Littoral Sand Research Articles

Microfossils and biofacies of the Vendian fossil biota in the southern Siberian Platform

Based on the study of the litho- and biofacies of the Vendian Nepa Horizon in the central area of the Siberian Platform inland, a paleoecological model for the Vendian microbiota has been developed. The sedimentation environments of the Katanga saddle have been reconstructed, and three sedimentary systems have been recognized: (1) lower continental, formed by the deposits of proluvial fans and riverbeds of temporary streams; (2) middle transgressive, made up of littoral sand facies in the lower part and of fine-clastic shelf strata in the upper part; and (3) upper, of sea highstand, composed of alternating sand bank facies and fine-clastic lagoon deposits. Four biofacies have been recognized in the fine-terrigenous deposits of the Nepa Horizon: (1) Appendisphaera, represented by a Doushantuo-Pertatataka acanthomorph assemblage; (2) Transitional, with a great diversity of plankton and benthic (including complex) taxa; (3) Vanavarataenia, dominated by Vanavarataenia complex benthic algae; and (4) Oscillatoriopsis, represented by taxonomically poor biotas with morphologically simple (mainly prokaryotic) remains. These biofacies are confined to the following sedimentation environments: Appendisphaera is widespread in the distal open-sea areas; the Transitional biofacies is localized in the distal environments of the semi-isolated inner basin; Vanavarataenia occurs in the proximal areas; and Oscillatoriopsis is typical of the shallow-water environments, both extended (corresponding to the highstand period) and local.

ANewMethod forZoning ofCoastalBarriersbased onHydro-geomorphological andClimateCriteria

This paper presents a new methodology for the zoning of littoral sand barriers on the basis oftheir groundwater dynamic responses to the local mean water level (MWL) climate. The method is based onthe comparison of the characteristic scales of the horizontal drainage and recharge processes. Using theBoussinesq equation and basic hydro-geomorphological parameters, this methodology identifies the climaticevents that markedly affect the groundwater dynamics across the barrier, along with the location of the mostaffected cross sections. The application of the methodology to the Mar Menor sand barrier shows that asignificant number of cross sections can be particularly vulnerable to the effects of local storm events typicalof the Mediterranean coast during the spring and winter seasons. The location of the affected cross sections isanalyzed as a function of both the local MWL forcing and hydrogeological parameters. This case studyhighlights the possible use of the proposed methodology for land use planning and the environmentalmanagementof coastal sand bars.

Dynamic stratigraphy of composite peripheral unconformity in a foredeep basin

AbstractThis study employs facies analysis and basic principles of sequence stratigraphy to correlate isolated outcrop sections and reveal the depositional history of the Chmielnik Formation – a prominent mid‐Serravalian clastic wedge formed on the basinward forebulge flank of the Polish Carpathian Foredeep. The coarse‐grained clastic wedge, up to 30 m thick and spanningca1·1 Ma within biozoneNN6, consists of fluvio‐deltaic, foreshore and shoreface deposits with a range of large littoral sand bars, all enveloped in muddy offshore‐transition deposits. Its dynamic stratigraphy indicates rapid shoreline shifts and environmental changes due to the interplay of forebulge tectonism, sediment supply and third‐order eustatic cycles. A similar interplay of tectonism and eustasy is recognizable in the whole middle Miocene sedimentary succession deposited on the forebulge flank, demonstrating an extreme case of an accommodation‐controlled shelf and indicating tectonic cycles of the forebulge uplift and subsidence spanningca800 to 900 ka. The episodes of forebulge uplift correlate with the main pulses of orogen thrusting. The resulting composite peripheral unconformity differs markedly from the idealized model of a ‘steady‐state’ stepwise onlap driven by forebulge continuous retreat. It is concluded that the foredeep peripheral unconformities, instead of being simplified in accordance with this idealized model, should rather be studied in detail because they bear a valuable high‐resolution record of regional events and give unique insights into the local role of tectonics, eustasy and sediment supply.

Ultra-long distance littoral transport of Orange sand and provenance of the Skeleton Coast Erg (Namibia)

Abstract Quantitative provenance analysis based on high-resolution bulk-petrography and heavy-mineral data on beach and dune sands, integrated with detrital-zircon geochronology and chemical analyses of pyroxene, garnet and staurolite, demonstrates that sand derived from the Orange River is carried by powerful and persistent longshore currents as far as northern Namibia and southern Angola, 1750 km north of its mouth. This is the longest cell of littoral sand transport documented so far. Compositional forward modeling indicates that ≥ 80% of dune sand in the Skeleton Coast is Orange-derived, the remaining ≤ 20% being supplied by slow erosion of the Damara Orogen chiefly via the Swakop River. A decrease in basaltic rock fragments and pyroxene with relative enrichment in garnet, staurolite, tourmaline and other metamorphic minerals north of Walvis Bay indicates that only one-third of beach sand in the 350 km dune-free gap between the Namib and Skeleton Coast Ergs is Orange-derived, the remaining two-thirds being supplied largely by the Swakop River draining the Damara Orogen. Although volcanic gravel becomes dominant in beaches of the Skeleton Coast north of the Uniab mouth, detritus from Cretaceous Etendeka lavas accounts for only 4% of beach sand, reflecting limited sand generation in the arid catchment. Contributions from the Kunene River to either dune or beach sands is ruled out, indicating that gem diamonds found in placer deposits along the coast of Namibia are all derived from the Orange River.

New Brazilian species of Archisotoma Linnaniemi, 1912 (Collembola: Isotomidae)

A new species of Archisotoma Linnaniemi, 1912 is described and illustrated based on material from littoral sand of Praia do Sossego beach, Niterói municipality, Rio de Janeiro State, Brazil. The new species Archisotoma arariboia sp.nov., the second of the genus described from Brazil, can be distinguished from its congeners by a set of characters: 6+6 eyes, maxillary capitulum unidentate and subequal to internal lamellae, 10 posterior chaetae on manubrium, 4–5 chaetae between the anterior trichobothria and lastly by 1 thick and curved chaetae on trochanter and femur of leg III and 1–2 on femur and 1–2 on tibiotarsus of leg I.

A New Species ofIsotogastrura(Collembola: Isotogastruridae) from Northeastern Brazil

A new species of Isotogastrura from marine littoral sand of Fernando de Noronha Islands, Pernambuco State, Brazil is described and illustrated. It is characterized by the absence of prelabral setae, presence of a spatulate mucro, fusion of the dorsal part of Abd. IV and V and one pair of long circumgenital setae on males. It is similar to I. veracruzana from México. A key for the 9 species known in the genus is provided.

20th century sediment budget trends on the Western Gulf of Lions shoreface (France): An application of an integrated method for the study of sediment coastal reservoirs

This paper presents a shoreface sediment budget established for the 20th century (1895–1984–2009) along the microtidal wave-dominated coast of the western Gulf of Lions (Languedoc-Roussillon, Mediterranean Sea, SE France). The implementation of a diachronic bathymetric approach, coupled with the definition of sand reservoirs (upper sand unit — USU) by very high-resolution seismic surveys and the results of LiDAR investigations, offers a new means of defining precisely the magnitude and change trends of the sediment budget. The aim of this study is to link the Large Scale Coastal Behaviour (LSCB) of the littoral prism (expressed in terms of shoreface sediment budget, shoreface sediment volume and spatial distribution pattern of cells) to climatic change, river sediment input to the coast, longshore sediment transport distribution, impact of hard coastal defence structures and artificial beach nourishment. The results show a significant reduction of the volume of the western Gulf of Lions littoral prism over 114years (−26.1±4.6×106m3). From 1895 to 1984, the overall budget is slightly positive, with a volume estimated at 4.1±3.5×106m3. For 1984–2009, however, the estimated sediment budgets clearly indicate that erosion is dominant over the last 25years, with a volume loss of −30.2±4.2×106m3. In relation to the long-term sediment budget and longshore drift pattern, the long-term trend of the USU volume distribution displays strong spatio-temporal contrasts linked to longshore sediment drift, spatial distribution of fluvial sediment inputs and hard engineering structures. Locally, the sedimentary reservoir is significantly eroded within a century (−80% of USU), since the initial amount present was low and not sustainable. The emphasis is on the importance of considering the volume changes of available sediment reservoirs rather than their losses and gains. Erosion of the Languedoc-Roussillon shoreface is likely to continue in the future due to the “natural” decrease of river sediment input and the sand removal for human purposes. Consequently the littoral sand prism results in sedimentary reservoirs that are gradually being used up.

A new genus and species of Tullbergiidae (Collembola) from the Pacific Mexican coast

The new genus Mexicaphorura, and its type species Mexicaphorura guerrerensis sp. n. are described from marine littoral sand. The new genus is similar to Sensilatullbergia Thibaud & Ndiaye, 2006 in having very big sensilla on sensorial organ of antennal segment III, but of different shape, the kind of postantennal organ, with simple vesicles in Mexicaphorura versus composed vesicles in Sensilatullbergia and formulae of the pseudocelli.

Historical Sediment Transport Pathways and Quantities for Determining an Operational Sediment Budget: Mississippi Sound Barrier Islands

Byrnes, M.R.; Rosati, J.D.; Griffee, S.F., and Berlinghoff, J.L., 2013. Historical sediment transport pathways and quantities for determining an operational sediment budget: Mississippi Sound barrier islands. In: Brock, J.C.; Barras, J.A., and Williams, S.J. (eds.), Understanding and Predicting Change in the Coastal Ecosystems of the Northern Gulf of Mexico, Journal of Coastal Research, Special Issue No. 63, pp. 166–183, Coconut Creek (Florida), ISSN 0749-0208.Historical shoreline and bathymetric survey data were compiled for the barrier islands and passes fronting Mississippi Sound to identify net littoral sand transport pathways, quantify the magnitude of net sand transport, and develop an operational sediment budget spanning a 90-year period. Net littoral sand transport along the islands and passes is primarily unidirectional (east-to-west). Beach erosion along the east side of each island and sand spit deposition to the west result in an average sand flux of about 400,000 cy/yr (305,000 m3/yr) throughout the barrier island system. Dog Keys Pass, located updrift of East Ship Island, is the only inlet acting as a net sediment sink. It also is the widest pass in the system (about 10 km) and has two active channels and ebb shoals. As such, a deficit of sand exists along East Ship Island. Littoral sand transport decreases rapidly along West Ship Island, where exchange of sand between islands terminates because of wave sheltering from the Chandeleur Islands and shoals at the eastern margin of the St. Bernard delta complex, Louisiana. These data were used to assist with design of a large island restoration project along Ship Island, Mississippi.

Seismic evidence for the preservation of several stacked Pleistocene coastal barrier/lagoon systems on the Gulf of Valencia continental shelf (western Mediterranean)

The focus of this study is the analysis of coastal sand barriers and associated coastal lagoons on the inner continental shelf of the Gulf of Valencia (western Mediterranean), based on two W–E seismic profiles recorded seaward of the Albufera de Valencia coastal lagoon. Seismic facies identified include a number of coastal sand barriers with landward lagoons draped by contemporary continental shelf deposits. The barrier systems have been grouped into two sedimentary systems tracts, the older one corresponding to a prograding/aggrading highstand systems tract involving at least four paleo-coastal sand barrier/lagoon systems, followed landward by a transgressive systems tract comprising three such systems. All the systems have been allocated a Tyrrhenian age, the formation of individual barrier systems having been associated with successive sea-level stillstands, and their present-day position being explained by the very high regional subsidence rate. In summary, this study demonstrates that the Quaternary stratigraphic record of the Gulf of Valencia inner continental shelf is composed of littoral sand facies, in particular coastal sand barrier and lagoon deposits. These findings are in agreement with corresponding observations on other continental shelves of the western Mediterranean, showing that the formation of coastal sand barriers was a characteristic feature of this region during the Quaternary.

SHOREWARD SAND TRANSPORT OUTSIDE THE SURFZONE, NORTHERN GOLD COAST, AUSTRALIA

To date, no suitable theoretical basis has been derived to predict with reliable accuracy the shoreward sand transport under waves in the deeper water outside the surf zone. This is important for understanding the rate of recovery of beaches after major storm erosion and, in some circumstances, to quantify net shoreward supply of sand to the shoreline from the active lower shore-face below the depth of storm erosion bar development. Even a relatively low rate of long term shoreward net supply may contribute to shoreline stability where it offsets a gradient in the longshore sand transport that would otherwise lead to recession. This paper outlines the results of analysis of a 41 year dataset of beach and nearshore profile surveys to quantify annual average rates of shoreward net sand transport in 6-20m water in an area where the profiles are not in equilibrium due to the existence of a residual river mouth ebb delta bar lobe. Additionally, an empirical adaptation of the sheet flow relationship of Ribberink and Al-Salem (1990) to provide for the effects of ripples has been derived from large wave flume data and correlates well with the measured Gold Coast transport rates. These have been applied to a new coastline modelling system developed as part of research into the long term evolution of Australia’s central east coast region in response to sea level change and longshore sand transport processes, which combines the one-line concept of shoreline profile translation within the zone of littoral sand transport with cross-shore profile evolution across the deeper shore-face profile below that zone. It demonstrates the importance of providing for both the shoreward supply from the continental shelf and the varying profile response time-scale across the shore-face in predicting shoreline evolution.

The sedimentary conditions of Middle-Upper Tithonian Limestones of the Demerdzhi Plateau (Mountain Crimea)

As a result of a microfacies analysis of the section of Middle-Upper Tithonian Limestones of the Demerdzhi Plateau (Mountainous Crimea), 12 microfacies types of rocks were distinguished. It has been established that these microfacies types formed in the facies zones of an inner carbonate platform: the littoral, restricted and open marine and sand shoals of the margins of a carbonate platform. The sequence of the microfacies types in the vertical section corresponds to a transgressive trend with minor sea-level fluctuations of the second order. In addition, the communities of microencrusters, which played an active role in sediment stabilization and formation of small bioherms, have been studied. These communities are considered to be evidence of the shallowness of the paleobasin

A fish otolith assemblage from the Late Miocene (Pannonian) deposits of Lake Pannon (Doba, NW Hungary)

A fish otolith assemblage consisting of 173 specimens was recovered from littoral sand deposits of the brackish Lake Pannon at Doba, western Hungary. Co-occurring endemic mollusks indicate the Lymnocardium ponticum Zone, thus an age of about 9 million years (Late Miocene) can be assumed. Most of the investigated otoliths belong to the family Sciaenidae. The otoliths are assigned to 6 taxa: Umbrina cirrhosoides, Umbrina aff. cirrosa, Umbrina sp., “genus aff. Umbrina” kokeni, Morone kuehni?, Gadidae indet. This otolith assemblage does not represent the ancient fish fauna of Lake Pannon, because the common prey fish (gobiids) have not been identified. Generally, the Pannonian fish fauna is characterized by a high number of sciaenid otoliths which indicate a shallow estuarine environment. The fish fauna from Doba is a unique otolith assemblage due to the co-occurrence of several Umbrina species.

Middle-Late Pleistocene marine terraces and fault activity in the Sant’Agata di Militello coastal area (north-eastern Sicily)

The coastal sector of Sant’Agata di Militello (north-eastern Sicily) is characterized by a flight of raised Middle-Upper Pleistocene marine terraces occurring at different heights with respect to present sea level. In particular, the geomorphological survey and the analysis of stereo-pairs of aerial photographs allowed to recognize at least five main orders of well preserved Quaternary surfaces and relative deposits mostly located at the hanging wall and at the footwall of the Pleistocene northwest-dipping Capo d’Orlando normal fault, which controlled the geomorphological evolution of the coastal area. The marine terraces show an overall good morphological continuity and are formed by marine platforms overlain by littoral deposits made up of yellow littoral sand and gravels in a sandy matrix. The continental sedimentary cover of the 3rd order terrace contains mammal-bearing deposits that were previously dated 200±40ka BP by isoleucine epimerization method, allowing to relate them to MIS 7.1 high-stand. In order to better define the whole terrace chronology, deposit samples were analyzed by Optically Stimulated Luminescence (OSL) methodology, a conventional SAR protocol used with sand-sized quartz. New datings, together with the detailed morphostructural analysis, allow to relate the 2nd and 4th order terraces to MIS 5.5 and 8.5, respectively, and to reconstruct the tectonic evolution of this coastal area, constraining the activity of the Capo d’Orlando fault.

Proposal of Fiersiphontina gen. nov., redescription of Fiersiphontina sensillata comb. nov., and new data on the distribution of Spiniferaphonte (Copepoda, Harpacticoida, Laophontidae)

The taxonomic position and original description of Laophontina sensillata Wells & Rao, 1987 are reviewed based on specimens collected from the interstitial fauna of littoral coral sands of several islands of the Philippines and New Caledonia, and the species is designated as the type species of Fiersiphontina gen. nov. The new genus is proposed based on the total loss of sexual dimorphism in the P3 and P4 exopods, and the sexual dimorphism in P2 exopod. Fiersiphontina is highly adapted to the littoral interstitial habitat of coral sandy beaches, and is related to Laophontina Norman & T. Scott, 1905, Wellsiphontina Fiers, 1991 and Spiniferaphonte Gheerardyn & Fiers, 2007. The shared characteristics that indicate a strong affinity of Fiersiphontina to Spiniferaphonte are the robust, dorsally bent, and strongly sclerotised caudal seta V and the morphology of the genital field. The phylogenetic relationship of Spiniferaphonte and Fiersiphontina is also suggested by the analysis of the last ontogenetic phases of the species of the two genera. We re-describe here the adults of both sexes, describe the last three copepodid stages of Fiersiphontina sensillata (Wells & Rao, 1987) comb. nov., and provide notes on the biogeography of the four related genera, and on the ecology of Fiersiphontina and Spiniferaphonte.

Physical Pore Properties and Grain Interactions of SAX04 Sands

During the 2004 Sediment Acoustic eXperiment (SAX04), values of sediment pore properties in a littoral sand deposit were determined from diver-collected cores using traditional methods and image analysis on X-ray microfocus computed tomography (XMCT) images. Geoacoustically relevant pore-space properties of sediment porosity, permeability, and tortuosity were evaluated at scales ranging from the pore scale to the core scale from “mud-free” sediments collected within the 0.07-km2 study area. Porosity was determined from water-weight-loss measurements to range from 0.367 to 0.369, from 2-D image analysis to range from 0.392 to 0.436 and from 3-D image analysis to range from 0.386 to 0.427. The range of permeability from all measurements was 2.8 × 10-11 m2 to 19.0 × 10-11 m2, however the range of permeability within each technique was much narrower. Permeability was determined using a constant head (CH) apparatus (krange = 2.88 to 3.74 × 10-11 m2), from a variant of the Kozeny-Carman (KC) equation (krange = 12.4 to 19.0 × 10-11 m2), from an effective medium theory technique (krange = 5.60 to 13.3 × 10-11 m2) and from a network model (krange = 8.49 to 19.0 × 10-11 m2 ). Permeability was determined to be slightly higher in the horizontal than in the vertical direction from the network model. Tortuosity ranged from 1.33 to 1.34. Based upon the small coefficients of variation for the conventionally determined pore-space properties, the sand sediment within these core samples was deemed homogeneous at all of the SAX04 sites. Additionally, grain interactions, specifically grain coordination number and grain contact areas, were determined from XMCT images. Grain contacts ranged in size from small point contacts of 136 μm2 to large-area contacts the size of grain faces ( >4500 × μm2). The mean coordination number was similar to that of a cubic packing (six), but sometimes exceeded 12, which is the coordination number for a hexagonal close packing of spheres.

Composition, age, and depositional rates of shoreface deposits under barriers and beach plains of the Columbia River littoral cell, USA

The Columbia River littoral cell (CRLC) consists of four subcells (totaling 160 km in length) that are unique in the West Coast of the United States, in that they contain prograded barriers and beach plains, reaching 0.5–3 km in width (Fig. 1). The prograded beach deposits (1–5 ka in age) overlie shoreface deposits (1–8 ka in age), as identified in 18 ground penetrating radar profiles, and sampled from 24 boreholes. Two competing hypotheses were initially proposed to account for the origins of these unique, progradative shorelines: (1) cross-shore feeding by onshore wave transport of pre-Holocene sand from the submerged shelf, and (2) longshore dispersal of nearshore sand that was supplied to the littoral system by bedload sediment discharge from the Columbia River during the Holocene. The CRLC sand forming the shoreface deposits is fine (diameter 0.2 ± 0.02 mm) and rich in lithic fragments (20–40% by volume). Gravel and shell lag layers are uncommon in most of the CRLC shoreface deposits, but they show greater abundance locally near ravinement surfaces, tidal inlets, and in the Clatsop subcell, located south of the Columbia River mouth. Gravel and granule layers increase upsection in barriers south of the Columbia River and downsection in barriers at the northern end of the littoral system. These trends suggest different mechanisms of shoreface sediment feeding within the four subcells. However, borehole samples from all four subcells show the same sand provenance, i.e., post-glacial Columbia River sand, which is identified by high ratios of hypersthene:augite in heavy-mineral fractions. Selected shoreface sections were dated (0.5–8 ka) by AMS radiocarbon analysis of articulated-shell and wood fragments recovered from auger flights (3–22 m depth subsurface). Relatively young shoreface deposition (2.5 ka at − 6.5 m elevation NGVD88) in the Clatsop subcell south of the Columbia River shows a net-southward beach transport that fed shoreface and beachface progradation into deeper water. Older and deeper shoreface deposition (4.4 ka at − 7.1 m elevation) in the Long Beach subcell north of the Columbia River was a result of the filling of innermost-shelf accommodation space prior to beachface progradation. The total volume of shoreface sand deposited under the barrier spits and beach plains of the CRLC is estimated to be 6–7 km 3 deposited since 6–8 ka. There was a net-northward transport of littoral sand (∼ 1 × 10 6 m 3 year− 1) along the nearshore and inner-shelf; subsequently some of this sand was transported onshore to feed beaches of the northernmost subcells. Columbia River sand was also the source for the formation of the offshore shelf wedge above the transgressive ravinement surface, and for the filling of major tidal basins located north of the Columbia River. In summary, the unique progradational history of the CRLC barriers and beach plains derives from the combination of (1) longshore dispersal of fine sand discharged from the Columbia River during Holocene time, and (2) across-shore feeding of beaches at the northern end of the littoral system from fine sand carried north along the nearshore and the inner-shelf.

Les collemboles (Collembola) interstitiels des sables littoraux de l'île d'Espiritu Santo (Vanuatu)

Thibaud J.-M. 2009. — Les collemboles (Collembola) interstitiels des sables littoraux de l'île d'Espiritu Santo (Vanuatu). Zoosystema 31 (3): 499-505.Collembola of coastal sands on Espiritu Santo island (Vanuatu).Populations of Collembola from coastal sands on Espiritu Santo island (Vanuatu) were studied. An annotated list of species is provided. Half on the species also occur on New Caledonia beaches. A trans-oceanic dispersal of these species inhabiting littoral sand is proposed.

Pre-late Weichselian podzol soil, permafrost features and lithostratigraphy at Penttilänkangas, western Finland

The ridge-shaped Penttilankangas accumulation in western Finland is glaciofluvial in origin: probably an ice-marginal subaquatic fan, reworked and covered with younger deep water, littoral, eolian and glacial sediments. Two cycles of glacial melting and land emergence to littoral and subaerial conditions can be distinguished at Penttilankangas, separated by one glacial advance. Special attention is paid to a fairly well preserved buried podzol soil profile and to permafrost features. The podzol soil is interpreted as having developed in littoral sand and subsequently covered with eolian sand and till. The permafrost features are observable in all the sediments below the covering till, indicating prolonged periglacial ice-free conditions after the soil formation and before the latest glacial advance. The physical properties (content of <0.0625 mm fraction, magnetic susceptibility, colour, dry bulk density and LOI) of the buried podzol soil profile are compared to the Holocene podzol soil with similar parent material in the same area. These properties, as well as the identified microfossils and cell tissue, imply that the paleosol probably developed over a longer period and/or in similar or warmer and moister conditions than the Holocene soil. The podzolisation possibly initiated in the Eemian Stage (MIS 5e), and according to the OSL datings, it ceased in the beginning of the Middle Weichselian Substage (MIS 4). After that, ice-free permafrost conditions prevailed for several thousand years before the Weichselian ice-sheet advanced to western Finland around 65 ka at the earliest.

Evidence of high‐energy events in shelly layers interbedded in coastal Holocene sands in Cadiz Bay (south‐west Spain)

AbstractIn some present‐day coastal areas, recent inactive deposits now outside the reach of sea agents can be observed. These deposits, although formed under different climatic and sea level conditions, often show similar facies to current littoral deposits. They are frequently interpreted as old dunes and beach ridges, or as abandoned spit bars, representative of previous dynamic stages. Nevertheless, in coastal areas which have been subjected to highly‐energetic events similar deposits can also be found. When a dynamic event acts on several adjacent environments, the transported and re‐deposited sediments can create homogeneous deposits with similar facies, that are easily confused.In this work, shelly layers interbedded in relict littoral sands located in the La Algaida pinewood, on the edge of the Rio San Pedro tidal channel, in Cadiz Bay (south‐west Spain) have been studied. The main constituents of the shelly beds are Glycimeris valves, organisms which are no longer found as live specimens in the present‐day sea bed of Cadiz Bay.From their appearance, the origin of these shelly beds could be related to sea washovers generated by tidal or storm action, but their disposition and height over the present sea level implies that even higher energy agents were involved in their formation, such as major storms or tsunami waves. The most significant process was the mobilization of the sub‐tidal and littoral sediments and their dispersal and re‐sedimentation, both lengthways and widthways of the coast, giving way to homogeneous sandy deposits in all littoral environments some of which are now outside the reach of current sea agents. The exception is the present‐day shore of the San Pedro tidal channel, where the sediments are being reworked by tidal and small wave action.If the proximity of the study zone to the limit of the African and Iberian plates is considered, where several historical earthquakes and tsunamis have taken place, it is possible to think that these deposits could be a consequence of sporadic and successive washovers, generated by tsunamis occurring between ad 800 and ad 1200. Sedimentological and historical data indicate an increase in seismic and tsunami activity during this period of time, while the shelly layers would be the consequence of the most intense pulses occurring during these high‐energy events. Copyright © 2009 John Wiley &amp; Sons, Ltd.