in Geochem Trans 17:1–22, 2016), principal components analysis and correlation analysis reveal that salinization explains most variation in surface water compositions, whereas progressive reduction of buried surface water by dissolved organic carbon is responsible for the nonconservative behavior of S, Fe, and As and changes in Eh and alkalinity of groundwater.

Transition from incised valley to barrier island systems during MIS 5e in the northern Chiba area, Kanto Plain, central Japan

Facies analysis, together with palynostratigraphic studies of the Kioroshi Formation, which lies beneath the northern Chiba area of the Kanto Plain of central Japan, reveal the sedimentary processes in the fluvial to coastal settings during marine isotope stage (MIS) 5e in this region. The depositional sequence of the Kioroshi Formation comprises a lower incised valley system and an upper barrier island system. The incised valley system consists of fluvial, estuary, and central basin facies with sandbar facies in the baymouth area. In the study area, the overlying barrier island system includes flood tidal delta, spit, and lagoon facies. These facies form part of the barrier island system known as Paleo-Tokyo Bay, which covered the present-day Kanto Plain. On the basis of palynostratigraphic correlation with the oxygen isotope curve in an offshore core (MD01-2421), the fluvial and estuary facies, together with the lower half of the central basin facies of the incised valley system are interpreted to have accumulated during a sea level rise in the early stage of MIS 5e. The baymouth sandbar also developed and retrograded with the rising sea level and was abruptly covered by the central basin facies during the middle stage of MIS 5e, probably due to the migration of the sandbar to the outer-bay area. The abrupt change corresponds with the initial stage of development of the barrier island system. The landward (westward) retreating bay ravinement erosion resulted in the transition to the barrier island system during middle MIS 5e. Maximum transgression also occurred during middle MIS 5e. The back-barrier lagoon was infilled during the late stage of MIS 5e. The sedimentary processes of the Kioroshi Formation are almost in accordance with the marine isotope curve.

Middle Ordovician acritarchs and problematic organic-walled microfossils from the Saq-Hanadir transitional beds in the QSIM-801 well, Saudi Arabia

Core samples from the QSIM-801 water well, drilled in central Saudi Arabia, cover a 93-foot interval spanning the transition between the Sajir Member of the Saq Formation, that consists mainly of sandstones of tidal sand flat environments, and the Hanadir Member of the Qasim Formation, characterized by argillaceous graptolitic mudstones, corresponding to a tidal delta front. The samples contain abundant, exceptionally well-preserved and diverse palynomorphs, which include cryptospores, acritarchs and chitinozoans, other problematic organic-walled microfossils as well as other organic particles such as cuticle-like fragments. The studied interval is biostratigraphically well constrained by the presence of chitinozoans of the formosa and pissotensis Zones of late-early to late Darriwilian age (Middle Ordovician) in the uppermost Saq Formation and Hanadir Member. The biostratigraphic age of the Sajir Member considered to span the Dapingian–Darriwilian boundary, is re-discussed based on the results herein. The uppermost part of the Sajir Member yielded the ichnofossil, Phycodes fusiforme. Acritarch assemblages from the Sajir Member of the Saq Formation are poorly diversified and dominated by sphaeromorphs. More diverse assemblages of acritarchs, associated with enigmatic forms, occur in the Hanadir Member of the Qasim Formation. The contact between the two formations and the transition between the palynomorph assemblages are sharp, suggesting a stratigraphic hiatus. A quantitative analysis allows us to discuss the paleoenvironmental changes and possibly climatic changes associated with an hypothesis of ice house conditions during this period. Among the diagnostic acritarch taxa observed are Frankea breviuscula, F. longiuscula, Baltisphaeridium ternatum, Dasydorus cirritus, Dicrodiacrodium ancoriforme, Poikilofusa ciliaris, Pterospermopsis colbathii and Uncinisphaera fusticula. These are associated with other typical forms known to range across the Lower–Middle Ordovician boundary, such as Aremoricanium rigaudae, Aureotesta clathrata, Barakella fortunata, B. rara, Baltisphaeridium klabavense, Glaucotesta latiramosa and Striatotheca spp. Galeate and peteinoid acritarchs are also well represented, as well as tiny forms of ultraplanctonic size. Three new species of acritarchs are proposed: Frankea longiuscula var. darriwilense var. nov, Micrhystridium regulum sp. nov, and Tyrannus proteus sp. nov. Repeated occurrences throughout the section of cryptospores, problematic microfossils such as organic filaments, cuticle-like tissues, striated and pigmented leiospheres frequently in clusters, are interpreted to reflect recurrent terrestrial and freshwater inputs in the depositional environment. Single-specimen, high-resolution analyses using Confocal Laser Scanning Microscopy on the enigmatic form Tyrannus proteus sp. nov. show fluorescence emission spectra and microstructural properties significantly different from those of typical marine acritarchs from the same levels.

Facies architecture, morphostratigraphy, and sedimentary evolution of a rapidly-infilled Holocene incised-valley estuary: The lower Manawatu valley, North Island New Zealand

Water well logs and vibracores recovered from the lower Manawatu valley record the facies architecture, morphostratigraphy, and sedimentary evolution of this drowned river incised-valley estuary over the past c. 30,000years. The Manawatu incised-valley estuary was rapidly infilled during the Holocene, and as a result preserves a complete record of Holocene estuary evolution. The base of the Holocene fill is marked by fluvial gravels deposited by the Manawatu River during the lowstand of the Last Glacial Maximum. The gravels are overlain by back-stepping transgressive fluvial silts and clays. The transgressive fluvial unit is present only in the lower reaches of the valley due to accelerated sea-level rise during the latter stages of the marine transgression. Large volumes of transgressive marine sediment were delivered to the mouth of the valley by rising sea-levels during the early Holocene, forming a large subaqueous tidal delta. This delta accreted and subsequently evolved to form a subaerial barrier in the mid-Holocene, as sea-levels stabilised during the sea-level highstand. The central basin of the estuary was rapidly infilled by marine and fluvial sediment, and consequently lacks the mud facies characteristic of less-rapidly-evolved estuaries. The transition from the sandy central basin estuarine environment to the overlying floodplain environment was extremely rapid. In some locations in the valley the transitional fluvial bay-head delta is missing completely, while in most locations it is present only as a very thin deposit, indicative of this rapid environmental transition. Infilling of the estuarine central basin was extremely rapid, taking only 2000–2700years. The unique features of the Manawatu estuary's evolutionary history are interpreted to primarily reflect the rapid infilling of the palaeo-estuary driven by the high rates of sediment supplied to the valley from both marine and fluvial sources.

Geochemistry and related studies of Clyde Estuary sediments

ABSTRACTGeochemical and related studies have been made of near-surface sediments from the River Clyde estuary and adjoining areas, extending from Glasgow to the N, and W as far as the Holy Loch on the W coast of Scotland, UK. Multibeam echosounder, sidescan sonar and shallow seismic data, taken with core information, indicate that a shallow layer of modern sediment, often less than a metre thick, rests on earlier glacial and post-glacial sediments. The offshore Quaternary history can be aligned with onshore sequences, with the recognition of buried drumlins, settlement of muds from quieter water, probably behind an ice dam, and later tidal delta deposits. The geochemistry of contaminants within the cores also indicates shallow contaminated sediments, often resting on pristine pre-industrial deposits at depths less than 1m. The distribution of different contaminants with depth in the sediment, such as Pb (and Pb isotopes), organics and radionuclides, allow chronologies of contamination from different sources to be suggested. Dating was also attempted using microfossils, radiocarbon and 210Pb, but with limited success. Some of the spatial distribution of contaminants in the surface sediments can be related to grain-size variations. Contaminants are highest, both in absolute terms and in enrichment relative to the natural background, in the urban and inner estuary and in the Holy Loch, reflecting the concentration of industrial activity.