A relationship between alluvial backswamps and avulsion cycles: an example from the Willwood Formation of the Bighorn Basin, Wyoming

Stratigraphically limited intervals from the Lower Eocene Willwood Formation contain laterally extensive carbonaceous shales and ribbon sandstone networks associated with channel avulsion. We present data from one such interval that documents the avulsion sequence. Vertical sections measured along the outcrop of this interval are similar and comprise a basal carbonaceous shale overlain by fine-grained deposits on which weakly developed, hydromorphic paleosols formed. The paleosols enclose and are locally incised by ribbon sandstones, some of which cut down to and partly through the carbonaceous shale. The ribbons have width/thickness ratios between 3 and 13. Some ribbons cluster at a particular stratigraphic level, which, together with paleocurrent trends, suggests that they formed channel networks. Sections are capped by yellow-brown paleosols showing moderate pedogenic development. We suggest that the carbonaceous shales developed in low-lying topogeneous swamps in distal portions of the floodplain far from the trunk channel. Such a location set limits on the sediment that they received. The mudrocks with weakly developed paleosols and associated ribbon sandstones are interpreted as crevasse-splay complexes resulting from avulsion of the trunk river. The ribbon sandstones represent ancient feeder channels of the avulsion complex. The rapid influx of avulsion deposits appears to have been crucial to preserving the organic material, and this study reveals an important and as yet uncharacterized link between trunk channel processes and the accumulation of organic-rich deposits in distal alluvial swamps. Similar deposits are found in other stratigraphic units in the Rocky Mountain region, and the development of these and other organic-rich deposits should be reassessed in terms of channel avulsion.     

Multi‐scale analysis of a migrating submarine channel system in a tectonically‐confined basin: The Miocene Gorgoglione Flysch Formation,southern Italy

The Miocene Gorgoglione Flysch Formation records the stratigraphic product of protracted sediment transfer and deposition through a long‐lived submarine channel system developed in a narrow and elongate thrust‐top basin of the Southern Apennines (Italy). Channel‐fill deposits are exposed in an outcrop belt approximately 500 m thick and 15 km long, oriented oblique to the palaeoflow, which was roughly south‐eastward. These exceptional exposures of channel‐fill strata allow the stacking architectures and the evolution of the channel system to be analyzed at multiple scales, enabling the effects of syn‐sedimentary thrust tectonics and basin confinement on the depositional system development to be deciphered. Two end‐member types of elementary channel architecture have been identified: high‐aspect‐ratio, weakly‐confined channels, and low‐aspect‐ratio, incisional channels. Their systematic stacking results in a complex pattern of seismic‐scale depositional architectures that determines the stratigraphic framework of the deep‐water system. From the base of the succession, two prominent channel complex sets have been recognized, namely CS1 and CS2, consisting of amalgamated incisional channel elements and weakly‐confined channel elements. These channelized units are overlain by isolated incisional channels, erosional into mud‐prone slope deposits. The juxtaposition of different channel architectures is interpreted to have been governed by regional thrust‐tectonics, in combination with a high subsidence rate that promoted significant aggradation. In this scenario, the alternating ‘in sequence’ and ‘out of sequence’ tectonic pulses of the basin‐bounding thrusts controlled the activation of coarse‐clastic inputs in the basin and the resulting stacking architectures of channelized units. The tectonically‐driven confinement of the depositional system limited the lateral offset in channel stacking, preventing large‐scale avulsions. This study represents an excellent opportunity to analyze the stratigraphic evolution of a submarine channel system in tectonically‐active settings from an outcrop perspective. It should find wide applicability in analogous depositional systems, whose stratigraphic architecture has been influenced by tectonically‐controlled lateral confinement and associated lateral tilting.     

Controls on the accumulation of coal and on the development of anastomosed fluvial systems in the Cretaceous Dakota Formation of southern Utah

Alluvial strata of the Cretaceous Dakota Formation of southern Utah are part of a transgressive systems tract associated with a foreland basin developed adjacent to the Sevier orogenic belt. These strata contain valley fill deposits, anastomosed channel systems and widespread coals. The coals constitute a relatively minor part of the Dakota Formation in terms of sediment volume, but may represent a substantial amount of the time represented by the formation. The coals are separated by clastic units up to 20 m thick. The stratigraphically lowest clastic unit of the Dakota Formation lies above an unconformity cut into Jurassic rocks. Incised valleys associated with the unconformity are up to 12 m deep. Two discrete episodes of valley fill sedimentation are recognized, including a lower sandstone unit with conglomerate layers, and an upper, discontinuous, coal-bearing unit. After the valleys filled, the area became one of low relief where extensive mires formed. Peat accumulation was interrupted at least three times by deposition of clastic sediment derived from the west. The clastic units consist of sandstone, mudstone or heterolithic ribbon bodies, stacked tabular sandstones, and laminated mudstones, and contain minor coal beds less than 0·35 m thick. Ribbon bodies are 1–9 m thick and 15–160 m wide, have pronounced basal scours, and are filled with both lateral and vertical accretion deposits. An anastomosed channel complex is suggested by the large number of coeval channels of varying dimensions, the variation in the structure and grain size of channel fills, and the presence of abundant tabular sandstones interpreted as crevasse splays. Although some sandstone bodies have well developed lateral accretion surfaces, the overall ribbon geometry indicates that mature meandering streams were not well developed. This is in contrast to modern anastomosed systems, which are commonly thought to be a transitional morphology caused by avulsion of a meander belt to a new position on its floodplain. Rather than being a transitional channel pattern related to river avulsion, the anastomosed channels of the Dakota Formation may have formed part of a large inland delta that episodically invaded widespread mires. The mires developed during periods when clastic influx was reduced either by high rates of subsidence close to the thrust belt or by deflection of rivers by emergent thrusts.     

Geological controls on the geometry of incised‐valley fills: Insights from a global dataset of late‐Quaternary examples

Incised valleys that develop due to relative sea‐level change are common features of continental shelves and coastal plains. Assessment of the factors that control the geometry of incised‐valley fills has hitherto largely relied on conceptual, experimental or numerical models, else has been grounded on case studies of individual depositional systems. Here, a database‐driven statistical analysis of 151 late‐Quaternary incised‐valley fills has been performed, the aim being to investigate the geological controls on their geometry. Results of this analysis have been interpreted with consideration of the role of different processes in determining the geometry of incised‐valley fills through their effect on the degree and rate of river incision, and on river size and mobility. The studied incised‐valley fills developed along active margins are thicker and wider, on average, than those along passive margins, suggesting that tectonic setting exerts a control on the geometry of incised‐valley fills, probably through effects on relative sea‐level change and river behaviour, and in relation to distinct characteristics of basin physiography, water discharge and modes of sediment delivery. Valley‐fill geometry is positively correlated with the associated drainage‐basin size, confirming the dominant role of water discharge. Climate is also inferred to exert a potential control on valley‐fill dimensions, possibly through modulations of temperature, peak precipitation, vegetation and permafrost, which would in turn affect water discharge, rates of sediment supply and valley‐margin stability. Shelves with slope breaks that are currently deeper than 120 m contain incised‐valley fills that are thicker and wider, on average, than those hosted on shelves with breaks shallower than 120 m. No correlation exists between valley‐fill thickness and present‐day coastal‐prism convexity, which is measured as the difference in gradient between lower coastal plains and inner shelves. These findings challenge some concepts embedded in sequence stratigraphic thinking, and have significant implications for analysis and improved understanding of ‘source to sink’ sediment route‐ways, and for attempting predictions of the occurrence and characteristics of hydrocarbon reservoirs.     

Down‐slope facies variability within deep‐water channel systems: Insights from the Upper Cretaceous Cerro Toro Formation,southern Patagonia

In southern Patagonia, outcrops of the Upper Cretaceous Cerro Toro Formation preserve a >150 km long deep‐water axial channel belt in the Magallanes–Austral Basin, providing a unique opportunity to investigate longitudinal variations in the depositional characteristics of a deep‐water channel system. This study documents sedimentological, stratigraphical and geochronological data from the Cerro Toro Formation in the Argentine sector of the basin. New results are integrated with previous work from the Chilean basin sector to conduct a basin‐scale comparison of the timing of deposition, provenance and lithofacies proportions. The Cerro Toro channel belt includes a nearly 1000 m thick section characterized by high‐density turbidites and mass‐wasting deposits. Two ash beds from the base of the section yield U–Pb zircon ages of 90·4 ± 2 Ma and 88·0 ± 3 Ma, indicating similar initiation ages as documented in the Chilean sector. The U–Pb detrital zircon age spectra from samples in the study area reveal similar provenance trends to samples from the Chilean basin sector, with peak age populations at 310 to 260 Ma, 160 to 135 Ma and 110 to 82 Ma. The maximum depositional age of the channel belt in the Argentine sector is 87·8 ± 1·5 Ma and all new geochronology data corroborate an 86 to 80 Ma depositional age for the main Cerro Toro channel belt. Statistical analyses of 7370 beds from nearly 8000 m of new and previously published stratigraphic sections along the entire outcrop belt suggest progressive variations in the down‐system proportion of lithofacies. In the up‐slope region, lithofacies representing mass wasting processes (for example, debris‐flow and mass‐transport deposits) account for ca 29% of the stratigraphic thickness, as opposed to 5% in the down‐slope region of the channel belt, where turbidity current deposits are more prevalent. The proportion of beds >1 m thick also decreases systematically down slope, particularly for conglomeratic turbidite deposits. This work highlights that: (i) the proportion of thick beds and distribution of lithofacies are key down‐system changes in the stratigraphic fill of this deep‐water channel belt; (ii) detrital zircon trends suggest a relatively well‐mixed longitudinal depositional system; and (iii) geochronology of the main Cerro Toro outcrop belt supports but does not necessitate the model of a single, roughly age‐equivalent, channel system. This study has implications for understanding the downslope variability in depositional processes, stratigraphic architecture and reservoir quality of submarine channel systems.     

Climatic versus halokinetic control on sedimentation in a dryland fluvial succession

Fluvial systems and their preserved stratigraphic expression as the fill of evolving basins are controlled by multiple factors, which can vary both spatially and temporally, including prevailing climate, sediment provenance, localized changes in the rates of creation and infill of accommodation in response to subsidence, and diversion by surface topographic features. In basins that develop in response to halokinesis, mobilized salt tends to be displaced by sediment loading to create a series of rapidly subsiding mini‐basins, each separated by growing salt walls. The style and pattern of fluvial sedimentation governs the rate at which accommodation becomes filled, whereas the rate of growth of basin‐bounding salt walls governs whether an emergent surface topography will develop that has the potential to divert and modify fluvial drainage pathways and thereby dictate the resultant fluvial stratigraphic architecture. Discerning the relative roles played by halokinesis and other factors, such as climate‐driven variations in the rate and style of sediment supply, is far from straightforward. Diverse stratigraphic architectures present in temporally equivalent, neighbouring salt‐walled mini‐basins demonstrate the effectiveness of topographically elevated salt walls as agents that partition and guide fluvial pathways, and thereby control the loci of accumulation of fluvial successions in evolving mini‐basins: drainage pathways can be focused into a single mini‐basin to preserve a sand‐prone fill style, whilst leaving adjoining basins relatively sand‐starved. By contrast, over the evolutionary history of a suite of salt‐walled mini‐basins, region‐wide changes in fluvial style can be shown to have been driven by changes in palaeoclimate and sediment‐delivery style. The Triassic Moenkopi Formation of the south‐western USA represents the preserved expression of a dryland fluvial system that accumulated across a broad, low‐relief alluvial plain, in a regressive continental to paralic setting. Within south‐eastern Utah, the Moenkopi Formation accumulated in a series of actively subsiding salt‐walled mini‐basins, ongoing evolution of which exerted a significant control on the style of drainage and resultant pattern of stratigraphic accumulation. Drainage pathways developed axial (parallel) to salt walls, resulting in compartmentalized accumulation of strata whereby neighbouring mini‐basins record significant variations in sedimentary style at the same stratigraphic level. Despite the complexities created by halokinetic controls, the signature of climate‐driven sediment delivery can be deciphered from the preserved succession by comparison with the stratigraphic expression of part of the system that accumulated beyond the influence of halokinesis, and this approach can be used to demonstrate regional variations in climate‐controlled styles of sediment delivery.     

Turbidites in the Upper Carboniferous Ross Formation, western Ireland: reconstruction of a channel and spillover system

ABSTRACT The Upper Carboniferous deep‐water rocks of the Shannon Group were deposited in the extensional Shannon Basin of County Clare in western Ireland and are superbly exposed in sea cliffs along the Shannon estuary. Carboniferous limestone floors the basin, and the basin‐fill succession begins with the deep‐water Clare Shales. These shales are overlain by various turbidite facies of the Ross Formation (460 m thick). The type of turbidite system, scale of turbidite sandstone bodies and the overall character of the stratigraphic succession make the Ross Formation well suited as an analogue for sand‐rich turbidite plays in passive margin basins around the world. The lower 170 m of the Ross Formation contains tabular turbidites with no channels, with an overall tendency to become sandier upwards, although there are no small‐scale thickening‐ or thinning‐upward successions. The upper 290 m of the Ross Formation consists of turbidites, commonly arranged in thickening‐upward packages, and amalgamated turbidites that form channel fills that are individually up to 10 m thick. A few of the upper Ross channels have an initial lateral accretion phase with interbedded sandstone and mudstone deposits and a subsequent vertical aggradation phase with thick‐bedded amalgamated turbidites. This paper proposes that, as the channels filled, more and more turbidites spilled further and further overbank. Superb outcrops show that thickening‐upward packages developed when channels initially spilled muds and thin‐bedded turbidites up to 1 km overbank, followed by thick‐bedded amalgamated turbidites that spilled close to the channel margins. The palaeocurrent directions associated with the amalgamated channel fills suggest a low channel sinuosity. Stacks of channels and spillover packages 25–40 m thick may show significant palaeocurrent variability at the same stratigraphic interval but at different locations. This suggests that individual channels and spillover packages were stacked into channel‐spillover belts, and that the belts also followed a sinuous pattern. Reservoir elements of the Ross system include tabular turbidites, channel‐fill deposits, thickening‐upward packages that formed as spillover lobes and, on a larger scale, sinuous channel belts 2·5–5 km wide. The edges of the belts can be roughly defined where well‐packaged spillover deposits pass laterally into muddier, poorly packaged tabular turbidites. The low‐sinuosity channel belts are interpreted to pass downstream into unchannellized tabular turbidites, equivalent to lower Ross Formation facies.     

Controls on large‐scale patterns of fluvial sandbody distribution in alluvial to coastal plain strata: Upper Cretaceous Blackhawk Formation,Wasatch Plateau,Central Utah,USA

Current models of alluvial to coastal plain stratigraphy are concept‐driven and focus on relative sea‐level as an allogenic control. These models are tested herein using data from a large (ca 100 km long and 300 m thick), continuous outcrop belt (Upper Cretaceous Blackhawk Formation, central Utah, USA). Many channelized fluvial sandbodies in the Blackhawk Formation have a multilateral and multistorey internal character, and they generally increase in size and abundance (from ca 10% to ca 30% of the strata) from base to top of the formation. These regional, low‐resolution trends exhibit much local variation, but are interpreted to reflect progressively decreasing tectonic subsidence in the upper Blackhawk Formation and overlying Castlegate Sandstone. The trend may also incorporate progressively more frequent channel avulsion during deposition of the lower Blackhawk Formation. Laterally extensive coal zones formed on the coastal plain during shallow‐marine transgressions, and define the high‐resolution stratigraphic framework of the lower Blackhawk Formation. Large (up to 25 m thick and 1 to 6 km wide), multistorey, multilateral, fluvial channel‐complex sandbodies that overlie composite erosion surfaces occur at distinct stratigraphic levels, and are interpreted as fluvial incised valley fills. Low amplitude (<30 m) relative sea‐level variations are interpreted as the dominant control on stratigraphic architecture in the lower Blackhawk Formation, which was deposited up to 50 km inland from the coeval shoreline. In contrast, the high‐resolution stratigraphy of the upper Blackhawk Formation is poorly defined, and channelized fluvial sandbodies are poorly organized. Vertical and laterally offset stacking of a small proportion (<10%) of sandbodies produced ‘clusters’ that are not confined by ‘master’ erosion surfaces. Avulsion is interpreted to dominate the stratigraphic architecture of the upper Blackhawk Formation. This data‐driven analysis indicates that alluvial to coastal plain stratigraphic architecture reflects a combination of various allogenic controls and autogenic behaviours. The relative sea‐level control emphasized in sequence stratigraphic models is only rarely dominant.     

Origin,evolution and anatomy of silt‐prone submarine external levées

Submarine external levées are constructional features that develop outside slope channel systems, and are a volumetrically significant component of continental margins. However, detailed observations of their process sedimentology and depositional architecture are rare. Extensive exposures of external levées at multiple stratigraphic intervals and well‐constrained palaeogeographic positions in the Fort Brown Formation, Karoo Basin, South Africa, have been calibrated with research boreholes. This integrated data set permits their origin, evolution and anatomy to be considered, including high‐resolution analysis of sedimentary facies distribution and characterization of depositional sub‐environments. An idealized model of the stratigraphic evolution and depositional architecture of external levées is presented, and variations can be attributed to allogenic (for example, sediment supply) and autogenic (for example, channel migration) factors. Initiation of external levée construction is commonly marked by deposition of a basal sand‐rich facies with sedimentary structures indicating rapid deposition from unconfined flows. These deposits are interpreted as frontal lobes. Propagation of the parent channel, and resultant flow confinement, lead to partial erosion of the frontal lobe and development of constructional relief (levées) by flow overspill and flow stripping. Overall fining‐upwards and thinning‐upwards profiles reflect increased flow confinement and/or waning flow magnitude through time. Identification of a hierarchy of levée elements is not possible due to the absence of internal bounding surfaces or sharp facies changes. The down‐slope taper in levée height and increasing channel sinuosity results in increasing numbers of crevasse lobe deposits, and is reflected by the increased occurrences of channel avulsion events down‐dip. External levées from the Fort Brown Formation are silt‐rich; however their stratigraphic evolution and the distribution of many components (such as sediment waves and crevasse lobe) share commonalities with mud‐rich external levées. This unique integrated data set has permitted the first high‐resolution characterization of external submarine levée systems.     

Storm‐dominated shelf‐edge delta successions in a high accommodation setting: The palaeo‐Orinoco Delta (Mayaro Formation), Columbus Basin,South‐East Trinidad

Pliocene age deposits of the palaeo‐Orinoco Delta are evaluated in the Mayaro Formation, which crops out along the western margin of the Columbus Basin in south‐east Trinidad. This sandstone‐dominated interval records the diachronous, basinwards migration of the shelf edge of the palaeo‐Orinoco Delta, as it prograded eastwards during the Pliocene–Pleistocene (ca 3·5 Ma). The basin setting was characterized by exceptionally high rates of growth‐fault controlled sediment supply and accommodation space creation resulting in a gross basin‐fill of around 12 km, with some of the highest subsidence rates in the world (ca 5 to 10 m ka^?1). This analysis demonstrates that the Mayaro Formation was deposited within large and mainly wave‐influenced shelf‐edge deltas. These are manifested as multiple stacks of coarsening upward parasequences at scales ranging from tens to hundreds of metres in thickness, which are dominated by storm‐influenced and wave‐influenced proximal delta‐front sandstones with extensive, amalgamated swaley and hummocky cross‐stratification. These proximal delta‐front successions pass gradationally downwards into 10s to 100 m thick distal delta front to mud‐dominated upper slope deposits characterized by a wide variety of sedimentary processes, including distal river flood and storm‐related currents, slumps and other gravity flows. Isolated and subordinate sandstone bodies occur as gully fills, while extensive soft sediment deformation attests to the high sedimentation rates along a slope within a tectonically active basin. The vertical stratigraphic organization of the facies associations, together with the often cryptic nature of parasequence stacking patterns and sequence stratigraphic surfaces, are the combined product of the rapid rates of accommodation space creation, high rates of sediment supply and glacio‐eustasy in the 40 to 100 Ka Milankovitch frequency range. The stratigraphic framework described herein contrasts strikingly with that described from passive continental margins, but compares favourably to other tectonically active, deltaic settings (for example, the Baram Delta Province of north‐west Borneo).     

Stratigraphy and architecture of the Upper Triassic Ischigualasto Formation,Ischigualasto Provincial Park,San Juan,Argentina

The Ischigualasto Formation in northwestern Argentina contains abundant fluvial channel sandstones, overbank mudstones, and paleosols that were deposited in a northwest-trending continental-rift basin during Late Triassic time. In the study area the formation progressively thins from ～700 m in the west to ～400 m in the east, over a distance of 7 km. This thinning is accompanied by a relative decrease in the abundance of fluvial channel sandstones and an increase in mud-rich overbank deposits and paleosols. While preserved channel deposits in the formation are highly variable in terms of their size and stratigraphic distribution, four general channel forms can be recognized based on their overall cross sectional geometry and internal sedimentary structures. Of these, the dominant channel-body types are interpreted as the deposits of sandy multi-channel fluvial systems. The internal stratigraphic architecture of the Ischigualasto Formation indicates that during deposition, the central part of the basin was the location of a long-lived, north flowing, fluvial channel belt that received relatively continuous channel and proximal overbank deposition. To the east, however, channel-related deposition was more infrequent, resulting in enhanced pedogenic modification of alluvial deposits. The overall thickness and facies trends observed in the Ischigualasto Formation most likely correspond to variations in fault-related accommodation development within the basin during the time of deposition.     

Signatures of climate vs. sea-level change within incised valley-fill successions: Quaternary examples from the Texas GULF Coast

The passive margin Texas Gulf of Mexico Coastal Plain consists of coalescing late Pleistocene to Holocene alluvial–deltaic plains constructed by a series of medium to large fluvial systems. Alluvial–deltaic plains consist of the Pleistocene Beaumont Formation, and post-Beaumont coastal plain incised valleys. A variety of mapping, outcrop, core, and geochronological data from the extrabasinal Colorado River and the basin-fringe Trinity River show that Beaumont and post-Beaumont strata consist of a series of coastal plain incised valley fills that represent 100 kyr climatic and glacio-eustatic cycles.

Valley fills contain a complex alluvial architecture. Falling stage to lowstand systems tracts consist of multiple laterally amalgamated sandy channelbelts that reflect deposition within a valley that was incised below highstand alluvial plains, and extended across a subaerially-exposed shelf. The lower boundary to falling stage and lowstand units comprises a composite valley fill unconformity that is time-transgressive in both cross- and down-valley directions. Coastal plain incised valleys began to fill with transgression and highstand, and landward translation of the shoreline: paleosols that define the top of falling stage and lowstand channelbelts were progressively onlapped and buried by heterolithic sandy channelbelt, sandy and silty crevasse channel and splay, and muddy floodbasin strata. Transgressive to highstand facies-scale architecture reflects changes through time in dominant styles of avulsion, and follows a predictable succession through different stages of valley filling. Complete valley filling promoted avulsion and the large-scale relocation of valley axes before the next sea-level fall, such that successive 100 kyr valley fills show a distributary pattern.

Basic elements within coastal plain valleys can be correlated with the record offshore, where cross-shelf valleys have been described from seismic data. Falling stage to lowstand channelbelts within coastal plain valleys were feeder systems for shelf-phase and shelf-margin deltas, respectively, and demonstrate that falling stage fluvial deposits are important valley fill components. Signatures of both upstream climate change vs. downstream sea-level controls are therefore interpreted to be present within incised valley fills. Signatures of climate change consist of the downstream continuity of major stratigraphic units and component facies, which extends from the mixed bedrock–alluvial valley of the eroding continental interior to the distal reaches, wherever that may be at the time. This continuity suggests the development of stratigraphic units and facies is strongly coupled to upstream controls on sediment supply and climate conditions within hinterland source regions. Signatures of sea-level change are critical as well: sea-level fall below the elevation of highstand depositional shoreline breaks results in channel incision and extension across the newly emergent shelf, which in turn results in partitioning of the 100 kyr coastal plain valleys. Moreover, deposits and key surfaces can be traced from continental interiors to the coastal plain, but there are downstream changes in geometric relations that correspond to the transition between the mixed bedrock–alluvial valley and the coastal plain incised valley. Channel incision and extension during sea-level fall and lowstand, with channel shortening and delta backstepping during transgression, controls the architecture of coastal plain and cross-shelf incised valley fills.     

Evolution and sedimentology of a channel fill in the sandy braided South Saskatchewan River and its comparison to the deposits of an adjacent compound bar

The depositional stratigraphy of within‐channel deposits in sandy braided rivers is dominated by a variety of barforms (both singular ‘unit’ bars and complex ‘compound’ bars), as well as the infill of individual channels (herein termed ‘channel fills’). The deposits of bars and channel fills define the key components of facies models for braided rivers and their within‐channel heterogeneity, knowledge of which is important for reservoir characterization. However, few studies have sought to address the question of whether the deposits of bars and channel fills can be readily differentiated from each other. This paper presents the first quantitative study to achieve this aim, using aerial images of an evolving modern sandy braided river and geophysical imaging of its subsurface deposits. Aerial photographs taken between 2000 and 2004 document the abandonment and fill of a 1·3 km long, 80 m wide anabranch channel in the sandy braided South Saskatchewan River, Canada. Upstream river regulation traps the majority of very fine sediment and there is little clay (< 1%) in the bed sediments. Channel abandonment was initiated by a series of unit bars that stalled and progressively blocked the anabranch entrance, together with dune deposition and stacking at the anabranch entrance and exit. Complete channel abandonment and subsequent fill of up to 3 m of sediment took approximately two years. Thirteen kilometres of ground‐penetrating radar surveys, coupled with 18 cores, were obtained over the channel fill and an adjacent 750 m long, 400 m wide, compound bar, enabling a quantitative analysis of the channel and bar deposits. Results show that, in terms of grain‐size trends, facies proportions and scale of deposits, there are only subtle differences between the channel fill and bar deposits which, therefore, renders them indistinguishable. Thus, it may be inappropriate to assign different geometric and sedimentological attributes to channel fill and bar facies in object‐based models of sandy braided river alluvial architecture.     

前陆盆地沉降机理和地层模型

前陆盆地形成的主要原因是造山带负载导致的岩石圈挠曲。逆冲作用造成地壳增厚,造山带的巨大质量又导致下部岩石圈的区域均衡沉降,从而临近和平行于造山带发育了凹陷。另外,前陆盆地的演化也受到沉积物供应、盆地内沉积物扩散能力、岩石圈强度、造山带逆冲速率、全球海平面变化、和俯冲有关的动力沉降及俯冲负载等众多其它因素的影响。本文阐述了这些因素与前陆盆地沉降的关系,介绍了与幕式逆冲有关的地层模型和欠补偿-过补偿地层模型。希望本文能够对中国西北地区前陆盆地的研究起到一定的借鉴意义。     

Conceptual models for short‐eccentricity‐scale climate control on peat formation in a lower Palaeocene fluvial system,north‐eastern Montana (USA)

Fluvial systems in which peat formation occurs are typified by autogenic processes such as river meandering, crevasse splaying and channel avulsion. Nevertheless, autogenic processes cannot satisfactorily explain the repetitive nature and lateral continuity of many coal seams (compacted peats). The fluvial lower Palaeocene Tullock Member of the Fort Union Formation (Western Interior Williston Basin; Montana, USA ) contains lignite rank coal seams that are traceable over distances of several kilometres. This sequence is used to test the hypothesis that peat formation in the fluvial system was controlled by orbitally forced climate change interacting with autogenic processes. Major successions are documented with an average thickness of 6·8 m consisting of ca 6 m thick intervals of channel and overbank deposits overlain by ca 1 m thick coal seam units. These major coal seams locally split and merge. Time‐stratigraphic correlation, using a Cretaceous–Palaeogene boundary event horizon, several distinctive volcanic ash‐fall layers, and the C29r/C29n magnetic polarity reversal, shows consistent lateral recurrence of seven successive major successions along a 10 km wide fence panel perpendicular to east/south‐east palaeo‐flow. The stratigraphic pattern, complemented by stratigraphic age control and cyclostratigraphic tests, suggests that the major peat‐forming phases, resulting in major coal seams, were driven by 100 kyr eccentricity‐related climate cycles. Two distinct conceptual models were developed, both based on the hypothesis that the major peat‐forming phases ended when enhanced seasonal contrast, at times of minimum precession during increasing eccentricity, intensified mire degradation and flooding. In model 1, orbitally forced climate change controls the timing of peat compaction, leading to enhancement of autogenic channel avulsions. In model 2, orbitally forced climate change controls upstream sediment supply and clastic influx determining the persistence of peat‐forming conditions. At the scale of the major successions, model 2 is supported because interfingering channel sandstones do not interrupt lateral continuity of major coal seams.     

Large‐scale architecture in non‐marine basins: the response to the interplay between accommodation space and sediment supply

Sedimentary successions of non‐marine basins can be considered in terms of accommodation space and sedimentary supply changes. Changes in accommodation space controlling the large‐scale architecture of non‐marine basins are different in areas with high and low sedimentary supplies. Uplift of intrabasinal monoclines and anticlines reduced the available accommodation space, resulting in changes in both the geometry of the depositional sequences and the large‐scale architecture of fluvial, mudflat and shallow carbonate lacustrine deposits. Main drainage fluvial systems record areas with a high sedimentary supply, while mudflats and shallow fluctuating lakes represent areas that received less sediment. Two end members in the large‐scale architecture of main drainage fluvial system in the Almazán Basin (Spain) are: (i) ribbon‐shaped channel fills with low interconnectivity which pass laterally into mudflats dominated by mudstones and evaporites and into palustrine and shallow carbonate lacustrine deposits (mainly in the A2 depositional sequence); and (ii) sheet‐like channel fills with high interconnectivity laterally correlated with stacked calcretes in the marginal mudflats (in the upper part of A3). Ribbon‐shaped channel fills formed in areas of high accommodation space and sheet‐like channel fills formed in areas of reduced accommodation space.     

A three‐dimensional numerical model of sediment transport,erosion and deposition within a network of channel belts,floodplain and hill slope: extrinsic and intrinsic controls on floodplain dynamics and alluvial architecture

A three‐dimensional numerical model of sediment transport, erosion and deposition within a network of channel belts and associated floodplain is described. Sediment and water supply are defined at the upstream entry point, and base level is defined at the downstream edge of the model. Sediment and water are transported through a network of channels according to the diffusion equation, and each channel has a channel belt with a width that increases in time. The network of channels evolves as a result of channel bifurcation and abandonment (avulsion). The timing and location of channel bifurcation is controlled stochastically as a function of the cross‐valley slope of the floodplain adjacent to the channel belt relative to the down‐valley slope, and of annual flood discharge. A bifurcation develops into an avulsion when the discharge of one of the distributaries falls below a threshold value. The floodplain aggradation rate decreases with distance from the nearest active channel belt. Channel‐belt degradation results in floodplain incision. Extrinsic (extrabasinal, allogenic) and intrinsic (intrabasinal, autogenic) controls on floodplain dynamics and alluvial architecture were modelled, and sequence stratigraphy models were assessed. Input parameters were chosen based on data from the Rhine–Meuse delta. To examine how the model responds to extrinsic controls, the model was run under conditions of changing base level and increasing sediment supply. Rises and falls in base level and increases in sediment supply occurred over 10 000 years. Rising base level caused a wave of aggradation to move up‐valley, until aggradation occurred over the entire valley. Frequency of bifurcations and avulsions increased with rate of base‐level rise and aggradation rate. Channel‐belt width varied with water discharge and the lifespan of the channel belt. Wide, connected channel belts (and high channel‐deposit proportion) occurred around the upstream inflow point because of their high discharge and longevity. Less connected, smaller channel belts occurred further down‐valley. Such alluvial behaviour and architecture is also found in the Rhine–Meuse delta. During base‐level fall, valley erosion occurred, and the incised valley contained a single wide channel belt. During subsequent base‐level rise, a wave of aggradation moved up‐valley, filling the incised valley. Bifurcation and avulsion sites progressively moved upstream. Relatively thin, narrow channel belts bordered and cut into the valley fill. These results differ substantially from existing sequence stratigraphy models. The increase in sediment supply from upstream resulted in an alluvial fan. Most bifurcations and avulsions occurred at the fan apex (nodal avulsion), and channel belts were the widest and the thickest here (giving high channel‐deposit proportion) due to their high discharge and longevity. The width and thickness of channel belts decreased down‐valley due to decreased discharge, longevity and aggradation rate. This behaviour occurs in modern alluvial fans. Intrinsic controls also affect floodplain dynamics and alluvial architecture. Variation of aggradation rate, bifurcation frequency and number of coexisting channel belts occurred over periods of 500 to 2000 years, compared with 10 000 years for extrinsic controls. This variation is partly related to local aggradation and degradation of channel belts around bifurcation points. Channel belts were preferentially clustered near floodplain margins, because of low floodplain aggradation rate and topography there.     

Stratigraphic organization and predictability of mixed coarse‐grained and fine‐grained successions in an upper slope Pleistocene turbidite system of the Peri‐Adriatic basin

The early Pleistocene clastic succession of the Peri‐Adriatic basin, eastern central Italy, records the filling of a series of piggyback sub‐basins that formed in response to the development of the eastward‐verging Apennine fold‐thrust belt. During the Gelasian (2·588 to 1·806 Ma), large volumes of Apennine‐derived sediments were routed to these basins through a number of slope turbidite systems. Using a comprehensive outcrop‐based dataset, the current study documents the depositional processes, stratigraphic organization, foraminiferal age and palaeodepth, and stratigraphic evolution of one of these systems exposed in the surroundings of the Castignano village. Analysis of foraminiferal assemblages consistently indicates Gelasian deposition in upper bathyal water depths. Sediments exposed in the study area can be broken into seven main lithofacies, reflecting specific gravity‐induced depositional elements and slope background deposition: (i) clast‐supported conglomerates (conglomerate channel‐fill); (ii) amalgamated sandstones (late stage sandstone channel‐fill); (iii) medium to thick‐bedded tabular sandstones (frontal splay sandstones); (iv) thin to thick‐bedded channelized sandstones (sandy channel‐fill); (v) medium to very thin‐bedded sandstones and mudstones (levée‐overbank deposits); (vi) pebbly mudstones and chaotic beds (mudstone‐rich mass‐transport deposits); and (vii) massive mudstones (hemipelagic deposits). Individual lithofacies combine vertically and laterally to form decametre‐scale, disconformably bounded, fining‐upward lithofacies successions that, in turn, stack to form slope valley fills bounded by deeply incised erosion surfaces. A hierarchical approach to the physical stratigraphy of the slope system indicates that it has evolved through multiple cycles of waxing then waning flow energy at multiple scales and that its packaging can be described in terms of a six‐fold hierarchy of architectural elements and bounding surfaces. In this scheme, the whole system (sixth‐order element) is comprised of three distinct fifth‐order stratigraphic cycles (valley fills), which define sixth‐order initiation, growth and retreat phases of slope deposition, respectively; they are separated by discrete periods of entrenchment that generated erosional valleys interpreted to record fifth‐order initiation phases. Backfilling of individual valleys progressed through deposition of two vertically stacked lithofacies successions (fourth‐order elements), which record fifth‐order growth and retreat phases. Fourth‐order initiation phases are represented by erosional surfaces bounding lithofacies successions. The component lithofacies (third‐order element) record fourth‐order growth and retreat phases. Map trends of erosional valleys and palaeocurrent indicators converge to indicate that the sea floor bathymetric expression of a developing thrust‐related anticline markedly influenced the downslope transport direction of gravity currents and was sufficient to cause a major diversion of the turbidite system around the growing structure. This field‐based study permits the development of a sedimentological model that predicts the evolutionary style of mixed coarse‐grained and fine‐grained turbidite slope systems, the internal distribution of reservoir and non‐reservoir lithofacies within them, and has the potential to serve as an analogue for seismic or outcrop‐based studies of slope valley fills developed in actively deforming structural settings and under severe icehouse regimes.     

Basin‐scale predictive models of alluvial architecture: Constraints from the Palaeocene–Eocene,Bighorn Basin,Wyoming, USA

Basin‐scale models are required to interpret ancient continental sedimentary successions, and reduce uncertainty in assessing geological resources in basins. Recently, modern studies show distributive fluvial systems to comprise a substantial proportion of modern sedimentary basins, but their role in ancient basin fills has yet to be quantitatively documented at the basin scale. This study analysed key fluvial characteristics to construct a detailed basin‐wide model of the Palaeogene Fort Union and Willwood formations (Bighorn Basin, Wyoming), using observations from modern studies, and ancient system scale studies of distributive fluvial systems, to guide interpretations. Mapping showed these formations to be highly heterogeneous with channel‐body proportion (from 12 to 81%) and geometry types (large amalgamated bodies to isolated channels), grain size (silt to conglomerate), average channel‐body thickness (4 to 20 m) and average storey thickness (3 to 10 m) varying significantly across the basin. Distributive fluvial systems in the form of alluvial and fluvial fans in transverse configurations were recognized as well as a wide axial system, with heterogeneity in the formations being closely aligned to these interpretations. Furthermore, numerous individual depositional systems were identified within the formations (Beartooth Absaroka, Washakie, Owl Creek and axial). Predicted downstream distributive fluvial system trends (i.e. downstream decrease in channel proportion, size and grain size) were identified in the Beartooth, Absaroka and Owl Creek systems. However, predicted trends were not identified in the Washakie system where intrabasinal thrusting disturbed the sequence. Importantly, a wide axial fluvial system was identified, where reverse downstream distributive fluvial system trends were present, interpreted to be the result of the input of transverse systems of variable size. This study provides a new level of detail in the application of basin‐scale models, demonstrating their usefulness in trying to understand and predict alluvial architecture distribution and heterogeneity, with important implications for economic resources and palaeogeographic reconstructions.