Bar Deposits Research Articles

Compound-clinoform model for tide-influenced deltas in the mixed siliciclastic–carbonate Cisco Group, Upper Pennsylvanian (Virgilian), Eastern Shelf of the Permian Basin, U.S.A.

ABSTRACT Deltas in the Cisco Group contain significant volumes of potential reservoir sandstones in the Upper Pennsylvanian (Virgilian stage) mixed siliciclastic–carbonate succession in the Eastern Shelf of the Permian Basin. While early studies predominantly supported a river-dominated model for these deltas, our findings establish the presence of tide-influenced deltas characterized by the following features: i) predominant heterolithic lithology, ii) diagnostic structures such as flaser bedding, bidirectional ripple cross-lamination, or double-mud drapes, iii) an abundance of interpreted fluid muds, and iv) elongated bars on the subaqueous platform. Through analysis of 290.2 m of core, we identify six facies associations related to tide-influenced deltaic systems in highstand and falling-stage intervals of the proximal Eastern Shelf: delta plain–tidal flat, distributary channel and distributary-mouth bar (delta front) deposits on the subaerial delta; subaqueous delta topset, subaqueous delta foreset, and prodelta deposits on the subaqueous delta. Moreover, two additional facies associations, transgressive shelf deposits and subtidal shallow-water carbonates, record continuous episodic transgressions in the deltaic packages. Well-log cross sections reveal repetitive stacking of deltas, typical of icehouse systems, and sandstone mapping suggests elongated tidal bars forming the interpreted subaqueous delta platforms. These features suggest a compound-clinoform morphology of the Cisco Group deltas akin to modern tide-influenced deltas that have carbonate banks such as the Gulf of Papua. This compound-clinoform morphology has implications for the elongate geometry and isolation of sandstone reservoirs, impacting fluid migration and storage. Moreover, these insights extend beyond the Cisco Group to other Pennsylvanian and Permian levels on the Eastern Shelf, as well as to other deltas in mixed siliciclastic–carbonate systems in icehouse conditions.

Uncovering the hidden world of riverbed sediments: The role of sediment heterogeneity and cross-bar channel fills in the hydrogeochemical dynamics of the hyporheic zone

Groundwater-surface water interaction (hyporheic exchange) is critical in numerous hydrogeochemical processes; however, hyporheic exchange is difficult to characterize due to the various spatial (e.g., sedimentary architecture) and temporal (e.g., stage fluctuations) variables that influence it. This interdisciplinary study brings forth novel insights by integrating various methodologies including geophysical surveys, physical and chemical sediment characterization, and water chemistry analysis to explore the interplay of the numerous facets governing hyporheic zone processes within a compound bar deposit. The findings reveal distinct sedimentary facies and geochemical zones within the compound bar, driven by the sedimentary architecture. Cross-bar channel fills are identified as critical structures influencing hydrogeochemical dynamics, acting as baffles to groundwater flow and modulating nutrient transformations. Geophysical imaging and hydrogeochemical analyses highlight the complex interplay between sediment characteristics and subsurface hydraulic connectivity, emphasizing the role of sediment heterogeneity in controlling hyporheic exchange and solute mixing. The study concludes that sediment heterogeneity, particularly the presence of cross-bar channel fills, plays a pivotal role in the hydrogeochemical dynamics of the hyporheic zone. These structures significantly influence hyporheic flow paths, solute residence times, and nutrient cycling, underscoring the necessity to consider the fine-scale sedimentary architecture in models of hyporheic exchange. The findings contribute to a deeper understanding of riverine ecosystem processes, offering insights that can inform management strategies for water quality and ecological integrity.

Permafrost Formation in a Meandering River Floodplain

AbstractPermafrost influences 25% of land in the Northern Hemisphere, where it stabilizes the ground beneath communities and infrastructure and sequesters carbon. However, the coevolution of permafrost, river dynamics, and vegetation in Arctic environments remains poorly understood. As rivers meander, they erode the floodplain at cutbanks and build new land through bar deposition, creating sequences of landforms with distinct formation ages. Here we mapped these sequences along the Koyukuk River floodplain, Alaska, analyzing permafrost occurrence, and landform and vegetation types. We used radiocarbon and optically stimulated luminescence (OSL) dating to develop a floodplain age map. Deposit ages ranged from modern to 10 ka, with more younger deposits near the modern channel. Permafrost rapidly reached 50% areal extent in all deposits older than 200 years then gradually increased up to ∼85% extent for deposits greater than 4 Kyr old. Permafrost extent correlated with increases in black spruce and wetland abundance, as well as increases in permafrost extent within wetland, and shrub and scrub vegetation classes. We developed an inverse model to constrain permafrost formation rate as a function of air temperature. Permafrost extent initially increased by ∼25% per century, in pace with vegetation succession, before decelerating to <10% per millennia as insulating overbank mud and moss slowly accumulated. Modern permafrost extent on the Koyukuk floodplain therefore reflects a dynamic balance between widespread, time‐varying permafrost formation and rapid, localized degradation due to cutbank erosion that might trigger a rapid loss of permafrost with climatic warming.

The Fate of Bars in Braided Rivers

Ancient river deposits are important archives of past landscape conditions on planetary surfaces. On Earth, they host valuable groundwater, energy resources, and carbon-storage potential. Reconstructing details of paleochannel forms and movements refines our understanding of the controls on river behavior under different climate, landcover, and tectonic conditions, and improves predictions and models of subsurface reservoirs. While studies have shown detailed connections between channel kinematics and bar-deposit architecture in meandering river systems, similar connections between braided river movements and preserved braided river deposits have not been established. Here we explore the potential for connecting braided river deposits to paleochannel movements, form, and flow conditions, and we evaluate the controls on bar preservation using synthetic stratigraphy generated with a numerical morphodynamic model. We investigate how attributes of channel morphodynamics, like channel widening or braiding intensity, impact bar deposits’ preservation, scale, geometry, and architecture. We then assess how the scale, preservation, and facies composition of bar deposits reflect formative flow conditions of the channel. Our results demonstrate that no diagnostic signature of braided channel morphodynamics is recorded in bar-deposit geometry, facies, or preservation patterns. Rather, the unique local history of thread movements combines stochastically to preserve or rework bar deposits, and the timing of channel avulsion is the dominant control on bar preservation. Our results also show that representative paleochannel flow conditions will likely be accurately reflected in aggregate observations of braid bar deposits within channel-belt sandbodies at a regional or member/formation scale. These results demonstrate the need for broad sampling and statistical approaches to subsurface prediction and paleo-flow reconstruction in ancient, braided river deposits.

Spit bar deposits from the Upper Cretaceous (Cenomanian) transgressive sequence in NE Bohemia (Czechia)

We propose a spit bar setting as the possible palaeoenvironment of the basal Late Cretaceous transgressive sequence in NW Bohemia. A new Cenomanian transgression model for the Bohemian Basin is also proposed. The uppermost Devět Křížů Sandstone, which has been conventionally referred to the Bohdašín Formation, probably represents the middle or lower upper Cenomanian (Upper Cretaceous), not the Triassic as previously supposed. We assume that this controversial unit was deposited before the main latest Cenomanian–early Turonian transgression. The spit bars were likely overgrown by vascular plants during their emergence in the late Cenomanian, and then inundated during the latest Cenomanian and early Turonian transgressive phases. The studied deposits had been intensively bioturbated, and the cf. Taenidium suite was recognized for the first time in them alongside the Thalassinoides assemblage (T. paradoxicus, T. suevicus, Thalassinoides isp., cf. Thalassinoides), which are characteristic of the Scoyenia and Glossifungites ichnofacies, respectively. The bioturbated, rhizolith-bearing horizon was presumably a paleosol.

Four Years of Meander‐Bend Evolution Captured by Drone‐Based Lidar Reveals Lack of Width Maintenance on the White River, Indiana, USA

AbstractMeandering rivers experience fluctuations in width whenever riverbanks migrate in different directions or at different rates, which can be observed after individual floods. However, meandering rivers maintain approximately constant widths over decadal timescales. This implies some timescale below which width fluctuates as banks migrate independently, and above which width is maintained by a bank‐coupling process. This coupling is thought to occur either as point bar deposition events induce cutbank erosion (bar‐push), or as cutbank erosion events induce point bar deposition (bank‐pull). This coupling, however, has been challenging to observe in natural rivers due to limited event‐scale field data. We present results from a 4.5‐year campaign with 22 drone‐based lidar surveys of a single point bar and cutbank (∼0.35 km2 in area) on the White River near Worthington, Indiana, USA. The middle point bar experienced net erosion (5,400 m3), but net aggradation (17,100 m3) between 2019 and 2022 when including perennially submerged regions. This aggradation was less than the 35,700 m3 of cutbank erosion over the same period. Combined, we have observed widening (1.58 m/yr bend‐averaged; 3.08 m/yr near apex) over the study period as point bar deposition has not kept up with cutbank erosion. Finally, we suggest that the difference between bar‐push and bank‐pull as width‐maintenance mechanisms may not be resolvable by observing bend widening or narrowing alone without an advancement of current theory, such as determining a long‐term equilibrium width and measuring deviations relative thereto.

Tidal versus fluvial point bars: Key features from the integration of outcrop, core and wireline log information of Triassic examples

AbstractThe Triassic red beds of the Tabular Cover of the Iberian Meseta are an excellent reservoir outcrop analogue, a direct consequence of high‐quality exposures, which offer exceptional three‐dimensional outcrops, as well as a wide variability of depositional environments. Fluvial and transitional with tide‐influenced and wave‐influenced settings are recognised. Three point bar geobodies of similar scale, but influenced by different processes, were selected from this succession. Point bar geobody 1 was influenced by purely fluvial processes while geobodies 2 and 3 were tide‐influenced. Both types of geobody were developed as point bar deposits in sinuous channels. A fully integrated study was carried out on these geobodies, utilising both outcrop and subsurface‐based approaches, to characterise the key differences between fluvial and tidal point bars in the sedimentary record. The outcrop‐based component involved traditional field data collection methods alongside digital techniques and data capture, including the use of digital outcrop models. Additionally, subsurface‐based methods were employed, utilising core and wireline logs obtained from wells drilled in close proximity to the outcrop. The integration of these approaches aims to accurately differentiate the depositional settings of the three different geobodies, which while apparently very similar in many key respects also exhibit considerable differences when considered from the perspective of subsurface management of potentially similar geobodies. This study also emphasises the need to clearly distinguish high‐sinuosity deposits based on their depositional sub‐environment in order to properly evaluate their potential for subsurface management. Additionally, it highlights the presence and importance of internal baffles that may well influence fluid migration and indeed even compartmentalise geobodies. Three point bar geobodies of similar scale, but influenced by different processes, have been selected in this succession. A fully integrated study was carried out on these geobodies, utilising both outcrop‐based and subsurface‐based approaches, to characterise the key differences between fluvial and tidal point bars in the sedimentary record.

Depositional setting and trace fossil suites of the early Cambrian (series 2, stage 4) Khussak Formation, east-central salt range, north-west sub-Himalayas, Pakistan

This is the first integrated ichnologic-sedimentologic study of the Khussak Formation at the Salt Range, Pakistan. This integrated approach led to identifying thirteen lithofacies and six recurring facies associations. Facies association 1 (FA-1) is made up of massive to parallel laminated mudstone with rare interbeds of sandstone. FA-1 is moderately bioturbated with trace fossils belonging to the Cruziana ichnofacies (Cruziana, Planolites, Rusophycus). FA-1 is interpreted to record deposition in the upper intertidal mud flat setting. Facies association 2 (FA-2) consists of thinly interbedded sandstone (wavy and current rippled) and mudstone (massive to parallel-laminated). The heterolithic wavy bedding is the dominant bedding type. FA-2 is variably bioturbated (BI = 0–5). Trace fossils of moderate diversity of the Cruziana ichnofacies are present. FA-2 represents deposition in the middle intertidal mixed flat setting. Facies association 3 (FA-3) comprises gradational-based, laterally extensive, crudely bedded to massive, medium to lower fine-grained sandstone with thin (cm-scale) mudstone beds. FA-3 is intensely bioturbated (BI = 3–5). Robust and diverse trace fossils belonging to the Skolithos ichnofacies are present (Arenicolites, Cylindrichnus, Diplocraterion, Phycodes, Planolites, Siphonichnus, Skolithos, and Treptichnus). FA-3 records deposition in the lower intertidal sand flat setting. Facies association 4 (FA-4) consists of muddy to silty lower-upper fine-grained sandstone with rare mudstone drapes. FA-4 displays inclined heterolithic stratification (IHS). Tidal couplets (mud and mica drapes) and mud clasts are common. Bioturbation is absent to sparse (BI = 0–2) with low-diversity trace fossils of Skolithos ichnofacies (Cylindrichnus, Diplocraterion, Palaeophycus, Planolites, Skolithos, and rare fugichnia). FA-4 is interpreted to represent deposition in an intertidal creek point bar. Facies association 5 (FA-5) consists of well sorted sandstone (upper medium to fine-grained). The sandstone is erosionally based, and laterally extensive. Planar parallel lamination, trough cross-stratification, reactivation surfaces, and mud drapes are present only locally. The degree of biogenic reworking is low to moderate (BI = 0–4), and traces are sporadically distributed. Trace fossils present are of moderate diversity and abundance, including Arenicolites, Cylindrichnus, Diplocraterion, Monocraterion, Palaeophycus, Planolites, Siphonichnus, and Skolithos. FA-5 is interpreted as subtidal bar deposits. Facies association 6 (FA-6) is characterized by bioturbated muddy sandstone, heterolithic sandstone, and massive mudstone. Physical bedforms are commonly obliterated by bioturbation. Planar horizontal laminae, wavy laminae, small ripples, and synaeresis cracks may occur locally. FA-6 is moderate to intensely bioturbated (BI = 3–6). The distribution of traces (within beds and across the FA) is heterogeneous (sporadically) to homogeneous (locally). Robust Bergaueria, Cylindrichnus, Planolites, Rhizocorallium, Siphonichnus, Teichichnus and Zoophycos are present. FA-6 records deposition in the subtidal bay region. The tidal flat deposits of the Khussak Formation show a distinctive ichnofacies trend. The Cruziana ichnofacies characterizes the lower energy proximal deposits (intertidal mixed and mud flat). In contrast, the high-energy distal deposits (lower intertidal sand flat and shallow subtidal) are characterized by Skolithos ichnofacies. The ichnologic variation between the mud flat, mixed flat, sand flat, and subtidal deposits are gradational, but they can be used as facies boundary indicators. The bioturbation intensity and diversity of trace fossils within the lower Cambrian (Series 2, Stage 4) tidal flat deposits of the Khussak Formation suggest elevated levels of bioturbation intensity during stable environmental conditions in the Cambrian. Furthermore, it implies that the Agronomic Revolution was already emplaced in lower Cambrian (Series 2, Stage 4) tidal flat settings.

Bank strength variability and its impact on the system-scale morphodynamics of the upper Amazon River in Brazil

Large anabranching rivers form channels in sediments of varying strength, resulting from erosional and depositional processes that act over geological time scales. Although bank strength variability is known to affect channel morphodynamics, its impact on the migration of large sand-bed rivers remains poorly understood. We report the first in situ measurements of bank strength from an ∼100-km-long reach of the Solimões River, the Brazilian Amazon River upstream of Manaus. These show that cohesive muds in Pleistocene terraces along the river’s right margin have bank strengths as much as three times greater than Holocene floodplain deposits composing the left bank. Image analysis suggests these resistant outcrops determine channel-bar dynamics: channel widening and bar deposition are inhibited, which lowers planform curvature and reduces erosion of the opposing bank. Planform analysis of the 1600-km-long Solimões River between 1984 and 2021 shows that where the channel is associated with Pleistocene terraces, lower rates of bank erosion and bar deposition are evident. Heterogeneity in bank strength is thus a first-order control on the large-scale morphodynamics of the world’s largest lowland river.

Sedimentary evolutionary processes, architecture, and sedimentary model of a lobate shallow-water delta: insights from flume simulation experiments

By using equipment such as time-lapse cameras and 3D laser scanners, this study conducted flume simulation experiments to obtain topographic elevation, the amount of sediment increment, sedimentary cross sections, and other related data and diagrams. Quantitative software was then utilized for sedimentological analysis to clarify the depositional evolution, architecture, and depositional model of lobate shallow-water deltas. The research results show that under the influence of hydraulic conditions and main distributary channels, lobate shallow-water deltas primarily undergo three evolution stages: initial channel formation, distributary channel system formation, and continuous stable evolution stage. The architecture elements of lobate shallow-water deltas can be divided into distributary channels dominated by channel deposition, proximal river mouth bars, transitional bars, and distal bars. The lateral composition and downstream deposition of the river-bar combination are the main structural units of the shallow water delta. The proximal profile distributary channels exist in the form of thick channel sand bodies with multiple repetitive intercalation cycles, and thick proximal bar deposits form on both sides of the distributary channels. The bifurcation of distributary channels in the middle profile leads to a reduction in channel scale, while large-scale proximal bars still develop on both sides of the channel. In the distal profile, a substantial river mouth bar complex is formed at the break of the delta front slope, and with the gradual progradation of the delta, the river mouth bar deposition zone becomes flattened. Based on the insights of the flume simulation experiments, a sedimentary model for lobate shallow-water delta was established.

Sedimentary architecture of thin-layer beach bar sand bodies in the G oilfield, Niger

The G oilfield in the southeastern Termit Basin of Niger is characterized by thin-bed beach bar deposits exhibiting strong reservoir heterogeneity and suboptimal production efficiency, necessitating internal structural dissection of the beach bar sand bodies. Employing a well-seismic integration approach, we systematically dissect the architecture of these sand bodies layer by layer to determine their spatial distribution. A classification scheme for beach bar architecture is proposed, with core and log data analysis identifying essential architectural elements and their logging responses. Seismic amplitudes, thin bed delineation, frequency decomposition inversion attributes, and attribute fusion technology delineate the architectural boundaries. Integrating five indicators from four-level architectural recognition at wellbores—shallow lake mudstone appearance, bar margin/beach microfacies occurrence, logging curve morphology differences, beach bar thickness variations, and elevation differences between adjacent bars—enables detailed dissection of the beach bar architecture, corroborated by connectivity analysis. In the study area, beach bar distribution primarily develops in two modes: vertical stacking (accumulation of multiple main bars from different episodes) and isolated (stable mudstone interlayers between main bar sand bodies appearing relatively isolated). This research provides a basis for dissecting beach bar architecture reservoirs under sparse well conditions.

Early Cretaceous continental sedimentatary environments, palaeogeography and biota of southeastern Western Siberia

Abstract The Ilek Formation was distinguished by Ragozin in 1936 in the SE of Western Siberia, as a sequence of continental deposits accumulated in the alluvial and lacustrine environments, dating Barremian–Early Albian. At the end of the twentieth century a series of sites, comprising rich and diverse Early Cretaceous vertebrates, was discovered, which forced an intensive comprehensive research of the Ilek Formation. The stratotype section of the Bolshoi Ilek represents an abnormal section of the Ilek Formation, comprising the deposits of prodelta, delta front and mouth bars. Three regions within the area of the Ilek Formation reflect the evolution of the fluvial system. The eastern region represents the oldest part of the section, preliminarily dated to the Barremian based on spore and pollen analysis in addition to lithological characteristics. The western region was probably deposited during ocean anoxic event (OAE) 1a. The central region represents the final stage of the tectonic cycle, deposited within the environments of the calm plain river and fluvial delta. The vertebrate assemblages distinguished for the Kiya and Bolshoi Kemchug basins have specific features, which can support the heterochrony of the Ilekian deposits.

The evolution of meandering rivers in sedimentary basins: Insights from the lower Drava (Hungary/Croatia)

AbstractRivers flowing through sedimentary basins are subjected to a variety of controls. The main goal of our study was to identify the effect of external (e.g. climate changes, tectonics) and internal controls (e.g. sediment transport, deposition, vegetation cover) on the evolution of meandering rivers flowing through sedimentary basins using the example of the lower Drava River (Europe, Hungary/Croatia). Field research was conducted along a 50‐km‐long section of the valley. Sedimentary data from boreholes and corings, 35 km of ground‐penetrating radar surveys and analyses of digital maps were conducted to reconstruct channel planform changes. Traces of four meander belts were identified, and 39 AMS radiocarbon dates were used to distinguish the chronology of the fluvial events. The evolution of the lower Drava River comprised alternating periods of deposition (formation of aggrading meander belt) and avulsions. The channel belts were formed owing to upstream sediment delivery and floodplain storage. Changes in climate humidity and the occurrence of high flows influenced the planform of the meanders within particular channel belts. The oldest channel was active at least ~40 000 cal. BP before being reworked by subsequent meanders active between the Late Pleniglacial (30 000–14 700 cal. BP) and Late Glacial (~11 000 cal. BP) periods. The channel belts shifted to the south in the Holocene, between ~11 000 and 250 cal. BP due to the presence of a thrust fault situated diagonally to the Drava Valley. Results show channel width, channel belt width and the surface area of point bar deposits increased in the succeeding generations of meanders and that the style and sedimentary architecture of the channel belts were dominantly dependent on autogenic controls, that is, sediment delivery, aggradation and erosional events (e.g. formation of chute cut‐offs).

Quantifying Formative Processes in River‐ and Tide‐Dominated Deltas for Accurate Prediction of Future Change

AbstractAlthough most deltas are expected to lose land due to climate change and urbanization, tide‐dominated deltas have been suggested to gain land. The processes of land change in such deltas are, however, not well understood, and tide‐dominated deltas with all their known morphological attributes have not been simulated before. Our Delft3D simulations successfully reproduce tide‐dominated delta morphology, and show that tide‐dominated deltas with seaward widening and stable channels, elongate tidal bars and a funnel shape form when bidirectional tidal flow occurs along the whole delta. In contrast, tide‐influenced but river‐dominated deltas protrude from shoreline, and are efficient at gaining land by mouth bar deposition and bifurcations. These differences are a function of tidal‐fluvial interactions expressed by month‐averaged seaward and lateral velocities. Identification of delta types is thus critical for predicting future land change. We propose morphological metrics to differentiate river‐ and tide‐dominated deltas and test metrics on 40 modern deltas.

Tracing past extreme floods on an alluvial fan using geophysical surveying

SummarySedimentary units in alluvial fans may record gradual transport and deposition during multiple floods or sediment‐laden flows or, conversely, during few catastrophic events. While outcrops are a valuable source of direct information to constrain past geomorphic and hydrologic processes, such exposures are scarce, especially along aggrading rivers or those that have been subject to recent catastrophic sedimentation. In this context, near‐surface geophysical techniques can constrain the dimensions, internal architecture, composition, and petrophysical properties of different sedimentary units. We consider the Grimmbach alluvial fan in the cuesta landscape of southwestern Germany, which was heavily impacted by sediment and wood loads during a flash flood in 2016; published radiocarbon dates indicate that at least three floods similar to the one in 2016 may have occurred since the 17th century. To test whether and to which detail near‐surface geophysics might reveal the sedimentary legacy of these floods, we survey the Grimmbach alluvial fan using detailed topographic data and geophysical imaging based on electromagnetic induction, electrical resistivity tomography (ERT), and ground‐penetrating radar. Our geophysical results indicate former channel courses and two coarse bar deposits up to 3 m below the surface, which are comparable with the more extensive bar deposits of the 2016 flood. From the ERT models, we interpret coarse, up to 5 m thick, gravel lag overlying bedrock at a maximum depth of 10 m. Our geophysical results also highlight patches of finer materials derived from gradual sedimentation and soil development. Overall, our results show that the Grimmbach alluvial fan may have formed and reshaped during catastrophic flows, which likely caused channel avulsions. Our findings point to the need to reconsider flash flood and debris‐flow hazards in similar headwaters and fans of this seemingly quiescent cuesta landscape in southern Germany.

Mud retention in hydrologically closed basins promoted pre-vegetation meandering: evidence from the Neoproterozoic Diabaig Formation, Scotland

Abstract Fluvial point-bar deposits are described from the c. 1 Ga Diabaig Formation, representing some of the most compelling architectural evidence of planform sinuosity recognized in pre-vegetation sedimentary strata to date. The stratigraphical architecture of the deposits indicates different planform transformations in response to meander-bend expansion and downstream migration. Point-bar architectural motifs are locally abundant in the unit and this is attributed to elevated amounts of mud-calibre cohesive sediment. Mud afforded landscape stability that, together with limited stream power, was sufficient to corral sediment into single-thread, sinuous channels. Significant volumes of fine-grained sediment were encouraged to accumulate by the local basin setting of several topographically confined endorheic depressions. These conditions ceased once the hydrologically closed valleys were filled, with the overlying sandstone-dominated Applecross Formation subsequently deposited by low-sinuosity, braided rivers that bypassed fine-grained sediment oceanward. Other endorheic basins conducive to the formation of depositional meanders should be expected to have been relatively widespread on pre-vegetation Earth, though many individually limited in size, confined by their inherited topographies. Depositional architectures representative of sinuous channel planforms remain scarce within more extensive, externally drained basins.

Flow depth estimates and avulsion behaviour in alluvial stratigraphy (Willwood Formation, Bighorn Basin, Wyoming, USA)

AbstractThe size and geometry of river channels play a central role in sediment transport and the character of deposition within alluvial basins across spatiotemporal scales spanning the initiation of grain movement to the filling of accommodation generated by subsidence. This study compares several different approaches to estimating palaeoflow depths from fluvial deposits in the early Palaeogene Willwood Formation of north‐west Wyoming, USA. Fluvial story heights (n = 60) and mud plug thicknesses (n = 13) are statistically indistinguishable from one another and yield palaeoflow depth estimates of 4 to 6 m. The vertical relief on bar clinoforms (n = 112) yields smaller flow depths, by a factor of ca 0.3, with the exception that the largest bar clinoforms match story heights and mud plug estimates. This observation is consistent with modern river data sets that indicate unit bar clinoforms do not capture the reach‐mean bank‐full flow depths except in rare circumstances. Future studies should use story heights (i.e. compound bar deposits) and mud plugs to estimate bank‐full flow depths in alluvial strata. Additionally, the thickness of multi‐storied fluvial sandbodies (n = 102) and overbank cycles composed of paired crevasse splay and palaeosol deposits (n = 45) were compared. The two depositional units display statistically indistinguishable mean and median values. Building upon previous depositional models, these observations suggest basin rivers aggraded approximately one flow depth prior to major avulsion. This avulsion process generated widespread crevasse splay deposition across the floodplain. Once the main river channel stem was reestablished, overbank flooding and palaeosol development dominated floodplain settings. The depositional model implies river aggradation autogenically generated topography in the basin that was effectively filled during the subsequent avulsion. This constitutes a meso‐timescale (103–104 years) compensational pattern driven by morphodynamics that may account for the high completeness of fossil and palaeoclimate records recovered from the basin.

A world-class example of a Late Palaeozoic glaciated landscape in Chad

Continental glacial deposits are rarely preserved in nature. Recent studies of the LPIA reveal the presence of periglacial deposits in Northern Chad covering an area of 20,000 km2. Using satellite maps, this project managed to examine the entirety of the Ennedi Plateau. 3D reconstructions of this mountain range reveal complex glacial morphological features. This paper presents large-scale subglacial features interbedded with proglacial fluvial features and discussed the variables at play during the progressive, repetitive, and perhaps cyclical nature of the Carboniferous glaciation. Using remote sensing techniques, this paper shows that there are three major ice advance–retreat cycles that led to the deposition of mega-scale glacial lineations with a complex fluvial channel belt. Palaeo-geomorphological mapping of satellite images in northern Chad reveals that the Late Carboniferous Chadian ice sheet covered at least 20,000 km2, a much greater extent than previously thought (ca. 6000 km2). Glacial lineations (GLs) are dominant and extensive on the plateau. They are arranged in widespread ice stream networks and developed at multiple stratigraphic levels. This broadly indicates palaeo-ice flow to the north. We report a newly discovered pristine channel belt, covering at least 300 km2 of a plateau in the Ennedi Plateau. A swarm of N–S trending channels, up to 250 m wide, is recognised. These are of both braided and meandering characters and are organised into channel belts of up to 500 m width. The mapped palaeo-morphology shows complex crosscutting relationships. The channel belt is intercalated with zones of GLs; hence, ice activity preceded and succeeded fluvial activity. Preserved planform sinuous channel geometries show distinctive point bar deposits and scroll bar geometries therein, which testify to a terrestrial, subaerial environment, rather than to their evolution in the subglacial realm, e.g. as eskers. Therefore, we find that cross-cutting, amalgamated channel systems record the complex phases of meltwater release during glacial retreat in the advance–retreat cycles of the Late Palaeozoic Ice Age in Chad.

Reconstruction of 3D Reservoir Lithological Model Using 2D Facies Profiles in SU 36-11 Area of Ordos Basin, China

In the middle and late stages of gas field development, the establishment of a fine reservoir lithological model is an important basis for drilling well pattern adjustment and potential exploitation. The SU 36-11 area of the Ordos basin in China is developing braided channel sediment with rich gas resources. However, the success rate of drilling wells is low due to the complex reservoir heterogeneity and the lack of a fine reservoir lithological model. In this paper, the complex internal structure of the reservoir sand body is revealed using the architectural element analysis method. Three sand body models, that is, isolated channel, superimposed channel, and cut superimposed channel, can be recognized. The effective sand body is mainly the channel bar deposit with a thickness of 2–5 m, a width of 200–500 m, a length of 400–700 m, a width ratio of 50–120, and a length-to-width ratio of 1.5–2. The 2D maps of the lithofacies (architectural elements) were then digitized to create 2D training images (TI) for the construction of the 3D model. The 2D data template was selected to scan the TI to obtain the 2D multi-point probability. The 3D multi-point probability was then generated using the probability fusion theory. The Monte Carlo sampling was used to predict the lithological type between wells. Finally, the 3D reservoir lithological model was built directly using the 2D lithological profiles. From the model, the geometry of the braided channel, channel bar, and flood plain was well revealed, and the spatial distribution of effective reservoir sand bodies was accurately predicted. The cross-validation test shows that the error of the channel bar is 6.5% on average, which improves the accuracy of the prediction of lithology in the sub-surface and can be used to guide the subsequent development of residual gas.

The Albian – Cenomanian boundary on the southern Tethyan margin: Abu Gharadig Basin, Northern Western Desert, Egypt

Lithostratigraphy and biostratigraphy sometimes provide contradictory interpretations, especially when trying to establish a chrono-lithological boundary between two reservoir formations from different paleoenvironments. One of these contradictions is the definition of the boundary between the Kharita and Bahariya formations and the Albian and Cenomanian Stages in the Abu Gharadig Basin of the Western Desert, Egypt. an integrated approach has been applied to this problem using borehole image logs, palynology, and conventional wireline logs. authors also calibrate the stratigraphic sections with global chronostratigraphic charts, using nannoplankton, planktonic foraminifera, and dinoflagellates to construct a better understanding of this basin. The abundances of pollen, spores, and dinoflagellates in the interpreted reservoir intervals suggest that hydrocarbons trapped in the Albian-Cenomanian reservoirs are composed of type II/III kerogen. The interpreted intervals are subdivided into seven depositional cycles: one in the Kharita Formation, five in the Bahariya Formation, and one in the Abu Roash Formation. The interpreted environments are, from bottom to top: I) fluvio-deltaic distributary channels, II) and III) intertidal to subtidal deposits, IV) offshore bar deposits, V) intertidal to subtidal deposits, VI) shoreface deposits, and VII) shallow marine carbonate deposits. Diagnostic palynomorphs reflect climatic change from arid to humid conditions and also enable changes in salinity to be determined.Much of the published work in the southern Tethyan margin, particularly in the Western Desert, is either related to stratigraphy, palynology, or reservoir evaluation with a focus on a narrow field. Tectonostratigraphic studies and analyses of petroleum system elements have often been very regional in nature. The work presented here is of an intermediate scale linking regional frameworks with local details to provide comprehensive information about the Albian-Cenomanian succession in the Northern Western Desert. The focus has been the Abu Gharadig basin but this integrated approach has implications for other basins in North Africa. A specific example is the similarity of Western Desert and Libyan Sirt basin Albian-Cenomanian key palynomorphs which would facilitate a similar integrated chrono-lithologic intepretation.