Sequence stratigraphy of Holocene incised-valley fills and coastal evolution in the Gulf of Cádiz (southern Spain)

This first sedimentary interpretation of two incised-valley fills in the Gulf of Cádiz (southern Spain), which accumulated during the last fourth-order eustatic cycle in response to fluvial incision, changes of sea level, and correlative deposition, relates the filling of the estuarine basins and their barriers with four regional progradation phases, H₁ to H₄. The cases studied are the wave-dominated Guadalete, and the mixed, tide and wave-dominated Odiel-Tinto estuaries. The sequence boundary is a type-1 surface produced during the lowstand of the Last Glacial period ca. 18 000 ¹⁴C yr BP. No fluvial lowstand deposits were found in the area. Due to rapid transgression the valley fills consist of transgressive and highstand sediments. The maximum landward advance of the estuarine barriers occurred ca. 6500–6000 ¹⁴C yr BP during the maximum of the Flandrian transgression, but there is no evidence of sea level rising appreciably above the present. A large part of the estuaries was filled during H₁ (ca. 6500–4400 ¹⁴C yr BP) but ravinement by shifting tidal inlets destroyed most of the coeval barriers. During the H₂ phase (ca. 4200–2550 ¹⁴C yr BP) sedimentation was favoured by arid conditions and concentrated in the axial estuarine zones and the barriers. Between H₂ and H₃ prevailing winds changed from W to WSW, increasing spit growth to the east and south-east. Progradation of bay-head deltas and flood-plains during H₃ (ca. 2300–800 ¹⁴C yr BP) and H₄ (500 yr ago to the present) further reduced the accommodation space in the largely-filled valleys, and sediment by-passed the estuaries and accumulated in the estuarine barriers as fast-growing spits. Arid conditions and increasing human activity have caused rapid coastal modifications.     

Understanding historical coastal spit evolution: A case study from Spurn,East Yorkshire,UK

Globally sandy coastlines are threatened by erosion driven by climatic changes and increased storminess. Understanding how they have responded to past storms is key to help manage future coastal changes. Coastal spits around the world are particularly dynamic and therefore potentially vulnerable coastal features. Therefore, how they have evolved over the last few centuries is of great importance. To illustrate this, this study focuses on the historical evolution of a spit at Spurn on the east coast of the UK, which currently provides critical protection to settlements within the Humber estuary. Through the combination of digitized historical mapping and luminescence dating, this study shows that Spurn has been a consistent coastal feature over at least the past 440 years. No significant westward migration was observed for the last 200 years. Results show a long-term extension of the spit and a decrease in its overall area, particularly in the last 50 years. Breaches of the neck cause temporary sediment pathway changes enabling westward extension of the head. Use of digitized historical maps in GIS combined with OSL dating has allowed a more complete understanding of long-term spit evolution and sediment transport modes at Spurn. In doing so it helps inform future possible changes linked to pressures, such as increases in storm events and sea-level rise. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd     

ON THE DEPOSITIONAL DYNAMICS OF GUANHE ESTUARY

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Sequence stratigraphy of the Upper Oligocene–Lower Miocene of eastern Jylland, Denmark: role of structural relief and variable sediment supply in controlling sequence development

The Upper Oligocene–Lower Miocene succession in eastern Jylland can be subdivided into three sequences (A–C from older to younger) deposited on and around the Ringkøbing-Fyn High. The development of the sequences reflects a complex interaction between eustatic sea-level changes, physiography and variable sediment supply. Superimposed on this, frequent storms promoted longshore sediment transport and the development of spit systems adjacent to structural highs. As a consequence, sequence boundaries and flooding surfaces are not always expressed as portrayed in conventional sequence models; sequence boundaries or flooding surfaces may only be marked by subtle changes in depositional environment that can only be revealed by careful integration of sedimentological observations with palynological data. The influence of the topography resulted in the development of brackish water basins that were sufficiently large to permit the deposition of hummocky cross-stratified sands with muds. These deposits are overlain by clean hummocky and swaley cross-stratified sands that were deposited in a fully marine, high-energy environment. This evolution from mud-rich, storm-influenced sediments to sand-dominated shoreface sediments resulted from a rise in sea level and was not the result of shoreface progradation and downstepping during a sea level fall. In addition to the topographic control on sequence development, sediment supply to the study area changed significantly during the deposition of the three sequences. Initially the basin was sediment-starved, favouring the formation of glaucony-rich sediments. The sediment input gradually increased and the influence of structural highs and lows became less significant with time. Consequently, both sequence boundaries and flooding surfaces are characterized by more conventional features in the younger part of the succession, where a basinward displacement of the shoreline resulted in thick lowstand delta deposits.     

The formation and closure of the Big Breach of Sacalin spit associated with extreme shoreline retreat and shoreface erosion

We document a case of exceptionally large natural breaching of a sandy spit (Sacalin spit, Danube Delta) using multiannual to seasonal surveys of topography and bathymetry on successive cross-barrier and shoreface profiles, LiDAR data, satellite imagery, and wind and wave data. The large breach, which quickly reached 3.4 km in May 2014, is attributed to morphological preconditioning of the narrow (50–150 m) barrier, which was susceptible to breaching even during moderate storm conditions. The event switched the barrier's decadal evolution from low cross-shore transport to high cross-shore transport over the barrier, which is an order of magnitude larger than during the non-breach period. Upper shoreface erosion, as indicated by the extensive erosion down to −4 m, indicates that this zone is a significant source for the rebuilding of the barrier. Barrier recovery and widening trigger a negative feedback which limits the back-barrier sediment transfer. As a result, back-barrier deposition decreases whilst the barrier aggradation through overwash becomes more frequent. The Big Breach (TBB) closed naturally in three years. The very high deposition rate of sediment in the breach is a testimony of the high sediment volumes supplied by the longshore transport and the high sediment released through shoreface retreat, and resulted in widening the barrier to a maximum of 1 km. Since the newly-formed barrier shoreline retreated 500 m, this reveals that barrier breaching is an important mechanism which significantly accelerates the landward migration of the barrier system and is a proof of the highly non-linear morphodynamics involved in the barrier island translation. © 2019 John Wiley & Sons, Ltd.     

Gravelly spit deposits in a transgressive systems tract: the Pleistocene Higashikanbe Gravel, central Japan

The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well‐sorted, pebble‐ to cobble‐size gravel beds with minor sand beds. The gravel includes large‐scale foreset beds (5–10 m high) and overlying subhorizontal beds (0·5–3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert‐type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low‐energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large‐scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea‐level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross‐sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large‐scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf‐margin systems tracts.     

The geomorphology and evolution of a large barrier spit: Farewell Spit,New Zealand

Farewell Spit is a 25 km long barrier spit that marks the end of a littoral drift system, almost 1000 km in length that runs along South Island, New Zealand. The spit is composed of barchan dunes over 20 m high, sand sheets over 1 km wide and vegetated linear dunes. Analysis of aerial photography indicates a rapid colonization of the spit by vegetation which has expanded in area by 75% since 1950. Vegetation colonization preferentially occurs on the southern side of the spit, with its northern margin characterized by barchan dunes which migrate at rates of up to 64 m/yr. Sand sourced from longshore drift appears to be the primary source of beach sediment, which is then transported into the dune field by the persistent westerly winds of the Roaring 40s. While there has been significant dune roll‐over on the surface of the spit, its overall area has remained much the same for the past 54 years. Occasional cyclone events cause erosion, but this is balanced by aeolian sediment transport. It would appear that extension of the subaerial portion of the spit is related to the development of shells banks at its downdrift end which are periodically welded to the main spit by dune extension. Farewell Spit therefore provides an ideal example of a barrier environment where longshore sediment supply and aeolian transport dominates geomorphic evolution. This differentiates the study site from other barrier environments where overwash or tidal inlet development often characterizes recent landform evolution. Copyright © 2010 John Wiley & Sons, Ltd.     

Anatomy and sedimentary model of a hooked spit (Sylt,southern North Sea)

A sedimentary model for hooked spit depositional systems based on ground‐penetrating radar and sedimentological data is presented. The recurved main spit of Sylt Island (southern North Sea) is dominated by migrating sand dunes; the investigated hooked spit exhibits a system of foredune ridges, oriented perpendicular to the dunes of the recurved spit. The development of the hooked spit is related closely to the presence of an adjacent tidal inlet, where strong tidal currents and a steep bathymetry prevent a further northward progradation of the main spit and trigger a deflection from northerly‐directed to easterly‐directed net sediment transport. Ground‐penetrating radar data and shallow sediment cores reveal the sedimentary architecture of the hooked spit in high resolution and allow the proposition of a genetic stratigraphic model. It is shown that the growth of the hooked spit is controlled by the interplay of alongshore migrating foreshore beach drifts under fair‐weather conditions and strong erosional events, interpreted as the result of rare severe storms. These storms may excavate scarps in the backshore, which play an important role in the development of foredune ridges. Accelerator mass spectrometry ¹⁴C ages indicate an absolute age of at least 1300 years for the hooked spit, which possibly correlates with strengthened erosion of the main spit. In contrast to the main spit, where the sediment budget is negative nowadays, growth of the hooked spit beach accelerated significantly during the last decades. This effect can probably be attributed to enhanced beach‐nourishments updrift along the main spit and makes the investigated hooked spit a natural laboratory to study the influence of increasing sediment supply into a system developing under the conditions of sea‐level rise. The study shows that the same external forces lead to distinct progradational processes along one barrier‐spit system.     

海南博鳌玉带沙海滩冲淤演变及脆弱性分析

玉带沙是海南岛东部博鳌万泉河口的一条细长沙嘴,也是我国重要旅游海滩之一。为了揭示近期玉带沙的冲淤演变趋势,选择了1988年以来4个年份的RS图像,利用现场GPS监测的海滩地形推算出校正后的低潮位水边线,并使用GIS软件计算各年份低潮线以上的玉带沙面积。结果表明：1988～2005年玉带沙呈显著蚀退趋势,平均蚀退70m（4.1m/a）,面积减少35%。分析认为：玉带沙蚀退主要原因是海平面上升和万泉河入海泥沙的减少。在气候变化与人类活动的双重作用下,玉带沙呈现高度脆弱性,值得关注。