Formation and evolution of back‐barrier perched lakes in rocky coasts: An example of a Holocene system in north‐west Spain

Coastal back‐barrier perched lakes are freshwater bodies that are elevated over sea‐level and are not directly subjected to the inflow of seawater. This study provides a detailed reconstruction of the Doniños back‐barrier perched lake that developed at the end of a small river valley in the rocky coast of the north‐west Iberian Peninsula during the Holocene transgression. Its sequence stratigraphy was reconstructed based on a core transect across the system, the analyses of its lithofacies and microfossil assemblages, and a high‐resolution radiocarbon‐based chronology. The Doniños perched lake was formed ca 4·5 ka bp . The setting of the perched lake was favoured by Late Holocene sea‐level stabilization and the formation of a barrier and back‐barrier basin, which was contemporaneous with the high systems tract period. This basin developed over marine and lagoonal sediments deposited between 10·2 ka bp and 8·0 ka bp , during rapidly rising sea‐level characteristic of the transgressive systems track period. At 1·1 ka bp , the barrier was breached and the perched lake was partially emptied, causing the erosion of the back‐barrier basin sediments and a significant sedimentary hiatus. Both enhanced storminess and human intervention were likely to be responsible for this event. After 1 ka bp , the barrier reclosed and the present‐day lake was reformed, with the water level reaching as high as 5 m above mean sea‐level. The depositional evolution of the Doniños system serves as a model of coastal back‐barrier perched lakes in coastal clastic systems that have developed over gently seaward‐dipping rugged substrates at small distances from the shoreline and under conditions of rising sea‐level and high sediment supply. A review of estuaries, back‐barrier lagoons, pocket beaches and back‐barrier perched lakes in the rocky coast of north‐west Spain shows that the elevation of the bedrock is the main factor controlling the origin and evolution of these systems.     

Holocene relative sea‐level changes in Harris,Outer Hebrides,Scotland, UK

Evidence for relative sea‐level changes during the middle and late Holocene is examined from two locations on the Atlantic coast of Harris, Outer Hebrides, Scotland, using morphological mapping and survey, stratigraphical, grain size and diatom analysis, and radiocarbon dating. The earliest event identified is a marine flood, which occurred after 7982–8348 cal. a (7370 ± 80 ¹⁴C a) BP, when the sea crossed a threshold lying at ?0.08 m Ordnance Datum Newlyn (OD) (?2.17 m mean high water springs (MHWS)) before withdrawing. This could have been due to a storm or to the Holocene Storegga Slide tsunami. By 6407–6122 cal. a (5500 ± 60 ¹⁴C a) BP, relative sea levels had begun to fall from a sandflat surface with an indicated MHWS level of between 0.08 and ?1.96 m (?2.01 to ?4.05 m). This fall reached between ?0.30 and ?2.35 m (?2.39 to ?4.44 m) after 5841–5050 cal. a (4760 ± 130 ¹⁴C a) BP, but was succeeded by a relative sea‐level rise which reached between 0.54 and ?1.57 m (?1.55 to ?3.66 m) by 5450–4861 cal. a (4500 ± 100 ¹⁴C a) BP. This rise continued, possibly with an interruption, until a second sandflat surface was reached between 2.34 and ?0.26 m (0.25 to ?2.35 m) between 2952–3375 cal. a (3000 ± 80 ¹⁴C a) and 1948–2325 cal. a (2130 ± 70 ¹⁴C a) BP, before present levels were reached. The regressive episode from the earliest sandflat is correlated with the abandonment of the Main Postglacial Shoreline. It is maintained that the fluctuations in relative sea level recorded can be correlated with similar events elsewhere on the periphery of the glacio‐isostatic centre and may therefore reflect secular changes in nearshore sea surface levels. Despite published evidence from trim lines of differential ice sheet loading across the area, no evidence of variations in uplift between the locations concerned could be found. Copyright © 2009 John Wiley & Sons, Ltd.     

Late Devensian and Holocene relative sea‐level change in North Wales,UK

Deglacial sea‐level index points defining relative sea‐level (RSL) change are critical for testing glacial isostatic adjustment (GIA) model output. Only a few observations are available from North Wales and until recently these provided a poor fit to GIA model output for the British‐Irish Ice Sheet. We present results of an integrated offshore geophysical (seismic reflection), coring (drilling rig), sedimentological, micropalaeontological (foraminifera), biostratigraphical (palynology) and geochronological (AMS ¹⁴C) investigation into a sequence of multiple peat/organic sediment horizons interbedded within a thick estuarine–marine sequence of minerogenic clay‐silts to silty sands from the NE Menai Strait, North Wales. Ten new sea‐level index points and nine new limiting dates from the Devensian Late‐glacial and early Holocene are integrated with twelve pre‐existing Holocene sea‐level index points and one limiting point from North Wales to generate a regional RSL record. This record is similar to the most recent GIA predictions for North Wales RSL change, supporting either greater ice load and later deglaciation than in the GIA predictions generated before 2004, or a modified eustatic function. There is no evidence for a mid‐Holocene highstand. Tidally corrected RSL data indicate initial breaching of the Menai Strait between 8.8 and 8.4 ka BP to form a tidal causeway, with final submergence between 5.8 and 4.6 ka BP. Final breaching converted the NE Menai Strait from a flood‐dominated estuary into a high energy ebb tidal delta with extensive tidal scouring of pre‐existing Late‐glacial and Holocene sequences. The study confirms the value of utilising offshore drilling/coring technology to recover sea‐level records which relate to intervals when rates of both eustatic and isostatic change were at their greatest, and therefore of most value for constraining GIA models. Copyright © 2011 John Wiley & Sons, Ltd.     

Mid‐ to late Holocene relative sea‐level change in Poole Harbour,southern England

A foraminiferal transfer function for mean tide level (MTL) is used in combination with AMS radiocarbon dated material to construct a record of relative sea‐level (RSL) change from Poole Harbour, southern Britain. These new data, based on multiple cores from duplicate sites, indicate four phases of change during the last 5000 cal. (calendar) yr: (i) rising RSL between ca. 4700 cal. yr BP and ca. 2400 cal. yr BP; (ii) stable to falling RSL from ca. 2400 cal. yr BP until ca. 1200 cal. yr BP; (iii) a brief rise in RSL from ca. 1200 cal. yr BP to ca. 900 cal. yr BP, followed by a period of stability; (iv) a recent increase in the rate of RSL rise from ca. 400–200 cal. yr BP until the present day. In addition, they suggest that the region has experienced long‐term crustal subsidence at a rate of 0.5 mm C¹⁴ yr^?1. Although this can account for the overall rise in MTL observed during the past 2500 yr, it fails to explain the changes in the rate of rise during this period. This implies that the phases of RSL change recorded in the marshes of Poole Harbour reflect tidal range variations or ‘eustatic’ fluctuations in sea‐level. Copyright © 2001 John Wiley & Sons, Ltd.     

Late Holocene great earthquakes and relative sea‐level change at Kenai,southern Alaska

Kenai, located on the west coast of the Kenai Peninsula, Alaska, subsided during the great earthquake of AD 1964. Regional land subsidence is recorded within the estuarine stratigraphy as peat overlain by tidal silt and clay. Reconstructions using quantitative diatom transfer functions estimate co‐seismic subsidence (relative sea‐level rise) between 0.28±0.28 m and 0.70±0.28 m followed by rapid post‐seismic recovery. Stratigraphy records an earlier co‐seismic event as a second peat‐silt couplet, dated to ～1500–1400 cal. yr BP with 1.14±0.28 m subsidence. Two decimetre‐scale relative sea‐level rises are more likely the result of glacio‐isostatic responses to late Holocene and Little Ice Age glacier expansions rather than to co‐seismic subsidence during great earthquakes. Comparison with other sites around Cook Inlet, at Girdwood and Ocean View, helps in constructing regional patterns of land‐level change associated with three great earthquakes, AD 1964, ～950–850 cal. yr BP and ～1500–1400 cal. yr BP. Each earthquake has a different spatial pattern of co‐seismic subsidence which indicates that assessment of seismic hazard in southern Alaska requires an understanding of multiple great earthquakes, not only the most recent. All three earthquakes show a pre‐seismic phase of gradual land subsidence that marked the end of relative land uplift caused by inter‐seismic strain accumulation. Copyright © 2005 John Wiley & Sons, Ltd.     

Late Quaternary sea‐level change and evolution of Belfast Lough,Northern Ireland: new offshore evidence and implications for sea‐level reconstruction

The interplay of eustatic and isostatic factors causes complex relative sea‐level (RSL) histories, particularly in paraglacial settings. In this context the past record of RSL is important in understanding ice‐sheet history, earth rheology and resulting glacio‐isostatic adjustment. Field data to develop sea‐level reconstructions are often limited to shallow depths and uncertainty exists as to the veracity of modelled sea‐level curves. We use seismic stratigraphy, 39 vibrocores and 26 radiocarbon dates to investigate the deglacial history of Belfast Lough, Northern Ireland, and reconstruct past RSL. A typical sequence of till, glacimarine and Holocene sediments is preserved. Two sea‐level lowstands (both max. ?40 m) are recorded at c. 13.5 and 11.5k cal a bp . Each is followed by a rapid transgression and subsequent periods of RSL stability. The first transgression coincides temporally with a late stage of Meltwater Pulse 1a and the RSL stability occurred between c. 13.0 and c. 12.2k cal a bp (Younger Dryas). The second still/slowstand occurred between c. 10.3 and c. 11.5k cal a bp . Our data provide constraints on the direction and timing of RSL change during deglaciation. Application of the Depth of Closure concept adds an error term to sea‐level reconstructions based on seismic stratigraphic reconstructions.     

Deep incision in an Aptian carbonate succession indicates major sea‐level fall in the Cretaceous

Long‐term relative sea‐level cycles (0·5 to 6 Myr) have yet to be fully understood for the Cretaceous. During the Aptian, in the northern Maestrat Basin (Eastern Iberian Peninsula), fault‐controlled subsidence created depositional space, but eustasy governed changes in depositional trends. Relative sea‐level history was reconstructed by sequence stratigraphic analysis. Two forced regressive stages of relative sea‐level were recognized within three depositional sequences. The first stage is late Early Aptian age (intra Dufrenoyia furcata Zone) and is characterized by foreshore to upper shoreface sedimentary wedges, which occur detached from a highstand carbonate platform, and were deposited above basin marls. The amplitude of relative sea‐level drop was in the order of tens of metres, with a duration of <1 Myr. The second stage of relative sea‐level fall occurred within the Late Aptian and is recorded by an incised valley that, when restored to its pre‐contractional attitude, was >2 km wide and cut ≥115 m down into the underlying Aptian succession. With the subsequent transgression, the incision was backfilled with peritidal to shallow subtidal deposits. The changes in depositional trends, lithofacies evolution and geometric relation of the stratigraphic units characterized are similar to those observed in coeval rocks within the Maestrat Basin, as well as in other correlative basins elsewhere. The pace and magnitude of the two relative sea‐level drops identified fall within the glacio‐eustatic domain. In the Maestrat Basin, terrestrial palynological studies provide evidence that the late Early and Late Aptian climate was cooler than the earliest part of the Early Aptian and the Albian Stage, which were characterized by warmer environmental conditions. The outcrops documented here are significant because they preserve the results of Aptian long‐term sea‐level trends that are often only recognizable on larger scales (i.e. seismic), such as for the Arabian Plate.     

A new Holocene relative sea‐level curve for Terra Nova Bay,Victoria Land,Antarctica

More than 100 radiocarbon dates of penguin guano and remains, shells and seal skin afford ages for raised beaches adjacent to Terra Nova Bay, Antarctica. These dates permit construction of a new relative sea‐level curve that bears on the timing of deglaciation. Recession of the Ross Sea ice‐sheet grounding line from Terra Nova Bay occurred no earlier than 7200 ¹⁴C yr (8000 cal. yr) BP. Retreat along the Victoria Land coast may have been rapid, possibly contributing to eustatic sea‐level rise centred at ca. 7600 cal. yr BP. The presence of a significant amount of ice remaining in the Ross Sea Embayment in Holocene time lessens the chance that Antarctica contributed significantly to meltwater pulse 1A several thousand years earlier. Copyright © 2004 John Wiley & Sons, Ltd.     

Relative sea‐level change and postglacial isostatic adjustment along the coast of south Devon,United Kingdom

Previous sea‐level studies suggest that southwest Britain has the fastest subsiding coastline in the United Kingdom, but tide‐gauge data, GPS and gravity measurements and geophysical models show little evidence of anomalous subsidence in this region. In this paper we present 15 new sea‐level index points from four coastal barrier systems in south Devon. Eight are from compaction‐free basal sediments and others were corrected for autocompaction. Our data suggest that relative sea level along the south Devon coastline has risen by 21 ± 4 m during the past 9000 years. Sea‐level rise slowed during the middle and late Holocene and a rise of 8 ± 1 m has occurred since ca. 7000 cal. yr BP. Anomalous ages for many rejected points are attributed to sediment reworking during barrier transgression. The relative sea‐level history during the early and middle Holocene shows a good fit with geophysical model predictions, but the geological and modelled data diverge in the later Holocene. Unlike the geophysical models, sea‐level index points cannot differentiate between late Holocene relative sea‐level histories of south Devon and southwest Cornwall. It is suggested that this discrepancy can be resolved by obtaining additional high‐quality sea‐level index points covering the past 4000 years. Copyright © 2008 John Wiley & Sons, Ltd.     

东海陆架全新统高分辨率层序地层学研究

在高分辨率14C测年、岩石、生物、化学、同位素、气候及磁性地层学研究成果基础上,通过不同沉积背景典型钻孔的沉积学分析,运用层序地层学理论,对东海陆架全新统进行了高分辨率层序划分及对比;建立了全新世层序地层格架及海平面变化过程;提出了相应的层序成因模式。研究结果表明,东海陆架全新统相当于一个发育中的六级（1~10ka）层序,可进一步划分为3个七级（0.1~1ka）层序和若干个更次级层序。代表1个六级或3个七级周期相对海平面变化过程中叠加有若干更次级的海平面波动,它们与地球旋回谱系中的太阳带、历法带密切相关。七级层序具有与三级层序相近的内部构型和成因格架。在东海陆架全新世沉积演化过程中,长江三角州至少有3次不同程度地越过东海陆架进入冲绳海槽,并滞留有至少3层海侵改造"残积砂"沉积。在东海陆架全新世海平面变化期间,最大海平面时期为约距今6~5ka,大致高于现今海平面2~4m,最低海平面在距今10ka左右,大约低于现今海平面130m。目前,由于温室效应的影响,海平面仍呈小幅度波浪式上升。事实证明,层序地层学不仅丰富了现代海洋沉积学的内容,而且解决了许多海洋沉积学未能解决的问题.     

Holocene and Lateglacial relative sea‐level change in north‐west England: implications for glacial isostatic adjustment models

Relative sea‐level (RSL) change is reconstructed for central Cumbria, UK, based on litho‐ and biostratigraphical analysis from the Lateglacial to the late Holocene. The RSL curve is constrained using ten new radiocarbon‐dated sea‐level index points in addition to published data. The sea‐level curve identifies a clear Lateglacial sea‐level highstand approximately 2.3 m OD at c. 15–17 k cal a BP followed by rapid RSL fall to below ?5 m OD. RSL then rose rapidly during the early Holocene culminating in a mid‐Holocene highstand of approximately 1 m OD at c. 6 k cal a BP followed by gradual fall to the present level. These new data provide an important test for the RSL predictions from glacial isostatic adjustment models, particularly for the Lateglacial where there are very little data from the UK. The new RSL curve shows similar broad‐scale trends in RSL movement predicted by the models. However, the more recent models fail to predict the Lateglacial sea level highstand above present reconstructed by the new data presented here. Future updates to the models are needed to reduce this mismatch. This study highlights the importance for further RSL data to constrain Lateglacial sea level from sites in northern Britain. Copyright © 2012 John Wiley & Sons, Ltd.     

Relative sea level changes during the Holocene in the Sisimiut area,south‐western Greenland

Holocene relative sea level (RSL) changes have been investigated by analysing and dating isolation sequences from five lakes near Sisimiut in south‐western Greenland. The transitions between marine and lacustrine sediments were determined from elemental analyses and analyses of macroscopic plant and animal remains. Radiocarbon dating was used to provide minimum ages for the transitions and to construct a RSL curve. Dating of a shell of the marine bivalve Macoma balthica indicates that deglaciation of the lowlands occurred in the early Holocene, at around 10 900 cal a BP. The RSL curve shows initial rapid regression from the marine limit at around 140 m, implying strong glacio‐isostatic rebound. We suggest that the margin of the Greenland Ice Sheet was located at the shelf break during the Last Glacial Maximum. Frequent remains of the ostracode Sarcypridopsis aculeata, which is a thermophilous brackish water species that is unknown from the extant fauna of Greenland, in one of the basins around 8500 cal a BP may mark the beginning of the Holocene thermal maximum in the region. Copyright © 2011 John Wiley & Sons, Ltd.     

Beach erosion under rising sea‐level modulated by coastal geomorphology and sediment availability on carbonate reef‐fringed island coasts

This study addresses gaps in understanding the relative roles of sea‐level change, coastal geomorphology and sediment availability in driving beach erosion at the scale of individual beaches. Patterns of historical shoreline change are examined for spatial relationships to geomorphology and for temporal relationships to late‐Holocene and modern sea‐level change. The study area shoreline on the north‐east coast of Oahu, Hawaii, is characterized by a series of kilometre‐long beaches with repeated headland‐embayed morphology fronted by a carbonate fringing reef. The beaches are the seaward edge of a carbonate sand‐rich coastal strand plain, a common morphological setting in tectonically stable tropical island coasts. Multiple lines of geological evidence indicate that the strand plain prograded atop a fringing reef platform during a period of late‐Holocene sea‐level fall. Analysis of historical shoreline changes indicates an overall trend of erosion (shoreline recession) along headland sections of beach and an overall trend of stable to accreting beaches along adjoining embayed sections. Eighty‐eight per cent of headland beaches eroded over the past century at an average rate of ?0·12 ± 0·03 m yr^?1. In contrast, 56% of embayed beaches accreted at an average rate of 0·04 ± 0·03 m yr^?1. Given over a century of global (and local) sea‐level rise, the data indicate that embayed beaches are showing remarkable resiliency. The pattern of headland beach erosion and stable to accreting embayments suggests a shift from accretion to erosion particular to the headland beaches with the initiation of modern sea‐level rise. These results emphasize the need to account for localized variations in beach erosion related to geomorphology and alongshore sediment transport in attempting to forecast future shoreline change under increasing sea‐level rise.     

Evolution and architecture of a West Mediterranean Upper Pleistocene to Holocene coastal apron‐fan system

The Quaternary deposits of tectonically stable areas are a powerful tool to investigate high‐frequency climate variations (<10 ka) and to distinguish allogenic and autogenic factors controlling deposition. Therefore, an Upper Pleistocene–Holocene coastal apron‐fan system in north–western Sardinia (Porto Palmas, Italy) was studied to investigate the relations between climate changes, sea‐level fluctuations and sediment source‐supply that controlled its development. The sedimentary sequence records the strong influence of local (wet/dry) and worldwide (sea‐level) environmental variations in the sedimentation and preservation of the deposits. A multi‐disciplinary approach allowed subdivision of the succession into four major, unconformity‐bounded stratigraphic units: U1 U2, U3 and U4. Unit U1, tentatively dated to the warm and humid Marine Isotopic Stage (MIS) 5, consists of sandy, gravelly coastal/beach deposits developed during high sea‐level in low‐lying areas. Unit U2 consists of debris‐flow dominated fan‐deposits (ca 74 ka; MIS 4), preserved as partial fills of small valleys and coves. Unit U2 is mainly composed of reddish silty conglomerate to pebbly siltstones sourced from the Palaeozoic metamorphic inland hills (bedrock), superficially disintegrated during the preceding warm, vegetation‐rich MIS 5. The cold and semi‐arid climate strongly reduced vegetation cover along the valley flanks. Therefore, sediment gravity‐flow processes, possibly activated by rainstorms, led to deposition of debris‐flow dominated fans. Unit U3 consists of water‐flow dominated alluvial‐fan deposits (ca 47 to 23 ka; MIS 3), developed on a slightly inclined coastal plain. Unit U3 is composed of sandstone and sandy conglomerate fed from two main sediment sources: metamorphic inland bedrock and Quaternary bioclastic‐rich shelf‐derived sands. During this cold phase, sea‐level dropped sufficiently to expose bioclastic sands accumulated on the shelf. Frequent climate fluctuations favoured inland aeolian transport of sand during dry phases, followed by reworking of the aeolian bodies by flash floods during wet phases. Bedrock‐derived fragments mixed with water‐reworked, wind‐blown sands led to the development of water‐flow dominated fans. The Dansgaard–Oeschger events possibly associated with sand landward deflation and main fan formations are Dansgaard–Oeschger 13 (ca 47 ka), Dansgaard–Oeschger 8 (ca 39 ka) and Dansgaard–Oeschger 2 (ca 23 ka). No record of sedimentation during MIS 2 was observed. Finally, bioclastic‐rich aeolianites (Unit U4, ca 10 to 5 ka; MIS 1), preserved on a coastal slope, were developed during the Holocene transgression (ca 10 to 5 ka; MIS 1). The studied sequence shows strong similarities with those of other Mediterranean sites; it is, however, one of the few where the main MIS 4 and MIS 3 climatic fluctuations are registered in the sedimentary record.     

黄海中部泥质区底栖有孔虫群落对早全新世海平面快速上升的响应

末次冰消期以来全球海平面经历了很大变动,本文通过陆架泥质区资料,对黄海中部泥质区5个沉积物柱样进行了AMS ¹⁴C年代测试,结合沉积物粒度组成与底栖有孔虫资料探讨黄海中部泥质区底栖有孔虫对早全新世海平面快速上升的响应。结果表明,约13.0 ka以来底栖有孔虫群落对海平面变化的响应呈现明显的3个阶段：早期阶段(约13.0~10.0 ka期间),底栖有孔虫以近岸浅水组合为主,与这一时期海平面相对较低对应,泥质区整体呈滨岸环境,水动力环境较强,有机质等食物供给相对丰富,底栖有孔虫丰度相对较高;中期阶段(约10.0~7.0 ka期间),底栖有孔虫群落几乎消失,可能是由于海平面快速上升,动荡的海洋环境不利于有机质沉降,造成底栖有孔虫食物供给不稳定,不利于底栖有孔虫生存所致;晚期阶段(约7.0 ka以来),底栖有孔虫群落逐渐繁盛,以黄海中部冷水团特征组合为主,反映了区域海平面相对稳定、泥质沉积的形成以及黄海冷水团核心区域的出现等海洋环境变化为底栖有孔虫带来了稳定的有机质供给,故而底栖有孔虫丰度迅速增加到与现在相当的水平。黄海中部泥质区多柱样底栖有孔虫群落变化对早全新世海平面上升有较一致的响应,为该区全新世早期海平面快速上升以及黄海现代环流体系的形成提供了更直接的证据。

     

Weichselian sediments at Foss‐Eikeland,Jæren (southwest Norway): sea‐level changes and glaciation history

The Jæren area in southwestern Norway has experienced great changes in sea‐levels and sedimentary environments during the Weichselian, and some of these changes are recorded at Foss‐Eikeland. Four diamictons interbedded with glaciomarine and glaciofluvial sediments are exposed in a large gravel pit situated above the post‐glacial marine limit. The interpretation of these sediments has implications for the history of both the inland ice and the Norwegian Channel Ice Stream. During a Middle Weichselian interstadial, a large glaciofluvial delta prograded into a shallow marine environment along the coast of Jæren. A minor glacial advance deposited a gravelly diamicton, and a glaciomarine diamicton was deposited during a following marine transgression. This subsequently was reworked by grounded ice, forming a well‐defined boulder pavement. The boulder pavement is followed by glaciomarine clay with a lower, laminated part and an upper part of sandy clay. The laminated clay probably was deposited under sea‐ice, whereas more open glaciomarine conditions prevailed during deposition of the upper part. The clay is intersected by clastic dykes protruding from the overlying, late Weichselian till. Preconsolidation values from the marine clay suggest an ice thickness of at least 500 m during the last glacial phase. The large variations in sea‐level probably are a combined effect of eustasy and glacio‐isostatic changes caused by an inland ice sheet and an ice stream in the Norwegian Channel. Copyright © 2002 John Wiley & Sons, Ltd.     

High‐frequency cycles of brachiopod shell beds on subaqueous delta‐scale clinoforms (early Pliocene,south‐east Spain)

During the early Pliocene, subaqueous delta‐scale clinoforms developed in the Águilas Basin, in a mixed temperate carbonate–siliciclastic system. The facies distribution is consistent with the infralittoral prograding wedge model. Stacking patterns and bounding surfaces indicate that the clinoforms formed during the highstand and falling sea‐level stages of a high rank cycle. Twenty‐two prograding clinothems were recognized over a distance of ≥1 km. Biostratigraphic data indicate a time span shorter than 700 kyr for the whole unit (MPl3 biozone of the Mediterranean Pliocene). Cyclic skeletal concentrations and occasional biostromes of suspension feeders (terebratulid brachiopods, modiolid bivalves and adeoniform bryozoan colonies), slightly evolved glauconite and occasional Glossifungites ichnofacies formed on the clinoforms during high‐frequency pulses of relative sea‐level rise. During such stages, increased accommodation space in the topsets of the clinoforms caused a strong reduction of terrigenous input into the foresets and bottomsets. This provided favourable conditions for the development of these suspension feeder palaeocommunities. During stillstand stages, however, reduced accommodation space in the topsets eventually resumed progradation in the foresets. There, the abundance of Ditrupa tubes indicates frequent siltation events that extirpated the terebratulid populations and other epifaunal suspension feeders in the foreset and bottomset subenvironments. The occurrence of shell beds on the clinoforms suggests that this case study represents lower progradation rates than standard examples where shell beds bound the clinobedded units at their base and top only. Importantly, the distributions of biofacies and ichnoassemblage associations contribute significantly to the understanding of the effects of relative sea‐level fluctuations on the evolution of subaqueous delta‐scale clinoform systems.     

Holocene coastal environmental changes on the periphery of an area of glacio‐isostatic uplift: an example from Scapa Bay,Orkney, UK

The first detailed investigation of a deep, coastal, sedimentary basin in Orkney reveals a complex Holocene history of back‐barrier morphodynamics. At Scapa Bay, the sea flooded a freshwater marsh after ca. 9400 yr BP at ca. ?5.4 m OD. Before ca. 7800 BP, abundant sediment from nearby cliffs was mobilised inland into a series of gravel barriers across the valley mouth. By ca. 7500 BP, direct marine influence was restricted in the back‐barrier area, although saltmarsh persisted until ca. 5900 BP. By then, at least four gravel ridges had enclosed the backing lagoon, where freshwater inputs became dominant. As terrestrial sediments filled the basin, another freshwater marsh developed. The multiple barrier complex demonstrates progradation resulting from continuous sediment supply in a sheltered embayment. The progressively rising height of the barrier crests seawards probably resulted from a combination of factors such as barrier morphodynamics, increased storminess and long‐term rising relative sea levels. The dominant vegetation surrounding Scapa Bay changed from open grassland to scrub ca. 9400 BP, then to deciduous woodland ca. 7800 BP, and to dwarf‐shrub heath ca. 2600 BP, the latter probably a response to a combination of climate change and human activity. Copyright © 2007 John Wiley & Sons, Ltd.