Overbank Fines Research Articles

Late Pleistocene alluvial plain sedimentation in Lower Narmada Valley, Western India: Palaeoenvironmental implications

Late Pleistocene fluvial sediments that were deposited in a slowly sinking basin are now exposed as 30–50 m high incised vertical cliffs all along the Lower Narmada Valley in western India. The exposed fluvial deposits have been classified into two sediment packages, alluvial fan sediments overlain by alluvial plain sediments. The alluvial plain sequence has not been studied previously. It consists mainly of sands and silts and is dominated by overbank deposits. Occurrence of large scale bedforms in the alluvial plain sequence points to the existence of a large sand bed river in an alluvial plain setting. The major sedimentary facies in stratigraphic order include large channel fills, giant epsilon cross bedded strata, overbank fines occurring in horizontal, massive and undulatory stratified forms associated with crevasse splay and backswamp deposits, and a reddish brown palaeosol overlain by thinly stratified sands and silts at the top of the exposed sediment succession. Large sized channel fills occur at two stratigraphic levels, which are morphologically similar and are indicative of high rates of deposition and avulsion. The large channel fill structures and the giant epsilon cross bedded strata indicate a large single channel river that was consistently 10–15 m deep and about 70–80 m wide even during the dry seasons. These dimensions are larger than those of the present day Narmada River at low discharge levels. The overbank sediments indicate rapid deposition through frequent overbank floods and floodplain aggradation by a laterally shifting river. Available chronologic data suggests that the reddish brown palaeosol correlates with a regional phase of pedogenesis in the alluvial plain of Gujarat prior to the Last Glacial Maximum (LGM). The thinly stratified sands and silts overlying the palaeosol were deposited by a considerably depleted but perennial river during the arid phase of the Last Glacial Maximum. Overall, the alluvial plain sediments of the Lower Narmada valley, particularly those below the palaeosol, have been attributed to a hyper-avulsive large river with low sinuosity whose high discharge levels were determined primarily by a large catchment area further to the east and not by the semiarid climate prevailing in the Gujarat alluvial plain during the upper part of the Late Pleistocene. The study concludes that the Narmada River has maintained a large catchment at least since the last 100 ka, however, the river was characterised by a much bigger channel during much of the Late Pleistocene with discharge levels higher than the present day.

Fluvial facies architecture in small-scale river systems in the Upper Dupi Tila Formation, northeast Bengal Basin, Bangladesh

The late stage basin-fill history of the fluvial Dupi Tila Group (Plio-Pleistocene) is described. These rocks have been deposited in the Sylhet trough, a sub-basin of the Bengal Basin, in a foreland basin setting. This outcrop study, carried out in Sylhet, Bangladesh, presents the first detailed facies analysis of the Upper Dupi Tila Formation. Four facies have been identified: trough cross-bedded sandstone (St), ripple cross-laminated sandstone (Sr), finely laminated mud with ripples (Fl), and massive mud with rootlets (Fm). Facies analysis supplemented with embedded Markov chain analysis, reveals small-scale fining-upward cycles (average 4.5 m thick). Facies architectural elements include channel (CH), lateral accretion (LA), sandy bedforms (SB), and overbank fines (OF) with limited vertical and lateral connectivity of the sand bodies. The average channel depth and width is 5 and 30 m, respectively. Sand body geometry ranges from tabular, to sheet, to shoestring with a 0.45 net to gross ratio. This study shows that the Upper Dupi Tila Formation is composed of small-scale, mudstone-reach meandering river deposits. In Bangladesh, the Dupi Tila Formation is the main aquifer presently being utilized. Understanding of facies architecture and sand body geometry of this Formation is crucial in examining the issue of arsenic and other contaminations of ground water in Bangladesh.

FLUVIAL ARCHITECTURE AND THE TECTONIC CONTROL ON DEPOSITION OF ONSHORE EUMERALLA FORMATION, OTWAY RANGES, VICTORIA: IMPLICATIONS FOR EXPLORATION IN THE EARLY CRETACEOUS OTWAY BASIN

Spectacular outcrops of the terrestrial Aptian-Albian Eumeralla Formation are exposed in the Skenes Creek– Wongarra region, located on the eastern margin of the Otway Ranges, Victoria. The succession comprises mudstone-dominated floodplain and lacustrine successions and fluvially-derived sandstones. Lithofacies observed in the study area comprise intraformational and exotically derived conglomerate, massive and planar laminated sandstone, trough and tabular cross-bedded sandstone, ripple laminated sandstone, interbedded sandstone and mudstone, massive to laminated mudstone and thin coal seams. Architectural analysis of the fluvial system reveals these lithofacies are arranged into architectural elements that include channel elements, sandy bedforms, downstream and lateral accretion elements, laminated sand sheets and overbank fines elements.The fluvial system is characterised by low-sinuousity, braided river channels with high width to depth ratios. Palaeocurrent data indicates that the generally westward palaeoflow is interpreted to have been diverted into local axial-through drainage patterns by active northeast trending normal faults. One of these, the Skenes Creek Fault, is also likely to have structurally isolated floodplain and lacustrine successions from the main channel belt, leading to the deposition of an anomalously thick coal measure sequence in the hanging wall of the fault. The local study therefore provides insight into regional lithofacies and potential source rock distributions, and the associated tectonostratigraphic evolution of the Eumeralla Formation in the eastern Otway Basin. While the nature of the Eumeralla Formation sandstone does not lend itself to good reservoir properties, the geometry and internal structure of the sands provide an excellent model for other fluvial sandstone reservoir reconstructions.

Sedimentology and reservoir characteristics of a Middle Jurassic fluvial system, Datong Basin, northern China

ABSTRACT An outcrop of the Middle Jurassic Yungang Formation in Datong Basin of northern China was studied in order to provide a better understanding of the architecture and heterogeneity of hydrocarbon reservoirs associated with braided-fluvial deposits. The main section of outcrop, which is continuously exposed over 350 m, is interpreted to have been deposited in the middle-stream reaches of a sandy braided river in a proximal area at the eastern edge of the basin. Seventeen lithofacies were identified, based on petrology and sedimentary structures. These lithofacies were grouped into seven architectural elements, including channel fill, transverse bar, longitudinal bar, diagonal bar, abandoned channel fill, levee, and overbank fines. These sediments were deposited in three stages (active, abandonment, and reactivation), which are related to sediment channel switching and waxing and waning of the fluvial system. The sandstones are mainly arkoses or lithic arkoses, with high matrix content, low compositional and textural maturity, and a variety of cements. A total of 282 specimens were collected from outcrops and analyzed for porosity and permeability. The results show that sandstones from longitudinal bar and channel fills have the highest porosity and permeability while abandoned channels and levee sediments have the lowest. In cross-section, porosity and permeability are generally highest in the middle of the sand bodies, reflecting the spatial distribution of architectural elements and petrophysical properties. End_Page 105------------------------

Environmental change and fluvial activity during the Younger Dryas in central Germany

Late Pleistocene climatically induced environmental changes are reconstructed by applying a multidisciplinary approach on floodplain sediments of small- to medium-sized catchments in central Germany. Radiocarbon dating , pollen analysis and the presence of an accurately dated tephra layer allow the establishment of a reliable chronology . The beginning of the Younger Dryas is marked by a change in fluvial activity that lasts for several hundred years. During this period gravels and sands were deposited by a braided river system. Fluvial systems at this time were predominantly controlled by the climatic conditions of the surrounding uplands, where the climatic deterioration led to a lowering of the forest limit and enhanced periglacial slope processes. An open pine forest prevailed in the basin areas and no evidence of slope wash and solifluction was found. The second part of the Younger Dryas is characterised by a meandering fluvial system and the deposition of overbank fines. The rapid transition from Younger Dryas to Preboreal coincides with an increase in organic deposition.

Middle Terrace Deposits of the Tagus River in Alpiarça, Portugal, in Relation to Early Human Occupation

The stratigraphy and sedimentology of the Q3 middle terrace alluvial sequence in the lower Tagus river valley, Portugal, were studied near the village of Alpiarça, approximately 40 km upstream from the estuarine area. Two main stratigraphic units were recognized, separated by an important uncomformity. The Lower Gravels unit (LG) consists of intercalations of medium to coarse gravel deposits, mainly quartzitic, with coarse sandy matrix, organized in tabular bodies. The overlying Upper Sands unit (US) consists of tabular sandy channel deposits and overbank fines, the latter containing well-developed paleosols and backswamp deposits, showing a general aggrading trend, apparently with varying rates; available data indicate that deposition of the US took place under temperate climatic conditions. Within US deposits are several paleolithic archaeological sites, the lower ones in the alluvial stratigraphy being Middle Acheulian, whereas those embedded in overlying deposits are, from bottom to top, Upper Acheulian and Micoquian. Some of these sites have been recently excavated. The quartzite artifacts were apparently abandoned by early humans on the flood plain surface during deposition of the US unit and were subjected to limited reworking during their incorporation in the alluvium. TL/OSL dating of sandy-silty sediments, though imprecise, support archaeological evidence pointing to an age of 150,000 to 70,000 yr B.P. for the US unit.

Spatial variability of floodplain sedimentation at the event scale in the Rhine–Meuse delta, The Netherlands

This article addresses spatial variability of comtemporary floodplain sedimentation at the event scale. Measurements of overbank deposition were carried out using sediment traps on 11 floodplain sections along the rivers Waal and Meuse in The Netherlands during the high-magnitude flood of December 1993. During the flood, sand sheets were locally deposited behind a natural levee. At distances greater than 50 to 100 m from the river channel the deposits consisted mainly of silt- and clay-sized material. Observed patterns of deposition were related to floodplain topography and sediment transporting mechanisms. Though at several sites patterns were observed that suggest transport by turbulent diffusion, convection seems the dominant transporting mechanism, in particular in sections that are bordered by minor embankments. The average deposition of overbank fines ranged between 1·2 and 4·0 kg m−2 along the river Waal, and between 1·0 and 2·0 kg m−2 along the river Meuse. The estimated total accumulation of overbank fines (not including sand sheets) on the entire river Waal floodplain was 0·24 Mton, which is 19 per cent of the total suspended sediment load transported through the river Waal during the flood. © 1998 John Wiley & Sons, Ltd.

Facies architecture and sedimentology of a meandering fluvial system: a Palaeogene example from the Weisselster Basin, Germany

The Schleenhain open pit coal mine, located 30 km south of Leipzig, Germany, exposes Upper Eocene and Oligocene non‐marine strata representing fluvial deposition in the centre of the Weisselster Basin. Active mining and successive cuts provided the rare opportunity to obtain a three‐dimensional perspective of laterally extensive surface outcrops. These were used to construct a detailed fence diagram, which provided the basis for recognition of key architectural elements in the weakly consolidated meandering stream deposits. In addition to the eight basic architectural elements of Miall (1985), the element SL (shallow lake deposits) was newly defined and the element CH (channel) was subdivided into CHg (palaeo‐river system) and CHk (small channel). The profiles contain parts of two fining‐upward cycles, which are separated by an unconformity spanning the Early Oligocene. Deposits of the first cycle begin with transverse sand bars (downstream accretion deposits‐DA) and point bars (lateral accretion deposits‐LA). The upper part of the cycle is represented by overbank fines (OF) and the element SL, which consists of laterally discontinuous lenses of dark, plant‐bearing, kaolinite‐rich clays, that were deposited in shallow lakes adjacent to the active channel. Coal seams interlayered with palaeosols are the main constituents of element OF. Sheetlike bodies of medium to fine gravels (gravel bars and bedforms‐GB) on an erosive coal surface mark the beginning of the second cycle. Dissolution of underlying Permian salts and sulphates prior to, during, and after the deposition of the Palaeogene strata caused the development of two synclines within the outcrop. Coal seams and clay horizons which thicken and dip towards the centre of the synclines, provide evidence for their chronological development.

The infill of a supradetachment(?) basin: the continental to shallow-marine Upper Permian succession in the Dolomites and Carnia (Italy)

In the Dolomites and Carnia (eastern Southern Alps), the Upper Permian succession is represented by red beds of Val Gardena Sandstone, grading upwards and eastwards into the evaporitic and carbonate deposits of the Bellerophon Formation. An overall transgressive pattern is shown by the general trend of the depositional setting, which evolved from flashy alluvial fans, through multiple-channel bedload rivers, mixed-load sinuous rivers, terminal fans, coastal sabkha and evaporitic lagoon, to a shallow, low-gradient marine ramp. The inferred fluvial regime was subject to rapid and erratic fluctuations in discharge. Palaeosols are represented by calcic soils, and suggest a warm to hot, semi-arid or dry subhumid climate with strongly seasonal rainfall. Vertic features are associated with more inland alluvial complexes: they are missing in the terminal-fan deposits, suggesting greater aridity in lowland (coastal) areas. The Bellerophon Fm. consists of two units: a lower evaportte-bearing unit, deposited in a barred basin, and an upper shallow-marine carbonate unit, laid down on a very low-energy, low-gradient ramp. Five third-order sequences and the lower part of a sixth sequence, collectively showing a backstepping pattern, have been identified in the succession. Due to the presence of a very low-gradient ramp margin, and the consequent ineffectiveness of relative sea-level falls in producing large-scale erosion in coastal areas, it is suggested that, rather than eustatic changes, degradational episodes marking sequence boundaries in the red-bed succession reflect changes in the climate-modulated intrinsic variables of discharge and sediment supply, and/or tectonic uplift. Base level rises in, red-bed sequences are recorded by upward change from thick channel-belt sandstone bodies with an often high connectedness ratio, to progressively thinner channel deposits, ranging from isolated ribbons to channel-belt sandstone bodies potentially reaching high width/thickness ratios, encased in a comparably greater volume of overbank fines. This trend is thought to reflect the change from a confined geomorphic setting, with a limited area of potential avulsion, to an unrestricted setting with rivers free to move extensively; in addition, it documents the transition from an inland fluvial system with laterally migrating perennial or semi-perennial channels, to an ephemeral network of randomly migrating and frequently avulsing small terminal-fan distributaries, through a drastic downstream decrease in channel depth and discharge. Identification of key sequence-stratigraphic relationships within the red-bed succession was mostly aided by the presence of easily recognizable and regionally correlated marine and marginal marine bed packages, traceable landwards into alluvial deposits showing faint traces of tidal activity, interpreted as the equivalent of marine maximum-flooding sediments. They may grade upwards into progradational fluvial packages showing basinward increase in thickness. The Upper Permian deposits of the Southern Alps are considered part of an Upper Permian-Lower Triassic, second-order, structurally controlled sequence. The location of the basin on a thickened, previously active crust, affected by thermal perturbance after the last stages of the Variscan orogeny, the relatively reduced thickness of the basin fill, and predominantly long, transverse drainage networks, mostly derived from the denuded Insubric footwall, all suggest that sedimentation took place in a supradetachment basin, with a major detachment fault located in the palaeo-Insubric belt.

Sedimentary sequences, seismofacies and evolution of depositional systems of the Oligo/Miocene Lower Freshwater Molasse Group, Switzerland

ABSTRACTMagnetostratigraphic chronologies, together with sedimentological, petrological, seismic and borehole data derived from the Oligo/Miocene Lower Freshwater Molasse Group of the North Alpine foreland basin enable a detailed reconstruction of alluvial architecture in relation to Alpine orogenic events. Six depositional systems are recorded in the Lower Freshwater Molasse Group. The bajada depositional system comprises 200–400‐m‐thick successions of ribbon channel conglomerates and overbank fines including mud‐ and debris‐flows which were derived from the Alpine border chain. The alluvial megafan depositional system is made up of massive pebble‐to‐cobble conglomerates up to 3 km thick which reveal a fan‐shaped geometry. This depositional environment grades downcurrent into the conglomerate channel belt depositional system, which comprises an ≈2‐km‐thick alternation of channel conglomerates and overbank fines. The sandstone channel belt depositional system is bordered by the 100–400‐m‐thick overbank fines assigned to the floodplain depositional system. At the feather edge of the basin, 50–400‐m‐thick lacustrine sediments in both clastic and carbonate facies represent the lacustrine depositional system.The spatial and temporal arrangement of these depositional systems was controlled by the geometrical evolution of the Molasse Basin. During periods of enhanced sediment supply and during phases of stable sliding of the entire wedge, >2000‐m‐thick coarsening‐and thickening‐upward megasequences comprising the conglomerate channel belt, alluvial megafan and bajada depositional systems were deposited in a narrow wedge‐shaped basin. In the distal reaches of the basin, however, no sedimentary trend developed, and the basin fill comprises a <500‐m‐thick series of sandstone meander belt, floodplain and lacustrine depositional systems. During phases of accretion at the toe of the wedge, the basin widened, and prograding systems of multistorey channel sandstones extended from the thrust front to the distal reaches of the basin.The rearrangement of the depositional systems as a function of changing orogenic conditions created discordances, which are expressed seismically by onlap and erosion of beds delimiting sedimentary sequences. Whereas stable sliding of the wedge succeeded by accretion at the toe of the wedge is recorded in the proximal Lower Freshwater Molasse by a coarsening‐and thickening‐upward megasequence followed by erosion, the opposite trend developed in the distal reaches of the Molasse. Here, fine‐grained sandstones and mudstones were deposited during periods of stable sliding, whereas phases of accretion caused a coarsening‐ and thickening‐up megasequence to form.

AN INTEGRATED APPROACH TO THE STUDY OF THE EARLY TRIASSIC REWAN GROUP, BOWEN BASIN

This study of the Early Triassic Rewan Group in the Bowen Basin was an integrated approach using seismic stratigraphic principles with outcrop, wireline log, bios- tratigraphy and core data. It resulted in a coherent and useable stratigraphic framework that assisted in defining facies and potential reservoirs below seismic resolution within the Rewan Group. The Rewan Group consists of the Sagittarius Sandstone and Arcadia Formation. Depositional systems of the Sagittarius Sandstone and Arcadia Formation were affected by major tectonic and climatic changes. Each formation is marked at its base by a third-order sequence boundary. The Sagittarius Sandstone is overall regressive and was deposited in a domi- nantly lacustrine environment. The base of the overlying Arcadia Formation is marked by the Brumby Sandstone Member. It was deposited in a fluvially dominated shoreface setting. Red beds of fluvial origin in the Arcadia Formation are characterised by overbank fines and lenticular channel sandstone. High-frequency incised valley fills occur in the uppermost Arcadia Formation. The Arcadia Formation is in turn erosively overlain by pebbly sandstone of the Clematis Group. Sandstone-filled incised valleys at the base of the Sagittarius Sandstone and in the upper Arcadia Formation offer the best reservoir potential in the Rewan Group.

Stratigraphic architecture resulting from Late Quaternary evolution of the Riverine Plain, south‐eastern Australia

ABSTRACTThe Riverine Plain of south‐eastern Australia is the result of prolonged Cenozoic fluvial activity. Single thread, anabranching and distributary channels and floodplains, and associated aeolian dunes, characterize the uppermost sequences. Based on detailed interpretations of Late Quaternary fluvial sedimentation and surficial stratigraphy for this 77 000‐km2 basin, earlier ‘prior stream’ and ‘ancestral stream’ models of fluvial deposition, deduced from limited stratigraphic and chronological evidence, are replaced with aggradational palaeochannel and migrational palaeochannel models. Thermoluminescence dating reveals four distinct phases of palaeochannel activity between 105 and 12 ka; the first (Coleambally phase) late in Oxygen Isotope Stage 5, the second (Kerarbury phase) in Stage 3, the third (Gum Creek phase) before and the fourth (Yanco phase) after the Last Glacial Maximum (LGM) in Stage 2. The first three of these phases were characterized by mixed‐load laterally migrating sinuous palaeochannels with occasional transitions to a straighter bedload‐dominated mode, and vice versa. The first two phases concluded with a bedload‐dominated episode resulting in aggradational palaeochannels on the surface of the Plain, and the third phase (prior to the LGM) did also in its downstream reaches. The phase following the LGM was characterized entirely by large mixed‐load sinuous migrational palaeochannels. These exhibited no terminating bedload episode, because the onset of Holocene climates reduced the size of the flood peaks, greatly diminished the supply of bedload from the upper catchments and resulted in streams evolving to their present highly sinuous suspended load form. The result is a complex stratigraphic architecture consisting of vertically and laterally accreted units extending over hundreds of kilometres in the form of channel‐sand stringers, sand sheets and derivative aeolian dunes partially or wholly encased in overbank fines.

Channel migration patterns and related sequences in some alluvial fan systems

The pendular displacement of a channel system, consisting of an area of higher channel density (trunk channel zone) on both sides of which channel density progressively decreases (secondary channel zone), can, under favourable subsidence conditions, lead to the development of alluvial fans. Characteristic sequences are found in these fans, depending on their position in the sedimentary body. A marginal position in the fan is thus recognized by the superposition of fining and thinning upwards (FTU) cycles, the upper part of which is made up of important thicknesses of overbank fines, all the backsets of bars dipping in the same direction (towards the centre of the cone). On the other hand, a central position is characterized by a higher number of FTU cycles, which are incomplete due to erosion of the upper parts (corresponding to the higher concentration of overbank fines), and the backsets in each cycle dip alternately in opposite directions. The displacement of the channel system in a constant direction may be caused by the preferential accumulation of bars on one of the banks of the channel. The change in migratory direction giving rise to the pendular movement is caused by the trunk channel reaching the basin margin or the sedimentation area of an adjoining fan. Palaeogeographic reconstructions of fans using this technique contribute to the analysis of ancient basins: small-radius fans with a high sweep angle (A S) are characteristic of basin margins subjected to a low sediment supply/subsidence ratio (S S /S B), whereas large-radius fans with a low A S characterize periods with a high S S /S B ratio on the basin margin. This model can be applied in economic geology studies, as the location of an ancient cone permits delimitation of the axial strip (with a higher proportion of coarse, highly porous, channelled facies) and the marginal sectors (where thick layers of less porous overbank fines are intercalated). We here present the example of the alluvial fans in the Guadix Fm. (Betic Cordillera, Spain), which lie on a basin margin (ESE) where the S S /S B ratio increased progressively throughout the Lower Pliocene. The detailed analysis of palaeocurrents on the basis of different criteria throughout the Arroyo de Gor section helps to locate more precisely and confirm the palaeogeographical distribution of the Pliocene cone, as deduced from sequential analysis.

Fine-Grained Splay Deposition in the Avulsion Belt of the Lower Saskatchewan River, Canada

ABSTRACT In or about 1883, a northward avulsion of the meandering lower Saskatchewan River at the Cumberland Marshes initiated a broad belt of alluvial sedimentation that has continued to evolve to the present, and now covers some 500 km2 The avulsion belt comprises a complex alluvial terrain dominated by individual and coalesced splays and associated wetlands connected by networks of active and abandoned channels of various sizes. The Windy Lake splay, a predominantly fine-grained feature typical of large parts of the avulsion belt, was examined to investigate the relationships between splay evolution and facies development. The splay, now largely inactive, mostly formed over a 35-year period, depositing nearly 2 m of sediment over a 7.2 km2 area. The splay surface is ch racterized by stable, well-defined anastomosed distributary channels that separate interchannel wetland basins. Heights of channel levees diminish downstream, and the levees merge with channel-mouth bars whose progradation forms the locus of lakeward channel extension and the base for subsequent subaerial levee deposition. The anastomosed channel pattern was created when the mouth bars of separate distributary channels merged during progradation. Vibracore and soil-probe profiles show the splay to be dominated by upward-coarsening lacustrine-prodeltaic mouth-bar facies sequences capped by levees and organic-rich interchannel wetland deposits. Channels, now containing sand only sporadically in narrowed or slightly migrated reaches, will eventually fill with fine sediment when abandonment s complete. The Windy Lake splay is typical of stage III splays (Smith et al. 1989), whose deposits and associated lacustrine fills form at least half of the avulsion belt. Thin, widespread, predominantly upward-coarsening sequences capped by either nondepositional horizons or true overbank fines may be characteristic of many avulsive suites in the ancient alluvial record.

Fell Sandstone lithostratigraphy south-west of Berwick-upon-Tweed: implications for the regional development of the Fell Sandstone

SUMMARY The Fell Sandstone Group in north-east England forms part of a fluvio-deltaic Lower Carboniferous Visean sequence that was deposited within the Northumberland Basin and the smaller Tweed Basin following their initiation by rifting and subsidence in the late Devonian/early Carboniferous. The Fell Sandstone was deposited by a large braided-river complex which drained westwards into a shallow, low-energy tideless sea. Outcrop studies have indicated that the Fell Sandstone Group is composed almost entirely of sandstone with a paucity of overbank fines (less than 20%). However, a number of boreholes drilled for a hydrogeological study in the Tweed Basin indicate that in this region the Fell Sandstone Group may contain 50 to 60% mudstone. Seven mappable sandstone units have been identified within the field area, separated by laterally persistent mudstone units. Pumping test data indicate that many of the sandstone units are in good communication, whereas others are in isolation. Thickness variations between wells indicate that many of the sand units have a lenticular geometry. Regional compilation of borehole data through the Fell Sandstone across the Northumberland Basin highlights a number of lateral facies changes which cannot be assigned simply to a relationship of distance from source. In the early Carboniferous a number of faults within the Northumberland Basin were active during the deposition of the Fell Sandstone. Local variations in facies and sandstone thickness within the Fell Sandstone Group can be explained using simple tilted fault-block models. Where a particular fault changes polarity of throw local thickness and extent of

Tectonic setting and sedimentology of Ganga River sediments, India

The Ganga basin provides a present‐day example of a peripheral foreland basin. The course of the river is controlled by Himalayan tectonics. Three main types of architectural elements, such as channels (CH), sandy bedforms (SB) and overbank fines (OF) have been developed in Ganga River sediments. The channels (CH) include gravelly (Gs) and sandy channel (Ss) lithofacies. The sandy bedforms (SB) include trough cross‐stratified (St), planar cross‐stratified (Sp), horizontal stratified (Sh), sandy massive (Sm) and climbing ripple cross‐laminated (Sr) lithofacies, all of which are active channel deposits. The overbank fines (OF) include massive silt and clay (Fm), parallel laminated silt and clay (FI) and climbing ripple cross‐laminated (Sr) lithofacies. Mega units have been developed in the lower part of the active channel deposits, while small units have been developed in the upper part of active channel deposits, in inactive channel deposits and overbank fines. This study illustrates the seasonal and tectonic control on sedimentation. Petrofacies studies of the sediments indicate a recycled orogen provenance. The sediments are derived from rapidly uplifted fault blocks comprising granite, gneiss and basic and ultrabasic rocks. Lack of textural and compositional maturity suggests a local source of derivation. The principal control on sand composition is source lithology. The hot and humid climate may slightly increase the content of quartz in sand derived from reworked foreland basin sediments. but the effect is neither sufficient to shift the sand compositions out of the recycled orogen field nor does it obscure composition mixing patterns.

Distal alluvial deposits in a foreland basin setting—the Lower Freshwater Miocene), Switzerland: sedimentology, architecture and palaeosols

ABSTRACTThe Lower Freshwater Molasse (Untere Susswasser Molasse) crops out over a wide area of the Swiss Molasse Basin. Coarse grained alluvial fan conglomerates dominate in proximal basin areas along the Alpine front. These conglomerates pass northwards into sandstones and mudstones of an extensive northeastward draining meandering river system which ran parallel to the basin axis. Sedimentological study of outcrops, quarry exposures and boreholes in the basal Miocene (‘Aquitanian‘) has permitted detailed facies analysis of this distal alluvial sequence. The distal Aquitanian is made up of distinct ‘architectural elements’characterized by their geometries and sedimentary structures. Each may be assigned to a particular depositional setting: meander belt, levees, crevasse channels and splays, overbank fines and palaeosols, and lacustrine.Meander belt sandstones were deposited in mixed load channels with a dominant bedload component. Sandstones commonly comprise amalgamated and locally stacked ribbon bodies 2–15 m thick and 150–1500 m wide. Interbedded rippled, laminated and mottled fine grained levee sandstones and siltstones form lenticular packages up to 3 m thick and 30–100 m across. Small scale crevasse channel sandstones 2–4 m thick and 5–10 m across pass laterally into metre scale, medium to fine grained crevasse sandstone sheets. Rare laminated lacustrine siltstones occur only in the north‐east part of the basin. Floodplain mudstones and marls make up the remainder of the succession. These display a variety of pedogenic features recording cyclical palaeosol development. Palaeosols show strong variations in morphology and maturity both laterally across the floodplain and downstream along the basin axis, reflecting local variation in aggradation rate associated with proximity to alluvial channel courses as well as regional variation in subsidence and floodplain drainage.Analysis of the organization and distribution of the various sediment bodies permits reconstruction of the fluvial system and allows development of a model for the sedimentary architecture of the Lower Freshwater Molasse in the study area. Integration of palaeosol studies into a well defined architectural framework assists recognition of areal facies belts and may aid location of sand‐prone sequences in the subsurface.

The chemical and physical speciation of trace metals in fine grained overbank flood sediments in the Tyne basin, north-east England

Deposition of fine sediment contaminated by heavy metals on floodplains in the Tyne basin, north-east England, during a major flood in August 1986 provided an unusual opportunity to investigate the chemical and physical speciation of metals in overbank fines. Selective extractions and heavy-liquid analyses were used to assess the chemical forms of sediment-associated metal and the relationship between grain size, density and metal concentration and to identify sources of metalliferous material. Generally the highest concentrations of trace metals were found in the 500–250 μm size range and in the heavier density fractions (>3.3 gml −1), though more than 60% of metals reported to the lightest (<3.3 gml −1) density product. The principal source of metals appeared to be reworked alluvium contaminated by historic mining activity. Metal concentrations in flood sediment decrease downstream from former mining areas though at varying rates depending on the physical propeties and chemical phase of individual metals. With the exception of Cu (dominated by lithogenic fraction), metals are primarily associated with Fe/Mn oxides (Pb, Zn) and to a lesser extent with carbonates (Cd, Zn), organic matter (Zn, Cu) and an operationally defined exchangeable (Cd) component. In the upper and middle parts of the Tyne basin physical processes of downstream sediment metal content reduction were found to predominate, while in the lower reaches of the Tyne sediment metal transport appears to be governed by chemical sorption-desorption processes associated with Fe/Mn oxides and organic material. Finally the results of the study are discussed in a wider context of particulate metal transport in rivers and recommendations are made for refining density-based models of metal dispersal.

Architectural-element analysis: A new method of facies analysis applied to fluvial deposits

Existing methods of facies analysis for fluvial deposits rely extensively on vertical profile analysis and comparisons with a limited array of fixed “end member” facies models. However, vertical profiles are not sufficiently diagnostic for this purpose because they cannot adequately represent three-dimensional variations in composition and geometry. A new method of analysis is proposed which subdivides fluvial deposits into local suites consisting of one or more of a set of eight basic three-dimensional architectural elements. These are channels, gravel bars and bedforms, sandy bedforms, foreset macroforms, lateral accretion deposits, sediment gravity flow deposits, laminated sand sheets and overbank fines. Twelve fluvial styles are selected to illustrate possible combinations of these elements. It is suggested that the same methodology could be used for other clastic facies. The better documentation of three-dimensional facies variability that can be obtained should be of considerable use in interpreting sedimentary controls and in carrying out petroleum field development, reservoir engineering or ore grade studies.

Neogene Paleogeography of Western Snake River Plain, Idaho and Oregon: ABSTRACT

Analysis of Miocene through Pleistocene siliciclastic and volcaniclastic sequences in the western Snake River Plain of Idaho and Oregon allows detailed paleogeographic reconstruction of sedimentation associated with the development of a rapidly subsiding continental basin. Extensional tectonism was accompanied by voluminous outpourings of basaltic and silicic volcanic material. These in turn were reworked basinward by marginal alluvial fan-braided stream networks into basin-center fluviolacustrine systems. Episodic influxes of both felsic and basaltic tephra are recorded in fossiliferous lacustrine silt and claystones of the Poison Creek and Chalk Hills Formations, radiometrically bracketed between 12.5 to 5 m.y.B.P. Basinward-fining facies indicate deposition in a large acustrine complex fed by at least 2 major fluvial systems. Complex interfingering of coarse-grained strandline deposits with offshore fine-grained sediments suggests repeated expansion and contraction of the lake system and record the dynamic interplay between basin tectonism and sedimentation. A major unconformity and presence of extensive interbasinal basaltic volcanism (Banbury Basalt) reflects draining of this lacustrine system and renewed extensional tectonism in the basin approximately 4 m.y.B.P. Reestablishment of a major fluviolacustrine complex is represented by deposits of the Pliocene-Pleistocene Glenns Ferry Formation (4-1.8 m.y.B.P.). The fossiliferous mudstone, sandstone, and conglomerate represent alluvial fan, meandering stream, flood-basin, and lake margin systems. Inter edded with these are a series of basaltic tuffs and flows reflecting continuing volcanism along the basin margin and basin center. Continued structural adjustments along high-angle normal faults that bound the basin are indicated by stratigraphic relationships as well as the style of fluvial sedimentation. Periods of rapid basin subsidence are recorded by thin, discontinuous channel sandstone encased by thick overbank siltstone and claystone. More stable periods allowed development of sheet-like channel sandstone with subordinate amounts of overbank fines. Basin-wide structural adjustments have continued to the present, as indicated by the incisement of these Neogene deposits by the modern Snake River. End_of_Article - Last_Page 518------------