Autostratigraphic modelling of the growth of alluvial‐shelf systems during steady base‐level cycles: Two‐dimensional tank experiments

Autostratigraphy is the stratigraphy generated by large‐scale autogenesis, developed based on the full recognition of the non‐equilibrium behaviour of depositional systems in response to steady external forcing. The existing autostratigraphic concepts were derived mostly from studies of river deltas growing during a single rise or fall of base level (or relative sea level). The present study challenges to extend the autostratigraphic framework to the alluvial‐shelf system growing through steady base‐level cycles by two‐dimensional tank experiments. During each experimental run, the base level was changed symmetrically, wherein through cycles, the rise and fall had the same constant rate (|R_bl|) and period (T_bl), and thus the same constant amplitude (A_bl), but with no basin tectonism. In total, nine runs with different combinations of |R_bl| and A_bl were performed. The experimental results brought the following implications. (i) The shelf‐transiting active depositional system takes non‐equilibrium responses in earlier base‐level cycles, during each of which the system experiences episodes of degradation with base‐level fall. (ii) After the system has sufficiently grown through cycles, non‐equilibrium responses change into equilibrium responses, whereby the shelf‐transiting alluvial system, whether retrogradational or progradational, is free from degradation and continues to aggrade but with a gradually decreasing rate of aggradation. (iii) The alluvial topset river tentatively but autogenically attains a graded state during the falling limb of an intermediate cycle, which separates the earlier degradation‐inclusive and later aggradation‐sustainable cycles. (iv) The number (or duration) of cycles elapsed prior to this phase‐transition is linearly proportional to the amplitude (or the square of the period) of base‐level cycles, with a coefficient defined by the rates of base‐level change and sediment supply. Such a growth pattern does not necessarily hold when considering long‐term tectonic subsidence or uplift. These notions help to understand the stratigraphic architectures of natural alluvial‐shelf systems evolved through base level cycles.     

Gilbert‐type deltas recording short‐term base‐level changes: Delta‐brink morphodynamics and related foreset facies

Gilbert‐type deltas are sensitive recorders of short‐term base‐level changes, but the delta‐front record of a base‐level rise tends to be erased by fluvial erosion during a subsequent base‐level fall, which renders the bulk record of base‐level changes difficult to decipher from the delta‐front deposits. The present detailed study of three large Pleistocene Gilbert‐type deltas uplifted on the southern coast of the Gulf of Corinth, Greece, indicates a genetic link between the delta‐front morphodynamic responses to base‐level changes and the delta‐slope sedimentation processes. Sigmoidal delta‐brink architecture signifies a base‐level rise and is accompanied by a debrite‐dominated assemblage of delta foreset deposits, thought to form when the aggrading delta front stores sediment and undergoes discrete gravitational collapses. Oblique delta‐brink architecture tends to be accompanied by a turbidite‐dominated assemblage of foreset deposits, which are thought to form when the delta‐front accommodation decreases and the sediment carried by hyperpycnal effluent bypasses the front. This primary signal of the system response to base‐level changes combines further with the secondary ‘noise’ of delta autogenic variation and possible allogenic fluctuations in fluvial discharge due to regional climatic conditions. Nevertheless, the evidence suggests that the facies trends of delta foreset deposits may be used to decipher the delta ‘hidden’ record of base‐level changes obliterated by fluvial topset erosion. Early‐stage bayhead deltas may be an exception from the hypothetical model, because their narrow front tends to be swept by river floods irrespective of base‐level behaviour and their subaqueous slope deposits are thus mainly turbidites.     

Shoal‐water deltas in high‐accommodation settings: Insights from the lacustrine Valimi Formation (Gulf of Corinth,Greece)

The stratigraphic architecture of shoal‐water deltaic systems developed in low‐accommodation settings is relatively well‐known. In contrast, the features of shoal‐water deltas developed in high‐accommodation settings remain relatively poorly documented, especially when compared with the available data sets for Gilbert‐type deltaic systems developed in the same settings. The lacustrine Valimi Formation (Gulf of Corinth, Greece) provides an opportunity to investigate the facies assemblage and architectural style of shoal‐water deltaic systems developed in high‐accommodation settings. The studied interval accumulated during the Pliocene and Pleistocene and represents part of the early syn‐rift Gulf of Corinth succession. Six facies associations, each described in terms of depositional processes and geometries, have been identified and interpreted to represent a range of proximal to distal deltaic sub‐environments: delta plain, distributary channel, mouth‐bar, delta front, prodelta and open lake. The facies associations and their architectural elements reveal characteristics which are not common in traditionally described shoal‐water deltas. Of note, different facies arrangements are observed in the distributary channels in different sectors of the delta, passing from thick single‐storey channel fills embedded within delta‐plain fines in landward positions, to thin, amalgamated and multi‐storey channels closer to the river mouth. This study proposes a new depositional model for shoal‐water deltas in high‐accommodation settings documenting, for the first time, that shoal‐water delta deposits can form a substantial part of stratigraphic successions that accumulate in these settings. The proposed depositional model provides new criteria for the recognition and interpretation of these deposits; the results of this study have applied significance for reservoir characterization.     

Unravelling the conundrum of river response to rising sea‐level from laboratory to field. Part I: Laboratory experiments

The most recent deglaciation resulted in a global sea‐level rise of some 120 m over approximately 12 000 years. In this Part I of two parts, a moving boundary numerical model is developed to predict the response of rivers to this rise. The model was motivated by experiments at small scale, which have identified two modes describing the transgression of a river mouth: autoretreat without abandonment of the river delta (no sediment starvation at the topset–foreset break) and sediment‐starved autoretreat with abandonment of the delta. In the latter case, transgression is far more rapid and its effects are felt much further upstream of the river mouth. The moving boundary numerical model is checked against experiments. The generally favourable results of the check motivate adaptation of the model to describe the response of the much larger Fly‐Strickland River system, Papua New Guinea to Holocene sea‐level rise; this is done in the companion paper, Part II.     

A three‐dimensional numerical model of sediment transport,erosion and deposition within a network of channel belts,floodplain and hill slope: extrinsic and intrinsic controls on floodplain dynamics and alluvial architecture

A three‐dimensional numerical model of sediment transport, erosion and deposition within a network of channel belts and associated floodplain is described. Sediment and water supply are defined at the upstream entry point, and base level is defined at the downstream edge of the model. Sediment and water are transported through a network of channels according to the diffusion equation, and each channel has a channel belt with a width that increases in time. The network of channels evolves as a result of channel bifurcation and abandonment (avulsion). The timing and location of channel bifurcation is controlled stochastically as a function of the cross‐valley slope of the floodplain adjacent to the channel belt relative to the down‐valley slope, and of annual flood discharge. A bifurcation develops into an avulsion when the discharge of one of the distributaries falls below a threshold value. The floodplain aggradation rate decreases with distance from the nearest active channel belt. Channel‐belt degradation results in floodplain incision. Extrinsic (extrabasinal, allogenic) and intrinsic (intrabasinal, autogenic) controls on floodplain dynamics and alluvial architecture were modelled, and sequence stratigraphy models were assessed. Input parameters were chosen based on data from the Rhine–Meuse delta. To examine how the model responds to extrinsic controls, the model was run under conditions of changing base level and increasing sediment supply. Rises and falls in base level and increases in sediment supply occurred over 10 000 years. Rising base level caused a wave of aggradation to move up‐valley, until aggradation occurred over the entire valley. Frequency of bifurcations and avulsions increased with rate of base‐level rise and aggradation rate. Channel‐belt width varied with water discharge and the lifespan of the channel belt. Wide, connected channel belts (and high channel‐deposit proportion) occurred around the upstream inflow point because of their high discharge and longevity. Less connected, smaller channel belts occurred further down‐valley. Such alluvial behaviour and architecture is also found in the Rhine–Meuse delta. During base‐level fall, valley erosion occurred, and the incised valley contained a single wide channel belt. During subsequent base‐level rise, a wave of aggradation moved up‐valley, filling the incised valley. Bifurcation and avulsion sites progressively moved upstream. Relatively thin, narrow channel belts bordered and cut into the valley fill. These results differ substantially from existing sequence stratigraphy models. The increase in sediment supply from upstream resulted in an alluvial fan. Most bifurcations and avulsions occurred at the fan apex (nodal avulsion), and channel belts were the widest and the thickest here (giving high channel‐deposit proportion) due to their high discharge and longevity. The width and thickness of channel belts decreased down‐valley due to decreased discharge, longevity and aggradation rate. This behaviour occurs in modern alluvial fans. Intrinsic controls also affect floodplain dynamics and alluvial architecture. Variation of aggradation rate, bifurcation frequency and number of coexisting channel belts occurred over periods of 500 to 2000 years, compared with 10 000 years for extrinsic controls. This variation is partly related to local aggradation and degradation of channel belts around bifurcation points. Channel belts were preferentially clustered near floodplain margins, because of low floodplain aggradation rate and topography there.     

干旱条件下冲积扇的沉积构型和演化过程: 基于水槽模拟实验

利用延时相机以及3D激光扫描仪等设备,通过水槽实验获得了74组扇面地貌数据,并采用定量化的软件对这些数据进行了精细的沉积学分析,明确了干旱条件下冲积扇的沉积演化过程及其控制的沉积构型。研究表明: (1)干旱条件下冲积扇沉积演化过程中水动力、水流样式、扇体增生规律等均存在明显的差异性,根据这些差异将整个模拟实验分为早期、中期、晚期3个阶段。(2)实验早期,扇体水动力较强,主控沉积作用为片流沉积,根据片流沉积的分布范围,可将其分为整体片流沉积和局部片流沉积。(3)实验中期,扇体中远端水动力及扇面扩大速率均有所减小,主控沉积作用为非限定性水道,主要沉积体为朵体沉积,非限定性水道主要分布在近源端,而朵体主要分布在水道末端的开阔地带。(4)实验晚期,受巨大的扇体影响,水动力进一步减小,主控沉积作用为限定性水道沉积。(5)通过实验研究,建立了具有明显3层结构的干旱条件下冲积扇沉积构型模式,其中底层是片流朵体复合体、中层为非限制性水道与末端朵体复合体、顶层是水道和小型末端朵体复合体。     

Scale considerations in using diatoms as indicators of sea‐level change: lessons from Alaska

This paper assesses variations in quantitative reconstructions of late Holocene relative sea‐level (RSL) change arising from using modern diatom datasets from different spatial scales, applied to case studies from Alaska. We investigate the implications of model choice in transfer functions using local‐, sub‐regional‐ and regional‐scale modern training sets, and produce recommendations on the creation and selection of modern datasets for reconstructing RSL change over Holocene timescales in tidal marsh environments comparable with those in Alaska. We show that regional modern training sets perform best in terms of providing fossil samples with good modern analogues, and in producing reconstructions that most closely match observations, where these are available. Local training sets are frequently insufficient to provide fossil samples with good modern analogues and may over‐estimate the precision of RSL reconstructions. This is particularly apparent when reconstructing RSL change for periods beyond the last century. For reconstructing RSL change we recommend using regional modern training sets enhanced by local samples. Copyright © 2012 John Wiley & Sons, Ltd.     

River‐dominated,shelf‐edge deltas: delivery of sand across the shelf break in the absence of slope incision

Shelf‐edge deltas record the potential magnitude of sediment delivery from shallow water shelf into deep water slope and basin floor and, if un‐incised, represent the main increment of shelf‐margin growth into the basin, for that period. The three‐dimensional complexity of shelf‐edge delta systems and along‐strike variability at the shelf edge in particular, remains understudied. The Permian–Triassic Kookfontein Formation of the Tanqua Karoo Basin, South Africa, offers extensive three‐dimensional exposure (>100 km²) and therefore a unique opportunity to evaluate shelf‐edge strata from an outcrop perspective. Analysis of stratal geometry and facies distribution from 52 measured and correlated stratigraphic sections show the following: (i) In outer‐shelf areas, parasequences are characterized by undeformed, river‐dominated, storm‐wave influenced delta mouth‐bar sandstones interbedded with packages showing evidence of syn‐depositional deformation. The amount and intensity of soft‐sediment deformation increases significantly towards the shelf edge where slump units and debris flows sourced from collapsed mouth‐bar packages transport material down slope. (ii) On the upper slope, mouth‐bar and delta‐front sandstones pinch out within 2 km of the shelf break and most slump and debris flow units pinch out within 4 km of the shelf break. (iii) Further down the slope, parasequences consist of finer‐grained turbidites, characterized by interbedded, thin tabular siltstones and sandstones. The results highlight that river‐dominated, shelf‐edge deltas transport large volumes of sand to the upper slope, even when major shelf‐edge incisions are absent. In this case, transport to the upper slope through slumping, debris flows and un‐channellized low density turbidites is distributed evenly along strike.     

A combined study of radar facies,lithofacies and three‐dimensional architecture of an alpine alluvial fan (Illgraben fan,Switzerland)

Alluvial fans serve as useful archives that record the history of depositional and erosional processes in mountainous regions and thus can reveal the environmental controls that influenced their development. Economically, they play an important role as groundwater reservoirs as well as host rocks for hydrocarbons in deeply buried systems. The interpretation of these archives and the evaluation of their reservoir architecture, however, are problematic because marked heterogeneity in the distribution of sedimentary facies makes correlation difficult. This problem is compounded because the accumulated sedimentary deposits of modern unconsolidated fan systems tend to be poorly exposed and few such systems have been the focus of investigation using high‐resolution subsurface analytical techniques. To overcome this limitation of standard outcrop–analogue studies, a geophysical survey of an alpine alluvial fan was performed using ground‐penetrating radar to devise a scaled three‐dimensional subsurface model. Radar facies were classified and calibrated to lithofacies within a fan system that provided outcropping walls and these were used to derive a three‐dimensional model of the sedimentary architecture and identify evolutionary fan stages. The Illgraben fan in the Swiss Alps was selected as a case study and a network of ca 60 km sections of ground‐penetrating radar was surveyed. Seven radar facies types could be distinguished, which were grouped into debris‐flow deposits and stream‐flow deposits. Assemblages of these radar facies types show three depositional units, which are separated by continuous, fan‐wide reflectors; they were interpreted as palaeo‐surfaces corresponding to episodes of sediment starvation that affected the entire fan. An overall upward decline in the proportion of debris‐flow deposits from ca 50% to 15% and a corresponding increase in stream‐flow deposits were identified. The uppermost depositional unit is bounded at its base by a significant incision surface up to 700 m wide, which was subsequently filled up mostly by stream‐flow deposits. The pronounced palaeo‐surfaces and depositional trends suggest that allocyclic controls governed the evolution of the Illgraben fan, making this fan a valuable archive from which to reconstruct past sediment fluxes and environmental change in the Alps. The results of the integrated outcrop–geophysical approach encourage similar future studies on fans to retrieve their depositional history as well as their potential reservoir properties.     

Geological controls on the geometry of incised‐valley fills: Insights from a global dataset of late‐Quaternary examples

Incised valleys that develop due to relative sea‐level change are common features of continental shelves and coastal plains. Assessment of the factors that control the geometry of incised‐valley fills has hitherto largely relied on conceptual, experimental or numerical models, else has been grounded on case studies of individual depositional systems. Here, a database‐driven statistical analysis of 151 late‐Quaternary incised‐valley fills has been performed, the aim being to investigate the geological controls on their geometry. Results of this analysis have been interpreted with consideration of the role of different processes in determining the geometry of incised‐valley fills through their effect on the degree and rate of river incision, and on river size and mobility. The studied incised‐valley fills developed along active margins are thicker and wider, on average, than those along passive margins, suggesting that tectonic setting exerts a control on the geometry of incised‐valley fills, probably through effects on relative sea‐level change and river behaviour, and in relation to distinct characteristics of basin physiography, water discharge and modes of sediment delivery. Valley‐fill geometry is positively correlated with the associated drainage‐basin size, confirming the dominant role of water discharge. Climate is also inferred to exert a potential control on valley‐fill dimensions, possibly through modulations of temperature, peak precipitation, vegetation and permafrost, which would in turn affect water discharge, rates of sediment supply and valley‐margin stability. Shelves with slope breaks that are currently deeper than 120 m contain incised‐valley fills that are thicker and wider, on average, than those hosted on shelves with breaks shallower than 120 m. No correlation exists between valley‐fill thickness and present‐day coastal‐prism convexity, which is measured as the difference in gradient between lower coastal plains and inner shelves. These findings challenge some concepts embedded in sequence stratigraphic thinking, and have significant implications for analysis and improved understanding of ‘source to sink’ sediment route‐ways, and for attempting predictions of the occurrence and characteristics of hydrocarbon reservoirs.     

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.     

Unravelling the conundrum of river response to rising sea‐level from laboratory to field. Part II. The Fly–Strickland River system,Papua New Guinea

The most recent deglaciation resulted in a global sea‐level rise of some 120 m over ca 12 000 years. A moving boundary numerical model is developed to predict the response of rivers to this rise. The model was motivated by experiments at small scale, which have identified two modes describing the transgression of a river mouth: (i) autoretreat without abandonment of the river delta (no sediment starvation at the topset–foreset break); and (ii) sediment‐starved autoretreat with abandonment of the delta. In the latter case, transgression is far more rapid, and its effects are felt much further upstream of the river mouth. A moving boundary numerical model that captures these features in experimental deltas is adapted to describe the response of the Fly–Strickland River system, Papua New Guinea. In the absence of better information, the model is applied to the case of sea‐level rise without local climate change in New Guinea. The model suggests that: (i) sea‐level rise has forced the river mouth to transgress over 700 km since the last glacial maximum; (ii) sediment‐starved autoretreat has forced enough bed aggradation to block a tributary with a low sediment load and create the present‐day Lake Murray; (iii) the resulting aggradation was sufficient to move the gravel–sand transition on the Strickland River upstream; (iv) the present‐day Fly Estuary may be, in part, a relict river valley drowned by sea‐level rise and partially filled by tidal effects; and (v) the Fly River is presently reforming its bankfull geometry and prograding into the Fly Estuary. A parametric study with the model indicates that sediment concentration during floods plays a key role in determining whether or not, and to what extent, transgression is expressed in terms of sediment‐starved autoretreat. A sufficiently high sediment concentration can prevent sediment‐starved autoretreat during the entire sea‐level cycle. This observation may explain why some present‐day river mouths are expressed in terms of deltas protruding into the sea, and others are wholly contained within embayments or estuaries in which water has invaded landward.     

Autogenic avulsion,channelization and backfilling dynamics of debris‐flow fans

Alluvial fans develop their semi‐conical shape by quasi‐cyclic avulsions of their geomorphologically active sector from a fixed fan apex. On debris‐flow fans, these quasi‐cyclic avulsions are poorly understood, partly because physical scale experiments on the formation of fans have been limited largely to turbidite and fluvial fans and deltas. In this study, debris‐flow fans were experimentally created under constant extrinsic forcing, and autogenic sequences of backfilling, avulsion and channelization were observed. Backfilling, avulsion and channelization were gradual processes that required multiple successive debris‐flow events. Debris flows avulsed along preferential flow paths given by the balance between steepest descent and flow inertia. In the channelization phase, debris flows became progressively longer and narrower because momentum increasingly focused on the flow front as flow narrowed, resulting in longer run‐out and deeper channels. Backfilling commenced when debris flows reached their maximum possible length and channel depth, as defined by channel slope and debris‐flow volume and composition, after which they progressively shortened and widened until the entire channel was filled and avulsion was initiated. The terminus of deposition moved upstream because the frontal lobe deposits of previous debris flows created a low‐gradient zone forcing deposition. Consequently, the next debris flow was shorter which led to more in‐channel sedimentation, causing more overbank flow in the next debris flow and resulting in reduced momentum to the flow front and shorter runout. This topographic feedback is similar to the interaction between flow and mouth bars forcing backfilling and transitions from channelized to sheet flow in turbidite and fluvial fans and deltas. Debris‐flow avulsion cycles are governed by the same large‐scale topographic compensation that drives avulsion cycles on fluvial and turbidite fans, although the detailed processes are unique to debris‐flow fans. This novel result provides a basis for modelling of debris‐flow fans with applications in hazards and stratigraphy.     

Evaluating delta asymmetry using three‐dimensional facies architecture and ichnological analysis,Ferron ‘Notom Delta’, Capital Reef,Utah, USA

Delta asymmetry occurs where there is strong wave influence and net longshore transport. Differences in the morphology and facies architecture between updrift and downdrift sides of asymmetric deltas are potentially significant for exploration and exploitation of resources in this class of reservoirs. Although delta asymmetry has been recognized widely from modern wave‐influenced deltaic shorelines, there are few documented examples in the ancient record. Based on an integrated sedimentological and ichnological study, the along‐strike variability and delta asymmetry within a single parasequence (Ps 6) is documented in continuously exposed outcrops of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation near Hanksville in southern Utah. Two intra‐parasequence discontinuity surfaces are recognized which allow subdivision of the parasequence into three bedsets, marked as Ps 6‐1 to Ps 6‐3. Four facies successions are recognized: (i) wave/storm‐dominated shoreface; (ii) river‐dominated delta front; (iii) wave/storm‐reworked delta front; and (iv) distributary channel and mouth bar. Dips of cross‐strata within distributary‐mouth bars and shorefaces show a strong downdrift (southward) component. Ps 6‐3 predominantly consists of river‐dominated delta‐front deposits, whereas Ps 6‐1 and Ps 6‐2 show an along‐strike facies change with shoreface deposits in the north, passing into heterolithic, river‐dominated delta‐front successions south to south‐eastward, and wave/storm‐reworked delta‐front deposits further to the south‐east. Trace fossil suites correspondingly show distinct along‐strike changes from robust and diverse expressions of the archetypal Cruziana Ichnofacies and Skolithos Ichnofacies, into suites characterized by horizontal, morphologically simple, facies‐crossing ichnogenera, reflecting a more stressed, river‐dominated environment. Further south‐eastward, trace fossil abundance and diversity increase, reflecting a return to archetypal ichnofacies. The overall facies integrated with palaeocurrent data indicate delta asymmetry. The asymmetric delta consists of sandier shoreface deposits on the updrift side and mixed riverine and wave/storm‐reworked deposits on the downdrift side, similar to that observed in the modern examples. However, in contrast to the recent delta asymmetry models, significant paralic, lagoonal and bay‐fill facies are not documented in the downdrift regions of the asymmetric delta. This observation is attributed to a negative palaeoshoreline trajectory during delta progradation and subsequent transgressive erosion. The asymmetric delta was induced by net longshore transport from north to south. The forced regressive nature of the delta precludes significant preservation of topset mud.     

Nature,origin and evolution of a Late Pleistocene incised valley‐fill,Sunda Shelf,Southeast Asia

Understanding the stratigraphic fill and reconstructing the palaeo‐hydrology of incised valleys can help to constrain those factors that controlled their origin, evolution and regional significance. This condition is addressed through the analysis of a large (up to 18 km wide by 80 m deep) and exceptionally well‐imaged Late Pleistocene incised valley from the Sunda Shelf (South China Sea) based on shallow three‐dimensional seismic data from a large (11 500 km²), ‘merge’ survey, supplemented with site survey data (boreholes and seismic). This approach has enabled the characterization of the planform geometry, cross‐sectional area and internal stratigraphic architecture, which together allow reconstruction of the palaeo‐hydrology. The valley‐fill displays five notable stratigraphic features: (i) it is considerably larger than other seismically resolvable channel forms and can be traced for at least 180 km along its length; (ii) it is located in the axial part of the Malay Basin; (iii) the youngest part of the valley‐fill is dominated by a large (600 m wide and 23 m deep), high‐sinuosity channel, with well‐developed lateral accretion surfaces; (iv) the immediately adjacent interfluves contain much smaller, dendritic channel systems, which resemble tributaries that drained into the larger incised valley system; and (v) a ca 16 m thick, shell‐bearing, Holocene clay caps the valley‐fill. The dimension, basin location and palaeo‐hydrology of this incised valley leads to the conclusion that it represents the trunk river, which flowed along the length of the Malay Basin; it connected the Gulf of Thailand in the north with the South China Sea in the south‐east. The length of the river system (>1200 km long) enables examination of the upstream to downstream controls on the evolution of the incised valley, including sea‐level, climate and tectonics. The valley size, orientation and palaeo‐hydrology suggest close interaction between the regional tectonic framework, low‐angle shelf physiography and a humid‐tropical climatic setting.     

Sea‐level changes,river activity,soil development and glaciation around the western margins of the southern North Sea Basin during the Early and early Middle Pleistocene: evidence from Pakefield,Suffolk, UK

This paper outlines evidence from Pakefield (northern Suffolk), eastern England, for sea‐level changes, river activity, soil development and glaciation during the late Early and early Middle Pleistocene (MIS 20–12) within the western margins of the southern North Sea Basin. During this time period, the area consisted of a low‐lying coastal plain and a shallow offshore shelf. The area was drained by major river systems including the Thames and Bytham. Changes in sea‐level caused several major transgressive–regressive cycles across this low‐relief region, and these changes are identified by the stratigraphic relationship between shallow marine (Wroxham Crag Formation), fluvial (Cromer Forest‐bed and Bytham formations) and glacial (Happisburgh and Lowestoft formations) sediments. Two separate glaciations are recognised—the Happisburgh (MIS 16) and Anglian (MIS 12) glaciations, and these are separated by a high sea level represented by a new member of the Wroxham Crag Formation, and several phases of river aggradation and incision. The principal driving mechanism behind sea‐level changes and river terrace development within the region during this time period is solar insolation operating over 100‐kyr eccentricity cycles. This effect is achieved by the impact of cold climate processes upon coastal, river and glacial systems and these climatically forced processes obscure the neotectonic drivers that operated over this period of time. © British Geological Survey/Natural Environment Research Council copyright 2005. Reproduced with the permission of BGS/NERC. Published by John Wiley & Sons, Ltd.     

Aeolian construction and alluvial dismantling of a fault‐bounded intracontinental aeolian dune field (Teruel Basin,Spain); a continental perspective on Late Pliocene climate change and variability

An aeolian dune field migrating to the east encroached on the toes of alluvial fans in the Teruel Basin (eastern Spain) during a short interval in the Late Pliocene (ca 2·9 to 2·6 Ma), when Northern Hemisphere glaciation and strong glacial–interglacial cycles began. Preservation of the dune field was controlled by syn‐sedimentary activity of a normal fault. Ephemeral water discharge eroded aeolian sands and formed V‐shaped channels in which aeolian sandstone blocks accumulated. The incorporation of loose aeolian sand in wadi waters modified the sediment/water ratio, changing the physical properties of the flows as they penetrated the aeolian dune field. The erosion and cover of aeolian dune foresets by sheetflood deposits suggest that dune‐damming caused the intermittent ponding of water behind the dunes and its flashy release. The arid climate in the Late Pliocene western Mediterranean realm favoured the transport of windblown sediments from northern Africa and western Mediterranean land masses into the Mediterranean. The formation of the studied aeolian dune field (2·9 to 2·6 Ma) and possibly others (for example, the Atacama, Namib and Sahara deserts) correlates with a strong increase of the influence of obliquity, which can be attributed to the combination of a regional expression related to the reduced effect of precession due to a minimum in the long‐period (2·3 Ma) eccentricity cycle and a remote expression of the onset of the Northern Hemisphere glaciation.