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ABSTRACT Fluvial facies models based on river planform are flawed and not fully fit for purpose. An alternative approach to classifying fluvial deposits is based on river discharge characteristics. A recent paper showed that the co‐efficient of variance of annual peak discharge (CVQp) correlates well with the characteristics of preserved alluvium for a suite of modern rivers. Here, the database of rivers is expanded and augmented, and the utility of this statistic confirmed. For modern rivers, plotting the running‐value of CVQp for a set interval (for example, 30 years) over a longer discharge record shows that the statistic varies both spatially and temporally. The CVQp can rise abruptly at a given location in response to a single flow event if that event is significantly larger than is common in that river. Such abrupt changes in CVQp are superimposed on longer‐term patterns of more gradual change stimulated by environmental factors or anthropogenic effects such as dam construction. If the period of calculation for CVQp is relatively short, ‘anomalous’ events appear more frequently. Sequential studies of modern rivers indicate that such anomalous events have a high probability of being preserved in the alluvial record of the river, and suggest that, over the timeframes of channel filling and abandonment, they may constitute the dominant (characteristic) portion of the record in high CVQp systems. Since flow events with strongly peaked hydrographs (flashy discharges) are more common in high CVQp systems and leave a record dominated by upper flow regime stratification styles, it follows that the alluvial record of high CVQp rivers will be dominated by upper flow regime stratification. In contrast, in lower CVQp systems the occurrence of an event that changes the running CVQp value significantly may form a distinct event deposit, and the temporarily changed CVQp is unrepresentative for the duration of the channel's life. The dominance of upper flow regime stratification, together with decreased preservation of identifiable macroform structure, increased lateral lithological heterogeneity, and in many cases increased preservation of in situ tree fossils in channel deposits of higher CVQp rivers, forms the basis for a series of new facies models for low, moderate, high and very high/ultra‐high CVQp alluvial systems and deposits. The new models can be applied to ancient successions either in combination with traditional, planform‐based models, or not. These models provide information about ancient palaeoenvironmental conditions, and changes in such conditions over time at various scales via stratigraphic investigations. [ABSTRACT FROM AUTHOR]

For more extensive and efficient sea‐level and lake‐level reconstructions and correlations in siliciclastic systems, this paper presents a new tool called SediRate‐Fischer plot (SR‐Fischer plot). In SR‐Fischer plots, the linear‐corrected sedimentary rate of each terrigenous sedimentary cycle is plotted from the top of the previous cycle to the top of the current cycle. This process is repeated cycle by cycle, constructing curves of cumulative departure from mean sedimentary rate (CDMR). Based on the assumption that relative sea‐level and lake‐level changes may control sedimentary rates of subaqueous terrigenous sediments within basins, the CDMR could change inversely with relative sea or lake level. Thus, this tool could be interpreted as an indicator of relative sea‐level and lake‐level changes in terrigenous successions. SediRate‐Fischer plots can be obtained from either lithological or proxy cycles formed by sea‐level and lake‐level changes in the case of availability of orbitally tuned data. The SR‐Fischer plots constructed based on different materials are compared with other methods in two cases, as follows: first for the lacustrine Mingshui Formation of the Songliao Basin in China; and second for the deep‐marine Arcillas de Gibraleón Formation of the Guadalquivir Basin in Spain. The SR‐Fischer plots of the Mingshui Formation exhibit low and high CDMR values for shallow‐lake intervals and lakeshore intervals, respectively, and vary inversely with the lake‐level curves derived from changes in sedimentary facies. For the Arcillas de Gibraleón Formation, the CDMR values illustrated by the SR‐Fischer plots exhibit opposite variations with sea‐level curves indicated by dinocyst/pollen ratios. The negative Z‐scores derived from runs tests indicate that these SR‐Fischer plots are not randomly stacked and are minimally influenced by different cycle‐splitting strategies. Thus, the results of comparison with other methods and runs tests demonstrate the validity of this method. Nonetheless, the other allogenic inflows (tectonics and climates) may limit interpretations of SR‐Fischer plots. [ABSTRACT FROM AUTHOR]

To better understand the sedimentary facies of tide‐dominated deltas, a core dataset from the Ba Lai palaeochannel in the Mekong River Delta was obtained and studied. Nine sedimentary facies were identified and interpreted as representing the Late Holocene evolution of the Ba Lai palaeochannel, including its pre‐abandonment and post‐abandonment phases, as well as pre‐channel phases. The channel formed at 2.6 ka as a distributary channel connected to the deltaic network and was abruptly abandoned and rapidly infilled with sediment at 0.7 ka. The channel deposits are up to ca 11 m thick and overlie shelfal shell layers, which, in turn, overlie Mid‐Holocene and Pleistocene deposits. The active‐channel fill and mouth‐bar deposits consist of sand and mud with cyclical patterns, bidirectional lenses and abundant mud layers, suggesting deposition mainly driven by river and tidal processes. The abandoned‐channel fill consists entirely of organic‐rich mud, suggesting a predominance of tidal processes. Other sedimentary facies include tidal‐flat and marsh deposits; they mostly consist of mud and formed in shallow to subaerial areas near the channel margins or on barforms. Depending on the exact location of the core in this depositional setting, three possible stratigraphic successions and facies models are presented herein. Near channel margins, the deposits show an upward gradual change from heterolithic to mud with a well‐developed fining‐upward trend. Away from the channel margins, the deposits show an upward sharp change from heterolithic to mud due to the channel abandonment. The mouth‐bar‐area facies model shows an upward gradual change from heterolithic to heterolithic/sand to mud deposits with a coarsening‐upward to fining‐upward trend. Although differences among systems likely exist, the authors suggest that the sedimentary facies described in this study and the resulting facies models should be used to better understand tide‐dominated deltaic systems and to improve their interpretation in the geological record. [ABSTRACT FROM AUTHOR]

Early Triassic temperatures were some of the hottest of the Phanerozoic, sea‐surface temperatures approached 40°C, with profound consequences for both the sedimentology and faunal distributions in the oceans. However, the impact of these temperatures in terrestrial settings is unclear. This study examines shallow lacustrine sediments from the Lower Triassic succession of North China. These consist of diverse fluvial to shallow lacustrine sandstones and also spectacular, coarse conglomerates composed of diverse, intraformational clasts reworked from the interbedded sediments. The conglomerate beds can show inverse grading and high angle, flat‐pebble imbrication in their lower part and vertically orientated flat pebbles in their upper part. The cobbles include cemented and reworked conglomerate intraclasts and sandstone concentrically‐laminated concretions that record multi‐step histories of growth and reworking, pointing to rapid cementation of the sandy lake bed (likely facilitated by high temperatures). The conglomerates record frequent, high‐energy events that were capable of brecciating a lithified lake bed and transporting cobbles in wave‐influenced sediment‐gravity flows. Initially, powerful oscillatory flows brecciated and deflated the lake bed and subsequently helped to sustain turbulence during short‐distance lateral flow. It is possible that hurricanes, originating from the adjacent hyper‐warm, Palaeo‐Tethyan Ocean travelled into the major lakes of the North China continent during the Early Triassic. [ABSTRACT FROM AUTHOR]

The tide‐dominated Messina Strait (southern Italy) is a 3 km wide marine passageway, whose block‐faulted borders form steep subaqueous zones incised by canyons and gullies. These erosional features retreat towards the shorelines and are often in direct connection with subaerial valley‐bounded river deltas. High‐energy density‐flows generated by river floods periodically enter the canyon heads, attaining supercritical‐flow regime and accreting large, upslope‐migrating bedforms. Although these bedforms have been documented in recent studies, little attention has been paid to the definition of the type of delta entering canyon heads, the internal features of river‐influenced deposits accumulated in the nearshore zone, and their interplay with tidal currents flowing axially to the strait. This study focuses on a Lower Pleistocene coarse‐grained succession exposed along the north‐eastern margin of the modern Messina Strait, investigated using conventional facies analysis and sedimentological logging, integrated with photogrammetric techniques and interpretation of drone‐acquired imagery. Facies confinement between basement blocks suggests a subaqueous delta complex shed from the tectonically controlled margin of the ancient strait and entering shallowly submerged canyon heads. Basal breccias, conglomerates and pebbly sandstones exhibiting channel‐form discontinuities and upslope dipping backsets are interpreted as cyclic‐step and antidune deposits. Units composed of these facies are comprised between master erosional surfaces and tidal ravinement surfaces. The tidal ravinements suggest that canyon infill occurred during a major phase of sea‐level rise, punctuated by minor falls and stillstands. These surfaces are overlain by mixed bioclastic–siliciclastic, arenitic, trough and planar cross‐strata, representing dunes migrating roughly parallel to the palaeo‐coastline and originated by tidal currents amplified by the narrowing of the ancient Messina Strait. Tidal‐influenced sedimentation dominated over the fluvial‐influenced processes during the late transgression, overfilling the canyon relief. The exceptionally good exposure of depositional architectures and facies characteristics is key to outline the general features of a specific type of delta system, fed by valley‐bounded rivers and entering canyon heads in the nearshore of tectonically‐controlled, tide‐influenced steep strait margins. The pre‐existing subaqueous incised topography forced the delta front to be split into lobe branches during the canyon infilling, hampering clinoform architectures and preserving large supercritical‐flow sedimentary structures. This study suggests these as possible criteria for the recognition of similar systems in outcrop or subsurface. [ABSTRACT FROM AUTHOR]

Tributary rivers can contribute significantly to alluvial‐plain construction by supplying large volumes of clastic material. Their relation to the main axial river strongly influences sediment deposition and preservation. The Po Plain is fed by the Po River and a dense network of transverse tributaries draining the nearby Alpine and Apennine chains. Stratigraphic, sedimentological, petrographic and geochemical analyses on 38 cores permitted detailed differentiation of Po and Apennine sedimentary units. Po River deposits are vertically stacked channel‐belt sand bodies with high contents of quartz–feldspar and metamorphic rock fragments, combined with high chromium levels. These sand bodies, 20 to 30 km wide, are replaced southward by finer‐grained deposits that represent the distal Apennine tributary‐rivers system. Apennine sands, confined in narrow ribbons, show lower quartz–feldspar contents, abundant sedimentary lithics and lower chromium levels. In the last 870 kyr, the boundary between the Po and the Apennine sediment delivery systems shifted along a north–south axis in response to distinct controlling factors. A 20 km northward shift of the Po channel belt, possibly related to a tectonic event, is recorded across a regional unconformity dating to the Marine Isotope Stage 12/11 transition. High sediment supply rates during glacial‐lowstand periods widened the Po channel belt southward towards the Apennine domain for a few kilometres. The Last Glacial Maximum channel‐belt sand body, 30 km wide and 40 m thick, progressively narrowed northward after the glacial culmination. During the Holocene, channel patterns became avulsive and distributive. Narrow channel belts (<3 km) formed along the Po River branches, and abundant swamp and poorly drained‐floodplain muds were preserved in interfluvial areas. Activation and deactivation of the Po branches resulted in sharp narrowing and widening of the area available for Apennine‐rivers sedimentation. This work provides insights into tributary‐trunk river relations which control grain‐size distribution and compositional characters of subsurface deposits. [ABSTRACT FROM AUTHOR]

Ancient dryland terminal fluvial systems are often recognized within the rock record for having a progressive downstream decrease in the size and amalgamation of channel elements and systematic downstream increase in sheet and overbank elements, alongside the downstream decrease in grain size that is displayed by most fluvial systems. The spatial distribution and downstream trends displayed by the fluvial sediments of the Lower Jurassic Kayenta Formation of south‐western USA, have been examined quantitatively. The results indicate many trends that are similar to those of a dryland terminal fluvial system, including; a lack of confinement of the fluvial system, a downstream decrease in channel and sheet element amalgamation and width‐to‐thickness ratios, a downstream decrease in grain size, albeit very small, and an increase in the percentage of overbank elements downstream. However, the study highlights several downstream relationships that are atypical. While some of these relationships may be the result of external factors inherent in this study, others, including the thicknesses of channel‐fill and sheet elements that display no significant relationships to distance downstream, and channel‐fill elements that display no significant variation in average grain size with distance downstream, may be a consequence of fluvial interaction with a competing and coeval aeolian system. This work demonstrates the inherent complexity in arid dryland fluvial systems and the downstream architectural and compositional relationships that they depict. Consequently, models for fluvial style may provide only a first‐order approximation for downstream trends in dryland systems, because the controlling factors upon these systems are inherently difficult to unravel, and the sedimentary detail is strongly dependent upon external setting and internal complexity. Consequently, a generalized model may not always be applicable to these systems. [ABSTRACT FROM AUTHOR]

An analysis of the sedimentary fill of a foreland basin can provide information on the relative effect of tectonics, base‐level fluctuations and climate during basin development. This study analyzes fluvial strata from the upper Cedar Mountain and Naturita formations of the mid‐Cretaceous foreland basin of Utah (USA) to determine the effects of spatial location within the foreland basin on fluvial planform and architecture. The study results document consistent planform and architecture across the distal foredeep and forebulge depozones during flooding of the Western Interior Seaway suggesting that fluvial planform and architecture did not change as a function of position within foreland basin depozone or as a result of base‐level rise. Outcrop data, virtual outcrop models and satellite imagery were used to perform facies and architectural analysis and to collect dimensional and geometric data. Fluvial channel deposits consist of coarse‐grained, laterally accreting bars deposited within narrow, low to moderate sinuosity channels, that were vertically stacked and encased in floodplain fines regardless of geographic location or stratigraphic position and may represent distal deposits of a distributive fluvial system. Bar elements increased in thickness over time indicating increasing channel depths and enhanced discharge. Palaeohydraulic calculations indicate an increase in discharge from between 315 and 1023 to between 1896 and 5270 m3/s, likely due to enhanced precipitation and/or drainage capture in the catchment and basin. Calculated drainage areas for the system increased from 104 to 105 km2 scale over time. The uppermost fluvial deposits were in close proximity (≤60 km) to contemporaneous shoreface systems as the Western Interior Seaway expanded; yet backwater effects are not observed, suggesting that rising base level had no significant effect on fluvial planform or architecture. These results show that fluvial planform and architecture remained constant despite rising base‐level, changing climate, increased discharge and position in different foreland basin depozones. [ABSTRACT FROM AUTHOR]

Tidal currents can propagate tens of kilometres landward from the shoreline, forming dense networks of meandering channels, which drain vegetated areas and range in width from tens to hundreds of metres. Sedimentary products of these inland tidal channels are poorly documented in the stratigraphic record, as most studies on tidal processes occurring in inland areas are focused on fluvio‐tidal interaction. The present study contributes to filling this gap, investigating Late Maastrichtian meandering‐channel deposits of the Tremp‐Graus Basin (Southern Pyrenees, Spain). These deposits accumulated in a growth syncline, which allowed the development of a 20 to 30 km wide, 80 to 100 km long, tidal embayment. Sporadic occurrence of poorly‐developed rhythmites and bidirectional currents confirms the occurrence of tidal processes, whereas their local dominance is inferred from several pieces of evidence, including: (i) disproportion between depth of the larger channels and related catchment area; (ii) landward‐accretion of point‐bar bodies; (iii) landward fining of point‐bar bodies, and (iv) reconstruction of palaeo‐flow patterns at the meander bend scale. This work shows that three‐dimensional architectural modelling and reconstruction of bar planform transformation styles can provide a critical contribution to the understanding of tidal control on sedimentation in inland areas. [ABSTRACT FROM AUTHOR]

Lake‐delta deposits in low‐accommodation conditions are only a few metres thick and in some cases non‐existent. Fluvial channels occur in many cases directly on top of fine‐grained lacustrine deposits with delta deposits apparently missing. The geomorphology and stratigraphy of modern deltas in Lake Eyre, Australia, and Lake Chad in Africa indicate that delta shorelines can develop in lakes, but in many cases build out into low‐accommodation settings due to highly dynamic lake‐water levels. This work suggests that low‐accommodation is critical in the development of lacustrine delta deposits and that high (frequency and magnitude) rates of lake‐level change result in 'telescoping' deltas, the deposits of which are thin but extensive. The telescopic deltas have conformable beds, but with abrupt vertical facies changes; this apparent stratigraphic contradiction is the result of long horizontal distance translation of the depositional system. Details of two lacustrine units, in Uinta Basin in the USA and Junggar Basin in China, both of which show characteristics of telescoping, are used to develop a process based stratigraphic model for the deposits of such lake deltas. Lacustrine facies comprise cross‐stratified, ripple cross‐laminated, structureless or graded and sharp‐based sandstone beds, mudstone (structureless, laminated, rippled, rich in organic content), carbonate mudstone, ooid packstone and stromatolites. Three facies associations are critical for lake delta stratigraphy: channelized distributary deposits that occur in the delta topset; lake plain deposits that represent the fluvial overbank deposits and the subaerially exposed (at low lake level) lake deposits; and fine‐grained, lacustrine deposits. A key corollary of the proposed low‐gradient, telescoping lacustrine delta model is that broad and shallow channels dominate the system, but that decimetre to metre thick, gently dipping delta‐front beds form subtle deposits along the margins of the lakes. These delta depocentres migrate basinward and landward at high rates (tens of kilometres each year), building thin (metres) but laterally extensive (tens of kilometres) sandstone units through multiple transgression–regression cycles. [ABSTRACT FROM AUTHOR]

Despite the fact that mud‐dominated composite particles have increasingly been recognized as important components of marine mudstones, the characteristics, types and origins of these composite particles remain poorly understood. This incomplete understanding of critical mudstone parameters (for example, depositional grain size, composition at the particle scale and provenance) severely limits the ability to accurately interpret the conditions (for example, depositional environment and climate) under which these rocks were deposited. Herein, detailed petrographic analysis was conducted in core samples from a transect of contemporaneous proximal to distal deposits from the Cenomanian Dunvegan Formation, a fluvial‐dominated delta system in the Western Canada Sedimentary Basin. Within the well‐developed depositional framework, mud‐dominated composite particles can be tracked along the depositional profile of the Dunvegan Formation from the fluvial to the marine realm (from incised valley to distal prodelta). By following these particles from source (hinterland erosion) to sink (offshore deposition), the origin and dispersal of various types of mud‐dominated composite particles in marine mudstones can be unequivocally determined. Mud‐dominated composite particles derived from a wide range of origins are differentiated. Allochthonous mud‐dominated composite particles, supplied by rivers draining the hinterland, comprise volcanic rock fragments (vitric or felsic in composition), chert fragments, shale lithics (clay‐mineral‐rich or quartz‐rich in composition), metamorphic rock fragments and chlorite/siderite clasts. Autochthonous mud‐dominated composite particles include rip‐up clasts (argillaceous, sideritic or siliceous in composition) formed from intra‐basinal erosion and contemporaneous floccules. The recognition of mud‐dominated composite particles shows that the Dunvegan prodelta 'mudstones' are much coarser grained and more heterogeneous in terms of their petrographic composition than the grain size of their component minerals would suggest. Results of this study call for a critical reappraisal of the composition and actual grain size of mudstones in general. Recognition of mud‐dominated composite particles can provide valuable insights to unravel their provenance, transport history and depositional setting, and will greatly enhance their utility as palaeoenvironmental archives. [ABSTRACT FROM AUTHOR]

The response of barrier islands to sea‐level rise is modulated by combinations of coastal processes, eco‐geomorphic feedbacks and structural controls, such as antecedent topography. Interactions among these drivers can lead to complex and non‐linear changes in island morphology and transitions between migrational, erosional or progradational states. This study seeks to constrain the morphological consequences of barrier islands migrating across complex antecedent topography in response to rising sea level. The stratigraphy of four barrier–backbarrier systems along the United States Mid‐Atlantic coast informs idealized geometries of diverse antecedent substrate. These outcomes are integrated into a cross‐shore morphodynamic model of barrier‐island migration to quantify the influence of this antecedent geology on barrier‐retreat behaviour. Additionally, this study explores the future response of specific barrier islands to various rates of sea‐level rise over multi‐decadal to millennial timescales. The results show that antecedent substrate slope plays a central role in barrier morphodynamic behaviour. In particular, migration across a subaqueous backbarrier ridge (for example, coastal barrier or dune deposits from earlier sea‐level highstands) can cause a succession of phase changes in a modern island. For example, the case studies illustrate that the steep slopes and decreased backbarrier accommodation associated with antecedent highs greater than 3 m in profile can greatly reduce island migration rates, effectively 'pinning' the island in place, even with sea‐level rise rates up to 6 mm yr−1. However, once the island migrates over the high, backbarrier accommodation increases, leading to enhanced overwash fluxes, more rapid landward migration, and possible drowning. Additionally, the results indicate that antecedent substrate may slow barrier‐island migration by providing sediment through both shoreface and inlet processes. The field and modelling insights from this study are presented as a conceptual model of the relative influence of various antecedent features on barrier‐island dynamics along sandy, siliciclastic coasts. [ABSTRACT FROM AUTHOR]

Wave‐dominated deltas and strandplains make up the majority of the world's depositional coastlines, provide an important record of sea‐level change and serve as hydrocarbon reservoirs worldwide. Satellite imagery forms a great source of data on the recent depositional history of modern deltaic systems. In the subsurface, three‐dimensional seismic and well data make the three‐dimensional assessment of large‐scale deltaic reservoir bodies possible but struggle to resolve internal heterogeneities away from wells. To bridge this gap in characterizing deltaic sedimentation, this study combines measurements from both the shallow, high‐resolution section of three‐dimensional seismic data of the Eocene Halibut Delta in the Outer Moray Firth, offshore Scotland, with information from Google Earth's satellite imagery and digital elevation model on south‐east Brazilian river deltas (São Francisco, Jequitinhonha, Doce and Paraíba do Sul) to present a means of predicting the location of fluvial sediment input points with respect to clinoform geometry. The key measurement for this study is the delta front and clinoform dip which has been measured at multiple locations along strike of the coastline of the examined deltas. Dip decreases away from the inferred river mouth for all deltas by 50% within 7.2 km. The river mouth location was inferred from the position of palaeo‐channels visible on the delta top and coarse sediment recorded in grab samples offshore for the south‐east Brazilian deltas, and from imprints of palaeo‐channels on attribute maps for the Eocene Halibut Delta. In summary, this study found that delta front dip is steepest at the location of the river mouth and decreases, along with grain size, away from it. This suggests that high dip values correlate with the proximity to the channel mouth and can be used to predict fluvial channel facies in modern deltaic systems and subsurface reservoirs. [ABSTRACT FROM AUTHOR]

Upper Permian to Lower Triassic coastal plain successions of the Sydney Basin in eastern Australia have been investigated in outcrop and continuous drillcores. The purpose of the investigation is to provide an assessment of palaeoenvironmental change at high southern palaeolatitudes in a continental margin context for the late Permian (Lopingian), across the end‐Permian Extinction interval, and into the Early Triassic. These basins were affected by explosive volcanic eruptions during the late Permian and, to a much lesser extent, during the Early Triassic, allowing high‐resolution age determination on the numerous tuff horizons. Palaeobotanical and radiogenic isotope data indicate that the end‐Permian Extinction occurs at the top of the uppermost coal bed, and the Permo‐Triassic boundary either within an immediately overlying mudrock succession or within a succeeding channel sandstone body, depending on locality due to lateral variation. Late Permian depositional environments were initially (during the Wuchiapingian) shallow marine and deltaic, but coastal plain fluvial environments with extensive coal‐forming mires became progressively established during the early late Permian, reflected in numerous preserved coal seams. The fluvial style of coastal plain channel deposits varies geographically. However, apart from the loss of peat‐forming mires, no significant long‐term change in depositional style (grain size, sediment‐body architecture, or sediment dispersal direction) was noted across the end‐Permian Extinction (pinpointed by turnover of the palaeoflora). There is no evidence for immediate aridification across the boundary despite a loss of coal from these successions. Rather, the end‐Permian Extinction marks the base of a long‐term, progressive trend towards better‐drained alluvial conditions into the Early Triassic. Indeed, the floral turnover was immediately followed by a flooding event in basinal depocentres, following which fluvial systems similar to those active prior to the end‐Permian Extinction were re‐established. The age of the floral extinction is constrained to 252.54 ± 0.08 to 252.10 ± 0.06 Ma by a suite of new Chemical Abrasion Isotope Dilution Thermal Ionization Mass Spectrometry U‐Pb ages on zircon grains. Another new age indicates that the return to fluvial sedimentation similar to that before the end‐Permian Extinction occurred in the basal Triassic (prior to 251.51 ± 0.14 Ma). The character of the surface separating coal‐bearing pre‐end‐Permian Extinction from coal‐barren post‐end‐Permian Extinction strata varies across the basins. In basin‐central locations, the contact varies from disconformable, where a fluvial channel body has cut down to the level of the top coal, to conformable where the top coal is overlain by mudrocks and interbedded sandstone–siltstone facies. In basin‐marginal locations, however, the contact is a pronounced erosional disconformity with coarse‐grained alluvial facies overlying older Permian rocks. There is no evidence that the contact is everywhere a disconformity or unconformity. [ABSTRACT FROM AUTHOR]

Coastal barriers provide sheltered, low‐energy settings for fine‐grained sediment deposition and retention, although the process of back‐barrier infilling and how tidal‐channel connectivity impacts this process is not well‐understood. Understanding how back‐barrier environments infill and evolve is necessary to predict how they will respond to future changes in sea‐level and sediment supply. With this motivation, in situ observations and sedimentary signatures from an Amazonian tidal‐channel system are interpreted to create a conceptual model of morphological evolution in a macrotidal back‐barrier environment that is rich in fine‐grained sediment, vegetated by mangroves and incised by tidal channels with multiple outlets. Results indicate that within a high‐connectivity back‐barrier channel, tidal processes dominate sedimentation and morphological development. Sediment cores (<60 cm) exhibited millimetre‐scale tidalites composed of sand and mud. High‐connectivity channels allow tidal propagation from multiple inlets, and in this case, the converging flood waves promote delivery of sediment fluxing through the system to the mangrove flats in the convergence zone. Sediment preferentially deposits in regions with adequate accommodation space and dense vegetation, and in these zones, sediment grain size is slightly finer than that transiting through the system. The greatest sediment‐accumulation rates (3 to 4 cm yr−1), calculated from steady‐state 210Pb profiles, were found in the convergence zone near the mangrove‐channel edge. As tidal flats aggrade vertically and prograde into the channels, accommodation space diminishes. In effect, the channel's narrowest stretch is expected to migrate along the path of net‐sediment flux towards regions with more accommodation space until it reaches the tidal‐convergence zone. The location of recent preferential infilling is evidenced by relatively rapid sediment‐accumulation rates, finer sediment and significant clustering of small secondary tidal channels. These findings shed light on how sediment transported through vegetated back‐barrier environments is ultimately preserved and how evidence preserved in surface morphology and the geological record can be interpreted. [ABSTRACT FROM AUTHOR]

Many published interpretations of ancient fluvial systems have relied on observations of extensive outcrops of thick successions. This paper, in contrast, demonstrates that a regional understanding of palaeoriver kinematics, depositional setting and sedimentation rates can be interpreted from local sedimentological measurements of bedform and barform strata. Dune and bar strata, channel planform geometry and bed topography are measured within exhumed fluvial strata exposed as ridges in the Ruby Ranch Member of the Cretaceous Cedar Mountain Formation, Utah, USA. The ridges are composed of lithified stacked channel belts, representing at least five or six re‐occupations of a single‐strand channel. Lateral sections reveal well‐preserved barforms constructed of subaqueous dune cross‐sets. The topography of palaeobarforms is preserved along the top surface of the outcrops. Comparisons of the channel‐belt centreline to local palaeotransport directions indicate that channel planform geometry was preserved through the re‐occupations, rather than being obscured by lateral migration. Rapid avulsions preserved the state of the active channel bed and its individual bars at the time of abandonment. Inferred minimum sedimentation durations for the preserved elements, inferred from cross‐set thickness distributions and assumed bedform migration rates, vary within a belt from one to ten days. Using only these local sedimentological measurements, the depositional setting is interpreted as a fluvial megafan, given the similarity in river kinematics. This paper provides a systematic methodology for the future synthesis of vertical and planview data, including the drone‐equipped 2020 Mars Rover mission, to exhumed fluvial and deltaic strata. [ABSTRACT FROM AUTHOR]

Ephemeral fluvial systems are commonly associated with arid to semi‐arid climates. Although their complex sedimentology and depositional settings have been described in much detail, depositional models depicting detailed lateral and vertical relationships, and interactions with coeval depositional environments, are lacking compared to well‐recognized meandering and braided fluvial systems. This study critically evaluates the applicability of current models for ephemeral fluvial systems to an ancient arid fluvial example of the Lower Jurassic Kayenta Formation of the Colorado Plateau, USA. The study employs detailed sedimentary logging, palaeocurrent analysis and photogrammetric panels across the regional extent of the Kayenta. A generic model that accounts for the detailed sedimentology of a sandy arid ephemeral fluvial system (drawing upon both ancient and geomorphological studies) is developed, along with analysis of the spatial and temporal interactions with the aeolian setting. Results show that the ephemeral system is dominated by laterally and vertically amalgamated, poorly channelized to sheet‐like elements, with abundant upper flow regime flat beds and high sediment load structures formed between periods of lower flow regime conditions. Through interaction with a coeval aeolian system, most of the fluvial deposits are dominated by sand‐grade sediment, unlike many modern ephemeral fluvial systems that contain a high proportion of conglomeratic and/or finer grained mudstone and siltstone deposits. During dominantly fluvial deposition, high width to thickness ratios are observed for channelized and sheet‐like elements. However, with increasing aridity, the aeolian environment becomes dominant and fluvial deposition is restricted to interdune corridors, resulting in lower width to thickness ratio channels dominated by flash‐flood and debris‐flow facies. The data presented here, coupled with modern examples of ephemeral systems and flood regimes, suggest that ephemeral flow produces and preserves distinctive sedimentological traits that can not only be recognized in outcrops, but also within core. [ABSTRACT FROM AUTHOR]

Aggradation and fluvial incision controlled by downstream base‐level changes at timescales of 10 to 500 kyr is incorporated in classic sequence stratigraphic models. However, upstream climate control on sediment supply and discharge variability causes fluvial incision and aggradation as well. Orbital forcing often regulates climate change at 10 to 500 kyr timescales while tectonic processes such as flexural (un)loading exert a dominant control at timescales longer than 500 kyr. It remains challenging to attribute fluvial incision and aggradation to upstream or downstream processes or disentangle allogenic from autogenic forcing, because time control is mostly limited in fluvial successions. The Palaeocene outcrops of the fluvial Lebo Shale Member in north‐eastern Montana (Williston Basin, USA) constitute an exception. This study uses a distinctive tephra layer and two geomagnetic polarity reversals to create a 15 km long chronostratigraphic framework based on the correlation of twelve sections. Three aggradation–incision sequences are identified with durations of approximately 400 kyr, suggesting a relation with long‐eccentricity. This age control further reveals that incision occurred during the approach of – or during – a 405 kyr long‐eccentricity minimum. A long‐term relaxation of the hydrological cycle related to such an orbital phasing potentially exerts an upstream climate control on river incision. Upstream, an expanding vegetation cover is expected because of an increasingly constant moisture supply to source areas. Entrapping by vegetation led to a significantly reduced sediment supply relative to discharge, especially at times of low evapotranspiration. Hence, high discharges resulted in incision. This study assesses the long‐eccentricity regulated climate control on fluvial aggradation and incision in a new aggradation–incision sequence model. [ABSTRACT FROM AUTHOR]

This study combines data from many published case studies to undertake a quantitative characterization of clastic parasequences, with the aim to determine how accommodation, sediment supply and autogenic sediment‐storage dynamics are recorded in their sedimentary architecture and stacking patterns. Results of this study are used to critically evaluate the validity of paradigms and models that are routinely used to explain and predict trends in the anatomy and arrangement of parasequences. Data on 957 parasequences from 62 case studies of clastic, shallow‐water successions were coded in a relational database, which includes outcrop and subsurface datasets of ancient and Quaternary examples. These units cover the preserved records of both river‐dominated deltas and wave‐dominated coasts, representing shoreline transits over a breadth of timescales, likely of both local and regional extent. The role of extant accommodation, rates of creation of accommodation and rates of sediment supply in determining parasequence architecture is assessed through analysis of relationships between: (i) proxies of these variables at different scales (rates of aggradation and progradation, facies‐belt shoreline trajectories, systems‐tract type, parasequence‐set stacking patterns, parasequence progradation angle and stratigraphic rise, size of feeder rivers); and (ii) parameters that describe the geometry and stacking style of parasequences, and associated shallow‐water sand bodies. Statistical analyses of database outputs indicate which proxies of accommodation, sediment supply and accommodation/sediment‐supply ratio are significant as predictors of parasequence architecture, and allow for interpretations of the importance of allogenic and autogenic factors. The principal results of this study reveal the following: (i) parasequence thickness varies as a function of water depth, accommodation generation and erosional truncation, and these variations are also reflected across types of systems tracts and parasequence sets; (ii) the dip length of parasequence sand bodies demonstrates scaling with measures of accommodation/sediment‐supply ratio at multiple scales, partly in relation to the possible effect of sediment supply on progradation rates; (iii) in systems tracts, stratigraphic trends in parasequence stacking due to autogenic mechanisms or to acceleration or deceleration in relative sea‐level fluctuations are not revealed quantitatively; (iv) some association is seen between the abundance of deltaic or river‐dominated parasequences and progradational stacking; (v) positive but modest correlation is observed between measures of river‐system size and the dip length of shallow‐marine parasequence sand bodies. The resulting insights can be applied to guide sequence stratigraphic interpretations of the rock record and the characterization of sub‐seismic stratigraphic architectures of subsurface successions. [ABSTRACT FROM AUTHOR]

Incised‐valley fills preserved within ancient coastal to shallow‐marine successions represent important archives of environmental and sea‐level change. Most current knowledge about the origin of incised valleys stems from Quaternary case studies; however, research on pre‐Quaternary examples can shed light on valley formation and evolution across longer timespans. This article describes different types of incised‐valley fills from Lower to Middle Pennsylvanian fluvio‐deltaic successions of the Breathitt Group (eastern Kentucky), accumulated in the Central Appalachian Foreland under prevalent glacioeustatic forcing driven by Gondwanan glaciations. Based on well‐established criteria for their recognition, numerous incised‐valley fills were identified from outcrop and subsurface data through more than 300 m of clastic successions consisting of fourth‐order stratigraphic sequences stacked into third‐order composite sequences. Incised‐valley fills were categorized into three archetypes based on lateral extent and aspect ratio (relatively wide versus narrow valley fills), nature of infill (fully continental versus mixed marine and continental facies associations) and relationships to underlying coal zones (truncating versus non‐truncating). The systematic occurrence of each incised‐valley fill type at specific stratigraphic positions within every third‐order sequence suggests control by a periodic allogenic factor. Valley‐fill archetypes are interpreted in terms of variable accommodation‐supply ratios driven by variable duration of formative base‐level cycles. For example, relatively wide incised‐valley fills with alluvial infill evolved during long‐lived cycles whose prolonged base‐level drawdown maintained low accommodation/supply ratios. Deeper valleys with low aspect ratios and mixed marine‐continental infills were generated by short‐lived base‐level drawdown that forced higher accommodation/supply ratios. Available chronological data for the studied successions consent to estimate base‐level cycles spanning 104–5 years that were likely modulated by interference patterns of orbital parameters (obliquity and eccentricity) via global climate and glacioeustatic fluctuations. This conceptual model, relating incised‐valley fill morphometry and internal architecture to orbital forcing patterns, provides a possible approach to predicting and interpreting incised‐valley fill variability through successions accumulated during icehouse conditions. [ABSTRACT FROM AUTHOR]

Fluvial red beds containing anatomically preserved large woody debris shed new light on seasonally dry biomes of the Pennsylvanian–Permian transition and elucidate the concurrence of river depositional systems and vegetation. As a result, the occurrence, distribution and preservation of petrified large woody debris accumulations are considered crucial to understanding the role of arborescent vegetation in shaping fluvial environments. This study reports sizeable silicified trunks and corresponding fluvial architectures from the uppermost Pennsylvanian (upper Gzhelian) Siebigerode Formation (Kyffhäuser, central Germany). The origin, taphonomy and depositional environment of the fossil woods are elucidated by using a multidisciplinary approach including geological mapping, lithofacies analysis, sediment petrography, wood anatomical studies and microstructure analyses. Results reflect the gradual burial of a gentle basement elevation by sand‐bed to gravel‐bed braided rivers at the north‐western margin of the perimontane Saale Basin. Facies architectures resulted from a complex interplay of syndepositional tectonics, repeated palaeorelief rejuvenation, high‐frequency channel avulsion, seasonally dry climate and woody debris–sediment interactions. The alluvial influx and cut‐bank erosion recruited trunks from adjacent semi‐riparian slope habitats vegetated by up to 40 m tall cordaitaleans and conifers. High discharge in wide braids facilitated uncongested transport of large woody debris. Trunk entombment and initial preservation resulted from grounding on barforms, anchoring by attached roots and subsequent burial. The post‐depositional two‐phase silicification was influenced by hydrothermal hematite mineralization and determined a selective wood preservation pattern known as 'pointstone'. Large woody debris‐induced sedimentary structures ('LWDISS') are introduced as a class of sediment structures formed by the biogenic impact on terrestrial deposition. [ABSTRACT FROM AUTHOR]

An outcrop dataset from the early Eocene Sunnyside Delta Interval of the Green River Formation in the Uinta Basin, Utah, USA, documents alluvial channel lithosomes. The abundance of Froude supercritical‐flow sedimentary structures, together with an abundance of high‐deposition‐rate sedimentary structures, in‐channel bioturbation and pedogenic modification, in‐channel muds and thick soft‐clast conglomerates, identify these lithosomes as deposits of variable‐discharge rivers. These recognition criteria are part of an emerging facies model for variable‐discharge rivers. This facies model, however, yet lacks robust recognition criteria for macro‐scale or bar‐scale stratal patterns of variable‐discharge rivers. This study presents a dataset that corroborates some known stratal patterns and provides examples of hitherto unknown bar‐scale stratal patterns of variable‐discharge rivers, including: (i) low‐angle downstream‐accretion sets that may form as washed‐out sheets in high sediment supply conditions or downstream of hydraulic jumps; (ii) high‐angle upstream‐accretion sets that imply deposition from systematically upstream‐migrating channel‐scale hydraulic jumps (cyclic steps); (iii) concave‐up, upward‐flattening high‐angle downstream‐accretion sets that are consistent with aggradation in channel‐scale hydraulic‐jump scours; (iv) upstream‐accretion and lateral‐accretion sets that may be linked to high‐magnitude flood reworking of point bars; and (v) aggradation or vertical‐accretion sets of ambiguous origin. These unconventional stratal patterns are compared to the established bar strata, such as those formed by point bars and braid bars and a discussion is provided on formative conditions for the here documented unconventional strata. This work highlights a need for further studies on the effect of discharge variability on bar formation and on the link between river morphology and bar types. [ABSTRACT FROM AUTHOR]

Inorganic aragonite occurs in a wide spectrum of depositional environments and its precipitation is controlled by complex physio‐chemical factors. This study investigates diagenetic conditions that led to aragonite cement precipitation in Cenozoic glaciomarine deposits of McMurdo Sound, Antarctica. A total of 42 sandstones that host intergranular cement were collected from the CIROS‐1 core, located proximal to the terminus of Ferrar Glacier. Standard petrography, Raman spectroscopy and electron microprobe analysis reveal a prominent aragonite cement phase that occurs as a pore‐filling blocky fabric throughout the core. Oxygen isotope compositions (δ18O = −30·0 to −8·6‰ Vienna Pee‐Dee Belemnite) and clumped isotope temperatures (TΔ47 = 13·1 to 31·5°C) determined from the aragonite cements provide precise constraints on isotopic compositions (δ18Ow) of the parent fluid, which mostly range from −10·8 to −7·2‰ Vienna Standard Mean Ocean Water. The fluid δ18Ow values are consistent with those of pore water, previously identified as cryogenic brine in the nearby AND‐2A core. Petrographic and geochemical data suggest that aragonite cement in the CIROS‐1 core precipitated from a similar brine. The brine likely formed and infiltrated sediments in flooded glacial valleys along the western margin of McMurdo Sound during the middle Miocene Climatic Transition, and subsequently flowed basinward in the subsurface. Consequently, the brine forms as a longstanding subsurface fluid that has saturated Cenozoic sediments below southern McMurdo Sound since at least the middle Miocene. Aragonite cementation in the CIROS‐1 core is interpreted to reflect its proximal position to sites of brine formation and greater likelihood of experiencing brines with sustained high carbonate saturation states and Mg/Ca ratios. This unusual occurrence expands the range of known natural occurrences of aragonite cement. Given the potential for cryogenic brine formation in glaciomarine settings, blocky aragonite, as the end member of the spectrum of aragonite cement morphology, may be more widespread in glaciomarine sediments than currently thought. [ABSTRACT FROM AUTHOR]

Coal seams preserve high‐resolution records of ancient terrestrial water table (base level) fluctuations in ancient peat accumulations, but little is known about base level change in anomalously thick coal seams. Using the Early Cretaceous 91 m anomalously thick No. 6 coal (lignite) seam in the Erlian Basin (north‐east China) as a case study, the origin and evolution of peat accumulation in a continental faulted basin is revealed by sedimentological, sequence stratigraphic and coal petrological analyses. The lignite is dominated by huminite, indicating oxygen‐deficient and waterlogged conditions in the precursor mire. Four types of key sequence stratigraphic surfaces are recognized, including paludification, terrestrialization, accommodation‐reversal and give‐up transgressive surfaces. Vertically, the No. 6 coal seam consists of fourteen superimposed wetting‐up and drying‐up cycles separated by key sequence stratigraphic surfaces, with each of these cycles having a mean duration of about 156 to 173 kyr. In a high accommodation peat swamp system, the wetting‐up cycles are generally characterized by an upward increase in mineral matter and inertodetrinite and an upward decrease in huminite with the paludification surface as their base and the give‐up transgressive surface or accommodation‐reversal surface as their top, representing a trend of upward‐increasing accommodation. In contrast, the drying‐up cycles are generally characterized by an upward decrease in mineral matter and inertodetrinite and an upward increase in huminite, with the terrestrialization surface as their base and the accommodation‐reversal surface as their top, representing a trend of upward‐decreasing accommodation. A multi‐phase mire stacking model for accumulation of the coal seam is proposed based on high‐frequency accommodation cycles and the stratigraphic relationships between coal and clastic sediments. High‐frequency accommodation cycles in the coal are closely related to water table fluctuations in the precursor mires and are driven by high‐frequency climate via changes in the intensity and seasonality of precipitation in a relatively stable subsidence regime. Recognition that the No. 6 coal seam is composed of multiple stacked mires has implications for studies addressing palaeoclimatic inferences and genesis of anomalously thick coals seams. [ABSTRACT FROM AUTHOR]

Depositional facies have been hypothesized to be linked to sequence stratigraphic positions. Also, shoreline systems are built by mixed processes, including rivers, storms, fair‐weather waves and tides. Resolving the complexity of shoreline deposition requires detailed quantitative facies analysis with particular attention to heterolithic successions. In this study, 71 sections in a 130 km long outcrop belt of the Cretaceous Gallup Formation in the north‐west of the San Juan Basin were measured. Five major facies associations were identified using sedimentological and iconological interpretations, including offshore shelf, non‐deltaic shoreline sandstones, deltas, coastal bayline and fluvial. Each facies association also comprises subordinate facies. Depositional facies interpretations are placed in a high‐resolution sequence stratigraphic framework that allows for reconstructions of the palaeogeography of individual parasequence sets that demonstrate temporal and spatial evolution of facies associations and depositional processes. The results show that the Gallup is a mixed‐process‐controlled depositional system with fair‐weather and storm‐wave dominance, river influence and tide‐effect, contrasting with previous interpretations of a solely fair‐weather wave‐dominated environment. Depositional processes and the resultant facies change with sequence stratigraphic positions in response to relative sea‐level changes – particular facies are only deposited in certain systems tracts. Distinction and transition between non‐deltaic shorefaces and wave‐dominated deltas have also been documented in this study. Non‐deltaic shorefaces are characterized by homogeneous sandstones with a wide‐range bioturbation index and the absence of mudstones. Wave‐dominated deltas are subject to river influence and contain prodelta facies. This study shows the importance of detailed facies analysis with high‐resolution sequence stratigraphic control using outcrops for documenting sedimentary processes of shallow marine shoreline systems. [ABSTRACT FROM AUTHOR]

Lower Palaeozoic fluvial systems tend to be more sand‐prone than those of later eras and the nature of coastal environments less certain. Field studies are presented that characterize the fluvial to marine transition over a distance of 80 km, in the Lower Cambrian of the Cotentin Peninsula, northern France. The sedimentary rocks are divided into six facies associations which represent deposition in proximal fluvial, distal fluvial, delta plain, delta front, pro‐delta and offshore carbonate bank environments. The basin fill is sandstone‐dominated and subdivided into three stratigraphic intervals. A 200 to 300 m thick basal interval contains very coarse‐grained fluvial sandstones deposited during a relative sea level lowstand. An overlying interval, 250 to 1500 m thick, is a facies mosaic. Fluvial strata in the north‐west pass laterally south‐east into deltaic and shallow marine pro‐delta sediments. The delta front deposits show repetitively stacked, upward‐coarsening parasequences, 8 to 10 m thick, which reflect the repeated progradation of lobate, fluvially‐dominated deltas onto a shallow marine shelf. The deltas formed following marine transgression and accumulated during a period of gradually rising relative sea level. An upper unit, 130 m thick, containing offshore stromatolitic and oolitic limestones, caps the study interval and represents deposition during a relative sea level highstand. The fluvial and delta distributary channel sandstones of the middle unit contain <1% mudstone. The cohesionless substrate determined that deltaic distributaries were predominantly braided in character and subject to common bifurcations which resulted in an ordered diminution of channel size and competence in a seaward direction. Terminal distributary channels show evidence of migratory levées and mouth‐bars and consistently delivered fine to medium‐grained sand to the delta front. The study highlights an example of pre‐vegetation deltaic sedimentation that was hydraulically organized and predictable, despite being fed by braided fluvial systems with high levels of peak discharge. [ABSTRACT FROM AUTHOR]

Unit bars are relatively large bedforms that develop in rivers over a wide range of climatic regimes. Unit bars formed within the highly‐variable discharge Burdekin River in Queensland, Australia, were examined over three field campaigns between 2015 and 2017. These bars had complex internal structures, dominated by co‐sets of cross‐stratified and planar‐stratified sets. The cross‐stratified sets tended to down‐climb. The development of complex internal structures was primarily a result of three processes: (i) superimposed bedforms reworking the unit bar avalanche face; (ii) variable discharge triggering reactivation surfaces; and (iii) changes in bar growth direction induced by stage change. Internal structures varied along the length and across the width of unit bars. For the former, down‐climbing cross‐stratified sets tended to pass into single planar cross‐stratified deposits at the downstream end of emergent bars; such variation related to changes in fluvial conditions whilst bars were active. A hierarchy of six categories of fluvial unsteadiness is proposed, with these discussed in relation to their effects on unit bar (and dune) internal structure. Across‐deposit variation was caused by changes in superimposed bedform and bar character along bar crests; such changes related to the three‐dimensionality of the channel and bar geometry when bars were active. Variation in internal structure is likely to be more pronounced in unit bar deposits than in smaller bedform (for example, dune) deposits formed in the same river. This is because smaller bedforms are more easily washed out or modified by changing discharge conditions and their smaller dimensions restrict the variation in flow conditions that occur over their width. In regimes where unit bar deposits are well‐preserved, their architectural variability is a potential aid to their identification. This complex architecture also allows greater resolution in interpreting the conditions before and during bar initiation and development. [ABSTRACT FROM AUTHOR]

The interaction of river and marine processes in the fluvial to marine transition zone fundamentally impacts delta plain morphology and sedimentary dynamics. This study aims to improve existing models of the facies distribution, stratigraphic architecture and preservation in the fluvial to marine transition zone of mixed‐process deltas, using a comprehensive sedimentological and stratigraphic dataset from the Middle Miocene Lambir Formation, Baram Delta Province, north‐west Borneo. Eleven facies associations are identified and interpreted to preserve the interaction of fluvial and marine processes in a mixed‐energy delta, where fluvial, wave and tidal processes display spatially and temporally variable interactions. Stratigraphic successions in axial areas associated with active distributary channels are sandstone‐rich, comprising fluvial‐dominated and wave‐dominated units. Successions in lateral areas, which lack active distributary channels, are mudstone‐rich, comprising fluvial‐dominated, tide‐dominated and wave‐dominated units, including mangrove swamps. Widespread mudstone preservation in axial and lateral areas suggests well‐developed turbidity maximum zones, a consequence of high suspended‐sediment concentrations resulting from tropical weathering of a mudstone‐rich hinterland. Within the fluvial to marine transition zone of distributary channels, interpreted proximal–distal sedimentological and stratigraphic trends suggest: (i) a proximal fluvial‐dominated, tide‐influenced subzone; (ii) a distal fluvial‐dominated to wave‐dominated subzone; and (iii) a conspicuously absent tide‐dominated subzone. Lateral areas preserve a more diverse spectrum of facies and stratigraphic elements reflecting combined storm, tidal and subordinate river processes. During coupled storm and river floods, fluvial processes dominated the fluvial to marine transition zone along major and minor distributary channels and channel mouths, causing significant overprinting of preceding interflood deposits. Despite interpreted fluvial–tidal channel units and mangrove influence implying tidal processes, there is a paucity of unequivocal tidal indicators (for example, cyclical heterolithic layering). This suggests that process preservation in the fluvial to marine transition zone preserved in the Lambir Formation primarily records episodic (flashy) river discharge, river flood and storm overprinting of tidal processes, and possible backwater dynamics. [ABSTRACT FROM AUTHOR]

Although general trends in transgressive to highstand sedimentary evolution of river‐mouth coastlines are well‐known, the details of the turnaround from retrogradational (typically estuarine) to aggradational–progradational (typically coastal/deltaic) stacking patterns are not fully resolved. This paper examines the middle to late Holocene eustatic highstand succession of the Po Delta: its stratigraphic architecture records a complex pattern of delta outbuilding and coastal progradation that followed eustatic stabilization, since around 7·7 cal kyr bp. Sedimentological, palaeoecological (benthic foraminifera, ostracods and molluscs) and compositional criteria were used to characterize depositional conditions and sediment‐dispersal pathways within a radiocarbon‐dated chronological framework. A three‐stage progradation history was reconstructed. First, as soon as eustasy stabilized (7·7 to 7·0 cal kyr bp), rapid bay‐head delta progradation (ca 5 m year−1), fed mostly by the Po River, took place in a mixed, freshwater and brackish estuarine environment. Second, a dominantly aggradational parasequence set of beach‐barrier deposits in the lower highstand systems tract (7·0 to 2·0 cal kyr bp) records the development of a shallow, wave‐dominated coastal system fed alongshore, with elongated, modestly crescent beaches (ca 2·5 m year−1). Third, in the last 2000 years, the development of faster accreting and more rapidly prograding (up to ca 15 m year−1) Po delta lobes occurred into 30 m deep waters (upper highstand systems tract). This study documents the close correspondence of sediment character with stratal distribution patterns within the highstand systems tract. Remarkable changes in sediment characteristics, palaeoenvironments and direction of sediment transport occur across a surface named the 'A–P surface'. This surface demarcates a major shift from dominantly aggradational (lower highstand systems tract) to fully progradational (upper highstand systems tract) parasequence stacking. In the Po system, this surface also reflects evolution from a wave‐dominated to river‐dominated deltaic system. Identifying the A–P surface through detailed palaeoecological and compositional data can help guide interpretation of highstand systems tracts in the rock record, especially where facies assemblages and their characteristic geometries are difficult to discern from physical sedimentary structures alone. [ABSTRACT FROM AUTHOR]