Subaqueous Dunes Research Articles

Very large subaqueous sand dunes on the upper continental slope in the South China Sea generated by episodic, shoaling deep-water internal solitary waves

Very large subaqueous sand dunes were discovered on the upper continental slope of the northern South China Sea. The dunes were observed along a single 40km long transect southeast of 21.93°N, 117.53°E on the upper continental slope in water depths of 160m to 600m. The sand dunes are composed of fine to medium sand, with amplitudes exceeding 16m and crest-to-crest wavelengths exceeding 350m. The dunes' apparent formation mechanism is the world's largest observed internal solitary waves which generate from tidal forcing on the Luzon Ridge on the east side of the South China Sea, propagate west across the deep basin with amplitudes regularly exceeding 100m, and dissipate extremely large amounts of energy via turbulent interaction with the continental slope, suspending and redistributing the bottom sediment. While subaqueous dunes are found in many locations throughout the world's oceans and coastal zones, these particular dunes appear to be unique for two principal reasons: their location on the upper continental slope (away from the influence of shallow-water tidal forcing, deep basin bottom currents and topographically-amplified canyon flows), and their distinctive formation mechanism (approximately 60 episodic, extremely energetic, large amplitude events each lunar cycle).

Large-scale bedforms along a tideless outer shelf setting in the western Mediterranean

High-resolution multibeam swath-bathymetry and sediment samples were collected across the outer shelf region of the Columbretes Islands (southern Ebro continental shelf, western Mediterranean Sea). Bathymetric data from the submerged part of the Columbretes volcanic system revealed the presence of three main relict sand bodies along the outer shelf, at 80–116 m depth range, above which asymmetric and slightly asymmetric large and very large 2D and 3D subaqueous dunes were observed. These bed features were recognized, mapped and quantified with the aim of evaluating their potential formation mechanisms in relation to the local hydrodynamic and morphologic settings of the area. Dunes range from 150 to 760 m in wavelength and from tens of centimeters to 3 m in height, and are among the longest ever recognized in an outer shelf region. These bedforms are mostly composed of medium-sized sandy sediments, presumably coming from the degraded relict sand bodies on top of which they have developed, mixed with fine fractions from the recent draping holocenic sediments. The orientation of the dunes is SSW, progressively turning W towards the southernmost sector of the area, following the trend of the shelf-edge. Contemporary hydrodynamic measurements at the Ebro continental shelf-edge show that recorded currents are insufficient to form the observed bedforms and that stronger currents are required for sediment mobilization and dune formation. Based on their morphology and orientation, it is proposed that these bedforms are produced by the action of the southward-flowing Liguro-Provençal-Catalan (LPC) geostrophic current. The LPC probably reaches high near-bottom currents during energetic hydrodynamic events through interactions with the seafloor morphology of the study area. Subaqueous dunes are expected to be basically inactive features with respect to present-day processes, although they can be reactivated during high-energy events. The small Δ h/ λ ratio measured in the dune fields of the Columbretes shelf revealed that the dune heights fall below the values predicted by the Flemming (1988) global equation, as observed in other outer shelf settings also dominated by unidirectional flows. This may suggest a different morphodynamic character of large dunes formed on outer shelves in a micro-tidal regime.

Occurrence of large subaqueous dunes in the Tsugaru Strait

Occurrence of large subaqueous dunes in the Tsugaru Strait

Aeolian dune interactions and dune-field pattern formation: White Sands Dune Field, New Mexico

Pattern formation is a fundamental aspect of self-organization in fields of bedforms. Time-series aerial photographs and airborne light detection and ranging show that fully developed, crescentic aeolian dunes at White Sands, New Mexico, interact and the dune pattern organizes in systematically similar ways as wind ripples and subaqueous dunes and ripples. Documented interactions include: (i) merging; (ii) lateral linking; (iii) defect repulsion; (iv) bedform repulsion; (v) off-centre collision; (vi) defect creation; and (vii) dune splitting. Merging and lateral linking are constructive interactions that give rise to a more organized pattern. Defect creation and bedform splitting are regenerative interactions that push the system to a more disorganized state. Defect/bedform repulsion and off-centre collision cause significant pattern change, but appear to be neutral in overall pattern development. Measurements of pattern parameters (number of dunes, crest length, defect density, crest spacing and dune height), dune migration rates, and the type and frequency of dune interactions within a 3500 m box transect from the upwind margin to the core of the dune field show that most pattern organization occurs within the upwind field. Upwind dominance by constructive interactions yields to neutral and regenerative interactions in the field centre. This spatial change reflects upwind line source and sediment availability boundary conditions arising from antecedent palaeo-lake topography. Pattern evolution is most strongly coupled to the pattern parameters of dune spacing and defect density, such that spatially or temporally the frequency of bedform interactions decreases as the dunes become further apart and have fewer defects.

A Mixed Bioclastic-Siliciclastic Flood-Tidal Delta in a Micro Tidal Setting: Depositional Architectures and Hierarchical Internal Organization (Pliocene, Southern Apennine, Italy)

Abstract Tide-dominated depositional systems are very common in macro tidal and meso tidal settings. They are less developed in micro tidal marine areas where sediments are dispersed mostly under the effects of waves and currents. In some specific coastal settings, the influence of the dominant hydrodynamics can be reduced or attenuated by the presence of promontories forming engulfed sectors or by the occurrence of submarine passageways or straits. In these conditions, as well as in micro tidal settings, the tidal influence can be amplified, producing a significant signature in the sedimentary record. A number to tide-influenced deposits can be recognized in the Neogene of the Southern Apennine, Italy, although the Mediterranean area is characterized by minor tidal ranges. Spectacular exposure of middle to upper Pliocene deposits cropping out around Tricarico allows the analysis of the architecture and internal complexities of a mixed bioclastic–siliciclastic succession deposited in a thrust-top basin. Undulations forming along the hinge of an anticline favored the onset of seaway conditions, which produced hydraulic amplification of marine currents flowing towards the chain and subjected to tidal influences. The mixed deposits of Tricarico exhibit prominent large-scale, unidirectional cross stratification and a suite of additional dune- and ripple-bedded structures of various dimensions. Cross stratification can be subdivided into four hierarchical levels based on their increasing degree of internal complexity of different ranks (from first-order to fourth-order sets). Processes invoked for the formation of such a complex suite of larger- and smaller-scale sedimentary structures are related to cyclical events, such as high-frequency sea-level oscillations and tidal cycles of various durations. First-order sets are interpreted as produced by migrating subaqueous dunes along a SSW–NNE-trending seaway at water depths below the wave base. These sets exhibit bed-thickness vertical patterns which have been related to the influence of high-frequency base-level oscillations that occurred during dune accumulation, producing alternating stages of accelerating and decelerating currents. Second-order cross stratification has been interpreted to have formed by dunes with varying sinuosity, superposition, and flow conditions, under the effect of varying current strengths but constant sediment production. Formset successions were produced by large compound dunes and are considered as the expression of low-energy and decaying dune fields that developed during times of decreasing sediment transport. Cross lamination of third-order and fourth-order sets shows series of bundles and couplets of coarser and finer laminae which, at a different scale, recorded repeated cycles of tidal ranges of different amplitudes. These considerations allow us to propose an original depositional model represented by a flood-tidal delta, which questions the absence of macro tidal sedimentation within the purportedly micro tidal oceanographic setting of the Mediterranean during the Pliocene.

Marine carbonate systems in the Sarmatian (Middle Miocene) of the Central Paratethys: the Zsámbék Basin of Hungary

AbstractThe study of eight stratigraphic sections at the margin of the semi‐enclosed Zsámbék Basin (Hungary) allows the sedimentary anatomy of oolitic–bioclastic systems in the Sarmatian of the Central Paratethys to be reconstructed. The mollusc, foraminiferal and ostracod associations indicate that the carbonate systems are Latest Badenian to Late Sarmatian in age. The Lower–Upper Sarmatian deposits are organized in superimposed subaqueous dunes prograding towards the basin on a low‐angle ramp. During the Late Sarmatian, the ramp underwent subaerial erosion linked to a moderate relative fall in sea‐level. Lagoonal deposits were later formed and microbial–nubeculariid–bryozoan–serpulid buildups were emplaced. The ‘abnormal’ marine conditions of the Sarmatian, conducive to the development of a poorly diversified flora and fauna and dominant non‐skeletal grains, are linked to fluctuating salinities, mesotrophic to eutrophic conditions and perhaps high alkalinity.

Hydraulic roughness over simple subaqueous dunes

Detailed studies of flow over subaqueous dunes in laboratory flumes were used to suggest a virtual near-bed layer of twice the dune height in which the mean velocity is accelerated towards the crest by contraction. The mean flow velocity in this layer above the crest, transformed into friction velocity by means of the surface skin roughness, is shown to give values consistent with measured values. The resulting dimensionless shear stress due to skin friction is depth-independent, in contrast to that derived by means of often cited traditional methods. As a result of the relationship between dune height and the thickness of the near-bed layer, an expression for the expansion loss behind dunes was formulated and used to relate form resistance directly to dune height.

Tide-driven dune migration and sediment transport on an intertidal shoal in a shallow estuary in Devon, UK

The migration of subaqueous dunes was investigated over a 3-month period on an intertidal shoal near the mouth of the shallow mixed wave/tide dominated Avon Estuary on the Channel coast of southwest Britain. The dunes, characterised by heights and lengths of 0.05–0.2 m and 5–10 m, respectively, migrated 10–20 m over the 3-month period, representing an average volumetric sediment transport rate of 0.5 m 3 per unit meter width. Compared to equilibrium models of dune dimensions based on the water depth, the dune height was predicted well, but the dune length was under-predicted. Sediment transport on the intertidal shoal was controlled by a distinct tidal asymmetry, with maximum flow velocities during flood (0.4–0.5 m s − 1 ) significantly stronger than during ebb (0.2–0.3 m s − 1 ). Predicted bedload transport based on theory underestimates the actual transport and this is attributed to the modest flow velocities encountered and, perhaps, the importance of suspended load transport. However, when empirically calibrated and related to tide range, the transport equation adequately accounts for observed rates. Using nearby tidal observations, the predicted annual volumetric sediment transport rate is 2.6 m 3 per unit meter width. This estimate compares favourably with the annual volumetric sediment transport based on dune measurements (2.0 m 3). Using the notion that the transport rate decreases on the intertidal shoal in the up-estuary direction to zero at its up-estuary end, a vertical accretion of 0.5 cm yr − 1 was predicted. The main implication of our study is that accurate and well-designed process measurements of dune dynamics and tidal currents can provide useful information on the longer term evolution of intertidal shoals in sandy, shallow (flood-dominant) estuaries.

Subaqueous Dunes of the Upper Slope of the Fraser River Delta (British Columbia, Canada)

Abstract Multibeam sonar data provide new high-resolution characterization of morphological and sedimentological features on the upper slope of the Roberts Bank section of the Fraser River delta. Subaqueous dunes are more abundant and broadly distributed over the slope than previously documented and include large asymmetric two-dimensional dunes (sinuous crested and straight crested), large asymmetric three-dimensional dunes (high relief, low relief) and irregularly-spaced dunes. The two-dimensional and three-dimensional dunes fall within the typical range of subaqueous dune heights and wavelengths and show dimensions that do not universally scale to water depth but are probably scaled to the dynamic conditions. The irregularly-spaced dunes fall outside the typical scaling, having excessively low height to wavelength ratios. Current data indicate that these irregularly-spaced dunes form under conditions where the ratio of suspended to bedload transport is too high to maintain fully-scaled bedforms. The or...

Large-scale sedimentary bedforms and sediment dynamics on a glaciated tectonic continental shelf: Examples from the Pacific margin of Canada

The Pacific margin of Canada has been subjected to tectonism, dramatic sea level change and vigorous storm and tidal energy since glacial times resulting in a complex seafloor. Extensive multibeam mapping of this shelf has provided an opportunity to understand how these processes have impacted sedimentology and morphology. Bathymetric restriction of the tidally dominated flow between the inland seas and the open Pacific has resulted in the development of very large subaqueous dune fields and terrace moats. For example, in the southern Strait of Georgia nearly symmetrical dunes with wavelengths between 100 and 300 m, dune heights up to 28 m, cover the seafloor in 170–210 m water depth. In northern Hecate Strait a 72 km 2 area of large 2D dunes occurs at the transition with Dixon Entrance which opens to the Pacific Ocean and steep (>10°) wave-cut terraces and drowned spits, a result of sea level changes during the Holocene, are now being undercut to generate moats 7 m deep, in a narrowing shelf trough. Currents, with velocities ranging between 0.2 and 2.2 m s −1, are dominated by semi-diurnal tidal streams that are continually modified by wind and estuarine circulation. There appears to be a clear association of grain size, water depth and flow velocity controlling the size of the subaqueous dunes.

Large and very large subaqueous dunes on the continental shelf off southern Vietnam, South China Sea

In the course of two regional side-scan sonar surveys on the continental shelf off southern Vietnam after the winter monsoon seasons of 2003 and 2004, and covering a total distance of over 1,000 km, the widespread occurrence of large and very large subaqueous dunes was discovered. On the basis of their size, shape, depth of occurrence and orientation, the dunes were grouped into five spatially distinct regions. In each case, a different height/wavelength relationship is observed. With the exception of region no. 3 where dune dimensions follow the mean global trend, the dimensions in the other regions lie below the mean global trend. The most plausible explanation for this is sediment starvation and/or insufficient time for the larger dunes to fully adjust to changing flow conditions. A good correlation is observed between average dune height in each region and water depth, although this is not the case for dune length. The orientation of the dunes corresponds to the direction of the current pattern induced by the regional winter monsoon winds (NE to SW and S). The generally well-developed asymmetrical shapes and the large size of the dunes suggest that the wind-induced currents are strong enough to reactivate most of the dunes during the winter monsoon season, a conclusion supported also by theoretical calculations of critical current velocities. The largest dunes, which seem to have reached their maximum sizes for the local water depths, may not be reactivated regularly but rather only by exceptionally strong episodic flows.

Temporal and spatial changes of dunes in the Changjiang (Yangtze) estuary, China

The response of subaqueous dunes to the variation of riverine sediment supply in an estuary is rarely reported. Five sets of field measurements of bed sediments and topography were made in the Changjiang estuary, China during dry and flood seasons for the past 9 years. Results show that dunes were mainly three-dimensional (3D) during flood seasons and two-dimensional (2D) dunes during dry seasons. Dunes were also much larger in flood seasons. Dunes mainly occurred within the main channels of the estuary although they extended 11.7 km seaward of their normal extent in the South Channel during dry seasons from 2002 to 2006. This extension of the dune field is likely due to a sharp decline of sediment discharge caused by large reservoirs such as Three Gorges.

Late Glacial to Preboreal sea-level rise recorded by the Rhône deltaic system (NW Mediterranean)

A unique late Glacial–Preboreal record of changes in sea-level and sediment fluxes originating from the Alps is recorded in the Rhône subaqueous delta in the Western Mediterranean Sea. The compilation of detailed bathymetric charts, together with high-resolution seismic profiles and long cores, reveals the detailed architecture of several sediment lobes, related to periods of decreased sea-level rise and/or increased sediment flux. They are situated along the retreat path of the Rhône distributaries, from the shelf edge and canyon heads up to the modern coastline. They form transgressive backstepping parasequences across the shelf, the late Holocene (highstand) deltas being confined to the inner shelf. The most prominent feature is an elongated paleo-shoreface/deltaic system, with an uppermost sandy fraction remolded into subaqueous dunes. A long piston core into the bottomsets of this prograding unit allows precise dating of this ancient deltaic system. In seismic data, it displays aggradation, starting at ∼ 15 cal kyr BP, followed by progradation initiated during the first phase of the Younger Dryas, a period of reduced sea-level rise or stillstand. The delta kept pace with resumed sea-level rise during the Preboreal (which is estimated at about 1 cm/yr), as a result of increased sediment supply from the Alps (melting of glaciers and more humid climate “flushing” the sediment down to the sea). Abandonment of the delta occurred around 10,500 cal yr BP, that is to say about 1000 yr after the end of the Younger Dryas, probably because of decreased sediment flux.

Bedload sediment transport dynamics in a macrotidal embayment, and implications for export to the southern Great Barrier Reef shelf

Keppel Bay is a macrotidal embayment on a tectonically stable, tropical coast, which links the Fitzroy River with the Great Barrier Reef continental shelf. Estuaries and deltas act as conduits between catchments and inner shelf environments. Therefore, understanding sediment transport pathways in these complex systems is essential for the management of ecosystems such as coral-reefs that are potentially vulnerable to enhanced river sediment loads. Furthermore, the morphology and sediment dynamics of subtidal sand ridges and dunes are relatively poorly characterised in macrotidal estuaries, particularly in turbid, episodic systems such as the Fitzroy River and Keppel Bay. Our sedimentological analysis of seabed samples, shear-stress modelling and three-dimensional acoustic imaging reveals that Keppel Bay is a mixed wave- and tide-dominated estuarine system. Areas of sediment starvation and shoreward transport characterise the offshore zone, whereas a complex of both active and relict tidal sand ridges, and associated subaqueous dunes, dominate the relatively protected southern Keppel Bay. Transport within this region is highly dynamic and variable, with ebb-dominated sediment transport through tidal channels into the outer bay where there is a switch to wave-dominated shoreward transport. Ultimately, bedload sediments appear to be reworked back inshore and to the north, and are gradually infilling the bedrock-defined embayment. Our characterisation of the Keppel Bay system provides a detailed example of the physiography of the seaward portion of a tide-dominated system, and shows that sediment transport in these areas is influenced by a variable hydrodynamic regime as well as relict channels and bedrock topography.

Fluid mud dynamics in the Weser estuary turbidity zone tracked by high-resolution side-scan sonar and parametric sub-bottom profiler

Fluid mud in estuarine turbidity maximum zones (TMZ) can pose considerable navigation risks due to potentially substantial reductions in nautical depth, coupled with an inherent difficulty of detection by conventional echo-sounders. Despite intensive research efforts, however, our knowledge about the spatial and temporal dynamics of fluid mud is still not sufficient. In this study, the combined use of a side-scan sonar (Sportscan®, Imagenex) and a parametric sub-bottom profiler (SES-2000®, Innomar Technology GmbH) has proved successful for high-resolution fluid mud detection and volumetric quantification in an estuarine environment. In 2004 and 2005, repeated surveys were conducted in the navigation channel of the upper meso- to lower macrotidal Weser estuary TMZ (German North Sea coast) at different tidal stages and river discharges. Current velocity data were simultaneously collected by 1,200-kHz broadband ADCP (RDInstruments) measurements. Ground-truthing was carried out by means of grab sampling and gravity coring, adapted to fluid mud conditions. It was found that fluid mud occurrence in the Weser estuary is highly variable on time scales of a few hours and spatial scales of several metres. The riverbed is characterised by sand and mud deposits, and a complex morphology including subaqueous dunes and smooth bed deposits intermittently overlain by fluid mud. Thus, a continuous, coherent fluid mud body covering the entire TMZ riverbed was not observed. Rather, spatial distribution was patchy and highly dependent on suspended particulate matter (SPM) concentrations in the water column, as a result of which local fluid mud deposits varied in thickness from centimetres to metres. The formation of fluid mud was largely restricted to slack water, although slack-water conditions were not necessarily associated with large-scale fluid mud appearance. Advective SPM transport of resuspended fluid mud seems to be the most plausible explanation for the high spatial variability observed, even between two successive tides. The amount of fluid mud deposited and resuspended in the course of a tidal cycle can reach several 10s of tons even in small riverbed depressions.

Decadal scale stability of sorted bedforms, German Bight, southeastern North Sea

Complex, large-scale sorted bedforms have been investigated in the southeastern North Sea. We present a unique time series of sidescan sonar and multibeam backscatter data spanning 26 years (1977–2003). In the storm-dominated study area, sorted bedforms surrounded by fine sand plains, are widespread. Although, wave and current shear stresses indicate a frequent remobilisation of fine sediment and an episodic remobilisation of coarse sediment, it appears that existing patterns have remained essentially stable over a period of 26 years. Moreover, we observe the birth of new sorted bedforms. The observed patterns are interpreted as the product of a feedback-related sorting process consistent with a recent explanation of sorted bedform formation. The observed sorted bedforms tend to be oriented perpendicular to the ambient tidal currents, indicating the importance of tidal flows in their shaping. Moreover, they are dominantly symmetric in cross-section, i.e. the boundaries between coarse and fine sediment are sharp. This finding is consistent with a reversing tidal current of almost equal strength during ebb and flood. Similar to subaqueous dunes, sorted bedforms might therefore be subdivided into symmetric and asymmetric types, depending on hydrodynamic forcing. However, the newly emerging sorted bedforms are asymmetric, with an orientation independent of tidal current flow but perpendicular to the direction from which the highest storm waves approach the study area. We thus conclude that extreme storm events may play a major role in the generation of sorted bedforms, whereas the quasi-continuous tidal currents form and maintain their final shape.

Monospecific Pliocene oyster buildups, Murray Basin, South Australia: Brackish water end member of the reef spectrum

Paleoestuarine, monospecific oyster bioherms and biostromes formed by the extant, soft-bottom oyster, Ostrea angasi, are prolific in the Pliocene Norwest Bend Formation, Murray Basin, South Australia. These buildups are pioneer communities that record “arrested ecological succession”, comprising only the Stabilization and Colonization Stages of reef development. Each of these stages is directly linked to a different phase of marine inundation. Stabilization is restricted to the early transgressive systems tract when pre-existing topographic highs were flooded and populated by dense aggregations of oyster clusters. Colonization is manifest throughout the transgressive and highstand systems tracts when increased accommodation allowed the vertical and lateral accretion of juvenile oyster communities into large buildups. Bioherms formed in areas of low terrigenous clastic sedimentation and seafloor disturbance, both of which encouraged rapid community growth. Biostromes in contrast, record buildup development on an unstable seafloor where episodic, catastrophic community burial by storm sedimentation created repeated conditions of seafloor stabilization and colonization. Subaqueous dunes formed as part of the falling stage systems tract when river and tidal currents reworked shell material from bioherms and biostromes into large-scale, subaqueous bedforms. Brackish water conditions in the palaeoestuary, a turbid water column, mesotrophic nutrient levels, and high spatial homogeneity of oyster buildups are interpreted to have prevented recruitment of specialized, stenohaline organisms that typify the Diversification Stage of other metazoan reefs. Comparison with modern, low diversity oyster communities imply that high fecundity and growth rates as well as exogenic nutrient cycling also made oysters in the Norwest Bend Formation highly adapted competitors capable of thriving in this nutrient-rich setting.

Flow and grain size control of depth‐independent simple subaqueous dunes

In the Grådyb tidal inlet (Danish Wadden Sea) simple medium to large dunes, which are superimposed on large to very large dunes, increase in size with decreasing grain size under quasi‐constant ebb dominated mean flow conditions. The mean height of the superimposed dunes increases from 0.1 m to 0.5 m and the mean dune length from 7 m to 12 m. This happens over a distance of 1.5 km from the inner to the outer part where mean depth (12 m) and the dominant ebb current (1.0 m s−1) remain constant. Over this reach, the grain size changes from 0.6 mm in the inner part to 0.3 mm in the outer part. As the water depth is constant and more than 1 order of magnitude larger than the height of the dunes, these are regarded as being depth‐independent. Relationships between dune dimensions and grain size established in previous studies are transformed into more general statements, bed form dimensions being directly related to the form‐corrected Shields parameter (θ′). In this way, dune dimensions and flow strength/grain size have, for the first time, been directly related to both bed load and free stream velocity in deeper flow systems. The former relationship is based on a recalibrated (form migration) version of the Meyer‐Peter and Müller bed load formula. Four diagrams describing flow conditions and dune migration rates as a function of grain size and dune length are presented.

Correction to “Development of subaqueous barchanoid-shaped dunes due to lateral grain size variability in a tidal inlet channel of the Danish Wadden Sea”

[1] High-resolution bathymetry at centimeter-scale accuracy acquired with a multibeam echo sounder system revealed the existence of barchanoid-shaped large (i.e., length 10–100 m) to very large (i.e., length >100 m) dunes in the Gradyb tidal inlet channel in the Danish Wadden Sea. The development of these dunes is due to an increase in dune celerity from 12 m/yr in the center of the channel to around 30 m/yr at the sides. This increase in dune celerity can be explained by the fact that dune heights decrease from 3.1 m in the center of the channel to 1.4 m at the sides, as a smaller sediment volume has to be moved per unit time for equal dune celerity. Water depth is uniform across the channel. Likewise, high-resolution acoustic Doppler current profiler measurements across the channel showed a uniform distribution of both ebb and flood flow. Thus no correlation between dune dimensions and water depth or flow velocity was established. Instead, high-accuracy bed sampling along the crests of the dunes showed a decrease in mean grain size from 0.63 mm in the center to 0.36 mm at the sides of the channel. The decrease in dune height is ascribed to this decrease in grain size, given that flow depth and flow velocity are uniform across the channel. The lateral decrease in grain size is suggested to result from sorting effects by secondary currents directed from the center toward the sides of the channel in the trough/lee side region of the barchanoid-shaped dunes.

Relationships Between Fault-Controlled Subsidence and Preservation of Shallow-Marine Small-Scale Cycles: Example from the Lower Pliocene of the Crotone Basin (Southern Italy)

Abstract The northern part of the Crotone Basin (southern Italy) was characterized by the activity of Pliocene growth faults bounding half-graben sub-basins. The thickest unit deposited during growth-fault activity is the lower Pliocene Belvedere Formation (up to 450 m thick), which shows a small-scale, highly regular rhythmic cyclicity. This sandstone unit was deposited in a shallow-marine environment, and its accumulation was mostly influenced by storm processes and locally by flows in tectonically confined areas. The meter-scale cycles are represented by rhythmic alternations between shell-rich intervals, characterized by storm structures or large-scale cross sets, and shell-poor sandy intervals that are commonly burrowed and poorly stratified. The better preserved cycles commonly show a fining-upward trend with minor progradation in their upper part, and may be bounded at the base by a shell lag. The deposition of large-scale cross sets is thought to be the consequence of the creation of fault scarps, which led to tectonic confinement in the half-graben basins. In the tectonically confined areas, tidal currents were enhanced and large subaqueous dunes migrated. The evidence suggests that the observed cyclicity was generated by sea-level and climatic variations that controlled the sediment input into the basin. The relationship between large-scale cross sets and tectonic confinement demonstrates that local extensional tectonics significantly influenced the sedimentation. The excellent preservation of the observed small-scale cycles is thought to be the consequence of deposition in the highly subsiding half-graben sub-basins. On the structural highs, this cyclicity is amalgamate and less appreciable. The data presented may be significant for a better understanding of the role of subsidence in preserving sedimentary successions in these growth-faulted settings.