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1.
The Pliocene–Pleistocene peripheral marine basins of the Mediterranean Sea in southern Italy, from Basilicata and western Calabria to northern and eastern Sicily, represent tectonically formed coastal embayments and narrow straits. Here, units of cross‐stratified, mixed silici–bioclastic sand, 25 to 80 m thick, record strong tidal currents. The Central Mediterranean Sea has had a microtidal range of ca 35 cm, and the local amplification of the tidal wave is attributed to tides enhanced in some of the bays and to the out‐of‐phase reversal of the tidal prism in narrow straits linking the Tyrrhenian and Ionian basins. The siliciclastic sediment was generated by local bedrock erosion, whereas the bioclastic sediment was derived from the contemporaneous, foramol‐type cool‐water carbonate factories. The cross‐strata sets represent small to medium‐sized (10 to 60 cm thick) two‐dimensional dunes with mainly unidirectional foreset dip directions. These tidalites differ from the classical tidal rhythmites deposited in mud‐bearing siliciclastic environments. Firstly, the foreset strata lack mud drapes and, instead, show segregation of siliciclastic and bioclastic sand into alternating strata. Secondly, the thickness variation of the successive silici–bioclastic strata couplets, measured over accretion intervals of 2 to 3 m and analysed statistically, reveal only the shortest‐term, diurnal and semi‐diurnal tidal cycles. Thirdly, the record of diurnal and semi‐diurnal tidal cycles is included within the pattern of neap‐spring cycles. Differences between these sediments and classical tidal rhythmites are attributed to the specific palaeogeographic setting of a microtidal sea, with the tidal currents locally enhanced in peripheral basins. It is suggested that this particular facies of mud‐free, silici–bioclastic arenite rhythmites in the stratigraphic record might indicate a specific type of depositional sub‐tidal environment of straits and embayments and the shortest‐term tidal cycles.  相似文献   

2.
Miocene siliciclastic sediments of the Marañón Foreland Sub‐basin in Peru record the sedimentary response to regional marine incursions into Amazonia. Contrary to previous interpretations, the Late Miocene Nauta Formation provides evidence of the last known marine incursion before the current Amazonia river basin became established. Sedimentological, ichnological and palynological data from well‐exposed outcrops along a ca 100 km road transect suggest that the Nauta Formation represents a shallow, marginal‐marine channel complex dominated by tidal channels developed in the inactive, brackish‐water portions of a delta plain. The main facies associations are: FA1 – slightly bioturbated mud‐draped trough cross‐stratified sand; FA2 – locally, pervasively bioturbated inclined heterolithic stratification (IHS); and FA3 – moderately bioturbated horizontally bedded sand–mud couplets. These identify subtidal compound dunes, tidal point bars and shallow subtidal to intertidal flats, respectively. Bi‐seasonal depositional cycles are ascribed to the abundant metre‐ to decimetre‐scale sand–mud couplets that are found mainly in the IHS association: semi‐monthly to daily tidal rhythmicity is inferred from centimetre‐ and millimetre‐scale couplets in the mud‐dominated parts of the decimetre‐scale couplets. The ichnology of the deposits is consistent with brackish depositional conditions; the presence of Laminites, a variant of Scolicia, attests to episodic normal marine conditions. Trace fossil suites are assigned to the Skolithos, Cruziana and mixed Skolithos–Cruziana ichnofacies. Pollen assemblages related to mangrove environments (e.g. Retitricolporites sp., Zonocostites sp., Psilatricolporites maculosus, Retitricolpites simplex) support a brackish‐water setting. Uplift of the Mérida Andes to the North and the consequent closure of the Proto‐Caribbean connection, and the onset of the transcontinental Amazon drainage, constrain the deposition of the Nauta sediments with around 10 to 8 Ma, probably contemporaneous to similar marine incursions identified in the Cuenca (Ecuador), Acre (Brazil) and Madre de Dios (Southern Peru) (sub)basins, and along the Chaco‐Paranan corridor across Bolivia, Paraguay and Argentina.  相似文献   

3.
The tube-building polychaete Lanice conchilega forms intertidal mounds in association with macroalgae. This assemblage produces structures that can record tidal and seasonal cycles in the stratigraphic record. They mark low-tide levels because the assemblage occurs below neap low tide and many of the structures form when water drains off the assemblage. Mounds are created by disturbance of the sediment surface around aggregations of the assemblage and accumulation of sediments within the aggregations. The initial aggregations may be relict patches of adult worms, new clumps of juveniles or both. Juvenile worms and drift algae easily settle and survive in high-density patches, whereas predators and waves more readily disturb low-density patches. Algae and tubes extend through accumulated sediment and create a new, higher surface. Through this interactive process the high-density patches increase height rapidly while the low-density patches erode. Regardless of density, when the initial distribution is regular the surface remains flat, rising evenly or eroding evenly. Accumulation and erosion increases during spring tides and decreases during neap tides. Mound development also follows a seasonal pattern. The recruitment of juvenile worms in spring facilitates algal settlement and initiates mound buildup. The rate of tube-building and algal growth increases in early summer as the erosive forces of storms decline. This leads to the greatest development of mounds in late summer just before the macroalgae begin to die. The loss of algae coincides with autumnal storms, and causes catastrophic erosion of the mounds into relict patches of dormant tube-worms. Patches of tubes that survive the winter enhance spring recruitment and renewal of mound development. The shape of the mounds is often obscured during burial and preservation. Because of this, it may be difficult to distinguish the original form of ancient mounds without contemporaneous cementing organisms. But cycles of deposition and erosion are recorded in Lanice tubes and other biogenic structures. These structures can be used in conjunction with physical structures to define tidal height, seasonality or current regime. In general however, the presence of tube aggregations per se is not diagnostic of a specific hydraulic environment.  相似文献   

4.
The morphology and migration rate of tidal bedforms are important because of their use in interpretation of modern and ancient sediment transport regimes. Tidal flow, megaripple morphology and migration were studied in the mesotidal Mawddach Estuary, North Wales, to examine the veracity of published flow-bedform relationships, quantify spatial variations in migration and assess consequences for palaeoflow reconstruction. Two transects were surveyed along a megarippled intertidal shoal (mean grain size 280 μm) for a period of 22 semi-diurnal tidal cycles. A vertical array of current meters recorded tidal current profiles at the centre of one of the transects. Flood tidal currents dominate at Fegla Fach shoal, with peak velocities over 1 m s?1 at spring tides, and 0.5 m s ?1 at neaps, and bed sediment transport was also flood-dominated. Over the lunar cycle, the morphology of the megaripples on the survey lines was divisible into three phases: 1 the neap mode-consisting of near-moribund two-dimensional (2-D) flood-orientated megaripples of wavelength c. 6 m and height c. 0.2 m; 2 a transitional mode-where, on rising tidal ranges, scour pits formed and developed into 3-D megaripples which underwent net migration with the flood tide; 3 the spring mode-consisting of 3-D megaripples of wavelength c. 4 m and height c. 0.2 m. Despite complete re-orientation by the ebb tide, these were recognizable from one low water survey to the next, and net migration was c. 1 m per tide with the flood tide. We infer the presence of the equilibrium ‘spring tidal form’ occurring as flood-orientated megaripples during the flood tide. The data support previously reported separation of 2-D and 3-D megaripples at a depth to grain size ratio of 8000, and at a depth-mean velocity of the dominant tide (Umaxdom) of 0.75-0.8 m s?1. A migration threshold exists at Umaxdom of c. 0.53-0.57 m s?1. Measures of migration which might be used on preserved sections have been applied to the data. These measures systematically overestimated bedform migration at most stages of the lunar cycle (by <25% at spring tides and <140% in the post-spring transition period), but were accurate when the megaripples had developed into their 'spring tidal form’. There is significant variation of migration rates within the survey populations. We conclude that whilst the occurrence of megaripple cross-sets may be used as a palaeoflow indicator, and sedimentary structures associated with 2-D to 3-D transitions may also be indicative of palaeoflows, there are likely to be significant uncertainties involved in using tidal bundles as an indication of sediment transport rates.  相似文献   

5.
Modern and ancient tidal straits are the least well understood of all tide‐dominated depositional systems. To provide an increased understanding of these systems, a facies‐based depositional model is assessed by comparing multibeam surveys of three present‐day tidally dominated seaways with a number of superbly exposed Neogene‐to‐Quaternary strait‐fill successions of Calabria (south Italy). The model points out the existence of four depositional zones, laterally adjacent from the narrowest strait centre to its terminations, distributed along symmetrical or asymmetrical seaways. These zones, whose signature is recorded in four facies associations in the Calabrian tidal straits, are as follows: (i) the strait‐centre zone, associated with the tidal current maxima and where sediments are scarce or absent; (ii) the dune‐bedded zone, where sediments form dune complexes due to tidal flow expansion; (iii) the strait‐end zone, where currents decelerate accumulating thinly bedded, fine‐grained deposits; and (iv) the strait‐margin zone, where sediment massflows descend tectonically active, steep margins towards the strait axis. In ancient, tectonically confined, narrow seaways, these facies generate a distinctive deepening‐upward vertical succession, where tidal currents are the dominant process in the sediment distribution.  相似文献   

6.
Many modern deltas show complex morphologies and architectures related to the interplay of river, wave and tidal currents. However, methods for extracting the signature of the individual processes from the stratigraphic architecture are poorly developed. Through an analysis of facies, palaeocurrents and stratigraphic stacking patterns in the Jurassic Lajas Formation, this paper: (i) separates the signals of wave, tide and river currents; (ii) illustrates the result of strong tidal reworking in the distal reaches of deltaic systems; and (iii) discusses the implications of this reworking for the evolution of mixed‐energy systems and their reservoir heterogeneities. The Lajas Formation, a sand‐rich, shallow‐marine, mixed‐energy deltaic system in the Neuquén Basin of Argentina, previously defined as a tide‐dominated system, presents an exceptional example of process variability at different scales. Tidal signals are predominantly located in the delta front, the subaqueous platform and the distributary channel deposits. Tidal currents vigorously reworked the delta front during transgressions, producing intensely cross‐stratified, sheet‐like, sandstone units. In the subaqueous platform, described for the first time in an ancient outcrop example, the tidal reworking was confined within subtidal channels. The intensive tidal reworking in the distal reaches of the regressive delta front could not have been predicted from knowledge of the coeval proximal reaches of the regressive delta front. The wave signals occur mainly in the shelf or shoreface deposits. The fluvial signals increase in abundance proximally but are always mixed with the other processes. The Lajas system is an unusual clean‐water (i.e. very little mud is present in the system), sand‐rich deltaic system, very different from the majority of mud‐rich, modern tide‐influenced examples. The sand‐rich character is a combination of source proximity, syndepositional tectonic activity and strong tidal‐current reworking, which produced amalgamated sandstone bodies in the delta‐front area, and a final stratigraphic record very different from the simple coarsening‐upward trends of river‐dominated and wave‐dominated delta fronts.  相似文献   

7.
Tidal currents and the spatial variability of tidally-induced shear stress were studied during a tidal cycle on four intertidal mudflats from the fluvial to the marine part of the Seine estuary. Measurements were carried out during low water discharge (<400 m3 s−1) in neap and spring tide conditions. Turbulent kinetic energy, covariance, and logarithmic profile methods were used and compared for the determination of shear stress. The cTKE coefficient value of 0.19 cited in the literature was confirmed. Shear stress values were shown to decrease above mudflats from the mouth to the fluvial part of the estuary due to dissipation of the tidal energy, from 1 to 0.2 N m−2 for spring tides and 0.8 to 0.05 N m−2 for neap tides. Flood currents dominate tidally-induced shear stress in the marine and lower fluvial estuary during neap and spring tides and in the upper fluvial part during spring tides. Ebb currents control tidally-induced shear stress in the upper fluvial part of the estuary during neap tides. These results revealed a linear relationship between friction velocities and current velocities. Bed roughness length values were calculated from the empirical relationship given by Mitchener and Torfs (1996) for each site; these values are in agreement with the modes of the sediment particle-size distribution. The influence of tidal currents on the mudflat dynamics of the Seine estuary was examined by comparing the tidally-induced bed shear stress and the critical erosion shear stress estimated from bed sediment properties. Bed sediment resuspension induced by tidal currents was shown to occur only in the lower part of the estuary.  相似文献   

8.
A study of the dynamics of a marine sandwave   总被引：3，自引：0，他引：3  
The movement of the crest of a sandwave was studied using cross-sectional profiles obtained from lines of sea-bed reference stakes. Measurements were made, over a six month period, before and after flood and ebb tides in relation to both spring and neap tides and surface wave conditions. Additional observations were obtained on a daily basis, over an equinoctial neap to spring to neap tidal period, in conjunction with boundary layer flow measurements. Tracer experiments were conducted to study the dispersion of sediment from the sandwave crest. The results showed that the sandwave was relatively stable at neap tides, whilst at higher tidal ranges, the crest position oscillated with successive flood and ebb tides. Net flank erosion occurred on the less steep, upstream slope during the dominant ebb tide. This, together with increased deposition on the lee slope, caused the crest to advance. It was not possible to extrapolate sandwave migration over long periods as the tidal dynamic trends were interrupted by wind stress and surface wave activity. High particle orbital velocities, generated at the sea-bed by storm waves, caused major reductions in crestal heights. Calculated volumes of sediment eroded and accreted were used, with boundary layer flow measurements, to calculate threshold velocities for the movement of the sediment and sediment transport rates.  相似文献   

9.
High-resolution current velocity and suspended sediment concentration (SSC) data were collected by using an Acoustic Doppler Current Profiler (ADCP) at two anchor stations and a cross-section in the South Channel of the Changjiang River mouth during meso and neap tides on Nov. 16, 2003. In addition, tidal cycle (13-hour) observation at two stations was carried out with traditional methods during the spring tide. Results indicated that resuspension occurred not only at the flood and ebb maximum, but also in the early phase of ebb in the meso and neap tide. When tidal current transited from high to ebb phase, current speed accelerated. Subsequently, fine-grained sediment with low critical threshold was resuspended and increased concentration. The river mouth area remained in siltation in the meso and neap tidal phase during the observation season, with calculated resuspension flux in the order of magnitude of 10−4–10−7 kg·m−2/s. Suspended sediment transport in the South Channel was dominated by freshwater discharge, but the Storks drift, vertical circulation and vertical shear effect due to tidal oscillation also played an important role in resuspension and associated sediment transport. In contrast, resuspension sediment flux in the spring tide was larger than that in meso and neap tide, especially at the ebb maximum and flood maximum. The present study revealed that intensive resuspension corresponded well with the larger current velocity during winter. In addition, the ‘tidal pumping’ effect and tidal gravity circulation were also vital for forming the turbidity maximum in the Changjiang River estuary.  相似文献   

10.
The Mono estuary is an infilled, microtidal estuary located on the wave-dominated Bight of Benin coast which is subject to very strong eastward longshore drift. The estuarine fill comprises a thick unit of lagoonal mud deposited in a ‘central basin’between upland fluvial deposits and estuary-mouth wave-tide deposits. This lagoonal fill is capped by organic-rich tidal flat mud. In addition to tidal flat mud, the superficial facies overlying the ‘central basin’fill include remnants of spits resting on transgressive/washover sand, an estuary-mouth association of beach, shoreface, flood-tidal delta and tidal inlet deposits, and a thin sheet of fluvial sediments deposited over tidal flat mud. After an initial phase of spit intrusion over the infilled central basin east of the present Mono channel, the whole estuary mouth became bounded by a regressive barrier formed from sand supplied by the Volta Delta during the middle Holocene eustatic highstand. Barrier progradation ceased late in the Holocene following the establishment of an equilibrium plan-form shoreline alignment that allowed through-drift of Volta sand to sediment sinks further downdrift. Over the same period, accretion, from fluvially supplied sediments, of the estuarine plain close to the limit of spring high tides, or, over much of the lower valley, into a fluvial plain no longer subject to tidal flooding, induced marked meandering of the Mono and its tidal distributaries in response to confinement of much of the tidal prism to these channels. The process resulted in erosion of spit/washover and regressive barrier sand, and in reworking of the tidal flat and floodbasin deposits. The strong longshore drift, equilibrium shoreline alignment and the year-round persistence of a tidal inlet maintained by discharge from the Mono and from Lake Ahémé have resulted in a stationary barrier that is reworked by a mobile inlet. The Mono example shows that advanced estuarine infill may result in considerable facies reworking, obliteration of certain facies and marked spatial imbrication of fluvial, estuarine and wave-tide-deposited facies, and confirms patterns of sedimentary change described for microtidal estuaries on wave-influenced coasts. In addition, this study shows that local environmental factors such as sediment supply relative to limited accommodation space, and strong longshore drift, which may preclude accumulation of sediments in the vicinity of the estuary mouth, may lead to infilled equilibrium or near-equilibrium estuaries that will not necessarily evolve into deltas.  相似文献   

11.
根据我国东部海岸现代潮坪沉积物和古代潮汐沉积物的研究,新发现一种潮汐沉积物所具有的特殊沉积构造,这种沉积构造可区分出三种类型,特征明显,易于鉴定,反映潮汐运动规律,命名为“潮汐周期层序”。用这种沉积构造作为古代潮汐沉积物的鉴定标志,将大大提高鉴定的可靠性。  相似文献   

12.
Turnagain Arm is a macrotidal fjord‐style estuary. Glacier Creek is a small, glacially fed stream which enters the estuary tangentially near Girdwood, Alaska. Trenches and daily sedimentation measurements were made in a mudflat along the fluvio–estuarine transition of Glacier Creek during several summers since 2003. Each year, the flats appear to erode during the winter and then accrete vertically in the spring and summer. In each of the years studied, tidal laminae in vertically thickening and thinning laminae bundles were deposited by twice daily tides in neap–spring tidal cycles. In 2004, bundles of thickening and thinning laminae couplets were noted in trenches cut into the flats. Five laminae bundles alternated between thicker and thinner bundles, corresponding to the perigean (high spring) and apogean (low spring) tides. Well‐preserved apogean–perigean cycles have rarely been documented in modern tidal flat sediments. At this location, vertical accretion of tidal rhythmites with well‐developed neap–spring cyclicity is possible because of the near‐complete removal of the flat from the previous year, which creates accommodation space for vertical accretion without significant reworking. Macrotidal conditions, no reworking by infaunal invertebrates, protection from the main tidal channel by a gravel bar and protection from storm waves and fluvial erosion by a recess in the sedge marsh that surrounds the flats all aid in preservation of rhythmites during aggradation. The position of the flats relative to tidal range allows for accumulation of complete spring cycles and incomplete neap cycles. In the summer of 2004, apogee and perigee were closely aligned with the new and full moons, resulting in successive strong perigee and apogee tides which probably aided in the accumulation of successive thick–thin spring cycles encoding the apogean and perigean tidal cycle. The apogean–perigean signal was not observed in subsequent years.  相似文献   

13.
Current understanding of bedform dynamics is largely based on field and laboratory observations of bedforms in steady flow environments. There are relatively few investigations of bedforms in flows dominated by unsteadiness associated with rapidly changing flows or tides. As a consequence, the ability to predict bedform response to variable flow is rudimentary. Using high‐resolution multibeam bathymetric data, this study explores the dynamics of a dune field developed by tidally modulated, fluvially dominated flow in the Fraser River Estuary, British Columbia, Canada. The dunes were dominantly low lee angle features characteristic of large, deep river channels. Data were collected over a field ca 1·0 km long and 0·5 km wide through a complete diurnal tidal cycle during the rising limb of the hydrograph immediately prior to peak freshet, yielding the most comprehensive characterization of low‐angle dunes ever reported. The data show that bedform height and lee angle slope respond to variable flow by declining as the tide ebbs, then increasing as the tide rises and the flow velocities decrease. Bedform lengths do not appear to respond to the changes in velocity caused by the tides. Changes in the bedform height and lee angle have a counterclockwise hysteresis with mean flow velocity, indicating that changes in the bedform geometry lag changes in the flow. The data reveal that lee angle slope responds directly to suspended sediment concentration, supporting previous speculation that low‐angle dune morphology is maintained by erosion of the dune stoss and crest at high flow, and deposition of that material in the dune trough.  相似文献   

14.
Late-Holocene evolution of the Mahakam delta, East Kalimantan, Indonesia   总被引：1，自引：0，他引：1  
The late-Holocene Mahakam delta, located along the tropical eastern shore of Kalimantan, Indonesia, is considered to be a textbook example of a mixed tide–fluvial dominated delta system. The delta prograded about 60 km during the past 5000 years, which led to the development of a distinct network of distributary and tidal channels. Wave action is low due the limited fetch in the narrow strait of Makassar. Mahakam River discharge is about a quarter of the Mississippi River discharge and is characterized by absence of flood surges. Therefore, natural levees, crevasse splays and avulsions are absent in the delta plain. For the past four decennia, both modern and ancient Mahakam delta deposits have been studied in detail in order to better understand subsurface Miocene and Tertiary Mahakam deposits, which host large volumes of hydrocarbons.

This study focuses on the dynamics and stratal patterns of delta plain, delta-front platform deposits and suspended sediments. Due to the predominance of semi-diurnal tides and the associated flow reversals, depositional patterns are highly variable which has resulted in the formation of characteristic sand–mud couplets. The distribution of the sand–mud couplets found in this study differs from previously proposed conceptual models. They are limited to the fluvial domain and form in the distributary channels (lateral channel bar) or at the fluvial dominated delta-front platform, which flanks the mouth bar deposits in offshore direction. The sand–mud couplets which formed as delta-front platform and lateral channel bar deposits are similar and can only be identified based on their 14C age. The sand content decreases significantly towards the tidal dominated areas due to limitation in transport capacity. Turbidity measurements taken in front of the river mouth also show rapid settlement of river plume sediments.

Some 22 new AMS 14C dates show that late Holocene sea level history resembles the eustatic sea level curve giving a first approximation of the Late Holocene sea level history for East Kalimantan. The dates suggest that the progradational delta system evolved under conditions of slowly rising sea level, which compares well to the eustatic sea level curve. In addition, calculated averaged deposition rates of the sand–mud couplets indicate that deposition is driven by the spring–neap tide cycles instead of the daily tidal cycle.  相似文献   


15.
Lower to Middle Turonian deposits within the Bohemian Cretaceous Basin (Central Europe) consist of coarse‐grained deltaic sandstones passing distally into fine‐grained offshore sediments. Dune‐scale cross‐beds superimposed on delta‐front clinoforms indicate a vigorous basinal palaeocirculation capable of transporting coarse‐grained sand across the entire depth range of the clinoforms (ca 35 m). Bi‐directional, alongshore‐oriented, trough cross‐set axes, silt drapes and reactivation surfaces indicate tidal activity. However, the Bohemian Cretaceous Basin at this time was over a thousand kilometres from the shelf break and separated from the open ocean by a series of small islands. The presence of tidally‐influenced deposits in a setting where co‐oscillating tides are likely to have been damped down by seabed friction and blocked by emergent land masses is problematic. The Imperial College Ocean Model, a fully hydrodynamic, unstructured mesh finite element model, is used to test the hypothesis that tidal circulation in this isolated region was capable of generating the observed grain‐size distributions, bedform types and palaeocurrent orientations. The model is first validated for the prediction of bed shear stress magnitudes and sediment transport pathways against the present‐day North European shelf seas that surround the British Isles. The model predicts a microtidal to mesotidal regime for the Bohemian Cretaceous Basin across a range of sensitivity tests with elevated tidal ranges in local embayments. Funnelling associated with straits increases tidal current velocities, generating bed shear stresses that were capable of forming the sedimentary structures observed in the field. The model also predicts instantaneous bi‐directional currents with orientations comparable with those measured in the field. Overall, the Imperial College Ocean Model predicts a vigorous tide‐driven palaeocirculation within the Bohemian Cretaceous Basin that would indisputably have influenced sediment dispersal and facies distributions. Palaeocurrent vectors and sediment transport pathways however vary markedly in the different sensitivity tests. Accurate modelling of these parameters, in this instance, requires greater palaeogeographic certainty than can be extracted from the available rock record.  相似文献   

16.
The Fraser River Delta exhibits distinct asymmetry in the sedimentological and neoichnological characteristics of the updrift (south) and downdrift (north) sides of the main distributary channel in water depths below storm‐wave base. The asymmetry is the result of net northward tidal flow. Tides erode sediments across the updrift delta front, whereas the downdrift delta front is an area of net deposition. A submarine channel prevents sand eroded from the updrift delta front from reaching the downdrift delta. The updrift delta front and updrift upper prodelta are composed of sand or heterolithic sand and mud that show a low density of burrowing (Bioturbation Index 0 to 3) and are dominated by simple traces. The downdrift delta front and prodelta, and the updrift lower prodelta are composed of homogeneous muds with significantly higher bioturbation intensities (Bioturbation Index 3 to 6), and a more diverse suite of traces akin to Cruziana Ichnofacies. Using the Fraser River Delta as an archetype and comparing the Fraser to the Amazon River Delta, a preliminary model for deep‐water (below storm‐wave base: ca 20 m) asymmetrical deltas is proposed. Firstly, deep‐water asymmetrical deltas are recognized from sediments deposited below storm‐wave base. At these depths, tidal and ocean currents are more likely to impact sediment transport, but wave processes are less effective as a sediment transport mechanism. Sediments deposited below storm‐wave base in deep‐water asymmetrical deltas will display the following: (i) the updrift delta front will be coarser‐grained (for example, sand‐dominated or heterolithic sand and mud), than the downdrift delta front (for example, mud‐dominated); and (ii) the updrift delta front should show low‐diversity suites of simple burrows. Depending on sedimentation rates, the downdrift delta front and prodelta may show either high diversity suites of traces that are dominated by both complex and simple burrows (low sedimentation rates) or low density and diversity suites akin to the updrift delta front (high sedimentation rates).  相似文献   

17.
On the south‐west coast of Vancouver Island, Canada, sedimentological and ichnological analysis of three beach–shoreface complexes developed along a strait margin was undertaken to quantify process–response relations in straits and to develop a model for strait‐margin beaches. For all three beaches, evidence of tidal processes are expressed best in the lower shoreface and offshore and, to a lesser extent, in the middle shoreface. Tidal currents are dominant offshore, below 18 m water depth (relative to the mean spring high tide), whereas wave processes dominate sediment deposition in the nearshore (intertidal zone to 5 m water depth). From 18 to 5 m water depth, tidal processes decrease in importance relative to wave processes. The relatively high tidal energy in the offshore and lower shoreface is manifest sedimentologically by the dominance of sand, of a similar grain size to the upper shoreface/intertidal zone and, by the prevalence of current‐generated structures (current ripples) oriented parallel to the shoreline. In addition, the offshore and lower shoreface of strait‐bound beach–shoreface complexes are recognized ichnologically by traces typical of the Skolithos Ichnofacies. This situation contrasts to the dominantly horizontal feeding traces characteristic of the Cruziana Ichnofacies that are prevalent in the lower shoreface and offshore of open‐coast (wave‐dominated) beach–shorefaces. These sedimentological and ichnological characteristics reflect tidal influence on sediment deposition; consequently, the term ‘tide‐influenced shoreface’ most accurately describes these depositional environments.  相似文献   

18.
The Middle Devonian Gauja Formation in the Devonian Baltic Basin preserves tide‐influenced delta plain and delta front deposits associated with a large southward prograding delta complex. The outcrops extend over 250 km from southern Estonia to southern Lithuania. The succession can be divided into 10 facies associations recording distributary channel belts that became progressively more tide influenced when traced southwards towards the palaeo‐shoreline, separated by muddy intra‐channel areas where deposition was characterized by crevasse splays, delta plain lakes, abandoned channel deposits and tidal gullies. Tidal currents influenced deposition over the entire delta plain, extending up to 250 km from the contemporary shoreline. Tidal facies on the upper delta plain differ from those on the lower delta plain and delta front. In the former case, deposition from river currents was only occasionally interrupted by tidal currents, e.g. during spring tides, resulting in mica and mudstone drapes, and distinctive graded cross‐stratification. The lower delta plain was dominated by tidal facies and tidal currents regularly influenced deposition. There was a change from progradation to aggradation from the lower to the upper part of the Gauja Formation coupled with a vertical decrease in tidal influence and a decrease in coarse‐grained sediment input. The Gauja Formation contrasts with established models for tide‐influenced deltas as the active delta plain was not restricted by topography. The shape of the delta plain, the predominant southward (basinward)‐directed palaeocurrents, and the thick sandstone succession, show that although tidal currents strongly influenced deposition at bed scale, rivers still controlled the overall morphology of the delta and the larger‐scale bedforms. In addition, there are no signs of wave influence, indicating very low wave energy in the basin. The widespread tidal influence in the Devonian Baltic Basin is explained by changes in the wider basin geometry and by local bathymetrical differences in the basin during progradation and aggradation of the delta plain, with changes in tidal efficiency accompanying the change in basin geometry produced by shoreline progradation.  相似文献   

19.
Isolated carbonate platforms occur throughout geological history, and commonly exhibit considerable spatial variability. To evaluate the controls on the nature of sediment accumulation across the expansive, shallow platform tops, this study systematically compares and contrasts patterns in surface sediments from several shallow (<10 m) Holocene Bahamian examples. Remote‐sensing data, field observations, petrographic characterization and quantitative grain‐size analyses reveal the spatial patterns of sediment accumulation on Crooked–Acklins Platform and the Berry Islands Bank. Integration of these data with synoptic observations of waves, tides and currents, along with regional geochemical data, provides a means to explore the factors that influence platform‐scale sedimentary patterns. These data illustrate that the platform interiors of both Crooked–Acklins Platform and Berry Islands Bank are blanketed with medium to coarse sand size sediment. Peloids are most common in the interior of Crooked–Acklins Platform, whereas the Berry Islands Bank includes more abundant composite grains. In both areas, very little mud is present, with surface sediments averaging <2% mud. Comparison of these results with published data from Little Bahama Bank, Great Bahama Bank and Caicos Platform suggest that, contrary to previous interpretations, the presence of open margins and/or brisk winds are not necessary for the occurrence of a platform top with little mud. Although the muddy sediment fraction of the interior can be suspended by elevated wave energy, wind‐generated current speeds in protected platform interiors are relatively low. Instead, in parts of the platform interiors, transport and winnowing of fines is enhanced greatly by tidal currents, which carry suspended sediments off the shallow platforms, even if shielded by islands. Beyond physical influences, however, regional geochemical compilations suggest that the Bahamian tides supply highly supersaturated waters rich in dissolved oxygen to these platform interiors. This exchange is interpreted to facilitate favourable conditions for calcium carbonate precipitation in the form of ooids, marine cements and hardened peloids across vast expanses of the platform interiors. Such fundamental controls on Holocene platform‐scale sediment dynamics are likely to have influenced carbonate systems through the geological record.  相似文献   

20.
The Moodies Group in the Dycedale Syncline, Barberton Greenstone Belt consists of a 100–130 m-thick upward-fining succession that exhibits a transition from fluvial to tide-modified sedimentation. A basal, 10–30 m-thick conglomerate–sandstone interval of alluvial origin is overlain by stacked upward-fining, decimeter- to meter-scale cycles within which three facies are recognized: 1) conglomerate lag; 2) cross-bedded sandstone; and 3) interlaminated sandstone–siltstone and mudstone. Within the cycles, the abundance of mudstone drapes increases upwards. Structureless conglomerates and cross-beds lacking mudstone drapes record braided-alluvial processes. In contrast, cross-beds with mudstone drapes and interlaminated sandstone–siltstone and mudstone are products of flows modified by various tidal beats. Sand and/or silt transport took place during the ebb and flood stages and mudstone accumulated during slack water phases. Alternating thick–thin laminations reflect dominant and subordinate, twice-daily tides. Thicker groupings of foresets and thicker intervals of vertically stacked sandstone–siltstone and mudstone laminations are interpreted as spring tide deposits whereas thinner groupings of foresets and thinner laminations record neap tides. Desiccated mudstone drapes on foresets indicate that bedforms rarely were locally exposed during some portion of the tidal cycle. Abundant exposure structures in the interlaminated sandstone–siltstone and mudstone facies indicate that the cycles are upward shoaling. The stacked upward-fining cycles are attributed to alternating subaerial exposure and fluvial influx followed by marine inundation, probably related to absolute sea level fluctuations. Lack of high-order vegetation on the Archean landscape promoted rapid lateral migration or avulsion of tidally influenced fluvial channels.

The association of facies within the 100–130 m-thick upward-fining succession is comparable to Holocene and ancient paleovalley fills characterized by basal alluvial gradational upwards into estuarine facies. However, in the absence of vegetation, the land–ocean interface in the Archean probably consisted of laterally extensive fan or braid deltas rather than point sources of sediment characteristic of most modern rivers. The abrupt up-section change from syntectonic, high-energy, alluvial–fluvial flash flood deposits to tide-influenced sedimentation implies a proximal source that provided sediment to a shoreline influenced by strong tidal action. Possible Holocene analogues are orogenic settings such as the Canterbury Plains of New Zealand, the Indo-Gangetic Plains of India and strike-slip settings such as the Gulf of Aqaba but all three examples lack a direct transition to tidally influenced sedimentation.  相似文献   


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