1,720,972 research outputs found
Sediment preservation and accretion rates of fluvial meander-belt deposits: variations with temporal scale and river size
No full textScale dependency in quantifications of the avulsion frequency of coastal rivers
Full text linkQuantification 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
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
A meta-study of relationships between fluvial channel-body stacking pattern and aggradation rate: implications for sequence stratigraphy
Full text linkA 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
Assessment of backwater controls on the architecture of distributary channel fills in a tide-influenced coastal-plain succession: Campanian Neslen Formation, USA
Full text linkThe 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
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
The Shallow-Marine Architecture Knowledge Store: A database for the characterization of shallow-marine and paralic depositional systems
No full textThe Shallow-Marine Architecture Knowledge Store (SMAKS) is a relational database devised for the storage of hard and soft data on the sedimentary architecture of ancient shallow-marine and paralic siliciclastic successions, and on the geomorphological organization of corresponding modern environments. The database allows incorporation of data from the published literature, which are uploaded to a common standard to ensure consistency in data definition. The database incorporates data on geological entities of varied nature and scale (i.e., surfaces, depositional tracts, architectural elements, sequence stratigraphic units, facies units, geomorphic elements), including attributes that characterize their type, geometry, spatial relations, hierarchical relations, and temporal significance. Furthermore, geological entities are assigned to depositional systems, or to parts thereof, that can be classified on multiple parameters (e.g., shelf width, delta catchment area) tied to metadata (e.g., data types, data sources). The SMAKS permits the quantitative characterization of modern and ancient shallow-marine and paralic clastic depositional systems. It aims to serves as a repository of analogue information for hydrocarbon-bearing successions, and as a research tool, applicable to aid the development of facies models or to assess the sensitivity of depositional systems to particular controlling factors, for example. To demonstrate the wide applicability of the database in fields of both fundamental and applied research, example database output is presented that (i) includes data from wave-, tide-, and fluvial-dominated shallow seas and sedimentary successions, and (ii) covers a wide depositional spectrum, from backshore to shelf-edge settings. The examples include information on the facies organization of different types of paralic sub-environments, on the hierarchical arrangement of architectural elements that form deltaic constructional units in Quaternary deltas, on the morphometry of modern and Quaternary tidal sand ridges, and on the geometry of parasequence-scale nearshore sandstone belts from the Upper Cretaceous of the Western Interior Seaway in Utah (USA)
Quantitative sedimentological analysis of the aeolian–fluvial Triassic Sherwood Sandstone Group, Cheshire Basin, UK
No full textSedimentological analysis of the outcropping Sherwood Sandstone Group, northern Cheshire Basin, UK, documents the preserved stratigraphic expression of water table-controlled aeolian-dominated erg-margin successions (Wilmslow Sandstone Formation and lower Helsby Sandstone Formation), overlain by dryland fluvial deposits (middle Helsby Sandstone Formation). A quantitative assessment of sedimentary architectures and a comparison with analogous depositional systems reveals that strata of the Sherwood Sandstone Group are similar to other aeolian-dominated successions that also accumulated in rift basins, in erg-margin settings and where the water table interacted with the sediment surface. In stratal successions of erg-margin origin in the Wilmslow Sandstone Formation and Helsby Sandstone Formation, the dominant facies associations comprise: (i) thin cross-bedded dune sets characterized by foreset–toeset grainflow strata; and (ii) lenticular bodies of dry and damp-to-wet interdune strata. Together, these associations record accumulation of small, rapidly migrating transverse or oblique dunes, and intervening isolated interdune depressions. Two types of interdune deposits are observed: (i) near-horizontal wind-ripple laminated sandstone; and (ii) irregular to wavy-laminated sandstone with adhesion strata. These deposits indicate dry and damp–wet surface conditions, respectively. Dune climbing is demonstrated by the interfingering between dune toesets and interdune strata; this was enabled by accommodation generation driven by fault-related subsidence and associated relative water-table rise. Accumulation via climbing at very low angle but net positive, else by non-climbing mechanisms, is demonstrated by erosional, sharp dune–interdune bounding surfaces, which likely developed during slowdowns in subsidence rate. Episodic absolute water-table rise may have occurred during wet seasons or due to intense precipitation events, potentially in a monsoon climate. Water-table fluctuations may have also occurred in response to longer-term climatic perturbations following the Permian–Triassic mass extinction event. This study helps to constrain the factors controlling the accumulation of Triassic aeolian and mixed aeolian–fluvial successions, and their long-term preservation. Quantitative models are proposed to explain aeolian dune–interdune facies architectures. These can be used to guide predictions of three-dimensional sedimentary heterogeneity in the subsurface
Anatomy and dimensions of fluvial crevasse-splay deposits: examples from the Cretaceous Castlegate Sandstone and Neslen Formation, Utah, U.S.A.
Full text linkCrevasse-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
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