Point-bar Deposits Research Articles

Paleoenvironmental reconstruction of the Lower Mogi Guaçu River Basin (São Paulo State — Brazil), morphopedosedimentary records and fluvial processes

Abstract The Mogi Guacu River rises in Bom Repouso (Minas Gerais State — Brazil) in the Mantiqueira Ridge, and flows into the Rio Pardo river at an elevation of 483 m in Pontal (Sao Paulo State — Brazil), after running a 530 km long course. Especially along the Lower Mogi Guacu Basin, the river morphology is extremely sinuous, characterized by intense processes of channel migration, avulsion, abandonment and reactivation of the channel, producing an extensive alluvial plain composed of a series of associated relief forms and sedimentary facies. Among these forms, point bar deposits by lateral accretion, abandoned meanders, paleo-channels and fluvial terraces are notable features. In this sense, the objective of this work was to investigate whether these features could be linked to environmental changes. To reach this goal, soil properties of a catena of the Jatai Ecological Station (Luiz Antonio — Sao Paulo State) were analyzed in four sectors: Slope, Terrace I, Terrace II and Alluvial Plain. The results from grain-size determination, geochemical and isotopic studies, dating, paleopalynology, coal fragments and micromorphology are presented in this paper. From these analyses, a paleo-environmental evolution divided in three stages is proposed for the area: 130,000 YBP (Upper Pleistocene), when the Mogi Guacu River base level was approximately 6 m above the present one; a drier second phase 10,250 years BP (Lower Holocene), when an organic horizon was formed inside of an abandoned meander (oxbow lake), and a third phase, 2096 YBP (Upper Holocene), of reactivation of a warm and humid climate that promoted the development of a two meters thick Typic Udifluvent in a sector where the Mogi Guacu River no more floods due to the incision of its thalweg, reaching more than six-meters depth in the last 130,000 years BP. Thus, this paper used a fluvial geomorphologic approach and its interplay with climate to understand how the landscape was shaped from Upper Pleistocene to Holocene, however, Neotectonics might have played a relevant role as well, not only in the Mogi Guacu River Basin, but also in the Parana Sedimentary Basin.

Fluvial trace fossils in the Middle Siwalik (Sarmatian-Pontian) of Darjeeling Himalayas, India

Trace fossils that record animal and plant activity are described for the first time from the Middle Siwalik, Neogene deposits of Darjeeling Himalaya. Sedimentary facies association attests to a channel–interchannel floodplain fluviatile setting. The intimate association of the burrows with phytoliths, rhizoliths, leaf compressions and coal lenses suggest that the tracemakers dominated a floodplain habitat. Point bar deposits host a low diversity Planolites-Naktodemasis-Macanopsis-Cylindricum equilibrium ichnocoenosis in the heterolithic fine sandstone-siltstone-shale facies that alternates with dense, monospecific colonization of Planolites as opportunistic pioneers relocating under stressed condition. Interlayered floodplain deposits in the fluvial successions preserve enigmatic large diameter, vertical tubes within thin to thick-bedded, dark silty shale facies. These tubes bear mixed characters assignable to both crayfish burrows and large-diameter rhizoliths. Further work on these tubes is necessary to make more accurate interpretations of those structures. Shallow to moderate burrow depths; intermittent, short-lived colonization events and preservation of rhizoliths and rhizohalos under fluctuating moisture content indicate short-term fluctuations of a relatively high water table (close to the paleosurface) in an imperfectly drained proximal floodplain setting. Ichnotaxa distribution and their inferred ethology provide significant faunal data that may put constraints on the reconstruction of Middle Siwalik depositional environment.

Mediative adjustment of river dynamics: The role of chute channels in tropical sand-bed meandering rivers

This paper examines processes of chute channel formation in four tropical sand-bed meandering rivers; the Strickland and Ok Tedi in Papua New Guinea, the Beni in Bolivia, and the lower Paraguay on the Paraguay/Argentina border. Empirical planform analyses highlight an association between meander bend widening and chute initiation that is consistent with recent physics-based modelling work. GIS analyses indicate that bend widening may be driven by a variety of mechanisms, including scour and cutbank bench formation at sharply-curving bends, point bar erosion due to cutbank impingement against cohesive terrace material, rapid cutbank erosion at rapidly extending bends, and spontaneous mid-channel bar formation. Chute channel initiation is observed to be predominantly associated with two of these widening mechanisms; i) an imbalance between cutbank erosion and point bar deposition associated with rapid bend extension, and ii) bank erosion forced by spontaneous mid-channel bar development. The work extends previous empirical analyses, which highlighted the role of bend extension (elongation) in driving chute initiation, with the observation that the frequency of chute initiation increases once bend extension rates and/or widening ratios exceed a reach-scale threshold. A temporal pattern of increased chute initiation frequency on the Ok Tedi, in response to channel steepening and mid-channel bar development following the addition of mine tailings, mirrors the inter- and intra-reach spatial patterns of chute initiation frequency on the Paraguay, Strickland and Beni Rivers, where increased stream power and sediment load are associated with increased bend extension and chute initiation rates. The process of chute formation is shown to be rate-dependent, and the threshold values of bend extension and widening ratio for chute initiation are shown to scale with measures of river energy, reminiscent of slope–ratio thresholds in river avulsion. Furthermore, Delft3D simulations suggest that chute formation can exert negative feedback on shear stress and bank erosion in the adjacent mainstem bifurcate, such that the process of chute formation may also be rate-limiting. Chute formation is activated iteratively in space and time in response to changes in river energy (and sediment load), predominantly affecting sites of rapid channel elongation, and thereby mediating the river response.

SAGD well orientation in point bar oil sand deposit affects performance

The creation and evolution of point bar reservoir systems are well understood in meandering river deposits. A large fraction of Athabasca oil sand deposits are ancient point bar systems characterized by bedded, sandstone-dominated strata with interbedded siltstone layers. The recovery process of choice for these deposits is the Steam-Assisted Gravity Drainage (SAGD) process due to the high viscosity of the oil, low solution-gas ratio, and often caps rock not sufficient to withstand injection pressures of Cyclic Steam Stimulation (CSS). However, because of the presence of siltstone interbeds, these reservoirs commonly have lateral and vertical lithological heterogeneity which interfere with the formation of uniform steam chambers along SAGD wellpairs. Other units in point bar deposits that impact SAGD chamber development within the formation include remnant channel succession and channel lag. The objective of this research is to construct a detailed three-dimensional point bar model to determine how its heterogeneity impacts SAGD performance. Here, the point bar model is based on the Lower Cretaceous Middle McMurray Formation in the Athabasca oil sand deposit in Alberta, Canada. Single SAGD wellpair submodels at different locations and orientations were extracted from the detailed point bar model. The results of the reservoir model simulation suggest that attention must be paid to SAGD wellpair placement in point bar systems.

The Role of Hydrologic Alteration and Riparian Vegetation Dynamics in Channel Evolution along the Lower Minnesota River

<abstract><title>Abstract.</title> The Minnesota River carries the largest load of sediment to the Mississippi River in Minnesota, most of which comes from channel sources. This study investigates bank retreat in the lower Minnesota River since 1938. Specifically we asked, How have changes to river form influenced sediment transport and deposition in the lower Minnesota River and how did hydrological and ecological processes affect channel change? It was hypothesized that channel straightening, reduction in floodplain access, and streamflow increases contribute to increased channel-derived sediment load and decreased point bar deposition. Secondly, it was hypothesized that hydrologic changes have reduced woody riparian vegetation on sandbars, further promoting channel widening. To quantify channel sediment and phosphorus loading rates in the lower Minnesota River, we analyzed historic aerial photos for evidence of channel change, we performed long-term monitoring of erosion and deposition rates within the river corridor, and we calculated channel sediment transport rates. Results from this study showed that the Minnesota River has widened by 52% between Mankato and St. Paul since 1938, on average contributing 280,000 Mg of sediment per year and 153 Mg total phosphorus. The river also shortened by 7% since 1938, increasing bankfull shear stress and stream power. Sediment deposition rates in the floodplain have increased since European settlement by an order of magnitude. Ecohydrological studies showed that establishment of woody riparian plants has been inhibited on sandbars by prolonged summer flow duration and scour at high flow, reducing potential point bar growth. Findings from this study will be useful in prioritizing sediment and vegetation management actions.

Elemental chemistry of sand-boil discharge used to trace variable pathways of seepage beneath levees during the 2011 Mississippi River flood

Shortly after peak stage of the 2011 Mississippi River flood, water samples were collected from the river, from sand boils near the toe of the levee, and from actively flowing relief wells over a 55 km stretch north of Vicksburg, MS. Two distinct pathways for seepage under the levee were identified based on the elemental composition of water samples. Sand boil discharge was similar to water from relief wells only at a location where the levee sits on ancient channel fill deposits. Seepage at this site is forced along a deeper pathway beneath the fine-grained channel fill. Where the levee sits on sandy point bar deposits, shallow flow beneath the levee is unimpeded. The chemical composition of discharge from sand boils at these sites was clearly not just river water, nor a simple mixture of river and groundwater, but appears to reflect unique weathering or redox interactions occurring within the upper portion of the alluvial aquifer. Distinguishing shallow and deep seepage pathways may prove useful for evaluating site specific risk of levee failure.

Analysis of Heavy Oil Recovery by Thermal EOR in a Meander Belt: From Geological to Reservoir Modeling

The objectives of this work is to assess the impact of reservoir heterogeneities on heavy oil recovery of a reservoir analogue of meander belt through the Steam Assisted Gravity Drainage (SAGD) process by using numerical models. These models are obtained with different scales of upscaling of the geological model. Meander belts consist of point bar deposits, characterized by a 3D complex internal architecture, with different scales of heterogeneities, which distribution is associated with the depositional processes. Based on a 3D outcrop description of a meander belt analogue to the Canadian heavy-oil fields, the approach includes three steps: 1) the construction of a reference static reservoir model based on a very fine description of the outcrops in terms of architecture and geological heterogeneities, 2) upscaling of the grid at different scales using different upscaling factors in order to evaluate their impact on the heterogeneity distribution in the reservoir, 3) reservoir SAGD simulations using horizontal well doublet (steam injector and producer) across the meander belt, so as to assess the impact of upscaling of heterogeneities on heavy oil production. The impact of heterogeneities on simulation results are evaluated for several upscaling stages. Results show that heterogeneity distribution has an impact on fluid flow at different stages of production. On the fine gridded model, small scale heterogeneities impact the steam chamber development and fluid flow in the wellbore vicinity at the beginning of the steam injection, whereas large scale heterogeneities strongly influence oil recovery during the whole recovery process and lower the efficiency of the reservoir drainage. On coarser grids, the effect of small-scale heterogeneities can be diminished, depending on the upscaling stage. The geomechanical effect is not taken into account in this work, the objective being to assess the impact of heterogeneities on oil recovery. The performance of SAGD is clearly linked to the steam chamber development, which depends on the degree of heterogeneities present in the reservoir. The simulation workflow and the sensitivity study on the upscaling method contribute to a better restoration of the heterogeneity distribution in the reservoir. The negative effect of these heterogeneities during the oil recovery must thus be quantified in order to monitor the thermal production at crucial periods of the production process.

Chute channel dynamics in large, sand‐bed meandering rivers

ABSTRACTMeander bends of many large, sand‐bed meandering rivers are partitioned by chute channels that convey permanent flow, and co‐exist with the mainstem for decades. As a first step toward understanding the dynamics and morphodynamic implications of these ‘bifurcate meander bends’, this study applied binary logistic regression analysis to determine whether it is possible to predict chute initiation based on attributes of meander bend character and dynamics. Regression models developed for the Strickland River, Papua New Guinea, the lower Paraguay River, Paraguay/Argentina, and the Beni River, Bolivia, revealed that the probability of chute initiation at a meander bend is a function of the bend extension rate (the rate at which a bend elongates in a direction perpendicular to the valley axis trend). Image analyses of all rivers and field observations from the Strickland suggest that the majority of chute channels form during scroll–slough development. Rapid extension is shown to favour chute initiation by breaking the continuity of point bar deposition and vegetation encroachment at the inner bank, resulting in widely‐spaced scrolls with intervening sloughs that are positively aligned with primary over‐bar flow. The rivers plot in order of increasing chute activity on an empirical meandering‐braided pattern continuum defined by potential specific stream power (ωpv) and bedload calibre (D50). Increasing stream power is considered to result in higher bend extension rates, with implications for chute initiation. In addition, chute stability is shown to depend on river sediment load relative to flow discharge (Qs/Q), such that while the Beni may plot in the region of highly braided rivers by virtue of a high potential specific stream power, the formation of stable chute channels is suppressed by the high sediment load. This tendency is consistent with previous experimental studies, and results in a planform that is transitional between single‐thread meandering and braided. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Seismic geomorphology and high-resolution seismic stratigraphy of inner-shelf fluvial, estuarine, deltaic, and marine sequences, Gulf of Thailand

Pleistocene fluvial, estuarine, marine, and deltaic depositional systems were identified in the uppermost 80 m (262 ft) of the central Gulf of Thailand modern continental shelf, situated approximately 70 m (230 ft) below sea level. Integration of offshore three-dimensional (3-D) seismic reflection data, high-resolution shallow-penetration two-dimensional (2-D) seismic reflection sparker and boomer profiles, and shallow geotechnical borehole measurements enabled the identification of seven depositional sequences. The 3-D plan-view images at successive time slices exhibit single meandering channels (as much as 600 m [1969 ft] wide) and channel belts (as much as 10 km [6.2 mi] wide) deposited in the shelf during times of subaerial exposure. Additional geomorphic features imaged include incised valleys, interfluves, oxbow lakes, neck and chute cutoffs, and point-bar meander scrolls showing evidence of expansion and translation. The high-resolution 2-D profiles, with a tuning thickness of approximately 25 cm (9.8 in.), enabled the discrimination of high-frequency stratigraphic discontinuities (sequence boundaries) and allowed a detailed bed-scale seismic facies characterization of fluvial (point bars), deltaic (clinoforms), estuarine, and marine deposits within a sequence-stratigraphic context. The complete succession shows that most fluvial systems lie within incised valleys in the lower parts of each depositional sequence, fluvial channels show a degradational stacking pattern, and no evidence of fluvial aggradation is observed; aggradation is limited to hemipelagic sedimentation during marine incursions. A shallow (35 m [115 ft]) single-story incised valley was described in detail, placing particular emphasis on the recognition criteria and the controls on valley formation and preservation potential of different systems tracts in an inner-shelf location. The 3-D characterization of this system allowed differentiation of sand-prone point-bar deposits and mud-prone abandonment channel facies. The sinuous but continuous mud-filled channel may act as a lateral muddy barrier or baffle that can potentially subdivide a reservoir system into discrete compartments.

Recognition of down-valley translation in tidally influenced meandering fluvial deposits, Athabasca Oil Sands (Cretaceous), Alberta, Canada

The vast Athabasca Oil Sands of Alberta, Canada has an estimated resource of more than 1.7 trillion barrels of bitumen in-place, the majority of which is hosted in the Lower Cretaceous McMurray Formation. Despite its economical significance the depositional environment of the formation, and particularly the middle part that is the primary reservoir in most areas, is still a matter of considerable debate. These strata of interest are widely known to comprise fluvial and estuarine point bar deposits that were subject to varying degrees of marine influence. The orientation of point bar strata from the formation is tabulated and the majority is observed to dip paleo-basinward, to the north. This observation has proven difficult to explain by geologists attempting to build predictive models for the formation over the last two decades. However, the basinward-dipping point bar strata can be attributed to widespread down-valley translation of point bars in confined north-south oriented valleys, which have previously been delineated in the region. Differentiating the deposits of lateral point bar migration and down-valley translation in the rock record is not possible at the facies scale, and thus translated point bars have not been previously recognized or interpreted in the rock record, despite their prevalence in numerous modern fluvial valley systems. Their identification in the McMurray Formation has important implications for the delineation and development of Canada’s economically significant oil sands resources.

Fluvial architecture of Belgian river systems in contrasting environments: implications for reconstructing the sedimentation history

AbstractAccurate dating is necessary to get insight in the temporal variations in sediment deposition in floodplains. The interpretation of such dates is however dependent on the fluvial architecture of the floodplain. In this study we discuss the fluvial architecture of three contrasting Belgian catchments (Dijle, Geul and Amblève catchment) and how this influences the dating possibilities of net floodplain sediment storage. Although vertical aggradation occurred in all three floodplains during the last part of the Holocene, they differ in the importance of lateral accretion and vertical aggradation during the entire Holocene. Holocene floodplain aggradation is the dominant process in the Dijle catchment. Lateral reworking of the floodplain sediments by river meandering was limited to a part of the floodplain, resulting in stacked point bar deposits. The fluvial architecture allows identifying vertical aggradation without erosional hiatuses. Results show that trends in vertical floodplain aggradation in the Dijle catchment are mainly related to land use changes. In the other two catchments, lateral reworking was the dominant process, and channel lag and point bar deposits occur over the entire floodplain width. Here, tracers were used to date the sediment dynamics: lead from metal mining in the Geul and iron slag from ironworks in the Amblève catchment. These methods allow the identification of two or three discrete periods, but their spatial extent and variations is identified in a continuous way. The fluvial architecture and the limitation in dating with tracers hampered the identification of dominant environmental changes for sediment dynamics in both catchments. Dating methods which provide only discrete point information, like radiocarbon or OSL dating, are best suited for fluvial systems which contain continuous aggradation profiles. Spatially more continuous dating methods, e.g. through the use of tracers, allow to reconstruct past surfaces and allow to reconstruct reworked parts of the floodplain. As such they allow a better reconstruction of past sedimentation rates in systems with important lateral reworking.

Cyclicity in Lower Cretaceous point bar deposits with implications for reservoir characterization, Athabasca Oil Sands, Alberta, Canada

The Lower Cretaceous McMurray Formation comprises the majority of the Athabasca Oil Sands deposit, which is one of the largest heavy oil accumulations in the world. Point bar deposits account for a significant proportion of the subsurface reservoir, and are characterized by bedded, sandstone-dominated strata. In-situ development of heavy oil is sensitive to lateral and vertical lithological heterogeneity in the form of interbedded siltstone beds, which are locally common in the formation and must be taken into account in order to understand permeability distribution within the reservoir. The objectives of this research are to integrate qualitative sedimentological observations with quantitative analyses of wireline log data in order to constrain the distribution of siltstone interbeds within the point bar deposit, to use this information to understand the deposition history of the fluvial system, and to provide insight into reservoir modeling. Quantitative statistical analyses of wireline log data, including autocorrelation analysis, Fourier transform analysis, and continuous wavelet transform analysis, reveal lithological cyclicity within the heterogeneous point bar sediments examined. Lithological cyclicity is evident at wavelengths ranging from 2 to 10 m, and generally increases in strength toward the downstream direction of the ancient point bar deposit where more heterogeneous sediments are prevalent. Cyclicity also increases in amplitude in the youngest layers of the laterally accreted point bar deposit; the overall finer-grained sediments correspond to decreasing energy levels and waning flow over the evolution of the fluvial system. Overall, the highest amplitude cycles were calculated at a wavelength of 3.5 m, and, when correlated to core observations, are represented as interbedded packages of thick, massive to cross-bedded sandstone that fine upward to siltstone. The control on cyclic sedimentation is difficult to discern from Cretaceous-aged point bar deposits. Through consideration of sedimentation rates from analogous modern fluvial systems, the smallest cycles observed might be attributed to seasonal variations in climate and associated flow fluctuation. However, it is more likely that the cyclicity recorded is linked to longer scale climatic signals, such as El-Niño Southern Oscillation (ENSO-like) fluctuations or variations in solar output (decadal cyclicity). The mappable variations in siltstone bed frequency and thickness across the ancient point bar deposit have implications for heavy oil development. Low permeability siltstone beds associated with the shortest wavelength cycles (i.e., < 2 m cycles) have limited impact on fluid flow within the reservoir; however, those associated with cycles longer than 4 m in wavelength may serve to compartmentalize portions of the reservoir, leading to reduced oil recovery.

Subsurface and outcrop characterization of large tidally influenced point bars of the Cretaceous McMurray Formation (Alberta, Canada)

Outcrop and subsurface investigations highlight that the main reservoirs within the Cretaceous McMurray Formation, Alberta, Canada, were located in an ancient estuary and that the major economic targets are represented by large point bar deposits. The underlined morphological characteristics of the associated McMurray-valley drainage show morphometric similarity to modern river systems like the Mississippi River. Hydrological parameters estimated by some geomorphologic data suggest that a paleodischarge of about 15,000m3/s (including the tidal prism) and a paleomeander migration rate of between approximately 30m/year and 65m/year characterized the McMurray.However, tidal influence and marginal-marine affinity are evidenced by abundant erosion and reactivation surfaces, mud clast breccia deposits, cleaner channel sands and bioturbated heterolithic sands interfingered with cleaner channel sands. Thus, internal structure observed on the Steepbank River outcrops appears to be more complex than the usual bi-partite system composed of cross-stratified sands at the base and inclined heterolithic stratified sands upward. The highlighted internal structures of large tidally influenced point bars of the McMurray Formation are quite similar to compound dune deposits in which inclined heterolithic sands (IHS) are interfingered with clean sands. These deposits are also associated with ichnological associations characterizing a brackish environment for the McMurray-valley drainage.A modern analog of the tidally influenced point bar deposits of the McMurray Formation is the meandering tidally influenced channels of the Garonne River (located in the Aquitaine basin, SW France). Facies described in cores acquired in both areas are very similar, which allows a possible comparison of these point bars, in spite of the size difference between rivers. The Garonne river point bar is located 95km from estuary mouth near the city of Bordeaux. In this area, tidal currents are the dominant dynamic agent and can reach velocities up to 2m/s. In the point bar core, the facies of the IHS exhibit 3 typical sedimentary structures: mud clast breccias more abundant at the base of the channel, heterolithic stratified sands recording semi-lunar cycles (up to 28 tides) and thick clay layers at the top of the channel. In the overbank deposits, classic tidal bedding recording the semi-diurnal and semi-lunar cycles is observed. Levees and crevasse splays are absent.This study aims to provide a depositional model and geometric framework for tidally influenced point bar reservoirs.

Determination of Hydraulic Conductivity of Unconsolidated River Alluvium from Permeameter Tests, Empirical Formulas and Statistical Parameters Effect Analysis

Hydraulic conductivity is an important parameter for the investigation of flow processes through streambeds. This paper presents a comparison of hydraulic conductivity estimates of unconsolidated sandy samples collected at four sites of the Nestos River point-bar deposits, Greece. Values of K were determined using laboratory permeameter tests as well as the widely applied empirical formulas of Hazen, Slichter, Terzaghi, Beyer, Sauerbrei, Krueger, Kozeny, Zunker, Zamarin, USBR, Alyamani and Sen, Shepherd, and Loudon, based on the grain-size distribution. In a statistical regression analysis, the relation between several graphical statistical grain size parameters and the determined K values, and the effect of these parameters on K estimates was investigated. For this study site it was found that the empirical formula of Loudon provided the most reliable K estimates for the upper 0.25 m of the streambed.

Sedimentology and stratigraphic architecture of a point bar deposit, Lower Cretaceous McMurray Formation, Alberta, Canada

Abstract A single channel and point bar deposit are examined in the subsurface of northeastern Alberta from the Lower Cretaceous McMurray Formation. High-quality 3-D seismic, core and wireline log data were used in order to constrain the stratigraphic architecture and the facies distribution. Mudstone-dominated abandoned channel fill provides a mold of the 32–36 m deep paleo-channel; seismic time slices reveal that these channels were 500–584 m wide. The associated point bar deposit ranges in thickness from 30–40 m. Seven facies are identified in the point bar strata studied, including massive sandstone with siltstone rip-up clasts at the base of the point bar sequence, cross-stratified sandstone that is common in upstream locations on the point bar, various interbedded sandstone and siltstone units that comprise IHS packages, and siltstone. Reservoir quality in the McMurray Formation is directly related to numerous factors, including the overall thickness of bar deposits, and the distribution of siltstone beds that can act as barriers or baffles to fluid flow. The overall thickness of the point bar deposit studied is greatest where increased scour around the apex of the paleo-channel bend resulted in increased accommodation; amalgamated sandstone units are up to 23–30 m thick in these areas. Stratigraphic cross-sections and mapping of lithological trends across the point bar deposit demonstrate that siltstone beds increase in abundance upward through the section, and in downstream reaches of the point bar. The average cumulative thickness of siltstone beds increases from 18 cm to 67 cm in the downstream direction, which corresponds to a decrease in siltstone bed frequency. Small-scale upward-fining packages that range from 5–12 m thick are locally observed within the point bar deposit, and are interpreted to represent erosion and reactivation following storm events or episodes of meander-bend rotation.

Alluvial stratigraphy and geoarchaeology in the Big Fork River Valley, Minnesota: human response to Late Holocene environmental change

Abstract The Late Quaternary geomorphology and stratigraphy of the Big Fork River valley, within the Rainy River basin of northern Minnesota, reveals evidence of prehistoric human interaction with late Holocene riverine environments. By 11 000 14 C BP, deglaciation made the region inhabitable by human groups using Clovis artefacts. Human habitation would also have been possible during the Moorhead low-water stage of glacial Lake Agassiz, starting at 10 500 14 C BP. Near its confluence with the Rainy River, the valley floor of the Big Fork valley consists of a floodplain complex and two terraces. The multi-component stratified Hannaford site is situated within the active floodplain. Overbank deposits contain artefacts in primary context, whereas artefacts within the point bar deposits are in secondary archaeological context; these deposits are associated with changing alluvial settings as the river moved eastward. Aggradation of the valley fill beneath the lowest surface (T 0 , floodplain complex) began by 3000 years ago and is associated with human activities focused on seasonal fishing and the use of riparian resources from 1300 to 650 14 C BP.

Sedimentary Architecture in Meanders of a Submarine Channel: Detailed Study of the Present Congo Turbidite Channel (Zaiango Project)

Abstract Sinuous deep-water channels are recognized in most large deep-sea fans in the world. They present a particular interest to oil companies, since they are significant hydrocarbon reservoirs in deep offshore environments. The understanding of their geometries and their internal sedimentary architecture is necessary to better characterize reservoir heterogeneity of sinuous submarine channels. Therefore, numerous studies have been undertaken recently to better understand the behavior and sedimentary architecture of deep-water channels. The aim of this paper is to present our results concerning the development of the meandering channel of the present Congo turbidite system (or Zaire turbidite system). The study is based on high-resolution data including multibeam bathymetry, seismic lines, echosounder profiles, high-resolution side-scan sonar images, and gravity cores, collected by IFREMER along the submarine Congo channel between 1994 and 2000, during Guiness and ZaiAngo surveys. The present Congo turbidite channel is a long incised turbidite channel. It is presently active. It has been built gradually by progradation of the distal depositional area. The most distal part of the channel is the youngest part and shows an immature morphology: the channel presents a low incision and a low sinuosity. In contrast, the upper part of the channel has undergone a long evolutionary history. Its pathway is mature and complex, with numerous abandoned meanders visible in the morphology. This paper presents evidence of progressive channel migration and meander development of the Congo channel. It describes and explains the presence of terraces inside the channel. The detailed characterization of channel morphology and migration geometry shows that the evolution of the channel path is very similar to fluvial meandering systems with (1) lateral meander extension or growing, (2) downstream translation of the thalweg, and (3) meander cutoff. Seismic and 3.5 kHz echosounder profiles show that the terraces, which are visible in the seafloor morphology, are not the imprints of incisional processes. Terraces are true depositional units infilling the channel. They are built during and after the lateral migration of the channel. They are composed of (1) point-bar deposits and (2) inner-levee deposits aggrading above the point bar deposits. Point-bar deposits are characterized by low-angle oblique reflectors forming deposits with a sigmoidal shape. They seem very similar to those observed in fluvial systems. The similarity between fluvial and turbidite point bars suggests that the basal part of the turbidity currents flowing in this channel can be considered as very similar to river flow. With the high-resolution dataset collected in a present Congo turbidite channel, we provide a new description of the channel morphology and evolution, at a “reservoir” scale, intermediate between outcrop observations and 2D and 3D seismic data. The detailed interpretation of intrachannel sedimentation, associated with lateral channel migration, also provides new data for interpretation of flow dynamics in submarine meandering channels.

Sedimentological and Palyno-environmental appraisal of the late quaternary sediments, north-eastern Bornu Basin

An integrative study has been carried out on some Quaternary sediments of the Chad Formation in Tuma-1 and Sa-1 wells. They are located between 11°N and 13° 43′ 38″N; 8° 21′ 49″and 14° 40′ 22″E. Sedimentological study entailed textural analysis and thin section preparation on the disaggregated samples, while the palynological study involves the recovering of the age diagnostic species which gave clues to the paleoenvironmental settings for these sediments.The sediments consist predominantly of terrigenous and argillaceous mudstone with thin sandstone interbeds of point bar deposits of laterally migrating, moderate to high sinuosity river channels. The dominant sandstones are angular to subrounded, moderately poor sorted, fine modal mineralogically and texturally immature. The presence of Botryococcus braunii, freshwater algae, suggests a freshwater-lacustrine environment.

Three-Dimensional Connectivity of Point-Bar Deposits

Abstract The geometry of heterogeneities within fluvial channel-belt deposits is predicted using an existing model of flow and sediment transport in river channel-meander bends. The thickest and coarsest-grained sediment accumulations are deposited near a channel-bend apex, and finer-grained sediments accumulate higher on the downstream end of a channel bar. Shale drapes deposited on individual beds during low flows are preserved mostly on downstream-dipping surfaces in finer-grained bar-top deposits. Extensive finer-grained deposits also accumulate in concave-bank areas in the lee of point bars when meanders migrate downstream. The coarsest deposits occur as elongate bodies parallel to the channel-belt axis when channel bends migrate mostly by downstream translation and are more circular when channel bends simply increase in sinuosity. The character of deposits preserved in concave-bank areas of the channel, the style of channel cutoff and filling during abandonment, and the amount of channel-belt aggradation during bar migration influence whether coarser-grained bodies of adjacent bar deposits are well connected. This 3D gridded model allows reservoir simulation studies that predict effects of types of deposit heterogeneity on patterns of subsurface flow through reservoirs and aquifers. Subsurface flow simulations suggest that channel belts with coarser-grained abandoned fills and significant vertical aggradation during bar migration constitute reservoirs with appreciably higher recovery factors than those with finer-grained channel abandoned fills and no vertical aggradation. Model results highlight the need to better understand connections between different types of facies associations in fluvial channel belts to improve predictions of fluvial-reservoir behavior.

Recognition and significance of sharp-based mouth-bar deposits in the Eocene Green River Formation, Uinta Basin, Utah

Sandstone bodies in the Sunnyside Delta Interval of the Eocene Green River Formation, Uinta Basin, previously considered as point bars formed in meandering rivers and other types of fluvial bars, are herein interpreted as delta mouth-bar deposits. The sandstone bodies have been examined in a 2300 m long cliff section along the Argyle and Nine Mile Canyons at the southern margin of the Uinta lake basin. The sandstone bodies occur in three stratigraphic intervals, separated by lacustrine mudstone and limestone. Together these stratigraphic intervals form a regressive-transgressive sequence. Individual sandstone bodies are texturally sharp-based towards mudstone substratum. In proximal parts, the mouth-bar deposits only contain sandstone, whereas in frontal and lateral positions mudstone drapes separate mouth-bar clinothems. The clinothems pass gradually into greenish-grey lacustrine mudstone at their toes. Horizontally bedded or laminated lacustrine mudstone onlaps the convex-upward sandstone bars. The mouth-bar deposits are connected to terminal distributary channel deposits. Together, these mouth-bar/channel sandstone bodies accumulated from unidirectional jet flow during three stages of delta advance, separated by lacustrine flooding intervals. Key criteria to distinguish the mouth-bar deposits from fluvial point bar deposits are: (i) geometry; (ii) bounding contacts; (iii) internal structure; (iv) palaeocurrent orientations; and (v) the genetic association of the deposits with lacustrine mudstone and limestone.