Coupled ‘storm‐flood’ depositional model: Application to the Miocene–Modern Baram Delta Province,north‐west Borneo

The Miocene to Modern Baram Delta Province is a highly efficient source to sink system that has accumulated 9 to 12 km of coastal–deltaic to shelf sediments over the past 15 Myr. Facies analysis based on ca 1 km of total vertical outcrop stratigraphy, combined with subsurface geology and sedimentary processes in the present‐day Baram Delta Province, suggests a ‘storm‐flood’ depositional model comprising two distinct periods: (i) fair‐weather periods are dominated by alongshore sediment reworking and coastal sand accumulation; and (ii) monsoon‐driven storm periods are characterized by increased wave‐energy and offshore‐directed downwelling storm flow that occur simultaneously with peak fluvial discharge caused by storm precipitation (‘storm‐floods’). The modern equivalent environment has the following characteristics: (i) humid‐tropical monsoonal climate; (ii) narrow (ca <100 km) and steep (ca 1°), densely vegetated, coastal plain; (iii) deep tropical weathering of a mudstone‐dominated hinterland; (iv) multiple independent, small to moderate‐sized (10² to 10⁵ km²) drainage basins; (v) predominance of river‐mouth bypassing; and (vi) supply‐dominated shelf. The ancient, proximal part of this system (the onshore Belait Formation) is dominated by strongly cyclical sandier‐upward successions (metre to decametre‐scale) comprising (from bottom to top): (i) finely laminated mudstone with millimetre‐scale silty laminae; (ii) heterolithic sandstone–mudstone alternations (centimetre to metre‐scale); and (iii) sharp‐based, swaley cross‐stratified sandstone beds and bedsets (metre to decimetre‐scale). Gutter casts (decimetre to metre‐scale) are widespread, they are filled with swaley cross‐stratified sandstone and their long axes are oriented perpendicular to the palaeo‐shoreline. The gutter casts and other associated waning‐flow event beds suggest that erosion and deposition was controlled by high‐energy, offshore‐directed, oscillatory‐dominated, sediment‐laden combined flows within a shoreface to delta front setting. The presence of multiple river mouths and exceptionally high rates of accommodation creation (characteristic of the Neogene to Recent Baram Delta Province; up to 3000 m Ma⁻¹), in a ‘storm‐flood’‐dominated environment, resulted in a highly efficient and effective offshore‐directed sediment transport system.     

Shelf‐margin clinothem progradation,degradation and readjustment: Tanqua depocentre,Karoo Basin (South Africa)

Degradation of basin‐margin clinothems around the shelf‐edge rollover zone may lead to the generation of conduits through which gravity flows transport sediment downslope. Many studies from seismic‐reflection data sets show these features, but they lack small‐scale (centimetre to metre) sedimentary and stratigraphic observations on process interactions. Exhumed basin‐margin clinothems in the Tanqua depocentre (Karoo Basin) provide seismic‐reflection‐scale geometries and internal details of architecture with depositional dip and strike control. At the Geelhoek locality, clinothem parasequences comprise siltstone‐rich offshore deposits overlain by heterolithic prodelta facies and sandstone‐dominated deformed mouth bars. Three of these parasequences are truncated by a steep (6 to 22°), 100 m deep and 1·5 km wide asymmetrical composite erosion surface that delineates a shelf‐incised canyon. The fill, from base to top comprises: (i) thick‐bedded sandstone with intrabasinal clasts and multiple erosion surfaces; (ii) scour‐based interbedded sandstone and siltstone with tractional structures; and (iii) inverse‐graded to normal‐graded siltstone beds. An overlying 55 m thick coarsening‐upward parasequence fills the upper section of the canyon and extends across its interfluves. Younger parasequences display progressively shallower gradients during progradation and healing of the local accommodation. The incision surface resulted from initial oversteepening and high sediment supply triggering deformation and collapse at the shelf edge, enhanced by a relative sea‐level fall that did not result in subaerial exposure of the shelf edge. Previous work identified an underlying highly incised, sandstone‐rich shelf‐edge rollover zone across‐margin strike, suggesting that there was migration in the zone of shelf edge to upper‐slope incision over time. This study provides an unusual example of clinothem degradation and readjustment with three‐dimensional control in an exhumed basin‐margin succession. The work demonstrates that large‐scale erosion surfaces can develop and migrate due to a combination of factors at the shelf‐edge rollover zone and proposes additional criteria to predict clinothem incision and differential sediment bypass in consistently progradational systems.     

Loading fractures and Liesegang laminae: new sedimentary structures found in the north-western North Alpine Foreland Basin (Oligocene–Miocene, south-west Germany)

Abstract Spectacular sedimentary structures recently found in the Molasse Basin (Oligocene–Miocene) in southern Germany were produced by soft‐sediment deformation under highly unusual conditions. These large, apparently wedge‐like structures –‘loading fractures’– cut down into beds of marl and are filled with coarse sand and intraclasts of shale. Wrapping the sides of the structures is a thin, continuous bed of layered dark claystone – the ‘DCB’. The upper and lower layers of this bed are an organic‐rich clay; the middle layer is a laminated quartzite. The precursor of the DCB was a lacustrine gyttja rich in diatom frustules. It was supersaturated in silica as it was buried. Subsequent diffusion of oxygen into this gyttja at a burial depth of only a few metres resulted in the formation of Liesegang laminae of quartz. These laminae grew and amalgamated, forming the layer of laminated quartzite. The sediments overlying the DCB were eventually removed by erosion, probably in a high‐energy marine environment. This erosion cut down to the DCB but was unable to penetrate it. The DCB remained exposed on the sea floor until a sudden depositional event occurred – the deposition of a 2·5 metre thick bed of coarse sand with shale intraclasts. Although the DCB had been able to resist the submarine erosion, it could not support the load of this new bed. The quartzite layer in it therefore fractured, transferring that load down onto the underlying, still‐unconsolidated marl. The intraclast‐rich sands were forced down into this marl, carrying ahead of them the partly broken remains of the DCB.     

The role of shelf morphology on storm‐bed variability and stratigraphic architecture,Lower Cretaceous,Svalbard

The dominance of isotropic hummocky cross‐stratification, recording deposition solely by oscillatory flows, in many ancient storm‐dominated shoreface–shelf successions is enigmatic. Based on conventional sedimentological investigations, this study shows that storm deposits in three different and stratigraphically separated siliciclastic sediment wedges within the Lower Cretaceous succession in Svalbard record various depositional processes and principally contrasting sequence stratigraphic architectures. The lower wedge is characterized by low, but comparatively steeper, depositional dips than the middle and upper wedges, and records a change from storm‐dominated offshore transition – lower shoreface to storm‐dominated prodelta – distal delta front deposits. The occurrence of anisotropic hummocky cross‐stratification sandstone beds, scour‐and‐fill features of possible hyperpycnal‐flow origin, and wave‐modified turbidites within this part of the wedge suggests that the proximity to a fluvio‐deltaic system influenced the observed storm‐bed variability. The mudstone‐dominated part of the lower wedge records offshore shelf deposition below storm‐wave base. In the middle wedge, scours, gutter casts and anisotropic hummocky cross‐stratified storm beds occur in inferred distal settings in association with bathymetric steps situated across the platform break of retrogradationally stacked parasequences. These steps gave rise to localized, steeper‐gradient depositional dips which promoted the generation of basinward‐directed flows that occasionally scoured into the underlying seafloor. Storm‐wave and tidal current interaction promoted the development and migration of large‐scale, compound bedforms and smaller‐scale hummocky bedforms preserved as anisotropic hummocky cross‐stratification. The upper wedge consists of thick, seaward‐stepping successions of isotropic hummocky cross‐stratification‐bearing sandstone beds attributed to progradation across a shallow, gently dipping ramp‐type shelf. The associated distal facies are characterized by abundant lenticular, wave ripple cross‐laminated sandstone, suggesting that the basin floor was predominantly positioned above, but near, storm‐wave base. Consequently, shelf morphology and physiography, and the nature of the feeder system (for example, proximity to deltaic systems) are inferred to exert some control on storm‐bed variability and the resulting stratigraphic architecture.     

柴达木盆地西南缘新近系与地震沉积有关的软沉积物变形构造及其地质意义

在区域构造背景研究和岩心观察的基础上,在柴达木盆地西南缘新近纪地层中识别出与地震沉积有关的软沉积物变形构造。软沉积物变形构造包括液化砂岩脉、泄水构造、重荷模、火焰构造、震积砂枕、砂球构造、枕状层、层内错断、地裂缝、串珠状构造、震褶层、混合层及地震角砾状构造等。液化砂岩脉有喉道状、脉络状、飘带状、尖突状及“V”字形五种,主要是由振动流体化作用、振动液化挤压作用和振动拉张裂缝充填作用形成的;重荷模、火焰构造、枕状构造、球状构造是受地震颤动在砂、泥岩界面上由于砂层下沉、泥层上穿形成的;地裂缝、层内错断、震褶层是地震颤动直接引起的断裂、错断和褶皱;枕状层是地震振动引起的砂层脱水、下沉、变形形成的;混合层构造的完整性取决于地震强度和地震持续时间;地震角砾状构造是由地震振动使原始沉积层断裂形成的自碎屑角砾、脆性角砾和塑性角砾组成。该成果从沉积学角度证明了新近纪是昆仑山造山带北侧断裂活动较强烈时期,也为柴达木盆地新生代构造演化研究提供了依据。地震作用极大地提高了储层的渗透率,改善了油气储层的储集物性。     

Downslope‐migrating sandwaves and platform‐margin clinoforms in a current‐dominated,distally steepened temperate‐carbonate ramp (Guadix Basin,Southern Spain)

During the Late Tortonian, platform‐margin‐prograding clinoforms developed at the south‐western margin of the Guadix Basin. Large‐scale wedge‐shaped deposits here comprise 26 rhythms of mixed carbonate–siliciclastic bedset packages and marl beds. These sediments were deposited on a shallow‐water, temperate‐carbonate distally steepened ramp. A downslope‐migrating sandwave field developed in this ramp, with sandwaves moving progressively down the ramp to the ramp‐slope, where they destabilized, folded and occasionally collapsed. Downslope sandwave migration was induced by currents flowing basinwards. During the Late Tortonian, the Guadix Basin was open north to the Atlantic Ocean via the Dehesas de Guadix Strait and connected east to the Mediterranean Sea through the Almanzora Corridor. According to the proposed current circulation model for the Guadix Basin for this time, surface marine currents from the Atlantic entered the basin from the northern seaway. These currents moved counter‐clockwise and shifted the sediment on the ramp, forming sandwaves that migrated downslope. The development of platform‐margin prograding clinoforms by the basinward sediment‐transport mechanisms inferred here is known relatively poorly in the ancient sedimentary record. Moreover, these wedge‐shaped geometries are similar to those found in some shelves in the Western Mediterranean Sea and could represent an outcrop analogue to (sub)‐recent, platform‐margin clinoforms revealed by high‐resolution seismic studies.     

Evidence for Late Messinian seismites,Nijar Basin,south‐east Spain

The Feos Formation of the Nijar Basin comprises sediments deposited during the final stage of the Messinian salinity crisis when the Mediterranean was almost totally isolated. Levels of soft‐sediment deformation structures occur in both conglomeratic alluvial sediments deposited close to faults and the hyposaline Lago Mare facies, a laminated and thin‐bedded succession of whitish chalky marls and intercalated sands alternating with non‐marine coastal plain deposits. Deformation structures in the coarse clastics include funnel‐shaped depressions filled with conglomerate, liquefaction dykes terminating downwards in gravel pockets, soft‐sediment mixing bodies, chaotic intervals and flame structures. Evidence for soft‐sediment deformation in the fine‐grained Lago Mare facies comprises syndepositional faulting and fault‐grading, sandstone dykes, mixed layers, slumping and sliding of sandstone beds, convolute bedding, and pillar and flame structures. The soft‐sediment deformed intervals resemble those ascribed elsewhere to seismic shaking. Moreover, the study area provides the appropriate conditions for the preservation of deformation structures induced by seismicity; such as location in a tectonically active area, variable sediment input to produce heterolithic deposits and an absence of bioturbation. The vertical distribution of soft‐sediment deformation implies frequent seismic shocks, underlining the importance of seismicity in the Betic region during the Late Messinian when the Nijar Basin became separated from the Sorbas Basin to the north. The presence of liquefied gravel injections in the marginal facies indicates strong earthquakes (M ≥ 7). The identification of at least four separate fissured levels within a single Lago Mare interval suggests a recurrence interval for large magnitude earthquakes of the order of millennia, assuming that the cyclicity of the alternating Lago Mare and continental intervals was precession‐controlled. This suggestion is consistent with the present‐day seismic activity in SE Spain.     

Macro‐ and micro‐sedimentology of a modern melt‐out till – Matanuska Glacier,Alaska, USA

The macro‐ and micro‐sedimentology of a supraglacial melt‐out till forming at the Matanuska Glacier was examined in relationship to the properties of the stratified basal zone ice and debris from which it is originating. In situ melting of the basal ice has produced a laminated to bedded diamicton consisting mainly of silt. Macroscopic properties include: discontinuous laminae and beds; lenses of sand, silt aggregates and open‐work gravel; deformed and elongate clasts of clay; widely dispersed pebbles and cobbles, those that are prolate usually with their long axes subparallel to parallel to the bedding. Evidence for deformation is absent except for localized bending of beds over or under rock clasts. Microscopic properties are a unique element of this work and include: discontinuous lineations; silt to granule size laminae; prolate coarse sand and rock fragments commonly with their long axis subparallel to bedding; subangular to subrounded irregular shaped clay clasts often appearing as bands; sorted and unsorted silt to granule size horizons, sometimes disrupted by pore‐water pathways. Limited deformation occurs around rock clasts and thicker parts of lamina. This study shows that in situ melting of debris‐rich basal ice can produce a laminated and bedded diamicton that inherits and thereby preserves stratified basal ice properties. Production and preservation of supraglacial melt‐out till require in situ melting of a stagnant, debris‐rich basal ice source with a low relief surface that becomes buried by a thick, stable, insulating cover of ice‐marginal sediment. Also required are a slow melt rate and adequate drainage to minimize pore‐water pressures in the till and overlying sediment cover to maintain stability and uninterrupted deposition. Many modern and ancient hummocky moraines down glacier of subglacial overdeepenings probably meet these process criteria and their common occurrence suggests that both modern and pre‐modern supraglacial melt‐out tills may be more common than previously thought.     

Soft-sediment Deformation Structures Related to Earthquake from the Devonian of the Eastern North Qilian Mts. and Its Tectonic Significance

Devonian in the North Qilian orogenic belt and Hexi Corridor developed terrestrial molasse of later stage of foreland basin caused by collision between the North China plate and Qaidam microplate. The foreland basin triggered a intense earthquake, and formed seismites and earthquake-related soft-sediment deformation. The soft-sediment deformation structures of Devonian in the eastern North Qilian Mts. consist of seismo-cracks, sandstone dykes, syn-depositional faults, microfolds (micro-corrugated lamination), fluidized veins, load casts, flame structures, pillow structures and brecciation. The seismo-cracks, syn-depositional faults and microfolds are cracks, faults and folds formed directly by oscillation of earthquake. The seismic dykes formed by sediment instilling into seismic cracks. Fluidized veins were made by instilling into the seismo-fissures of the fluidized sands. The load casts, flame structures and pillow structures were formed by sinking and instilling caused from oscillation of earthquake along the face between sandy and muddy beds. The brecciation resulted from the oscillation of earthquake and cracking of sedimentary layers. The seismites and soft-sediment deformations in Devonian triggered the earthquake related to tectonic activities during the orogeny and uplift of North Qilian Mts.     

Sequence stacking patterns in the Western Canada foredeep: influence of tectonics, sediment loading and eustasy on deposition of the Upper Cretaceous Kaskapau and Cardium Formations

The Kaskapau and Cardium Formations span Late Cenomanian to Early Coniacian time and were deposited on a low‐gradient foredeep ramp. The studied portion of the Kaskapau Formation spans ca 3·5 Myr and forms a mudstone‐dominated wedge thinning from 700 to <50 m from SW to NE over ca 300 km. In contrast, the Cardium Formation spans about 2·1 Myr, is about 100 m thick, sandstone‐rich and broadly tabular. The Kaskapau and Cardium Formations are divided, respectively, into 28 and nine allomembers, each bounded by marine flooding surfaces. Kaskapau allomembers 1 to 7 show about 200 km of offlap from the forebulge, accompanied by progradation of thin sandstones from the eroded forebulge crest. In contrast, Kaskapau allomembers 8 to 28 and Cardium allomembers C1 to C9 show overall onlap onto the forebulge of about 350 km, and contain no forebulge‐derived sandstones. This broad pattern is interpreted as recording a latest Cenomanian pulse of tectonic loading which led to shoreline back‐step in the proximal foredeep and coeval uplift of the forebulge, leading to erosion. The advance of the sediment wedge after Kaskapau allomember 7 is attributed primarily to the isostatic effect of a distributed sediment load; the advance of the orogenic wedge had a subordinate effect on subsidence of the forebulge. For Kaskapau allomembers 1 to 6, isopachs trend north to south, suggesting a load directly to the west; allomembers 7 to 28 show an abrupt rotation of isopachs to NW–SE, suggesting that the load shifted several hundred kilometres to the south. This re‐orientation might be related to a change from an approximately orthogonal to a dextral transpressive stress regime. Within the longer‐term offlap–onlap cycle recorded by the Kaskapau and Cardium Formations, individual allomembers are grouped into packages reflecting higher‐frequency onlap–offlap cycles, each spanning ca 0·5 to 0·7 Myr. Offlap from the forebulge tends to be accompanied by more pronounced transgression in the foredeep, whereas onlap onto the forebulge is accompanied by progradation of tongues of shoreface sandstone. This relationship suggests that changes in deformation rate in the orogenic wedge modulated proximal subsidence rate, enhancing or suppressing shoreline progradation, and also causing subtle uplift or subsidence of the forebulge region. Wedge‐shaped allomembers in the Kaskapau Formation contain shoreface sandstone and conglomerate that prograded, respectively, <40 and <25 km from the preserved basin margin; progradation of coarse clastics was limited by rapid flexural subsidence. Tabular allomembers of the Cardium Formation imply a low flexural subsidence rate and contain sandy and conglomeratic shoreface deposits that prograded up to ca 180 km from the preserved basin margin. This relationship suggests that low rates of flexural subsidence promoted steeper alluvial gradients, more vigorous gravel transport and more extensive shoreface progradation. Overall, observed stratal geometry and facies distribution is explained readily in terms of current elastic flexural models. Most shoreface sandstones in the proximal foredeep show evidence of forced regression. Eustasy provides the most plausible explanation for relative sea‐level rise–fall cycles on the 125 kyr allomember timescale. Geometric relationships suggest eustatic oscillations of about 10 m. Forced regressive shoreface development was suppressed during Kaskapau allomembers 1 to 10 when the rate of flexural subsidence was at its highest.     

Hyperpycnal delivery of sand to the continental shelf: Insights from the Jurassic Lajas Formation,Neuquén Basin,Argentina

Hyperpycnal currents are river‐derived turbidity currents capable of transporting significant volumes of sediment from the shoreline onto the shelf and potentially further to deep ocean basins. However, their capacity to deposit sand bodies on the continental shelf is poorly understood. Shelf hyperpycnites remain an overlooked depositional element in source to sink systems, primarily due to their limited recognition in the rock record. Recent discoveries of modern shelf hyperpycnites, and previous work describing hyperpycnites deposited in slope or deep‐water settings, provide a valuable framework for understanding and recognizing shelf hyperpycnites in the rock record. This article describes well‐sorted lobate sand bodies on the continental shelf of the Neuquén Basin, Argentina, interpreted to have been deposited by hyperpycnal currents. These hyperpycnites of the Jurassic Lajas Formation are characterized by well‐sorted, medium‐grained, parallel‐laminated sandstones with hundreds of metre extensive, decimetre thick beds encased by organic‐rich, thinly laminated sandstone and siltstone. These deposits represent slightly obliquely‐migrating sand lobes fed by small rivers and deposited on the continental shelf. Hyperpycnites of the Lajas Formation highlight several unique characteristics of hyperpycnal deposits, including their distinctively thick horizontal laminae attributed to pulsing of the hyperpycnal currents, the extraction of coarse gravel due to low flow competence, and the extraction of mud due to lofting of light interstitial fluid. Recognition of shelf hyperpycnites in the Lajas Formation of the Neuquén Basin allows for a broader understanding of shelf processes and adds to the developing facies models of hyperpycnites. Recognizing and understanding the geometry and internal architecture of shelf hyperpycnites will improve current understanding of sediment transfer from rivers to deeper water, will improve palaeoenvironmental interpretations of sediment gravity‐flow deposits, and has implications for modelling potentially high‐quality hydrocarbon reservoirs.     

Aseismic controls on in situ soft-sediment deformation processes and products in submarine slope deposits of the Karoo Basin, South Africa

Intervals of soft‐sediment deformation features, including vertical fluid escape and load structures, are common and well‐exposed in Permian lower slope deposits of the Tanqua Depocentre, Karoo Basin. The structures mainly comprise elongated flames and load structures associated with ruptured sandstones and structureless siltstones, observed over a range of scales. The presence of an upper structureless siltstone layer linked to the flames, interpreted as a product of the debouching of fine‐grained material transported through the flame onto the palaeo‐seabed, together with the drag and upward folding of lower sandstone layers is evidence that the flames were formed in situ by upward movement of sediment‐rich fluids. Flames are oriented parallel to the deep‐water palaeoslope in lateral splay deposits between two major slope channel complexes. Statistical correlation and regression analyses of 180 flame structures from seven stratigraphic intervals suggest a common mechanism for the deformation and indicate the importance of fluidization as a deformation mechanism. Importantly, deformation occurred in an instantaneous and synchronous manner. Liquefaction and fluidization were triggered by incremental movement of sediment over steeper local gradients that were generated by deposition of a lateral splay on an inherited local north‐west‐facing slope. Seismic activity is not invoked as a trigger mechanism because of the restricted spatial occurrence of these features and the lack of indications of earthquakes during the time of deposition of the deep‐water succession. The driving mechanisms that resulted in the final configuration of the soft‐sediment deformation structures involved a combination of vertical shear stress caused by fluidization, development of an inverse density gradient and a downslope component of force associated with the local slope. Ground‐penetrating radar profiles confirm the overall north‐east orientation of the flame structures and provide a basis for recognition of potential larger‐scale examples of flames in seismic reflection data sets.     

Characterization of hard‐to‐differentiate dune stratification types in the Permian Coconino Sandstone (Arizona,USA)

Dune stratification types, which include grainfall, grainflow and ripple lamination, provide a record of the fine‐scale processes that deposited sediment on palaeo‐dune foresets. While these facies are relatively easy to distinguish in some cross‐bedded sandstones, for others – like the Permian Coconino Sandstone of northern and central Arizona – discrete stratification styles are hard to recognize at the bedding scale. Furthermore, few attempts have been made to classify fine‐scale processes in this sandstone, despite its renown as a classic aeolian dune deposit and Grand Canyon formation. To interpret depositional processes in the Coconino Sandstone, cross‐bed facies were characterized using a suite of sedimentary textures and structures. Bedding parameters were described at multiple scales via a combination of field and laboratory methods, including annotated outcrop photomosaics, strike and dip measurements, sandstone disaggregation and laser‐diffraction particle analysis, high‐resolution scans of thin sections, and scanning electron microscopy. Cross‐beds were observed to be laterally extensive along‐strike, with most dip angles ranging from the mid‐teens to mid‐twenties. While some cross‐bed sets are statistically coarser near their bases, others exhibit no significant vertical sorting trends. Both massive and laminated textures are visible in high‐resolution scans of thin sections, but laminae contacts are commonly indistinct, making normal and reverse grading difficult to define. Diagenetic features, such as stylolite seams and large pores, are also present in some samples and might indicate alteration of original textures like detrital clay laminae and carbonate minerals. Observed textures and sedimentary structures suggest that the cross‐beds may consist of grainflow and grainfall deposits, but these remain difficult to differentiate at outcrop and thin‐section scales. This characterization of fine‐scale processes will play a critical part in the development of depositional models for the Coconino Sandstone and elucidate interpretations for similar cross‐bedded formations.     

Tsunami backwash deposits with Chicxulub impact ejecta and dinosaur remains from the Cretaceous–Palaeogene boundary in the La Popa Basin,Mexico

The La Popa Basin in north‐eastern Mexico features outstanding, continuous three‐dimensional exposures of the Cretaceous–Palaeogene boundary event deposit in shallow shelf environments pierced by salt stocks. In the area to the south‐east of the El Papalote diapir, the Cretaceous–Palaeogene deposit consists of two superimposed sedimentary units and erosively overlies upper Maastrichtian sand‐siltstones with soft‐sediment deformation and liquefaction structures. The basal unit 1 is an up to 8 m thick chaotic, carbonate‐rich bed that discontinuously fills incised gutters and channels. Besides abundant silicic and carbonate ejecta spherules from the Chicxulub impact, unit 1 includes large sandstone boulders and abundant shallow‐water debris (for example, mud clasts, algae, bivalve shells, gastropod shells and vertebrate remains). Unit 1 is conformably overlain by unit 2. Distal to the diapir, unit 2 consists of a centimetre to decimetre‐thick conglomeratic, coarse bioclast and spherule‐bearing sandstone bed. Closer to the diapir, unit 2 becomes a metre‐thick series of four to eight conglomeratic to fine‐grained graded sandstone beds rich in shell debris and ejecta spherules. Unit 2 is conformably overlain by structureless to parallel laminated sandstone beds that may mark the return to the pre‐event depositional regime. The sedimentary characteristics of the Cretaceous–Palaeogene deposit, including its erosive base, its sheet‐like geometry, the presence of multiple, graded beds, evidence for upper flow regime conditions and the absence of bioturbation, support an origin by a short‐term multiphase depositional event. The occurrence of soft‐sediment deformation structures (for example, liquefaction) below the Cretaceous–Palaeogene deposit suggests that earthquakes were the first to occur at La Popa. Then, shelf collapse and strong backflow from the first tsunami waves may have triggered erosion and deposition by violent ejecta‐rich hyperconcentrated density flows (unit 1). Subsequently, a series of concentrated density flows resulting from tsunami backwash surges may have deposited the multiple‐graded bedding structures of unit 2. The specific depositional sequence and the Fe‐Mg‐rich as well as Si‐K‐rich composition of the ejecta spherules both provide a critical link to the well‐known deep marine Cretaceous–Palaeogene boundary sites in the adjacent Burgos basin in north‐eastern Mexico. Moreover, the pulse‐like input of Chicxulub ejecta material at the base of the event deposit allows for correlation with other Cretaceous–Palaeogene boundary sites in the Gulf of Mexico and the Atlantic, as well as in Central and Northern America. The presence of diverse dinosaur and mosasur bones and teeth in the event deposit is the first observation of such remains together with Chicxulub ejecta material. These findings indicate that dinosaurs lived in the area during the latest Maastrichtian and suggest that the tsunami waves not only eroded deltas and estuaries but the coastal plain as well.     

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.     

A pre‐LGM sandur at Fiskarheden in NW Dalarna,central Sweden – sedimentology and glaciotectonic deformation

The Fiskarheden quarry, situated in NW Dalarna, central Sweden, reveals thick coarse‐grained sediments of Scott type facies association representing a sandur deposited in an ice‐proximal proglacial environment. Optically Stimulated Luminescence (OSL) dating of the sandur sediments suggests a pre‐Last Glacial Maximum (LGM) age. Most acquired ages are pre‐Saalian (>200 ka) and we regard each of these ages to represent non/poorly bleached sediment except for one small‐aliquot OSL age of 98±6 ka. This age comes from the top surface of an arguably well‐bleached sand bed deposited on the lee‐side of a braid‐bar, putting the sandur build‐up into the Early Weichselian. Large‐scale glaciotectonic structures show an imbricate thrust fan involving both ductile and brittle deformation. The deformation was from the WNW, which largely coincides with the formative trend of the predominating streamlined terrain and Rogen moraine tracts surrounding Fiskarheden. It is suggested that the deformation of the sandur sediments took place when the advancing glacier approached and pushed its own proglacial outwash sediment, during an ice‐marginal oscillation either at the inception of one of the Early Weichselian glaciations in the area, or during a general ice retreat amid a deglacial phase. The Fiskarheden sandur deposits are covered by a subglacial traction till deposited from the NE/NNE. This direction corresponds with younger streamlined terrain flowsets cross‐cutting the older NNW–SSE system and probably represents deglaciation in the area following the LGM. This study will add to the understanding of the formation and deformation of Pleistocene sandur successions and their relationship to past ice‐sheet behaviour.     

Sedimentological–ichnological model for tide‐dominated shelf sandbodies: Lower Cambrian Gog Group of western Canada

Dunes and bars are common elements in tide‐dominated shelf settings. However, there is no consensus on a unifying terminology or a systematic classification for thick sets of cross‐stratified sandstones. In addition, their ichnological attributes have hardly been explored. To address these issues, the properties, architecture and ichnology of compound cross‐stratified sandstone bodies contained in the Lower Cambrian Gog Group of the southern Canadian Rocky Mountains are described here. In these transgressive sandstones, five types of compound cross‐stratified sandstone are distinguished based on foreset geometry, sedimentary structures and internal heterogeneity. These represent four broad categories of subtidal sandbodies: (i) compound‐dune fields; (ii) sand sheets; (iii) sand ridges; and (iv) isolated dune patches; tidal bars comprise a fifth category but are not present in the Gog Group. Compound‐dune fields are characterized by sigmoidal and planar cross‐stratified sandstone in coarsening‐upward and thickening‐upward packages (Type 1); these are mostly unburrowed, or locally contain representatives of the Skolithos ichnofacies, but are intercalated with intensely bioturbated sandstone containing the archetypal Cruziana ichnofacies. Sand‐sheet complexes, also composed of compound dunes, cover more extensive subtidal areas, and comprise three adjacent subenvironments: core, front and margin. The core is characterized by thick‐bedded sets of cross‐stratified sandstone (Type 2). A decrease of bedform size at the front is recorded by wedges of thinner‐bedded, low‐angle and planar cross‐stratified sandstone (Type 3) exhibiting dense Skolithos pipe‐rock ichnofabric. The margin is characterized by interbedded sandstone and mudstone, and hummocky cross‐stratified sandstone. Sand‐sheet deposits exhibit clear trends in trace‐fossil distribution along the sediment transport path, from non‐bioturbated beds in the core to Skolithos ichnofacies at the front, and a depauperate Cruziana ichnofacies at the margin. Tidal sand ridges are large elongate sandbodies characterized by large sigmoid‐shaped reactivation surfaces (Type 4). Sand ridges display clear ichnological trends perpendicular to the axis of the ridge, with no bioturbation or a poorly developed Skolithos ichnofacies in the core, a depauperate Cruziana ichnofacies in lee‐side deposits, and Cruziana ichnofacies at the margin. While both tidal ridges and tidal bars migrate by means of lateral accretion, the latter occur in association with channels while the former do not. Because tidal bars tend to occur in brackish‐water marginal‐marine settings, their ichnofauna are typically of low diversity, representing a depauperate Cruziana ichnofacies. Isolated dune patches developed on sand‐starved areas of the shelf, and are represented by lenticular sandbodies with sigmoidal reactivation surfaces (Type 5); they typically lack trace fossils, but the interfingering muddy deposits are intensely bioturbated by a high‐diversity fauna recording the Cruziana ichnofacies. The variety of sandbody types in the Gog Group reflects varying sediment supply and location on the inner continental shelf. These, in turn, governed substrate mobility, grain size, turbidity, water‐column productivity and sediment organic matter which controlled trace fossil distribution.     

Tepee-shaped agglutinated microbialites: an example from a Famennian carbonate platform on the Lennard Shelf, northern Canning Basin, Western Australia

Distinctive, metre‐scale antiformal structures are well developed in a Famennian carbonate platform in the Chedda Cliffs area of the Lennard Shelf reef complexes. The structures are distinguished by chevron‐shaped crests and thickened cores and contain abundant non‐skeletal allochems (ooids/pisoids, peloids and intraclasts) of silt to pebble size and variably developed laminations and fenestrae. The internal morphology and pervasive occurrence of fenestral clotted and wavy laminated fabrics suggest that these structures are microbial mounds composed of agglutinated stromatolites and thrombolites. Microbial fabrics most probably originated through sediment trapping and binding by microbial mats with early lithification involving microbial calcification and cementation of trapped sediment. The facies and stratigraphic context of the mounds support a shallow subtidal, transitional backreef to reef‐flat setting; however, alone these mounds do not provide unequivocal environmental information. Other large antiformal structures in Famennian platforms on the Lennard Shelf, previously described as tepee structures, show morphological similarities to the Chedda Cliffs mounds, which suggests that these other structures may also be microbial mounds. The presence of microbial mounds in platform successions further highlights the importance of microbial communities in the Lennard Shelf reef complexes.     

Laminated–bioturbated cycles in Maastrichtian chalk of the North Sea: oxygenation fluctuations within the Milankovitch frequency band

The Maastrichtian chalk of the southern Central Graben, Danish North Sea, is a homogeneous pure white coccolithic chalk mudstone deposited in a deep epeiric shelf sea, which covered large parts of northern Europe. The sediment displays a pronounced cyclicity marked by decimetre‐thick bioturbated beds alternating with slightly thinner non‐bioturbated, mainly laminated beds. The laminated half‐cycles consist of alternating millimetre‐thick, graded, high‐porosity laminae and non‐graded, low‐porosity laminae. The cyclicity has been interpreted previously as caused by periods of slow background sedimentation and bioturbation interrupted by periods of rapid deposition of laminated beds, with the latter reflecting random and local resedimentation processes. Based on textural and structural analysis, the millimetre‐scale, non‐graded laminae are interpreted as having been deposited directly from pelagic rain of pelleted coccoliths representing the primary production. The graded laminae were deposited from small‐volume, low‐density turbidity currents and suspension clouds. The sedimentation rates of the cyclical chalk are similar to those known elsewhere, and the lamination is interpreted as having been preserved from destruction through bioturbation by anoxic conditions at the seafloor. Bioturbated–laminated cycles are thus formed by slow sedimentation during alternating seafloor redox conditions probably on a Milankovitch scale. A direct implication of this interpretation is that the cycles are areally widespread, probably extending throughout the southern Central Graben area and may be useful for correlation and high‐resolution cyclostratigraphy in the chalk fields of the Danish North sea. If the laminated half‐cycles represent a few rapid resedimentation events, with a high sedimentation rate as suggested by most workers, then the sediment would not be truly cyclic, but would represent event sedimentation within a pelagic background represented by the bioturbated beds. In this case, the cycles would have very limited potential for correlation.     

Identification and micro‐stratigraphy of hyperpycnites and turbidites in Cretaceous Lewis Shale,Wyoming

Sandy hyperpycnal flows and their deposits, hyperpycnites, have been documented in modern environments and, more recently, in Cretaceous and Tertiary strata; they may be more common in the rock record, and within petroleum reservoirs, than has been previously thought. Muddy hyperpycnites also occur within the rock record, but these are more difficult to document because of their finer‐grained nature and lack of common sedimentary structures. This paper documents the presence of submarine slope mudstone and siltstone hyperpycnites (and muddy turbidites) in the delta‐fed, Upper Cretaceous Lewis Shale of Wyoming; based on field measurements, analyses of rock slabs and thin sections, and laser grain‐size distributions. Four lithofacies comprise laminated and thin‐bedded mudstones that are associated with levéed channel sandstones: (L1) grey, laminated, graded mudstone with thin siltstone and sandstone interbeds; (L2) dark grey to tan, laminated mudstone with very thin siltstone and sandstone stringers; (L3) light grey, laminated siltstones; and (L4) laminated mudstones and siltstones with thin sandstone interbeds. Two styles of mudstone grain‐size grading have been documented. The first type is an upward‐fining interval that typically ranges in thickness from 2·5 to 5 cm. The second type is a couplet of a lower, upward‐coarsening interval and an upper, upward‐fining interval (sometimes separated by a micro‐erosion surface) which, combined, are about 3·8 cm thick. Both individual laminae and groups of laminae spaced millimetres apart exhibit these two grain‐size trends. Although sedimentary structures indicative of traction‐plus‐fallout sedimentary processes associated with sandier hyperpycnites are generally absent in these muddy sediments, the size grading patterns are similar to those postulated in the literature for sandy hyperpycnites. Thus, the combined upward‐coarsening, then upward‐fining couplets are interpreted to be the result of a progressive increase in river discharge during waxing and peak flood stage (upward increase in grain‐size), followed by waning flow after the flood begins to abate (upward decrease in grain‐size). The micro‐erosion surface that sometimes divides the two parts of the size‐graded couplet resulted from waxing flows of sufficiently high velocity to erode the sediment previously deposited by the same flow. Individual laminae sets which only exhibit upward‐fining trends could be either the result of waning flow deposition from either dilute turbidity currents or from hyperpycnal flows. The occurrence of these sets with the size‐graded couplets suggests that they are associated with hyperpycnal processes.