Downstream Accretion Research Articles

Facies analysis, architectural elements, and paleoenvironmental reconstruction of alluvial deposits of the low terraces and floodplains in the Middle Sebou river (Eastern Saïss foreland basin, Morocco)

This paper focuses on the study of the floodplain and low terrace deposits of the Middle Sebou Valley, which crosses the south-rif corridor and constitutes the eastern limit of the Saïss Basin. The study includes the identification of facies, the determination of architectural elements, the reconstruction of paleoenvironments, and the proposal of a model for deposit formation. The work is based on detailed sedimentological analysis of the deposits, focusing on lithology, grain size, primary sedimentary structures, and the types of contact between facies. Facies analysis identified 21 facies grouped into three associations corresponding to nine architectural elements whose identification was based on the study of their lateral and vertical profiles. The architectural elements identified include the channel (CH), gravel bars and bedforms (GB), sandy bedforms (SB), downstream accretion (DA), lateral accretion (LA), sediment gravity flows (SG), natural levee (LV), floodplain (FF), and abandoned channels and meanders (FF (CH)). The alluvium is organized into rhythmic and fining-up elementary sequences reflecting its deposition in different fluvial environments: alluvial fan, straight to slightly sinuous and braiding-tending fluvial environment, and meandering fluvial environment. Alluvial deposition is controlled by lithology, climate, and tectonics. Lithology and tectonics controlled the slope, the degree of valley confinement, the accommodation space, and the alluvial fan development. Climate controlled the flow regime, sediment dynamics, and the formation of paleosol and calcrete.

Soft X-Ray Excess from Shocked Black Hole Accretion

A certain class of luminous galaxies hosting supermassive black holes (BHs) is known to exhibit an excess of “soft X-rays” in their X-ray spectra. However, its physical identity and origin are yet to be understood. In this work, we systematically study a sample of 9 well-documented narrow-line Seyfert 1 active galactic nuclei by utilizing the archival XMM-Newton/EPIC spectra in the context of general relativistic magnetohydrodynamic model. In this scenario, thermal accretion disk photons (in UV) are Compton up-scattered by nonthermal energetic electrons in the hot downstream accretion due to shock compression, producing the observed soft excess. Our spectral model consists primarily of the shock Comptonization component and the underlying continuum including reflection from the accretion disk. Based on χ 2-statistics, we successfully constrain the model parameters, most notably electron energy of the downstream flow kT e , effective disk blackbody temperature kT bb, and inclination angle θ for a given BH spin. The disk temperature is commonly found to be kT bb 10 eV and the electron energy ranges from kT e 75–160 keV (for Schwarzschild BHs) to 126–232 keV (for Kerr BHs) depending on inclination. Our analyses imply that the characteristics of the observed soft excess are strongly dependent on the properties of the downstream accretion flow and BH spin.

Sedimentary Characteristics and Internal Architecture of a Sandy Braided River: Insights from a Numerical Model

Sandy braided river deposits are widely observed in ancient stratigraphic records and modern sedimentary basins. Frequent channel migration shapes intricate internal architecture, while research on its influence on fine sedimentary architecture remains limited. This study utilizes DELFT3D to simulate and analyze the sedimentary processes and architecture of the channel bar in a sandy braided river. The results show that: (1) The channel bar consists of downstream accretion (DA), lateral accretion (LA), and aggradational accretion (AA). (2) Channel bars can be divided into three types: unit bars, compound bars, and complex compound bars. Unit bars are lobate-shaped and consist of multiple accretions, including DA in the core, AA layered above, and LA deposited on the sides. Compound bars result from the migration and amalgamation of multiple unit bars, characterized by a greater number of accretions. Compound bars can further be classified as DA-dominated or LA-dominated, depending on the dominant accretion type. Complex compound bars form from the merging of multiple unit bars and compound bars. In this paper, insights from a braided river on its sedimentary processes and evolution may help to improve river management and water resources research. The in-depth understanding of sedimentary architecture on channel bars can be utilized for detailed subsurface oil and gas reservoir characterization.

A comprehensive view of sedimentation and erosion processes in small reservoirs: A case study of the Pocheň reservoir, Czech Republic

This article focuses on the need to improve current knowledge of sedimentation and erosion processes in small reservoirs on the example of the Pocheň reservoir (Czech Republic). We combined data from sediment cores, with unmanned aerial vehicle imaging and ground-penetrating radar survey. The results show the distinct sediment distribution and the gradient in the grain-size, petrophysical properties, and geochemistry. Sediment thickness increases in the proximal-to-distal direction (from the inflow to the dam) with the frequent erosion near the inflow, especially during floods. It contributed to the sediment relocation towards distal sites with the tendency to downstream accretion. After reservoir flushing, sedimentation processes were replaced by drawdown-enhanced erosion. Considerable erosion occurred along a temporary drawdown channel, resulting in headward erosion and channel incision. Lateral erosion resulted in drawdown-channel widening. The most extensive lateral erosion took place in the middle part of the reservoir, where the channel created a complex multichannel-branched system.

Architecture Elements of the Lemat Formation of the Lubuk Bernai Region, Batang Asam District, Tanjung Jabung Barat Regency, Jambi Province

The study of facies analysis and interpretation of the depositional environment of the Lemat Formation located at Bukit Tiga Puluh aims to clarify the problems found in Paleogene sediments in the South Sumatra Basin. A detailed analysis of the fluvial facies has been carried out in the Lubuk Bernai track. A number of observations, profiling, and measured stratigraphic section have been carried out. The stratigraphy of the study area can be divided into 5 units from old to young, namely the Mentulu-metamorphic sandstone unit, Lemat conglomerate unit, Lemat gravel-sandstone unit, Benakat volcanic-siltstone unit, and the alluvial deposit unit. The stratigraphic relationship between Mentulu metamorphic-sandstone unit and Lthe emat conglomerate unit is nonconformity. The relationship between Lthe emat conglomerate unit and Lemat gravel-sandstone unit and Benakat volcanic-siltstone unit is interfingering. The relationship between alluvial deposit units with Lemat conglomerate units and Lemat gravel-sandstone units is aan ngular unconformity. Analysis of lithofacies and architectural elements of the study area showed a fluvial depositional environment. The resulting architectural elements are 8 associations: SG (sedimentary gravity flow), GB (gravel bedform), SB (sandy bedform), CH (channel), DA (downstream accretion), HO (scour-hollow fill), AC (abandoned channel) and FF (overbank fine). The depositional environment of the Lemat Formation; Lemat conglomerate unit in alluvial fans environment, Lemat gravel sandstone unit in the shallow environment, gravel-bed braided river. In the Benakat tuffaceous-siltstone unit, it was deposited in a flashy, ephermal, sheet flood, sand-bed river environment.

Fluvial architecture of the Upper Cretaceous Nubia Sandstones: An ancient example of sandy braided rivers in central Eastern Desert, Egypt

The present paper focuses on the fluvial architecture and depositional facies model of the Late Cretaceous large sandy braided-river system of the Nubia Formation in the central Eastern Desert of Egypt. Six lithofacies types and two paleosol pedotypes (entisol and vertisol) are arranged in channel and overbank architectural elements that display first- to fifth-order bounding surfaces. Two distinctive fluvial architectural styles are recognized. The Fluvial architecture of the lower member identifies a low sinuous and energetic sandy braided river, in which the channels cover the entire of fluvial landforms with limited overbank areas. The suggested depositional model resembles the classic Platte-type braided river with its broad, low sinuosity streams, rich sand bedload and minor overbank sediments. Recurrent autogenic fining-upward cycles, rarely preserved overbank deposits, dominant entisol pedotype, prevailing trough cross-bedded sandstones, sporadic to absent planar cross-bedded sandstone, and dominant downstream accretion also imply perennial, sandy braided rivers. By contrast, the fluvial architecture of the upper member is characterized by fining-upward cycles, downstream and lateral accretion elements, and commonly preserved overbank deposits (overprinted by the mature vertisol pedotype). This architecture style is ascribed to a perennial sandy braided river characterized by sinuous energetic streams and laterally developed overbank areas, similar to the modern South Saskatchewan River in Canada. The fluvial depositional system and sequences in the Nubia Formation are strongly controlled by Late Cretaceous base-level fluctuations and tectonic pulses of the Laramide Orogeny.

Fluvial architecture and sequence stratigraphy of the Burro Canyon Formation, southwestern Piceance Basin, Colorado

The Lower Cretaceous Burro Canyon Formation in the southwestern Piceance Basin, Colorado, is composed of deposits that represent a braided fluvial system with high net to gross that transitions stratigraphically upward into a low net-to-gross, low-sinuosity, meandering fluvial system. The fluvial deposits are composed of multiple upward-fining, conglomeratic-to-sandstone successions forming bars and bar sets that exhibit inclined heterolithic strata that we have interpreted to have formed by oblique and downstream accretion. We used well-exposed outcrops, detailed measured sections, and unmanned aerial system-based imagery to describe the fluvial architecture of the Late Cretaceous formation using a hierarchical approach. We described the Burro Canyon Formation as comprising sandstone-rich amalgamated channel complexes (ACC) overlain by non- to semiamalgamated channel complexes. The lower interval of the formation is composed of ACC that contain channel-fill elements with cross-stratification and numerous truncated contacts. These stacked channel-fill elements exhibit an apparent width range of 137–1300 ft (40–420 m) and a thickness range of 5–60 ft (1.5–18 m). The upper interval of the Burro Canyon Formation comprises mudstone-prone intervals of the nonamalgamated channel complex with isolated channel-fill elements interbedded with floodplain mudstones that represent a period of relatively high base level. Associate channel fill elements range in apparent width from 200 to 1000 ft (60 to 300 m) and thickness from 20 to 30 ft (6 to 18 m). The characteristics and spatial distribution of architectural elements of the Burro Canyon Formation correspond to one depositional sequence. The erosional basal surface of the formation, as well as lateral changes in thickness and net to gross, suggest that the Burro Canyon Formation within this study area was deposited as an incised valley fill. Fluvial deposits of the Burro Canyon Formation serve as outcrop analogs for subsurface interpretations in similar reservoirs.

Controls on the deposition and preservation of architectural elements within a fluvial multi-storey sandbody

Architectural elements of fluvial multi-storey sandbodies provide principal controls on the distribution of meso-scale (100–101 m scale) heterogeneity and reservoir quality. Consequently, it is valuable to understand the deposition and preservation of sedimentary architecture in such systems in relation to autogenic (stream capture and avulsion) and allogenic controls (subsidence rates, climate and sediment supply). The aims of this study are to quantify the architectural and erosional nature of a fluvial multi-storey sandbody and to establish the effects of downstream distance and subsidence rates upon the preservation of architectural elements, using the Lower Castlegate Sandstone, Utah, USA, as an example. Quantitative architectural element analysis and palaeodischarge reconstructions were undertaken from eight locations using sedimentary logs and three terrestrial photogrammetric outcrop datasets along a 150 km down-dip profile. These observations were supplemented by burial history analysis of ten wells across the same profile. Results show the Lower Castlegate comprises channel-fill, downstream accretion, lateral accretion, upstream accretion and overbank elements. From these observations, calculations of sinuosity and flow depths along with architectural geometric analysis provide evidence of stream capture contemporaneous with foreland basin subsidence. The preservation of lateral accretion and overbank elements is limited within the distal portion of the multi-storey sandbody, as a result of local avulsion and limited subsidence rates. Results demonstrate that complex sedimentary architecture can form in fluvial multi-storey sandbodies as a product of variable discharge rates, the fluvial graded profile and spatially variable aggradation rates, driven principally by subsidence rates. The use of meso-scale architectural analysis, with analysis of in-channel sinuosity and hydrodynamics, along with erosional bounding surfaces, has helped to complemented basin-scale interpretations of fluvial architecture.

Complex alluvial architecture, paleohydraulics and controls of a multichannel fluvial system: Bajo Barreal Formation (Upper Cretaceous) in the Cerro Ballena anticline, Golfo San Jorge Basin, Patagonia

A sedimentological-architectural review, and a palaeohydrological study was performed in a 380 m thick succession of the Bajo Barreal Formation (Upper Cretaceous) in the Cerro Ballena anticline, Golfo San Jorge Basin, Patagonia. The studied exposure shows, at the base, isolated sandstone channel belts of small scale and low connectivity within siliciclastic floodplain strata (Section A, 185 m thick). It changes upwards into sheet-like, sandstone channel fills with larger interconnectivity that form channel-belt complexes intercalated with thinner floodplain fines of volcaniclastic origin (Section B, 195 m thick). The succession contains seventeen lithofacies and twelve architectural elements that provide information on the alluvial organization of channel and overbank deposits. Palaeoflow data of 218 fluvial channels (Az. 187°; n = 3777) indicate the presence of a depocenter located southward. Seven fluvial styles were identified: 1) multistorey bodies dominated by downstream accretion, 2) multilateral, broad sheets dominated by downstream accretion, 3) narrow sheets dominated by lateral accretion, 4) ribbon or sheet-like bodies infilled by resedimented volcaniclastics, 5) ribbon-shaped fixed channels with attached splays, 6) unconfined, isolated sheet complexes, and 7) unconfined, moderately amalgamated sheet complexes. The variable geometry and style of palaeo-rivers in coeval levels, and dominance of avulsion deposits support that the fluvial system was anabranching, and locally anastomosed. Using palaeohydrological parameters estimated from cross-bed set thickness, a climatic shift toward wetter and more humid climatic conditions from Section A to Section B was demonstrated. We find support to assess the climate as the main external forcing factor on the alluvial organization and stacking density of the Bajo Barreal Formation.

Braided–meandering system evolution in the rock record: Implications for climate control on the Middle–Upper Jurassic in the southern Junggar Basin, north‐west China

This paper presents the current body of knowledge regarding the evolution of a braided–meandering system in the southern Junggar Basin, north‐west China, based on the preserved architecture evidences. The response to climate change was also discussed. Herein, 10 types of lithofacies are summarized, including information of their primary sedimentary structures, characteristic lithology, and geometry. This study focuses on recognizing the architectural elements within the channels (migrating channel: CHm, switching channel: CHs, transiting channel: CHt, and filling channel: CHf), the bar units (downstream accretion: DA, lateral accretion: LA, unit bar: UB, and compound bar: CB), and fine‐grained clastic deposits (floodplain fines: FF and overbank fines: OF). Three representative sections of outcrops were selected, and detailed observation and analysis of these sections revealed 3 stages can be divided: the braided stage, in which the deposits show an evidence of CHm, DA elements which are the mainly architectural elements, multichannel with aggradational filling; The transformation between braided–meandering stage, during which the fluvial system consists of CHt and CHm elements with lateral accretion; and meandering stage which is characterized by dominance of CHm, LA elements, and the channels become isolated and meandering. Finally, this paper concludes with an interpretation of climate events as manifested by the fluvial archives of the colours of sedimentary rocks, special mineral, palaeontology, geochemistry spanning the Middle–Upper Jurassic (between 166.1 and 152.1 Ma).

Precambrian snapshots: Morphodynamics of Torridonian fluvial braid bars revealed by three‐dimensional photogrammetry and outcrop sedimentology

AbstractPre‐vegetation fluvial channels have long been considered predominantly sheet‐like in geometry, owing to hydraulic sections that rapidly widened rather than incise during floods. This motif has been paralleled to that of modern dryland rivers subject to sharp discharge fluctuations during ephemeral floods. However, a number of Precambrian fluvial successions have been recently appraised as the product of deep‐channelled systems characterized by relatively stable – probably perennial – discharge regimes. One such example is the ca 1·0 Ga Applecross Formation, part of the well‐studied Torridon Group of Scotland. To contribute to this debate and to provide refined morphodynamic models for the Applecross Formation, this article presents an integration of three‐dimensional photogrammetry and outcrop sedimentology applied to key exposures at Stoer Peninsula, north‐western Scottish Highlands. Analysis of selected sandbodies reveals that high‐relief fluvial sand bars, both mid‐channel and bank‐attached, evolved within deep, braided‐channel belts. These bars grew through complex mechanisms of accretion and reactivation related to different flood stages: upstream and downstream accretion probably occurred during waning‐flood stages characterized by high hydrograph levels and abundant sediment availability; lateral accretion took place during later waning‐flood stages, and it was associated in some cases with helical recirculation and increase in bend sinuosity. Overall, the depicted morphodynamics are consistent with prolonged flood events that cannot be reconciled with sharply fluctuating discharge regimes. Critical comparisons between the internal geometry of the studied bars and modern counterparts corroborate the findings herein. Thus, this study recommends stricter comparisons between the products of modern braided channels and Precambrian fluvial rock records featuring thick and well‐developed bar forms.

Facies and architecture of a coarse-grained alluvial-dominated incised valley fill: a case study from the Oligocene Gebel Ahmar Formation, Southern Tethyan-shelf (northern Egypt)

Exposures of multistorey, alluvial deposits from the Oligocene Gebel Ahmar Formation in the Cairo-Suez province (north Eastern Desert, Egypt) show the architecture of an up to 35m thick continuously prograding fluvial/alluvial filling of an incised valley. The Oligocene base level fall resulted in cannibalization of the Eocene bedrock. Subsequent baselevel rise created accommodation space that was filled by deposition of four stacked storeys: lower storeys (1-2) of low sinuosity sandy braid plains and upper storeys (3-4) of gravelly braid plain. These braid plains were sourced from exposed Upper Cretaceous-Eocene and Paleozoic-Lower Cretaceous siliciclastic successions to the south. These successions dominate the Galala-Araba inverted structures. The sandy braid plain channel belts mainly downstream accretion (DA), downstream oblique accretion(DLA), lateral accretion (LA), sandy bedforms (SB), channel (CH), and Hollow (HO) elements, while the gravelly braid plain consists mainly of gravel bars and sheets (GB), gravel-sandstone foresets (GSF), gravel-sand couplets (GSC), and scour pool filling (SPF) architectures. Incised valley incision is potentially linked to a global drop of sea level caused by glaciation, although hinterland tectonism (i.e. Late Cretaceous-Paleogene tectonic inversion and Late Eocene-Oligocene crustal updoming in the source terrains) as well as Late Oligocene-Miocene rifting play a significant role in the subsequent filling. The hinterland tectonism as well as the climate controls the sediment supply. The understanding of the nature of the Oligocene incised valley fill helps in the constrain potential down depositional dip hydrocarbon reservoirs in Nile Delta, East Mediterranean basins, and similar settings in passive continental margins.

Highly Variable Precambrian Fluvial Style Recorded In the Nelson Head Formation of Brock Inlier (Northwest Territories, Canada)

Abstract Spectacular canyon exposures of the ∼ 1 Ga Nelson Head Formation along the modern Brock River, Northwest Territories, provide a rare opportunity to assess the deposits of pre-vegetation, braided to sinuous-channelized fluvial systems. We analyze the sedimentology, architecture, and depositional evolution of 16 stacked fluvial-channel belts at this site, demonstrating greater variability in sedimentary style and morphodynamics than is typically interpreted for Precambrian rivers. Inferences on fluvial planview based on integrated analysis of depositional architecture and paleoflow reveal complex patterns of channel-planform evolution, and four depositional stages are recognized: (1) floodbasin-splay progradation with bounding episodes of eolian reworking; (2) deposition in wandering-channel belts characterized by deep anabranches and prominent lateral accretion; (3) progressive shift towards shallow, braided-channel belts confined within alluvial valleys and dominated by downstream accretion; and (4) deposition by marine-influenced braided-channel belts with mixed downstream, lateral, and upstream accretion. Overall, the studied fluvial-channel belts point to significant morphodynamic complexity and geomorphic variability, challenging the assumption that all pre-Silurian rivers shared poor channelization and low sinuosity. Observed channel bodies also have width:thickness ratios strongly overlapping with those of rivers postdating the rise of vegetation. In both wandering- and braided-channel belts, complex patterns of channel migration, bar accretion, and dissection were the result of co-acting processes such as development of transverse-velocity zones within the channels, local paleoflow disturbance induced by depositional topography, and avulsion. In concert with the regional extent and paleoflow, and increasingly distal character of correlative stratigraphic units towards the northwest, the magnitude of the observed trunk-fluvial channels is consistent with a mature drainage capable of transecting the Laurentian craton over thousands of kilometers.

Forced regressive wedge in the Mesoproterozoic Koldaha Shale, Vindhyan basin, Son valley, central India

The present paper highlights the sequence development within the Mesoproterozoic Koldaha Shale Member of the Kheinjua Formation, Vindhyan Supergroup which records the occurrence of a forced regressive wedge and associated discontinuity surfaces at the base of the wedge. Nine lithofacies have been identified within the study area that are grouped into three lithofacies associations varying in depositional setting from outer shelf, through shoreface-foreshore-beach to continental braidplain. The outer shelf sediments are aggradational to slightly progradational representing highstand systems tract. The rapidly progradational, wedge-shaped shoreface to foreshore-beach succession occurs sharply or erosively above the outer shelf sediments and is bounded by a regressive surface of marine erosion (RSME) at the base and by a subaerial unconformity at the top. This, along with its downstepping trajectory, supports deposition of this sedimentary wedge during falling sea level. A laterally extensive soft sediment deformation zone occurs at the base of the wedge.The forced regressive wedge is incised by fluvial braidplain deposits that rest on an erosive surface representing a sequence boundary. The thin braidplain deposits are the product of aggradation during a subsequent early rise in relative sea level, and thus, they are inferred to represent a lowstand systems tract. The constituent architectural elements that characterize the braidplain deposits are downstream accretion elements and small channel elements. Further landward, the base and top of the shoreface wedge merge to form an unconformity across deposits that rest directly on the outer shelf sediments. The identification of forced regressive wedges has significant economic importance in view of the potential occurrence of hydrocarbons within the Proterozoic formations.

Planview style and palaeodrainage of Torridonian channel belts: Applecross Formation, Stoer Peninsula, Scotland

The sedimentary products of forty-three Neoproterozoic fluvial-channel belts are superbly exposed along the seaboard of Stoer Peninsula, Scotland, UK. The deposits belong to the Applecross Formation, part of the Torridonian succession, and their planview style and palaeodrainage are reconstructed at three sites. In-channel deposits record both active and abandonment stages, while soft-sediment deformation calls for local failure of cohesion-less deposits and/or effects of seismic events. Channel belts comprise compound-bar sheets and large foreset bars, as well as channel bodies with rarely preserved levees. In most cases bars show downstream accretion, though lateral and upstream growth is also evident along reactivated bar flanks and heads, respectively. While these features are distinctive of active braided-channel belts, scattered trunk-channel bodies with preserved levees point to local embankment of deep streams with somewhat limited lateral mobility.Profusion of thick cross-bedding calls for perennial discharge in channels with depth in the order of 13m, suggesting a mature, large-scale fluvial drainage capable of transporting sediment for hundreds to thousands of km. While entire channel belts yield width:thickness ratios exceeding 320, single channel bodies with ratios of 20 to 80 better reconcile with a channelled rather than sheet braided fluvial model, a feature in contrast with classic models for pre-vegetation rivers. Though some degree of uncertainty exists when applying Phanerozoic-derived models to Torridonian rivers, the latter could have been controlled by cycles of discharge to sediment supply that alternated aggradation and degradation, with patterns of bar buildup and reworking comparable to those observed in modern braided rivers.

Depositional and erosional architectures of gravelly braid bar formed by a flood in the Abe River, central Japan, inferred from a three-dimensional ground-penetrating radar analysis

We conducted a ground-penetrating radar (GPR) survey of gravelly braid bars in the Abe River, central Japan, to clarify the three-dimensional (3D) variations in their depositional facies under various geomorphologic conditions. In September 2011, a ten-year return-period flood in the study area reworked and deposited braid bars. After the flood, we surveyed three bars with different geomorphologies using a GPR system with a 250-MHz antenna and identified seven fundamental radar depositional facies: Inclined reflections (facies Ia and Ib), horizontal to subhorizontal reflections (facies IIa and IIb), discontinuous reflections (facies IIIa and IIIb), and facies assemblage with a large-scale channel-shaped lower boundary (facies IV). Combinations of these facies in\\dicate bar formation processes: channel filling, lateral aggradation, and lateral and downstream accretion. In the Abe River, aerial photographs and airborne laser scanning data were obtained before and after the flood. The observed changes of the surface topography are consistent with the subsurface results seen in the GPR sections. This study demonstrated that the erosional and depositional architecture observed among bars with different channel styles was related to river width and represented depositional processes for high-sediment discharge. The quantitative characterizations of the sedimentary architecture will be useful for interpreting gravelly fluvial deposits in the rock record.

Architectural Styles and Sedimentology of the Fluvial Lower Beaufort Group, Karoo Basin, South Africa

Abstract The Permo-Triassic Beaufort Group comprises in excess of 3000 m of sandstone and mudstone-rich fluvial deposits that vary both laterally and vertically in abundance, architecture, and clustering styles. The lower 1370 m has been studied in detail. Detailed stratigraphic logging and photographic interpretation have been carried out on river cliffs and hillside sections, and sedimentary structures have been mapped by close inspection of clean outcrops. Seven fluvial architectural styles have been identified, encompassing sand body and sand body cluster scales: 1) flat-topped sheets dominated by lateral accretion, 2) concave-up topped sheets dominated by lateral accretion, 3) heterolithic ribbons dominated by lateral accretion, 4) flat-topped ribbons with attached splays dominated by downstream accretion, 5) unconfined moderately amalgamated concave-up topped complexes, 6) unconfined strongly amalgamated sheet complexes, and 7) valley-confined strongly amalgamated ribbon complexes. The internal architecture of each indicates mixed lateral and downstream accretion modes on several scales with local variability. Lateral-accretion deposits differ from typically published facies models in that trough cross-bedding is rare, possibly the result of an inability to generate dunes due to the dominant very fine sandstone grain size. The observed architecture and sedimentary structures are the product of: 1) a semiarid climate, producing fluvial and debris flows, and cementation of sediment packages at the surface (which exerts a strong control on the magnitude of channel scour, the nature of channel fill, and compaction history); 2) local variability in subsidence rate causing variation in the vertical spacing of channel belts, channel slope, and plan form, and variation in preservation of topographically higher features (e.g., levees); 3) relative position in a fluvial megafan influencing suspended load to bedload partitioning from proximal to distal locations, and formation of channel belt complexes; and 4) relative sea-level changes causing incised-valley formation and fill only in the most distal parts of the system.

Inferring Fluvial Morphodynamics and Overbank Flow Control From 3D Outcrop Sections of A Pleistocene Point Bar, Dandiero Basin, Eritrea

Abstract Sedimentological and architectural data from three-dimensional larger vertical outcrops of Pleistocene fluvial deposits of the Dandiero Basin (Eritrean Danakil depression) are here used to reconstruct planform evolution of a meander bend and investigate the role of overbank flooding in sediment distribution on point bars. The point bar accumulated as part of a lowstand systems tract at the base of an incised valley cut into lacustrine mud. Selected outcrops transect a 1-km-long channel belt exposed parallel to the main fluvial transport direction. Sedimentological logs, outcrop line drawings, and paleocurrent and bedding measurements show that this outcrop belt represents a transverse section of an asymmetric fluvial point bar, which accumulated through migration of a channel 6 m deep and 70 m wide. This bar was characterized by early expansion followed by downstream accretion, which was abruptly interrupted by lacustrine deposition. The onset of downstream accretion started when the channel expanded to reach the more resistant valley wall. Sediment accreted on the downstream side of the point bar and accumulated both during bankfull discharge and overbank floods, with significant modification of the channel flow where the floodwater re-entered the main channel (i.e., riffle zone). In this zone, a secondary circulation cell triggered a shift of erosion toward the inner bank and scrolling on the downstream side of the point bar.

Seismic geomorphology and sedimentology of a tidally influenced river deposit, Lower Cretaceous Athabasca oil sands, Alberta, Canada

The bitumen of the Lower Cretaceous McMurray Formation in Alberta arguably represents one of the most important hydrocarbon accumulations in the world. In-situ development relies on heat transfer through the reservoir via horizontal steam injection wells placed 4 to 6 m (13–20 ft) above horizontal producers near the base of the sandstone reservoirs. Given this technology, understanding the distribution of the resource is paramount for a successful development program. Sedimentary facies provide a direct control on bitumen distribution and recovery. Most facies models developed to describe and predict sedimentary units of the McMurray Formation consider fluvial, estuarine, and/or deltaic depositional settings. In-situ development, however, requires a particularly high-resolution sedimentologic interpretation. High-quality three-dimensional seismic reflection data and extensive drill cores from acreage located approximately 50 km (31 mi) south of Fort McMurray provide important insights into the sedimentologic organization of reservoir and nonreservoir deposits in the upper one third (40 m [131 ft]) of the reservoir interval. Geomorphologic characteristics of the strata observed in seismic time slices reveal that a fluvial depositional setting was prevalent. Ichnologic and palynologic data, as well as sedimentary structures suggestive of tidal processes, indicate a marine influence in the upper reaches of a fluvial system characterized by channels that were 390 to 640 m (1280–2100 ft) wide and 28 to 36 m (92–118 ft) deep. The complex stratigraphic architecture consists of a mosaic of large-scale depositional elements, including abandoned channels or oxbow lake fills, point bars associated with lateral accretion, point bars associated with downstream accretion, counter point bars, and sandstone-filled channels. Reservoir deposits are primarily associated with point bars and sandstone-filled channels.

Architecture and development of the alluvial sediments of the Upper Jurassic Tordillo Formation in the Cañada Ancha Valley, northern Neuquén Basin, Argentina

The Upper Jurassic Tordillo Formation at Cañada Ancha area, northern Neuquén Basin, Argentina, comprises a multi-stage suit of predominantly alluvial sediments that is heterolithic in nature. In that suit, several lithofacies, architectural elements, and bounding surfaces of different order have been identified and their lateral and vertical distribution characterized. This analysis allowed the differentiation of 3 main units (lower, middle and upper), 20 subunits (C-1 to C-20), and the characterization of their alluvial styles. The lower unit (which comprises subunits C-1 to C-4) is mainly formed by fine- to medium-grained sandstones, which become medium- to coarse-grained towards the top. These sandstones characterize settings ranging from floodplains with isolated, unconfined flows, to more complex, vertically stacked, multi-storey sheet sandstones of braided fluvial systems. The middle unit (C-5 to C-10) is dominated by pale brown-grey fine-to coarse-grained sands and medium size subangular to angular conglomerates, which reflect amalgamated complexes of sandstone sheets and downstream accretion macroforms. Remarkably, this alluvial sedimentation was episodically punctuated by volcaniclastic flows. The upper unit (C-11 to C-20) consists of finer sediments, mainly pink to white fine-to medium grained sandstones and red to green siltstones. Towards the top, bioturbation becomes important, and also the presence of volcanosedimentary flows is noticeable. Fluvial settings include braided sheet sandstones with waning flood deposits evolving to isolated high-sinuosity fluvial systems, with flash flood deposits. At the top of this unit, facies may suggest marine influence. Vertical changes in the fluvial style result from both climatic and tectonic controls. A semiarid to arid climate and the active tectonism linked to the eastward migration of the Andean volcanic arc determined major bounding surfaces, fluvial style evolution and the presence of the volcano-sedimentary deposits. Different stages of high and low subsidence rates has been deduced from the vertical stacking of sediments.