Gravel Sheets Research Articles

Fluvial gravel facies architecture of late Pleistocene age at Po Chue Tam, Lantau Island, Hong Kong: tectonic versus climatic control?

A cross-section of fluvial gravel deposits of late Pleistocene age exposed at Po Chue Tam, Lantau Island, Hong Kong contains two facies: a lower facies of planar cross-bedded gravel (Gp) and an overlying facies of clast-supported, massive gravels (Gcm). The Gp gravels include five gravel couplets. Each couplet consists of a clast-supported, coarse gravel-dominated bed and an overlying clast-supported, fine gravel-dominated bed with a discrete bounding surface. Tectonic uplift predating the last interglacial transgression produced a large amount of detritus in the source area. Excessive peak rainfall intensity resulting from enhanced seasonality of monsoonal precipitation in the following glacial period triggered catastrophic floods, which transported mature detritus in large quantities into a fault-controlled piedmont basin. The Gp gravels were deposited by pulsating flood flows. In relation to kinematic waves of particles, bedload sediment was longitudinally sorted and segregated into a train of gravel sheets. They draped over each other and accreted laterally due to expansion of flow, producing planar cross stratifications that are characteristic of recurrent, couplet-style coarse/fine cross beds. In contrast, Gcm gravels were laid down as a single, nearly horizontal bed by a catastrophic flood that was not subject to flow pulsation.

Depositional response of a headwater stream to channelization, East Central Illinois, USA

AbstractChannelization typically modifies the energy regime and sediment transport capacity of rivers, triggering morphologic adjustments. Most past studies of channelization have focused on erosional responses involving channel incision and widening. Depositional adjustments to channelization, although noted in previous work, have not been documented in detail. This study investigates the depositional response of the Spoon River, a headwater agricultural stream in Illinois, USA, to channelization. Historical aerial and ground‐based photography show that channelization of the Spoon River in the early 20th century produced a wide, deep trapezoidal drainage channel. Following this channelization, unvegetated alternate and mid‐channel bars developed on the bottom of the ditch. Sedimentological analysis of bar stratigraphy indicates that the bars grew through vertical accretion of horizontal sheets of sand‐and‐gravel bedload and organic‐rich drapes of fine‐grained suspended load. The horizontal sheets of sand and gravel are consistent with the braided conditions shown on historical photographs. Late‐stage bar growth appears to have been dominated by progressive overbank deposition of suspended load as indicated by the presence of a thick, fine‐grained organic‐rich A horizon immediately below the surface of each bar. The development of a soil layer also suggests that the bars are stable—an inference supported by the thick grass cover on the bar surfaces. The net result of the depositional response is the formation of a meandering channel flanked by a discontinuous floodplain on the bottom of the ditch.The construction of a wide ditch relative to original stream size is a key factor promoting a depositional response to channelization. Allowing or actively promoting floodplain elements to form in overwidened ditches may be a viable management option for improving the environmental quality of channelized agricultural streams. The Spoon River has a diverse fish community compared to channelized streams in East Central Illinois that lack a meandering low‐flow channel. This morphological configuration apparently enhances geomorphological and ecological variability while sustaining the drainage function of the ditch. Copyright © 2003 John Wiley & Sons, Ltd.

Three-dimensional GPR analysis of various Quaternary gravel-bed braided river deposits (southwestern Germany)

Abstract Closely spaced (1 m) ground penetrating radar (GPR) profiles were used for a three-dimensional characterization and comparison of glaciofluvial gravel-bed deposits in palaeodischarge zones of the Würmian Rhine glacier (southwestern Germany). Previous sedimentological outcrop investigations revealed three regionally reoccurring architectural styles of gravel bodies. For each of these styles a three-dimensional GPR dataset has been acquired in active gravel pits in order to calibrate the radar profiles with outcrop walls and to analyse, in three dimensions, the depositional elements and their stacking pattern in the subsurface. The GPR data were interpreted by mapping reflection terminations in order to delineate genetically related units. In particular, radar facies types and radar sequence boundaries were used to define and map depositional elements. Both accretionary and cut-and-fill depositional elements could be identified. Accretionary elements are characterized by horizontally to low-angle inclined (1–3°) and moderately continuous reflections (5–30 m) terminating on flat sequence boundaries; they represent the deposits of gravel sheets and traction carpets. In contrast, cut-and-fill elements are characterized by low to steeply inclined (3–25°), often discontinuous reflections terminating on concave to trough-shaped lower truncation boundaries; these are interpreted as scour-pool fills and small dissection elements (e.g. chutes and lobes). The three basic architectural styles of gravel bodies can be distinguished on the basis of the size and proportion of cut-and-fill elements mapped within the radar images. One type of gravel body is composed of an amalgamation of large cut-and-fill elements whereas the other two types are dominated by accretionary elements and differ by the proportion of smaller cut-and-fill elements. The results show that GPR is an adequate technique to illuminate the sedimentary architecture of the various types of gravel bodies. GPR data allow detailed three-dimensional reconstruction of depositional elements and their stacking pattern in the subsurface.

Facies architecture within a regional glaciolacustrine basin: Copper River, Alaska

This paper defines the principal architectural elements present within the Pleistocene, glaciolacustrine basin-fill of the Copper River Basin in Alaska. The Copper River drains an intermontane basin via a single deeply incised trench through the Chugach Mountains to the Gulf of Alaska. This trench was blocked by ice during the last glacial cycle and a large ice-dammed lake, referred to as Lake Atna, filled much of the Copper Basin. Facies analysis within the basin floor allows a series of associations to be defined consistent with the basinward transport of sediment deposited along calving ice margins and at the basin edge. Basinward transport involves a continuum of gravity driven processes, including slumping, cohesive debris flow, hyperconcentrated/concentrated density flows, and turbidity currents. This basinward transport results in the deposition of a series of subaqueous fans, of which two main types are recognised. (1) Large, stratified, basin floor fans, which extend over at least 5 km and are exposed in the basin centre. These fans are composed of multiple lobes, incised by large mega-channels, giving fan architectures that are dominated by horizontal strata and large, cross-cutting channel-fills. Individual lobes and channel-fills consist of combinations of: diamict derived from iceberg rainout and the ice-marginal release of subglacial sediment; multiple units of fining upward gravels which grade vertically into parallel laminated and rippled fine sands and silts, deposited by a range of density flows and currents derived from the subaqueous discharge of meltwater; and rhythmites grading vertically into diamicts deposited from a range of sediment-density flows re-mobilising sediment deposited by either iceberg rainout or the ice-marginal release of sediment. (2) Small, complex, proximal fans, which extend over less than 2 km, and are exposed in the southern part of the basin. These fans are composed of coalescing and prograding lobes of diamict and gravel deposited both directly by subaqueous meltwater and from sediment-density flows. These lobes are cross-cut by a range of sand and gravel-filled troughs and channels cut by subaqueous outwash, and either overlie or are overlain by horizontal sheets of gravel and diamict deposited from a range of sediment-density flows. The fans are, therefore, characterised by a complex, and laterally variable facies, architecture. Water depth, proglacial topography, stability of meltwater portals and sediment supply may all be important in determining the type of subaqueous fan present at any one location. We suggest that the Copper River basin-fill is dominated by packages of sediment containing multiple subaqueous fans with individual fans separated by units of diamict. Each sediment package is in turn separated from the next by a palaeo-landsurface shaped by interstadial/interglacial fluvial processes and by volcanic debris flows.

Sheet-gravel evidence for a late Holocene tsunami run-up on beach dunes, Great Barrier Island, New Zealand

A semi-continuous sheet of granule to cobble-size clasts forms a distinctive deposit on sand dunes located on a coastal barrier in Whangapoua Bay, Great Barrier Island, New Zealand. The gravel sheet extends from the toe of the foredune to 14.3 m above mean sea level and 200 m landward from the beach. Clasts are rounded to sub-rounded and comprise lithologies consistent with local bedrock. Terrestrial sources for the gravel are considered highly unlikely due to the isolation of the dunes from hillslopes and streams. The only source for the clasts is the nearshore to inner shelf of Whangapoua Bay, where gravel sediments have been previously documented. The mechanism for transport of the gravel is unlikely to be storm surge due to the elevation of the deposit; maximum-recorded storm surge on this coast is 0.8 m above mean high water spring tide. Aeolian processes are also discounted due to the size of clasts and the elevation at which they occur. Tsunami is therefore considered the most probable mechanism for gravel transport. Minimum run-up height of the tsunami was 14.3 m, based on maximum elevation of gravel deposits. Optical ages on dune sands beneath and covering the gravel allow age bracketing to 0–4.7 ka. Within this time frame, numerous documented regional seismic and volcanic events could have generated the tsunami, notably submarine volcanism along the southern Kermadec arc to the east-southeast of Great Barrier Island where large magnitude events are documented for the late Holocene. Radiocarbon ages on shell from Maori middens that appear to have been reworked by tsunami run-up constrain the age of this event to post ca. 1400 AD. Regardless of the precise age of this event, the well-preserved nature of the Whangapoua gravel deposit provides for an improved understanding of the high degree of spatial variability in tsunami run-up.

A review of geological records of large tsunamis at Vancouver Island, British Columbia, and implications for hazard

Large tsunamis strike the British Columbia coast an average of once every several hundred years. Some of the tsunamis, including one from Alaska in 1964, are the result of distant great earthquakes. Most, however, are triggered by earthquakes at the Cascadia subduction zone, which extends along the Pacific coast from Vancouver Island to northern California. Evidence of these tsunamis has been found in tidal marshes and low-elevation coastal lakes on western Vancouver Island. The tsunamis deposited sheets of sand and gravel now preserved in sequences of peat and mud. These sheets commonly contain marine fossils, and they thin and fine landward, consistent with deposition by landward surges of water. They occur in low-energy settings where other possible depositional processes, such as stream flooding and storm surges, can be ruled out. The most recent large tsunami generated by an earthquake at the Cascadia subduction zone has been dated in Washington and Japan to AD 1700. The spatial distribution of the deposits of the 1700 tsunami, together with theoretical numerical modelling, indicate wave run-ups of up to 5 m asl along the outer coast of Vancouver Island and up to 15–20 m asl at the heads of some inlets. The waves attenuated as they moved eastward along Juan de Fuca Strait and into Puget Sound and the Strait of Georgia. No deposits of the 1700 event or, for that matter, any other tsunami, have yet been found in the Strait of Georgia, suggesting that waves were probably no more than 1 m high in this area. If a tsunami like the 1700 event were to occur today, communities along the outer Pacific coast from southern British Columbia to northern California would be severely damaged. There would be little time to evacuate these communities because the tsunami would strike the outer coast within minutes of the first ground shaking. Fortunately, such tsunamis are infrequent — perhaps as few as seven have occurred in the last 3500 yr. Other tsunamis that are much smaller and more localized, although probably more frequent, are caused by local crustal earthquakes and landslides along the British Columbia coast. Two such tsunamis have occurred in British Columbia in recent years, one in 1946 in the Strait of Georgia and another in 1975 at the head of a fiord on the northern mainland coast.

Towards realistic aquifer models: three-dimensional georadar surveys of Quaternary gravel deltas (Singen Basin, SW Germany)

Abstract Understanding sedimentary heterogeneities has nowadays become crucial for sophisticated aquifer management. The methods used so far for aquifer characterization (e.g. pump test, coring analysis) cannot provide the needed resolution for a sufficient prediction of the subsurface properties (e.g. Sudicky, 1986 ). This calls for the application of the `outcrop analogue-approach' to aquifers (e.g. Macfarlan et al., 1994 ; Aigner, 1995 ). In this project we used georadar (ground-penetrating radar or GPR) to acquire the subsurface information for high-resolution three-dimensional images and radar facies mapping of glaciolacustrine delta sediments in the Singen Basin (SW Germany). A three-dimensional reconstruction of sedimentary heterogeneities for a detailed aquifer characterisation becomes possible by combining GPR profiling and outcrop information. A case study is presented, showing a complex, three-dimensional architecture of a Quaternary gravel body, consisting of the following three types of architectural elements: (1) glaciolacustrine delta foresets, (2) erosional surfaces with scour pool fills, and (3) horizontal glaciofluvial gravel sheets. Although this seems a relatively simple architectural inventory, the three-dimensional georadar profiling shows considerable internal heterogeneities. The radar data are interpreted following the seismic stratigraphic approach, i.e. the delineation of radar facies types and of radar sequences. The three-dimensional heterogeneities are visualized by means of time slices, 3-D block or chair diagrams, or series of radar facies maps.

Using two- and three-dimensional georadar methods to characterize glaciofluvial architecture

Abstract The threat of pollution in the shallow subsurface has led to an increasing need to understand how complex heterogeneities in gravel aquifers influence groundwater transport. To characterize these heterogeneities, we have conducted extensive two-dimensional (2-D) and three-dimensional (3-D) ground-penetrating radar (GPR or georadar) surveys in two quarries within the Rhine valley of northeastern Switzerland. The quarries comprised gravel and sand deposited in a proglacial braided river system that followed the maximum Wurm-stage glaciation. After the surveys, the terrace walls beneath the two study sites were photographed as they were being excavated. By combining information extracted from the 2- and 3-D georadar images with the outcrop photographs, it was possible to correlate georadar facies with the various glaciofluvial architectural elements. The dominant elements were scour pools formed at or near the confluences of two stream channels and horizontally bedded and massive gravel sheets deposited during moderate to high water flow conditions and exposed during low flow conditions. Architectural elements were generally elongated parallel to the mean flow directions of the ancient river system. Variations in the strike of their long axes reflected lateral and vertical changes in the local flow direction. Time slices showed structural trends not evident on 2-D georadar images and photographs. Data interpretation was quicker, more complete and less ambiguous when georadar facies analyses were based on a combination of georadar vertical profiles and time slices than when based on georadar vertical profiles alone.

Sequential architecture and cyclicity in Permian desert deposits, Brodick Beds, Arran, Scotland

This study of ancient desert deposits (Permian Brodick Beds) tests the possibility that cyclicityin these interbedded aeolian, fluvial and lacustrine deposits was orbitally controlled. The 740 m thicksuccession of desert deposits forms one sequence which can be divided into three genetic units. A majordeflation surface forms the basal sequence boundary and represents a drop in base-level (groundwatertable). On top of the sequence boundary are rather uniform aeolian dune and sand-sheet sediments (Unit1) formed in an erg during low base-level with increasing rates of base-level rise. They are followed byinterbedded aeolian dune, aeolian sand-sheet and flood-reworked aeolian sediments (Unit 2) representingerg margin deposits formed during increasing rates of base-level rise. The uppermost part of the sequenceis composed of gravel sheets (Unit 3) representing alluvial fan deposition during a high base-level withdecreasing rates of base-level rise. The succession is cyclic, and spectral analysis of sedimentary andgeochemical parameters reveal cycles with a thickness of 25.7–29.3 m, 34.5 m, and 64.5 m, and fieldworkreveals cycles of 155 m and 675 m. These sedimentary cycles are thought to reflect climatic oscillations. The lack of detailed time control makes it diffcult to link these climatic fluctuations to orbital cycles, although the detected cyclicity matches well with Milankovitch theory.

Clast assembling, bed-forms and structure in gravel beaches

AbstractBoth textural maturity and structure acquired by gravels on beaches are largely a response to the interaction between beach surface and wave-backwash energy. The turbulent energy driving the processes of particle rejection andselection at the surface of growing gravel sheets is partly controlled by the grain size of the sheet itself, so the process is to a large extent self-regulating. Beach surfaces are seen to comprise many discrete sheets of gravel competing for growth at their boundaries, but each characterised by a uniform or uniformly gradational texture.There are two main types of gravel sheet: (1) selection pavements which occur on low beach slopes, showing little areal grain-size or grain-shape variation, and (2) turbulence shadows which occur on steeper slopes and produce assemblages of clasts which may show perfect lateral shape and size gradation.The clasts which make up these various gravel sheets are termed ‘clast assemblages’, and such assemblages are the fundamental units from which beaches are constructed. Clast assemblages are classified in terms of their textural maturity—the degree to which they exhibit uniformity in clast size and shape. In beach sections they are, either singly or in combination, bounded by planes of discontinuity (bedding planes) to form beds.Repeated combinations of either clast assemblages or beds in a genetic association are regarded as sedimentary structures, many of which are diagnostic of the gravel beach environment. Growth of beaches involves the stacking of sedimentary structures, and four growth patterns have been identified. The beach structure is, therefore, classified in a hierarchy comprising clast assemblage, bed, structures and growth form, and it is the growth pattern which may be related to tidal range. P.ecognition of the processes which generate beach gravels through the structure of the gravels permits an analysis of the internal structure of major gravel bodies such as barrier beaches. It adds another set of criteria which may further lead to an understanding of the processes responsible for the generation and evolution of these large gravelforms.Three types of gravel lithosomes have been identified. (1) Regressive barrier bars which form a series of gravel ridges separated by lagoonal deposits. Barriers are built initially by swash bars which grow in size and coarsen through time to become wave-resistant forms. They form as a response to times when the sediment, unable to be evenly distributed and sorted on the beach surface, forms a discrete bar seaward of the active beach. This is the result of a punctuated or continuously high sediment supply compared with the wave energy available to disperse the sediment, or a falling sea level which shifts the locus of sediment accretion. (2) In contrast, regressive (prograding) gravel sheets form as a response to a continuous supply of sediment to the beach surface, allowing it to build seaward by the uniform accretion of sediment which is sorted and retained on its surface. (3) Gravel sheets produced in transgression are characterised by an abundance of spherical clasts and are often overlapped by the sand beaches which occur seaward of them.

Characteristics and Depositional Processes of Large-scale Gravelly Gilbert-Type Foresets in the Miocene Doumsan Fan Delta, Pohang Basin, Se Korea

ABSTRACT The Doumsan fan delta in the Miocene Pohang Basin (SE Korea) includes large-scale gravelly Gilbert-type foresets that are more than 150 m high and dip at about 20°. The foresets consist of six sedimentary facies: medium- to thick-bedded sandy gravel deposits with variable grading patterns and bed geometries (Facies A), thin- to medium-bedded, commonly inversely graded sandy gravel deposits (Facies B), sheet-like layers, a few grains thick, of pebble gravel (gravel sheets; Facies C), lensoidal layers, a few grains thick, of cobble to boulder gravel (gravel lenses; Facies D), thin-bedded sand with outsized clasts (Facies E), and very thick-bedded (> 10 m thick) disorganized gravel deposits in the toeset area (Facies F). These facies are indicative of deposition from cohesionless ebris flows (Facies A and B), debris falls (Facies C and D), turbulent flows (Facies E), and very thick debris flows that are related to large-scale foreset failure (Facies F). These facies are in close association with one another, except for Facies F, and are either vertically superposed or laterally juxtaposed within single sedimentation units. This suggests that these facies originated from a series of evolving sediment gravity flows. A cohesionless debris flow generated at the topset-foreset boundary or on the middle of the foreset slope segregated its sediments into a pebble-rich lower division and a sandy upper division with sparse cobble-to-boulder clasts by preferential upward drift of large clasts and surface transformation of fine-grained material. The pebbly lower division was emplaced by frictional freezing, forming Facies A beds. On the other hand, the sediments in the upper division continued to move as a sandy turbulent flow and bouldery debris fall, resulting in Facies E and D deposits downslope. The cohesionless debris flow occasionally transformed into a series of thinner flows by development of roll waves along the top of the flow, producing Facies B layers on the margin of a Facies A bed. Otherwise the cohesionless debris flow transformed into grain-assemblage debris fall and single-grain debris fall consecutively by removal of interstitial sand via downward percolation and stripping into ambient water. The debris falls produced gravel sheets (Facies C) and gravel lenses (Facies D). During this flow transformation, cobble- to boulder-size clasts and sandy material were selectively transported farther downslope, resulting in prominent textural bimodality of the prodelta deposits, which comprise isolated large clasts and lensoidal deposits of cobble-to-boulder gravel set in sandy background material.

Ground Penetrating Radar for Assessing Sediment Structures in the Hyporheic Zone of a Prealpine River

Ground penetrating radar (GPR) was used to investigate the structure of the sediments in a floodprone gravel-bed river. Measurements were made on 2 gravel bars and in the adjacent channel. We buried a metallic object to calibrate depth and to visualize changes in GPR-profiles taken before and after digging. In addition, along 2 GPR-profiles the structure of the alluvial sediment of a gravel bar was visually verified by digging a trench down to the groundwater table. The GPR method enables one to distinguish sedimentary units of different texture with a vertical resolution of about 0.25 m. The investigation showed the depth of the alluvial valley fill, and it revealed geomorphological features such as lateral point bar accretion, fills of former pools, or open framework textures. These structures form a complex 3-dimensional mosaic of sediment patches. The GPR method also allowed us to document the changes resulting from bed-moving floods, such as in the formation of homogeneous gravel sheets during high magnitude floods. At some sites we observed intercalations of coarse organic matter between adjacent sedimentary units. Based on GPR data and the analysis of geomorphological structures near the study site, we developed a diagrammatic model of the sediments of this site in the lower Necker Valley.

Nearly pure sorting waves and formation of bedload sheets

Bedload sheets are coherent migrating patterns of bed material recently observed both in flume studies and in field streams with beds of coarse sand and fine gravel. This newly recognized feature is inherently associated with the heterogeneous character of the sediment and consists of sorting waves with distinct coarse fronts only one or two coarse grains high.The question of the formation of bedload sheets poses an interesting and peculiar stability problem for the grain size distribution. Sorting waves are essentially two-dimensional migrating perturbations associated with variations of this distribution. We show that their growth is strictly associated with grain sorting. In fact the latter gives rise to perturbations of bedload transport which drive small perturbations of bottom elevation the amplitude of which scales with grain size. The sorting wave also induces spatial variations of bottom roughness, and consequently alters the fluid motion, which conversely exerts a spatially varying stress on the bed. The feature of bedload sheets which allows them to be distinguished from dunes over beds with coarse sand or fine gravel is then the fact that sorting is the dominant effect controlling their growth, rather than being a relatively small perturbation of the mechanism which gives rise to dunes in the case of uniform sediment.The requirement that perturbations should not alter the sediment budget leads to an integral condition which gives rise to an integro-differential mathematical problem. With the help of recently developed bedload relationships suitable for mixtures, as well as appropriate modelling of turbulent channel flow over a bed with spatially periodic perturbations of bottom elevation and roughness we are able to derive a general dispersion relation which can be readily solved in terms of undisturbed size densities in the form of sums of Dirac distributions.Perturbations are found to be unstable within a range of wavenumbers depending on the relative roughness and Froude number. We show that when the effects of perturbations of bottom elevation are neglected the unstable region corresponds to the range of conditions where the bottom stress leads bottom roughness, a range distinct from that which characterizes the formation of dunes. This result is given a physical explanation which depends crucially on the deviation from equal mobility of different grain sizes in the surface layer. The effect of perturbations of bottom elevation is however not negligible when the bottom roughness is fairly large compared to depth. In the latter case perturbations of bottom elevation and of bottom roughness are equally important, and gravel sheets are not easily distinguished from small-amplitude dunes.Comparison with the field observations of Whiting et al. (1985, 1988) is satisfactory insofar as the bedload sheet mode is unstable under the conditions of the experiments, and the predicted wavelengths fall within the experimental range. The laboratory observations of Kuhnle & Southard (1988), on the other hand, appear to fall within a range of bottom roughness where the observed bedforms do not exhibit features unambiguously distinct from those of small-amplitude dunes.

Sharply-Topped Alluvial Gravel Sheets in the Palaeogene Promina Basin (Dinarides, Croatia)

The upper part of the Promina Beds at their western extent is represented by two alluvial units: the Kunovac Beds and the Upper Alluvial Unit. The Kunovac Beds contain a high proportion of fine-grained sediments, and generally lacks debris flow deposits. The principal architectural components of the Kunovac Beds are (1) complex (multilateral, multi-storey) gravel-dominated sheets, which originated by the advance of mobile-channel belts, and were terminated by sudden abandonment, (2) smaller heterogeneous gravelly-sandy sheets, which originated in fluvial belts from a combination of sheet flows and channelized flows, (3) small isolated ribbons reflecting the filling of small channels, and (4) floodplain mudstones and sandstones. The basic style by which the Kunovac Beds, as well as the most part of the Promina alluvium were built up are the repeated advances and abandonments of the alluvial belts, and related stacking of coarse-grained sheets and floodplain deposits. The most important factors responsible for the dominance of this sheet-like geometry are high sediment supply and high aggradation and subsidence rates. Deposition of the Kunovac Beds occurred on alluvial plains, situated between the basin-margin proximal alluvium (including fans) and marginal-marine zone of the Promina Basin. The alluvial Promina Beds represent a transverse type of basin-fill pattern, whereas the deepest portion of the basin experienced longitudinal palaeotransport.

Fluvialism or diluvialism? Changing views on superfloods and landscape change

In a series of recent articles, the possibility was raised that, owing to the occasional impact of asteroids and comets in the oceans, lowlands on continental margins might be prone to superflooding (Huggett, 1988a; 1988b; 1989a). It was suggested that a number of landscape features, including diverted drainage systems, gorges, valley meanders and extensive sheets of gravel might have been created by these superfloods. At the conclusion of the 1989 article it was noted that many landscape changes that might be ascribed to the action of superwaves are the same as the landscape changes accredited to the Noachian Deluge by the old school of diluvialists led at its zenith in the 1820s by William Buckland (see Huggett, 1989b). Thus, the bombardment hypothesis, through its prediction of superwaves and superfloods, leads to a new brand of diluvialism. This article explores the nature of neodiluvialism a little further, drawing attention to a growing body of evidence suggesting that, owing to various agencies, truly catastrophic floods have occurred in the past. It also discusses landscape features which can be expected to have been fashioned by diluvial, rather than by fluvial, action.

Channel sediment variability along a river: A case study of the Siret River (Romania)

AbstractThe Siret River has the largest drainage basin (42 274 km2) in Romania. It gathers all the rivers from the eastern part of the Eastern Carpathians, a fact that causes marked asymmetry of the basin. This study is principally concerned with changes in the form of the longitudinal profile and the grain size variability introduced by the Carpathian tributaries. Channel sediment analyses considered the petrography, granulometry, and morphometry of the pebbles, relating these to the river bed and floodplain geometry and to some properties of the drainage basin.The following conclusions arise. The Siret River undergoes an intense regrading of its longitudinal profile, with marked aggradation between transects 24 and 26 (see Figures 1 and 2). This reflects selective accumulation of coarse material due to the massive contribution of the Carpathian tributaries. This phenomenon has been continuous throughout the Holocene, resulting in the gravel sheet formation of the Pericarpathian piedmont.

Particle over‐passing on depth‐limited gravel bars

ABSTRACTAn 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.

Growth and burial of the English Channel unconformity

The recent suggestion that the geological record of the future will contain no English Channel sediments dated to the present day, except in a few favoured and untypical situations, is examined critically. It is concluded that modern sheets of sand and gravel, in equilibrium with peak tidal current strength, cover about half of the rock floor of the English Channel now, as well as being widespread in the adjacent part of the Celtic Sea. Modern faunas are providing a continuing supply of bioclastic material, so that the sand and gravel sheets will thicken in time. The modern deposits are underlain, in part, by older Holocene deposits. The Holocene deposits as a whole rest unconformably on the flattish eroded surface of the rock floor. The remaining half of the English Channel is only partly armoured by pebbles and cobbles. The rock floor is already rough in some areas and the strong currents, aided by boring activity, are expected to cause further erosion before a new equilibrium surface is established, in part covered with new deposits.

Raised channel systems as indicators of palaeohydrologic change: a case study from Oman

The aim of this paper is to determine the extent to which “raised” channel systems may be used as indicators of long-term paleohydrologic change, and hence of broader paleoenvironmental changes, in arid areas during the Cenozoic. The paper reviews the paleohydrologic significance of former drainage systems in arid areas, and demonstrates the application of paleohydrologic methods to the analysis of the long-term evolution of a complex series of exhumed, multistorey, raised channel systems in interior Oman. The paper discusses the range of possible autocyclic and allocyclic controls on changes in channel behaviour through time, as a means of establishing the paleoenvironmental significance of observed changes in paleochannel pattern, morphology, sedimentology and stability. The paper identifies some of the major allocyclic controls, including climatic changes, and particularly the seasonality, magnitude, frequency and distribution of rainfall events during the Cenozoic, together with the degree of continentality and hence the strength and persistence of arid wind systems. Base-level changes, due to eustatic fluctuations and/or tectonic activity, may also act to control long-term patterns of aggradation followed by large-scale landscaping lowering. However, long-term fan ageing associated with progressive aggradation and steepening of paleochannel systems, may also eventually lead to fanhead trenching, independent of any external controls. In addition, progressive expansion of the drainage network and resulting river capture, can lead to marked increases in water and sediment supplies to the fluvial system. The paper also emphasizes the difficulties involved both in differentiating the wide range of possible controls on paleohydrologic response, and also in developing an accurate and precise chronology for desert paleodrainage systems, and in their correlation with non-fluvial sedimentological sequences. The raised channel systems of interior Oman consist of heavily desert varnished chert-and/or ophiolite-rich gravels, dating from some time during the Plio-Pleistocene period. The paleochannels from sinuous gravel ridges rising <30 m above the surounding plains. At least 12 successive generations of superimposed paleochannel systems have been identified, representing major periods of fluvial activity, each of varying duration and character. The earlier generations form typically broad, sinuous, single-thread channels, comprising fine-grained, chert-rich gravels, emented by clear crystalline calcite, and associated with bankfull paleodischarges of up to 1400 m 3 s −1. The later generations become increasingly narrow, steep and less sinuous, with single-thread distributary channels which narrow rapidly downfan. The gravels are coarser grained and dominated by increasing concentrations of ophiolites. The latest stage is associated with fanhead entrenchment, and formation of terrace gravel sheets, poorly cemented by CaCO 3, and dominated by ophiolites. Peak paleoflows may have exceeded 13,000 m 3 s −1. The observed changes have resulted from a complex combination of both internally and externally generated environmental changes during the Cenozoic. Progressive extension of the drainage network on to the ophiolite massif of the mountain source area resulted in river capture and a concomitant increase both in paleodischarges and in supplies of ophiolitic sediments to the paleodrainage system. The early sinuous channels may have developed under a humid to sub-humid climate, in which perennial or seasonal flows allowed the distinct meander paleochannels to form. Ephemeral flows, characterized by flash flood regimes, were probably associated with the later distributary systems, which may reflect semi-arid to arid climatic conditions. The large-scale deflation of the interfluve sediments, which resulted in the eventual exhumation of the cemented, gravel-capped, multistorey paleochannel sequence, is also likely to have occurred during periods of marked continentality, probably during periods of low sea-level associated with the Quaternary glaciations.

Sedimentation in a meandering gravel‐bed river: The River Tywi, South Wales

AbstractPrevious studies of meandering gravel‐bed rivers have illustrated a wide range of bar types. The River Tywi of South Wales shows that significant variations of accretionary style can also occur within a single river. There is a downstream decrease in the proportion of lateral bars to point bars and changes in the morphological characteristics of these point bars. Three types are recognized: simple, linguoid and multi‐unit point bars. Sedimentation on the concave sides of meander bends is locally important. The changes of bar type are accompanied by different styles of channel behaviour.The River Tywi is interpreted to have deposited multilateral gravel sheets, composed of partially reworked and abandoned bars and dissected by palaeochannels and sloughs. Bar deposits consist of parallel‐bedded gravel, inclined laterally‐accreted gravel, local angle‐of‐repose foresets and inclined lenses of heterolithic beds. The proportion of the various sedimentary structures and the geometry of the abandoned bars varies along the Tywi valley because of the patterns of bar distribution and channel behaviour.The deposits of this river have strong affinities with Tertiary sequences in the Italian Apennines, previously interpreted as the deposits of meandering gravel‐bed rivers. This type of river is not readily distinguished from ‘Scott type’ braided streams in the geological record, unless exposures are particularly good. In this respect, the presence of abundant, inclined heterolithic wedges and lenses may be a useful diagnostic criterion.