Sheetflow fluvial processes in a rapidly subsiding basin, Altiplano plateau, Bolivia

Although facies models of braided, meandering and anastomosing rivers have provided the cornerstones of fluvial sedimentology for several decades, the depositional processes and external controls on sheetflow fluvial systems remain poorly understood. Sheetflow fluvial systems represent a volumetrically significant part of the non‐marine sedimentary record and documented here are the lithofacies, depositional processes and possible roles of rapid subsidence and arid climate in generating a sheetflow‐dominated fluvial system in the Cenozoic hinterland of the central Andes. A 6500 m thick succession comprising the Late Eocene–Oligocene Potoco Formation is exposed continuously for >100 km along the eastern limb of the Corque syncline in the high Altiplano plateau of Bolivia. Fluvial sandstone and mudstone units were deposited over an extensive region (>10 000 km²) with remarkably few incised channels or stacked‐channel complexes. The Potoco succession provides an exceptional example of rapid production of accommodation sustained over a prolonged period of time in a non‐marine setting (>0·45 mm year⁻¹ for 14 Myr). The lower ≈4000 m of the succession coarsens upward and consists of fine‐grained to medium‐grained sandstone, mudstone and gypsum deposits with palaeocurrent indicators demonstrating eastward transport. The upper 2500 m also coarsens upward, but contains mostly fine‐grained to medium‐grained sandstone that exhibits westward palaeoflow. Three facies associations were identified from the Potoco Formation and are interpreted to represent different depositional environments in a sheetflow‐dominated system. (i) Playa lake deposits confined to the lower 750 m are composed of interbedded gypsum, gypsiferous mudstone and sandstone. (ii) Floodplain deposits occur throughout the succession and include laterally extensive (>200 m) laminated to massive mudstone and horizontally stratified and ripple cross‐stratified sandstone. Pedogenic alteration and root casts are common. (iii) Poorly confined channel and unconfined sheet sandstone deposits include laterally continuous beds (50 to >200 m) that are defined primarily by horizontally stratified and ripple cross‐stratified sandstone encased in mudstone‐rich floodplain deposits. The ubiquitous thin‐sheet geometry and spatial distribution of individual facies within channel sandstone and floodplain deposits suggest that confined to unconfined, episodic (flash) flood events were the primary mode of deposition. The laterally extensive deposition and possible distributary nature of this sheetflow‐dominated system are attributed to fluvial fan conditions in an arid to semi‐arid, possibly seasonal, environment. High rates of sediment accumulation and tectonic subsidence during early Andean orogenesis may have favoured the development and long‐term maintenance of a sheetflow system rather than a braided, meandering or anastomosing fluvial style. It is suggested here that rapidly produced accommodation space and a relatively arid, seasonal climate are critical conditions promoting the generation of sheetflow‐dominated fluvial systems.     

Carbonate pond deposits related to semi-arid alluvial systems: examples from the Tertiary Madrid Basin, Spain

Carbonate pond deposits occur associated with alluvial sediments in Miocene sequences of the Madrid Basin, central Spain. The ponds developed near the basin margins, either in floodplain environments (north) or mud-flat settings (south). Three main facies assemblages are recognized: (1) floodplain/mud-flat, (2) palaeosols and (3) pond deposits. In the northern part of the basin, ponds developed on the floodplain of terminal fluvial systems. The floodplain facies are typically red mudstones with interbedded sandstones and siltstones. Palaeosols associated with the ponds show a pedofacies relationship, the maturity of soils increasing with distance from the main channel. Carbonate pond deposits consist mainly of limestones, which display typical ‘palustrine’features. The formation and further accumulation of carbonate in the ponds took place in periods of reduced clastic sediment input and it is suggested that recharge into the pond areas was mainly from groundwater. In the south, ponds developed on mud-flats located between sheet-flood-dominated alluvial fans and evaporite lakes. Mud-flat facies consist of red mudstone that exhibits evidence of progressive soil development near both edges and beneath the carbonate pond lenses. Carbonate in the ponds is mainly dolomite and comprises two subfacies, mottled and laminated dolomicrites. This mineralogy, together with the presence of gypsum crusts below and in the lower part of the carbonate body, suggests higher evaporation rates and/or more saline waters filling the ponds in this part of the basin. In spite of differences in depositional setting and, to some extent, climatic conditions between the two areas of the basin, both facies associations and the sequential arrangement of the ponds show strong similarities that allow the proposal of a facies model for carbonate pond deposits related to semi-arid alluvial systems. The sequences recognized from the pond deposits record a set of facies clearly different to those forming in swampy lakes associated with many permanent fluvial systems developed in more humid climates.     

Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes

Flood‐generated sandy siltstones are under‐recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490 m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone ‘cementstones’ and gypsum from a coastal–alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation (Scotland, UK); they are matrix‐supported siltstones with millimetre‐sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil‐bearing deposits, such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking, and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment transported across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non‐marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems.     

Infilling of the Younger Kathmandu–Banepa intermontane lake basin during the Late Quaternary (Lesser Himalaya,Nepal): a sedimentological study

The Kathmandu and Banepa Basins, Central Nepal, are located in a large syncline of the Lesser Himalayas. The Older Kathmandu Lake evolved during the Pliocene and early Pleistocene; the Younger Kathmandu Lake, which is the focus of this study, is infilled with late Quaternary sediments. Three formations, arranged in stratigraphical order, the Kalimati, Gokarna and Thoka Formations formed during the infilling stage of this lacustrine basin. Structural and textural sedimentological analyses, a chemical survey across the basin and mineralogical investigations of fine‐grained sediments form the basis of this palaeogeographical study. The basin under investigation was covered by a perennial freshwater lake before 30 000 yr BP. The lake was infilled with alluvial and fluvial sediments delivered mainly from the mountains north of the basin. A fairly low gradient was favourable for the formation of diatomaceous earths, carbonaceous mudstones and siltstones, which were laid down in the centre of the lake and in small ponds. Towards the basin edge, lacustrine sediments gave way to deltaic deposits spread across the delta plain. Crevasse splays and anastomosing rivers mainly delivered suspended load for the widespread siltstones and mudstones. The proximal parts of the alluvial–fluvial sedimentary wedge contain debris flows that interfinger with fine‐grained floodplain deposits. Three highstands of the water‐level (>30 000 yr BP, 28 000–19 000 yr BP, 11 000–4000 yr BP (?)) have been recognised in the sedimentary record of the younger Kathmandu Lake in the Late Quaternary. Second‐order water‐level fluctuations are assumed to be triggered by local processes (damming by tectonically induced landslides). First‐order water‐level fluctuations are the result of climatic changes. Copyright © 2003 John Wiley & Sons, Ltd.     

Architecture of vertically stacked fluvial deposits,Atane Formation,Cretaceous, Nuussuaq,central West Greenland

Excellent exposures of thick, multistorey, fluvial deposits from the deltaic Atane Formation on south‐east Nuussuaq, central West Greenland, show the architecture of up to 100 m thick continuously aggrading fluvial depositional complexes. The succession comprises vertically stacked channel belt sandstones separated by thin floodplain deposits, with little to no incision between storeys. Architectural elements and palaeocurrent patterns of channel deposits indicate deposition in large, relatively stable, low‐sinuosity rivers, probably located within an incised valley. Gradual transitions from channel to floodplain deposits accompanied by a gradual change from floodplain to spillover sand suggest avulsion on the floodplain as a possible mechanism for the vertically alternating channel and floodplain deposits. Despite its relative proximity to contemporaneous sea‐level (ca 35 km upstream from the palaeo‐shoreline) the depositional complex is entirely non‐marine. The aggrading nature of the deposits suggests a continuously rising base level coupled with a high and steady sediment supply. Vertical alternations between floodplain and channel deposits may be forced by subtle interruptions in this balance or autocyclic mechanisms on the floodplain. This study provides an example of aggrading lowstand/non‐marine transgressive systems tract deposits.     

X‐ray tomography of tsunami deposits: Towards a new depositional model of tsunami deposits

X‐ray computed microtomography is used to obtain high resolution imagery of a historical tsunami deposit in Andalusia, Spain (1755 Lisbon tsunami). The technique allows characterization of grain‐size distribution, structures, component analysis and sedimentary fabric of fine‐grained unconsolidated tsunami deposits at resolutions down to particle scale. The results are validated by comparing to data obtained using other techniques such as laser diffraction, anisotropy of magnetic susceptibility and X‐ray microfluorescence on the same deposits. Specific technical details such as sampling, scanning and image processing methods, and further improvements are addressed. The use of X‐ray computed microtomography provides new insights into the stratigraphy of the deposits and gives access to significantly more detailed view of key sedimentary features such as mudlines, rip‐up clasts, crude laminations, convolutions, floating outsized clasts and contacts between successive units. This analysis of the 1755 tsunami deposits using X‐ray computed microtomography allows the proposal of new hypotheses for the sedimentary processes forming tsunami deposits. Deposition by settling is limited and the section analysed here is dominated by a high shear stress leading to the development of traction carpets, with laminated mudlines corresponding to the basal frictional region of these carpets. The onset of the tsunami backwash is marked by a micro‐vortex resembling Kelvin–Helmoltz instabilities.     

Recognition of strong seasonality and climatic cyclicity in an ancient,fluvially dominated,tidally influenced point bar: Middle McMurray Formation,Lower Steepbank River,north‐eastern Alberta,Canada

Inclined heterolithic stratification in the Lower Cretaceous McMurray Formation, exposed along the Steepbank River in north‐eastern Alberta, Canada, accumulated on point bars of a 30 to 40 m deep continental‐scale river in the fluvial–marine transition. This inclined heterolithic stratification consists of two alternating lithologies, sand and fine‐grained beds. Sand beds were deposited rapidly by unidirectional currents and contain little or no bioturbation. Fine‐grained beds contain rare tidal structures, and are intensely bioturbated by low‐diversity ichnofossil assemblages. The alternations between the sand and fine‐grained beds are probably caused by strong variations in fluvial discharge; that are believed to be seasonal (probably annual) in duration. The sand beds accumulated during river floods, under fluvially dominated conditions when the water was fresh, whereas the fine‐grained beds accumulated during the late stages of the river flood and deposition continued under tidally influenced brackish‐water conditions during times of low‐river flow (i.e. the interflood periods). These changes reflect the annual migration in the positions of the tidal and salinity limits within the fluvial–marine transition that result from changes in river discharge. Sand and fine‐grained beds are cyclically organized in the studied outcrops forming metre‐scale cycles. A single metre‐scale cycle is defined by a sharp base, an upward decrease in sand‐bed thickness and upward increases in the preservation of fine‐grained beds and the intensity of bioturbation. Metre‐scale cycles are interpreted to be the product of a longer term (decadal) cyclicity in fluvial discharge, probably caused by fluctuations in ocean or solar dynamics. The volumetric dominance of river‐flood deposits within the succession suggests that accumulation occurred in a relatively landward position within the fluvial–marine transition. This study shows that careful observation can reveal much about the interplay of processes within the fluvial–marine transition, which in turn provides a powerful tool for determining the palaeo‐environmental location of a deposit within the fluvial–marine transition.     

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.     

Pleistocene fluvial sediments,palaeontology and archaeology of the upper River Thames at Latton,Wiltshire, England

Pleistocene fluvial sediments of the Northmoor Member of the Upper Thames Formation exposed at Latton, Wiltshire, record episodic deposition close to the Churn–Thames confluence possibly spanning the interval from Marine Isotope Stages (MIS) 7 to 2. The sequence is dominated by gravel facies, indicating deposition by a high‐energy, gravel‐bed river. A number of fine‐grained organic sediment bodies within the sequence have yielded palaeoenvironmental and biostratigraphical data from Mollusca, Coleoptera, vertebrates, pollen and plant macrofossils. The basal deposit (Facies Association A) contains faunal material indicating temperate conditions. Most of the palaeontological evidence including a distinctive small form of mammoth (Mammuthus cf. trogontherii), together with the U‐series age estimate of >147.4 ± 20 kyr suggest correlation with MIS 7. The overlying deposits (Facies Associations B and C) represent deposition under a range of climatic conditions. Two fine‐grained organic deposits occurred within Association B; one (Association Ba) in the northern part of the pit as a channel fill and the other (Association Bb) in its southern part as a scour‐fill deposit. The coleopteran assemblages from Ba, indicate that it accumulated under temperate oceanic conditions, while Bb, which also yielded a radiocarbon age estimate of 39 560 ± 780 ¹⁴C yr BP, was formed under much colder and more continental climatic conditions. The sequence is considered to represent deposition within an alluvial fan formed at the Churn–Thames confluence; a depositional scenario which may account for the juxtaposition of sediments and fossils of widely differing age within the same altitudinal range. Copyright © 2006 John Wiley & Sons, Ltd.     

Matrix‐rich and associated matrix‐poor sandstones: Avulsion splays in slope and basin‐floor strata

A common facies observed in deep‐water slope and especially basin‐floor rocks of the Neoproterozoic Windermere Supergroup (British Columbia, Canada) is structureless, coarse‐tail graded, medium‐grained to coarse‐grained sandstone with from 30% to >50% mud matrix content (i.e. matrix‐rich). Bed contacts are commonly sharp, flat and loaded. Matrix‐rich sandstone beds typically form laterally continuous units that are up to several metres thick and several tens to hundreds of metres wide, and commonly adjacent to units of comparatively matrix‐poor, scour‐based sandstone beds with large tabular mudstone and sandstone clasts. Matrix‐rich units are common in proximal basin‐floor (Upper Kaza Group) deposits, but occur also in more distal basin‐floor (Middle Kaza Group) and slope (Isaac Formation) deposits. Regardless of stratigraphic setting, matrix‐rich units typically are directly and abruptly overlain by architectural elements comprising matrix‐poor coarse sandstone (i.e. channels and splays). Despite a number of similarities with previously described matrix‐rich beds in the literature, for example slurry beds, linked debrites and co‐genetic turbidites, a number of important differences exist, including the stratal make‐up of individual beds (for example, the lack of a clean sandstone turbidite base) and their stratigraphic occurrence (present throughout base of slope and basin‐floor strata, but most common in proximal lobe deposits) and accordingly suggest a different mode of emplacement. The matrix‐rich, poorly sorted nature of the beds and the abundance and size of tabular clasts in laterally equivalent sandstones imply intense upstream scouring, most probably related to significant erosion by an energetic plane‐wall jet or within a submerged hydraulic jump. Rapid energy loss coupled with rapid charging of the flow with fine‐grained sediment probably changed the rheology of the flow and promoted deposition along the margins of the jet. Moreover, these distinctive matrix‐rich strata are interpreted to represent the energetic initiation of the local sedimentary system, most probably caused by a local upflow avulsion.     

Lithostratigraphic development and neotectonic significance of the Quaternary sediments along the Kachchh Mainland Fault (KMF) zone,western India

The Kachchh Mainland Fault (KMF) is a major E–W trending seismically active fault of the Kachchh palaeorift basin whose neotectonic evolution is not known. The present study deals with the eastern part of the KMF zone where the fault is morphologically expressed as steep north facing scarps and is divisible into five morphotectonic segments. The Quaternary sediments occurring in a narrow zone between the E–W trending KMF scarps and the flat Banni plain to the north are documented. The sediments show considerable heterogeneity vertically as well as laterally along the KMF zone. (The Quaternary sediments for a northward sloping and are exposed along the north flowing streams which also show rapid decrease in the depth of incision in the same direction.) The deposits, in general, comprise coarse as well as finer gravelly deposits, sands and aeolian and fluvial miliolites. The Quaternary sediments of the KMF zone show three major aggradation phases. The oldest phase includes the colluvio-fluvial sediments occurring below the miliolites. These deposits are strikingly coarse grained and show poor sorting and large angular clasts of Mesozoic rocks. The sedimentary characteristics indicate deposition, dominantly by debris flows and sediment gravity flows, as small coalescing alluvial fans in front of the scarps. These deposits suggest pre-miliolite neotectonic activity along the KMF. The second aggradation phase comprises aeolian miliolites and fluvially reworked miliolites that have been previously dated from middle to late Pleistocene. The youngest phase is the post-miliolite phase that includes all deposits younger than miliolite. These are represented by comparatively finer sandy gravels, gravelly sands and sand. The sediment characteristics suggest deposition in shallow braided stream channels under reduced level of neotectonic activity along the KMF during post-miliolite time evidenced by vertical dips of miliolites and tilting of gravels near the scarps. The tectonically controlled incision and dissection of the Quaternary deposits is the result of neotectonic activity that continues at present day. The overall nature, sedimentary characteristics and geomorphic setting of the sediments suggest that the KMF remained neotectonically active throughout the Quaternary period.     

Sedimentological and deformational criteria for discriminating subglaciofluvial deposits from subaqueous ice‐contact fan deposits: A Pleistocene example (Ireland)

A pit located near Ballyhorsey, 28 km south of Dublin (eastern Ireland), displays subglacially deposited glaciofluvial sediments passing upwards into proglacial subaqueous ice‐contact fan deposits. The coexistence of these two different depositional environments at the same location will help with differentiation between two very similar and easily confused glacial lithofacies. The lowermost sediments show aggrading subglacial deposits indicating a constrained accommodation space, mainly controlled by the position of an overlying ice roof during ice‐bed decoupling. These sediments are characterized by vertically stacked tills with large lenses of tabular to channelized sorted sediments. The sorted sediments consist of fine‐grained laminated facies, cross‐laminated sand and channelized gravels, and are interpreted as subglaciofluvial sediments deposited within a subglacial de‐coupled space. The subglaciofluvial sequence is characterized by glaciotectonic deformation structures within discrete beds, triggered by fluid overpressure and shear stress during episodes of ice/bed recoupling (clastic dykes and folds). The upper deposits correspond to the deposition of successive hyperpycnal flows in a proximal proglacial lake, forming a thick sedimentary wedge erosively overlying the subglacial deposits. Gravel facies and large‐scale trough bedding sand are observed within this proximal wedge, while normally graded sand beds with developed bedforms are observed further downflow. The building of the prograding ice‐contact subaqueous fan implies an unrestricted accommodation space and is associated with deformation structures related to gravity destabilization during fan spreading (normal faults). This study facilitates the recognition of subglacial/submarginal depositional environments formed, in part, during localized ice/bed coupling episodes in the sedimentary record. The sedimentary sequence exposed in Ballyhorsey permits characterization of the temporal framework of meltwater production during deglaciation, the impact on the subglacial drainage system and the consequences on the Irish Sea Ice Stream flow mechanisms.     

Facies and basin architecture of the Late Carboniferous Salvan-Dorénaz continental basin (Western Alps, Switzerland/France)

The Salvan‐Dorénaz Basin formed during the Late Palaeozoic within the Aiguilles‐Rouges crystalline basement (Western Alps) as an asymmetric, intramontane graben elongated in a NE–SW direction and bounded by active faults. At least 1700 m of fluvial, alluvial fan and volcanic deposits provide evidence for a strong tectonic influence on deposition with long‐term, average subsidence rates of > 0·2 mm yr^?1. The early basin fill was associated with coarse‐grained alluvial fans that were dominated by braided channels (unit I). These issued from the south‐western margin of the basin. The fans then retreated to a marginal position and were overlain by muddy floodplain deposits of an anastomosed fluvial system (unit II) that drained towards the NE. Deposition of thick muds resulted from a reduction in the axial fluvial gradient caused by accelerated tectonic subsidence. Overlying sand‐rich meandering river deposits (unit III) document a reversal in the drainage direction from the NE to the SW caused by synsedimentary tectonism, reflecting large‐scale topographic reorganization in this part of the Variscides with subsidence now preferentially in the W and SW and uplift in the E and NE. Coarse‐grained alluvial fan deposits (unit IV) repeatedly prograded into, and retreated from, the basin as documented by coarsening‐upward cycles tens of metres thick reflecting smaller scale tectonic cycles. Volcanism was active throughout the evolution of the basin, and U/Pb isotopic dating of the volcanic deposits restricts the time of basin development to the Late Carboniferous (308–295 Ma). ⁴⁰Ar/³⁹Ar ages of detrital white mica indicate rapid tectonic movements and exhumation of the nearby basement. In unit I, youngest ages are close to that of the host sediment, but the age spectrum is wide. In unit II, high subsidence and/or sedimentation rates coincide with very narrow age spectra, indicating small, homogeneous catchment areas. In unit III, age spectra became wider again and indicate growing catchment areas.     

Fine‐grained meandering systems of the Lower Permian Clear Fork Formation of north‐central Texas,USA: Lateral and oblique accretion on an arid plain

Facies models that adequately represent the diverse range of fine‐grained fluvial systems are currently lacking from the literature. In this paper, the spectrum of these systems on the arid plains of western equatorial Pangea is explored, as well as the source and nature of the fine‐grained sediments. Eight fluvial elements in the Early Permian Clear Fork Formation of north‐central Texas represent channel systems up to 7 m deep with coarse basal deposits, three types of lateral‐accretion deposits and sandstone sheets, with laminated, disrupted and massive mudstones laid down in abandoned channels and on floodplains. The three fine‐grained fluvial styles represent a continuum between two end‐members: sustained lateral accretion of bedload composed of quartzose sediments and mud aggregates on point bars, and oblique accretion of suspended sediment on steep accretionary benches and banks with limited lateral migration. This spectrum is controlled, in part, by grain size and the proportion of suspended to bedload sediments. The presence of rarely documented swept ripples on exhumed accretion surfaces is attributed to rapid decline in water levels and downstream re‐entry of overbank floodwaters into the channel. Rill casts, roots and disrupted mudstones low down in channel bodies indicate periods of near‐dryness. Laterally extensive sheet sandstones were formed by episodic flows in broad, sandbed channels. The fluvial sediments were primarily intrabasinally sourced with extrabasinal sediments brought in during major floods from upland source areas or reworked from local storage in the basin, representing a supply limited system. The upward change in cement composition from mainly calcite and ankerite to dolomite and gypsum with minor celestine implies increasingly saline groundwater and progressive aridification, supporting Late Palaeozoic palaeoclimatic models. By integrating petrographic data with sedimentology, a plethora of information about ancient landscapes and climate is provided, allowing a fuller comparison between the Clear Fork Formation and modern dryland alluvial plains.     

Microstructures of sediment flow deposits and subglacial sediments: a comparison

Microstructural analysis of glacial deposits has recently been used as a research tool to determine sediment genesis. However, the occurrence of microstructures in deposits of known origin has not been sufficiently documented, hindering our ability to confidently interpret microstructures in sediments of unknown origin. Our objective is to present a calibration study of microstructures of recent sediment flow deposits and associated sediments from the Matanuska Glacier, Alaska, and to evaluate the degree of commonality with microstructures found in subglacially deformed sediments. Microstructures in sediment flow deposits can be formed as a result of sediment transport, deposition, and/or post-depositional processes, and are related to the viscosity regime of the source flow. Characteristic microstructures formed during brittle deformation include shears, faults, and brecciation; microstructures formed during ductile deformation include folds, pressure shadows, re-orientation of clasts around a 'core' stone, fine laminations, basal shear zones, imbrication, and flow fabrics. Other microstructures include fluid escape and injection structures, clast haloes, and fissility. The results of our comparison suggest that sediment flow deposits share many microstructures in common with subglacially deformed sediments.     

Kaolinite Claystones associated with the Wongawilli seam in the southern part of the Sydney basin

Kaolinite claystones that are similar in structure, texture and composition to the kaolin tonsteins of Western Europe, and to some of the flint clays of North America, are associated with the Wongawilli Seam in the southern part of the Sydney Basin, where they form thin persistent bands within the coal and somewhat thicker deposits immediately overlying the seam. The thin bands within the coal are fine grained and consist of brecciated to pelletal clasts composed of well‐ordered kaolinite set in a matrix of similar composition. The thicker deposits overlying the seam are much coarser grained and appear restricted in occurrence to the basin margins. They contain a predominance of oolites with kaolinite clasts bonded by a relatively sparse matrix and in places, remarkable ‘outgrowths’ of vermicular kaolinite. The origin of the deposits is discussed, and it is concluded that most of the unusual features of these claystones can be ascribed to a fluvial environment.     

Vented sediments and tsunami deposits in the Puget Lowland,Washington – differentiating sedimentary processes

The sandy deposits produced by tsunamis and liquefaction share many sedimentary features, and distinctions between the two are important in seismically active coastal zones. Both types of deposits are present in the wetlands bordering Puget Sound, where one or more earthquakes about 1100 years ago caused both tsunami flooding and sediment venting. This co‐occurrence allows an examination of the resulting deposits and a comparison with tsunami and liquefaction features of modern events. Vented sediments occur at four of five wetland field localities and tsunami deposits at two. In comparison with tsunami deposits, vented sediments in this study and from other studies tend to be thicker (although they can be thin). Vented sediments also have more variable thickness at both outcrop and map scale, are associated with injected dykes and contain clasts derived from underlying deposits. Further, vented sediments tend to contain a greater variety of sedimentary structures, and these structures vary laterally over metres. Tsunami deposits compared with vented sediments are commonly thinner, fine and thin landward more consistently, have more uniform thickness on outcrop and map scales, and have the potential of containing coarser clasts, up to boulders. For both tsunami deposits and vented sediments, the availability and grain size of source material condition the characteristics of the deposit. In the cases presented in this paper, both foraminifera and diatom assemblages within tsunami deposits and vented sediments consisted of brackish and marine species, and no distinction between processes could be made based on microfossils. In summary, this study indicates a need for more careful analysis and mapping of coastal sediments associated with earthquakes to avoid misidentification of processes and misevaluation of hazards.     

Architecture of an Oligocene fluvial ribbon sandstone in the Ebro Basin,North‐eastern Spain

Fluvial ribbon sandstone bodies are ubiquitous in the Ebro Basin in North‐eastern Spain; their internal organization and the mechanics of deposition are as yet insufficiently known. A quarrying operation in an Oligocene fluvial ribbon sandstone body in the southern Ebro Basin allowed for a three‐dimensional reconstruction of the sedimentary architecture of the deposit. The sandstone is largely a medium‐grained to coarse‐grained, moderately sorted lithic arenite. In cross‐section, the sandstone body is 7 m thick, occupies a 5 m deep incision and wedges out laterally, forming a ‘wing’ that intercalates with horizontal floodplain deposits in the overbank region. Three architectural units were distinguished. The lowest and highest units (Units A and C) mostly consist of medium‐grained to coarse‐grained sandstone with medium‐scale trough cross‐bedding and large‐scale inclined stratasets. Each of Units A and C comprises a fining‐up stratal sequence reflecting deposition during one flood event. The middle unit (Unit B) consists of thinly bedded, fine‐grained sandstone/mudstone couplets and represents a time period when the channel was occupied by low‐discharge flows. The adjoining ‘wing’ consists of fine‐grained sandstone beds, with mudstone interlayers, correlative to strata in Units A and C in the main body of the ribbon sandstone. In plan view, the ribbon sandstone comprises an upstream bend and a downstream straight reach. In the upstream bend, large‐scale inclined stratasets up to 3 m in thickness represent four bank‐attached lateral channel bars, two in each of Units A and C. The lateral bars migrated downflow and did not develop into point bars. In the straight downstream reach, a tabular cross‐set in Unit A represents a mid‐channel transverse bar. In Unit C, a very coarse‐grained, unstratified interval is interpreted as deposited in a riffle zone, and gives way downstream to a large mid‐channel bar. The relatively simple architecture of these bars suggests that they developed as unit bars. Channel margin‐derived slump blocks cover the upper bar. The youngest deposit is fine‐grained sandstone and mudstone that accumulated immediately before avulsion and channel abandonment. Deposition of the studied sandstone body reflects transport‐limited sediment discharges, possibly attaining transient hyperconcentrated conditions.     

Mesozoic carbonate rudites, megabreccias and associated deposits from central Greece

In the Kokkinovrakhos Formation of central Greece, poorly sorted, grain supported, lithoclastic carbonate rudites, megabreccias and olistoliths are associated with accumulations of fine-grained laminated carbonates. The laminae frequently exhibit grading and are interpreted as turbidity current deposits. The coarser grained rudites and megabreccias were also redeposited but were probably transported downslope by sliding and rolling, i.e. they are rock-fall deposits. Olistoliths were emplaced into deeper water by sliding. The fine-grained sediments of the laminated facies appear to have accumulated in pockets on an uneven sea floor during episodes of minimal rock fall activity. The sedimentary association is similar to that described from some modern fore-reef environments and is possibly characteristic of accumulations of sediment on, or near to, steep slopes adjacent to carbonate platforms, when episodes of tectonic activity lead to the shearing off and transport downslope of masses of well-cemented shallow marine carbonates.     

Facies characteristics, morphology and depositional models of clay-slide deposits in terraced fjord valleys, Norway

Many bedrock-confined fjord valleys along the Norwegian coast contain thick accumulations of fine-grained sediments that were deposited during and after the last deglaciation. The deposits gradually emerged above sea level due to glacioisostatic uplift, and fjord marine sedimentation was gradually followed by shallow marine and fluvial processes. During emergence terraces and river-cut slopes were formed in the valleys. Subsequent leaching of salt ions from the pore water in the marine deposits by groundwater has led to the development of quick clay. The deposits are subject to river erosion and destructive landslides involving quick clay. Most slides are of prehistoric age. Others are known from modern observations as well as from historic records.Landforms such as distinct slide scars or the hummocky terrain of slide deposits may be strongly modified by secondary processes. In addition, deposits from the most liquid part of quick clay slides may have planar surfaces. Clay-slide deposits on a fluvial or deltaic terrace, therefore, are not always easily recognized from morphology, and only exposures may reveal their internal structures and allow them to be distinguished from overbank flood sediments. Detailed sedimentological work shows that slide deposits in such setting consist of distinct facies containing reworked marine sediments. We propose three facies successions of clay-slide deposits that form a continuum. The dominant components of these succession types are: slightly deformed blocks of laminated clay and silt (A), highly deformed clay and silt with gravel clasts (B) and massive to stratified clay and silt with scattered clasts (C). We suggest that in many cases a basal muddy diamicton is a characteristic, and possibly diagnostic feature. Processes and depositional models are interpreted from the different succession types. The results may be relevant for identifying clay-slide deposits elsewhere and may be useful during general mapping of fjord marine deposits and characterization of slide-prone areas as well as during identification of prehistoric slides.