Flood-tidal Delta Research Articles

Erratum to: Neogene eastern Amazon carbonate platform and the paleoenvironmental interpretation

The Early Miocene Pirabas Formation represents extensive carbonate and siliciclastic belts deposited in marine coastal environments along the eastern Amazon coast. This formation was studied in its palynological, crustacean decapods and sedimentary facies, to develop a model of its depositional processes, faunal and floral assemblages and the sedimentary environments, as observed in the B17 mine (Capanema, Para). The sedimentary records consist mostly of light gray to greenish gray mudstones with tidal bedding, calciferous sandstones, stratified biocalcirudites and levels with conglomerates deposited in a carbonate/siliciclastic transitional system with lagoon, tidal flat, flood-tide delta and foreshore environments. 18 palynomorph species were identified in the tidal flat deposits including one alga, five pteridophytes and nine angiosperms. Among the latter, mangrove pollen and spores of Zonocostites ramonae, Zonocostites minor and Deltoidospora adriennis are reported. Nine species of crustacean decapods were recognized in the foreshore deposits, comprising one Callianassoidae, two Calappidae and six Portunidae, represented by Euphylax, †Necronectes, Portunus and Scylla, the ecology of which is associated with beaches of marginal lagoons. The main factor in the formation of these carbonate environments is assumed to have been related to the lack of a large-scale drainage system such as the Amazon River basin in the transition Oligocene–Miocene, which may also have influenced carbonate production and resulted in a strong decrease of paleodischarges from incipient river systems along the Amazon coast.

Remarked morphological change in a large tidal inlet with low sediment-supply

Sediment transport within small tidal inlets is sensitive to natural processes, whilst large tidal inlets are relatively robust systems because of their large tidal prism. However, remarked morphological changes may be initiated even under the condition of low sediment supply, as illustrated by Jiaozhou Bay, a large coastal embayment on the Shandong Peninsula, eastern China. Jiaozhou Bay is characterized by its relatively slow rate of natural change, and while the embayment has a flood-dominated entrance channel and muddy seabed, the suspended sediment concentration is generally low due to the lack of abundant source material. Observations of sediment dynamics show that net suspended sediment transport is directed towards outside of the bay, with an order of magnitude of 103t during a tidal cycle. The export of sediment associated with this flood-dominated environment implies that the net transport pattern is controlled by tidal exchange processes rather than the strength of the seabed shear stress. Sediment budget calculations show that supply of artificial sediment into the bay can account for up to 72% of the total input, which is in agreement with the 210Pb and 137Cs radioisotope geochronologies, and this leads to accumulation rates of 100–101mmyr−1; without this, the deposition rate would be low under natural conditions. The flood tidal delta area is also influenced by the input of anthropogenic material, and acts as a depocenter with relatively high accumulation rates. Furthermore, although the inlet system has not yet reached its equilibrium state (i.e., the entrance cross-sectional area is still larger than the equilibrium cross-sectional area), land reclamation activities have resulted in a rapid reduction of the embayment area (by 37%) over the last 80 years. Our findings indicate that the rapid changes observed in the tidal basin area and seabed morphology are mainly the result of human activity rather than natural processes.

Estuarine foraminiferal biofacies pattern compared to the brackish ichnofacies model: Port Stephens, southeast Australia

Port Stephens, located in SE Australia, is a shallow estuary with an outer and inner basin separated by a narrow channel. Foraminiferal biofacies distribution reveals this complex physical environment. The flood-tide delta forms a transition between the wave-dominated open coast and the microtide-dominated estuary. Sand wave dynamics cause a shifting substrate, resulting in allochthonous assemblages of calcareous shelf species transported into the bay through a wide entrance. The muddy central basin is heavily bioturbated, and allows for an in-situ, mixed calcareous and agglutinated assemblage of a progressively more estuarine character. Burial of organic matter and increased bioturbation permits infaunal species to increase in abundance, but promotes calcium carbonate dissolution. The bayhead delta of the Karuah River is characterized by lithic-rich substrates that are associated with robust calcareous species. Tidal cycles, combined with low river-flux, provide nearly fully marine conditions far upstream into the Karuah River. Marginal regions, from sandy/muddy tidal flats to marsh, show distinct assemblages of opportunistic species that respond to tidal exposure, substrate variations, organic matter flux and salinities. Early diagenetic taphonomic loss, closely linked with bioturbation, needs to be considered in interpretations of microfossil assemblages. Foraminiferal biofacies analysis, in combination with the brackish-water ichnology model, is a powerful tool for recognizing Mesozoic marginal marine environments. Port Stephens, with its environmental complexity, offers a valuable modern analogue for estuarine deposits of the geological past.

Unusual Barrier/Inlet Behaviour Associated with Active Coastal Progradation and River-Dominated Estuaries

ABSTRACT Green, A.; Cooper, J.A.G., and LeVieux, A., 2013. Unusual barrier/inlet behaviour associated with active coastal progradation and river-dominated estuaries. The mesoscale behaviour of unusual barrier inlets on the east coast of South Africa is described. The inlets are associated with river-dominated estuaries and are consequently subject to periodic closure during low river flow. They are also located on a prograding barrier coast. Deflection of the estuary channel of a small river in the direction of longshore transport by 1230 m resulted in its capture by the estuarine channel of a larger river. The larger catchment river was not deflected by coastal progradation but maintained its position by periodic breaching of a channel through the barrier during floods. Flood breaches reseal by landward reworking of ephemeral deltas and may undergo limited migration. A tidal inlet at the northern end of the barrier is maintained by a small tidal prism and has a flood-tidal delta. It exhibits limited late...

Sequence Stratigraphy of the Upper Cretaceous Sego Sandstone Member Reveals Spatio-Temporal Changes In Depositional Processes, Northwest Colorado, U.S.A

Abstract The Upper Cretaceous Sego Sandstone Member of the Mesaverde Group has been extensively studied in the Book Cliffs area of Utah and Colorado, and has been the focus of stratigraphic reconstruction aimed at developing an understanding of the evolution of the Western Cretaceous Interior Seaway. The Sego Sandstone Member was deposited in a marginal marine, tide-influenced environment of the Cretaceous Seaway. This study documents the sequence stratigraphy of the Sego Sandstone Member in northwestern Colorado, just north of Rangely, and compares and contrasts it with equivalent strata in the Book Cliffs area in Utah. The Sego Sandstone Member in the study area contains three sequences characterized by progradational and aggradational stacking patterns. The stratigraphically lowest sequence consists of a prograding, tide-influenced delta overlain by marine mudstones, which represents a retrogradation and flooding surface. The second sequence is composed of multiple parasequences and consists of an incised valley filled with stacked tidal bars which then pass into a largely aggradational stacking pattern, composed of barrier-island deposits with back-barrier, flood-tidal-delta deposits, and wave-dominated-shoreface deposits. The third sequence is a broad, tide-dominated distributary-mouth system with a sharp, incisional basal contact. The three sequence boundaries documented in northwestern Colorado are consistent with the three main sequence boundaries identified in the Book Cliffs. However, whereas barrier islands and flood-tidal deltas are characteristic of the Sego Sandstone Member in northwestern Colorado, similar deposits are not as prevalent in the Book Cliffs of Utah, suggesting different depositional processes and paleogeography. Tidal and fluvio-deltaic processes are the dominant controls on deposition of the Sego Sandstone Member north of Rangely, Colorado. The transition from a tide-dominated fluvio-deltaic system to a mixed wave–tide-influenced coastline indicates a fundamental change in processes and depositional environment in the upper part of Sequence 2. Such change from a prograding fluvio-deltaic system to a more passive tide-modified coastline is not observed in the Book Cliffs, and may be the result either of large scale transgression or of relocation of the river system through large-scale avulsion, which is not observed in the Book Cliffs. Our study shows significant stratigraphic variability between rocks exposed in the Book Cliffs versus time-equivalent rocks exposed in northwestern Colorado in the Upper Cretaceous, which has implications for the regional basin architecture and stratigraphic correlations.

Neogene Eastern Amazon carbonate platform and the palaeoenvironmental interpretation

The Early Miocene Pirabas Formation represents extensive carbonate and siliciclastic belts deposited in marine coastal environments along the Eastern Amazon coast. This Formation was studied in its palynological, palaeocarcinological and sedimentary facies, in order to develop a model of its depositional processes, faunal and floral assemblages and the sedimentary environments, as observed in the B17 mine (Capanema, Pará). The sedimentary records consist mostly of light gray to greenish gray mudstones with tidal bedding, calciferous sandstones, stratified biocalcirudites and levels with conglomerates deposited in a carbonate/siliciclastic transitional system with lagoon, tidal flat, flood-tide delta and foreshore environments. Eighteen palynomorph species were identified in the tidal flat deposits including 1 alga, 3 fungi, 5 pteridophytes, and 9 angiosperms. Among the latter, mangrove pollen and spores of Zonocostites ramonae, Zonocostites minor, and Deltoidospora adriennis are reported. Nine species of crustacean decapods were recognized in the foreshore deposits, comprising 1 Callianassoidae, 2 Calappidae, and 6 Portunidae, represented by Euphylax, †Necronectes, Portunus, and Scylla, the ecology of which is associated with beaches of marginal lagoons. The main factor of the formation of these carbonate environments is assumed to have been related to the lack of a large-scale drainage system such as the Amazon River basin in the transition Oligocene–Miocene, which may also have influenced carbonate production and resulted in a strong decrease of palaeodischarges from incipient river systems along the Amazon coast.

Reconstructed Holocene sea level curve from the Akkeshi barrier system in eastern Hokkaido, Japan

An active barrier system is present in Akkeshi Bay and a lagoonal area along the Pacific coast of eastern Hokkaido, Japan. This rare feature consists of a so-called lagoon (the Akkeshi-ko lagoon), a flood tidal delta, a barrier, and a tidal inlet that leads into the outer sea of Akkeshi Bay and the Pacific Ocean. Large oyster reefs were living on the flood tidal delta until their sudden extinction in the early 1980s. The processes involved in the formation of this barrier system during a postglacial transgression period are not well understood. Here, we use sedimentological, paleontological, and radiocarbon dating methods to analyze Holocene drillcore obtained from the Akkeshi fishing port in 2009 and borehole log data previously obtained from public works around the Akkeshi lowlands. The data obtained from both drillcore and borehole logs indicates that: (1) postglacial transgression occurred in this area some 11,400 years ago and reached 50 m below the present sea water level, with the barrier system in the study area established 8800 years ago. (2) The preservation of this barrier system is due to the stable sea level in this area. The sea level has been constant for the past 5500 years, and has led to the development of oyster reefs on the tidal flood delta. (3) The current topographically distinctive barrier system has been affected by sudden uplifts associated with giant earthquakes. These events occur every few hundred years, with the barrier undergoing prolonged but relatively rapid post-earthquake subsidence at about 1 cm/year, as observed after a giant 17th century earthquake.

Modelling the effects of tidal range and initial bathymetry on the morphological evolution of tidal embayments

Tidal embayments are characterized by a wide variety of landscape features, often including either complex tidal channel networks or extensive flood-tidal deltas. The origin of these features and the influence of hydrodynamic drivers and initial geological setting on their long-term characteristics are essentially unexplored. A model was applied to simulate the long-term morphological evolution of tidal embayments, with the purpose of providing insight into the environmental conditions that lead to the differences in tidal embayment morphology. Numerical simulations indicated that the interaction between hydrodynamics, sediment transport, and the evolving topography gives rise to the formation of channel networks. The tidal range and the depth of the initially unchannelized tidal basin controlled the way in which the morphology evolved and determined the timescale over which channels and intertidal areas developed. Channel network formation occurred more rapidly when the tidal range increased and/or when the initial basin depth decreased. Tidal basins with a large initial depth showed the development of a flood-tidal delta and for these deep basins channel incision could remain absent over long timescales. Both tidal range and initial bathymetry affected final basin hypsometry and channel network characteristics, including the channel density and the fraction of the basin occupied by the channels. All the simulated morphologies, with different combinations of the tidal range and depth of the basin, evolved towards a state of less morphodynamic activity for which the relative intertidal area was proportional to the ratio of tidal amplitude to basin depth.

Changing physiography of rift basins as a control on the evolution of mixed siliciclastic–carbonate back-barrier systems (Barremian Iberian Basin, Spain)

A detailed analysis of both sedimentary facies and stratigraphic architecture of a mixed-siliciclastic carbonate depositional system in the synrift Cretaceous Galve sub-basin (eastern Spain), is carried out. Two different stages in the sedimentary evolution are recognised from the stratigraphic architecture of the back-barrier system: (1) extensive back-barrier mud flats with tidal creeks, and minor washover fans interbedded with the lagoonal carbonates and influenced by local synsedimentary tectonics (thickness variations, rotated blocks and angular unconformities), and (2) a back-barrier with flat-lying architecture and characterised by washover fan deposits interbedded with lagoonal carbonates, well-developed ebb- and flood-tidal delta deposits and a complete absent of back-barrier tidal mud flats and associated creeks. Evidence suggests that synsedimentary extensional tectonics modified the basin configuration, and tectonically-induced physiographic changes controlled the distribution and areal extension of barrier-island sub-environments (open marine, barrier, and lagoon) and their resultant stratigraphic architecture. Physiographic changes in basin configuration ultimately modulated the effect of tides, which produced changes in depositional sub-environments determining the stacking pattern of depositional systems.

Seasonal and episodic variability in the morphodynamics of an ephemeral inlet, Zinkwazi Estuary, South Africa

Bond, J., Green, A.N., Cooper, J.A.G. and Humphries, M.S., 2013. Seasonal and episodic variability in the morphodynamics of an ephemeral inlet, Zinkwazi Estuary, South Africa. In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 446-451, ISSN 0749-0208. The inlet dynamics of estuary and lake systems with ephemeral inlets have been little studied compared to systems with semi-permanent tidal inlets. Here we document the meso-scale dynamics of a barrier and its associated ephemeral inlet. The inlet is typically open during the rainy season and experiences closure during winter low flow periods. It does not migrate. The inlet is formed by fluvial overtopping of the barrier and when active, forms a small delta seaward of the channel and a small flood tide delta in the back-barrier. The tidal prism is insufficient to maintain the inlet and it is quickly sealed through wave-reworking of sand from the ephemeral delta when fluvial discharge diminishes. During a large ocean swell event coupled with abnormally high tides in 2007, a departure from this seasonal behaviour occurred. The barrier migrated 100 m landwards and formed a gently (1.74 o ) seaward dipping sand sheet. Whereas the barrier had previously been narrow, it was widened by 80 m and rose 1.5 m in elevation. A breach later occurred and rapidly migrated northwards, establishing a significantly deeper inlet. This closed following wave reworking of a large ephemeral delta. Since then the barrier has maintained its landward position yet the inlet has continued to function as it did prior to the storm surge. This episodic barrier retreat appears to represent crossing of a morphodynamic threshold that triggered historically unprecedented rates of barrier rollover, creating a new set of inlet morphodynamic processes. These were short-lived and the system appears to have reverted to the typical seasonal morphodynamic processes despite such rapid rollover.

LOWER CAMBRIAN ANEMONE BURROWS FROM THE UPPER MEMBER OF THE WOOD CANYON FORMATION, DEATH VALLEY REGION, UNITED STATES: PALEOECOLOGICAL AND PALEOENVIRONMENTAL SIGNIFICANCE

Large (up to 7 cm in diameter) and deep-penetrating (up to 30 cm) vertical burrows are described from the lower Cambrian Wood Canyon Formation of the Death Valley region, United States, and their paleoenvironmental and paleoecological implications are addressed. Trace fossils occur as dense populations that precede the earliest occurrences of Skolithos pipe rock in the region. These trace fossils are assigned to the ichnogenera Bergaueria, Conichnus, and Dolopichnus, and each represents the burrowing behavior of an anemone-like organism. These ichnotaxa occur within oolitic and sandy dolostones of a flood-tidal delta and lagoonal environment, respectively, and burrow fill is dominated by echinoderm ossicles. Ichnofabric indices of 4–5 and a tidally influenced position place these ichnotaxa within the same nearshore locus of bioturbation as Skolithos pipe rock; however, unlike Skolithos pipe rock, the tracemaker of these trace fossils can be identified. This study reveals that anemone-like animals were responsible for extensive modification of the marine substrate and this style of deep-penetrating bioturbation appears first in nearshore environments. This is in accord with the observation that many evolutionary novelties originate in nearshore environments, and specifically that deep-penetrating bioturbation originated nearshore before expanding offshore. This study also provides insight into the paleoecology of earliest Cambrian noncalcified cnidarians and their behavior and paleoenvironmental distribution during the Cambrian radiation.

Study of the surface water circulation in San Blas channel (Argentina) using landsat imagery

This paper deals with the application of satellite images to study turbidity and water circulation patterns in San Blas channel during a theoretical tidal cycle. Eight Landsat TM and ETM images acquired under clear-sky conditions and representing different tidal stages were selected from a pool of Landsat images provided by the argentinean National Commission of Space Activities (CONAE) and the US Geological Survey. Standard digital image processing techniques were used to perform geometric and radiometric corrections on the visible and near-infrared bands. An image-based atmospheric correction (COST method by CHAVEZ, 1996) was applied. An ISODATA unsupervised classification was performed in order to identify different turbidity levels throughout the channel and adjacent areas. The results suggest that suspended sediment transport towards the channel mouth by ebb currents occurs along both flanks. These currents carry suspended sediment into the open sea, generating an ebb tidal delta which tends to rotate in a clockwise direction. Flood currents trigger turbidity mostly over the southern flank of the channel, generating a flood tidal delta with elongated banks extending in the direction of the tidal currents. From the elongated shape of the turbidity plumes, general tidal circulation patterns were identified.

Barrier island response to late Holocene climate events, North Carolina, USA

The Outer Banks barrier islands of North Carolina, USA, contain a geologic record of inlet activity that extends from ca. 2200 cal yr BP to the present, and can be used as a proxy for storm activity. Optically stimulated luminescence (OSL) dating (26 samples) of inlet-fill and flood tide delta deposits, recognized in cores and geophysical data, provides the basis for understanding the chronology of storm impacts and comparison to other paleoclimate proxy data. OSL ages of historical inlet fill compare favorably to historical documentation of inlet activity, providing confidence in the technique. Comparison suggests that the Medieval Warm Period (MWP) and Little Ice Age (LIA) were both characterized by elevated storm conditions as indicated by much greater inlet activity relative to today. Given present understanding of atmospheric circulation patterns and sea-surface temperatures during the MWP and LIA, we suggest that increased inlet activity during the MWP responded to intensified hurricane impacts, while elevated inlet activity during the LIA was in response to increased nor'easter activity. A general decrease in storminess at mid-latitudes in the North Atlantic over the last 300 yr has allowed the system to evolve into a more continuous barrier with few inlets.

Sedimentología v paleoecología de un estuario del litoral Pacífico del Istmo Centroamericano Primordial (Mioceno Medio, Costa Rica)

The rise of the Central American landbridge is characterized by a Campanian to Eocene island arc stage, an Oligocene to Miocene swell stage, and a Pliocene to Recent mountain chain stage. During the Oligocene to Miocene swell stage the volcanic edifices and insular shelves of the foregoing stage gradually grew together, forming the primordial Central American isthmus. Thus, extensive tidal flat areas established which occupied, especially during the Middle Miocene, large areas of the actual territory of Costa Rica. The present study deals with a Middle Miocene section situated at Punta Judas on the Pacific coast of Costa Rica. This section provides a nearly complete record of the depositional environments that characterize the Pacific estuaries of the primordial Central American isthmus. The 1300 m of mainly litharenitic volcaniclastic sediments are extre-mely fossiliferous, especially in mollusks, echinoids, and trace fossils. Thus, sedimentological interpretations can be choroughly controlled and complemented by paleoecological data. 10 different facies types could be identified that reflect a great variety of depositional environments ranging from open estuary through intertidal shoals and mangrove swamps to mesohaline marginal shallows, and even slightly hypersaline lagoons. Facies character and pattern in the subtidal area was largely controlled by the temporariness and volume of the terrigenous input which was transferred across the deltaic depocenter and overtaken by the estuarine circulatory regime Ecological successions that developped in response to the intermittent sediment supnlv commonly are characterized by native background communities (susnenpion feeiers - mollusks) and opportunistic after-event communities (sediment feeders-cetossils) senau SEILACHER (1980). In the intertidal area lower flats are typified by crosslaminated sand-stones, whereas in mid and upper flats mangrove swamp deposits are predominant. This fades unit frequently interfingers with granule and pebble conglomerates that entered from minor lateral river mouths. The coarse river bed load. is redistributed both in meandering tidal channels and in straight beach-parallel channels. The latter show an unidirecional filling pattern, the size ranges being fashioned into large graded sigmoidal foresets that end up in welldevelopped toe sets; mud drapes prove that these features have to be considered as ti, dal bundles. Tempestites are fairly frequent within the mangrove swamp deposits, but occur also in nearly all other facies types. Following these criteria it mustbe concluded that the Punta Judas estuary was laterally bounded by a mountainous hinterland and headed by a stream system that supplied large amounts of fine- to medium-sized volcaniclastic sediments. Despite the low salinities in its uppermost sector, the estuary was strongly controlled by esotidal currents which obviously followed an asymmetrical circulatory pattern. It was an essentially sandy estuary and must have reached dimension, comparable to the actual Gulf of Nicoya. There is no evidence for the existence of an outer bar, of barrier islands, or ebb- and flood-tidal deltas. Hence, thp Middlo Miocene Punta Judas estuary has to be considered as an open embayment. The Punta Judas section documents at least 2 complete filling cycles (pro-gradations). The return from shoreline to open estuarine environments always came off very abruptly. This suggests rapid sealevel rises, followed by gra-dual lowerings. The absence of both angular unconformities and hardgrounds proves that subsidence was fairly constant. Most probably, the Punta Judas estuarine embayment was a direct graben-like tectonic feature. These data coincide with the tectonical and geographical framework of the actual Pacific estuaries of Central America. Differences exist, however, in that the Punta Judas estuary reached salinities as low as the -mesohaline range. This is not common to the actual ones which are essentially normal marine to high polyhaline. Thus, we propose an environmental model which falls midway bet-ween the strongly tectonically controlled Pacific centroamerican type and the epicontinental type which characterizes mid latitude Atlantic coastal plains.

Analyzing Spatial Patterns in Modern Carbonate Sand Bodies From Great Bahama Bank

Abstract Three carbonate sand bodies on Great Bahama Bank, which show a range of depositional facies patterns typifying modern deposits as well as their ancient counterparts, are quantitatively interrogated to broaden our perspective of the types of information that can be derived from studies of modern environments. Rimming the southern end of Tongue of the Ocean (TOTO) is the broadest expanse of “high-energy” sands found in the Bahamas, characterized by narrow sandbars separated by wide, deep channels and a lack of islands. A variation of the tidal-bar motif with broader and more irregular sandbars, relatively narrow channels, and few small islands occurs at the northern end of Exuma Sound (Schooners). Sands associated with tidal channels and the numerous islands of the Exumas chain along the western edge of Exuma Sound occur primarily as flood tidal deltas. The geometry of sandbars that inhabit the three sand bodies is quantified using Landsat remote sensing and assembled, along with optically derived bathymetric surfaces, into a GIS. This database is quantitatively examined using a suite of morphometric tools to compare and contrast geometric character of sandbars within and between the three sand bodies. Considered are properties such as the size, shape, complexity, distribution, orientation, and topography of the individual sandbars. Profiles and spatial analysis tools enable sandbar and channel spacing, position relative to the platform margin, connectedness, separation distances, and density to be characterized. As has been previously reported for reef-dominated environments, certain aspects of the geometry of the three systems are found to behave in a systematic and hence predictable manner, though important mathematical differences are revealed between the scaling of reefal landscapes and the grainy geobodies considered by this study. Because the three sand bodies are disparate in their overall extent and depositional settings (e.g., orientation, prevalence of islands), this predictable behavior has the potential to impart considerable insight to the characterization of grainstone systems.

Sediment and pollutant distribution along the Negro River: Patagonia, Argentina

The Negro River extends from the Andes to the Atlantic Ocean supplying water to several cities and to the most important fruit-bearing productive region of Patagonia. Floodplains from the Upper Valley are artificially irrigated with water delivered from the main channel. Pesticides are increasingly used without strict government control. Dams constructed at the tributary rivers retain sediment and therefore the clean water delivered becomes free of sediment and therefore is able to erode certain levees during high-discharge events. Bedload–transport fraction (sand dunes) increases at the Upper Valley and is fixed at the levees of the Lower Valley by riparian vegetation. This riparian vegetation is dominated by Salix humboldtiana and Schoenoplectus californicus while Spartina sp is abundant at the estuarine marsh area. Organochlorine compounds (OCCs) were measured by GC-ECD including dichloro-diphenyl-trichloroethane (DDT), dichlorophenyl-dichloroethylene (DDE) and dichloro-dyphenyl-dichloroethane (DDD) endosulfans, hexachlorocyclohexanes (HCH) and chlordanes. OCCs were preferentially accumulated at the Upper Valley in relation to the agricultural activities developed. The dominance of DDE and the parental endosulfan can be explained by their past and current use, respectively. PCBs (different congener groups) showed low levels as a consequence of chronic pollution related to the mud fraction. The high concentration in pollutants at the Upper Valley is significantly reduced to the Middle and Lower Valleys due to filtering effects, assumed to be induced by the riparian vegetation. At the estuary, very low OCCs levels were measured probably due to the dilution induced by mesotidal dynamics. The sedimentation rate of the lower floodplain is of 1 mm/year. The bottom of the main channel is dominated by 2D sand dunes. At the inlet, a sandy gravel bank is composed of megaripples 0.5 m high. Tidal currents guarantee the stability of the inlet, and particularly the asymmetric flood-tidal delta. Wave action induces a net longshore sand transport towards the north.

Process-based modeling of morphodynamics of a tidal inlet system

The morphodynamic evolution of an idealized inlet system is investigated using a 2-D depth-averaged process-based model, incorporating the hydrodynamic equations, Englund-Hansen’s sediment transport formula and the mass conservation equation. The model has a fixed geometry, impermeable boundaries and uniform sediment grain size, and driven by shore-parallel tidal elevations. The results show that the model reproduces major elements of the inlet system, i.e., flood and ebb tidal deltas, inlet channel. Equilibrium is reached after several years when the residual transport gradually decreases and eventually diminishes. At equilibrium, the flow field characteristics and morphological patterns agree with the schematized models proposed by O’Brien (1969) and Hayes (1980). The modeled minimum cross-sectional entrance area of the tidal inlet system is comparable with that calculated with the statistical P-A relationship for tidal inlets along the East China Sea coast. The morphological evolution of the inlet system is controlled by a negative feedback between hydrodynamics, sediment transport and bathymetric changes. The evolution rates decrease exponentially with time, i.e., the system develops rapidly at an early stage while it slows down at later stages. Temporal changes in hydrodynamics occur in the system; for example, the flood velocity decreases while its duration increases, which weakens the flood domination patterns. The formation of the multi-channel system in the tidal basin can be divided into two stages; at the first stage the flood delta is formed and the water depth is reduced, and at the second stage the flood is dissected by a number of tidal channels in which the water depth increases in response to tidal scour.

Transition of a regressive to a transgressive barrier island due to back-barrier erosion, increased storminess, and low sediment supply: Bogue Banks, North Carolina, USA

Although back-barrier erosion is a prevalent process of island narrowing, it is often overlooked in conceptual models of barrier island evolution. In many wave-dominated barrier island settings, the absence of overwash precludes landward movement of the back-barrier shoreline and hence the island as a whole from sustaining its width. Typically, regressive barriers are wide and exhibit high elevations with the most seaward dune ridge possessing the highest elevation. This morphology may prevent overwash and inlets from breaching the barrier, and therefore the associated sediment from reaching the back-barrier shoreline for millennia. Back-barrier shoreline erosion from sea-level rise and storms can be permanent in the absence of sediment supply from rivers, overwash, and inlet processes. With continued narrowing and lowering of the island, regressive barriers may reach a critical state, making overwash and inlet formation imminent and transitioning the island to a transgressive barrier. The modern day morphologic variability along the 40-km long island of Bogue Banks, North Carolina includes both regressive and transgressive segments, making this setting ideal for examining whether the transition between these barrier island types is gradual or threshold-driven. Bogue Banks consists of two discrete segments characterized by high-elevation beach ridges, large island widths, and stratigraphy consistent with regressive barrier islands. These regressive-island segments are separated by a broad, narrow section of the island devoid of any washover fans, flood-tide deltas or other transgressive elements. Analyses of seismic data from the inner continental shelf reveal paleochannels intersecting the wide sections of the island, while the narrow central part of the island occupies an inter fluvial area. Reworking of fluvial sediment from paleochannels was an important sediment source for the barrier during regression. Optically Stimulated Luminescence (OSL) dates from the most landward beach ridges constrained initiation of island regression at ~ 3000 cal yr BP, close to when there was a significant decrease in the rate of relative sea-level rise from ~ 5 mm/yr to ~ 0.8 mm/yr. Transects of cores, seismic data, ground-penetrating radar data, and radiocarbon and OSL dates show that prior to ~ 1500 cal yr BP the central narrow section of the island was wide and progradational, similar to adjacent areas. Back-barrier erosion of the central part of the barrier primarily caused island narrowing as a result of increased storminess, which occurred around the Medieval Climate Anomaly (~ 1100 cal yr BP). This part of the island was more vulnerable to erosion than adjacent areas due to increased bay-ravinement (Bogue Sound is widest there) and its lower elevation (farther away from paleochannel-sediment source). Relict inlet channels exist along the central portion of the island, formed within the last 250 yr, and likely closed shortly after formation. The presence of historical inlets along the narrow central section of the island indicates Bogue Banks may be nearing a critical width threshold and will subsequently transition to a transgressive barrier. Because the change in barrier morphology associated with back-barrier erosion occurred over a period of time when the rate of sea-level rise was relatively low, low sediment supply and increased storm frequency are shown to be the main forcing mechanisms of island narrowing. These impacts, in addition to a predicted increase in sea-level rise rates and human modifications (e.g. maintenance of a high-elevation foredune, closing of inlets, prevention of island overwash and associated sediment delivery to the back-barrier shoreline) will likely promote rapid transition of regressive barrier islands to those dominated by transgressive processes.

Flood-tide Delta Morphological Change at the Gippsland Lakes Artificial Entrance, Australia (1889–2009)

Analysis of historical survey and navigation charts, aerial imagery, and digital hydro-data (representing a time-series of 1889–2009) allows the morphological evolution of a flood-tide delta at the artificial entrance to the Gippsland Lakes (Victoria, Australia) to be visualised and quantified. Analysis shows that flood-tide delta growth has imposed progressively greater demands upon port managers for publicly funded maintenance dredging. This flood-tide delta growth corresponds with a progressive diminution of Gippsland Lakes catchment river discharge volumes, and dampening of flow variability, due to both regional precipitation pattern changes and changes in catchment water resource allocation. Future climate change predictions for the Gippsland Lakes catchment and coastal area suggest both further decreases in catchment river discharge, and changing sediment flux along this sector of the Ninety Mile Beach. Thus, scope for flood-tide delta nourishment will probably increase, as will the demand for mitigation of the inevitable effects upon entrance channel navigability. Information derived from the analysis of time-series bathymetry used for this study offers baseline information in support of stakeholder consensus building regarding options for maintaining navigability. Analysis points to the merit of considering (and testing via morphological modelling) alternative options for navigation channel maintenance than those in present practice. We argue that the circumstances prevailing at the artificial entrance call for consideration of engineered configuration changes practised elsewhere by entrance managers faced with similar issues.

Geological characteristics and spatial distribution of paleo-inlet channels beneath the outer banks barrier islands, North Carolina, USA

Nearly 200 km of high-resolution ground penetrating radar (GPR) data were acquired along the Outer Banks barrier island system of North Carolina, USA. GPR data combined with lithofacies and biofacies data reveal multiple depositional facies including inlet channel, flood-tide delta, overwash, peat and inner shelf. Previously undocumented paleo-inlet channels constitute a significant portion of the shallow geologic framework between Oregon Inlet and Cape Hatteras. GPR data reveal the complex stratigraphy associated with multiple sequences of cut-and-fill within inlet channels. Two types of paleochannels (non-migrating and migrating) were classified based on geometry and fill-patterns. Sediments and foraminifera collected from vibracores were correlated to GPR data to define the regional shallow stratigraphic framework. Channel-fill facies are characterized by clinoform packages, sometimes bounded by erosional surfaces, indicating variable sediment transport directions from the ocean and sound sides. Channels are incised into older flood-tide delta deposits corresponding to older inlet activity when barriers existed further seaward. Flood-tide delta deposits are capped with marsh peat and overwash units. Migrating inlet facies occur under the widest portions of the island, whereas narrow portions of the island are underlain by the non-migrating inlet facies or flood-tide delta/overwash facies. This geologic/geomorphic relationship is attributed to the successional stage of island evolution during transgression, and sediment transport processes associated with the different inlet types. The radar facies, lithofacies, and biofacies provide a comprehensive dataset that will permit more precise identification of barrier island facies in the geologic record.