The thickness variability of fluvial cross‐strata as a record of dune disequilibrium and palaeohydrology proxy: A test against channel deposits

Strata produced by fluvial dunes can provide insight into the hydrological regime of ancient rivers. Recent experiments indicate that conditions of disequilibrium between bedforms and formative flows may be inferred from the coefficient of variation of preserved dune cross-set thickness, suggesting that this quantity may act as a proxy for the flashiness of river floods relative to the time required for full bedform translation. To assess whether this idea is applicable to interpretations of the stratigraphic record, this study examines published data relating to more than 2600 cross-sets from 53 sedimentary units of 19 river systems. The presented analyses must not be over interpreted, because the considered rivers span different environmental settings, the data sources are heterogeneous in terms of type and dimensionality, and some variables were established by applying empirical relationships. Yet, significant findings are revealed. Larger rivers exhibit discharge and bedform characteristics that are more conducive to disequilibrium; however, a modest increase in the coefficient of variation of cross-set thickness, CV(Dst), as opposed to the expected decrease, is seen as a function of river size. Crucially, smaller CV(Dst) values are not systematically associated with conditions that should favour dune disequilibrium. Meanwhile, only ca 25% of the studied examples demonstrate cross-set thickness statistics compatible with quantitative formulations of the autogenic control by variable dune topography – the notion of ‘variability-dominated’ preservation. These findings indicate that the variability in cross-set thickness may be a poor predictor of discharge variability, perhaps because of the multiplicity of factors controlling dune preservation, such as bedform hierarchy, transport stage and depth-dependent variations in dune disequilibrium. To improve interpretations of cross-stratified deposits, further research is needed to: (i) establish the value of process-to-product models for reverse product-to-process interpretations; and (ii) to define representative samples for preserved dune deposits accounting for temporal and spatial variability in preservation potential

Application of a Training-Image Library to Reservoir Modeling Using Multi-Point Statistics Based on Quantitative Fluvial Facies Characterization

Facies modelling seeks to reproduce the geometry and distribution of the reservoir forming sedimentary bodies in three dimensions to provide a framework for the construction of property and flow models. However, variogram-based facies modelling techniques are not well suited to the reproduction of complex geological shapes (e.g., sinuous fluvial channels), whereas object-based simulations may fail to honour conditioning data (e.g., well data). New workflows have been developed for the generation of fluvial reservoir models with improved geological realism compared to outputs of conventional methods. These workflows are suitable for modelling reservoirs that comprise fluvial meander-belt deposits, and can therefore provide the models of spatial heterogeneity (training images) required to apply simulation techniques based on multi-point statistics (MPS), which are then useful to integrate complex geological patterns. A library of training images from which MPS modelling algorithms replicate geological patterns has been developed using quantitative information derived from a relational database of geological analogues (Fluvial Architecture Knowledge Transfer System, FAKTS), and a forward stratigraphic modelling tool that simulates fluvial meander-bend evolution and resulting point-bar facies organization (PB -SAND). The devised training images incorporate fundamental features of the facies architecture of fluvial point-bar elements and larger meander belts composed of these and related elements. The application of training images has been optimized to three widely used MPS algorithms: SNESIM, DEESSE and FILTERSIM. A quantitative and qualitative quality check of MPS realizations has been performed whereby facies proportions, facies relationships, element geometries, dimensions, control of non-stationarity and runtime are optimized for particular fluvial successions being modelled. The sensitivity of multiple simulation results to input parameters has been analysed to define preferred modelling recipes, paired to each training image and to each MPS modelling algorithm. Research outcomes are the development of an extensive library of training images for MPS simulations of the architecture of subsurface successions deposited by a variety of types of meandering fluvial systems. Devised workflows are applicable to multiple MPS algorithms, and enable off-the-shelf training-image selection for the effective establishment of a hierarchical approach to facies modelling

Autogenic evolution of valley-confined deltas during sea-level rise: Insights from numerical and physical modelling

Nearshore incised valleys are important conduits for the transport of sediment, nutrients, pollutants and organic carbon from the continents to the sea. Therefore, it is essential to understand the autogenic evolution of deltas confined within incised valleys and how such evolution is affected by relative sea-level rise. To date, limited research has focused on how deltas constrained by incised valleys or other forms of antecedent topography respond to rising sea level. An existing theory of autostratigraphy envisages scenarios in which two-dimensional or unconfined three-dimensional fan deltas can experience three evolutionary stages under constant rates of relative sea-level rise and sediment supply: progradation, autoretreat and post-autobreak transgression. In this work, an integrated study of geometric numerical models and physical experiments is undertaken to investigate autostratigraphic delta evolution for a variety of incised-valley geometries, under conditions of constant rates of relative sea-level rise and sediment supply. Results indicate that interplays of antecedent topography (valley geometries) and sediment mass balance expressed in resultant deltaic geometries can result in autogenic changes in shoreline dynamics and river avulsion frequency on deltas. The following primary findings arise. (i) Compared to valleys with rectangular and trapezoidal cross-sectional profiles, valleys with triangular cross-sections tend to contain deltas that experience faster rates of progradation, autoretreat and post-autobreak transgression under rising sea level, and exhibit a more prominent convex-seaward shoreline trajectory. (ii) The shoreline trajectory is also related to delta topset geometry, becoming more convex-seaward under decreasing topset slopes. (iii) River avulsion frequency on deltas with rising sea level varies markedly across valleys with different geometries, even under the same rate of relative sea-level rise; this is attributed to the difference in temporal evolution of shoreline migration for different valley geometries and the resultant difference in the delta topset aggradation. This study highlights complexities in responses of sedimentary systems under the confinement of different topographic configurations that have hitherto largely been overlooked in sequence-stratigraphic models. The findings provide insight into future shoreline behaviour and river avulsion hazard on confined deltas, and for decoding the stratigraphic record

Sedimentary cycles in a Mesoproterozoic aeolian erg-margin succession: Mangabeira Formation, Espinhaço Supergroup, Brazil

Aeolian systems were abundant and widespread in the early Proterozoic, post-2.2 Ga. However, the majority of aeolian successions of such great age are intensely deformed and are preserved only in a fragmentary state meaning that, hitherto, few attempts have been made to apply a sequence stratigraphic approach to determine mechanisms of aeolian construction, accumulation and preservation in such systems. The Mangabeira Formation is a well preserved Mesoproterozoic erg successions covering part of the São Francisco Craton, northeastern Brazil. The lower unit of the Mangabeira Formation (~ 500 m thick) comprises aeolian deposits of dune, interdune, and sand-sheet origin, as well as some of waterlain origin. These deposits are organized into vertically stacked depositional cycles, each 6 to 20 m thick, and characterized by aeolian sandsheet and waterlain deposits succeeded by aeolian dune and interdune deposits indicative of a drying-upward trend. Aeolian cross-strata exhibit a mean dip direction to the north. Each of these cycles likely arose in response to climatic oscillation from relatively humid to arid conditions, possibly related to orbital forcing. The lower unit of the Mangabeira Formation comprises up to 14 erg sequences. The accumulation and preservation of each was determined by the relative rate of water-table rise and the availability of sand for aeolian transport, both of which changed through time, resulting in the preservation of a succession of repeated drying-upward cycles

The Precambrian continental record: A window into early Earth environments

The Precambrian was characterized by unique palaeoenvironmental conditions in the Earth’s atmosphere, biosphere and geosphere. This study presents a global quantitative analysis of Precambrian sedimentary successions of aeolian, alluvial, fluvial, lacustrine and glacigenic origins, examined in the broader context of Earth evolution. In the Precambrian, an apparent scarcity of aeolian successions is observed. This may be linked to: (1) differences in atmospheric density, which controlled wind erosion and sedimentation; (2) different astronomical configurations, which may have influenced tides and atmospheric circulation, thereby affecting sand availability and the width of subtropical zones; (3) potentially hotter and more humid climates, restricting dry-sand availability; (4) a lack of vascular vegetation that could prevent reworking of aeolian deposits; (5) poor preservation potential; (6) misinterpretation of the Precambrian record. Mixed aeolian-alluvial strata are more abundant, perhaps because their preservation in the geological record was favoured by water tables sustained by incursions of alluvial systems into otherwise aeolian dominated environments. Aeolian deposits were preferentially accumulated during phases of supercontinental breakup, where rapidly subsiding rift basins provided accommodation suitable for preservation. Other than in the Neoproterozoic record, where glacigenic deposits dominate, alluvial strata are the most common and thickest type of continental deposit in the Precambrian. Precambrian braided alluvial systems were more widespread than in the Phanerozoic. Major alluvial systems formed preferentially during phases of supercontinent assembly, whereby alluvial systems drained major orogens, and long drainage pathways developed from supercontinent interiors to coastlines. In the Paleoproterozoic, ephemeral, saline to partly arid lakes developed extensively in the desertic interior of Columbia. Glacial deposits preferentially formed in the breakup phase of supercontinental cycles; this supports theories invoking enhanced chemical weathering of uplifted rift shoulders as a driver of carbon dioxide sequestration, global cooling, and glaciation. Overall, the number of identified continental successions increases towards the Precambrian-Phanerozoic boundary. This may be an artefact of an increasingly more complete stratigraphic record as time progresses. However, the abundance of continental successions varies on a quasi-periodic cycle of 500 – 700 Myr, with peaks coinciding with the tenure and breakup of Precambrian supercontinents

A 3D forward stratigraphic model of aeolian dune evolution for prediction of lithofacies heterogeneity

Within aeolian systems, complex dune morphologies can develop due to the interplay of a variety of allogenic and autogenic controls. As a result, the preserved sedimentary record of aeolian dune deposits is highly varied, exhibiting an array of sedimentary architectures and facies heterogeneities. However, reconstructions of such aeolian sedimentary architectures are usually based on limited information from one-dimensional borehole data or two-dimensional outcropping successions; as such, it is challenging to predict three-dimensional architectures and the distribution of small-scale facies heterogeneities of aeolian sedimentary successions. To address this, a novel rule-based forward stratigraphic model, the Dune Architecture and Sediment Heterogeneity model (DASH), has been developed to reproduce three-dimensional sedimentary bodies, bounding surfaces and associated facies distributions formed by a wide range of dune morphologies and morphodynamic behaviours. The model generates architectural frameworks produced by dune and interdune migration and aggradation, based on a long-established modelling approach; it then applies a series of rules that reflect geological understanding or observations from geological analogues to populate the three-dimensional space with facies domains. The model has been applied to simulate the stratal architectures and facies organization of (i) three idealized examples of successions produced by different dune morphologies, and (ii) a real-world case example from the Triassic Helsby Sandstone Formation, Cheshire Basin, UK. The results demonstrate how the model can be used to predict likely facies distributions in three dimensions, which themselves can be used to constrain models of petrophysical properties constructed with geostatistical techniques. The model can therefore be applied to assist reconstructions of subsurface architectures and petrophysical heterogeneity

A database approach for constraining stochastic simulations of the sedimentary heterogeneity of fluvial reservoirs

Quantitative databases storing analog data describing the geometry of sedimentologic features are commonly used to derive input for geostatistical simulations of reservoir sedimentary architecture; however, geometrical information alone is inadequate for the detailed characterization of sedimentary heterogeneity. A relational database storing fluvial architecture data has been developed and populated with literature- and field-derived data from modern rivers and ancient successions. The database scheme characterizes fluvial architecture at three different scales of observation-recording style of internal organization geometries and spatial relationships of genetic units-classifying data sets according to controlling factors (e.g. climate type) and context-descriptive characteristics (e.g. river pattern). The database can therefore be filtered on both architectural features and boundary conditions to yield outputs tailored on the system being modeled to generate input to object- and pixel-based stochastic simulations of reservoir architecture. When modeling heterogeneity with stochastic simulations, the choice of input parameters quantifying spatial variation is problematic because of the paucity of primary data and the partial characterization of supposed analogs. This databasedriven approach permits the definition of various constraints referring to either genetic units (e.g., architectural elements) or material units (i.e., contiguous volumes of sediment characterized by the same value of a given categorical or discretized variable; e.g., same lithofacies type, clay and silt content, and others), which permit the realistic description of fluvial architecture heterogeneity. Applications of this database approach include the computation of relative dimensional parameters and the generation of auto- and cross-variograms and transition-probability matrices, which are necessary to effectively model spatial complexity. Copyrigh

Ultra‐long‐distance transport of aeolian sand: The provenance of an intermontane desert, south‐east China

Intermontane deserts are an important type of arid-climate sedimentary system. Although rare at present, the sedimentary records of intermontane deserts reveal their widespread development in past greenhouse periods, and they might develop in the near future in response to ongoing global warming. Determination of the provenance of sand supplied for the construction of intermontane deserts is important to gain improved understanding of the potential impact of future climate on environmental evolution in arid and semi-arid regions. During the Cretaceous, a typical intermontane desert developed in the Xinjiang Basin, south-east China. In this study, the origin, spatial variability, and transport pathways of both aeolian and alluvial–fluvial sediments in the Xinjiang intermontane desert are investigated by analyses of bulk-rock petrography and detrital-zircon U–Pb geochronology. These results demonstrate that the sand in the Xinjiang intermontane desert succession was mainly of extraneous origin and wind-derived. The nearby South China Block and South China Magmatic Belt were primary sources, and the 1000 km distant western margin of Yangtze Block was an important secondary source. During the Late Cretaceous, the westerlies were stronger in the northern than in the southern hemisphere with doubled wind speeds. In such a climatic context, the results herein suggest that the ultra-long-distance aeolian sediment transport was likely further enabled by two factors: (i) the strengthening of intermittent westerly winds during short-lived glacial episodes; and (ii) the presence of a low-relief corridor that served as a transport pathway from source to sink

Influence of basin physiography upon the character and distribution of hybrid event beds

It is increasingly recognized that sediment gravity flow deposits often defy classical turbidite-debrite models and are wide ranging in character (e.g., slurry beds, hybrid event beds and transitional flow deposits). Such deposits, here collectively referred to as hybrid event beds (HEBs) are of importance as they cause heterogeneities in reservoir quality distribution from intra-bed to system scales. HEBs are inferred to record deposition beneath flow characterised by discrete internal rheological zones, ranging from turbulent to laminar, whose relative proportions within the flow evolved both spatially and temporally. The high clay concentration in these deposits could reflect preferential deposition of the coarser sand fraction, and hence a relative clay enrichment, and/or an actual enrichment through entrainment of muddy substrate into the flow. In topographically-influenced systems it has been suggests HEBs may indicate proximity to confining slopes due to their localised occurrence and systematic depositional variation towards such features. New research from a range of systems developed within basins of differing physiography (e.g., unconfined, confined and contained [ponded]) has begun to highlight contrasts in the depositional character and distribution of HEBs between these settings. Specifically; 1) the relative degree of flow transformation undergone by the parent hybrid flow, as expressed by the degree of textural and compositional segregation within HEBs, and how argillaceous (clay-rich) their deposits are; 2) the degree of lateral variation in HEB depositional character over short (10s \u2013 100s metres) and longer (100s \u2013 1000s metres) length scales, and whether such variation is systematic or not; 3) whether HEBs are localized to confining sea-floor topography and exhibit depositional trends towards such features; 4) the range of potential stratigraphic stacking patterns developed in these systems. Characteristics of HEBs in different system types are thought to reflect variations in the initial flow character, substrate entrainment, flow confinement, flow containment and flow run-out distance achieved and their influences upon the processes that promote flow evolution and transformation. This work highlights the range of boundary conditions to be considered when attempting to predict the spatial occurrence and depositional character of these non-classical deposits, and thus reservoir quality distribution, within deep-water systems