Point-bar Deposits Research Articles

Facies variation in the Corallian Group (U. Jurassic) of the Faringdon-Shellingford area (Oxfordshire) and the rockground base to the Faringdon Sponge Gravels (L. Cretaceous)

We describe the sedimentology of four cores which passed down from the Aptian Faringdon Sponge Gravels into several formations of the Oxfordian Corallian Group. The facies of the Corallian Group in the cores differ somewhat from those seen at outcrop at Shellingford and Stanford Quarries: biosparite lenses are absent from the Coral Rag, no unit of the Urchin Marls is present though the Osmington Oolite is noticably muddier, as also is the Highworth Grit. At outcrop, the lower division (Shellingford Member) of the Highworth Grit is interpreted as deltaic distributary, point bar deposits which are truncated by an intraformational wedge of oolitic sands and clays floored by Diplocraterion . Intra-Corallian Group faulting is suggested between deposition of the Osmington Oolite and Coral Rag, with reactivation prior to cutting of the unconformity surface.

Abstract: Lateral Variability of Ichnofacies in Point-bar Deposits at Willapa Bay, Washington

Abstract: Lateral Variability of Ichnofacies in Point-bar Deposits at Willapa Bay, Washington

Channel avulsions and related processes, and large-scale sedimentation patterns since 1875, Rio Grande, San Luis Valley, Colorado

Analysis of a section of the Rio Grande in south-central Colorado using data from 1875 to the present shows that along four reaches relatively abrupt shifts of the channel to a new location (avulsions) have been common, but in five other reaches the channel has been laterally stable. In the unstable reaches, repeated avulsions have led to the development of subequally spaced, lozenge-shaped internodes composed of coarse-grained point-bar deposits and both coarse- and fine-grained abandoned-channel fill. In the stable nodes that separate the active internodes deposition has been minimal. In map view, the large-scale depositional geometry of these nodes and internodes is analogous to a string of lozenge-shaped, linked sausages, the links representing the nodes and the lozenges representing the internodes. Two end-member geometries could result from the aggradation of the nodes and internodes: (1) laterally swelling and thinning, mostly coarse-grained, channel-derived deposits (internodes) enclosed by fine-grained overbank sediments in systems dominated by overbank sedimentation, and (2) vertically and laterally amalgamated nodes and inter-nodes composed primarily of coarse-grained deposits in systems dominated by channel-derived sedimentation. Rio Grande mean annual discharge decreased by 60%–70% from 1875 to 1925 because of irrigation withdrawals upstream from the study area. From 1875 to the present the following changes have occurred: (1) Meander wavelength decreased from about 500 m to about 320 m. Empirical data from other studies suggest that this was probably a result of decreased discharge. (2) Sinuosity increased from about 1.2 to 1.7, and the number of two-channel reaches appears to have decreased, probably as a direct result of the avulsion process. During an avulsion, the new channel evolves from low to high sinuosity due to rapid meander growth at the same time that dis-charge shifts from the old to the new channel. As a result, early in an avulsion two channels of different sinuosity generally are present, but later a single high-sinuosity channel develops. Thus, the observed decrease in avulsion frequency may be responsible for the present high number of single channel, high sinuosity reaches. (3) The number of avulsions that occurred decreased from about 19 (1875–1941) to 2 (1941-present). Data are inadequate to show what, if any, relationship exists between the decrease in discharge and the change in avulsion frequency.

On the origin of non‐tidal flaser bedding in point bar deposits of the river Ajay, Bihar and West Bengal, NE India

ABSTRACTFlaser bedding of non‐tidal origin has been described from point bar deposits of the Ajay river, NE India. Mud drapes are deposited during waning flood phases. The preferential accumulation of curled, desiccated mud laminae in ripple troughs during periods of exposure produces mud streaks resembling classical flaser bedding. This process of non‐tidal flaser formation is unlike the common tidal origin related to flood and ebb cycles, and the less common mechanism of formation in fluvial systems by deposition of mud in ripple troughs after floods.

Exploration in the Gulf of Thailand in deltaic reservoirs, related to the Bongkot Field

Abstract Statoil has been involved in exploration in the Gulf of Thailand since 1985, following an invitation by PTTEP (National Exploration and Production Oil Company of Thailand) to assist in the evaluation and possible development of the Bongkot gas and condensate Field. Statoil and PTTEP jointly studied the field in 1985 and 1986. In 1990 PTTEP farmed 60% out to a group consisting of Total, British Gas and Statoil. Development work on the field started in 1990 with the acquisition of 3D seismic data. The field came on stream in June 1993, and production increased from the original planned 150 10 6 ft 2 /day to 350 10 6 ft 2 /day at present. In the joint PTTEP/Statoil study the Oligocene to Middle Miocene reservoirs were described as fluvial point bar sandstones, deposited in a meandering river system located on a delta top or coastal plain. Based on available data it was not possible to map the individual reservoirs. 22 wells found intermingled gas- and water-bearing sandstones without obvious relation to structure. No field fluid contacts were found, and the pressure regime was thought to be hydrostatic. Statoil simulated a reservoir model consisting of numerous pointbar deposits on a muddy delta plain. The pointbars were assumed to be isolated sandbodies which favoured stratigraphic trapping. Structural mapping of existing 2D and old 3D data sets was thought to give reliable structural maps, although it was difficult to correlate single events. Stratigraphic traps were interpreted as the main traps in the field because no relationship was found between structural closure and gas pools. Since 1990 when field development started with Total as operator around 2500 km 2 of 3D data has been acquired, and approximately 20 exploration wells and 45 development wells have been drilled. These data resulted in a much more accurate description of the field, especially through the use of high-resolution 3D seismic data. The well information allowed the identification of high GR intervals with a continuous seismic expression. Also individual sand reservoirs could be correlated. It became evident from the detailed 3D seismic interpretation that structural traps form the main trapping mechanism, although a minor part of the reserves are found in isolated channel sands, which could be detected from amplitude maps. It has been difficult to identify stratigraphic traps but it is hoped that ongoing seismic attribute analyses will help. In 1993 Statoil farmed in to licence B10/32, operated by Ampolex. The Bongkot model of isolated fluvial channels, with traps formed by several faults with or without structural closure was accepted as an attractive play type. Not withstanding this, the first wildcat well, Karawake-1 was a structural test. The well failed to find hydrocarbons because it intersected a series of overlapping alluvial fans; these lack internal seal and would have precluded hydrocarbon migration and entrapment. Ampolex made a reinterpretation of the block based on the existing 2D seismic, and concentrated on seismic facies mapping. The mapping resulted in two prospect areas where fluvial facies could be recognized above a lacustrine source rock; and also the results of the Karawake-1 well could be better explained. A second well should prove the model.

STORAGE OF MINING-RELATED ZINC IN FLOODPLAIN SEDIMENTS, BLUE RIVER, WISCONSIN

This study investigates spatial patterns of sediment-associated zinc (Zn) storage in floodplain deposits and the potential reintroduction of these metal-contaminated sediments to the active channel by bank erosion. We estimate patterns of Zn mass storage by combining longitudinal trends in Zn concentrations with measurements of sediment mass storage in overbank and point-bar deposits. Overbank deposits are the largest contaminant sink, storing five times more Zn than the point-bar deposits. While Zn concentrations decrease downstream because of dilution effects, the total mass of Zn stored in floodplains is greatest in both the upstream reaches and in the wider lower valleys where low channel gradients promote rapid sedimentation. Zn storage is low in middle reaches where steep, narrow valleys with high stream power favor sediment transport over deposition. Overall, more than half of the Zn released by mining remains stored in floodplain deposits within the watershed. The remobilization of Zn from storage is more likely in the upstream and mid-basin reaches where high stream power increases rates of lateral channel migration. Channels in the lower valley lack the stream power to migrate laterally and remobilize the large Zn mass stored in overbank sediments. [Key words: floodplains, Zn contamination, mining, stream power, Wisconsin.]

The evolution of fluvial systems in the Port Hood Formation (Upper Carboniferous), western Cape Breton Island, eastern Canada

The sandstones of the Port Hood Formation (Westphalian A) of western Cape Breton Island, Nova Scotia, eastern Canada have previously been interpreted, mainly from work on the upper part of the stratotype, to be the product of deposition from broad meandering rivers within a continental setting. Advances in methodology, namely the development of bounding surface hierarchies and interpretive architectural element analysis, allow for a reassessment of fluvial style in the formation as a whole. This has been accomplished through the detailed lateral profile analysis of coastal outcrops from various stratigraphic levels within the formation, six of which are presented herein. The first profile (P1), is from the base of the Port Hood Formation, and the sandstones are interpreted to be the result of ephemeral, very flashy, fluvial discharge in shallow, poorly confined channels of low sinuosity. The next three profiles, P2, P3, and P4, indicate that larger, perennial, low-sinuosity, braided channel systems, complete with large in-channel macroforms separated by major (4th order) bounding surfaces, persisted during deposition of overlying lower Port Hood Formation sandstones. Profiles from higher up in the formation (P5 from the ‘middle’, and P6 from the upper Port Hood Formation) again suggest the presence of in-channel bars rather than point-bar deposits, and hence a low-, rather than a high-sinuosity fluvial planform is proposed. However, sedimentary structures are by this stage more commonly indicative of deposition in the lower flow regime. Palaeocurrent data collected from throughout the Port Hood Formation in western Cape Breton Island further indicate that fluvial channels traversed a lower-gradient floodplain by the time `middle' and upper Port Hood Formation strata were deposited. Nevertheless, an overall southeasterly regional palaeoflow persists throughout the Port Hood Formation. This is more correlative with palaeocurrent directions obtained from Lower Carboniferous strata in the region, at a time when depocentres were localized in rapidly subsiding half-grabbes in a transtensional tectonic setting, than with data from other Upper Carboniferous strata (mostly of later Westphalian-Stephanian age), when a prevailing regional northeasterly palaeoflow developed over a large floodplain. Soft-sediment deformation in the fluvial sandstones can be explained by the rapid accretion that took place during basin filling but it might also have been a result of earthquakes that possibly accompanied graben maintenance.

Deposits of a rapidly eroding meandering river: Terrace cut and fill in the Taupo Volcanic Zone

Abstract River terraces in a section of the upper Rangitaiki River have formed in the last 1850 years as the river rapidly regrades following a major volcanic eruption (Taupo Tephra Formation). Terrace treads are cut at the migrating outside edge of meander loops and are subsequently filled by scroll and point bar deposits associated with the same loop. Entrenchment rates as high as 5 mm/yr cause the meanders to be permanently abandoned following cutoff. This results in the widespread preservation of fluvial cover sequences up to 6 m thick in an otherwise strongly erosive regime.

A Simulation Model for Meandering Rivers

A computer model for the dynamics of meandering rivers has been used to study the interplay between the migrating river and the changing sedimentary environment created by the meandering river itself. The model is based on the theory of Ikeda et al. [1981] and is closely related to that proposed by Howard [1983]. Coarser sands, which are often associated with high credibility, are deposited in the point bars formed when the river migrates away from its former bank. Fine‐grained material eventually fills the oxbow lakes created by cutoff processes and forms erosion‐resistant plugs. In the simulations, geometric forms of individual meanders observed in different natural sedimentary environments have been reproduced by changing the credibility of the corresponding sedimentary materials, such as point bar deposits, flood plain deposits, and oxbow lake deposits. The simulations indicate that the typical meander wavelength is determined mainly by hydraulic factors such as the flow in the channel and the inclination of the underlying flood plain and is independent of the difference in the credibilities of sedimentary deposits. The computational approach permits exploration of long‐term changes in the floodplain geology, mediated by the meandering river. As an initial demonstration, the formation of meander belts is investigated using the model. The results suggest that a meander belt will be formed by a river's own cutoff loops only if the characteristic time of deposition and solidification of an oxbow lake is longer than the typical time that it takes the river to migrate downstream over the distance of a meander‐loop wavelength.

Point bar deposits in narrow sharp bends

Meandering channels typically deposit sediments near their inner bank to form point bars. Laboratory experiments with narrow sharp bends show that under identical flow conditions the configuration of point bars depends on particle size. Explanation to this grain-size effect is found in a three-dimensional particle stability. Despite favorable secondary currents, coarse grains tend to move toward the thalweg and coarse-grained point bar deposits are eroded as the side-slope angle approaches the angle of repose of the bed material. Calculated results coincide with experimental observations. Point bars are considerably smaller for coarse-sand deposits than fine sands.

Accommodation-Based Controls on Fluvial-Deltaic Reservoir Compartmentalization: Examples from the Oligocene Frio Formation, South Texas: ABSTRACT

Accurate prediction of compartment architecture and intracompartment heterogeneity is necessary to locate and recover the estimated 15 billion barrels of mobile oil remaining in U.S. fluvial-dominated deltaic reservoirs. To improve this prediction, facies-specific relationships between accommodation trends and sand-body architecture that were established by outcrop studies in the western interior were Successfully applied in a study of Oligocene Frio Formation reservoirs along the Vicksburg Fault Zone of South Texas. Compartment architecture and internal heterogeneity were determined for mature upper and lower delta-plain reservoirs within two 4th-order genetic units to document reserve growth potential. In lower delta-plain settings, low-accommodation conditions at the base of genetic units produce narrow distributary channels that feed many small mouth bars, whereas high-accommodation conditions near the top of genetic units yield broader distributaries arid an expansive wave-dominated delta front. In proximal upper delta-plain settings, low accommodation produces few narrow fluvial channels that are commonly internally trough cross-strata predominate, with the finer grained silty upper point-bar deposits being removed by successive scour events during which base level has not risen substantially. In contrast, high accommodation yields many broad internally heterogeneous channels in which high and erosion of fine-grained channel-top facies. This study documents the profound effect thatmore » accommodation conditions can have suiting production behavior. Similar studies in other settings are needed to identify these relationships for the entire spectrum of depositional systems.« less

Transition from alluvial to coastal deposits (Permian–Triassic) on the Island of Mallorca, western Mediterranean

AbstractNon-marine redbeds (Permian and Triassic) in the island of Mallorca consist of a 0.4 km-thick alluvial succession that passes upwards into siliciclastic–carbonate coastal deposits. Tectonics and sealevel changes have been the main influences in their evolution. Low in this succession (the ‘Areniscas y lutitas de Port des Canonge’ unit) sandstone sheets with lateral accretion surfaces (macroscale inclined strata) and mudstones with frequent exposure structures are interpreted as the products of a sinuous alluvial system and floodplain. Climatic fluctuations are considered to be responsible for some significant up-section changes in the evolution of the alluvial deposits. Low-angle or horizontally stratified sandy units, interpreted as the result of flash floods, alternate upwards with point-bar deposits in the ‘Areniscas de Asá’. The hydrological response to minor climatic changes was evidently nearly instantaneous due to the lack of significant vegetation cover.During accumulation of mudstones and sandstones of the overlying ‘Lutitas y Areniscas de Son Serralta’ unit, the interpreted environment of deposition changed from a distal braidplain, mainly constructed by superposition of sandy bedforms with straight or linguoid crestlines in low sinuosity river channels, into a coastal plain with evidence of both continental and marine influences. The overlying carbonate platform (Muschelkalk) marks the development of a more homogeneous marine environment resulting from the Tethyan transgressive event that affected the whole peri-Mediterranean realm during the Anisian (middle Triassic).

Problems of dating alluvium using buried subfossil tree trunks: lessons from the 'black oaks' of the Vistula Valley, Central Europe

Dating of alluvia of large rivers based on subfossil tree trunks is discussed using examples from the Vistula Valley near Kraków and from literature referring to Central Europe. The method, which has been used hitherto for dating alluvia based exclusively on single trunks or single generations, has led usually to erroneous results because the majority of trunks in the alluvia were redeposited. Den drochronology is helpful in solving problems of dating alluvia, which should use only in situ trunks, i.e., trunks with bark, sapwood and branches. Equally suitable materials are stumps in growth position, which were cut down with an axe and which are preserved by an aggradating river. These stumps possess well-preserved root systems with roots of a lower order. Redeposition of trunks in alluvia is associated with lateral migration of a river channel. Channel migration leads to the washing out of subfossil trunks of different ages from older alluvia. These trunks, heavier than water, are redeposited at the level of channel lag. Simultaneously, waterside trees undermined by the river accumulate at the water level in the upper parts of point bar deposits. This results in one series of channel deposits with two layers of trunks ('a sequence of simulate aggradation'). In recent centuries, after deforestation of floodplains, this mechanism lead to the formation of a single level of black oaks at the level of channel lag. Nevertheless, the spatial pattern of redeposited trunks may be helpful in reconstruction of the direction of palaeochannel migration.

Block hydraulic conductivity of cross‐bedded fluvial sediments

This paper concerns the three‐dimensional block hydraulic conductivity of blocks of limited size in cross‐bedded sediments of meandering rivers. Based on a simplified sedimentological model of a single point bar, a stochastic model is proposed to simulate the spatial distribution of core scale conductivities within point bar deposits. The required parameters for these simulations are obtained from actual hydraulic conductivity measurements and sedimentological field data. The simulations are then used to calculate the three‐dimensional block conductivity tensor with a numerical upscaling procedure. The stochastic simulation model is used to investigate the influence of slope and aspect angle of the cross‐beds and the core scale anisotropy ratio on the block hydraulic conductivity. It is expected that the off‐diagonal elements of the block conductivity tensor generally cannot be neglected. The core scale anisotropy ratio, which reflects the small scale layering within the cross‐beds, can potentially have a significant impact on the elements of the block conductivity tensor.

Flow structure and development of circular meander pools

Flow structure, bank strength, and scour and fill in one of a series of atypical river bends on Mansfield Creek, Cattaraugus County, New York are studied. These bends are characterized by large, deep, circular pools located at the apices of extremely tight bends. The pattern of flow through the bends is typified by the presence of a large countercurrent occupying approximately half of the pool on the outside of the bend. This differs greatly from the typical river bend in which flow is generally downstream and subparallel to the channel banks throughout the bend. Deposition in the bend also does not follow the conventional pattern, but instead occurs primarily on the concave bank in the form of a concave bank bench. Erosion is concentrated on the convex bank and no point bar deposits exist. Shifting of erosion to the convex bank has been noted elsewhere in the literature in extremely tight bends with ratios of mean radius to mean width of less than 2.0, but not in association with circular meander pools. Several possible theories are suggested that may explain development of the circular meander pools at this site. It is likely that the high, cohesive banks of the study reach are an important factor, slowing channel migration and preventing cutoff chutes from forming, allowing development of highly sinuous meanders.

Twin Morrow Field: A case study

Upper Morrow sandstones in the Texas panhandle are often laterally discontinuous and small. Exploration and development of these sandstones can be precarious due to the rapid change from reservoir quality sandstone to shale. Successful incorporation of seismic data into reservoir delineation and characterization of the Upper Morrow sandstones could increase the success ratio and thereby enhance the economic feasibility of this type of exploration and development venture. Therefore, in 1989, Unocal shot a high resolution vibroseis line across Twin Morrow Field to analyze the seismic response of the productive Upper Morrow Sandstone, locally known as the Buckhaults Sandstone. The producing reservoir is an interpreted point bar deposit, three miles long and one mile wide with a maximum thickness of 55 ft. A reflector which is correlatable to the Buckhaults is present on the high resolution seismic data, but this sandstone is too thin for thickness resolution with current seismic methods.

Reservoir Facies Architecture of Two Upper Point Bar Deposits: ABSTRACT

Reservoir Facies Architecture of Two Upper Point Bar Deposits: ABSTRACT

Stratigraphy, palaeontology and palaeoenvironment of the Wessex Formation (Wealden Group, Lower Cretaceous) at Yaverland, Isle of Wight, southern England

The section at Yaverland in Sandown Bay exposes over 50 m of sediments belonging to the upper part of the Lower Cretaceous Wessex Formation. These are described and a new stratigraphie section is presented. The sequence is dominated by alluvial silty muds, with subordinate point bar deposits, possible crevasse splay sands and organic-rich pond or lake fills. The latter contain life assemblages of non-marine bivalves as well as storm-generated beds of allochthonous plant and vertebrate material. The palaeontology of these fossiliferous units is reviewed. Dinosaur footprints and footcasts occur at several levels throughout the succession and are attributable to iguanodontids and theropods, as well as sauropods or ankylosaurs. These sometimes form parts of extensive trampled surfaces.

Liquefaction-Induced Ground Damage in Dagupan in the July 16, 1990 Luzon Earthquake

ABSTRACT Geotechnical aspects of the ground damage caused by widespread occurrence of liquefaction at the time of the 1990 Luzon, Philippines earthquake are highlighted by examining the geomorphological regime and boring data obtained in the city area of Dagupan. The comparison of old and present locations of the river courses indicates that the liquefaction in the city of Dagupan had taken place in two geomorphologically different areas: namely, the terrain of naturally buried old river channels and the area of point bar deposits. Although impossible to distinguish soil profile characteristics between these two types of terrains, the common feature is that there exists a surface layer composed of unliquefiable silty soils, underlain by sand deposits which are loose enough to provoke liquefaction. As a result of simple analysis of liquefaction, it was shown that the upper part of this sand deposits developed liquefaction, leading to the disastrous consequences in the city of Dagupan. By compiling a number of SPT data and the data of Swedish cone penetration tests, an attempt was made to clarify the relationship between the thickness of unliquefiable surface layer and that of the underlying liquefied layer which can or can not bring about surface manifestation of liquefaction, accompanied by disastrous damage on the ground surface. Lastly, the lateral spreads of the ground near the riverside were analyzed by considering the soil profiles established by the Swedish cone tests. The residual strength determined by back-calculation at three typical cross sections indicated a general coincidence with those estimated previously for other data sets.

The Relationship Between Lithofacies and Contaminant Migration Pathways at a Site Contaminated with Heavy Metals

ABSTRACT Lithofacies distribution is the primary influence on the migration of an acidic, heavy metal plume of contaminated groundwater at a site in southeast Louisiana. The site, located north of Lake Pontchartrain, is situated on the youngest of the Pleistocene terraces. The presume source of contamination is an inactive hazardous waste impoundment formerly used to store spent acid from a battery recycling operation. Another potential source, located approximately 300 southeast of the impoundment, is a battery waste pile which has since been removed. Sulfuric acid has mobilized heavy metals such as arsenic, cadmium, chromium, and lead derived from onsite smelting operations. Heavy metals, sulfates, and low pH (acidic) groundwater characterize the nature of the subsurface contamination. Geotechnical borings at the site reveal that the shallow stratigraphy (0-30) is composed primarily of point bar deposits. A fluvial depositional model has been developed in this study to explain the distribution of the various lithofacies. The main depocenters associated with point bar deposits are on the inside (concave) bend of the meander. These deposits pinch out laterally into flood basin clay deposits. Groundwater contaminants, due to intrinsic porosity and permeability characteristics, are concentrated primarily in the coarser grained point bar deposits. Potentiometric maps reveal that the direction of groundwater flow is controlled primarily by the distribution of point bar deposits. Pinchout of sand in a northerly direction causes groundwater flow vectors to shift from a predominantly northerly to a westerly direction where the pinchout occurs. End_of_Record - Last_Page 519-------