Sets Of Beach Ridges Research Articles

Architecture of Shelly beach ridges in a sheltered beach environment in Punta Perdices beach, San Antonio bay, Argentina

On the northern coast of the San Matías Gulf, Argentina, San Antonio Bay is developed, with an approximate area of 160 km2. The region has a diversity of depositional systems, both inside and outside the bay, which are morphologically characterized by the deposits of plains and tidal channels, beach ridges, beaches and dunes. In this environment relatively protected by the Villarino Peninsula, against the high energy of waves coming from the gulf, the circulation of water is governed by a regime of semidiurnal macro-tides with syzygy amplitudes of up to 9 m, which enter through the single channel, communicating the bay to the ocean. The edges of the Villarino Peninsula have a large number of beach ridges, in the form of extensive modern bioclastic deposits, where bivalves with whole, fragmented, and gastropod valves predominate. The objective of this work is the development of models of depositional architecture of the coquinas of the Villarino Peninsula, through terrain mapping (DGPS) and acquisition of lines by ground penetration radar (GPR), in addition to understanding the Holocene-modern sedimentary dynamics of this region. Surveys with GPR were carried out in the sector of Punta Perdices Beach in the Peninsula Villarino sectors of bioclastic deposits, in a total of 23 GP R lines, where there are several sets of beach ridges (Holocene) with an average width of 100 m, which extend for more than 1 km, located in the sheltered area of the bay. The processed and interpreted GPR data were used to investigate the lateral and vertical relationships of the facies, the depositional geometries and geophysical signatures, developed by progradation mechanisms induced by the hydrodynamics of waves and tides. The genesis of these a lot of bioclasts deposits are discussed, mainly due to the macro tide regime of the region and the littoral drift of the semicircular bay, where the geomorphological features of an actual coastal terrace were described. Depositional models of coquinas in Pleistocene, Holocene, and actual beach environments are scarce in the literature. The obtained results can be analyzed and compared to analogues of hydrocarbon deposits.

Coastal barrier morphogenic categories in Mississippi Delta plain development

Coastal barriers are defined as shore-parallel, aggraded-prograded supratidal beach ridge sets and individual single ridge-landforms, composed of sand and/or gravel. Island, spit, and mainland barriers represent the three major topographic subdivisions. In terms of bathymetric and hydrographic characteristics, bottom sediment types and associated habitats, barriers separate the marine offshore from inshore facies. This definition is not uniformly accepted and applied in the literature. Barriers impose reduced salinities inshore, diminish storm impact and moderate inland inundation, protect critical habitats and may mark ancient longshore transport directions, shore and sea-level positions. A wide range of dimensions, topography, and other factors characterize mainland strandplains. Shoal aggradation appears to be the prime island-forming mechanism, followed by accretion around semi-consolidated, elevated cores. Smaller islands may also form by barrier spit fragmentation. Detachment of dune ridges from the mainland shore is an often cited but at best unlikely mode of island genesis. Review of the literature and remote-sensed images created new insights, utilized barrier definitions and genetic theories, as applied in the Mississippi Delta Complex. A three-stage concept, involving dune ridge or barrier spit-detachment by marine encroachment and landward shift of fragmented transgressive islands forms the basis of a recent invited Marine Geology research article. That model assumes spit-growth from delta headlands, followed by splintering into landward-migrating islands. Submerged, they become permanent shoals. For several reasons, this “island-cycle” hypothesis is questionable and a reassessment of the geomorphic and stratigraphic context of this delta coast is required.Contradicting the geomorphic setting of assumed Stage 1, key component of the cycle-model, evidence is lacking for subrecent delta lobes and erosional headland in spawning barrier islands. In addition, the delta headland-flanking islands reveal only progradational (regressive) development and in situ disintegration,. Transgressive landward migrations is known only from the Chandeleur chain's prehistory. Sand from decaying Terrebonne-Lafourche 6 Subdelta headland was the dominant source for the Caminada Strandplain and for the shoal /or island predecessor of Grand Isle. Wave refraction around an inferred Lafourche delta lobe (dated provisionally between ∼2900 - ∼ 2300 yrs. BP) and its decayed erosional headland remnant resulted in diametric reversal of littoral sand transport. An earlier proferred ∼750–300 yrs. BP Caminada Strandplain age range is now replaced by a loosely defined ∼2200- to- ∼800 yrs. BP value, based on ages of Grand Isle and reworked delta hinterland sediments incorporated into the Strandplain.

A 5700-year-old beach-ridge set at Cape Canaveral, Florida, and its implication for Holocene sea-level history in the southeastern USA

The Cape Canaveral Peninsula is the largest Holocene coastal sand deposit composed of beach ridges on the Atlantic coast of Florida. It is composed of 16 beach-ridge sets that are separated by erosional surfaces. Despite its prominence as a Holocene coastal depocenter, there are a limited amount of chronological data constraining the timing of its formation. In this study, we apply optically stimulated luminescence (OSL) dating on sand-sized quartz and radiocarbon dating on individual marine shells to develop a refined chronology of the Cape Canaveral beach-ridge plain with particular focus on constraining the depositional age of the northwesterly-most, and geographically oldest, beach-ridge set on the peninsula. We obtain an average OSL age of 5680 ± 240 years ( n = 4) for the initiation of coastal deposition at Cape Canaveral. The new ages, and the organization of beach ridges into 16 distinct sets indicates that the Cape Canaveral beach-ridge plain experienced an ~5700-year history of alternating deposition and erosion, with 75% of present-day Cape Canaveral (Beach-ridge Sets 5–16) deposited over the past 2000 years and Beach-ridge Sets 8–16 comprising 50% of the area over the past 1000 years. Because the minimum swale elevations of the ~5700-year Beach-ridge Set 1, and those of all the younger beach-ridge sets, are within several decimeters of present-day mean higher high water, we hypothesize that all the beach ridges present at Cape Canaveral could have been deposited at or within decimeters of present-day sea level. There is no evidence for Holocene “highstand” events over the past 5700 years in the published sea level curves from northeast and south Florida, which are based on subsurface estuarine foraminifera/leaf litter and mangrove peat data, respectively. This dichotomy illustrates the need to integrate both subaerial and subsurface data to produce a more realistic Holocene sea-level curve for the southeastern United States.

Effects of Holocene climate changes and anthropogenic river regulation in the development of a wave-dominated delta: The São Francisco River (eastern Brazil)

The São Francisco River is the fourth longest river in South America and one of the most regulated. Severe coastal erosion has affected the delta shoreline since 1985, leading to the complete destruction of Cabeço village between 1997 and 1999. In this study, we mapped and radiocarbon dated the beach ridge sets occurring on the delta plain and performed a detailed analysis of the delta shoreline changes since 1960. During the Holocene, the delta plain construction was punctuated and took place during episodes of higher river discharges coincident with Bond events 4, 2 and 1 and periods of higher precipitation in the river basin, as reconstructed by δ18O measurements in cave speleothems. The last major episode of delta construction apparently ended at approximately 1.0 ka cal. BP. Since that time, riverine sediment input has been just sufficient to maintain the shoreline. A comparison of historical maps and aerial photographs showed that from 1853 to 1960, the shoreline at the river mouth remained in approximately the same position. A decrease in rainfall in combination with river regulation, particularly after 1985, triggered extensive erosion at the delta shoreline. This erosion was not caused by sediment retention behind the major dams but instead resulted from changes in the backwater/drawdown effects deriving from river regulation. Shoreline erosion mostly affected the river mouth. The mobilized sediments caused progradation of the downdrift shoreline. Updrift of the river mouth, the shoreline remained stable, as it had already reached an equilibrium orientation in which the net longshore transport was zero.

Comparative morphosedimentary framework of small subtropical islands offshore Kuwait

Five small islands offshore of the State of Kuwait in the northern Arabian Gulf (Miskan, Awhah, Kubbar, Qaruh, and Umm Al-Maradem) occur in differing structural settings and have evolved in a dynamic environment of shifting fluvial depocenters, fluctuating Holocene sea level, and variable tidal conditions. As a first step in reconstructing the origin and evolution of the islands, bathymetric charts, historical photographs, topographic profiles, and sediment samples were used to investigate their geomorphology and sedimentary environments. The subaerial portion of the islands varies in area from 0.04 to 0.57 km2. The plan-shape ellipticity ranges from nearly circular (0.10) to oval-shaped (0.54). The northern two islands (Miskan and Awhah) experience mesotidal conditions and are situated on a shallow platform (<5 m below mean sea level), which is part of a larger Failaka Island structure. In contrast, the three southern sites have a microtidal regime and sit atop small isolated platforms of aggrading perched reef origin, which descend steeply to more than 20–30 m depth within 3 km of the islands. Topographic profiles indicate that island interiors are composed of beach-ridge sets, whereas modern shoreline consists of segments of beachrock platforms alternating with depositional salients and flying spits in areas of longshore transport convergence. The proportion of total shoreline length represented by exposed beachrock varies from 4 to 46%. Beach sediments consist of very coarse to medium sand and variable sorting, primarily due to the degree of attrition of bioclastic material. A simple conceptual model is proposed to describe the general evolutionary trajectory of end-member islands. Both their accretion during the Late Holocene sea-level stillstand and decadal-scale persistence of sediment transport convergence sites (salients) suggest a morphosedimentary stability of the islands in the near future.

Geomorphic origin of Merritt Island-Cape Canaveral, Florida, USA: A paleodelta of the reversed St. Johns River?

The Merritt Island-Cape Canaveral (MICCSC) sedimentary complex consists of a series of adjacent, non-conformable, beach ridge sets that suggest a multi-phase constructional history, but the feature's geomorphic and sedimentary origins are not well-understood. In spite of its notable sedimentary volume (surface area = 1200 km2), the MICCSC lacks a clear sediment source, or supply mechanism, to explain its presence today. Previously published U/Th, radiocarbon and OSL dates indicate that beach ridge deposition was active during MIS 5 (130–80 ka) on Merritt Island, but has occurred over a shorter, younger time interval on Cape Canaveral proper (6 ka to present). In this paper, it is proposed that the MICCSC is an abandoned paleodelta whose fluvial source provided a sediment supply sufficient for coastal progradation. Although the MICCSC, today, does not receive an appreciable sediment supply, the nearly 23,000 km2 drainage basin of the St. Johns River may well have provided such a sediment supply during MIS 5 times. This low-gradient fluvial system currently empties to the Atlantic Ocean some 200 km north of the MICCSC (near Jacksonville, Florida) but may have flowed southward during the time of MICCSC sedimentary construction, then experienced flow reversal since MIS 5 times. Three possible uplift mechanisms are proposed to explain the northward down-tilting that may have reversed the flow direction of the St. Johns, abandoning deltaic construction of the MICCSC: (1) karst-driven, flexural isostatic uplift in response to carbonate rock dissolution in central Florida, (2) glacio-hydro-isostatic tilting/back-tilting cycles during loading and unloading of the Laurentide ice sheet during the Pleistocene, and (3) mantle convection-driven dynamic topography operating within southeastern North America since the Pliocene. This example testifies to the sensitivity of low-gradient, low-relief landscapes to various sources of uplift, be they isostatic or otherwise.

Geomorphic response to sea level and climate changes during Late Quaternary in a humid tropical coastline: Terrain evolution model from Southwest India.

The coastal lands of southern Kerala, SW India in the vicinity of Achankovil and Thenmala Shear Zones reveal a unique set of geomorphic features like beach ridges, runnels, chain of wetlands, lakes, estuaries, etc. The chain of wetlands and water bodies that are seen in the eastern periphery of the coastal lands indicates the remnants of the upper drainage channels of the previously existed coastal plain rivers of Late Pleistocene age that are later broadened due to coastal erosion under the transgressive phase. The terrain evolutionary model developed from the results of the study shows that the Late Pleistocene transgressive events might have carved out a major portion of the land areas drained by the coastal plain rivers and as a result the coastal cliff has been retreated several kilometers landwards. The NNE—SSW trending beach ridges located close to the inland wetlands indicate the extent of shoreline shift towards eastwards during Late Pleistocene period. The present beach parallel ridges in the younger coastal plain indicate the limit of the Mid Holocene shoreline as the transgression was not so severe compared to Late Pleistocene event. The zone of convergence of the two sets of beach ridges coincides with the areas of economically viable heavy mineral placers that resulted from the size and density based sorting under the repeated transgressive events to which the coast had subjected to. The chain of wetlands in the eastern side of the study area has been evolved from a mega lagoon existed during Late Pleistocene. The Pallikkal River that links discrete eastern wetland bodies has been evolved into its present form during Early Holocene.

Progradational and backstepping shoreface deposits in the Ladinian to Early Norian Snadd Formation of the Barents Sea

AbstractShoreface sandstone deposits within the Early Carnian part of the Snadd Formation of the Norwegian Barents Sea can be traced for hundreds of kilometres in the depositional strike direction and for tens of kilometres in the depositional‐dip direction. This study uses three‐dimensional seismic attribute mapping and two‐dimensional regional seismic profiles to visualize the seismic facies of these shoreface deposits and to map their internal stratigraphic architecture at a regional scale. The shoreface deposits are generally elongate but show variable width from north‐east to south‐west, which corresponds to a sediment source in the northern part of the basin and a southward decrease in longshore sediment transport. The Snadd Formation presents an example of how large‐scale progradational shoreface deposits develop. The linear nature of its shoreface deposits contrasts with more irregular, cuspate wave‐dominated deltaic shorelines that contain river outlets, and instead implies longshore drift as the main sediment source. In map view, discrete sets of linear features bounded by truncation surfaces scale directly to beach ridge sets in modern counterparts. The shoreface deposits studied here are characteristic in terms of scale and basin‐wide continuity, and offer insight into the contrast between shallow marine deposition under stable Triassic Greenhouse and fluctuating Holocene Icehouse climates. Findings presented herein are also important for hydrocarbon exploration in the Barents Sea, because they describe a hitherto poorly understood reservoir play in the Triassic interval, wherein the most prominent reservoir plays have so far been considered to be found in channelized deposits in net‐progradational delta‐plain strata that form the topsets to shelf‐edge clinoforms. The documented presence of widespread wave‐dominated shoreface deposits also has implications for how the relative importance of different sedimentary processes is considered within the basin during this period.

Evolution and morphodynamics of a prograded beach‐ridge foreland, northern baffin island, canadian arctic archipelago

Landward retreat (marine transgression) is a common response of coastal systems to rising relative sea level. However, given sufficient sediment supply, the coast may advance seaward. The latter response of gravel barriers has been recorded in parts of southeastern and northwestern Canada, where seaward‐rising sets of beach ridges are observed in areas of Holocene RSL rise. Cape Charles Yorke, northern Baffin Island, is a 5 km long gravel foreland characterized by seaward‐rising beach‐ridge crest elevations. The prograded morphology of the Cape Charles Yorke foreland is a prime example of coastal response to a combination of rising RSL and abundant sediment supply, an unusual and little‐documented pattern in the Canadian Arctic. The main gravel supply to Cape Charles Yorke is likely from eroding bedrock and raised marine deposits southwest of the foreland. Although not the dominant sediment source, the Cape Charles Yorke delta contributed to the formation of the foreland by sheltering it from easterly storm waves and providing an anchor point for the prograding ridges. The truncation of relict ridges by the modern shoreline suggests a recent regime shift from continuous deposition to predominant erosion. The cause and timing of this shift are unknown but could result from a recent dwindling in sediment supply, increased accommodation space, increased wave energy, and/or an accelerated rise of relative sea level.

Palaeohydrographic reconstructions from strandplains of beach ridges in the Laurentian Great Lakes

Abstract The current temporal and spatial context of water-level change, drivers of change, and possible future scenarios of the Laurentian Great Lakes is controversial. Palaeohydrographs are being constructed from measured subsurface elevations of palaeo-swash zones and modelled ages in strandplains of beach ridges that are preserved in embayments along the lakes’ edge. More than 800 elevations and 200 ages have been collected from 15 strandplains to construct site strandplain palaeohydrographs. Palaeo-beach elevations from whole strandplains or sets of correlative palaeo-beaches within strandplains are then used to establish an outlet palaeohydrograph for each lake. Adjusting strandplain palaeohydrograph elevations to account for glacial isostatic adjustment and refining age models help define the outlet palaeohydrograph. Common basin-wide water-level patterns and changes in outlet location or conveyance can then be interpreted. Systematic patterns of elevation and geomorphic/sedimentologic properties in individual, groups and sets of beach ridges in strandplains suggest that long-term patterns of water-level change and sediment supply occurred on decadal, centennial and millennial scales. Outlet palaeohydrograph construction for Lake Superior revealed discrepancies between geological and historical rates of glacial isostatic adjustment. These differences are currently being investigated using new data from Lake Huron.

Ground-Penetrating Radar Study of North Padre Island: Implications for Barrier Island Internal Architecture, Model for Growth of Progradational Microtidal Barrier Islands, and Gulf of Mexico Sea-Level Cyclicity--Discussion

Garrison et al. (2010) are the latest among several authors to claim significant Gulf level fluctuations in the last several millennia (see discussions in Otvos 2001, 2004, 2005a). The onus in their particular case rests on the rather unusual assumptions by Tanner (1992), who linked granulometric kurtosis values and his subjective age estimates of the progradation rate of a St. Vincent Island, NW Florida ridgeplain sequence to late Holocene climate and sea-level changes. The recurring major sea-level and shoreline shifts proposed by Garrison et al. can not be reconciled with the straightforward subsurface documentation presented. ### Proposed Evidence for Late Holocene Sea-Level Fluctuations Tanner (1992), on whose assumptions Garrison et al. base their sea-level chronology starts with three sweeping claims: (1) Essentially all St. Vincent strandplain ridge lithosomes were wave-built. Eolian contribution (so-called “dune decoration”) of unspecified thickness was not significant. (2) Low beach-ridge sets formed during lower sea levels; taller ones during highstands. Tanner provided no specific ridge elevation figures. Lacking corroborating dates of his own, he has arbitrarily assigned ridge-set ages by postulating a subjectively conceived rate of steady strandplain progradation. (3) Based on statistical moment-measure calculations, Tanner assumed a direct relationship between ridge sets of different average elevations, fluctuating late Holocene sea levels, and values of granulometric mean kurtosis from ridge sands. As foundation for their own contention, Garrison et al. (their Fig. 16) also repeat misstatements by Tanner (1992) that late Holocene highstands and lowstands are also reflected in Mississippi subdelta complexes. Frazier (1967), Otvos and Giardino (2004), Otvos (2005a), Tornqvist et al. (2004), Rogers et al. (2009), and others provided no stratigraphic base to support this contention. A similar assertion, equally unfounded, was applied to the SW Louisiana chenier plain as well (Tanner 1992). However, significant lateral elevation differences along these shell-dominated ridges are …

Tectonic Control of High-Frequency Holocene Delta Switching and Fluvial Migration in Lingayen Gulf Bayhead, Northwestern Philippines

Remotely sensed images, maps, charts, and historical accounts document the evolution of the Lingayen Gulf bayhead plain in the northwestern Philippines. Beach ridges and relict channel patterns record delta progradation and switching that, together with meander belt migration, constructed the bayhead plain. The latest delta switching occurred after 1935, when the downstream portion of the Agno River, the largest river discharging into the bayhead, was artificially diverted to a more direct route. The shoreline retreated in the abandoned delta; in contrast, paired sets of beach ridges that diverge toward the new river mouth formed a cuspate delta. In the more landward portions of the coastal plain, similar but older pairs of wedge-shaped beach ridge sets occur between more continuous to parallel sets that also truncate their apical ends. The pair of wedge-shaped sets was used to map paleodelta lobes; the continuous-parallel sets record transgressive events. Within 7 kilometers of the bayhead plain, at least 15 paleodelta lobes were alternately, successively, or simultaneously built by three river systems. These paleodeltas formed during the sea level fall from 2.4 ka to the present. Before this, westward tilting because of movements along the two faults that bound the alluvial plain and switching of the main distributary channel of the Agno alluvial fan caused the lower Agno River to migrate episodically to the southwest. Continued channel avulsions in the alluvial fan changed the sediment loads of the multiple river pathways created by meander belt migration, leading to delta switching downstream. Contemporaneous 220-year recurrences of delta switching and high-magnitude Philippine Fault earthquakes indicate tectonic control of the bayhead plain evolution.

Geomorphic and sedimentologic evidence for the separation of Lake Superior from Lake Michigan and Huron

A common break was recognized in four Lake Superior strandplain sequences using geomorphic and sedimentologic characteristics. Strandplains were divided into lakeward and landward sets of beach ridges using aerial photo- graphs and topographic surveys to identify similar surficial features and core data to identify similar subsurface features. Cross-strandplain, elevation- trend changes from a lowering towards the lake in the landward set of beach ridges to a rise or reduction of slope towards the lake in the lakeward set of beach ridges indicates that the break is associated with an outlet change for Lake Supe- rior. Correlation of this break between study sites and age model results for the strandplain sequences suggest that the outlet change occurred sometime after about 2,400 calendar years ago

Quantifying rates of coastal progradation from sediment volume using GPR and OSL: the Holocene fill of Guichen Bay, south‐east South Australia

AbstractGuichen Bay on the south‐east coast of South Australia faces west towards the prevailing westerly winds of the Southern Ocean. The bay is backed by a 4 km wide Holocene beach‐ridge plain with more than 100 beach ridges. The morphology of the Guichen Bay strandplain complex shows changes in the width, length, height and orientation of beach ridges. A combination of geomorphological interpretation, shallow geophysics and existing geochronology is used to interpret the Holocene fill of Guichen Bay. Six sets of beach ridges are identified from the interpretation of orthorectified aerial photographs. The ridge sets are distinguished on the basis of beach‐ridge orientation and continuity. A 2·25 km ground‐penetrating radar (GPR) profile across the beach ridges reveals the sedimentary structures and stratigraphic units. The beach ridges visible in the surface topography are a succession of stabilized foredunes that overlie progradational foreshore and upper shoreface sediments. The beach progrades show multiple truncation surfaces interpreted as storm events. The GPR profile shows that there are many more erosion surfaces in the subsurface than beach ridges on the surface. The width and dip of preserved beach progrades imaged by GPR shows that the shoreface has steepened from around 2·9° to around 7·5°. The changes in beach slope are attributed to increasing wave energy associated with beach progradation into deeper water as Guichen Bay was infilled. At the same time, the thickness of the preserved beach progrades increases slightly as the beach prograded into deeper water. Using the surface area of the ridge sets measured from the orthophotography, and the average thickness of upper shoreface, foreshore and coastal dune sands interpreted from the GPR profile, the volume of Holocene sediments within three of the six sets of beach‐ridge accretion has been calculated. Combining optically stimulated luminescence (OSL) ages and volume calculations, rates of sediment accumulation for Ridge Sets 3, 4 and 5 have been estimated. Linear rates of beach‐ridge progradation appear to decrease in the mid‐Holocene. However, the rates of sediment accumulation calculated from beach volumes have remained remarkably consistent through the mid‐ to late Holocene. This suggests that sediment supply to the beach has been constant and that the decrease in the rate of progradation is due to increasing accommodation space as the beach progrades into deeper water. Changes in beach‐ridge morphology and orientation reflect environmental factors such as changes in wave climate and wind regime.

Sea-Level Variation During the Holocene Deduced from the Morphologic and Stratigraphic Evolution of Morgan Peninsula, Alabama, U.S.A.

Abstract Recent Gulf of Mexico shoreline studies interpret middle to late Holocene sea level as falling from a level above present elevation or stable at present elevation; however, the architecture of Morgan Peninsula, Alabama, does not support this. Morgan Peninsula is a beach-ridge strandplain composed of two obliquely aligned Holocene beach-ridge sets. Ground-penetrating radar profiles discriminate between parallel, even to wavy reflectors of the eolian dune environment and the underlying seaward-dipping, complex sigmoidal-oblique reflectors of the foreshore and upper-shoreface environment. The contact between foreshore and eolian facies in beach ridges can be used as a sea-level indicator. The average elevation of this contact in Morgan Peninsula rises throughout shoreline accretion, which occurred throughout the last 5.4 ka, suggesting that there was continual sea-level rise during this time. Morgan Peninsula is a useful modern analog to ancient shoreface–shelf parasequences and demonstrates the significant internal complexities that can exist in these deposits. Erosional discontinuities imaged in the Holocene foreshore–upper shoreface environment are laterally continuous, extend to elevations above mean sea level, and correlate to beach ridges and the transition between beach-ridge sets. An increase in the wave regime or fluctuations in sediment supply appear to be likely mechanisms for forming erosional discontinuities below beach ridges. The erosional surface separating beach-ridge sets may have formed by increased storm activity and associated barrier breaching, or a reconfiguration of the Mobile Bay tidal-delta complex superimposed on a gradual rate of sea-level rise. This boundary is recognized by a change in beach-ridge orientation at the surface and an increase in the aggradational component of shoreline accretion.

Sequence morphodynamics at an emergent barrier island, middle Atlantic coast of North America

The southern Delmarva Peninsula is located along the middle Atlantic Coastal Plain of the United States. The axial highland of the peninsula formed in four stages of Pleistocene spit progradation. The landward shoreline of the peninsula is on the Chesapeake Bay. The seaside shoreline of the peninsula is on the Atlantic Ocean. The coast of the peninsula is composed of five landscape sections described as a headland, a left-hand spit, a right-hand spit, a wave-dominated barrier island, and tide-dominated barrier islands. Fisherman Island is a barrier island located at the southern end of the southern Delmarva Peninsula. The landscape features on Fisherman Island do not illustrate a direct linkage to (1) the sediment dispersion from the Delaware headland or (2) the influence of local antecedent topography. The island has a bipolar progradational history that is normal to the axis of the southerly sediment dispersion pattern from the Delmarva headlands. During the late Holocene, sea-level rise flooded the low-elevation land at the distal end of the southern Delmarva Peninsula. The submerged area formed a shallow platform in the entrance to the Chesapeake Bay. Two sediment dispersion tracts affected the development of this area. On the ocean side of the peninsula, sediment moved southward along the lower shoreface to the Chesapeake Bay entrance. On the west side of the peninsula, southerly moving bay currents also dispersed sediment to the entrance of the bay. The two tracts converged on the northern side of the bay entrance forming a broad sand shoal. Wave diffraction and refraction around the margins of the shoal “swept” sediment into linear sand bars that migrated back toward the peninsula. By the middle of the 19th century, the fusion of sand bars on the shoal surface produced a permanent nucleus for island development. Wave refraction caused wave crests to “wrap around” the island core producing separate easterly and westerly components of shore aggradation. The westerly aggradational history is recorded in closely spaced sets of beach ridges. The easterly aggradational history is recorded in broadly spaced hammocks.

Responses of Stable Bay-Margin and Barrier-Island Systems to Holocene Sea-Level Highstands, Western Gulf of Mexico

ABSTRACT The microtidal, wave-dominated coast of the western Gulf of Mexico displays a variety of Holocene geomorphic features indicating higher-than-present water levels that were previously attributed to storm processes while geoidal sea level was at its present position. Field and aerial-photograph examinations of bay margins, barrier islands, and beach-ridge plains following major hurricanes show that the elevated features are inundated periodically by high storm surge. Despite their inundation, these highstand features are not modified by modern storm processes. Instead, storm-related erosion and deposition are always seaward of and lower than the highstand features and are always limited to the extant shorezone, where elevations typically are less than 1.5 m above present sea level. Bay-margin and lagoonal highstand indicators include raised marshes and subtidal flats, wave-cut benches, abandoned wave-cut scarps with fringing marshes and/or beach ridges, and accretionary islands and recurved spits. Other emergent marine features include abandoned compound flood-tidal delta and washover fan complexes attached to barrier islands and anomalously high beach ridges within both the barrier-island complexes and beach-ridge plains. The highest beach ridges, raised marshes and flats, and erosional scarps and benches are manifestations of one or more rising phases and highstands in sea level, whereas the lower marshes and accretionary topography are mainly products of the falling phases and shoreface adjustment to present sea level. Different elevations of beach-ridge sets, discordant truncation of beach ridges, and elevated marine- and brackish-water faunal assemblages preserved in beach ridges, raised marshes and flats, and natural levees are compelling evidence of sea-level fluctuations of ±1 to 1.5 m from about 5500 to 1200 cal yr BP. Independent evidence from studies of geodynamic, climatic, and glacio-eustatic processes can explain the mid-Holocene highstands and late Holocene lowering of sea level that is observed in tectonically stable coastal regions far from former centers of glaciation.

Louisiana Cheniers: Settling from High Water

ABSTRACT There are two kinds of cheniers in Louisiana: one made largely of shell debris, the other mostly quartz sand. This report deals with the second. In other study areas, where swash-built beach ridges are made of quartz sand, suite-mean values of grain-size kurtosis show near-shore wave-energy level, and the extent to which settling from water was important. Kurtosis close to 3-3.5 indicates moderate-to-high wave energy density, and little or no settling. Kurtosis about 4 indicates low energy. As K rises above 4, the settling component increases. Louisiana cheniers have a very high kurtosis (10), and a standard deviation of kurtosis (9.5) far higher than any other sandy beach ridges now known (nothing else > 6). These numbers put the cheniers in a class with settling deposits (from waning stages of storms) just seaward of the surf zone. A plot of kurtosis against standard deviation (of the grain diameter) places the Louisiana examples with other known products of settling such as the horizontally-bedded settling-lag (not swash-built) beach ridges on Mesa del Gavilan, near the mouth of the Rio Grande (Texas). One narrow sandy swash-built beach ridge does not represent a single storm, but decades of slow deposition by the swash. Each ridge indicates a small slow rise-and-fall pair in sea-level, perhaps 5-40 cm. Beach ridges commonly occur in sets of 5 to 20 or more. Differences in the heights of ridge sets mark larger changes in sea level. High ridge sets represent a century or more of high sea level; low ridge sets indicate a century or more of low sea level. The vertical difference between set elevations is typically less, to much less, than three meters. The Louisiana cheniers -- except where splayed -- are like beach ridge sets, rather than like the individual swash-built beach ridges that are seen in other study areas.

The Holocene evolution of Rheban Spit, Tasmania. Age structure, geomorphic development and sediment characteristics

Rheban Spit is a small prograded sand barrier located on the east coast of Tasmania in the lee of Maria Island. It consists of four disconformable sets of beach ridges. Shallow drilling at seven sites towards the eastern end of the spit yielded sediment samples formineralogic and granulometric analyses and detrital shell for radiocarbon dating. The latter indicates that the three oldest sets of ridges were formed about 5500,4200 and 3 100 calendar years ago, the youngest set is known to have formed this century. This age structure is unusual for a coastal barrier in eastern Australia because most started to prograde at the end of the Postglacial Marine Transgression (c. 7000 calendar years ago) and ceased about 2000 years ago. Rheban Spit has been severely eroded by northeasterly waves during its forma tion, probably as a result of periods of increased storminess in the Tasman Sea. If the sand isthmus between north and south Maria Island was also removed during these episodes, wave refraction/ diffraction and sediment movement patterns near the eastern end of Rheban Spit would have changed significantly and this may account for the disconformable nature of the successive sets of beach ridges which form the spit.

BEACH RIDGES AND PALEOGEOGRAPHY, CENTRAL BAJA CALIFORNIA, MEXICO

Two spatially separated sets of beach ridges were studied in Baja California. Each set contains several well-preserved beach ridges and each is adjacent to an arroyo mouth. Modern beach ridges mark coastal location whereas abandoned paleobeach ridges record coastal change. Paleobeach ridges were dated by absolute and relative techniques for rate-change, chronology, and longshore correlation purposes. With time, shape becomes more planar, foreslope angle is reduced, and there is a progressive tendency for constituent shingle to disintegrate and develop both rinds and surficial roughness. One paleobeach ridge on the lowest emergent terrace was assigned a 1,150 ± 230 yr. B.P. radiocarbon date whereas another beach ridge 18 km farther north has a partially overlapping date. Older but undated beach ridges also exist on the lowest emergent terrace as well as on the highest persistent terrace. Those beach ridges on the highest persistent terrace are highly weathered and Pleistocene in age. All beach ridges have ...