Bar Deposits Research Articles

APPLICATIONS OF MODERN AND ANCIENT GEOLOGICAL ANALOGUES IN CHARACTERISATION OF FLUVIAL AND FLUVIAL-LACUSTRINE DELTAIC RESERVOIRS IN THE COOPER BASIN

Reservoir characterisation in fluvial and fluvial- lacustrine delta successions is enhanced by the use of appropriate modern and ancient analogues to understand subsurface reservoir architecture and to help build appropriately scaled reservoir models. Two case studies of reservoir characterisation in the Cooper Basin are used to illustrate the value of analogues. Firstly the Late Permian Toolachee Formation crevasse splay reservoirs of the Cooper Basin, southwest Queensland are outlined, and analogues from the Ob River in Western Siberia illustrate the relative scale of crevasse splay deposits within avulsion belts in a cool-temperate peat-forming environment. The South Blackwater coal mine in the Permian Bowen Basin is used as an analogue to quantify the 3D geometry and reservoir architecture of crevasse splays and to highlight subsurface reservoir heterogeneity.Secondly, the Early Permian Epsilon Formation shallow water lacustrine delta reservoirs are outlined, and analogues from the extant geometry of the distributary channels and relict mouth bar deposits from the fluvial dominated Neales Delta in Lake Eyre are used to interpret flow rate decline trends and probable reservoir architecture. The subsurface Tertiary lacustrine deltaic complex of the Sirikit Field from the Phitsanulok Basin, central Thailand, is selected as an ancient analogue for the multistorey reservoirs developed within amalgamated mouth bar complexes intersected in the lower Epsilon Formation.

O-243 浅海域における砂の集積過程とストームによる再動作用 : 四国南西部中新統三崎層群の例

O-243 浅海域における砂の集積過程とストームによる再動作用 : 四国南西部中新統三崎層群の例

Architecture of a tide‐influenced river delta in the Frontier Formation of central Wyoming, USA

The Frewens sandstone is composed of two elongate tide‐influenced sandstone bodies that are positioned directly above and slightly landward of a more wave‐influenced lobate sandstone. The 20‐km‐long, 3‐km‐wide Frewens sandstone bodies coarsen upwards and fine away from their axes, have gradational bases and margins and have eroded tops abruptly overlain by marine shales. These sandstones are superbly exposed in large cliffs on the banks of the South Fork of the Powder River in central Wyoming, USA. The deposits change upwards from thinly interbedded sandstones and mudstones to metre‐thick heterolithic cross‐strata and, finally, to metres‐thick sandstone‐dominated cross‐strata. There is abundant evidence for tidal modulation of depositional flows; however, palaeocurrents were strongly ebb‐dominated and nearly parallel the trend of sandstone‐body elongation. Detailed mapping of stratal geometry and facies across these exposures shows a complex internal architecture. Large‐scale bedding units within sandstone bodies are defined by alternations in facies, bed thickness and the abundance of shales. Such bedsets are inclined (5°–15°) in walls oriented parallel to palaeoflow and gradually decrease in dip over hundreds of metres as they extend from the sandstone‐dominated deposits higher in a sandstone body to muddier deposits lower in the body. Where viewed perpendicular to palaeoflow, bedsets are 100‐metre‐wide lenses that shingle off the sandstone‐body axis towards its margins. The sandstone bodies are interpreted as sand ridge deposits formed on the shoreface of a tide‐influenced river delta. Metres‐thick cross‐strata in the upper parts of sandstone bodies resemble deposits of bars (sandwaves) formed where tidal currents moved across shallows and the tops of tidal ridges. Heterolithic deposits lower in sandstone bodies record fluctuating currents caused by ebb and flood tides and varying river discharge. Erosion surfaces capping sandstone bodies record tidal ravinement. The tidal ridges were abandoned following transgression and covered with marine mud as waters deepened.

An ice-contact rhythmite (turbidite) succession deposited during the November 1996 catastrophic outburst flood (jökulhlaup), Skeiðarárjökull, Iceland

Abstract This paper presents new evidence of coarse-grained deposition from a turbulent suspension within a low-energy `slack-water' location during a recent well-documented, volcanically related, high-magnitude glacier outburst flood or jokulhlaup, Skeiðararjokull, Iceland. This study uses established spatial and temporal constraints on processes observed during the November 1996 jokulhlaup to interpret resultant flood sediments. Coarse-grained sediments were deposited within flood slack-water conditions from suspension load by repeated turbulent flow pulses with durations of seconds–minutes. Depositional processes are thought to be analogous to turbidity currents. A minimum thickness of 15 m of rhythmites record suspended sediment dynamics at a conduit mouth during the late rising and early falling stages of the jokulhlaup. The morphology and sedimentology of deposits in a large ice-walled chamber are consistent with previous models proposed for bar deposition during cataclysmic floods. Flood flows within subglacial conduits and ice-walled open channels were sufficiently powerful to carry material up to boulder size in suspension. Macro-turbulent flows carried huge quantities of suspended sediment to high levels within the main outlet channel. High-frequency, short-duration flow pulses entering a complex ice-walled channel geometry generated highly unsteady and non-uniform flows capable of both erosional and depositional work. This study lends support to the deposition of multiple rhythmites per flood under main flow slack-water conditions. Identification of high-magnitude jokulhlaups in the ice-marginal sedimentary record will depend on the identification and correct interpretation of feeder channel and slack-water turbidite sediments. The results described here may also have application to rhythmic deposits formed by large turbulent flows in other environmental settings.

DEPOSITIONS. FACES IN LATE CRETACEOUS-?LOWER TERTIARY DEPOSITS FROM NORTHWESTERN MARANHAO STATE, BRAZIL

Late Cretaceous-?Lower Tertiary deposits from northwestern Maranhao State (northern Brazil) were investigated using facies analysis in outcrops in combination with subsurface data provided from gamma-ray profiles. Four facies associations were recognized in this area and attributed to: a) mouth bar (facies association 1); b) braided fluvial/distributary channel (facies association 2); c) distal mouth bar/crevasse lobe (facies association 3); and d) interdistributary bay/prodelta (facies association 4). Mouth bar deposits consist of well-sorted, fine- to very-fine grained sandstone with a variety of sedimentary structures, mostly including: convolute lamination; overturned foresets; sigmoidal, trough, and tabular cross stratifications; and massive bedding. Growth faults and slumpings are common features associated with these deposits. Braided fluvial/distributary channel deposits are characterized by poorly-sorted, coarse-grained cross-stratified sandstone with sharp, locally concave-up, basal surfaces mantled by lags. Distal mouth bar/crevasse lobe deposits consist of a succession of strata formed by coarsening upward cycles represented by massive and cross-laminated sandstones that are interbedded with mudstones containing abundant plant remains. Finally, interdistributary bay/prodelta deposits include mudstones with abundant plant remains and streaks of silt and very-fine grained sandstones. The four facies associations described above characterize depositional environments typical of delta settings, an interpretation consistent with the funnel-shaped gamma-ray patterns observed in well logs. Correlation of these deposits with a wave-dominated estuarine system (i.e., Cujupe Formation) exposed in the eastern of Sao Luis Basin led to attribute them to a bayhead delta complex located in the innermost portion of this estuary. The sedimentological and stratigraphic data presented in this paper support the continuity of the Cujupe estuarine system southward of the Ferrer-Urbano Santos Arch. These results thus suggest a paleogeographic reconstruction in which the Sao Luis and Grajau basins behaved as a unique basin at least from the late Cretaceous on.

Unusual coarse, clastic, wave-dominated shoreface deposits, Pliocene to middle Pleistocene, northern Chile; implications for coastal facies analysis

ABSTRACT Pliocene to mid-Pleistocene shallow-marine strata exposed at Gatico, northern Chile (22° 30 S, 70° 14 W) display unusual characteristics for coarse-grained wave-dominated shoreface deposits. A 15 m2000 m strike section of Pliocene to mid-Pleistocene strata exposed within small, late Pleistocene sea cliffs contain four facies: beach, foreshore channel, upper shoreface, and rip channel. Beach and upper shoreface facies predominate. The latter consists of locally bioturbated, coarse-grained, pebbly sandstone with (1) single sets of planar to tangential cross-strata, up to 3.5 m thick, with reactivation surfaces, and (2) sets of medium-to large-scale cross-strata that were deposited on inclined set surfaces (up to 15°). Some of the inclined set surfaces extend up-dip of cross-set terminations. Inclined parallel laminae and bioturbated beds lie subparallel to the inclined set surfaces. Interpretation of these unusual bedforms has been made through analogy with the present-day microtidal wave-dominated coastline at Gatico, by facies analysis, and by comparison with modern-day wave-dominated shorelines. The large-scale sets of cross-strata record the migration of simple, two-dimensional (2-D) dunes and represent the slipface deposits of nearshore bars. Parallel laminae represent the bar surface. Medium-scale dunes migrated up the inclined set-bounding surfaces, which lie subparallel to older preserved bar surfaces. Paleocurrent data indicate that bar migration was largely directed obliquely onshore towards the southeast. The bar slipface deposits are considered to record the recovery of a nearshore bar immediately following storm conditions. Bar recovery involves re-establishment of a slipface and rapid onshore migration. Erosion surfaces that appear as the extension of lower set boundaries beyond the termination of cross-strata sets, and reactivation surfaces, are considered to be storm-generated. The episodic nature of bar migration is indicated by the development of heavily bioturbated horizons. The small-scale ascending cross-sets may reflect fair-weather conditions with northwestward dune migration up the modified bar slipface. Despite the abundance of barred shorelines around the world at present, their deposits are not widely described from the rock record, suggesting that they are either not preserved or not recognized. The scale and internal characteristics of these nearshore bars closely resemble those of some tide-generated bedforms (e.g., compound cross-strata in marine sediment with bimodal-bipolar paleocurrents and evidence of episodic bedform migration). The similarity of these north Chilean deposits to documented tidal deposits suggests that an element of caution may be required in the identification of tidal deposits from the rock record.

Geology and Stratigraphy of Fluvio-Deltaic Deposits in the Ivishak Formation: Applications for Development of Prudhoe Bay Field, Alaska

Significant remaining reserves in Prudhoe Bay field are confined within deltaic rocks at the base of the Triassic Ivishak sandstone. The initial stratigraphic characterization of the Prudhoe Bay reservoir was lithostratigraphically based, and it depicted this basal reservoir interval as tabular zones between marine shale and overlying coarse-grained, fluvial sandstones. A reassessment of this interval based on cores and genetic-stratigraphic correlations depicts en echelon, offlapping, fluvially dominated deltaic wedges. Reservoir-quality rocks occur in distributary mouth bar, distributary channel, and fluvial facies associations. A paleogeographic reconstruction of one delta lobe includes an alluvial plain crossed by channels of possibly braided or low-sinuosity rivers. This alluvial plain graded into a delta plain cut by distributary channels that fed distributary mouth bars on a broad delta front. River dominance is inferred from the abundance of unidirectional current structures, normally graded beds, soft-sediment deformation, and general absence of wave-formed, tidal, and biogenic structures. Slumping and growth faulting locally replaced coarsening-upward deltaic successions with sharp-based, overthickened mouth bar and distributary channel deposits. Mudstones deposited following delta-lobe abandonment form laterally extensive flow barriers between lobes. Compartmentalization is most pronounced distally, where deltaic sandstones are overlain by and pass laterally into marine shale. Proximally, fluvial and deltaic sandstones are juxtaposed across erosional contacts, improving reservoir continuity. This stratigraphic interpretation is corroborated by production and surveillance data plus an interference test. Locally, stratigraphy and poor waterflood performance reflect completions in diachronous sandstones that originated in separate deltaic lobes. Previously, poor well performances were attributed to sandstone pinch-outs. In some cases, production can be enhanced with recompletions rather than infill drilling. Nonconventional wells planned and completed with the benefit of detailed facies-association correlations currently are recovering millions of barrels of previously bypassed oil.

Depositional facies in Late Cretaceous-?Early Tertiary deposits in northwestern Maranhão, Brazil

Late Cretaceous-?Lower Tertiary deposits from northwestern Maranhao State (northern Brazil) were investigated using facies analysis in outcrops in combination with subsurface data provided from gamma-ray profiles. Four facies associations were recognized in this area and attributed to: a) mouth bar (facies association 1); b) braided fluvial/distributary channel (facies association 2); c) distal mouth bar/crevasse lobe (facies association 3); and d) interdistributary bay/prodelta (facies association 4). Mouth bar deposits consist of well-sorted, fine- to very-fine grained sandstone with a variety of sedimentary structures, mostly including: convolute lamination; overturned foresets; sigmoidal, trough, and tabular cross stratifications; and massive bedding. Growth faults and slumpings are common features associated with these deposits. Braided fluvial/distributary channel deposits are characterized by poorly-sorted, coarse-grained cross-stratified sandstone with sharp, locally concave-up, basal surfaces mantled by lags. Distal mouth bar/crevasse lobe deposits consist of a succession of strata formed by coarsening upward cycles represented by massive and cross-laminated sandstones that are interbedded with mudstones containing abundant plant remains. Finally, interdistributary bay/prodelta deposits include mudstones with abundant plant remains and streaks of silt and very-fine grained sandstones. The four facies associations described above characterize depositional environments typical of delta settings, an interpretation consistent with the funnel-shaped gamma-ray patterns observed in well logs. Correlation of these deposits with a wave-dominated estuarine system (i.e., Cujupe Formation) exposed in the eastern of Sao Luis Basin led to attribute them to a bayhead delta complex located in the innermost portion of this estuary. The sedimentological and stratigraphic data presented in this paper support the continuity of the Cujupe estuarine system southward of the Ferrer-Urbano Santos Arch. These results thus suggest a paleogeographic reconstruction in which the Sao Luis and Grajau basins behaved as a unique basin at least from the late Cretaceous on.

Recent, arctic deltaic sedimentation: Olivier Islands, Mackenzie Delta, North‐west Territories, Canada

Despite a low tidal range and relatively low wave conditions, the Mackenzie Delta is not prograding seaward but rather is undergoing transgressive shoreface erosion and drowning of distributary channel mouths. In the Olivier Islands region of the Mackenzie Delta the resultant morphology consists of a network of primary and secondary channels separated by vegetated islands. New ground is formed through channel infilling and landward‐directed bar accretion. This sedimentation is characterized by seven sedimentary facies: (1) hard, cohesive silty clay at the base of primary channels which may be related to earlier, offshore deposition; (2) ripple laminated sand beds, believed to be channel‐fill deposits; (3) ripple laminated sand and silt, interpreted as flood‐stage subaqueous bar deposits; (4) ripple laminated or wavy bedded sand, silt and clay, representing the abandonment phase of channel‐fill deposits and lateral subaqueous bar deposition from suspension settling; (5) a well sorted very fine sand bed, presumed to result from a single storm event; (6) parallel or wavy beds of rooted silt, sand and clay, interpreted as lower energy emergent bar deposits; and (7) parallel or wavy beds of rooted silt and clay, believed to represent present‐day subaerial bar aggradation. The distribution of sedimentary facies can be interpreted in terms of the morphological evolution of the study area. Initial bar deposition of facies 3 and channel deposition of facies 2 was followed by lateral and upstream bar sedimentation of facies 3 and 4 which culminated with the deposition of the storm bed of facies 5. Facies 6 and 7 signify bar stabilization and abandonment.Patterned ground formed by thermal contraction and preserved in sediments as small, v‐shaped sand wedges provides the most direct sedimentological indicator of the arctic climate. However, winter ice and permafrost also govern the stratigraphic development of interchannel and channel‐mouth deposits. Ice cover confines flow at primary channel mouths, promoting the bypassing of sediments across the delta front during peak discharge in the spring. Permafrost minimizes consolidation subsidence and accommodation in the nearshore, further enhancing sediment bypass. Storms limit the seaward extent of bar development and promote a distinctive pattern of upstream and lateral island growth. The effects of these controls are reflected in the vertical distribution of facies in the Olivier Islands. The sedimentary succession differs markedly from that of a low‐latitude delta.

Origins of massive-type sandstones in braided river systems

This study details largely ignored massive-type, predominantly structureless sandstones preserved within braided fluvial successions of Carboniferous to Triassic age. Architectural element analysis reveals that these sediments were deposited within sand-dominated perennial systems of low braiding index. Cross-stratified braid bar deposits are interbedded with, and laterally equivalent to geometrically distinct, largely structureless massive-type sandbodies identified as two separate architectural elements: channel-like (SMC) and sheet-like (SMS). Sub-divisions within these broad categories define six geometric units which are texturally distinct from each other and from the structured sediments of the same lithological unit. Since massive-type sandstone elements have many features in common with the deposits of highly concentrated, laminar sediment/water flows, they are interpreted in terms of similar depositional processes. SMC elements form elongate channel-like features which trend both at high angles to, and parallel with, the palaeoflow of host fluvial channels. The lower bounding surfaces of SMC elements may be either erosive or non-erosive, and describe symmetrical cross-sections with margins dipping <50°. Concentric laminae are preserved parallel to the scour margins which grade into a structureless sandstone fill. Diffuse laminae and water escape structures are commonly preserved in the upper portion of these elements, which are interpreted as the deposits of sandy debris flows related to fluvial bank and/or bar collapse. SMS elements form sandsheets up to 8 m in thickness which may be traced >250 m parallel and transverse to the fluvial palaeoflow direction established from cross-stratified sandstones of adjacent architectural elements. The basal surface of SMS elements may either be undulose (where the sandbodies are termed SMSU) or erosional (where they are termed SMSE). Internally SMSU elements preserve parallel laminae marginal to basal scours, diffuse sweeping laminae, isolated cross-sets and water escape structures. The SMSU sediments are compositionally identical to the structured sandstones with which they are associated and are interpreted as the result of deposition from highly concentrated stream flows developed through flooding and entrainment of sandy floodplain material. Along the basal surface of SMSE elements scours in excess of 4.5 m are preserved which are elongate both parallel and oblique to fluvial flow. These are commonly lined with a mudclast breccia and/or massive sandstone with crude laminae developed parallel with the margin. In the upper portion of SMSE elements diffuse dune structures and sweeping laminae are preserved. SMSE units are texturally and compositionally immature with respect to the cross-stratified sandstones with which they are associated. Thus, they are believed to be of first cycle origin and are interpreted as the deposits of highly concentrated flows possibly generated by glacial outburst floods or jökulhaups.

Exchanges of sediment between the flood plain and channel of the Amazon River in Brazil

Sediment transport through the Brazilian sector of the Amazon River valley, a distance of 2010 km, involves exchanges between the channel and the flood plain that in each direction exceed the annual flux of sediment out of the river at Obidos (∼1200 Mt yr −1 ). The exchanges occur through bank erosion, bar deposition, settling from diffuse overbank flow, and sedimentation in flood-plain channels. We estimated the magnitude of these exchanges for each of 10 reaches of the valley, and combined them with calculations of sediment transport into and out of the reaches based on sediment sampling and flow records to define a sediment budget for each reach. Residuals in the sediment budget of a reach include errors of estimation and erosion or deposition within the channel. The annual supply of sediment entering the channel from bank erosion was estimated to average 1570 Mt yr −1 (1.3 × the Obidos flux) and the amount transferred from channel transport to the bars (380 Mt yr −1 ) and the flood plain (460 Mt yr −1 in channelized flow; 1230 Mt yr −1 in diffuse overbank flow) totaled 2070 Mt yr −1 (1.7 × the Obidos flux). Thus, deposition on the bars and flood plain exceeded bank erosion by 500 Mt yr −1 over a 10–16 yr period. Sampling and calculation of sediment loads in the channel indicate a net accumulation in the valley floor of approximately 200 Mt yr −1 over 16 yr, crudely validating the process-based calculations of the sediment budget, which in turn illuminate the physical controls on each exchange process. Another 300–400 Mt yr −1 are deposited in a delta plain downstream of Obidos. The components of the sediment budget reflect hydrologic characteristics of the valley floor and geomorphic characteristics of the channel and flood plain, which in turn are influenced by tectonic features of the Amazon structural trough.

PROSPECTIVITY OF THE OFFSHORE SYDNEY BASIN: A NEW PERSPECTIVE

Approximately 40 per cent of the 52,000 km2 Sydney Basin lies in shallow waters (less than 200 m) off the central New South Wales coast. Containing more than 5,000 m of Permo-Triassic marine and non-marine sediments, and having been the subject of several previous exploration campaigns, no wells have been drilled in the offshore despite widespread numerous occurrences of oil and gas onshore.The Sydney Basin, together with the Bowen and Gunnedah basins, form a major longitudinal Permo-Triassic basinal complex stretching 2,500 km down the eastern margin of Australia. Whereas the onset of this basinal development may have been extensional, reinterpretation of seismic and other geophysical data highlight the potential role played in the early development of the Sydney Basin by easterly directed compression. A compressional style is to be contrasted with the dominantly extensional style interpreted by others for the adjacent onshore areas. The most conspicuous structural element in the offshore, the Offshore Uplift, is interpreted to represent the western overthrust edge of the Currarong Orogen. Accepting the Panthalassan margin geometry of Veevers and Powell (1994) it follows that the Offshore Uplift and restored Dampier Ridge would have constituted a 'greater Currarong Orogen'. A series of progressive westerly directed thrust fronts may have been established across the Panthalassan margin, including the uplifted western margin of the Currarong Orogen, which over-rode and created a thrust load onto the eastern margin of the Lachlan Fold Belt. Much of the Early Permian development of the Sydney Basin therefore could have resulted as a consequence of foreland loading. This is consistent with depositional trends including the overall westerly directed marine transgression which dominated the sedimentary record of the Early Permian. Alternatively, this marine transgression may represent the sag phase induced along a segment of the Bowen-Sydney rift system that had been offset by the Hunter River Transverse Zone from the Gunnedah Basin to a site coincident with the Offshore Syncline.Previous interpretations identified structural development of the Currarong Orogen as either a Cretaceous (Tasman Sea rift related) or Middle to Late Permian phenomena. Early Permian structural growth of the offshore Uplift has important implications for petroleum exploration. The major impediment to exploration appears to be the perception that the Sydney Basin lacks suitable reservoir targets and is gas-prone. Potential source and seal sequences occur extensively within both Early Permian marine shales and siltstones and Early and Late Permian coal measure sequences. The emerging uplift provided a major sediment provenance area and represented a barrier behind which restricted anoxic conditions flourished, conditions favouring the preservation of organic matter. Late Permian and Triassic sequences are absent across the crestal portions of the uplift. However, the emerging, sea-ward facing flank of the uplift would have been subject to marginal and shallow marine, wave-base, barrier and strand bar deposition during the Lower Permian, conditions known in the onshore to favour better reservoir development.Gas demand to the greater Sydney region is anticipated to exceed supply by the year 2000, and new gas markets are being eagerly sought in time for the expiration, in 2006, of the current contract under which gas is supplied to Sydney via the Moomba pipeline.Cretaceous, Tasman Sea rift related, structuring is subordinate to that of the earlier compressional and wrench related structuring. Several new structural targets have been added to the existing inventory of prospects and leads, including some now considered optiminally located with respect to source rock and reservoir development.

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.

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.

Fácies sedimentares da Formação Rio Claro, neocenozóico da depressão periférica paulista

Os dados das fácies sedimentares da Formação Rio Claro aqui apresentados resultam de pesquisa enfocando os depósitos neocenozóicos da porção centro-leste da Depressão Periférica Paulista, a qual objetivou a compreensão da evolução geológica da região. Os sedimentos neocenozóicos são delgados e descontínuos, sua interpretação e classificação requereram uma abordagem levando em conta, também, aspectos geomórficos (relações com níveis planálticos), além dos conceitos litoestratigráficos usuais. Foram identificados cinco níveis planálticos de extensão regional, sendo' que os dois mais elevados ocorrem somente nas províncias geomorfológicas do Planalto Atlântico e Cuestas Basálticas (limítrofes com a Depressão Periférica), e os três mais jovens e rebaixados ocorrem dentro dos limites da Depressão Periférica. A associação de depósitos rudáceos e couraças ferruginosas com estes níveis planálticos é sugestiva de que eles correspondam a pediplanos elaborados em fases de climas secos. A Formação Rio Claro foi reconhecida na área do platô de Rio Claro, onde é mais contínua, e na borda leste da Depressão Periférica, com ocorrências descontínuas; ocorre sobre os dois níveis planálticos principais que nivelam as colinas da região; é constituida por depósitos de sistema fluvial meandrante, formados sob clima úmido, agrupados em quatro litofácies principais: lamitos de processos gravitacionais; cascalhos e areias de canais e barras fluviais; areias finas de rompimento de diques marginais; e argilas de transbordamento em planície de inundação. Na área do platô de Rio Claro a sedimentação foi controlada pela reativação neocenozóica de falhas com movimentação vertical na estrutura de Pitanga, alto estrutural ativo durante o Mesozóico, situado a jusante do sítio deposicional. Nas ocorrências da borda leste da Depressão Periférica, a sedimentação está mais claramente associada com barramentos litológicos da drenagem (soleiras e diques de diabásio)

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.

Modeling spatial and volumetric changes in the Atchafalaya delta, Louisiana

Abstract The Atchafalaya River has two outlets: the Wax Lake Outlet and Lower Reach. The Lower Reach empties into Atchafalaya Bay approximately 200 km west of the modern bird‐foot delta of the Mississippi River, while the Wax Lake Outlet empties into Atchafalaya Bay about 18 km further west. Historically, the last significant episode of sedimentation occurred in the Atchafalaya Bay area about 1,500 years ago. Between 1962 and 1972 the appearance of the first subaerial exposures of a new delta lobe began with a rapid expansion of the delta front environment, and progradation of a distal bar. Following waning flows of the 1975 flood events, an extensive network of distributary mouth bar deposits and complex branching of seaward extending distributaries emerged. Changes in the volume and areal extent of sediment issuing from the two mouths of the Atchafalaya River were examined using NASA's ELAS and Delta Data System's AGIS image processing and Geographic Information System software. CAMS (Calibrated Airborn...

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.

Sedimentological studies on the central Wadi Kalabsha kaolin deposits, Southwest of Aswan, Egypt

Kaolin deposits initially formed as a lateritic crust on a nearby basement complex, then were transported to the central Wadi Kalabsha and deposited as flood plain and river bar deposits. Repeated fluvial influx and drying out of the kaolin resulted in the following facies: a) Intraformational disorganized conglomerate over the flood plain and/or on the flat bar-tops; b) Intraformational partially organized and organized conglomerates on the slip faces of bars and river banks, and/or as lateral point bars in sinuous streams; and c) Pisolites of kaolinite, kaolinite-hematite, hematite-kaolinite and hematite composition, either in erosion hollows and/or on the slip faces at the downstream end of lognitudinal and diagonal bars in braided rivers. The surface of the fluvial kaolin deposits was subjected to a second phase of in situ lateritization which resulted in a partial or complete dissolution of quartz and the formation of pisolitic laterites containing traces of gibbsite, geothite, hematite and anatase. Isotopic analyses of δ18O and δD of separated kaolinites showed that the pisolitic kaolin deposits are sedimentary and weathering in origin. The original reworked sedimentary-lateritic pisolites may be deposited during the late Mesozoic while the pisolitic laterites may be formed later as a crust in the Tertiary.