The lithofacies organization of fluvial channel deposits: a meta-analysis of modern rivers

Environmental interpretations of subsurface fluvial successions are commonly based on facies observations from core and are often attempted by generalist geologists by reference to classic facies models. However, for fluvial channel deposits, the value of observations on lithofacies proportions for interpretations of depositional environment has yet to be assessed quantitatively. Here, a test is presented that is based on a comparative study of facies data from 77 reaches of 46 modern rivers. The analysis is undertaken on datasets from published case studies stored in a sedimentological database, with consideration of causes for observational bias, and with particular attention paid to sandy lithofacies. The observed variability in the proportion of facies assemblages in the channel deposits of sandy river systems is quantified for classes of environments categorized according to channel pattern (braided, low sinuosity, meandering), climatic setting (arid to perhumid), and discharge regime (ephemeral to perennial). By capturing the variability in facies organization within fluvial systems of certain types, these outputs serve as facies models that provide a measure of uncertainty to sedimentological interpretations. Concurrently, the statistical analysis presented enables a test of the significance of relationships between the relative proportions of channel lithofacies and parameters that either represent controlling factors (e.g., water-discharge characteristics) or covariates (e.g., channel pattern). For classes of river systems grouped by channel pattern, climate, and discharge regime, emerging features of facies organization can be identified. Statistically, it is observed that relationships exist (i) between channel pattern and the frequency of the preserved expression of bedforms, and (ii) between controls on river hydrology (climate, discharge regime and seasonal variability) and the record of upper and lower flow-regime conditions. Thus, the results corroborate existing qualitative facies models in some respects. However, observations of the relative dominance of facies in channel deposits demonstrate limited value for interpretations or predictions in subsurface or outcrop studies, as variability within each type of depositional system is significant. Corehole data of fluvial channel deposits may be commonly overinterpreted

On the geological significance of clastic parasequences

Parasequences recognized in clastic sedimentary successions of shallow-marine origin are considered by some geologists to be the fundamental building blocks of depositional sequences, even though problems in their definition and application have been identified by others, who instead advocate their abandonment as formal sequence stratigraphic units. To elucidate the geological significance of clastic parasequences and inform the debate on their use in stratigraphy, a quantitative characterization of the geometry, facies characteristics and timescale of deposition of 1163 parasequences has been undertaken based on a synthesis of data from outcrop and subsurface studies that are available in the scientific literature. Through a database compilation, the attributes of the studied parasequences are analysed with respect to the interpreted geological origin of the units, and with consideration of sources of bias and uncertainty. Particular emphasis is placed on assessing the following: (i) the importance of heuristics, and of data types and coverage in the recognition of parasequences; (ii) differences in parasequence characteristics observed across deltaic and shoreface depositional systems, and between the Quaternary and the ancient rock record; (iii) possible explanations for the range in timescales of deposition of parasequences; and (iv) the role of autogenic dynamics on the development of deltaic parasequences, partly based on a comparison with the recent evolution of modern deltas. The results demonstrate that parasequence definition and physical correlation suffer from subjectivity, and that significant variability exists in the spatio-temporal and architectural attributes of clastic parasequences. This gives rise to uncertainty that affects the use of parasequences as a framework for comparison of the architecture of packages of strata originating via shoreline regression: this uncertainty must be considered when using analogue data for subsurface predictions or when attempting comparative studies of clastic successions

Assessment of backwater controls on the architecture of distributary channel fills in a tide-influenced coastal-plain succession: Campanian Neslen Formation, USA

The backwater zone of a river is its distal reach downstream of the point at which the streambed elevation reaches sea level. Backwater hydraulics is believed to exert an important control on fluvio-deltaic morphodynamics, but the expressions with which this may be recorded in the preserved stratigraphic record are not well understood. The seaward reaches of modern rivers can undergo flow acceleration and become erosional at high discharges due to drawdown of the in-channel water surface near the river mouth, in relation to the fixed water surface at the shoreline. As coastal-plain distributary channels approach the shoreline they tend to be subject to a reduction in lateral mobility, which could be related to diminished sediment flux at low flow. Current understanding of channel morphodynamics associated with backwater effects, as based on observations from numerical models and modern sedimentary systems, is here used to make predictions concerning the architecture of coastal distributary channel fills in the rock record. On the basis of existing knowledge, distributary channel fills are predicted to be typically characterized by low width-to-thickness aspect ratios, by a clustering of scour surfaces toward their base, by an aggradational infill style, by a facies organization that bears evidence of drawdown-influenced scour filling, possibly resulting in the overprint of tidal signals toward their base, and by co-genetic sand-prone overbank units of limited occurrence, thickness and sand content. To test these predictions, fieldwork was carried out to examine sedimentological characters of channel bodies from an interval of the Campanian Neslen Formation (eastern Utah, USA), which comprises a succession of sandstone, carbonaceous mudstone, and coal, deposited in a coastal-plain setting, and in which significant evidence of tidal influence is preserved. Three types of channel bodies are recognized in the studied interval, in terms of lithology and formative-channel morphodynamics: sand-prone laterally accreting channel elements, heterolithic laterally accreting channel elements and sand-prone aggradational ribbon channel elements. This study concentrates on the ribbon channel bodies since they possess a geometry compatible with laterally stable distributaries developed in the zone of drawdown. Sedimentological and architectural characteristics of these bodies are analyzed and compared with the proposed model of distributary channel-fill architecture. Although conclusive evidence of the influence of backwater processes in controlling the facies architecture of distributary channel fills is not reached, the studied bodies display an ensemble of internal architecture, lithological organization, nature of bounding surfaces and relationships with other units that conforms to the proposed model to a certain extent. The analyzed ribbon sandbodies are all characterized by erosional cut-banks, very limited proportions of mudstone deposits, a lack of genetically related barform units, clustering of scour fills at their base, and a lack of relationships with co-genetic river-fed overbank sandstones. This work provides a guide to future research, which is required to better understand the role of backwater processes in controlling the architecture of distributary channel bodies, their down-dip variations, and how these are expressed in the stratigraphic evolution of prograding coastal plains

Quantitative analysis of the stratigraphic architecture of incised-valley fills: a global comparison of Quaternary systems

Facies models of the internal fills of incised valleys developed in shelf and coastal settings during cycles of relative sea-level change are largely conceptual, descriptive and qualitative in form; moreover, they are commonly bespoke to individual examples. Here, a database-driven quantitative statistical analysis of 87 late-Quaternary incised-valley fills (IVFs) has been undertaken to assess the general validity and predictive value of classical facies models for IVFs, and to investigate the relative importance of possible controls on their stratigraphic organization. Based on datasets from the published literature stored in a sedimentological database, the geometry and proportion of systems tracts, and of architectural elements of different hierarchies within IVFs are quantified. These variables were analysed to assess how they vary in relation to parameters that represent potential controlling factors: relative sea-level stage, continental-margin type, drainage-basin area, valley geometry, basin physiography and shoreline hydrodynamics. The stratigraphic organization of the studied coastal-plain IVFs is generally consistent with that represented in facies models, the primary control being the rate and magnitude of relative sea-level change. However, results from this study demonstrate significant variability in the stratigraphic architectures of IVFs, which is not accounted for by existing models. Variations in the facies architecture of coastal-plain and cross-shelf valley fills can be attributed to controls other than sea level, and expressed in relationships with continental-margin type, basin physiography, catchment area, river-system size and shoreline hydrodynamics. The following primary findings arise from this research. (i) Compared to their counterparts on passive margins, coastal-plain IVFs hosted on active margins contain, on average, a higher proportion of fluvial deposits and a lower proportion of central-basin estuarine deposits; estuarine deposits tend however to be thicker. This suggests a control on IVF stratigraphic architecture exerted by distinct characteristics of the tectonic setting of the host continental margins, notably basin physiography, rates and mode of sediment supply, and nature of sediment load. (ii) The thickness and proportion of lowstand systems tract are positively correlated with coastal-plain IVF dimensions, likely reflecting the role of drainage-basin area in dictating the scale of the fluvial systems that carved and infilled the valleys. (iii) Positive correlations are observed between the thickness of fluvial deposits, bayhead-delta deposits and central-basin estuarine deposits, versus coastal-plain IVF dimensions and valley catchment area. This suggests a control exerted by the river-system scale on sediment-supply rates and on the accommodation determined by valley size. (iv) Positive correlations between the thickness and proportion of barrier-complex deposits within cross-shelf IVFs versus mean shelf gradient indicate that the geometry of the shelf might control the establishment and preservation of barrier-island environments in incised valleys located on the shelf. (v) Correlations between the width of coastal-plain IVFs and present-day mean tidal range at the shoreline indicate that tidal dynamics may contribute to the widening of the incised valleys. Positive correlation is observed between the proportion of tide-dominated elements in highstand IVF deposits and IVF width, suggesting possible effects of interplays between hydrodynamic conditions and the geometry of incised valleys on their infills. This study highlights the complexity of the internal fills of incised valleys, which must be considered when attempting the application of facies models of IVFs to rock-record interpretations or as predictive tools in subsurface studies

The geometry of fluvial channel bodies: Empirical characterization and implications for object-based models of the subsurface

The distribution of channel deposits in fluvial reservoirs is commonly modeled with object-based techniques, constrained on quantities describing the geometries of channel bodies. To ensure plausible simulations, it is common to define inputs to these models by referring to geologic analogs. Given their ability to reproduce complex geometries and to draw upon the analog experience, object-based models are considered inherently realistic. Yet this perceived realism has not hitherto been tested by assessing the outputs of these techniques against sedimentary architectures in the stratigraphic record. This work presents a synthesis of data on the geometry of channel bodies, derived from a sedimentologic database, with the following aims: (1) to provide tools for constraining stochastic models of fluvial reservoirs in data-poor situations, and (2) to test the intrinsic realism of object-based modeling algorithms by comparing characteristics of the modeled architectures against analogs. An empirical characterization of the geometry of fluvial channel bodies is undertaken that describes distributions in (and relationships among) channel-body thickness, cross-stream width, and planform wavelength and amplitude. Object-based models are then built running simulations conditioned on six alternative, analog-informed parameter sets, using four algorithms according to nine different approaches. Closeness of match between analogs and models is then determined on a statistical basis. Results indicate which modeling approaches return architectures that more closely resemble the organization of fluvial depositional systems known from nature and in what respect. None of the tested algorithms fully reproduce characteristics seen in natural systems, demonstrating the need for subsurface modeling methods to better incorporate geologic knowledge

Impact of a pre-existing transverse drainage system on active rift stratigraphy: an example from the Corinth Rift, Greece

Models to explain alluvial system development in rift settings commonly depict fans that are sourced directly from catchments formed in newly uplifted footwalls, which leads to the development of steep‐sided talus‐cone fans in the actively subsiding basin depocentre. The impact of basin evolution on antecedent drainage networks orientated close to perpendicular to a rift axis, and flowing over the developing hangingwall dipslope, remains relatively poorly understood. The aim of this study is to better understand the responses to rift margin uplift and subsequent intrabasinal fault development in determining sedimentation patterns in alluvial deposits of a major antecedent drainage system. Field‐acquired data from a coarse‐grained alluvial syn‐rift succession in the western Gulf of Corinth, Greece (sedimentological logging and mapping) has allowed analysis of the spatial distribution of facies associations, stratigraphic architectural elements and patterns of palaeoflow. During the earliest rifting phase, newly uplifted footwalls redirected a previously established fluvial system with predominantly southward drainage. Footwall uplift on the southern basin margin at an initially relatively slow rate led to the development of an overfilled basin, within which an alluvial fan prograded to the southwest, south, and southeast over a hangingwall dipslope. Deposition of the alluvial system sourced from the north coincided with the establishment of small‐scale alluvial fans sourced from the newly uplifted footwall in the south. Deposits of non‐cohesive debris flows close to the proposed hangingwall fan apex pass gradationally downstream into predominantly bedload conglomerate deposits indicative of sedimentation via hyperconcentrated flows laden with sand‐ and silt‐grade sediment. Subsequent normal faulting in the hangingwall resulted in the establishment of further barriers to stream drainage, blocking flow routes to the south. This culminated in the termination of sediment supply to the basin depocentre from the north, and the onset of underfilled basin conditions as signified by an associated lacustrine transgression. The evolution of the fluvial system described in this study records transitions between three possible end‐member types of interaction between active rifting and antecedent drainage systems: (i) erosion through an uplifted footwall, (ii) drainage diversion away from an uplifted footwall, (iii) deposition over the hangingwall dip‐slope. The orientation of antecedent drainage pathways at a high angle to the trend of a developing rift axis, replete with intrabasinal faulting, exerts a primary control on the timing and location of development of overfilled and underfilled basin states in evolving depocentres. This article is protected by copyright. All rights reserve

Anatomy and dimensions of fluvial crevasse-splay deposits: examples from the Cretaceous Castlegate Sandstone and Neslen Formation, Utah, U.S.A.

Crevasse-splay deposits form a volumetrically significant component of many fluvial overbank successions (up to 90% in some successions).Yet the relationships between the morphological form of accumulated splay bodies and their internal facies composition remains poorly documented from ancient successions. This work quantifies lithofacies distributions and dimensions of exhumed crevasse-splay architectural elements in the Campanian Castlegate Sandstone and Neslen Formation, Mesaverde Group, Utah, USA, to develop a depositional model. Fluvial crevasse-splay bodies thin from 2.1 m (average) to 0.8 m (average) and fine from a coarsest recorded grain size of lower-fine sand to fine silt away from major trunk channel bodies. Internally, the preserved deposits of splays comprise laterally and vertically variable sandstone and siltstone facies associations: proximal parts are dominated by sharp and erosional-based sandstone-prone units, which may be Structureless or may comprise primary current lineation on beds and erosional gutter casts; medial parts comprise sets of climbing-ripple strata and small scale deformed beds; distal parts comprise sets of lower-stage plane beds and complex styles of lateral grading into fine-grained floodbasin siltstones and coals. Lithofacies arrangements are used to establish the following: (i) recognition criteria for crevasse-splay elements; (ii) criteria for the differentiation between distal parts of crevasse-splay bodies and flood plain fines; and (iii) empirical relationships with which to establish the extent (ca. 500 m long by 1000 m wide) and overall semi-elliptical planform shape of crevasse-splay bodies. These relationships have been established by high-resolution stratigraphic correlation and palaeocurrent analysis to identify outcrop orientation with respect to splay orientation. This permits lateral changes in crevasse-splay facies architecture to be resolved. Facies models describing the sedimentology and architecture of crevasse-splay deposits preserved in floodplain successions serve as tools for determining both distance from and direction to major trunk channel sandbodies

Accommodation and sediment-supply controls on clastic parasequences: a meta-analysis

This study combines data from many published case studies to undertake a quantitative characterization of clastic parasequences, with the aim to determine how accommodation, sediment supply and autogenic sediment‐storage dynamics are recorded in their sedimentary architecture and stacking patterns. Results of this study are used to critically evaluate the validity of paradigms and models that are routinely used to explain and predict trends in the anatomy and arrangement of parasequences. Data on 957 parasequences from 62 case studies of clastic, shallow‐water successions were coded in a relational database, which includes outcrop and subsurface datasets of ancient and Quaternary examples. These units cover the preserved records of both river‐dominated deltas and wave‐dominated coasts, representing shoreline transits over a breadth of timescales, likely of both local and regional extent. The role of extant accommodation, rates of creation of accommodation and rates of sediment supply in determining parasequence architecture is assessed through analysis of relationships between: (i) proxies of these variables at different scales (rates of aggradation and progradation, facies‐belt shoreline trajectories, systems‐tract type, parasequence‐set stacking patterns, parasequence progradation angle and stratigraphic rise, size of feeder rivers); and (ii) parameters that describe the geometry and stacking style of parasequences, and associated shallow‐water sand bodies. Statistical analyses of database outputs indicate which proxies of accommodation, sediment supply and accommodation/sediment‐supply ratio are significant as predictors of parasequence architecture, and allow for interpretations of the importance of allogenic and autogenic factors. The principal results of this study reveal the following: (i) parasequence thickness varies as a function of water depth, accommodation generation and erosional truncation, and these variations are also reflected across types of systems tracts and parasequence sets; (ii) the dip length of parasequence sand bodies demonstrates scaling with measures of accommodation/sediment‐supply ratio at multiple scales, partly in relation to the possible effect of sediment supply on progradation rates; (iii) in systems tracts, stratigraphic trends in parasequence stacking due to autogenic mechanisms or to acceleration or deceleration in relative sea‐level fluctuations are not revealed quantitatively; (iv) some association is seen between the abundance of deltaic or river‐dominated parasequences and progradational stacking; (v) positive but modest correlation is observed between measures of river‐system size and the dip length of shallow‐marine parasequence sand bodies. The resulting insights can be applied to guide sequence stratigraphic interpretations of the rock record and the characterization of sub‐seismic stratigraphic architectures of subsurface successions

A test of analog-based tools for quantitative prediction of large-scale fluvial architecture

Outcrop analogs are routinely used to constrain models of subsurface fluvial sedimentary architecture built through stochastic modeling or inter-well sandbody correlations. Correlability models are analog-based quantitative templates for guiding the well-to-well correlation of sand-bodies, whereas indicator variograms used as input to reservoir models can be parameterized from data collected from analogs, using existing empirical relationships. This study tests the value and limitations of adopting analog-informed correlability models and indicator-variogram models, and assesses the impact and significance of analog choice in subsurface workflows for characterizing fluvial reservoirs. A 3.2 km long architectural panel based on a Virtual Outcrop from the Cretaceous Blackhawk Formation (Wasatch Plateau, Utah, USA) has been used to test the methodologies: vertical 'dummy' wells have been constructed across the panel, and the intervening fluvial architecture has been predicted using correlability models and sequential indicator simulations. The correlability and indicator-variogram models employed to predict the outcrop architecture have been compiled using information drawn from an architectural database. These models relate to: (i) analogs that partially match with the Blackhawk Formation in terms of depositional setting, and (ii) empirical relationships relating statistics on depositional-element geometries and spatial relations to net-to-gross ratio, based on data from multiple fluvial systems of a variety of forms. The forecasting methods are assessed by quantifying the mismatch between predicted architecture and outcrop observations in terms of the correlability of channel complexes and static connectivity of channel deposits. Results highlight the effectiveness of correlability models as a check for the geologic realism of correlation panels, and the value of analog-informed indicator variograms as a valid alternative to variogram-model parameterization through geostatistical analysis of well data. This work has application in the definition of best-practice use of analogs in subsurface workflows; it provides insight into the typical degree of realism of analog-based predictions of reservoir architecture, as well as on the impact of analog choice, and draws attention to associated pitfalls

Quantitative analysis of aeolian stratigraphic architectures preserved in different tectonic settings

Despite a well-documented record of preserved sedimentary architectures of aeolian successions from a variety of different sedimentary basin types, relationships between tectonic setting and aeolian accumulation and preservation remain poorly constrained and largely unquantified. This study uses a database-informed approach to quantitatively document the variability in preserved sedimentary architecture of 56 globally distributed aeolian systems, and to relate these variations to differences in their basin settings. Three different tectonic settings are considered: intracratonic, foreland, and rift basins. Key finding are as follows. (1) Intracratonic basins are characterized by slow accommodation generation. They favour the accumulation and preservation of relatively thin aeolian genetic units; likely generated by dunes and interdunes that climbed at low angles, resulting in the accumulation and preservation of relatively thin dune and interdune elements. Depressed water tables – indicated by abundant dry interdune elements – left accumulated dune successions exposed above the erosional baseline, making them vulnerable to post-depositional reworking. This likely resulted in sporadic episodes of aeolian dune accumulation, between long episodes of sediment bypass or deflation, under conditions of low rates of net accommodation generation. (2) Rapid accommodation generation in the depocentres of foreland basins favours the preservation of thick dune-set and interdune elements, enabled by a rapid rate of rise of the accumulation surface that allowed bedform climb at relatively high angles. High rates of sediment supply associated with erosion of adjacent orogenic belts allowed the construction of large dunes. Elevated water tables – indicated by abundant wet interdune elements – may have allowed aeolian accumulations to be placed beneath the baseline of erosion shortly after deposition, thereby protecting them from potential post-depositional deflation. (3) Despite rift basins experiencing the highest rates of accommodation generation, their fills tend to be associated with the preservation of relatively thin dune and sandsheet elements. Elevated water tables, associated with rapid accommodation generation, create damp substrates and consequently restrict the availability of dry sand for dune construction. In the examples considered here, rapid accommodation generation outpaces sediment supply, favouring the construction and rapid migration of small dunes that consequently accumulate thin dune sets. Aeolian dunes in rift basins are also commonly reworked by fluvial and alluvial processes, in some cases likely related to orographic precipitation, associated with rift shoulder topography. Results of this analysis can be applied to improve predictions of the architecture of ancient aeolian successions at the basin scale, both in outcrop and in subsurface successions