Swale Topography Research Articles

The Geomorphic Development of Wreck Shoal, a Subtidal Oyster Reef of the James River, Virginia

Wreck Shoal is a subtidal oyster reef located in the James River estuary, Virginia. This estuary has moved upstream and landward in response to rising sea level. The recent geomorphic history of Wreck Shoal is analyzed based on bathymetric records from the 1850’s to the 1980’s. The data indicate that the shallow oyster reef areas have lost elevation in the last 130 yr. This is attributed to intense harvesting activity during the last century. The late Holocene evolution of Wreck Shoal is developed based on the results of sub-bottom profiles and coring data. These suggest that the Wreck Shoal oyster reef has developed on the ridge and swale topography of a point-bar formed during the late Pleistocene epoch. Contemporary biodeposition processes on Wreck Shoal are evaluated. The results indicate that sediments of biogenic origin (fecal and shell material) potentially accumulate at rates in excess of 50 cm 100 years−1. A model for subtidal oyster reef development is proposed that accounts for sea level rise, biodeposition, and the harvesting activity of man. The model is verified with field observations of reef elevation and radiocarbon dates of oyster shell material. The implications of these results are that oyster reefs should be considered a renewable natural resource, and therefore managed accordingly in concert with the oysters.

Seasonal changes in beach topography and beach volume in Delaware

A beach study was conducted along a shore segment of the Delaware coast from March 1982 through to April 1983. Results indicate that beach topography and beach volume varied with changes in wave-climatic regimes; the regimes in turn were generally in phase with the winter and summer seasons. Because both the magnitude and frequency of storm waves were higher during winter than in summer, the winter beach shape was concave upward. Following the winter storms of 1981–1982, swells rebuilt the beach in two stages from March through to August 1982. In the first stage, the beach vertically accreted (aggraded) and in the second, it laterally accreted (prograded). The aggrading stage involved the welding of a swash bar to the foreshore; thereafter, swash overtopping caused deposition on and landward of the bar. In time, the swash bar attained a berm and swale topography. The prograding stage started when the swash could no longer overtop the berm crest, which confined deposition to the foreshore. This stage occurred following a minor or moderate storm event that shifted the berm crest upward and landward, or during a neap tidal stage that occurred after the construction of a spring tide berm. Between March and the end of August 1982, 60 m3 of sediments per 1 m of beach were deposited on the shore. Of this value, 67% was attributed to aggradational processes, while the remaining 33% was attributed to progradational processes. Swells as well as storm waves caused vertical accretion; storm waves contributed 20% of the total sediment volume deposited on the beach. When the shore was at its maximum sediment volume for 1982, the beach topography was convex upward.An inverse relationship existed between the mean monthly sediment volume and the mean monthly surge variability. The surge level was used as an index of wave conditions; high surge levels were associated with large waves while low surge levels were associated with small waves. When surge variability was high during the winter, sediment volume on the beach was low, and conversely, when surge variability was low in summer, sediment volume was high. Correlation analysis showed that 80% of the variance of the mean monthly sediment volume was explained by the mean monthly surge variability.

Reappraisal of a 2200 Ma-old paleosol near Waterval Onder, South Africa

Abstract A fossil soil (paleosol) about 2200 Ma-old in a deep roadcut between the towns of Waterval Boven and Waterval Onder (Transvaal), here called the Waterval Onder clay paleosol, was formerly reported to have formed on the Hekpoort Basalt. It is here thought to have developed on a thick (formerly at least 5.5 m), upward-fining and thinning sequence of sandstone, siltstone and shale derived from pre-existing soils on Hekpoort Basalt, within the basal portion of the fluvial Dwaal Heuvel Formation. In contrast to paleosols developed on Hekpoort Basalt, the Waterval Onder clay paleosol is little deformed, lacks corestones, and shows compositional variation and berthierine-rich laminae interpreted as relict bedding. The paleosol has a ridge and swale topography (linear or wavy gilgai) and a subsurface structure (mukkara) typical of modern Vertisols. Since there is considerable relict bedding, it probably formed over a short period of time (less than 7000 years) and was not ‘self plowing’ to the extent seen in modern Vertisols. Clastic dikes filled with sandstone from the overlying rocks, are evidence that the profile was periodically dry to depths of at least 2 m. More drab colored rocks below that may represent the level of permanent water table. Iron and manganese stain of layers and claystone granules within the surface horizon of the profile have some characteristics of modern rock varnish, and could be taken as evidence for life in the soil. The high ratio of ferric to ferrous iron near the surface of the paleosol, together with evidence from clastic dikes that this horizon was not waterlogged, is an indication of an oxidizing atmosphere, although probably one in which oxygen was much less abundant than it is today. The very low amounts of acid titratable bases in the shaly parent material of this paleosol and surficial loss of these bases within the paleosol, appear to have been produced by decarbonation reactions comparable to those found today in soils of vascular land plants. A likely source of weathering acid during the Precambrian is an elevated abundance of atmospheric carbon dioxide. Considering these likely atmospheric differences, together with the distribution of comparable modern Vertisols and other geological evidence, the climate under which this soil formed was probably semiarid to subhumid, seasonally dry and temperate.

Carbonate Comparison of West Florida Continental Margin with Margins of Eastern United States: ABSTRACT

Temperate carbonate margins may have as many similarities to clastic margins as to other carbonate systems. An example is the west Florida continental margin north of Florida Bay, a vast area of more than 150,000 km/sup 2/. The facies of this area differ from those of other Holocene carbonates, such as the Bahama Banks, the Great Barrier Reef, and the Caribbean and Pacific bioherms. The west Florida margin is analogous to the predominantly clastic southeastern US in both physiology and sedimentary processes. The shelf facies is a veneer of carbonate sand, primarily molluscan shell fragments, with low sedimentation rates. It is similar to the southeastern US sand veneer with the clastic component removed. Like the US system, the west Florida shelf has a ridge and swale topography replete with sedimentary structures, such as sand waves, with a series of drainage systems incised into its surface at lower stands of sea level. On the outer edge, it is commonly bounded by outcrops with considerable positive relief. The upper slope of the west Florida margin is a calcilutite, a Holocene chalk deposit accumulating at rates of tens of centimeters/1000 years, comparable to the clastic lutite depositional rates of the eastern US continentalmore » slope, and two orders of magnitude higher than deep-sea oozes of similar composition. These relatively high rates are probably caused by fines pumped from and across the coarser shelf-sand sheets in both systems.« less

Late quaternary stratigraphy and environments of the west Mexican coastal plain

Abstract The geomorphic events that formed the west Mexican coastal plain and the stratigraphy and chronology for those events based on soils, pollen zones, and radiocarbon ages are discussed. Three phases of geomorphic development are recognized. The Alluvial Phase represents deposition of the coastal bajada during the Neogene; oxisols and saprolites have formed on this surface. The Teacapan Peninsula Phase began during the Sangamonian Inter‐glacial with deposition of the spit; an ultisol and an alfisol have developed on the peninsula. In the Prograding Beach Phase, beginning about 13,000 B.P., four sequences of beach ridges were deposited during eustatic sea level rise and periodic uplift. On the resulting ridge and swale topography are four soil units, three inceptisols on the initial, early, and middle beaches and an entisol on the late beach. The pollen stratigraphy and chronology are based on samples from eleven cores, twelve radiocarbon dates, and calculated deposition rates. The alternation in dom...

Evolution of a classic sand ridge field: Maryland sector, North American inner shelf

ABSTRACTThe ridge and swale topography of the Middle Atlantic Bight is best developed on the Delaware‐Maryland inner shelf. Here sand ridges can be seen in all stages of formation. Several aspects of the ridge field are pertinent to the problem of ridge genesis. The first is ridge morphology. There is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges, which reflects the changing hydraulic regime. As successively more seaward ridges are examined, maximum side slope decreases, the ratio of maximum seaward slope to maximum landward slope decreases, and the cross‐sectional area increases. These changes in ridge morphology with depth and distance from shore appear to be equivalent to the morphologic changes experienced by a single ridge during the course of the Holocene transgression.A second aspect is the change in bottom sediment characteristics that accompanies these large‐scale morphologic changes. Megaripples, sand waves and mud lenses appear in the troughs between nearshore and offshore ridges. These changes indicate that the storm flows which maintain ridges are less frequently experienced in the deeper sector, and that the role of high‐frequency wave surge becomes less important relative to the role of the mean flow component in shaping the sea‐floor.A third aspect is the systematic relationship of grain size to topography. Grain size is 90° out of phase with topography, so that the coarsest sand lies between the axis of the landward trough and the ridge crest, while the finest sand lies between the ridge crest and the axis of the seaward trough. This relationship is characteristic of large‐scale bedforms.Finally, flow was measured and transport calculated on the same ridge during a one‐month period (November 1976). Threshold was exceeded only during storm events. Mean transport was southerly and a little seaward with respect to both the ridge crest and the shoreline. These flow measurements are in conformity with the pattern of smaller bedforms. A 43‐year time series of bathymetric change for this ridge reveals a systematic pattern of landward flank erosion, seaward flank deposition, and seaward crest migration.Sand ridges are considered the consequence of constructive feedback between an initial topography and the resulting distribution of bottom shear stress. The relationship between grain size and topography supports this model, but does not account directly for the oblique angle of the ridge with respect to the coastline. This feature may be due to a more rapid alongshore migration rate of the inshore edge of the ridge than the offshore edge, and the relationship between this migration rate, and the rate of shoreface retreat.

Current structure on the Long Island inner shelf

Current velocity records from a deployment of 17 current meters across a ridge and swale topography with a 5 m amplitude, 6.5 km off the coast of Long Island, were analyzed to determine the interaction between the flow field and the topography which might maintain these special bottom features. No first‐order interactions were found; the flow field was generally parallel everywhere. Correlation, coherence, and empirical orthogonal modal (EOM) analysis showed no significant differences between meters as a function of alongshore or cross‐shore separation. EOM analysis showed that 79% of the variance was in the first barotropic mode with a strong vertical shear that was probably related to friction. Tide and wind response models can replicate about 64% of the variance in the alongshore velocity, of which about 30–40% is tidal. The residual record, uncorrelated to wind or tide, shows the effect of alongshore variations in the pressure field following a storm event by reversing the flow opposite to the diminishing wind stress. By regression of the averaged alongshore bottom velocities to wind stress, a linear bottom friction coefficient of 0.14 cm s−1 is found, which is in agreement with previous estimates, and a time average alongshore pressure gradient of 0.3 × 10−7 is found which is an order of magnitude smaller than previous estimates. By scaling the results of the wind response model, the linear bottom friction coefficient is also found to be 0.14 cm s−1.

Morphology and Processes Associated with the Accumulation of the Fine-Grained Sediment Deposit on the Southern New England Shelf

ABSTRACT A 13,000 km2 area of the southern New England Continental Shelf which is covered by anomalously fine-grained sediment has been surveyed by means of high-resolution, seismic-reflection and side-scan sonar techniques to map its morphology and structure, and a near-bottom instrument system contributed to understanding present activity of the deposit. Seismic-reflection profiles show that the fine-grained deposit, which is as much as 13 m thick, has accumulated during the last transgression because it rests on a reflector that is geomorphically similar to and continuous with the Holocene transgressive sand sheet still exposed on the shelf to the west. The ridge and swale topography comprising the sand sheet on the shelf off New Jersey and Long Island are relict in origin as the e same features are found buried under the fine sediment deposit. Southwestward migrating megaripples observed on the sonographs in the eastern part of the deposit are evidence that sediment is still actively accumulating in this area. In the western part of the deposit, where surface sediment is composed of silt plus clay, evidence of present sediment mobility consists of changes in the near-bottom, suspended-matter concentrations primarily associated with storms. Nantucket Shoals and Georges Bank are thought to be the sources for the fine-textured sediment. Storms and strong tidal currents in these shoal areas may still erode available fine-grained material, which then is transported westward by the mean drift to the southern New England Shelf, where a comparatively tranquil environmen permits deposition of the fine material.

Holocene Stratigraphy of Rapidly Transgressive Barrier Island, Cape Romain, South Carolina: ABSTRACT

Cape Romain, South Carolina, a barrier island-cuspate foreland just south of the Santee delta, shows a classic transgressive stratigraphic sequence. The island has eroded landward 150 m in the past 30 years. Today, 75% of the island shoreline is manifest as narrow washover fans and coalesced terraces migrating landward under the influence of normal spring high tides and storm events. The remainder of the island is occupied by eroding ridge and swale topography. Landward of the barrier is a lagoon-marsh system 11 km wide which abuts and overlies the Pleistocene mainland. Using a modified concrete compactor, 40 vibracores 5 to 9 m long were taken from three transects across the island. The coring system permits an undisturbed (preserved primary sedimentary structures and bioturbation) core 7.6 cm in diameter to be removed from any unconsolidated sediment. Cores have been taken in compacted clays, fine to coarse sands, shell lags, and oyster reefs. Recovery ranges from 90 to 100%. Additional data were obtained from box cores and short (1 to 2 m) vibracores taken by divers in specific subtidal locations. Eight meters under the present berm crest is a relict barrier-marsh complex overlain by a shell storm lag. Overlying the shell lag are flaser-bedded sands and muds of a very shallow-water high-energy lagoonal facies which grades upward into highly bioturbated sandy muds of a deeper water lagoonal facies. A tidal-flat oyster reef complex lies above, grading upward into a fringing marsh. The sequence is capped by washover sands and shells, beachface sands, and/or dunes of the present barrier. This sequence indicates a general rise in sea level during the Holocene through a combination of eustatic rise and a relatively high subsidence rate. The sequence shows a barrier island overtopped by the rising sea level. At some later time, another barrier was developed seaward on the shelf. This barrier transgressed landward to occupy the present position of Cape Romain. End_of_Article - Last_Page 518------------

Preliminary results of coincident current meter and sediment transport observations for wintertime conditions on the Long Island Inner Shelf

We have observed late fall and winter bedload sediment transport and the overlying current field in ridge and swale topography on the inner continental shelf south of Long Island, and can report movement of bed material at a water depth of 20 m to a distance of approximately 1500 m after several storm events. Movement over an 11‐week observation period was longshore and oblique to the ridge crest at the experimental site. Currents were also predominately longshore, but long term averages demonstrate that a vertical shear existed in the fluid motion. Although the number of sediment transport "events" suggested by the current meter data is nearly balanced in eastward and westward directions, both estimates of transport from current speeds and sand tracer dispersion patterns show that several westward flowing events dominated the transport during a two and one‐half month period. A quantitative upper bound of 31 cm/sec on the threshold velocity for sediment movement in this size range is also set by the data.

Sediment Response to the Present Hydraulic Regime on the Central New Jersey Shelf

ABSTRACT Petrographic data, from vibracores and grab samples collected on the Central New Jersey Shelf, suggest a substrate still actively responding to the hydraulic regime. Radiocarbon dates of shell material from the ridge and swale topography indicates aggradation of the ridge's crest during the last 500 years and exposure of earlier Holocene material in the deeper troughs of the area. The samples from both the cores and the surficial samples were investigated for heavy mineral percentages and grain size analysis in addition to radiocarbon dating. The concentration of heavy minerals into disseminated bands, as observed in the vibracores, is compatible with sediment transport by sand ripples on the ridge's flanks. The grain size variation was subjectively analyzed by applying a Q-mode facto analysis which produced three distinct groupings of the grain size distribution. Each grouping is found to characterize a particular part of the ridge topography. Fine sand and moderate sorting occurs on the flanks, medium to fine sand and moderate sorting occurs on the crests whereas two populations are found in the troughs; coarse, poorly sorted sands and very fine, well sorted sands. This textural variation supports a hypothesis of up-flank rheologic and suspensive transport of medium and fine sand during intense storms and subsequent down-flank winnowing of fine sand during less intense meterological events. The radiocarbon dates indicate that size fractionation and heavy mineral concentrations are subsequent to isolation from a beach environment.

Ridge and swale topography of the Middle Atlantic Bight, Nnorth America: Secular response to the Holocene hydraulic regime

The ridge and swale topography of the Middle Atlantic Bight was originally interpreted as a relict strand plain whose ridges reflect stillstands of the returning Holocene sea. However, close examination indicates that the ridges appear to be instead longitudinal bed forms, responses to a regime of intermittent, south-trending storm currents. Ridges may be initiated on the shore face and detached as the coast retreats to form fields of isolated ridges, or they may be molded into the shelf-transverse sand massifs that mark the retreat paths of littoral-drift depositional centers at estuary mouths and off cuspate forelands. The ridge and swale topography is thus a stable end configuration toward which a variety of near-shore constructional topographies have converged during the Holocene transgression. Morphologic evidence for readjustment of ridge topography to the deepening shelf flow field during the Holocene transgression is discernable. However, the extent to which the offshore topography continues to respond to hydraulic regime is unclear. The role of helical flow structure in the storm flow field remains to be documented. Resolution of these problems will require more detailed information of hydraulic process and substrate response on storm-dominated shelves.

Seismic reconnaissance of post-Miocene deposits, Middle Atlantic continental shelf—Cape Henry, Virginia to Cape Hatteras, North Carolina

A seismic reconnaissance survey was conducted to examine post-Miocene deposits on the Middle Atlantic Shelf between Cape Henry, Virginia and Cape Hatteras, North Carolina. Approximately 900 kilometers of continuous seismic reflection profiles were obtained, employing a 400-joule high resolution boomer system. The post-Miocene section unconformably overlies a furrowed and discordant Tertiary substrate. The basal unconformity appears to be of regional extent, and its configuration supports the presence of an eastward-trending ancestral Albemarle fluvial channel near the present mouth of Albemarle Sound. A basal sedimentary sequence of pre-Wisconsinan and Early Wisconsinan age is widespread, and comprises most of the post-Miocene section. It contains both transgressive fluvial deposits, and regressive coastal barrier lithosomes. An overlying sequence of Late Wisconsinan age was deposited in a regressive paralic-neritic environment. Holocene deposits, unconformably overlying the Pleistocene sequence, form a discontinuous sand sheet whose upper surface is molded into a north- or northeast-trending ridge and swale topography. The morphology of the outer shelf and shelf break is variable throughout the region. Promment wave-cut terraces reflecting Pleistocene glacio-eustatic movements are present, except in the southern third of the region. Most areas of the outer shelf exhibit ridge and swale systems which appear to be polygenetic. Variations in shelf break morphology suggest differential rates of Holocene sedimentation and shelf development. Shelf upbuilding and progradation are minimal near Cape Henry, and maximal near Cape Hatteras where Diamond Shoals may be actively advancing eastward across the shelf.

Anatomy of a shore face ridge system, False Cape, Virginia

The Middle Atlantic Shelf of North America is a broad sand plain, characterized by a subdued ridge and swale topography. Some ridges extend into or merge with the shore face. Three such ridges at False Cape, Virginia, trending southwest, have second order ridges on their flanks. Systematic asymmetry of secondary ridge crests away from major troughs, toward major ridges, indicates that the ridges are maintained by coast-parallel currents forming a pattern of helicoidal flow cells. Median diameters decrease to the southwest along the ridge system. In addition, grain size varies with topography. Troughs tend to be floored with a primary lag deposit of coarse, pebbly sand. Wave-winnowed crests consist of a secondary lag deposit of well-sorted, medium- to fine-grained sand. Flanks are fine- to very fine-grained sand winnowed out of the crests. Vibracores and pinger probe records reveal a three-fold stratigraphy. A basal unit consists of clayey fine-grained sand of probable Late Sangamon-Early Wisconsin age. An intermediate unit of relatively watery mud has yielded a radiocarbon date of 25,700 ± 800. The uppermost unit is the modern sand sheet into which the ridges are molded. The innermost ridge and trough appear to be actively forming in response to southtrending coastal “currents,” and the shelf tide during tropical and extra-tropical storms. The ridges appear to be moving westward, but it is not clear whether their longterm movement is more or less rapid than the rate at which the beach is retrograding. In any case, Bruun coastal retreat appears to be occurring, with sea floor aggradation at the expense of the adjacent shore face. In this manner the retreating coast is generating a shelf “relict” sand blanket with a ridge and swale topography. Coastal sediment movement has a southward component for fine sand through the ridge system, and may have, for coarser sand, a return north along the beach, by littoral drift. The ridge system may be moving south in response to headward erosion of the troughs.

Late Quaternary Stratigraphy of the Inner Virginia Continental Shelf: A Proposed Standard Section

A continuous seismic reflection survey indicates that the post-Miocene section of the inner Virginia Shelf is 27 m in average thickness and unconformably overlies a discordant Tertiary substrate. Vibracore data and a faunal analysis reveal three distinct sedimentary sequences separated by prominent unconformities. The oldest post-Miocene sequence has a radiocarbon age of more than 37,000 yrs B.P. It consists largely of a muddy, fine-grained sand and is characterized by lenticular stratification and prominent local channeling. It appears to represent a pre-Wisconsinan and early Wisconsinan assemblage, containing both a transgressive fluvial complex and a regressive coastal barrier complex. The overlying sequence has a radiocarbon age ranging from 25,700 ± 800 yrs, to possibly as young as 20,400 ± 850 yrs B.P. It is comprised primarily of mud, and is characterized by relatively uniform horizontal stratification. The assemblage appears to represent a regressive paralicneritic sequence developed at the end of the mid-Wisconsinan interstadial. The youngest sequence of the section has yielded a radiocarbon age of 4,200 ± 140 yrs B.P. It is comprised of a discontinuous sand sheet that mantles the modern sea floor, and which appears to have been generated through the shore face erosion of a retrograding coastal barrier complex during the Holocene transgression. Evidence suggests that it accumulated as a lag deposit on the sea floor, where it was subsequently molded into a ridge and swale topography by the Holocene hydraulic regime.

TEXTURAL DIFFERENTIATION ON THE SHORE FACE DURING EROSIONAL RETREAT OF AN UNCONSOLIDATED COAST, CAPE HENRY TO CAPE HATTERAS, WESTERN NORTH ATLANTIC SHELF

ABSTRACTIn order to evaluate a model of Holocene shelf sediment distribution requiring a nearshore modern sand facies and an offshore relict sand facies, we have undertaken a textural reconnaissance of the Virginia‐North Carolina Coast between Capes Henry and Hatteras. Grab samples were subjected to grain‐size analysis by means of a modified Woods Hole Rapid Sediment Analyser. Textural provinces were erected with the aid of factor vector analysis. These include medium‐grained sands of the beach and surf zones; seaward fining, fine‐grained sands of the shore face, and heterogenous sands of the sea floor. In this latter province, grain size is controlled by a ridge and swale topography, with coarser sand on the crests. Coast‐wise grain‐size trends on the beach and shore face can be explained by assuming that wave heights increase toward the south, and that the Pleistocene sediment source is exposed higher on the shore face in the north than it is in the south. The shore face is retrograding, except in the vicinity of Diamond Shoals. There is textural evidence for a former Albemarle River channel, which bisects the study area. A model for sediment fractionation on a retreating barrier coast with low sediment input is proposed, based on studies which indicate that on such coasts: (1) barrier superstructures retreat more or less continuously by upper shore‐face erosion and storm washover; and (2) lower shore‐face erosion results in an equal‐volume aggradation of the adjacent sea floor, and forms the leading edge of the Holocene transgressive sand sheet. The nearshore “modern” sands and offshore “relict” sands are both present in the study area, but the terms are unnecessarily restrictive. Both are “relict” in the sense of being derived from a Pleistocene substrate, and both are “modern” in the sense of having undergone adjustment to a modern hydraulic regime. While modern and relict are useful general terms, it is convenient in this area to refer to a Holocene barrier sand prism, versus a Holocene transgressive sand sheet.

Western Tasman Sea Floor

Abstract The western Tasman Sea floor is characterised by physiographic units that tend to parallel the coastline of south-eastern Australia. The margin has a narrow continental shelf and a steep continental slope. Sediment derived mainly from northern New South Wales and Queensland in the north, and from Tasmania and Victoria in the south, has built a system of fans forming the lower continental slope, continental rise, and Tasman Abyssal Plain. Areas of swale topography occurring along the distal margin of the 1,200-mile-long and 60—120-mile-wide Tasman Abyssal Plain represent regions where density-current-transported muds have overflowed on to abyssal hill topography and around the bases of seamounts. One to 2 km of flat-lying sediments form the abyssal plain and consist of graded beds of calcareous and terrigenous sand, silt, and clay in the uppermost 2–4 m. Calcareous ooze covers the Lord Howe Rise, the tops of seamounts, the continental slope of eastern Australia, and the abyssal hills at depths les...