Sedimentary cycles and facies architecture of aeolian–fluvial strata of the Upper Jurassic Guará Formation, southern Brazil

The Upper Jurassic Guará Formation comprises an 80–200 m thick continental succession exposed in the western portion of the Rio Grande do Sul State (Brazil). It comprises four distinct facies associations: (i) simple to locally composite crescentic aeolian dune sets, (ii) aeolian sand sheets, (iii) distal floodflows, and (iv) fluvial channels. The vertical stacking of the facies associations defines several 5–14 m thick wetting-upward cycles. Each cycle starts with aeolian dune sets followed by aeolian sand sheets deposits and culminating in either fluvial channels or distal flood strata. Within some cycles, aeolian sand sheets are absent and fluvial deposits rest directly above aeolian dune facies. The transitions from one facies association to another are abrupt and marked by erosive surfaces that delineate distinct episodes of sediment accumulation. The origin of both the wetting-upward cycles and the erosive surfaces was controlled by the ground-water table level, dry sand availability and aeolian and fluvial sediment transport capacity variations, related to climatic fluctuations between relatively arid and humid conditions. Preservation of the fluvial–aeolian deposits reflects an overall relative water table rise driven by subsidence.     

Distinct styles of fluvial deposition in a Cambrian rift basin

Process‐based and facies models to account for the origin of pre‐vegetation (i.e. pre‐Silurian) preserved fluvial sedimentary architectures remain poorly defined in terms of their ability to account for the nature of the fluvial conditions required to accumulate and preserve architectural elements in the absence of the stabilizing influence of vegetation. In pre‐vegetation fluvial successions, the repeated reworking of bars and minor channels that resulted in the generation and preservation of broad, tabular, stacked sandstone‐sheets has been previously regarded as the dominant sedimentary mechanism. This situation is closely analogous to modern‐day poorly vegetated systems developed in arid climatic settings. However, this study demonstrates the widespread presence of substantially more complex stratigraphic architectures. The Guarda Velha Formation of Southern Brazil is a >500 m‐thick synrift fluvial succession of Cambrian age that records the deposits and sedimentary architecture of three distinct fluvial successions: (i) an early rift‐stage system characterized by coarse‐grained channel elements indicative of a distributive pattern with flow transverse to the basin axis; and two coeval systems from the early‐ to climax‐rift stages that represent (ii) an axially directed, trunk fluvial system characterized by large‐scale amalgamated sandy braid‐bar elements, and (iii) a distributive fluvial system characterized by multi‐storey, sandy braided‐channel elements that flowed transverse to the basin axis. Integration of facies and architectural‐element analysis with regional stratigraphic basin analysis, palaeocurrent and pebble‐provenance analysis demonstrates the mechanisms responsible for preserving the varied range of fluvial architectures present in this pre‐vegetation, rift‐basin setting. Identified major controls that influenced pre‐vegetation fluvial sedimentary style include: (i) spatial and temporal variation in discharge regime; (ii) the varying sedimentological characteristics of distinct catchment areas; (iii) the role of tectonic basin configuration and its direct role in influencing palaeoflow direction and fluvial style, whereby both the axial and transverse fluvial systems undertook a distinctive response to syn‐depositional movement on basin‐bounding faults. Detailed architectural analyses of these deposits reveal significant variations in geometry, with characteristics considerably more complex than that of simple, laterally extensive, stacked sandstone‐sheets predicted by most existing depositional models for pre‐vegetation fluvial systems. These results suggest that the sheet‐braided style actually encompasses a varied number of different pre‐vegetation fluvial styles. Moreover, this study demonstrates that contemporaneous axial and transverse fluvial systems with distinctive architectural expressions can be preserved in the same overall tectonic and climatic setting.     

A quantitative approach to fluvial facies models: Methods and example results

Traditional facies models lack quantitative information concerning sedimentological features: this significantly limits their value as references for comparison and guides to interpretation and subsurface prediction. This paper aims to demonstrate how a database methodology can be used to generate quantitative facies models for fluvial depositional systems. This approach is employed to generate a range of models, comprising sets of quantitative information on proportions, geometries, spatial relations and grain sizes of genetic units belonging to three different scales of observation (depositional elements, architectural elements and facies units). The method involves a sequential application of filters to the knowledge base that allows only database case studies that developed under appropriate boundary conditions to contribute to any particular model. Specific example facies models are presented for fluvial environmental types categorized on channel pattern, basin climatic regime and water‐discharge regime; the common adoption of these environmental types allows a straightforward comparison with existing qualitative models. The models presented here relate to: (i) the large‐scale architecture of single‐thread and braided river systems; (ii) meandering sub‐humid perennial systems; (iii) the intermediate‐scale and small‐scale architecture of dryland, braided ephemeral systems; (iv) the small‐scale architecture of sandy meandering systems; and (v) individual architectural features of a specific sedimentary environment (a terminal fluvial system) and its sub‐environments (architectural elements). Although the quantification of architectural properties represents the main advantage over qualitative facies models, other improvements include the capacity: (i) to model on different scales of interest; (ii) to categorize the model on a variety of environmental classes; (iii) to perform an objective synthesis of many real‐world case studies; (iv) to include variability‐related and knowledge‐related uncertainty in the model; and (v) to assess the role of preservation potential by comparing ancient‐system and modern‐system data input to the model.     

Anatomy and facies distribution of terminal lobes in ephemeral fluvial successions: Jurassic Tordillo Formation,Neuquén Basin,Argentina

In terminal fluvial-fan systems, characteristic proximal to distal variations in sedimentary architectures are recognized to arise from progressive downstream loss of water discharge related to both infiltration and evaporation. This work aims to elucidate downstream trends in facies and architecture across the medial and distal zones of terminal-fan systems, which record transitions from deposits of channel elements to lobe-like and sheet-like elements. This is achieved via a detailed characterization of ancient ephemeral fluvial deposits of the well-exposed Kimmeridgian Tordillo Formation (Neuquén Basin, Argentina). The fine sand-prone and silt-prone succession associated with the medial to distal sectors of the system has been studied to understand relationships between depositional processes and resulting architectures. Facies and architectural-element analyses, and quantification of resulting sedimentological data at multiple scales, have been undertaken to characterize sedimentary facies, facies transitions, bed types, architectural elements and larger-scale architectural styles. Eight bed types with distinct internal facies transitions are defined and interpreted in terms of different types of flood events. Channelized and non-channelized architectural elements are defined based on their constituent bed types and their external geometry. The most common elements are terminal lobes, which are composite bodies within which largely unconfined sandy deposits are stacked in a compensational manner; a hierarchical arrangement of internal components is recognized. Proximal feeder-channel avulsion events likely controlled the evolution of terminal-lobe elements and their spatiotemporal shifts. Stratigraphic relations between architectural elements record system-wide trends, whereby a proximal sector dominated by channel elements passes downstream via a gradational transition to a medial sector dominated by sandy terminal-lobe elements, which in turn passes further downstream to a distal sector dominated by silty terminal lobe-margin and fringing deposits. This work enhances current understanding of the stratigraphic record of terminal fluvial systems at multiple scales, and provides insight that can be applied to predict the facies and architectural complexity of terminal fluvial successions.     

Aeolian dune‐field development in a water table‐controlled system: Skeiđarársandur,Southern Iceland

Aeolian dune fields characterized by partly vegetated bedforms undergoing active construction and with interdune depressions that lie at or close to the water table are widespread on Skei?arársandur, Southern Iceland. The largest aeolian dune complex on the sandur covers an area of 80 km² and is characterized by four distinct landform types: (i) spatially isolated aeolian dunes; (ii) extensive areas of damp and wet (flooded) interdune flat with small fluvial channels; (iii) small aeolian dune fields composed of assemblages of bedforms with simple morphologies and small, predominantly damp, interdune corridors; and (iv) larger aeolian dune fields composed of assemblages of complex bedforms floored by older aeolian dune deposits that are themselves raised above the level of the surrounding wet sandur plain. The morphology of each of these landform areas reflects a range of styles of interaction between aeolian dune, interdune and fluvial processes that operate coevally on the sandur surface. The geometry, scale, orientation and facies composition of sets of strata in the cores of the aeolian dunes, and their relationship to adjoining interdune strata, have been analysed to explain the temporal behaviour of the dunes in terms of their mode of initiation, construction, pattern of migration, style of accumulation and nature of preservation. Seasonal and longer‐term flooding‐induced changes in water table level have caused episodic expansion and contraction of the wet interdune ponds. Most of the dunes are currently undergoing active construction and migration and, although sediment availability is limited because of the high water table, substantial aeolian transport must occur, especially during winter months when the surface of the wet interdune ponds is frozen and sand can be blown across the sandur without being trapped by surface moisture. Bedforms within the larger dune fields have grown to a size whereby formerly damp interdune flats have been reduced to dry enclosed depressions and dry aeolian system accumulation via bedform climb is ongoing. Despite regional uplift of the proximal sandur surface in response to glacial retreat and unloading over the past century, sediment compaction‐induced subsidence of the distal sandur is progressively placing aeolian deposits below the water table and is enabling the accumulation of wet aeolian systems and increasing the likelihood of their long‐term preservation. Wet, dry and stabilizing aeolian system types all co‐exist on Skei?arársandur and the dunes are variously undergoing coeval construction, accumulation, bypass, stabilization and destruction as a result of interactions between localized factors.     

Styles of interaction between aeolian,fluvial and shallow marine environments in the Pennsylvanian to Permian lower Cutler beds,south‐east Utah,USA

The Pennsylvanian to Permian lower Cutler beds comprise a 200 m thick mixed continental and shallow marine succession that forms part of the Paradox foreland basin fill exposed in and around the Canyonlands region of south‐east Utah. Aeolian facies comprise: (i) sets and compound cosets of trough cross‐bedded dune sandstone dominated by grain flow and translatent wind‐ripple strata; (ii) interdune strata characterized by sandstone, siltstone and mudstone interbeds with wind‐ripple, wavy and horizontal planar‐laminated strata resulting from accumulation on a range of dry, damp or wet substrate‐types in the flats and hollows between migrating dunes; and (iii) extensive, near‐flat lying wind‐rippled sandsheet strata. Fluvial facies comprise channel‐fill sandstones, lag conglomerates and finer‐grained overbank sheet‐flood deposits. Shallow marine facies comprise carbonate ramp limestones, tidal sand ridges and bioturbated marine mudstones. During episodes of sand sea construction and accumulation, compound transverse dunes migrated primarily to the south and south‐east, whereas south‐westerly flowing fluvial systems periodically punctuated the dune fields from the north‐east. Several vertically stacked aeolian sequences are each truncated at their top by regionally extensive surfaces that are associated with abundant calcified rhizoliths and bleaching of the underlying beds. These surfaces record the periodic shutdown and deflation of the dune fields to the level of the palaeo‐water‐table. During episodes of aeolian quiescence, fluvial systems became more widespread, forming unconfined braid‐plains that fed sediment to a coastline that lay to the south‐west and which ran approximately north‐west to south‐east for at least 200 km. Shallow marine systems repeatedly transgressed across the broad, low‐relief coastal plain on at least 10 separate occasions, resulting in the systematic preservation of units of marine limestone and calcarenite between units of non‐marine aeolian and fluvial strata, to form a series of depositional cycles. The top of the lower Cutler beds is defined by a prominent and laterally extensive marine limestone that represents the last major north‐eastward directed marine transgression into the basin prior to the onset of exclusively non‐marine sedimentation of the overlying Cedar Mesa Sandstone. Styles of interaction between aeolian, fluvial and marine facies associations occur on two distinct scales and represent the preserved expression of both small‐scale autocyclic behaviour of competing, coeval depositional systems and larger‐scale allocyclic changes that record system response to longer‐term interdependent variations in climatic and eustatic controlling mechanisms. The architectural relationships and system interactions observed in the lower Cutler beds demonstrate that the succession was generated by several cyclical changes in both climate and relative sea‐level, and that these two external controls probably underwent cyclical change in harmony with each other in the Paradox Basin during late Pennsylvanian and Permian times. This observation supports the hypothesis that both climate and eustasy were interdependent at this time and were probably responding to a glacio‐eustatic driving mechanism.     

Late Quaternary paleoenvironments and paleoclimatic conditions in the distal Andean piedmont, southern Mendoza, Argentina

The Andean piedmont of Mendoza is a semiarid region covered by extensive and partially vegetated dune fields consisting of mostly inactive aeolian landforms of diverse size and morphology. This paper is focused on the San Rafael plain (SRP) environment, situated in the distal Andean piedmont of Mendoza (34° 30′S), and reports the sedimentology and OSL chronology of two representative exposures of late Quaternary deposits, including their paleoenvironmental and paleoclimatic significance. Eleven facies, including channel, floodplain, fluvio–aeolian interaction, and reworked pyroclastic and aeolian deposits, were described and grouped into two facies associations (FA1 and FA2). FA1 was formed by unconfined sheet flows, minor channelized streams and fluvial–aeolian interaction processes. FA2 was interpreted as aeolian dune and sand-sheet deposits. OSL chronology from the SRP sedimentary record indicates that between ca. 58–39 ka and ca. 36–24 ka (MIS 3), aggradation was governed by ephemeral fluvial processes (FA1) under generally semiarid conditions. During MIS 2, the last glacial maximum (ca. 24–12 ka), a major climatic shift to more arid conditions is documented by significant aeolian activity (FA2) that became the dominant sedimentation process north of the Diamante–Atuel fluvial system. The inferred paleoenvironmental conditions from the SRP sections are in broad agreement with regional evidence.     

Architecture and preservation in the fluvial to marine transition zone of a mixed-process humid-tropical delta: Middle Miocene Lambir Formation,Baram Delta Province,north-west Borneo

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.     

Facies model of a fine‐grained,tide‐dominated delta: Lower Dir Abu Lifa Member (Eocene), Western Desert,Egypt

Existing facies models of tide‐dominated deltas largely omit fine‐grained, mud‐rich successions. Sedimentary facies and sequence stratigraphic analysis of the exceptionally well‐preserved Late Eocene Dir Abu Lifa Member (Western Desert, Egypt) aims to bridge this gap. The succession was deposited in a structurally controlled, shallow, macrotidal embayment and deposition was supplemented by fluvial processes but lacked wave influence. The succession contains two stacked, progradational parasequence sets bounded by regionally extensive flooding surfaces. Within this succession two main genetic elements are identified: non‐channelized tidal bars and tidal channels. Non‐channelized tidal bars comprise coarsening‐upward sandbodies, including large, downcurrent‐dipping accretion surfaces, sometimes capped by palaeosols indicating emergence. Tidal channels are preserved as single‐storey and multilateral bodies filled by: (i) laterally migrating, elongate tidal bars (inclined heterolithic strata, 5 to 25 m thick); (ii) forward‐facing lobate bars (sigmoidal heterolithic strata, up to 10 m thick); (iii) side bars displaying oblique to vertical accretion (4 to 7 m thick); or (iv) vertically‐accreting mud (1 to 4 m thick). Palaeocurrent data show that channels were swept by bidirectional tidal currents and typically were mutually evasive. Along‐strike variability defines a similar large‐scale architecture in both parasequence sets: a deeply scoured channel belt characterized by widespread inclined heterolithic strata is eroded from the parasequence‐set top, and flanked by stacked, non‐channelized tidal bars and smaller channelized bodies. The tide‐dominated delta is characterized by: (i) the regressive stratigraphic context; (ii) net‐progradational stratigraphic architecture within the succession; (iii) the absence of upward deepening trends and tidal ravinement surfaces; and (iv) architectural relations that demonstrate contemporaneous tidal distributary channel infill and tidal bar accretion at the delta front. The detailed facies analysis of this fine‐grained, tide‐dominated deltaic succession expands the range of depositional models available for the evaluation of ancient tidal successions, which are currently biased towards transgressive, valley‐confined estuarine and coarser grained deltaic depositional systems.     

Braided rivers within an arid alluvial plain (example from the Lower Triassic,western German Basin): recognition criteria and expression of stratigraphic cycles

Based on a detailed sedimentological analysis of Lower Triassic continental deposits in the western Germanic sag Basin (i.e. the eastern part of the present‐day Paris Basin: the ‘Conglomérat basal’, ‘Grès vosgien’ and ‘Conglomérat principal’ Formations), three main depositional environments were identified: (i) braided rivers in an arid alluvial plain with some preserved aeolian dunes and very few floodplain deposits; (ii) marginal erg (i.e. braided rivers, aeolian dunes and aeolian sand‐sheets); and (iii) playa lake (an ephemeral lake environment with fluvial and aeolian sediments). Most of the time, aeolian deposits in arid environments that are dominated by fluvial systems are poorly preserved and particular attention should be paid to any sedimentological marker of aridity, such as wind‐worn pebbles (ventifacts), sand‐drift surfaces and aeolian sand‐sheets. In such arid continental environments, stratigraphic surfaces of allocyclic origin correspond to bounding surfaces of regional extension. Elementary stratigraphic cycles, i.e. the genetic units, have been identified for the three main continental environments: the fluvial type, fluvial–aeolian type and fluvial/playa lake type. At the time scale of tens to hundreds of thousands of years, these high‐frequency cycles of climatic origin are controlled either by the groundwater level in the basin or by the fluvial siliciclastic sediment input supplied from the highland. Lower Triassic deposits from the Germanic Basin are preserved mostly in endoreic basins. The central part of the basin is arid but the rivers are supplied with water by precipitation falling on the remnants of the Hercynian (Variscan)–Appalachian Mountains. Consequently, a detailed study of alluvial plain facies provides indications of local climatic conditions in the place of deposition, whereas fluvial systems only reflect climatic conditions of the upstream erosional catchments.     

Evidence for plunging river plume deposits in the Pahrump Hills member of the Murray formation,Gale crater,Mars

Recent robotic missions to Mars have offered new insights into the extent, diversity and habitability of the Martian sedimentary rock record. Since the Curiosity rover landed in Gale crater in August 2012, the Mars Science Laboratory Science Team has explored the origins and habitability of ancient fluvial, deltaic, lacustrine and aeolian deposits preserved within the crater. This study describes the sedimentology of a ca 13 m thick succession named the Pahrump Hills member of the Murray formation, the first thick fine‐grained deposit discovered in situ on Mars. This work evaluates the depositional processes responsible for its formation and reconstructs its palaeoenvironmental setting. The Pahrump Hills succession can be sub‐divided into four distinct sedimentary facies: (i) thinly laminated mudstone; (ii) low‐angle cross‐stratified mudstone; (iii) cross‐stratified sandstone; and (iv) thickly laminated mudstone–sandstone. The very fine grain size of the mudstone facies and abundant millimetre‐scale and sub‐millimetre‐scale laminations exhibiting quasi‐uniform thickness throughout the Pahrump Hills succession are most consistent with lacustrine deposition. Low‐angle geometric discordances in the mudstone facies are interpreted as ‘scour and drape’ structures and suggest the action of currents, such as those associated with hyperpycnal river‐generated plumes plunging into a lake. Observation of an overall upward coarsening in grain size and thickening of laminae throughout the Pahrump Hills succession is consistent with deposition from basinward progradation of a fluvial‐deltaic system derived from the northern crater rim into the Gale crater lake. Palaeohydraulic modelling constrains the salinity of the ancient lake in Gale crater: assuming river sediment concentrations typical of floods on Earth, plunging river plumes and sedimentary structures like those observed at Pahrump Hills would have required lake densities near freshwater to form. The depositional model for the Pahrump Hills member presented here implies the presence of an ancient sustained, habitable freshwater lake in Gale crater for at least ca 10³ to 10⁷ Earth years.     

关中盆地上新世地层形成时代、层序划分及沉积相特征研究

关中盆地位于华北板块西南缘,为一新生代断陷盆地,具典型地堑形态,沉积巨厚的新生代地层。针对盆地新近纪上新世地层形成时代有争议、地层层序划分不明确、沉积相界定较笼统等问题,利用钻测井资料及野外调研,结合前人研究成果,对盆地新生代地层进行了重新划分,明确了上新世各地层形成时代及其地层层序划分和沉积相类型。研究结果表明,上新统可划分为蓝田组(11Ma—7 Ma)、灞河组(7.3 Ma—2.6 Ma)、张家坡组(3.4 Ma—2.6 Ma);新生代层序地层可划分为古近系构造层序、中新统构造层序、上新统构造层序和第四系构造层序等4个一级层序,存在6个构造层序界面。蓝田组以"三趾马红黏土"的风成沉积为特征。灞河组沉积时期,关中盆地的西安凹陷为一套冲(洪)积相、河流、三角洲相沉积,固市凹陷为一套冲(洪)积、河流、三角洲及滨浅湖相沉积;盆地边部开始出现小范围的风成黏土沉积。张家坡组以河湖相沉积为主,沉积中心向南移;盆地南北两侧或边部为冲(洪)积扇相、河流相及三角洲相沉积,边部也有小范围的风成黏土沉积。     

Sedimentology,stratigraphy and palynological occurrences of the late Cretaceous Erlian Formation,Erlian Basin,Inner Mongolia,People's Republic of China

The Erlian Basin is one of the non-marine Cretaceous basins of north-east China that developed during the late Mesozoic continental extension in eastern Asia. This basin experienced two major tectonic events: (i) a syn-rift stage that was dominated by a fluvial–lacustrine depositional environment and (ii) a post-rift stage that was dominated by a fluvial environment. A new sedimentological study performed on Erlian Formation drill cores has led to the determination of an architectural model and to the subsequent characterisation of the stratigraphic evolution of this sedimentary unit during the late Cretaceous. The palynological occurrences that were identified in samples provided a possible stratigraphical age for the Erlian Formation.Sediments of the Erlian Formation occur at the top of the Cretaceous stratigraphic column of the Erlian Basin and were deposited during the post-rift stage. Facies architecture and the ideal succession of facies that were identified for this formation exhibit two different members, both dominated by a fluvial depositional environment: (i) the lower member, which is dominated by channels of a braided river system and (ii) the upper member, which is dominated by overbank deposits. The lower member expresses a tectonically induced uplift as indicated by channels clustering under negative accommodation, whereas a period of stratigraphic base-level rise that is associated with an increase of accommodation is identified in the upper member. Therefore the Erlian Formation highlights an alternation of short uplifts that were dominated by braided fluvial channel deposits with periods of stratigraphic base-level rise that were dominated by overbank deposits. This sedimentological architecture has significant metallogenic implications for the origin of confined permeable sandstone layers, which represent adequate host-rocks for roll front-type uranium deposits.The palynological assemblage Exesipollenites, Ulmipollenites/Ulmoideipites, Buttinia and Momipites that were recognised in two samples of the Erlian Formation has revealed a post-late Campanian age therefore more likely indicating a late Cretaceous age of deposition for the sediments of the Erlian Formation.     

Interaction of an antecedent fluvial system with early normal fault growth: Implications for syn‐rift stratigraphy,western Corinth rift (Greece)

Continental ‘overfilled’ conditions during rift initiation are conventionally explained as due to low creation of accommodation compared with sediment supply. Alternatively, sediment supply can be relatively high from the onset of rifting due to an antecedent drainage system. The alluvial Lower Group of the western Plio–Pleistocene Corinth rift is used to investigate the interaction of fluvial sedimentation with early rifting. This rift was obliquely superimposed on the Hellenide mountain belt from which it inherited a significant palaeorelief. Detailed sedimentary logging and mapping of the well‐exposed syn‐rift succession document the facies distributions, palaeocurrents and stratigraphic architecture. Magnetostratigraphy and biostratigraphy are used to date and correlate the alluvial succession across and between fault blocks. From 3·2 to 1·8 Ma, a transverse low sinuosity braided river system flowed north/north‐east to east across east–west‐striking active fault blocks (4 to 7 km in width). Deposits evolved downstream from coarse alluvial conglomerates to fine‐grained lacustrine deposits over 15 to 30 km. The length scale of facies belts is much greater than, and thus not directly controlled by, the width of the fault blocks. At its termination, the distributive river system built small, stacked deltas into a shallow lake margin. The presence of a major antecedent drainage system is supported by: (i) a single major sediment entry point; (ii) persistence of a main channel belt axis; (iii) downstream fining at the scale of the rift basin. The zones of maximum subsidence on individual faults are aligned with the persistent fluvial axis, suggesting that sediment supply influenced normal fault growth. Instead of low accommodation rate during the early rift phase, this study proposes that facies progradation can be controlled by continuous and high sediment supply from antecedent rivers.     

Fluvial sedimentation in a salt‐controlled mini‐basin: stratal patterns and facies assemblages,Sivas Basin,Turkey

The Sivas Basin, located on the Central Anatolian Plateau in Turkey, is an elongate Oligo‐Miocene basin that contains numerous salt‐walled mini‐basins. Through field analysis, including stratigraphic section logging, facies analysis and geological mapping, a detailed tectono‐stratigraphic study of the Emirhan mini‐basin and its 2·6 km thick sediment fill has been undertaken. Three main palaeoenvironments are recognized – playa‐lake, braided stream and lacustrine – each corresponds to a relatively long‐lived depositional episode within a system that was dominated overall by the development of a distributive fluvial system. At local scale, this affects the geometry of the succession and influences facies distributions within preserved sequences. Sequences affected by wedge geometries are characterized by localized channelized sandstone bodies in the area of maximum subsidence and these pass laterally to floodplain mudstone towards the diaper; several internal unconformities are recognized. By contrast, sequences affected by hook geometries display narrow and steep drape‐fold geometries with no evidence of lateral facies change and apparent conformity in the preserved succession. The sediment fill of the Emirhan mini‐basin records the remobilization of diapir‐derived detritus and the presence of evaporitic bodies interbedded within the mini‐basin, implying the growth of salt walls expressed at the surface as palaeo‐topographic highs. The mini‐basin also records the signature of a regional change in stratigraphic assemblage, passing from playa‐lake facies to large‐scale highly amalgamated fluvial facies that represent progradation of the fluvial system. The initiation and evolution of this mini‐basin involves a variety of local and regional controls. Local factors include: (i) salt withdrawal, which influenced the rate and style of subsidence and consequently temporal and spatial variation in the stratigraphic assemblage and the stratal response related to halokinesis; and (ii) salt inflation, which influenced the topographic expression of the diapirs and consequently the occurrence of diapir‐derived detritus intercalated within the otherwise clastic‐dominated succession.     

Interaction between aeolian, fluvial and playa environments in the Permian Upper Rotliegend Group, UK southern North Sea

The Permian Upper Rotliegend Group in offshore UK Quadrants 42, 43, 47 and 48 comprises a sequence of mixed aeolian/fluvial/playa deposits. These deposits are up to 300 m thick and contain a record of the interaction between desert fluvial systems and adjacent aeolian and playa environments. The relative dominance of water vs. wind transport and deposition in this stratigraphic package was a function of fluctuations in the discharge of ephemeral fluvial systems and changes in water table/playa level driven by a combination of climatic change and syndepositional tectonics. The Rotliegend sedimentary record is punctuated by numerous surfaces recording erosion by wind and water. The origin of these surfaces is mostly climatic, with periods of increased runoff resulting in fluvial incision, especially near active faults. During periods of reduced runoff, wind erosion of fluvial deposits occurred, with fluvially derived sand being reworked into expanding aeolian dune fields. Wind erosion also occurred as a rising water table isolated dunes from their sediment supply, resulting in deflation of dunes down to the water table. These surfaces formed in a basin that was subsiding. Thus, even in a background of overall increasing accommodation space, climatically driven falls in the water table allowed for periods of erosion. The occurrence of significant erosion, especially near syndepositional fault zones, resulted in a sedimentary record that shows pronounced lateral as well as vertical facies variations.     

Early Cambrian braid-delta deposits, MacKenzie Mountains, north-western Canada

Latest Neoproterozoic to earliest Cambrian strata in north-western Canada provide an example of a pre-vegetation braid-delta depositional system. Depositional environments represented in the succession include braided fluvial and braid-delta distributary channels, aeolian dune fields and interdistributary lagoons/bays, as well as mouth bar, beach to shoreface, and prodelta to distal shelf settings. Three formations have been investigated: the Ingta Formation formed in wave-dominated nearshore to offshore shelf environments with little or no apparent deltaic influence, whereas the overlying Backbone Ranges and Vampire formations contain an extensive record of braid-delta deposits ranging from braidplain to distal prodelta facies. On the braid-plain, river channels reached widths of up to several kilometres. Such channels terminated seaward in braid deltas that showed some shoreline protuberance and were characterized by fluvial-dominated mouth-bar deposition with lesser wave influence; wave-dominated deltaic successions are rare in the succession. Interdeltaic areas were characterized by wave-dominated prograding shorelines. Interdistributary lagoons probably formed primarily in abandoned distributary channels. Delta-front/prodelta deposits are silt-rich and contain abundant soft-sediment deformation, including slumps. The deposits in these formations illustrate the significantly different nature of sedimentation prior to the advent of land plants. This is illustrated in the dominance of braided fluvial deposition and of silt-rich offshore facies that may have resulted from enhanced aeolian transport of loess. The non-actualistic effects of limited bioturbation and extensive microbial binding apparently exerted relatively little control on the distribution of facies. However, the absence of extensive bioturbation is manifest in pristine preservation of primary sedimentary structures, while the hypothesized latest Proterozoic-earliest Cambrian decline in microbial binding may be reflected in the upward increase in the abundance of sole marks in the succession.     

Stacked fluvial and tide-dominated estuarine deposits in high-frequency (fourth-order) sequences of the Eocene Central Basin, Spitsbergen

Eighteen coastal-plain depositional sequences that can be correlated to shallow- to deep-water clinoforms in the Eocene Central Basin of Spitsbergen were studied in 1 × 15 km scale mountainside exposures. The overall mud-prone (>300 m thick) coastal-plain succession is divided by prominent fluvial erosion surfaces into vertically stacked depositional sequences, 7–44 m thick. The erosion surfaces are overlain by fluvial conglomerates and coarse-grained sandstones. The fluvial deposits show tidal influence at their seaward ends. The fluvial deposits pass upwards into macrotidal tide-dominated estuarine deposits, with coarse-grained river-dominated facies followed further seawards by high- and low-sinuosity tidal channels, upper-flow-regime tidal flats, and tidal sand bar facies associations. Laterally, marginal sandy to muddy tidal flat and marsh deposits occur. The fluvial/estuarine sequences are interpreted as having accumulated as a series of incised valley fills because: (i) the basal fluvial erosion surfaces, with at least 16 m of local erosional relief, are regional incisions; (ii) the basal fluvial deposits exhibit a significant basinward facies shift; (iii) the regional erosion surfaces can be correlated with rooted horizons in the interfluve areas; and (iv) the estuarine deposits onlap the valley walls in a landward direction. The coastal-plain deposits represent the topset to clinoforms that formed during progradational infilling of the Eocene Central Basin. Despite large-scale progradation, the sequences are volumetrically dominated by lowstand fluvial deposits and especially by transgressive estuarine deposits. The transgressive deposits are overlain by highstand units in only about 30% of the sequences. The depositional system remained an estuary even during highstand conditions, as evidenced by the continued bedload convergence in the inner-estuarine tidal channels.     

Desert sand plain and sabkha deposits from the Bunter Sandstone Formation (L. Triassic) at the northern margin of the German Basin

Facies studies of well cores from the Bunter Sandstone Formation in the Tønder area, Denmark indicate, that the formation is composed of two desert sand plain sequences associated with sabkha and inland basin (lake?) mudstones. The lower desert sand plain sequence consists of subaerial sand flat deposits overlain by aeolian sand sheet and dune facies topped by interbedded aeolian and ephemeral river deposits. The upper desert sand plain sequence consists of ephemeral river deposits partly interbedded with and overlain by sabkha and inland basin mudstones. Two shoreline sandstones occur in the uppermost part. Both sequences are interpreted mainly in terms of tectonic subsidence of the basin and related upheavals of the source regions. The lower sequence represents a rather continuous progradation of the desert sand plain followed by a rapid transgression of the waters from the inland basin. The upper sequence represents brief periods of fluvial progradation followed by a gradual retreat of the river plain. The most distal part of the sand plain was finally reworked by weak wave-action.