46,361 research outputs found
Scale dependency in quantifications of the avulsion frequency of coastal rivers
No full textQuantification of the frequency with which coastal-plain rivers avulse is important for elucidating autogenic dynamics and their role as controls on landscape change and stratigraphic architectures. An outstanding question exists, however, as to whether measures of avulsion frequency are inherently affected by the spatial and temporal scales at which they are evaluated, which has implications regarding our ability to make direct comparisons between different river systems or deltas. To address this problem, a quantitative analysis of the avulsion histories of 57 coastal-plain river systems is undertaken. Nine alternative measures of avulsion frequency are extracted. These are based on numbers of (i) avulsion events, (ii) active or abandoned channel courses, and (iii) delta lobes, all considered per unit time. Additional sets of avulsion-frequency proxies are established based on normalization of these numbers relative to the size of the area being studied, and to the number of distinct river systems that drain into that area. The sensitivity of these quantities to the spatial and temporal extent of study areas and time intervals, and their relationships with quantities describing the scale of the river systems, are assessed. All avulsion-frequency estimates demonstrate apparent negative relationships with the timespan over which they are determined; this may reflect global Holocene trends, or variations in resolution with the time window. Avulsion metrics that are not normalized by the planform extent of the study area do not show proportionality with the size of the study areas themselves, nor with the scale of the river systems; correspondingly, the spatio-temporal density of avulsion events tends to be higher for smaller rivers and associated study areas. This may be due to systematic variations in data resolution, to the influence of external controls that relate to the scale of deltas, or to inherent non-stationarity in the avulsion dynamics of lowland rivers, in association with forms of self-organization that do not vary with scale. Although non-normalized avulsion-frequency estimates do not scale with measures of river-system size, they are seen to correlate with progradation rates, which are themselves scaled to sediment discharge and catchment size. Practical considerations can be drawn on how avulsion frequency may be appropriately quantified to enable meaningful comparisons of the autogenic dynamics of coastal-plain rivers
Evaluation of Morphodynamic Controls on the Preservation of Fluvial Meander‐Belt Deposits
No full textThe way river morphodynamics influence the preservation of point-bar deposits at multiple scales is not fully understood. Employing time-lapse trajectories of natural rivers, a numerical model is used here to simulate planform evolutions of meander-belt reaches that embody different transformation behaviors and cutoff processes. Proxies for temporal durations are obtained considering the surface area over which a river migrated and channel migration rates that relate to average channel radius of curvature through constant, monotonic, and non-monotonic relationships. The preservation of meander-belt deposits over different timescales is assessed at three architectural hierarchies: (a) pairs and (b) sets of accretion packages, and (c) meander-belts. Results confirm that sediment preservation decreases in a predictable way with the accumulation time; however, accretion rates decay with time in a way that does not follow the expected power-law. This is interpreted to reflect the effect of the onset of geomorphic thresholds of channel transformation and cutoff
On the geological significance of clastic parasequences
No full textParasequences 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
Sediment preservation and accretion rates of fluvial meander-belt deposits: variations with temporal scale and river size
No full textAssessment of backwater controls on the architecture of distributary channel fills in a tide-influenced coastal-plain succession: Campanian Neslen Formation, USA
No full textThe 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
A meta-study of relationships between fluvial channel-body stacking pattern and aggradation rate: implications for sequence stratigraphy
No full textA quantitative comparison of 20 literature case studies of fluvial sedimentary successions tests common assumptions made in published models of alluvial architecture concerning (1) inverse proportionality between channel-deposit density and floodplain aggradation rates, and (2) resulting characteristics of channel-body geometries and connectedness. Our results do not support the relationships predicted by established stratigraphy models: the data suggest that channel-body density, geometry, and stacking pattern are not reliable diagnostic indicators of rates of accommodation creation. Hence, these architectural characteristics alone do not permit the definition of accommodation-based “systems tracts” and “settings”, and this calls into question current sequence stratigraphic practice in application to fluvial successions
A database of Aeolian Sedimentary Architecture for the characterization of modern and ancient sedimentary systems
No full textThe Database of Aeolian Sedimentary Architecture (DASA) records the architecture and spatio-temporal evolution of a broad range of modern and recently active aeolian systems, and of their preserved deposits in ancient successions. DASA currently stores data on >14,000 geologic and geomorphic entities (including bounding surfaces and transition relationships) extracted from >60 case-study examples documented in the published literature. DASA stores data on a variety of aeolian and associated non-aeolian entities of multiple scales, including attributes that characterize their type, geometry, spatial relations, hierarchical relations, temporal significance, and textural and petrophysical properties; associated metadata are also stored.
Database output describes (1) stratigraphic relationships between aeolian and associated fluvial, lacustrine and paralic depositional systems; (2) the geometry of aeolian architectural elements, and hierarchical and spatial relationships between them; (3) the probabilities of vertical and lateral transition from one type of deposit or landform to another; (4) the presence and nature of aeolian bounding surfaces at different scales, and their nested, hierarchical relationships; (5) aeolian lithofacies types, proportions and distributions, and (6) grain-scale textural parameters.
DASA is applied to quantitatively characterize and compare modern and ancient aeolian sedimentary systems. Examples of database outputs demonstrate how DASA outputs can be tailored for numerous applications, including: (1) the development of bespoke quantitative facies models, specifically tailored for particular sets of boundary conditions; (2) the empirical assessment of how aeolian systems, and associated preserved sedimentary architectures, represent a response to allogenic and autogenic forcings; and (3) the instruction of forward stratigraphic models and 3D geocellular subsurface models. DASA is a valuable tool for the characterization of subsurface aeolian successions, such that output can help to (1) predict three-dimensional lithological heterogeneity in subsurface successions that are resource targets, (2) constrain geocellular stochastic models, and (3) facilitate borehole correlations of aeolian dune sets or associated non-aeolian elements
Controls on fluvial meander-belt thickness and sand distribution: insights from forward stratigraphic modelling
No full textFluvial point‐bar evolution commonly involves multiple stages of bar development driven by changes in the style of meander transformations. Complicated planform morphologies are widely recognized in remote‐sensing imagery, but the relationships between meander‐bend evolutionary behaviour and stratigraphic architecture, facies distribution, and sand volumes remain poorly understood. This study applies a geometric forward stratigraphic model (Point‐Bar Sedimentary Architecture Numerical Deduction – ‘PB‐SAND’) to simulate the internal sedimentary architecture of 24 meander‐belt segments that evolved via a broad range of meander‐bend transformation styles. Modelling inputs are constrained by channel trajectories inferred from high‐resolution Light Detection and Ranging (LiDAR) datasets, lithological information from a sedimentological database (Fluvial Architecture Knowledge Transfer System – ‘FAKTS’) and geological knowledge of trends in point‐bar lithology (for example, decrease in sand proportion with sinuosity, downstream of bend apices, and beyond the transition from point‐bar to counter‐point‐bar deposits) and in channel bathymetry (depth variations across pools and riffles). Modelling results are used to explore how the relative distribution of sand and mud is controlled by the styles of point‐bar transformation, quantified by the relative degree of meander translation versus expansion, and by the amount of bend rotation. The 24 models are classified into three groups based on cluster analysis of their mean migration angle, mean apex rotation, mean sinuosity, standard deviation of channel circular variance and preservation ratio; these quantities are known to be controlled by meander transformation types. Quantitative comparisons across these groups and relationships between metrics of planform change and quantifications of point‐bar deposits demonstrate how meander planform evolution controls point‐bar thickness and sand volume. Locally, the thickness of sand in bar deposits is controlled by the interplay of facies trends and spatial variations in bar thickness that reflect bathymetric changes, both related to local hydrodynamics. The proposed workflow establishes linkages between planform morphologies and three‐dimensional facies distributions; it can be employed to characterize the distribution of subsurface porous volumes where the planform history of meander bends can be reconstructed
An integrated approach to determine three‐dimensional accretion geometries of tidal point bars: Examples from the Venice Lagoon
No full textLow rates of lateral migration (centimetres to decimetres per year) combined with relatively high rates of vertical accretion (millimetres to centimetres per year) recorded in microtidal channels of the Venice Lagoon (Italy) give rise to point‐bar geometries and internal facies arrangements that differ substantially from widely accepted models of point‐bar sedimentary architecture. In this study, field data from the Venice Lagoon are combined with a three‐dimensional forward stratigraphic model, the ‘Point‐Bar Sedimentary Architecture Numerical Deduction’ (PB‐SAND), to predict the stratal geometries of point bars formed in aggradational settings. The PB‐SAND uses a combined geometric and stochastic modelling approach that can be constrained by field evidence. The model applied determines the geometry of four point bars generated by 9 to 11 m wide channels cutting through salt marshes. An iterative best‐fit modelling approach has been used to obtain multiple simulations for each case‐study, each of which fits the observations derived from the analysis of time‐series historical aerial photographs and 44 sedimentary cores. Results demonstrate how the geometry of the bars is determined by the development of two key stratal surfaces: the point‐bar brink and channel‐thalweg surfaces. These surfaces are defined by the progressive translation and vertical shift of the point‐bar brink (i.e. break of slope between bar top and bar slope) and the channel thalweg (i.e. deepest part of the channel) during bar evolution. The approach is used to: (i) reconstruct three‐dimensional point‐bar geometries; (ii) propose alternative reconstructions; (iii) provide insight to drive the acquisition of additional data to better constrain the proposed models; and (iv) provide insight into the mechanism of bar growth for slowly migrating channels in settings subject to relatively high rates of aggradation. This study highlights how interaction between styles of planform transformation and latero‐vertical shifts of meandering channels can determine the geometry of related sedimentary bodies
Influence of Meander-Belt Sedimentary Architecture on Performance of Low-Enthalpy Geothermal Doublet: Insights From High-Resolution Heat-Transport Simulations
No full textThis work is a study on the impact of sedimentary heterogeneity on groundwater flow and heat transport with a focus on meander-belt fluvial successions acting as low-enthalpy geothermal reservoirs. Geocellular models were generated to represent three types of meander belts produced by rivers that were ca. 12 m deep and include km-scale long and wide point-bar elements. Well doublets of typical spacing were placed on individual point-bar elements. Groundwater-flow and heat transport models were developed using MODFLOW 2005 and MT3D-USGS, respectively. The results suggest a relationship between the shape of the thermal plume and aspects of sedimentary heterogeneity. Mud plugs are a significant cause of asymmetry on thermal plume propagation, whereas discontinuous mud drapes have limited impact. The presence of openwork gravels, which act as thief zones, does not necessarily cause a reduction in thermal breakthrough time, although their impact is dependent on their orientation relative to the well doublet. The results of this study can assist the optimization of well-doublet planning based on knowledge of the subsurface. Yet, the impact of sedimentary heterogeneity in the successions of meandering rivers warrants further investigation, to be undertaken considering a wider natural variability of meander-belt sizes, architecture and styles of facies heterogeneity
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