Large Storm- and Current-Shaped Sand Bodies on the Southeastern Australian Shelf: Abstract
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Abstract The coast of southeastern Australia has a high-energy embayed and cliffed coast that receives only minor amounts of modem sediment from streams. This setting, coupled with an apparently stable sea level for the past 6000 years, results in a thin erosional veneer of Holocenc sediment overlying truncated Pleistocene and older deposits on the shelf. In several locations, however, large sand bodies have accreted on the lower shoreface and these sand bodies are unlike those described from other shelves in the world. They arc 10 to 30 m thick, several kilometres wide, tens of kilometres long, and result in a convex shoreface profile. Radiocarbon dates indicate that most of the sand bodies were deposited in the past 6000 years after the sea level reached its present position. From the study of the texture and composition of long-core samples, it appears that the sand was derived from adjacent beaches and cliffs, and seismic-reflection profiles indicate progradation of sand bodies has occurred across the steep (2° to 5°) seaward flank into water depths of 60 to 80 m. Three major factors appear to control formation of these sand bodies: an initially steep inner-shelf profile; local high-energy conditions; and a long time period of a stable sea level. Based on sediment texture, seaward dipping reflectors, and surface channels, we infer that sediment is transported seaward from the upper shoreface to the sand bodies by storm-induced downwelling. In areas of strong regional flows, such as off Cape Byron, where the East Australian current impinges on the coast, these flows play a major role in modifying the shape and textural character of the sand bodies.Keywords:
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Large shore-parallel sand bodies on the lower shoreface of the embayed and cliffed coast of central New South Wales, Australia, provide indirect evidence of downslope transport of sand. These sand bodies are 10 to 30 m (33 to 100 ft) thick, extend discontinuously for 40 km (25 mi) along the coast, and add a pronounced convexity to the shoreface profile. Evidence from surface samples, deep borings, and side-scan sonar and high-resolution seismic reflection profiling from the sand bodies indicates that deposition occurred by downslope transport of sand from the upper shoreface and surf zone. The sand bodies consist entirely of locally derived quartz sand intermixed with marine mollusk fragments and microfauna. The internal acoustic structure of the sand bodies indicates growth by seaward progradation; the seaward face, or foreslope, locally attains a slope greater than 5°. Surface morphology indicates coalescing of individual sediment lobes and damming of bedload sediment against bed-rock ridges at the toe of the sand body in water depths of 70 to 80 m (230 to 262 ft). Channels and individual sediment lobes oriented normal to the horeline also indicate seaward transport. On this coast, the sea level reached its present position 6,000 years ago, and these sand bodies were produced by high-energy conditions on a steep shoreface during the stillstand period. Average waves here are 1 to 2 m (3 to 6.5 ft) and storm waves are much higher. Radiocarbon dates from analogous sand bodies elsewhere in New South Wales suggest continuous seaward progradation and upbuilding since the cessation of sea level rise. No direct observations or measurements of downslope transport are available, but we infer that sand derived from along the coast or the cliff face is gradually reworked seaward on the steep upper shoreface during fair-weather conditions. During storm periods, sand is flushed seaward by returning bottom flows onto and across the surface of the sand body. Figure In addition to downslope transport of sand to deeper water, there is strong evidence of concurrent modification of the sand bodies by processes acting parallel to the shoreline. This evidence consists of textural trends, shore normal sand waves, and the overall alongshore continuity of the sand bodies. End_of_Article - Last_Page 460------------
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Bedform
Marine transgression
Seabed
Beach ridge
Sand dune stabilization
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ABSTRACT The geologic framework and surficial morphology of the shoreface and inner continental shelf off the Wrightsville Beach, North Carolina, barrier island were mapped using high-resolution sidescan-sonar, bathymetric, and seismic-reflection surveying techniques, a suite of over 200 diver vibracores, and extensive seafloor observations by divers. The inner shelf is a sediment-starved, active surface of marine erosion; modern sediments, where present, form a patchy veneer over Tertiary and Quaternary units. The lithology of the underlying units exerts a primary control on the distribution, texture, and composition of surficial sediments, as well as inner-shelf bathymetry. The shoreface is dominated by a linear, cross-shore morphology of rippled scour depressions (RSDs) extending from just seaward of the surf zone onto the inner shelf. On the upper shoreface, the RSDs are incised up to 1 m below surrounding areas of fine sand, and have an asymmetric cross section that is steeper-sided to the north. On the inner shelf, the RSDs have a similar but more subdued cross-sectional profile. The depressions are floored primarily by shell hash and quartz gravel. Vibracore data show a thick (up to 1.5 m) sequence of RSD sediments that unconformably overlies ancient coastal lithosomes. In this sediment-starved inner shelf setting, rippled scour depressions probably form initially on preexisting coarse-sediment substrates such as modern lag deposits of paleofluvial channel lithosomes or ancient tidal inlet thalwegs. Interannual observations of seafloor morphologic change and the longer-term record contained in vibracores suggest that the present seafloor morphology is either relatively stable or represents a recurring, preferential morphologic state to which the seafloor returns after storm-induced perturbations. The apparent stability is interpreted to be the result of interactions at several scales that contribute to a repeating, self-reinforcing pattern of forcing and sedimentary response which ultimately causes the RSDs to be maintained as sediment-starved bedforms responding to both along-shore and across-shore flows. Sediment accumulation from over 30 years of extensive beach nourishment at Wrightsville Beach appears to have exceeded the local shoreface accommodation space, resulting in the leaking of beach and shoreface sediment to the inner shelf. A macroscopically identifiable beach nourishment sediment on the shoreface and inner shelf was used to identify the decadal-scale pattern of sediment dispersal. The nourishment sediment is present in a seaward-thinning wedge that extends from the beach over a kilometer onto the inner shelf to waters depths of 14 m. This wedge is best developed offshore of the shoreline segment that has received the greatest volume of beach nourishment.
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Shoal
Winnowing
Bedform
Marine transgression
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Two types of Holocene sediment bodies occur on the NC mid-continental shelf. (1) Gravelly sand bodies form as lag pavements in topographic lows between erosional scarps and on hardbottoms composed of Cenozoic rocks. The gravel within this sand consists of old corroded, bored, and encrusted shell debris and rock lithoclasts. (2) Fine sand bodies have four general occurrences: (a) perimeter sediment aprons around the base of rock scarps where there is little gravelly sand; (b) away from the scarps as irregular to linear sand sheets on top of the mega-rippled gravelly sands; (c) sand ramps that bury NE facing scarps and form sediment transport paths onto the upper hardbottoms; and (d) upper hardbottoms where they occur as thin, scattered, and ephemeral bodies. During pre-storm conditions, the sediment surfaces are characterized by high concentrations of organic components that form a bound sediment in the upper portion of the sand bodies. A major storm results in total sediment motion that breaks down the pre-existing organic binding agents. The post-storm distribution of sediment types is initially controlled by the large-scale hardbottom morphology. The gravelly sands are concentrated in the more open areas of lower hardbottoms in between erosional scarps. Fine sands settle outmore » on top as a grades sequence as the storm wanes. Redeposited fine sands lack organic binding agents; consequently, for a brief period following a storm, the surface of unbound fine sands is rippled and actively modified by tidal currents. Fine sands on the lower hardbottoms adjacent to scarps are protected from erosional modification; this results in the ubiquitous fine sand aprons at the base of most scarps. A growth of micro-algae and benthic infauna become re-established in the surface sands to form a bound sediment; then the rippled sands are broken down by vagrant epibenthos and pocks, pits, and trails develop on the surface. Sand bodies are preserved until the next major storm event.« less
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Abstract Sediment flux on the Middle Atlantic Shelf can be described as an advective-diffusive process. Sediment grains are released to the dispersal system from erosion of the shoreface. These grains move about on the shelf surface in a “random walk” manner under the impetus of repeated storm events. However, statistics describing the hydraulic climate show that the grain trajectories are biased, random walk trajectories; the preferred direction of movement is along the shelf to the southwest. Grain size gradients reflect the southwestward transport, and grain size within each major shelf sector becomes finer to the southwest. Grain shape is a more durable grain characteristic than grain size, and tends to reflect early Holocene subaerial drainage patterns rather than modern dynamics. However, grain shape patterns are fading due to the diffusive process. Bed form patterns on the Middle Atlantic Shelf are similarly a response to intermittent, along-shelf (southwestward) sediment transport by storm-generated currents. Flow-transverse bed forms occur at several scales. Ripples (6–300 cm spacing) are ubiquitous. Evidence from time-lapse photography suggests that current ripples that were formed during storms were remade as oscillation ripples in the post-storm period. Megaripples (3–20 m spacing) occupy perhaps 10–15% of the shelf surface. In areas of fine to very fine sand, this class appears as hummocky megaripples, a bed form probably equivalent to the hummocky cross-strata sets seen in ancient sedimentary strata. Sand waves (20–200 m spacing) arc widespread on the shelf surface. In most areas, they are of less than 1 m amplitude, due to the relative infrequency of major flow events; however, in areas where the storm flow field is constricted and accelerated, they may be as high as 7 m. Sand ridges are flow-oblique bed forms with a spacing of 0.5–4 km, and a height of 5–10 m. They make northwestward-opening angles of 10–45° with the shoreline. Both the flow-transverse bed forms and the large flow-oblique sand ridges have steeper and finer-grained down-current (southeastward) flanks. The Holocene transgressive sand sheet, into which these bed forms have been impressed, ranges from 0 to 20 m thick. Its leading edge commonly lies at the base of the shoreface. It grows landward in response to storm currents, which sweep sand off the shoreface. The sand sheet lies disconformably on marine marginal, early Holocene and Pleistocene strata. Stratification in the transgressive sand sheet is complex. Vibracores and box cores reveal stacked sand strata, which commonly range from 10 to 50 cm thick. About 40% of the beds exhibit horizontal to low-angle inclined lamination. Inclined lamination may be equivalent to hummocky cross-stratification, although the relationship is difficult to establish in cores. Laminated beds tend to be graded (tend to fine upward). Graded beds at the sea floor have ripple cross-lamination or have burrows near the top, but this uppermost zone is missing, perhaps due to erosion, in deeper, graded beds beneath the sea floor. Other beds are massive to indistinctly mottled, and are assumed to be bioturbated. Massive and mottled beds become increasingly abundant at the expense of laminated beds in a seaward direction. About 10% of the beds reveal high-angle cross-stratification. The strata of the transgressive sand sheet can be divided into four classes: 1. graded strata are single-event storm beds deposited from suspension; they are most abundant on the down-current flanks of sand ridges; 2. hummocky strata are also single-event suspension deposits, their depositional environment was characterized by a high ratio of wave orbital current to mean current; 3. cross-strata sets are multi-event beds which develop on the up-current flanks of sand ridges; 4. lag strata, consisting of coarse sand, pebbly sand, or fine gravel, occur in the troughs between ridges. The sand strata of the Atlantic Continental Shelf are tempestites, deposited by geostrophic storm currents. Relatively dense near-bottom suspensions of sediment are generated by storms, but velocities measured within these suspensions arc directed primarily along the shore, rather than offshore. There is no evidence for high-velocity turbidity currents on the Atlantic Continental Shelf.
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A depositional model for intertidal sand bodies indicative of tidal embayments was developed from 20 vibracores and 25 can cores taken at St. Helena Sound, South Carolina. This V-shaped embayment located 35 km south of Charleston, South Carolina, has a tidal range of 2.0 to 2.8 m. The intertidal shoals are formed and reworked by opposing tidal currents. Ebb currents usually exceed 100 cm/sec in the deep adjacent channels and produce the long linear features on the shoals. Flood currents rarely exceed 75 cm/sec and are dominant across the broad seaward sand flats. The range of sedimentary features gradually changes from a dominance of physical sedimentary structures on the exposed seaward sand flats to a dominance of biogenic sedimentary structures on the protected sand flats. The distribution of each feature is controlled by their relative position on the sand flats to maximum wave energy. Where biogenic sedimentary structures are abundant, protection from wave energy is afforded by the shoal crest. Laterally the shoals grade into ebb channels or lower subtidal mixed sand and mud flats. The shoals display a coarsening-upward sequence of wavy-bedded to flaser-bedded clays and sands overlain by clean well-sorted, cross-bedded to burrowed sands. The sands are composed of fine to very fine subangular quartz grains. The depositional history of the intertidal sand bodies indicates a vertical buildup of sediments and subsequent lateral accretion. Subtidal sand bodies were first deposited on preexisting bay-fill muds. With a decline in sea-level rise, an increase in vertical deposition occurred, producing incipient intertidal bars. As the bars became fully emergent, increasing wave energy and tidal currents reworked the shoals into their present shape. Continued sand deposition occurred as lateral accretion and infilled adjacent channels. The shoals are up to 10 m thick and cover an area of 1 to 4 km2. They extend 3 to 5 km seaward and are as much as 1 to 2 km in width. Because most of the shoals are subtidal to intertidal, preservation potential is high. As the embayment fills, prograding salt marshes will eventually cap the sand bodies. End_of_Article - Last_Page 622------------
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The Labrador Shelf is dissected by the Marginal Trough into a narrow inner rocky shelf, which is the submerged extension of the Precambrian Shield landmass, and an outer shelf zone consisting of broad, flat banks mantled by thick deposits of glacial drift. The inner shelf north of Groswater Bay is 35 km wide, with a highly irregular bedrock-dominated bottom topography. Unconsolidated materials consist of sand, and coarse gravel pavement deposits. Sand deposits are less than 1 m thick and limited in areal extent to the flat-bottomed, low-lying areas between bedrock highs. Coarse gravel deposits occur as veneer pavements on the flanks of highs. The sands are underlain either by cohesive muds (early Holocene?) which were deposited in former basinal depressions, or by coars gravels in local areas marginal to bedrock outcrops. The coarse gravels are probably relict lag deposits formed by the reworking of glacial drift, but the sands are thought to be derived from contemporary nearshore and beach sediments situated about 10 km west of the study area. The thin and patchy sand distribution suggests that transport mechanisms are more than sufficient to disperse the volume of sand that is being supplied to the inner shelf. Preliminary analysis of near-bottom velocity measurements indicates that the seabed is subjected to a strong southeasterly current (Labrador current) which induces a net southeasterly sand flux across the shelf. The predominantly resistant substrate of the shelf would likely be swept clean of sand, if it were not for the irregular bottom configuration which provides local and temporary sinks for sand deposition. The most important sediment-transport process on the inner shelf is the southeasterly directed Labrador current. Wave-generated currents are of lesser importance (except in shallow nearshore areas) as a sand-dispersal mechanism, and iceberg-scouring is more effective in redistributing sediment in areas seaward of the inner shelf edge. End_of_Article - Last_Page 771------------
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Siliciclastic
Seafloor Spreading
Bioturbation
Surf zone
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The continental shelf on the western margin of the Cretaceous Interior seaway was a muddy surface which bore abundant northwest-southeast trending sand bodies, up to 20 m (65 ft) thick and many km long (Medicine Hat, Mosby, Shannon, Sussex, Duffy Mountain, and Gallup Sandstones). These features resemble the storm-built or tide-built sand ridges reported from the modern Atlantic continental shelf, or from the Southern Bight of the North Sea. However, whereas modern sand ridges may rise from the Holocene transgressive sand sheet through overlying Holocene mud deposits to be exposed at the present sea floor, no modern examples are known where sand ridges are completely encased in mud, as the Cretaceous examples seem to have been. Hydrodynamical theory suggests that special circumstances may make it possible to build sand bodies from a storm flow regime whose transported load consists of sandy mud. Under normal circumstances, such a transport regime would deposit little clean sand. The sea floor is eroded as storm currents accelerate, but erosion ceases when the boundary layer becomes loaded with as much sediment as the fluid power expenditure will permit (flow reaches capacity). Deposition of the graded bed occurs as the storm wanes; the resulting deposit is liable to consist of a sequence of thin shale beds with basal sand laminae. However, slight topographic inequalities in the shelf floor may result in horizontal velocity gradients so that the flow undergoes acceleration and deceleration in space as well as in time. Fluid dynamical theory predicts deceleration of flow across topographic highs as well as down their down-current sides. The coarsest fraction of the transported load (sand) will be deposited in the zone of deceleration, and deposition will occur throughout the flow event. Relatively thick sand deposits, 20 to 50 cm (8 to 20 in.) can accumulate in this manner. Enhancement of initial topographic relief results in position feedback; as the bed form becomes higher, it extracts more sand from the transported load during each successive storm. Individual storm beds may tend to fine upward (waning current grading), but the sequence as a whole is likely to coarsen upward, reflecting increasing perturbation of flow by the bed form as its amplitude increases. Stability theory suggests that the end product of these processes should be a sequence of regularly spaced sand ridges on the shelf surface. However, sand bodies are localized in stratigraphic position and lateral distribution within Cretaceous shelf deposits. Upward-coarsening sequences are a widespread phenomenon in the Western Interior Cretaceous System, and the sand bodies appear to constitute localized sand concentrations within more extensive sandy or silty horizons. Especially widespread upward-coarsening sequences appear to be due to the close coupling between activity in the overthrust belt to the west and sedimentation in the foreland basin. In the proposed sequence of events, a thrusting episode increases relief in the source terrane as well as the load on the crust. Sedime tation at first dominates over subsidence, and initially the shelf on the western margin of the basin becomes shallower. As it does so, intensified wave scour on the shelf floor increases the amount of bypassing, which results in the deposition of increasingly coarser sediment, culminating in a sandy horizon. As relief in the hinterland wanes, subsidence overtakes sedimentation and the shelf subsides. Renewed thrusting begins the cycle anew. In a second mechanism for the formation of upward-coarsening sequences, tectonic uplift affects parts of the shelf as well as the hinterland. The initiation of Sevier or Laramide structural elements beneath the shelf, and the remobilization of other, older structures, creates submarine topographic highs. These highs cause slight sand enrichment over broad sectors by means of the process described above. The development of sand-enriched areas on the shelf floor by both mechanisms leads to the flow-substrate feedback behavior that builds large scale, elongate bodies of clean sand. End_of_Article - Last_Page 556------------
Sedimentation
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