Particle over-passing on depth-limited gravel bars

An experimental channel is used to examine the transport of mixed sand and gravel bedload over the crestal platform of ‘hump-back’ bars and along the top of planar gravel sheets. Hydraulic processes result in the simultaneous transport of cobbles and pebbles over a static closely packed bed consisting of like-sized and finer particles. For prescribed conditions, flat upper-stage plane sand-beds develop over the crestal location with pebbles rolling easily over the sandy bed. At the brinkpoint, flow separation ensures effective segregation of the gravel from the sand. Over the slip-face the deposition rate of the sand is insufficient to fill fully the interstices within the gravel foresets before rapid deposition of gravel further advances the bed-form. Consequently, distinctive vertical assemblages of open-work and closed contact framework gravels could be generated as another bar migrates over, and preserves, the initial structure. In respect to the observed mechanisms of sorting over the bars, a mathematical expression is developed to explain the critical conditions allowing coarse particle mobility over planar sand or gravel beds under upper-stage plane-bed conditions on the crestal platform. The model then is used to ascertain whether the depositional environment ascribed to certain facies in the Bunter Pebble Beds, described in a recent publication, is appropriate given the distinctive facies assemblages generated in this experiment and the known hydrodynamic control of the particle-segregation process.     

Gravelly flood deposits of Irvine Creek, Ontario, Canada

In May 1974 a powerful flood flushed the Grand River basin, Ontario. The effects on the bedload were drastic in a narrow (30 m) and deep (40 m) rock walled and floored gorge near Elora, Ontario. Along Irvine Creek, the tributary occupying the gorge, the gravel cover was reworked in several types of bars, predominantly transverse and point bars. The bars formed very rapidly in response to essentially steady, non-uniform flow that developed during a brief period of high flood. Superimposed on major bars are several minor sedimentary features such as coarse transverse ribs, chute channels and bars, longitudinal ribs, imbrication clusters, backsets with well developed imbrication, that were formed under very high stream discharge. Structures like imbrication clusters, transverse ribs and small riffle bars require a ‘live bed’ situation to form, and they develop when stones come to a stop either because they cluster during transport, or because keystone effects occur along shallow channels. In Irvine Creek, very few sedimentary features were formed during waning and low flood stages: only some shadow deposits and a few Ostler lenses. The few fines that were available were lost downstream or filled in lower parts of gravel beds. This study confirms that in streams that experience strong seasonal fluctuations in discharge, bedforms that develop during high floods have a high probability of preservation. In gravelly deposits, foreset structures and plane beds are most commonly preserved, although they may be difficult to recognize in old deposits, which may appear massive, particularly if the gravel has been infilled with finer pebbles and sand. In the case of Irvine Creek, all deposits are organized, and lateral and vertical variations in textures, particularly imbrication and packing, are very useful in the recognition of sedimentary structures.     

Umbrella structure and channel-wall stoping in the Cambrian St. Roch Formation,Quebec Appalachians: significance for particle support mechanisms and turbulence development in hyper-concentrated sediment gravity flows

Umbrella structure is a newly recognized sedimentary structure associated with large platy clasts in resedimented boulder-bearing pebble conglomerate with a sandy matrix. It consists of a sand rim that lacks pebbles on parts or the entire underside of platy boulders, whereas on the upper side, pebbles are in direct contact with the boulders. The depositing processes were high- to hyper-concentrated sediment gravity flows in a submarine channel or canyon on the Cambrian continental slope of North America bordering the Iapetus Ocean. The structure occurs predominantly where clasts dip moderately in the down-current direction. Based on the association of the structure with slightly forward dipping slabs, it is proposed that these down-current dipping slabs may have been in the process of counter-clockwise rotation that was aborted and may have generated a pressure shadow on the underside enabling the inrush of fluid and the infiltration of sand into the anomalous low-pressure zone. The structure has implications for particle support mechanisms in high- to hyper-concentrated sedimentary gravity flows, in that it redirects attention to the much debated mechanism of dispersive pressure and alternatives. It provides an observable sediment structure that supports dispersive pressure which so far depended on experimental evidence and theoretical arguments alone. Vrolijk and Southard’s (1997) concept of a ‘laminar sheared layer’ is here for the first time interpreted as having an upward-moving ‘free-surface’ layer effect during deposition from hyper-concentrated flows. Channel-wall stoping involves unlithified turbiditic spillover sand in the levee sediment of the canyon wall that was washed out by the upper diluted parts of the high-concentration flows coming down the channel and leaving a niche in the wall that was filled with coarser channel-axis facies by the same flow (or later flows) when its aggradation reached the level of the niche. The contact between turbidite and pebble conglomerate occurs now more than 2 m laterally into the exposed channel wall. Channel-wall stoping tracks turbulence development in hyper-concentrated gravity flows.     

Comparative analysis of megaflood deposits and alluvium of the Chuya and Katun’ river valleys (Gorny Altai)

Sedimentary indicators of catastrophic glacial megafloods—plane-bedded angular gravel, cobbles, and boulders—are described in several sections of the high terraces of the Chuya River valley. The principal difference of these sediments from typical alluvium of this area is demonstrated. The clast roundness, grain size of clasts, and sedimentary structures of the high-terrace deposits of the Chuya and Katun’ valleys indicate the same facies originated through megaflood deposition. These results are at odds with ideas of alluvial, glacial, or glaciofluvial genesis of the high-terrace deposits of the Chuya River.     

成都平原东郊台地中更新统合江组沉积特征及工程地质意义

通过地层沉积序列、粒度特征、砾石特征综合分析认为,成都平原东郊台地中更新统合江组为河流相沉积,垂向上具有明显的"河流二元"结构特征,主要以细粒沉积物为主,底部砾石层多为中-粗砾,磨圆较好,具有定向性。该组可细分为河道、边滩、堤岸-河漫3种沉积微相类型,对应发育3种岩相类型。河道砾石岩相沿十陵镇-红河镇-大面镇一带呈北西-南东向展布,边滩砂坝岩相集中发育于大面镇-西河镇东侧古河道凹岸处,堤岸-漫滩粘土岩相广泛分布于研究区内。这3种岩相特性及分布对工程建设的影响存在差异:河道砾石层作为地下水储集层的同时,水溶蚀使其地层结构稳定性被破坏,影响工程稳固技术的选择;边滩砂坝分布较为集中,对区内工程建设影响面最小;堤岸-河漫粘土广泛分布,是一种分布稳定的软弱层,对地下水有一定的封闭效果,同时软弱层的特性使其降低了边坡、坝体的稳定性。     

Sandy gravel accumulation in a fluvial environment

Particle size, pebble shape, pebble fabric, discharge and flow velocity data are used to introduce a model of sandy gravel formation in Welsh gravel-bed rivers. The development of contact-imbrication of the typically very bladed and very platy cobbles and larger pebbles subsequently acts to significantly affect the depositional modes and patterns of small pebbles and sand particles. An important distinction is drawn between sand deposition, which can occur at or below the bed surface, and pebble and cobble deposition, which is merely a surface phenomenon.     

Pebble segregation and bed lenticularity in wave-worked versus alluvial gravel*

Two aspects of bedding may permit visual discrimination between wave-worked and alluvial gravel. Pebbles in gravel worked by waves tend to be better segregated into discrete beds than those in alluvial deposits, and bedding in wave-worked gravel tends to be more laterally regular, or less lenticular, than that in stream gravel. Pebble segregation and lenticularity were quantitatively analysed in twenty-five different marine and fluvial deposits of Quaternary or Tertiary age in southwestern Oregon in order to evaluate their use as environmental indicators. The results indicate that these two factors combined provide a visual criterion for discriminating between many gravels of wave-worked and alluvial origin.     

Gravel-topped offshore bar sequences in the Lower Carboniferous of southern Ireland

Within the Kinsale Formation (Lower Carboniferous) of southern Ireland are pebbly sandstones and conglomerates contained in what is known locally as the Garryvoe conglomerate facies. In this facies there are three main groups of lithologies: (a) heterolithic mudrocks and sandstones characterized by a wide variety of wave-produced structures; (b) sandstones dominated by swaley cross-stratification (SCS), parallel lamination, and rare hummocky cross-stratification (HCS); and (c) pebbly sandstones and conglomerates occurring as discrete beds or as gravel clasts dispersed through SCS sets. Successions of the facies comprise units of heterolithic mudrock and rippled sandstone alternating repeatedly with coarsening-upward units of SCS pebbly sandstone capped by top-surface granule and pebble lags. The Garryvoe conglomerate facies accumulated in a system of offshore bars on a muddy shallow-marine shelf that was dominated by waves and currents generated by storms. Sands and gravels were bypassed from a contemporaneous northerly coastal zone to the shelf, where they were moulded by the storm-generated flow into low, broad, sand ridges (offshore bars). The elongate bars were spaced kilometres apart, oriented obliquely to the coast, and separated by muddy interbar troughs. Their surfaces were largely covered by hummocky and swaley forms. Long-term, gradual seaward migration of the offshore bars concentrated gravels on landward flanks from the dispersed pebbly sands that were on the crests and seaward flanks. Exceptionally intense storms could form laterally extensive winnowed gravel lags above thinned bar sequences. Such storms could also flush gravel-bearing turbidity currents into muddy interbar trough areas.     

Sedimentology of tsunami inflow and backflow deposits: key differences revealed in a modern example

Onshore tsunami deposits may consist of inflow and backflow deposits. Grain sizes can range from clay to boulders of several metres in diameter. Grain‐size distributions reflect the mode of deposition and may be used to explore the hydrodynamic conditions of transport. The absence of unique sedimentary features identifying tsunami deposits makes it difficult in some cases to distinguish inflow from backflow deposits. On Isla Mocha off central Chile, the 27 February 2010 tsunami left behind inflow and backflow deposits of highly variable character. Tsunami inflow entrained sands, gravels and boulders in the upper shoreface, beach, and along coastal terraces. Boulders of up to 12 t were transported up to 300 m inland and 13 m above sea‐level. Thin veneers of coarse sand were found up to the maximum runup at 600 m inland and 19 m above sea‐level. Backflow re‐mobilized most of the sands and gravels deposited during inflow. The orientation of erosional structures indicates that significant volumes of sediment were entrained also during backflow. A major feature of the backflow deposits are widespread prograding fans of coarse sediment developed downcurrent of terrace steps. Fan sediments are mostly structureless but include cross‐bedding, imbrication and ripples, indicating deposition from bedload traction currents. The sediments are poorly sorted, grain sizes range between medium to coarse sand to gravel and pebbles. An assessment of the backflow transport conditions of this mixed material suggests that bedload transport at Rouse numbers >2·5 was achieved by supercritical flows, whereas deposition occurred when currents had decelerated sufficiently on the low‐gradient lower coastal plain. The sedimentary record of the February 2010 tsunami at Isla Mocha consists of backflow deposits to more than 90%. Due to the lack of sedimentary structures, many previous studies of modern tsunami sediments found that most of the detritus was deposited during inflow. This study demonstrates that an uncritical use of this assumption may lead to erroneous interpretations of palaeotsunami magnitudes and sedimentary processes if unknowingly applied to backflow deposits.     

Boulder beds in the glaciogenic Permo-Carboniferous Dwyka Formation in South Africa

Single-layer and massive boulder beds, which include boulder pavements, are sporadically distributed in the glaciogenic Permo-Carboniferous Dwyka Formation. These matrix-supported beds consist of moderately to poorly sorted, rounded boulders, cobbles and pebbles with a clast composition similar to those in the underlying or overlying diamictite. Alternatively, the clasts are composed of monolithic basement rock-types. The clasts show a long-axis orientation which, in the case of the boulder pavements, is parallel to the striae on the pavements. The various types of boulder beds have a similar mode of deposition and their subglacial origin is evidenced by the clast orientation, clasts with stoss and lee sides, stacking of clasts, and the development of a cleavage in the matrix due to horizontal stresses exerted by the boulders in the subglacial sediment. Subglacial streams, kame mounds, subaqeously winnowed till, or boulder beaches supplied the coarse debris which was entrained in the basal ice by plastic flow and regelation. Selective lodgement of the transported boulders occurred down-glacier when the basal thermal conditions changed from cold-freezing to warm-melting. The formation of the different types of boulder beds is thought to depend primarily on the concentration of coarse debris in the basal ice.     

Gravelly spit deposits in a transgressive systems tract: the Pleistocene Higashikanbe Gravel, central Japan

The Pleistocene Higashikanbe Gravel, which crops out along the Pacific coast of the Atsumi Peninsula, central Japan, consists of well‐sorted, pebble‐ to cobble‐size gravel beds with minor sand beds. The gravel includes large‐scale foreset beds (5–10 m high) and overlying subhorizontal beds (0·5–3 m thick), showing foreset and topset structure, from which the gravel has previously been interpreted as deposits of a Gilbert‐type delta. However, (1) the gravel beds lack evidence of fluvial activity, such as channels in the subhorizontal beds; (2) the foresets incline palaeolandwards; (3) the gravels fill a fluvially incised valley; and (4) the gravels overlie low‐energy deposits of a restricted environment, such as a bay or an estuary. The foresets generally dip towards the inferred palaeoshoreline, indicating landward accretion of gravel. Reconstruction of the palaeogeography of the peninsula indicates that the Higashikanbe Gravel was deposited as a spit similar to that developed at the western tip of the present Atsumi Peninsula, rather than as a delta. According to the new interpretation, the large‐scale foreset beds are deposits on the slopes of spit platforms and accreted in part to the sides of small islets that are fragments of the submerging spit during relative sea‐level rise. The subhorizontal beds include nearshore deposits on the spit platform topsets and deposits of gravel shoals or bars, which are reworked sediments of the spit beach gravels during a transgression. The lack of spit beach facies in the subhorizontal beds results from truncation by shoreface erosion. Dome structure, which is a cross‐sectional profile of a recurved gravel spit at its extreme point, and sandy tidal channel deposits deposited between the small islets were also identified in the Higashikanbe Gravel. The Higashikanbe Gravel fills a fluvially incised valley and occupies a significant part of a transgressive systems tract, suggesting that gravelly spits are likely to be well developed during transgressions. The large‐scale foreset beds and subhorizontal beds of gravelly spits in transgressive systems tracts contrast with the foreset and topset beds of deltas, characteristic of highstand, lowstand and shelf‐margin systems tracts.     

Provenance and sediment characteristics of contemporary gravel deposits at Sellicks Beach,eastern shore of Gulf St Vincent,South Australia

Sellicks Beach, located on the eastern shore of Gulf St Vincent, South Australia, is subject to wave-dominated processes and northward longshore transport. During winter, when wave energy is typically vigorous, gravel deposits are exposed across most of the beach, and three step-like berms are well developed. Sand is restricted to a narrow strip that is exposed only at low tide. In contrast, during summer, when wave energy is generally moderate to low, much of the gravel is covered by a thin veneer of sand and only the high berm, on the landward edge of the beach, remains as an obvious feature. Steeply dipping Neoproterozoic to Cambrian strata that outcrop strongly across Sellicks Hill are the original source rocks for the beach gravel; distinctive sedimentary textures, structures and fossils in the cobble-size clasts can be confidently matched with those of the provenance rocks. Much of the sediment entered the modern beach environment as a consequence of coastal erosion of transitional alluvial fan sediments. The oldest alluvial fan sediments are of late Pliocene to earliest Pleistocene age. Mount Terrible Gully provides a conduit for the input of fluvial sediment at the mouth of Cactus Canyon, where clasts as large as boulders accumulate across the beach. Sellicks Beach gravels are subject to longshore transport northwards. Relatively softer clasts, such as those derived from the Heatherdale Shale, are rare beyond Cactus Canyon. In contrast, quartzite clasts are more abundant towards the north. This lithological differentiation is attributed to preferential survivorship of clasts that are physically harder and chemically less reactive. The change in the shapes of clasts northwards, from predominately shingle-like ‘very platy’ and ‘very bladed’ at Cactus Canyon, to more ‘compact’ towards the boat ramp, is in accord with the more massive fabric of the surviving quartzite clasts. At Sellicks Beach, preservation of uplifted, coarse gravels, with entire and comminuted marine molluscan shells, of last interglacial age, provides evidence of neotectonism. At the landward margin of the beach, imbricated gravels in which pore spaces have been infilled with mud, and which show no evidence of modern coastal erosion, may provide evidence of continuing uplift during the recent Holocene. The geological setting, geomorphic framework and modern sedimentary regime at Sellicks Beach combine to provide an exceptionally useful outdoor laboratory for education in field geology.     

Bed forms and stratification types of modern gravel meander lobes, Nueces River, Texas

All major streams draining the southwestern flank of the Edwards Plateau in south-central Texas transport large volumes of gravel and sandy muddy gravel and are developing meander lobe sequences consisting predominantly of coarse gravel. The largest of these streams, the Nueces River, has a sinuosity index of 1.3 and an average stream surface slope of 1.8 m/km in the study area. Stream discharge is variable and has ranged from no flow to more than 17,000 m³/s. Mean clast b-axis length for the ten largest clasts at thirteen sample sites ranged from 2.5 to 10.8 cm. Velocities of 2.7-4.4 m/s 1 m above the stream bed are required to transport these clasts. Stream velocities of these magnitudes occur about once in 8 years when discharge of the Nueces River exceeds 3300 m³/s. Mean grain size of Nueces River alluvium ranges from 1.2 to 3.4 cm. At a flow depth of 1 m, sediment of this size has a critical erosion velocity of 1.8-3 m/s. Velocities of this magnitude occur about once in two years when discharge exceeds 340 m³/s. Under these conditions flow is subcritical, with critical shear stresses on depositional surfaces ranging from 6.4 to 12.7 kg/m². Gravel clasts are imbricated and channel bed forms are predominantly transverse gravel bars with slip faces ranging up to 2 m high and wavelengths in excess of 100 m. Stratification includes graded planar crossbeds and horizontal beds. Lower lateral accretion face sediments are also predominantly transverse bars; upper lateral accretion face deposits occur as longitudinal gravel ridges deposited in the lee of vegetation and, less commonly, as chute bars. Near the upper limit of meander lobes where vegetation is heavy, mud and muddy sand occur as overbank deposits; in these deposits sedimentary structures other than desiccation cracks are rare. Sedimentary sequences in gravel meander lobe systems deposited by low sinuosity streams are graded or non-graded horizontal beds and planar cross-beds overlain by mud and muddy sand interbedded with horizontally bedded gravels. Sequences may be several metres thick, but probably do not exceed 8-10 m in thickness. These deposits in turn are overlain by overbank deposits of mud and muddy sand. Similar sedimentary sequences occur in the extensive Quaternary terraces that parallel the Nueces River.     

Precambrian non-marine stromatolites in alluvial fan deposits, the Copper Harbor Conglomerate, upper Michigan

Laminated cryptalgal carbonates occur in the Precambrian Copper Harbor Conglomerate of northern Michigan, which was deposited in the Keweenawan Trough, an aborted proto-oceanic rift. This unit is composed of three major facies deposited by braided streams on a large alluvial-fan complex. Coarse clastics were deposited in braided channels, predominantly as longitudinal bars, whereas cross-bedded sandstones were deposited by migrating dunes or linguoid bars. Fine-grained overbank deposits accumulated in abandoned channels. Gypsum moulds and carbonate-filled cracks suggest an arid climate during deposition. Stromatolites interstratified with these clastic facies occur as laterally linked drapes over cobbles, as laterally linked contorted beds in mudstone, as oncolites, and as poorly developed mats in coarse sandstones. Stromatolites also are interbedded with oolitic beds and intraclastic conglomerates. Stromatolitic microstructure consists of alternating detrital and carbonate laminae, and open-space structures. Radial-fibrous calcite fans are superimposed on the laminae. The laminae are interpreted as algal in origin, whereas the origin of the radial fibrous calcite is problematic. The stromatolites are inferred to have grown in lakes which occupied abandoned channels on the fan surface. Standing water on a permeable alluvial fan in an arid climate requires a high water table maintained by high precipitation, or local elevation of the water table, possibly due to the close proximity of a lake. Occurrence of stromatolites in the upper part of the Copper Harbor Conglomerate near the base of the lacustrine Nonesuch Shale suggests that these depositional sites may have been near the Nonesuch Lake.     

Some last glaciation drift deposits near Pontypridd,South Wales

Drift deposits exposed in a large open cut are described. The basal member of the sequence consists of bedded sandy gravels, and these are overlain by a grey silty sand containing abundant striated pebbles and boulders. The uppermost deposit consists of a brown poorly sorted drift containing silty and sandy lenses. Analysis of the texture, index properties and densities of the sediments, together with the shape, lithology and fabric of the contained pebbles suggests that the basal gravels were deposited by subglacial meltwater, the grey drift is a lodgement till, and the uppermost brown rubbly drift was formed by solifluction of the till, with the addition of frost shattered sandstone from the hilltop above. It is suggested that the glacigenic sediments are of Devensian age, and the solifluction layer Late Devensian.     

Depositional features of a late Weichselian outwash fan; central East Jylland,Denmark

Four major sedimentary facies are present in coarse-grained, ice-marginal deposits from central East Jylland, Denmark. Facies A and B are matrix-supported gravels deposited by subaerial sediment gravity flows as mudflows (facies A) and debris flows (facies B). Facies C consists of clast-supported, water-laid gravels and facies D are cross-bedded sand and granules. The facies can be grouped into three facies associations related to the supraglacial and proglacial environments: (1) the flow-till association is made up of alternating beds of remobilized glacial mixton (facies A) and well-sorted cross-bedded sand (facies D); (2) the outwash apron association resembles the sediments of alluvial fans in containing coarse-grained debris-flow deposits (facies B), water-laid gravel deposited by sheet floods (facies C) and cross-bedded sand and granules (facies D) from braided distributaries; (3) the valley sandur association comprises water-laid gravel (facies C) interpreted as sheet bars and longitudinal bars interbedded with cross-bedded sand and granules (facies D) deposited in channels between bars in a braided environment.The general coarsening-upward trend of the sedimentary sequences caused by the transition of bars and channel-dominated facies to debris-flow-dominated facies indicate an increasing proximality of the outwash deposits, picturing the advance and still stand of a large continental lowland ice-sheet. The depositional properties suggest that sedimentation was caused by melting along a relatively steep, active glacier margin as a first step towards the final vanishing of the Late Weichselian icesheet (the East Jylland ice) covering eastern Denmark.     

Wavy lamination in a mixed sand and gravel foreshore facies of the Pleistocene Hosoya Sandstone, Aichi, central Japan

Although sandy foreshore facies are generally characterized by parallel lamination, wavy lamination is predominant in the mixed sand and gravel foreshore facies of the Pleistocene Hosoya Sandstone, which crops out along the Pacific coast of the Atsumi Peninsula, Aichi, central Japan. The foreshore facies consists of three sedimentary subfacies; interbeds of gravel and parallel laminated sand of the lower foreshore facies, parallel laminated fine to medium sand beds containing scattered pebbles and cobbles of the middle foreshore facies, and wavy laminated fine to medium sand beds containing scattered pebbles and cobbles of the upper foreshore facies. A lack of erosional surfaces in the middle foreshore facies indicates the continuous accumulation of sand in flat beds under upper plane bed flow. The wavy laminated sands of the upper foreshore facies exhibit erosional surfaces indicative of repeated deposition and erosion. The erosional surfaces are undulatory, with depressions (10 cm wide and 3 cm deep) that contain scattered pebbles and cobbles. These depressions reflect backwash erosion of sand around and below the pebbles and cobbles. Sand draping over the undulating erosional surfaces forms the wavy lamination. The wavy laminated sand with scattered pebbles and cobbles is a key facies of an upper foreshore or swash zone, and is a good sea-level marker.     

Structures and textures in Triassic braided stream conglomerates ('Bunter' Pebble Beds) in the Sherwood Sandstone Group, North Staffordshire, England

ABSTRACT In the northern parts of the Needwood and Stafford/Eccleshall Basins, England, the Pebble Beds of the Sherwood Sandstone Group contain thick successions of texturally mature, fluvial pebble/cobble conglomerates which are organized into either horizontal or cross-stratified sets. The horizontally lying sets, generally coarser grained and more poorly sorted than the cross-bedded sets, are usually disorganized and either matrix- or clast-supported, although thin lenses of well-sorted, occasionally openwork units, interpreted as falling stage phenomena, are often present. The cross-stratified conglomerates have foresets exhibiting remarkable textural organization, with a coarse, bimodal (sometimes matrix-supported) part grading upwards or being abruptly overlain by a finer, well-sorted (occasionally openwork) part and finally capped by sandstone. These rhythmic textural changes are attributed partly to an avalanche process at high stage and partly to falling stage conditions. The most common types of vertical association are thick successions of horizontally bedded conglomerates (up to 20 m) and sequences of an upwards coarsening nature (2-12 m) in which cross-stratified sets are overlain by flat-lying sets. The environment of deposition of the gravels is interpreted as one in which water depths at high stage were greater than depths in most modern braided stream plains (proglacial or alluvial fan) but shallower than depths associated with the Pleistocene catastrophic floods from which texturally mature, giant gravel bars have been recorded. Recent braided streams with relatively confined channels and considerable bar/channel relief are better analogues. In particular, medial or mid-channel bars with a two-tier structure (subaqueous and partly emergent portions) may explain the upward-coarsening sequences in which horizontally lying conglomerates overlie cross-stratified conglomerates. The thicker sequences of horizontally stratified conglomerates represent proximal, longitudinal bar deposits. Sheets of pebbly sandstone and argillaceous sandstone lying between the conglomerates, and commonly occurring towards the top of the succession, largely represent deposition from sandwaves and dunes. Finer, interbedded, argillaceous sandstones, siltstones and mudstones are interpreted as overbank and waning-flood deposits. Basin-forming tectonism of increasing intensity probably caused the initial coarsening upwards of the lower part of the succession, whilst more stable tectonic conditions and decreasing relief on the margins of the basins and in the areas of provenance in the Midlands and the Hercynides, account for the upwards-fining of the upper part of the succession.     

Sedimentation of the Triassic (Scythian) Red Pebbly Sandstones in the Cheshire Basin and its margins

This magnalithofacies, which forms the bulk of two formations, the Bunter Pebble Beds and the Lower Keuper Sandstone, is described and interpreted as the deposits of former low-sinuosity, largely braided, streams which flowed across a regional palaeoslope inclined to the north-west. Arid blocks of the Welsh and Pennine Massifs lay to the west and east. Lithofacies A, conglomerate breccia and gravel, is interpreted as a deposit of former channel floors and sometimes bars. Lithofacies B crossbedded pebbly coarse sandstone, formed from large-scale ripples developed on channel (? braid) bars of flood courses, whilst lithofacies C, argillaceous finer sandstone, represents the large-scale ripples of quieter channels at lower stage and discharge. Lithofacies D, interbedded fine sandstone and shale, formed a riverplain topstratum deposit and, rarely, a levee. Lithofacies E is similar to D except that the sandstone is coarser and injects the mudstones repeatedly. Lithofacies F, red shales and mudstones (largely the ‘marl’ bands of former authors), may be laterally restricted or extensive. The former bands represent deposits of bar swales and abandoned cut-off channels of varying stage: the latter suggest the development of a riverplain topstratum. Lithofacies X, soft fine sandstone, is aeolian. These lithofacies are arranged in vertical repetitions of varying thicknesses which are often asymmetric. Allocyclic climatic influence, reflected in varying discharge and sediment load under conditions of net accretion and steady subsidence, is regarded as a dominant factor in the origin of miocycles, megacycles and magna-cycles, though in smaller repetitions the possibility of channel wandering, and in magnacycles the influence of tectonics, is possible.     

庐山地区第四纪泥砾沉积物的成因探讨

庐山第四纪泥砾沉积物的砾石组构特征表明, 砾石岩性与山体基岩基本一致, 砾石的等体积球径较小, 加权平均砾径较中值砾径大, 分选系数大于1, 分选性差, φ值相对较小, 平均为0.66, F值相对较大, 在2左右, F/φ比值介于2.75～4.75, ρ值集中且较小, 为22.75%～37.75%, 砾石以棱角、次棱角状为主, 磨圆较差; a轴和ab面都倾向沟谷下游, a轴倾角集中且较小, 为7～8°, ab面倾角为7～21°, a轴较ab面具有更为优势的组构倾向; 沉积物粒度频率曲线在4～5?范围内表现为峰; 石英砂颗粒为次棱角状, 贝壳状断口发育, 具平行擦痕、上翻解理片、V形坑和硅质沉淀。上述沉积学特征表明庐山第四纪泥砾沉积物系冰碛冰水沉积, 而非泥石流或河流堆积。