Asymmetrical Sand Waves Research Articles

Small‐scale macrobenthic community structure along asymmetrical sand waves in an underwater seascape

AbstractSand waves are dynamic and regular bedforms that are ubiquitous in sandy shelf seas. However, information about the ecological characteristics (e.g., benthic community structure) and their spatial variability within these habitats is very limited. To address this knowledge gap, we undertook a field campaign in summer 2017 to investigate the macrofaunal community composition of a sand wave area off Texel (Dutch part of the North Sea). Sand waves in this area were asymmetrical, with longer gentle slopes that were approximately double in length to the shorter steep slopes. The benthic distribution along the different parts of these sand waves was assessed by collecting a large number of box cores within a transect line (~1 km). We show considerable variability in the individual, biomass and taxon densities, which were all significantly higher on the steeper slopes of the sand waves. These results are consistent with the trends observed in both the abiotic parameters and video analysis that were measured in two recent studies at the same study area. Our results provide valuable insight into the small‐scale patterns of variability in asymmetrical dynamic bedform environments, where gentle slopes seem to be primarily controlled by physical forces, while steep slopes are more under biotic control.

The role of Internal Solitary Waves on deep-water sedimentary processes: the case of up-slope migrating sediment waves off the Messina Strait.

Subaqueous, asymmetric sand waves are typically observed in marine channel/canyon systems, tidal environments, and continental slopes exposed to strong currents, where they are formed by current shear resulting from a dominant unidirectional flow. However, sand-wave fields may be readily observed in marine environments where no such current exists; the physical processes driving their formation are enigmatic or not well understood. We propose that internal solitary waves (ISWs) induced by tides can produce an effective, unidirectional boundary “current” that forms asymmetric sand waves. We test this idea by examining a sand-wave field off the Messina Strait, where we hypothesize that ISWs formed at the interface between intermediate and surface waters are refracted by topography. Hence, we argue that the deflected pattern (i.e., the depth-dependent orientation) of the sand-wave field is due to refraction of such ISWs. Combining field observations and numerical modelling, we show that ISWs can account for three key features: ISWs produce fluid velocities capable of mobilizing bottom sediments; the predicted refraction pattern resulting from the interaction of ISWs with bottom topography matches the observed deflection of the sand waves; and predicted migration rates of sand waves match empirical estimates. This work shows how ISWs may contribute to sculpting the structure of continental margins and it represents a promising link between the geological and oceanographic communities.

An evaluation of along- and down-slope sediment transport processes between Goban Spur and Brenot Spur on the Celtic Margin of the Bay of Biscay

Multi-beam bathymetry and backscatter, 3.5 kHz pinger profiles, side-scan sonar and seabed samples have been examined to evaluate along- and down-slope sedimentary processes along the Celtic Margin shelf and upper slope in water depths of 200 to 1500 m. The continental shelf and slope are indented and dissected by major canyon systems. The shelf is characterised by major northeast–southwest trending sand banks that are orthogonal to the shelf edge. Along the shelf edge, several fields of asymmetric sandwaves oriented orthogonal to the canyon axes indicate sediment transport into the canyon heads. Less commonly, sandwaves with weak asymmetry suggest sediment transport onto the shelf. These may be reworked and are partly overprinted by more recent sandwaves. Down-slope sediment transport by turbidity currents is the dominant process through the major canyons. Recent faulting has also played a role in canyon development. Turbidity currents are most likely initiated by faulting, and/or slope failure of the walls that bound the canyon head drainage basins and sediment migration from the shelf. This leads to deep incision of sinuous thalwegs in the upper reaches of canyon floors and downcutting and sediment transport on the mid to lower continental slope. The canyons are V-shaped in the upper reaches and become U-shaped progressively down-slope, suggesting they represent either a transition from erosive to depositional processes or sediment bypass conduits carrying sediment between the shelf and abyssal plain. Over-bank spill from canyons leads to deposition of unconfined turbidite deposits (muds) on the intervening canyon spurs.

Hydrodynamics and sediment dynamics of North Sea sand waves and sand banks

Seabed drag coefficients have been measured at a site within the Norfolk Banks and at a site within the sand wave field in the southern North Sea, using pressure sensors and moored current meters. At the sand banks site a seabed tripod measuring turbulent flows within 1 m of the bed was also used. The results are generally in agreement with values used in numerical models. At the sand banks site, the drag coefficient increases with wind conditions, but at the sand waves site there is a reduction during the highest wave conditions, attributed to the drag reduction caused by sand resuspension from the bed. This result suggests that sediment effects must be considered if wave/current interaction is included in numerical models of the region. The drag coefficient for reversing tidal flows over the asymmetric sand waves is found to be larger for flow towards the steeper face, suggesting a small form drag component. Studies of sand movement used bedform mapping, fluorescent sand tracing and photography of migrating ripples. There is evidence for the early stages of formation of a new bank between Broken Bank and Well Bank. At the sand waves site, ripple migration is found to be a useful estimator of bedload transport under conditions of negligible suspension. Over the sand banks, a definite correlation of surface water properties, notably the turbidity, with the topography of the banks was observed. The mechanism for this surface effect is not known but it is consistent with the observation of features, seen in satellite visible light images, which outline the shapes of the banks.

Sandstream on the northeast Brazilian shelf

A preliminary characterization of seabed morphology and mapping of algal patches within a sandstream on the northeast Brazilian continental shelf off Cape Calcanhar (where the South American coast turns sharply to the west) is presented. The study area (30 km × 30 km) is swept by unidirectional, wind-driven, tidally rectified currents, under the influence of the North Brazil Current. The study was made by use of Landsat Thematic Mapper (TM) data of the seafloor down to 40 m depth, digital bathymetric data from smooth sheets furnished by the Brazilian Navy, echo-sounder profiling and underwater surveying by SCUBA diving. A mesoscale shore-parallel natural zonation of the sandstream was observable on the TM-images: (a) an inner zone dominated by sediment resuspension caused by wave-driven turbulence and tidal currents; (b) an intermediate zone dominated by sand ribbons and other longitudinal bedforms; (c) an outer zone dominated by large-scale sand waves (underwater sand dunes) limited offshore by shore-parallel sandbanks. Image-assisted in situ work, carried out on the latter zone, gave the following results: (1) Morphology and sand composition—the asymmetric sand waves are made of medium quartz sand, with almost straight crests of lengths up to 4 km, heights between 3 and 7 m, and avalanche lee slopes of 30°; on the stoss sides we recorded the presence of ripples and absence of megaripples and benthic ecosystems; on interdune areas, benthic ecosystem patches dominated by foliaceous and calcareous algae were found. (2) Sediment transport—active bedload transport was observable only during the winter season, when the strongest wind-forced currents induce flow separation at the crests and avalanche streams on the lee slope. Within the interdune areas, under the influence of the lee vortices, benthic community data confirms no appreciable abrasion due to sand transport. (3) Biology—species composition and biomass estimates from the algal patches in the first upstream trough are presented, and species composition of bryozoa observed as epibionts on live gastropod shells are analysed. This sandstream shows some similarities to one described from the southeast African continental shelf, which is also under the influence of unidirectional currents.

The modelling of the free surface flow of water over topography

A numerical model is developed for the flow of water over a bottom topography that is characteristic of sandwaves found in the coastal and river estuary environment. Turbulence closure is achieved at the level of the turbulence energy equation and the Reynolds stresses are parameterised in terms of a gradient transfer law and an assumed partitioning of the turbulence energy between different components of the flow. The pressure derives from both hydrostatic and hydrodynamic contributions. An experiment is performed for steady-state flow over a sequence of asymmetrical sandwaves and it is shown that there is flow separation over the lee slope of a sandwave. In this case, it is shown further that flow separation is a consequence of the inclusion of the hydrodynamic pressure contribution. A comparison is also made between non-hydrostatically and hydrostatically determined values of other mean and turbulent flow quantities.

The modelling of sandwave evolution resulting from suspended and bed load transport of sediment

A numerical model is used to determine changes in the bed morphology resulting from the bed load and suspended transport of sand. The ability of the model to predict mean and turbulent quantities and the suspended transport of sand is assessed by appeal to laboratory flume experiments and field observations. The model is then applied to investigate changes that may be expected to occur in an initial bedform corresponding to asymmetrical sandwaves characteristic of those found in the coastal and estuarine environment. It is found that suspended transport may frequently dominate over bed load transport in producing changes in the bedform structure. The predicted evolution of the bedform is reasonably consistent with observations made of sediment fluxes and sandwave migration in the Taw Estuary, England.

Marine Sand-Wave Complex in the Permian of Central Arizona

ABSTRACT The Permian Rancho Rojo Member of the Schnebly Hill Formation in central Arizona displays distinctive large-scale cross stratification in compound sets that are several meters thick and tens of meters long. Based on geometry, stratigraphic sequences, sedimentary structures, and the processes which formed these features, the Rancho Rojo is interpreted as a submarine or estuarine sand wave complex. Modern sand wave processes produce large-scale intraset cross-stratification and superimposed dunes which form complex internal sedimentary structures like those of the Rancho Rojo. The Rancho Rojo is subdivided into five facies. The planar-tabular foreset facies is composed of giant westward-dipping foresets that probably formed in large, steep, strongly asymmetrical sand waves. The intraset cross-stratified facies comprises complex compound cross stratification bounded by westward-dipping planes of erosion. It was deposited by large, less asymmetrical sand waves with superimposed dunes on the surface. The trough cross-stratified facies consists of complex trough-shaped intrasets. The internal geometry of this facies probably resulted from deposition in complex, nearly symmetrical sand waves that responded to oscillatory tidal currents. The ripple-laminated sand facies and homogeneous sand facies comprise ripple-laminated and bioturbated sandstone that formed du ing waning sand-wave deposition. Although the facies are complexly interbedded, their overall stratigraphic succession from oldest to youngest typically is similar to the order just presented. This suggests that through time large, asymmetrical, rather simple sand waves were succeeded by more complex, lower, less asymmetrical sand waves due to change of current regime from unidirectional to bidirectional. This interpretation of a sand body that was once thought to be part of a continental redbed sequence, requires revision of paleogeographic reconstruction for part of the Permian of central Arizona.

Stability of the Fraser River Delta front

The distributary front deposits of the delta consist mainly of interlayered sand and silt and lie in one of the most seismically active zones in Canada. Subsurface deposits at the north tend to be siltier than those at the south. Sediments on the surface of the delta slope range from clayey silts at the north to pure sands at the south. Slope angles vary from 23° at the head of the slope to 1–2° within 2 km beyond the tidal flats.Hydraulic bedforms observed on the sandy slopes southeast of the main channel gradually increase in size from small ripples to large asymmetric sand waves 2–3 m high and approximately 30 m long. They are generated primarily by flood tidal currents which scour the seabed to a water depth in excess of 100 m.Formation of gullies which crease the delta slope probably is initiated principally by failure of oversteepened deposits at a channel mouth. Previous studies have suggested that the large hummocks or ridges at the base of the slope off the Main Channel have been formed by the compressional folding of failed deposits. Recent reflection seismic surveys suggest that such a process has recurred in this area during the growth of the delta.Interpretation of SPT data, in the light of recent research findings, suggests that previous analyses have overestimated failure potential. It is estimated that the slope may withstand an earthquake with a duration corresponding to approximately 15 significant cycles of motion and a peak acceleration in the range of 11–13% G. Keywords: stability, Fraser River Delta front, mass wasting, erosion, liquefaction potential, cyclic mobility.

Carbonate and Siliciclastic Deposits on Slope and Abyssal Floor Adjacent to Southwestern Florida Platform: ABSTRACT

End_Page 484------------------------------Three distinct carbonate deposits have been identified on the slope and adjacent sea floor of the Southwestern Florida platform: (1) reef talus, recognized by shape and location, found on the upper slope of the Yucatan Channel and also east of the Marquesas Keys; (2) hemipelagic sediments, with complex sigmoid-oblique bed forms, filling the intervening gap between the channel and Keys and forming two lobes on the floor of the northern Florida Straits; and (3) turbidite deposits, with chaotic internal bed forms, covering siliciclastic Mississippi fan sediments at the base of the canyons in the Florida escarpment. The source of the talus, eroded and transported during the many storms which frequent the region, is the reef complexes which have formed on the platform rim. The sediment of he other two deposits is of foraminiferal tests, produced in nutrient-rich waters at the shelf edge. This sediment is deposited on the outer shelf and is vigorously transported southward as evidenced by 5 m (16 ft) high asymmetric sand waves. Geophysical, geochemical, and sedimentological data suggest that the spatial relationships of these deposits are related to sea level variations. During extreme lowstands, with much of the shelf exposed, the dominant sedimentation is siliciclastic deposition on the abyssal floor, and slope talus development at the edge of the shelf. During a subsequent rise in sea level, after carbonate production on the shelf is initiated, sediment is transported southward to the head of the canyons and funneled to the abyssal floor. Subsequent rising sea level shifts the axis of transport farther on the shelf, bypassing the canyons and funneling the sediment through breaks in the carbonate reef-banks at the southern edge of the platform. At the sites of both the hemipelagic and the turbidite deposit on, high-resolution seismic data indicate that at least three cycles of deposition have occurred. In the abyss, this cyclic nature has produced alternating layers of carbonate and noncarbonate sediments, recognizable in the sedimentary record as chaotic limestone units interlayered with fine shales. The hemipelagic deposits would be almost indistinguishable in the geologic record from deep sea foraminiferal oozes. End_of_Article - Last_Page 485------------

Sand waves and sediment transport around the end of a tidal sand bank

ABSTRACTTwo detailed surveys have been made of the north‐western end of the Haisborough Sand off the Norfolk coast using echosounder, 3‐5 kHz reflection profiler and side‐scan sonar. Asymmetrical sand waves indicate north‐westerly sand movement on the southern side and south‐easterly sand movement on the steeper northern side of the bank. Secondary, superimposed megaripples, which are probably better indicators of sediment movement, give evidence of a cross‐bank component. Between the north‐westerly and south‐easterly facing sand waves on the tip of the bank there is a zone of symmetrical sand waves. These are usually taken to indicate zero net transport, but in this case the oblique orientation of megaripples in their troughs indicates transport parallel to the sand wave crests. This suggests the route by which sand travels around the end of the bank to form a roughly closed circulation. Sediment textural parameters support the notion that sand is winnowed from the foot of the bank on both sides and is transported to the middle with an overall net transport from the south to the north. Analysis of charts dating back to 1886 shows that the bank is stable within the error limits of position fixing, though that could allow more than 0‐25 km shift to the north east in 100 years to pass undetected. A box model is drawn up for the estimated sediment fluxes around the end of the bank, and implications for residence times and circulation rates are drawn from it.

Palaeotidal speeds and ranges estimated from cross-bedding sets with mud drapes

Tidally formed cross-bedding sets in which mud drapes are spaced out among sandy foresets occur in Cretaceous1–4, Eocene5 and Holocene6 sands deposited subtidally by strongly asymmetrical sand waves. A striking feature of some sets4,6 is that the horizontal drape spacing, measured in the direction of sand-wave advance, varies cyclically on a time scale related to the spring–neap tidal cycle as indicated by the numbers of drapes. Each drape forms part of a distinctive lithological couplet, consisting of a group of sandy foresets recording avalanching on the steeper side of the sand wave, followed by the drape itself, typically divided into upper and lower parts by a centrally placed silt‐sand parting. The drapes have been assigned to slack water, but the groups of sandy foresets to episodes of vigorous flow, the combination suggesting a quantitative model based on the tide. Here I show how the peak speeds and ranges of ancient tides can be estimated by combining an empirical bedload transport function with appropriate measurements from such cross-bedding sets.

Carbonate Sand Bodies Along Contrasting Shallow Bank Margins Facing Open Seaways in Northern Bahamas

High-resolution seismic profiling, bottom sampling, and aerial and space imagery show that widely contrasting carbonate sand bodies are present along the shallow windward, leeward, and tide-dominated margins of Little and Great Bahama Banks which face open seaways--the Northwest Providence Channel and the northern Straits of Florida. This high diversity in sand bodies and their associated bank margins is due to variations in the level, duration, and magnitude of the physical energy flux across the edges of the platforms. In the Bahamas, the dominant winds, waves, and currents are from the east; hence easterly facing margins are windward, westerly facing margins are leeward. Where strong tidal currents are present (because of adjacent basin shape) the windward and leeward effects are diminished. Thus, bank-margin orientation relative to the climatic energy patterns appears to assert the major controls upon sedimentation. Antecedent topography in the form of islands or subtidal rock ridges creates energy barriers which can control the direction and volume of sediment flux on or off the banks. Along windward margins, energy barriers such as large islands not only block the bankward transport of sands but appear to augment offbank transport by setting up seaward flows. In contrast, along leeward margins, energy barriers prevent the vigorous offbank sand transport that normally occurs when these margins are open and water flow is unrestricted. In addition, sand-body geometry and grain type are controlled by bank-margin orientation. Along windward, open margins, skeletal sands accumulate in the lee of low-relief, mostly relict reefs. These margins are generally sediment-barren, having only a sporadic, thin veneer of sand. Along leeward, open margins a broad (10 × 10 km), thick (12 m), bankward-thinning blanket of sand rich in nonskeletal components (peloids, composite grains) lies on top of rock ridges and reefs. Large, asymmetric sand waves superimposed on this sand body indicate net offbank sand transport. Along the tide-dominated margin, large, broad, oolitic sand lobes have migrated onto adjacent lagoonal muddy sands and indicate net bankward transport. We suggest the following classification of modern, shallow, carbonate bank margins: (1) windward open; (2) windward protected; (3) leeward open; (4) leeward protected; (5) tide dominated. This classification seems to fit many ancient analogs and may be useful in paleoenvironmental reconstructions.

Internal Structures of Shallow-Marine Tidal Sand Waves: ABSTRACT

Asymmetric sand waves (average height 0.86 m, and average wavelength 46.6 m) with superimposed megaripples 0.05 to 0.2 m high, occur commonly in medium to coarse sand on tidal sandbars in the Bay of Fundy. Their internal structures are complex, but three main types can be distinguished: (1) inclined sets of descending and ascending cross-bedding (0.1 to 0.3 m thick) that have set boundaries dipping at an average angle of 9° in the dominant transport direction; (2) large-scale foresets having set thicknesses comparable to the sand wave lee-face height and average inclinations of only 20°; and (3) complex cosets up to 0.5 m thick of thin (0.05 to 0.15 m thick) cross-bedded sets with abundant herringbone cross-stratification. Types 1 and 2 are formed during sand wa e lee-face migration, whereas type 3, which overlies the lee-face structures, is produced by the superimposed megaripples during vertical growth of the sand waves following degradation by storms or winter ice. Inclined sets are the most common lee-face deposit in the Bay of Fundy. Their formation is favored by the high current speeds, low to intermediate sediment transport, and the migration of large megaripples (relative to the sand waves) which characterize this area. Large foresets are relatively rare, and extensive development of large-scale, angle-of-repose cross-bedding has never been observed. Large-scale foresets may be more abundant in other areas where there are larger sand waves, lower current speeds, and higher sediment transport, but they should contain numerous reactivation surfaces, and be overlain by vertically accreted complex cosets. The internal structures of tidal sand waves should differ significantly from those in aeolian dunes. End_of_Article - Last_Page 696------------

Longitudinal and transverse bedforms on a large tidal delta, Gulf of St. Lawrence, Canada

Both longitudinal and transverse bedforms occur on the flood-tidal delta in the Miramichi estuary, New Brunswick. Asymmetrical sand waves are the predominant transverse bedform, with heights up to 2 m and wavelengths to 60 m. They are flood-oriented (face landward), and floor most of the seaward-sloping region between the limbs of the parabolic shoal. Less stable transverse bedforms, megaripples and highly modified sand waves, occur on the seaward-tapering limbs of the shoal, areas which are affected by strong and reversing tidal currents. Current lineations (longitudinal bedforms), with heights up to 25 cm and spacings to 15 m, are present in an elongated zone extending across the asymmetrical sand-wave field, and lying parallel with the southern limb of the shoal. The distribution of transverse bedform types and the overall parabolic shape of the shoal, in general, reflect a simple tidal-current flow model, in which dominant flood currents traverse through the centre of the shoal, and dominant ebb currents, around the periphery. However, the presence of the current-lineation field indicates that this simple tidal-current flow pattern has become more complicated, and that the gross morphology of the shoal is no longer in equilibrium with this new flow pattern.

The geological history of the Sandettie-Fairy Bank area, southern North Sea

Tertiary strata with a low northeasterly dip, truncated by an erosion surface with low westerly dip, form the basement in the Sandettie—Fairy Bank gap except in the northwest part of the area where a basin is cut into the Tertiary strata. This basin is filled with a sequence of late Pleistocene sands overlain by Holocene sands which form the major topographic features of the Sandettie and Fairy Banks and the low sea-floor col linking them. The surface of the banks and col are moulded into transverse ridges up to 10 m in amplitude and 200 m in wavelength. Three types of ridges are present: at the north end of the col, linear transverse ridges of irregular amplitude and wavelength occur with a flint gravel base grading up into medium sand; these ridges are fossil features of Preboreal-Subboreal age. At the south end of the col there are symmetrical sandwaves of regular amplitude and wavelength which, from geological evidence, appear to be stable. To the west of Sandettie Bank and the col there are south-facing asymmetrical sandwaves which on geological evidence, are potentially mobile and which later sedimentological investigations revealed to be moving southwards in one area.

Sediment Dispersal Patterns of Oolitic Calcarenite in the Sundance Formation (Jurassic), Wyoming

ABSTRACT The Hulett Sandstone Member of the Sundance Formation (Callovian and Oxfordian, Jurassic) exposed along the eastern flank of the Bighorn Basin, Wyoming, occurs as lenticular and discontinuous units of oolitic calcarenite, whose dominant internal structure is trough cross-stratification. This crossbedding type indicates that the units may have formed as large-scale, asymmetrical sand waves. The near north-south bipolar distribution of crossbed inclination directions at individual localities suggests that tidal current action, probably in the subtidal zone, was responsible for the formation of the units. The primary mode of the directions measured perpendicular to small-scale symmetrical ripple marks is aligned near north-south and corresponds with crossbed data. An exhumed calcarenite body of the Hulett Sandstone that appears to be a composite fossil sand wave was found. The length of this body is roughly transverse to the local bipolar crossbed modes. The shape and structure of the body supports the view that it and other less well exposed bodies of the Hulett Sandstone are large-scale, composite sand waves.

Sand waves in the North Sea off the coast of Holland

Four quarterly surveys were made of the sand-wave field in 1968–1969. The data on trough to crest height of the sand waves have been expressed as significant wave heights. These show a maximum of 7 m, decreasing to 2 m in the north and towards the coast. Wavelengths are from 200 to 500 m. Variability in the position of the northern and eastern margins of the field is attributed to wave activity. The position of the western boundary to the sand-wave field is due to scarcity of sand and to the existence of a gravel bottom in parts of the deep-water channel. Symmetrical and asymmetrical sand waves, and sand waves with superimposed megaripples are the forms recognised. The maximum height of 7 m is in agreement with predictions using an equation given by Kennedy (1969). Together with northward decrease in height there is decrease in grain size and absence of megaripples on sand waves. The northern boundary to the sand-wave field is caused by decreasing asymmetry of the tidal ellipse. This does not occur in a narrow tongue extending along the coast of Texel where an extension of the sand-wave field is found. The northwards decrease in height is mainly caused by increase in suspension transport of sand. This agrees with a re-evaluation of Bagnold's (1966) suspension criterion which shows that large amounts of sand < 230 μ in diameter will be transported in suspension. Megaripples on sand waves are taken to be large scale analogues of ripples on dunes seen in flumes. The megaripples represent perturbation of bed-load transport rate and the sand waves perturbation of suspended load transport rate. Thus the northwards absence of megaripples on sand waves, decrease in grain size, decrease in height and increase in suspension transport, fit in a coherent dynamical explanation of the characteristics of the sand-wave field.

Sand Waves in the Tidal Entrance to Chesapeake Bay: Preliminary Observations

Sand waves are present on the bottom in the tidal entrance to Chesapeake Bay. As far as known they are confined largely to the northern part of the entrance in an area of sand shoals and tidal channels. The waves found are 5 to 10 feet in height, and 200 to 800 feet in crest-to-crest length. They occur in water 15 to 35 feet deep and are found both atop shoals and in channels. Symmetrical and asymmetrical profile types were observed. Near the tidal current thalweg in North Channel, sand waves are largely asymmetrical and face seaward with the ebb. At greater lateral distances away from the thalweg, symmetrical forms are more common, a situation suggestive of equal ebb and flood currents. With even greater distance, sand waves are not developed, a situation suggestive of current incompetence. In a closed-end flood channel, asymmetrical sand waves face landward toward the closed end of the channel with the flood current. On one sand shoal, marginally-located sand waves face convergently toward the higher center of the shoal where symmetrical waves are found. Sand wave migration is believed to be a factor in the maintenance and evolution of some tidal shoals.

OCCURRENCE OF HIGH‐ANGLE STRATIFICATION IN LITTORAL AND SHALLOW NERITIC ENVIRONMENTS, CENTRAL GEORGIA COAST, U.S.A.1

SUMMARYHigh‐angle stratification (greater than 20°) is produced in several areas of shallow marine sedimentation along the barrier islands of the central Georgia coast. The maximum angle of inclination is 30° which is the angle of repose for the saturated, fine‐grained, angular sand of this area. High‐angle stratification forms in the following locations:(1) The depositional margin of tidal channel inlets. Under some wave and current conditions, sand accumulates near low tide level and steepens the depositional interface to the angle of repose. (2) The steep face of asymmetrical megaripples developed by tidal currents. Ripples with amplitudes as much as 3 ft. and wave lengths of 20–40 ft. commonly develop in channel inlets and other areas of sand sediments. (3) The steep face of sand waves formed in channel inlets. These large asymmetrical ripples have amplitudes as great as 12 ft. and wave lengths of ca. 300 ft. Lengths along the crests are over 600 ft. (4) The landward side of low bars developed on the beach. Bars and troughs (ridges and runnels) are common on the beaches of this area. The bars, which are as much as 5 ft. high, shift landward by deposition on the steep landward face. (5) The oceanward side of large sand waves at the mouth of offshore tidal channels. Large sand waves are located 6 miles offshore from Doboy Sound inlet in 20–25 ft. of water. The steep face of these asymmetrical sand waves is orientated toward the ocean. Amplitude of these large ripples is as much as 17 ft. and length along the crests is over 1/2 mile.