Distinctive erosional and depositional structures formed at a canyon mouth: A lower Pleistocene deep‐water succession in the Kazusa forearc basin on the Boso Peninsula,Japan

The canyon mouth is an important component of submarine‐fan systems and is thought to play a significant role in the transformation of turbidity currents. However, the depositional and erosional structures that characterize canyon mouths have received less attention than other components of submarine‐fan systems. This study investigates the facies organization and geometry of turbidites that are interpreted to have developed at a canyon mouth in the early Pleistocene Kazusa forearc basin on the Boso Peninsula, Japan. The canyon‐mouth deposits have the following distinctive features: (i) The turbidite succession is thinner than both the canyon‐fill and submarine‐fan successions and is represented by amalgamation of sandstones and pebbly sandstones as a result of bypassing of turbidity currents. (ii) Sandstone beds and bedsets show an overall lenticular geometry and are commonly overlain by mud drapes, which are massive and contain fewer bioturbation structures than do the hemipelagic muddy deposits. (iii) The mud drapes have a microstructure characterized by aggregates of clay particles, which show features similar to those of fluid‐mud deposits, and are interpreted to represent deposition from fluid mud developed from turbidity current clouds. (iv) Large‐scale erosional surfaces are infilled with thick‐bedded to very thick‐bedded turbidites, which show lithofacies quite similar to those of the surrounding deposits, and are considered to be equivalent to scours. (v) Concave‐up erosional surfaces, some of which face in the upslope direction, are overlain by backset bedding, which is associated with many mud clasts. (vi) Tractional structures, some of which are equivalent to coarse‐grained sediment waves, were also developed, and were overlain locally by mud drapes, in association with mud drape‐filled scours, cut and fill structures and backset bedding. The combination of these outcrop‐scale erosional and depositional structures, together with the microstructure of the mud drapes, can be used to identify canyon‐mouth deposits in ancient deep‐water successions.     

Permian glaciated bedrock surfaces and associated sediments on Kangaroo Island,South Australia: Implications for local Gondwanan ice‐mass dynamics

Late Palaeozoic glaciated rock surfaces and associated sediments occur along the northeastern coast of Kangaroo Island. The erosional forms include glacially polished rock surfaces, striae, grooves, chatter marks, friction cracks, crescentic gouges, p‐forms, sichelwannen, miniature rock crag‐and‐tails and roches moutonnées. The distribution and orientation of these along with till fabrics indicate a general northwesterly ice flow in this part of the Troubridge Basin. The glacial erosional forms and the presence of thick lodgement till imply that the local basal ice was at pressure‐melting point during their formation. Temperate to subpolar glacial ice conditions, similar to those currently prevailing in glaciers in Spitsbergen, are inferred.     

Geometry,distribution and fill of erosional scours in a heterolithic,distal lower shoreface sandstone reservoir analogue: Grassy Member,Blackhawk Formation,Book Cliffs,Utah, USA

Many shoreface sandstone reservoirs host significant hydrocarbon volumes within distal intervals of interbedded sandstones and mudstones. Hydrocarbon production from these reservoir intervals depends on the abundance and proportion of sandstone beds that are connected by erosional scours, and on the lateral extent and continuity of interbedded mudstones. Cliff‐face exposures of the Campanian ‘G2’ parasequence, Grassy Member, Blackhawk Formation in the Book Cliffs of east‐central Utah, USA , allow detailed characterization of 128 erosional scours within such interbedded sandstones and mudstones in a volume of 148 m length, 94 m width and 15 m height. The erosional scours have depths of up to 1·1 m, apparent widths of up to 15·1 m and steep sides (up to 35°) that strike approximately perpendicular (N099 ± 36°) to the local north–south palaeoshoreline trend. The scours have limited lateral continuity along strike and down dip, and a relatively narrow range of apparent aspect ratio (apparent width/depth), implying that their three‐dimensional geometry is similar to non‐channelized pot casts. There is no systematic variation in scour dimensions, but ‘scour density’ is greater in amalgamated (conjoined) sandstone beds over 0·5 m thick, and increases upward within vertical successions of upward‐thickening conjoined sandstone beds. There is no apparent organization of the overall lateral distribution of scours, although localized clustering implies that some scours were re‐occupied during multiple erosional events. Scour occurrence is also associated with locally increased amplitude and laminaset thickness of hummocky cross‐stratification in sandstone beds. The geometry, distribution and infill character of the scours imply that they were formed by storm‐generated currents coincident with riverine sediment influx (‘storm floods’). The erosional scours increase the vertical and lateral connectivity of conjoined sandstone beds in the upper part of upward‐thickening sandstone bed successions, resulting in increased effective vertical and horizontal permeability of such intervals.     

Origin of the vertically orientated clasts in brecciated shallow‐marine limestones of the Chaomidian Formation (Furongian,Shandong Province,China)

Numerous shallow‐marine limestone layers of the Furongian (Late Cambrian) Chaomidian Formation in the Jiulongshan section (Shandong Province, China) are breccias. Some of these breccias show abundant vertical to sub‐vertical clasts. Typically, these clasts end abruptly at the contact plane with the overlying deposit, either abutting the overlying sedimentary bed or via an erosional plane that truncates the clasts. A few exposures show concentrations of clasts that must have been uplifted to the extent that they transgressed the then sedimentary surface or (possibly) penetrated the overlying sediment which, in this case, consists of muds or marls. The clasts tend to show clusters with respect to the enclosing fabric. All clasts are parallel to each other in a specific cluster, while the various clusters may show different orientations of the clasts. It is deduced that both the exceptional position and the exceptional orientation of the clasts must be ascribed to the upward movement of the clasts under the influence of pore water escaping under high pressure through fluidized sediment.     

Morphodynamics and depositional signature of low‐aggradation cyclic steps: New insights from a depth‐resolved numerical model

Bedforms related to Froude‐supercritical flow, such as cyclic steps, are increasingly frequently observed in contemporary fluvial and marine sedimentary systems. However, the number of observations of sedimentary structures formed by supercritical‐flow bedforms remains limited. The low number of observations might be caused by poor constraints on criteria to recognize these associated deposits. This study provides a detailed quantification on the mechanics of a fluvial cyclic step system, and their depositional signature. A computational fluid‐dynamics model is employed to acquire a depth‐resolved image of a cyclic step system. New insights into the mechanics of cyclic steps shows that: (i) the hydraulic jump is, in itself, erosional; (ii) there are periods over which the flow is supercritical throughout and there is no hydraulic jump, which plays a significant role in the morphodynamic behaviour of cyclic steps; and (iii) that the depositional signature of cyclic steps varies with rate of aggradation. Previous work has shown that strongly aggradational cyclic steps, where most of the deposited sediment is not reworked, create packages of backsets, bound upstream and downstream by erosive surfaces. Here, the modelling work is focussed on less aggradational conditions and more transportational systems. The depositional signature in such systems is dominated by an amalgamation of concave‐up erosional surfaces and low‐angle foresets and backsets creating lenticular bodies. The difference between highly aggradational cyclic steps and low‐aggradation steps can be visible in outcrop both by the amount of erosional surfaces, as well as the ratio of foreset to backset, with backsets being indicative of more aggradation.     

高压水射流破岩规律的数值模拟研究

运用动态非线性有限元法和Hoffman破碎准则,研究了高压水射流破碎岩石的规律,提出了高压射流破碎岩石过程分为射流冲蚀破碎和锤冲破碎及两个临界压力的概念。研究结果表明,提高射流冲击速度、喷射直径、横移速度、射流束数,在35°至40°之间合理选择射流入射角可以提高射流破碎岩石的效率。     

Repeated cycles of submarine channel incision, infill and transition to sheet sandstone development in the Alpine Foreland Basin, SE France

The Grès de Champsaur turbidite system, deposited in a distal setting in the Alpine Foreland Basin of south‐eastern France, exhibits a repeated upsection alternation in sand body geometry between incised channels and sheet sands. The channels form symmetric lenticular erosional features, of width 900–1000 m (measured between the lateral limits of incision) and depth 65–115 m, and can be traced axially for up to 5 km. In each case, the channel fill is capped by a laterally persistent sandy sheet‐form interval, which lies upon a fine‐grained substrate beyond the channel margins. No intrachannel elements have been traced into the substrate sequence, suggesting that, before infill, the channels acted as open sea‐floor conduits of essentially the same dimensions as the preserved channel deposits. The channels are vertically stacked, although axial erosion juxtaposes younger channel axis deposits against the fill of older channels and their channel‐capping sheet sandstones to produce an apparently well‐connected composite sandstone body geometry. The predominant channel‐fill facies comprises coarse‐grained, amalgamated sandstones, which are commonly parallel‐ or cross‐stratified. Subsidiary facies of finer grained sandstone–mudstone couplets and clast‐bearing muddy debrites are commonly preserved as erosional remnants, suggesting a complex channel history of aggradation and erosion. The repeated cycles of channel incision, infill and transition to sheet sandstone development indicate repetitive incision and healing of the palaeo‐sea floor. A model is proposed that links incision to the development of relatively steep axial gradients (parallel to the mean dispersal direction) and the return to sheet‐form deposition to the re‐establishment of lower axial gradients, with the repetitive switch between incisional channels and sheet sandstones driven by changes in sediment input rate against a background of ongoing sea‐floor tilting.     

Geomorphology of carbonate platform‐marginal uppermost slopes: Insights from a Holocene analogue,Little Bahama Bank,Bahamas

As the product of a variety of sediment sources and sedimentation (and re‐sedimentation) and erosion processes, the geomorphology and sedimentology of carbonate slopes are highly variable. The purpose of this study is to describe sub‐bottom profiles and side‐scan sonar, multibeam and optical data acquired by an autonomous underwater vehicle to explore variability in geomorphological and sedimentological character of the present‐day platform‐marginal, uppermost slope environments (< 240 m water depth) on the north, open‐ocean facing flank of Little Bahama Bank, Bahamas. Although at time scales of greater than 100 ka this margin is progradational, the data illustrate a complex juxtaposition of erosional and depositional processes and features. Erosion is evidenced by two prominent escarpments (70 m and 120 m) that expose eroded, bedded rocky outcrops. These escarpments are interpreted to represent relict features, related to past sea‐level positions, although they still may be shedding debris. Aside from erosional remnants, sedimentation and active transport is indicated by several features, including active bedforms (especially above the 70 m escarpment, but ripples occur to depths of ca 200 m), several mass transport complexes that overlie and cover the lower escarpment, gravity flow deposits and rare slump features. Similarly, a thick (up to 20 m) onlapping sediment wedge, interpreted to be Holocene in age, suggests lateral accretion of the slope by more than 75 m in this period. Data illustrate that this open‐ocean margin is distinct from windward margins in the Bahamas, which typically include near‐vertical walls of erosion or bypass, flanked downdip by rubble and talus, and leeward margins, which have onlapping muddy wedges, but that lack marked terraces or escarpments. Collectively, the results provide perspectives into the nature and controls on complex geomorphological patterns of erosion and deposition in Holocene uppermost slope systems, concepts potentially applicable to ancient analogues.     

Sedimentary architecture and 3D ground-penetrating radar analysis of gravelly meandering river deposits (Neckar Valley, SW Germany)

Sedimentological outcrop analysis and sub‐surface ground‐penetrating radar (GPR) surveys are combined to characterize the three‐dimensional sedimentary architecture of Quaternary coarse‐grained fluvial deposits in the Neckar Valley (SW Germany). Two units characterized by different architectural styles are distinguished within the upper part of the gravel body, separated by an erosional unconformity: (i) a lower unit dominated by trough‐shaped depositional elements with erosional, concave‐up bounding surfaces that are filled by cross‐bedded sets of mainly openwork and filled framework gravel; and (ii) an upper unit characterized by gently inclined sheets of massive and openwork gravels with thin, sandy interlayers that show lateral accretion on a lower erosional unconformity. The former is interpreted as confluence scour pool elements formed in a multi‐channel, possibly braided river system, the latter as extensive point bar deposits formed by the lateral migration of a meandering river channel. The lateral accretion elements are locally cut by chute channels mainly filled by gravels rich in fines, and by fine‐grained abandoned channel fills. The lateral accretion elements are associated with gravel dune deposits characterized by steeply inclined cross‐beds of alternating open and filled framework gravel. Floodplain fines with a cutbank and point bar morphology cover the gravel deposits. The GPR images, revealing the three‐dimensional geometries of the depositional elements and their stacking patterns, confirm a change in sedimentary style between the two stratigraphic units. The change occurred at the onset of the Holocene, as indicated by ¹⁴C‐dating of wood fragments, and is related to a re‐organization of the fluvial system that probably was driven by climatic changes. The integration of sedimentological and GPR results highlights the heterogeneity of the fluvial deposits, a factor that is important for modelling groundwater flow in valley‐fill aquifers.     

The influence of turbidity currents and contour currents on the distribution of deep‐water sediment waves offshore eastern Canada

Sediment waves are commonly observed on the sea floor and often vary in morphology and geometry according to factors such as seabed slope, density and discharge of turbidity currents, and the presence of persistent contour currents. This paper documents the morphology, internal geometry and distribution of deep‐water (4000 to 5000 m) bedforms observed on the sea floor offshore eastern Canada using high‐resolution multibeam bathymetry data and seismic stratigraphy. The bedforms have wavelengths of >1 km but fundamentally vary in terms of morphology and internal stratigraphy, and are distinguished into three main types. The first type, characterized by their long‐wavelength crescentic shape, is interpreted as net‐erosional cyclic steps. These cyclic steps were formed by turbidity currents flowing through canyons and overtopping and breaching levées. The second type, characterized by their linear shape and presence on levées, is interpreted as net‐depositional cyclic steps. These upslope migrating bedforms are strongly aggradational, indicating high sediment deposition from turbidity currents. The third type, characterized by their obliqueness to canyons, is observed on an open slope and is interpreted as antidunes. These antidunes were formed by the deflection of the upper dilute, low‐density parts of turbidity currents by contour currents. The modelling of the behaviour of these different types of turbidity currents reveals that fast‐flowing flows form cyclic steps while their upper parts overspill and are entrained westward by contour currents. The interaction between turbidity currents and contour currents results in flow thickening and reduced sediment concentration, which leads to lower flow velocities. Lower velocities, in turn, allow the formation of antidunes instead of cyclic steps because the densiometric Froude number (Fr′) decreases. Therefore, this study shows that both net‐erosional and net‐depositional cyclic steps are distributed along channels where turbidity currents prevail whereas antidunes form on open slopes, in a mixed turbidite/contourite system. This study provides insights into the influence of turbidity currents versus contour currents on the morphology, geometry and distribution of bedforms in a mixed turbidite–contourite system.     

青藏高原东缘龙门山晚新生代剥蚀厚度与弹性挠曲模拟

龙门山是青藏高原东缘边界山脉,具有青藏高原地貌、龙门山高山地貌和山前冲积平原三个一级地貌单元。利用数字高程模式图像和裂变径迹年代测定方法研究和计算龙门山晚新生代剥蚀厚度与剥蚀速率,结果表明:3.6 Ma以来龙门山的剥蚀厚度介于1.91-2.16 km之间,剥蚀速率介于0.53-0.60 mm/a之间。在此基础上,开展了该地区岩石圈的弹性挠曲模拟,结果表明龙门山的隆升机制具有以构造缩短隆升和剥蚀卸载隆升相叠合的特点。3.6 Ma之前,龙门山的隆升与逆冲推覆构造负载有关,以构造缩短驱动的构造隆升为特色;3.6 Ma之后,龙门山的隆升与剥蚀卸载驱动的抬升有关,并以剥蚀卸载隆升为特色,进而提出了龙门山晚新生代以来的隆升机制以剥蚀成山作用为主的认识。     

Metamorphism of the deepest exposed arc rocks in the Cretaceous to Paleogene Cascades belt, Washington: evidence for large-scale vertical motion in a continental arc

The Swakane Gneiss and the overlying Napeequa Complex in the North Cascade range, Washington, were metamorphosed and deformed during development of a Cretaceous‐Paleogene continental arc, and are among the structurally deepest exposed rocks within the Cordilleran arcs of North America. Peak metamorphic conditions in both the Swakane Gneiss and Napeequa Complex were c. 640–750 °C, 9–12 kbar. Clockwise paths and widespread evidence for high‐P metamorphism in meta‐supracrustal rocks (burial to >40 km) document major vertical tectonic motion during arc construction and unroofing. These and other moderately high‐pressure rocks in the North Cascades‐Coast Mountains experienced a dramatically different tectonometamorphic history than metamorphic rocks within other Cordilleran arcs. The exhumed arc complexes of the Sierra Nevada and Peninsular Ranges are dominated by relatively low‐P metamorphic and plutonic rocks (typically <6 kbar). There is no evidence that the northern Cordillera was thickened to a greater degree than these other belts, suggesting that the greater magnitude of vertical motion in the Cascades may have been related to exhumation mechanisms: Eocene extension in the northern Cordillera vs. erosional unroofing in the central and southern Cordillera.     

Stratigraphic organization and predictability of mixed coarse‐grained and fine‐grained successions in an upper slope Pleistocene turbidite system of the Peri‐Adriatic basin

The early Pleistocene clastic succession of the Peri‐Adriatic basin, eastern central Italy, records the filling of a series of piggyback sub‐basins that formed in response to the development of the eastward‐verging Apennine fold‐thrust belt. During the Gelasian (2·588 to 1·806 Ma), large volumes of Apennine‐derived sediments were routed to these basins through a number of slope turbidite systems. Using a comprehensive outcrop‐based dataset, the current study documents the depositional processes, stratigraphic organization, foraminiferal age and palaeodepth, and stratigraphic evolution of one of these systems exposed in the surroundings of the Castignano village. Analysis of foraminiferal assemblages consistently indicates Gelasian deposition in upper bathyal water depths. Sediments exposed in the study area can be broken into seven main lithofacies, reflecting specific gravity‐induced depositional elements and slope background deposition: (i) clast‐supported conglomerates (conglomerate channel‐fill); (ii) amalgamated sandstones (late stage sandstone channel‐fill); (iii) medium to thick‐bedded tabular sandstones (frontal splay sandstones); (iv) thin to thick‐bedded channelized sandstones (sandy channel‐fill); (v) medium to very thin‐bedded sandstones and mudstones (levée‐overbank deposits); (vi) pebbly mudstones and chaotic beds (mudstone‐rich mass‐transport deposits); and (vii) massive mudstones (hemipelagic deposits). Individual lithofacies combine vertically and laterally to form decametre‐scale, disconformably bounded, fining‐upward lithofacies successions that, in turn, stack to form slope valley fills bounded by deeply incised erosion surfaces. A hierarchical approach to the physical stratigraphy of the slope system indicates that it has evolved through multiple cycles of waxing then waning flow energy at multiple scales and that its packaging can be described in terms of a six‐fold hierarchy of architectural elements and bounding surfaces. In this scheme, the whole system (sixth‐order element) is comprised of three distinct fifth‐order stratigraphic cycles (valley fills), which define sixth‐order initiation, growth and retreat phases of slope deposition, respectively; they are separated by discrete periods of entrenchment that generated erosional valleys interpreted to record fifth‐order initiation phases. Backfilling of individual valleys progressed through deposition of two vertically stacked lithofacies successions (fourth‐order elements), which record fifth‐order growth and retreat phases. Fourth‐order initiation phases are represented by erosional surfaces bounding lithofacies successions. The component lithofacies (third‐order element) record fourth‐order growth and retreat phases. Map trends of erosional valleys and palaeocurrent indicators converge to indicate that the sea floor bathymetric expression of a developing thrust‐related anticline markedly influenced the downslope transport direction of gravity currents and was sufficient to cause a major diversion of the turbidite system around the growing structure. This field‐based study permits the development of a sedimentological model that predicts the evolutionary style of mixed coarse‐grained and fine‐grained turbidite slope systems, the internal distribution of reservoir and non‐reservoir lithofacies within them, and has the potential to serve as an analogue for seismic or outcrop‐based studies of slope valley fills developed in actively deforming structural settings and under severe icehouse regimes.     

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.     

Subglacial erosion forms in northwest Ireland

Subglacial erosional forms are commonly found on bedrock substrates inside the Late Weichselian ice margin in County Donegal, northwest Ireland, and can be used to provide detailed information on subglacial processes and environments. The erosional forms occur on spatial scales from whalebacks (tens of metres in scale), to asymmetric and channelized bedrock-cut scours (tens of cm in scale) and striations (mm scale). Processes responsible for development of subglacial erosional forms occur along a continuum, from free meltwater existing as a laterally extensive sheet at the ice-bed interface, to abrasion by basal ice. Channelized bedrock-cut scours are particularly common in County Donegal, and show asymmetric and meandering thalwegs, U-shaped cross-profiles and steep lateral margins. Innermost parts of the scours are highly polished and have striations that follow thalweg direction. In places, bedrock surfaces are overlain by a delicate polish and thin calcite cement, and are buried beneath glacial till. Based on their morphology, the bedrock scours are interpreted as s-forms caused by high-pressure subglacial meltwater erosion. Striations within the scoured channels reflect periods of ice-bed coupling and subglacial abrasion. The range of features observed here was used to consider relationships between subglacial topography, hydraulic processes and ice-bed coupling. Precipitation of calcite cement took place in depressions on the bedrock surface by CO₂ degassing. Infilling of depressions by glacial till formed a new type of 'sticky spot' related to spatial variations in subglacial water pressure. The temporal evolution of sticky spots reflects interactions within the subglacial environment between subglacial relief, hydraulic regime and ice-bed coupling.     

Architecture of a coarse‐grained channel–levée system: the Rosario Formation,Baja California,Mexico

Seafloor images of coarse‐grained submarine channel–levée systems commonly reveal complex braid‐plain patterns of low‐amplitude bedforms and zones of apparent bypass; however, mechanisms of channel evolution and the resultant channel‐fill architecture are poorly understood. At Playa Esqueleto the lateral relationships between various elements of a deep‐marine slope channel system are well‐exposed. Specifically, the transition from gravel‐dominated axial thalwegs to laterally persistent marginal sandstones and isolated gravel‐filled scours is revealed. Marginal sandstones pass into a monotonous thin‐bedded succession which built to form relatively low‐relief levées bounding the channel belt; in turn, the levées onlap the canyon walls. Three orders of confinement were important during the evolution of the channel system: (i) first‐order confinement was provided by the erosional canyon which confined the entire system; (ii) confined levées built of turbidite sandstones and mudstones formed the second‐order confinement, and it is demonstrated that these built from overspill at thalweg margins; and (iii) third‐order confinement describes the erosional confinement of coarse‐grained thalwegs and scours. Finer‐grained sediment was transported in suspension and largely was unaffected by topography at the scale of individual thalwegs. Facies and clast analyses of conglomerate overlying channel‐marginal scours reveal that they were deposited by composite gravity flows, which were non‐cohesive, grain‐dominant debris flows with more fluidal cores. These flows were capable of basal erosion but were strongly depositional; frictional freezing at flow margins built gravel levées, while the core maintained a more fluidal transport regime. The resultant architecture consists of matrix‐rich, poorly sorted levées bounding better‐sorted, traction‐dominated cores. The planform geometry is interpreted to have consisted of a low‐sinuosity gravel braid‐plain built by accretion around mid‐channel and bank‐attached bars. This part of the system may be analogous to fluvial systems; however, the finer‐grained sediment load formed thick suspension clouds, probably several orders of magnitude thicker than the relief of braid‐plain topography and therefore controlled by the levées and canyon wall confinement.     

The Meandu Creek Bentonite,Queensland

New data on the source of sands found in desert alluvia and adjacent dunes have led to a revival of the suggestion that the sand ridges of the Australian dunefields, and specifically those of the Great Victoria and Simpson Deserts, are predominantly of erosional origin. In this paper, geological and morphological evidence are cited against this wind rift hypothesis. In the Great Victoria Desert several arguments put for a local derivation of dune sand are incompatible with regional and local geology. In the Simpson Desert, the distribution, shape and colour of dunes, and the topographic relation of dunes to substrates argue against an erosional origin. Long‐distance transport of sand from numerous local depocentres and under a bidirectional wind regime better accounts for the field evidence.     

Architecture and formation of transgressive–regressive cycles in marginal marine strata of the John Henry Member,Straight Cliffs Formation,Upper Cretaceous of Southern Utah,USA

Marginal marine deposits of the John Henry Member, Upper Cretaceous Straight Cliffs Formation, were deposited within a moderately high accommodation and high sediment supply setting that facilitated preservation of both transgressive and regressive marginal marine deposits. Complete transgressive–regressive cycles, comprising barrier island lagoonal transgressive deposits interfingered with regressive shoreface facies, are distinguished based on their internal facies architecture and bounding surfaces. Two main types of boundaries occur between the transgressive and regressive portions of each cycle: (i) surfaces that record the maximum regression and onset of transgression (bounding surface A); and (ii) surfaces that place deeper facies on top of shallower facies (bounding surface B). The base of a transgressive facies (bounding surface A) is defined by a process change from wave‐dominated to tide‐dominated facies, or a coaly/shelly interval indicating a shift from a regressive to a transgressive regime. The surface recording such a process change can be erosional or non‐erosive and conformable. A shift to deeper facies occurs at the base of regressive shoreface deposits along both flooding surfaces and wave ravinement surfaces (bounding surface B). These two main bounding surfaces and their subtypes generate three distinct transgressive – regressive cycle architectures: (i) tabular, shoaling‐upward marine parasequences that are bounded by flooding surfaces; (ii) transgressive and regressive unit wedges that thin basinward and landward, respectively; and (iii) tabular, transgressive lagoonal shales with intervening regressive coaly intervals. The preservation of transgressive facies under moderately high accommodation and sediment supply conditions greatly affects stratigraphic architecture of transgressive–regressive cycles. Acknowledging variation in transgressive–regressive cycles, and recognizing transgressive successions that correlate to flooding surfaces basinward, are both critical to achieving an accurate sequence stratigraphic interpretation of high‐frequency cycles.