Scour Fills Research Articles

The contribution of grain sorting to the dynamics of the bedload active layer

AbstractDuring the last 20 years, flume and field experiments have shown that grain sorting contributes to bed‐level fluctuations and bedload pulses. In this work, we propose a new analysis of these experimental data. From the flume data, we derive a model for gravel‐bed rivers where both local (bedform‐scale) slope and bedload are known to fluctuate through space and time, in the so‐called ‘bedload active layer’. The model uses standard concepts and empirical tools with reach‐averaged data for the hydraulics and sediment transport. It considers a maximum slope for local armouring equal to the mean bed slope (reach scale) affected by a coefficient which expresses the difference in mobility of the coarse fraction considered alone or in a mixture. The minimum local slope for bed erosion is the mean bed slope corrected by a coefficient that depends on the armour ratio Ar (ratio of the surface to the subsurface grain diameter) and the reach‐averaged transport rate. The model is compared with a compilation of scour–fill depths measured in the field. Results suggests that the slope fluctuations in 1D flume experiments are consistent with in‐channel bed‐level fluctuations associated with scour–fill processes in the active layer. The model also suggests that although the length scale of the maximum scour depth δ is on the order of the bed surface D90, it is well explained by the product between the mean bed slope S and the active channel width W, with δ ≈ 1.4SW. For the pulse intensity, we provide a justification for the simplified squared slope equation for solid concentration C = Qs/Q ∝ S2 (with Qs the solid discharge, Q the water discharge and S the slope), which has often been used in place of standard bedload equations for modelling highly concentrated bedload transport events in mountain streams.

Recognition and three‐dimensional characteristics of ancient supercritical flow bedforms on a submarine slope: An example from the South China Sea

AbstractSupercritical flow bedforms are important elements of sedimentary environments, but their internal three‐dimensional structure has been elusive due to seismic imaging limitations. This article presents high‐resolution three‐dimensional seismic reflection data from Formosa Ridge – a ridge between two canyons that incise into the north‐eastern South China Sea margin. The ridge consists of 300 m thick submarine deposits including sediment waves that are manifested as crescentic depressions surrounded by elevated walls on the palaeo‐seafloor. Cross‐sectional profiles display scour fills that turn into step‐like sediment waves further downstream. These bedforms are 470 to 1370 m long and 30 to 140 m high. The three‐dimensional seismic data clearly show the step‐like bedforms that may be misinterpreted as faults or slumps on data with lower resolution. Despite exhibiting negative palaeo‐seafloor relief, they are overall depositional structures and have constructed at least part of the Formosa Ridge. The bedforms’ morphology and upslope migration suggest that they are the continuum of partially depositional to fully depositional cyclic steps formed by bottom currents travelling, based on a series of simple calculations, at least 2 m s−1. These currents are able to transport sediments with grain size up to coarse sand, and such dynamic processes might impact seafloor infrastructure safety, oil and gas reservoir systems, and the functioning of benthic ecosystems in similar settings.

Stochastic Assessment of Scour Hazard

Scour is the most frequent cause of bridge collapses in Mexico. Bridges located along the Mexican Pacific coast are exposed to extreme rainfall originating from tropical storms and hurricanes. Such environmental phenomena trigger sediment loss, which is known as scour. If maintenance actions are not taken after scouring events, the scour depth increases over time until the bridge collapses. A methodology to estimate the scour hazard considering both the scour–fill interaction and the Monte Carlo simulation method is proposed. The general extreme value probability distribution is used to characterize the intensity of the scouring events, the lognormal distribution is used to characterize the sedimentation process (fill), and a homogeneous Poisson process is used to forecast the occurrence of both types of events. Based on the above, several histories of scour–fill depths are made; such simulations are then used to develop time-dependent scour hazard curves. Different hazard curves associated with different time intervals are estimated for a bridge located in Oaxaca, Mexico.

Bedforms and sedimentary structures related to supercritical flows in glacigenic settings

AbstractUpper‐flow‐regime bedforms, including upper‐stage‐plane beds, antidunes, chutes‐and‐pools and cyclic steps, are ubiquitous in glacigenic depositional environments characterized by abundant meltwater discharge and sediment supply. In this study, the depositional record of Froude near‐critical and supercritical flows in glacigenic settings is reviewed, and similarities and differences between different depositional environments are discussed. Upper‐flow‐regime bedforms may occur in subglacial, subaerial and subaqueous environments, recording deposition by free‐surface flows and submerged density flows. Although individual bedform types are generally not indicative of any specific depositional environment, some observed trends are similar to those documented in non‐glacigenic settings. Important parameters for bedform evolution that differ between depositional environments include flow confinement, bed slope, aggradation rate and grain size. Cyclic‐step deposits are more common in confined settings, like channels or incised valleys, or steep slopes of coarse‐grained deltas. Antidune deposits prevail in unconfined settings and on more gentle slopes, like glacifluvial fans, sand‐rich delta slopes or subaqueous (ice‐contact) fans. At low aggradation rates, only the basal portions of bedforms are preserved, such as scour fills related to the hydraulic‐jump zone of cyclic steps or antidune‐wave breaking, which are common in glacifluvial systems and during glacial lake‐outburst floods and (related) lake‐level falls. Higher aggradation rates result in increased preservation potential, possibly leading to the preservation of complete bedforms. Such conditions are met in sediment‐laden jökulhlaups and subaqueous proglacial environments characterized by expanding density flows. Coarser‐grained sediment leads to steeper bedform profiles and highly scoured facies architectures, while finer‐grained deposits display less steep bedform architectures. Such differences are in part related to stronger flows, faster settling of coarse clasts, and more rapid breaking of antidune waves or hydraulic‐jump formation over hydraulically rough beds.

Interplay of sediment supply and lake-level changes on the margin of an intrabasinal basement high in the Late Miocene Lake Pannon (Mecsek Mts., Hungary)

Lake Pannon, covering the Pannonian Basin (Hungary) during the Late Miocene, had a complex lake bottom with deeper sub-basins and intrabasinal basement highs, sometimes emerging above the lake level as islands and peninsulas. Above structural highs, the basin fill sequence usually commenced with deposition of transgressional, locally sourced coarse-grained deltas. These deposits are overlain by distally-sourced deltaic bodies associated with the prograding delta system that gradually filled up the entire basin. The transition between these two distinct delta systems was studied in a large outcrop on the edge of a former basement high (Mecsek Mts., southern Hungary). The transgressional phase is represented by the deposition of a mass flow dominated fan delta body, fed by local material from a granitic catchment area. An overlying fossil-rich, clayey unit records an episode for which sediment delivery into the basin was curtailed, possibly due to submergence of the granite body. The deposition of these two units took place between 8 and 6.8 Ma. The onset of sedimentation associated with the arrival of the distally sourced regressive delta system (around 6.8 Ma) is represented by deposition of shoreface sediments. This unit is characterized by distinctive bioturbation and storm related, sand filled scour-fills. Resedimented local material that forms mm thick, coarse laminas in the scour fills is indicative of denudation due to tectonic events and implies coupled storm-flood sedimentation. The deposition of the two distinct delta bodies and the interplay between tectonic events and lake-level changes occurred during a relatively short, ca. 1.5 Ma long time interval.

Imaging scours in straightened and braided gravel‐bed rivers with ground‐penetrating radar

ABSTRACTThe genesis of alternate gravel bars in straightened rivers and braided rivers is closely coupled to the formation of scours. Scours can lead to riverbank failure, initiate lateral river dynamics and strongly impact the interaction between surface and subsurface flow and transport. It is, therefore, critical to account for scours in the design of flood protection measures. However, there is still little knowledge on the formation and characteristics of such scours, especially on the dynamic relationship between the riverbed morphology and the scours. Bathymetric riverbed surveys conducted at medium to low discharge may underestimate the real scour sizes and shapes if scours filled with sediments during waning discharge. Furthermore, the literature suggests that gravel bars and scours can jointly migrate on the riverbed, resulting in cut‐and‐fill sedimentary structures in the near subsurface. These cut‐and‐fill structures cannot be observed from the surface. We investigate with ground‐penetrating radar the presence of scour fills below the riverbed of two gravel‐bed rivers and study any indications of bar and scour migration. Thus, two ground‐penetrating radar surveys were conducted, one on the emerged part of an alternate bar of the channelized Alpine Rhine River (Switzerland) and the other on a flat gravel surface in a braided reach of the Tagliamento River (Italy) that is near the pristine state. At both sites, a scour fill could be clearly identified below a 2‐m‐thick sediment layer. The imaged part of the scour of the Alpine Rhine River (30 × 100 × 4.5 m) is located at the front end of the gravel bar next to the riverbank. This scour was only partially imaged by ground‐penetrating radar and is in reality significantly deeper and larger. The scour of the Tagliamento River (20 × 30 × 2.5 m) shows a clear internal structure consisting of inclined, planar cross‐beds that merge tangentially with the lower‐bounding erosional surface of the scour. At the light of the literature, we compare these two scours in terms of sedimentary processes resulting from the complex interplay between scours and gravel bars. The study findings offer a promising research avenue on the dynamic relationship between discharge, sediment transport, scour and bar formation and migration.

How to recognize crescentic bedforms formed by supercritical turbidity currents in the geologic record: Insights from active submarine channels

Submarine channels have been important throughout geologic time for feeding globally significant volumes of sediment from land to the deep sea. Modern observations show that submarine channels can be sculpted by supercritical turbidity currents (seafloor sediment flows) that can generate upstream-migrating bedforms with a crescentic planform. In order to accurately interpret supercritical flows and depositional environments in the geologic record, it is important to be able to recognize the depositional signature of crescentic bedforms. Field geologists commonly link scour fills containing massive sands to crescentic bedforms, whereas models of turbidity currents produce deposits dominated by back-stepping beds. Here we reconcile this apparent contradiction by presenting the most detailed study yet that combines direct flow observations, time-lapse seabed mapping, and sediment cores, thus providing the link from flow process to depositional product. These data were collected within the proximal part of a submarine channel on the Squamish Delta, Canada. We demonstrate that bedform migration initially produces back-stepping beds of sand. However, these back-stepping beds are partially eroded by further bedform migration during subsequent flows, resulting in scour fills containing massive sand. As a result, our observations better match the depositional architecture of upstream-migrating bedforms produced by fluvial models, despite the fact that they formed beneath turbidity currents.

3D architecture of cyclic-step and antidune deposits in glacigenic subaqueous fan and delta settings: Integrating outcrop and ground-penetrating radar data

Bedforms related to supercritical flows are increasingly recognised as important constituents of many depositional environments, but outcrop studies are commonly hampered by long bedform wavelengths and complex three-dimensional geometries. We combined outcrop-based facies analysis with ground-penetrating radar (GPR) surveys to analyse the 3D facies architecture of subaqueous ice-contact fan and glacifluvial delta deposits. The studied sedimentary systems were deposited at the margins of the Middle Pleistocene Scandinavian ice sheets in Northern Germany.Glacifluvial Gilbert-type deltas are characterised by steeply dipping foreset beds, comprising cyclic-step deposits, which alternate with antidune deposits. Deposits of cyclic steps consist of lenticular scours infilled by backset cross-stratified pebbly sand and gravel. The GPR sections show that the scour fills form trains along the delta foresets, which can locally be traced for up to 15m. Perpendicular and oblique to palaeoflow direction, these deposits appear as troughs with concentric or low-angle cross-stratified infills. Downflow transitions from scour fills into sheet-like low-angle cross-stratified or sinusoidally stratified pebbly sand, deposited by antidunes, are common. Cyclic steps and antidunes were deposited by sustained and surge-type supercritical density flows, which were related to hyperpycnal flows, triggered by major meltwater discharge or slope-failure events.Subaqueous ice-contact fan deposits include deposits of progradational scour fills, isolated hydraulic jumps, antidunes and (humpback) dunes. The gravel-rich fan succession consists of vertical stacks of laterally amalgamated pseudo-sheets, indicating deposition by pulses of waning supercritical flows under high aggradation rates. The GPR sections reveal the large-scale architecture of the sand-rich fan succession, which is characterised by lobe elements with basal erosional surfaces associated with scours filled with backsets related to hydraulic jumps, passing upwards and downflow into deposits of antidunes and (humpback) dunes. The recurrent facies architecture of the lobe elements and their prograding and retrograding stacking pattern are interpreted as related to autogenic flow morphodynamics.

Two million years of river and cave aggradation in NE Brazil: Implications for speleogenesis and landscape evolution

This study characterizes and provides ages for an extensive sedimentary record occurring in Lapa Doce and Torrinha caves, NE Brazil. With >40km of surveyed passages, these caves integrate a distributary cave system fed by allogenic recharge from the surrounding sandstone plateaus. Sediment petrography together with descriptions of depositional facies and architectural elements shows four depositional units related to fluvial and standing water environments. These include, from bottom to top: (1) a channel unit including lateral bars deposited during an ordinary flood regime; (2) a sandy flood unit including minor channel and scour fills derived from bank-full equivalent flood events; (3) mud caps deposited in standing water that often reach the ceiling; and (4) intraclast breccias associated with collapse of the mud caps under saturated conditions. The deposits were dated using a combination of cosmogenic nuclide burial dating and U-series dating of flowstone. Cosmogenic nuclide data point to fluvial aggradation being active since 1.91+0.12My and extending until 0.36±0.08My, with intensive cave and valley aggradation events between 0.78±0.10My and 0.44±0.12My. Long term alluviation of the cave system seems to be important in forming passages, determining their configuration, and setting up a general distributary pattern evident in passage morphology and sedimentary sequences. Mud caps overlapping the fluvial deposits are interpreted as the products of successive rising and lowering of the water table (static level). Radiometric ages of interstratified flowstones and speleothems show that these oscillations were active at least since 115ky ago and finally ceased around 12ky ago, indicated by the recurrent age obtained from uneroded capping flowstones. These long-term water table oscillations may drive paragenetic expansion of the whole cave system. Valleys, caves and other landforms in our study area are part of an ancient landscape with multiple episodes of burial and exhumation. Despite the possibility of much older sediment being preserved somewhere in the cave system, our data indicate a younger Quaternary age for the bulk of the sediment filling the study caves.

Assessment of Backwater Controls On the Architecture of Distributary-Channel Fills In A Tide-Influenced Coastal-Plain Succession: Campanian Neslen Formation, U.S.A.

Abstract 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 the architecture of distributary-channel fills. 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.

The stratigraphic expression of decreasing confinement along a deep-water sediment routing system: Outcrop example from southern Chile

The products of sediment-laden turbidity currents that traverse areas of decreasing confinement on submarine slopes include erosional and depositional features that record the inception and propagation of deep-sea channels. The cumulative stratigraphic expression and deposits of such transitions, however, are poorly constrained relative to depositional settings dominated by end-member confined (i.e., submarine channel fill) and unconfined (i.e., lobe) deposits. Upper Cretaceous strata of the Magallanes foreland basin in southern Chile are characterized by a variety of stratigraphic architectural elements in close juxtaposition both laterally and vertically, including: (1) low-aspect-ratio channelform bodies attributed to slope channel fills; (2) high-aspect-ratio channelform bodies interpreted as the deposits of weakly confined submarine channels; (3) lenticular sedimentary bodies considered to represent the infill of laterally coalesced scours; (4) discontinuous channelform bodies representing isolated scour fills; and (5) a cross-stratified, positive-relief sedimentary body, which is interpreted to record an upslope-migrating depositional bedform. These elements are interpreted to have formed at a submarine sediment routing system segment characterized by a break in slope, and an accompanying decrease in confinement. The various architectural elements examined are interpreted to record a unique stratigraphic perspective of turbidite channels at various stages of development, from early-stage discontinuous and isolated scour fills to low-aspect-ratio channel units.

A study of the scour–fill threshold based on Lane's equilibrium relation: The lower Yellow River

The scour–fill threshold in river channels is an expression of scour–fill equilibrium, which may be understood as the balance between the river's sediment carrying capability and the sediment load imposed by the drainage basin. In the present study, the above capability–load relation is quantified using Lane's relation, γQJ–QsD50, wherein Q is water discharge, J is channel slope, Qs is sediment transport rate, D50 is median size of bed material, and γ is the specific weight of water. On this basis, we have established scour–fill thresholds for the lower Yellow River based on data from 141 flood events. The results show that the QsD50–γQJ relation may well distinguish between the scour and fill flood events. The following equation has been established: SDRFE=0.00099(γQJ)0.90(QsD50)−0.60, wherein SDRFE is channel sediment delivery ratio. From the equation, the scour–fill threshold is established as: QsD50=0.00001(γQJ)1.5. Starting from this formula, four possible options are proposed for sediment management of the lower Yellow River.

Confluence Scours Versus Incised Valleys: Examples From the Cretaceous Ferron Notom Delta, Southeastern Utah, U.S.A

Abstract Confluence scours in braided rivers occur where channel threads join together, producing erosional relief that may be considerably deeper than average channel depth. Based on studies of the continental-scale Ganges–Brahmaputra river system, it has been observed that the maximum depth of confluence scours by autocyclic processes may reach up to four to five times the average depth of the incoming channels. Considering the possibility of such massive scours in an ancient fluvial system, it was argued that allogenically produced incised valleys at sequence boundaries should only be properly defined in ancient systems if the erosional relief is more than five times average channel depth. Based on cross-sectional geometries, a number of confluence scour fills were interpreted on well-exposed fluvial outcrops in a Cretaceous-age compound incised-valley system in the Notom delta complex of the Ferron Sandstone Member, Utah. Cross-sectional observations from outcrops and comparisons with modern rivers reveal that confluence scours have diagnostic fill facies (single set of large steep foresets) and do not produce multistory sand bodies. A confluence scour fill produces a single-story body in which a fifth-order scour is filled with unit bar foresets, which in turn are overlain by a fourth-order surface capped by compound bar deposits. The story thickness in the confluence scour does not represent the average channel depth because confluence scours allow preservation of the deepest parts of channels as well as fully preserved abnormally thick stories. Therefore, we argue that interpretation of incised valleys in the ancient system associated with a sequence boundary should not be based on the depth of the erosional surface versus the number of average preserved channel stories. Rather, it should be defined by the erosional relief that is significantly deeper than the thickest fully preserved stories, which in a braided stream are likely to represent confluence scour fills.

Revision of Platte River Alluvial Facies Model Through Observations of Extant Channels and Barforms, and Subsurface Alluvial Valley Fills

Abstract The Platte River of Nebraska, central USA, has been used by sedimentologists as a type example of a sandy, braided river system for some forty years. Despite this designation, however, the details of the river’s subsurface stratigraphy have never been evaluated, and so the extant facies model for the Platte River remains unvalidated. This paper analyzes the Holocene stratigraphy of the central and lower Platte River. It utilizes newly acquired data from wireline logs, sediment cores, aerial photography, and ground-penetrating radar (GPR) traverses to characterize subsurface geology, in addition to surface sediment and geomorphic mapping, sampling, and analysis of serial aerial imagery. Cores and GPR profiles show that the lithosome of the active Platte is 6–8 m in thickness and consists predominantly of medium sand. Although macroform features are common on the surface of the modern (active) central and lower Platte River in Nebraska, the shallow subsurface stratigraphy is dominated by mesoform cross-bedding and planar stratification overlying channel scour fills. This differs markedly from the vertical succession predicted by the Platte River facies model of Miall (1977). Data from inactive portions of the central and lower Platte River, however, show macroform cross-bedding indicative of the downstream and cross-stream migration of linguoid and transverse bars, which would be predicted by the published model. The typical architecture of the Holocene channel-belt sand body in the areas studied thus comprises a lower unit dominated by macroform-scale cross-bedding, overlain by an upper unit dominated by mesoform-scale cross-bedding and scour fills. Furthermore, time-series analysis of aerial photography of the river in the vicinity of Grand Island, Nebraska, shows that: (1) the river has become dominated by aggressively vegetated subaerially exposed channel-bed domains (termed “vegetated channel islands”) that superficially resemble bars; and (2) that this stretch is no longer dominated by downstream-accreting macroform bars, which were abundant in the 1960s and 1970s. Contraction of the active channel in this region has accompanied these changes, somewhat independently of discharge variation. New vertical sequence facies models are presented to account for these differences. Our data show the futility of seeking to characterize a fluvial system by means of a single vertical sequence model, and furthermore indicate that the Holocene facies architecture of the sandy, braided Platte River varies both in space and in time. The dominance of mesoform-scale structure in the shallow subsurface of the central and lower Platte River likely reflects the ongoing process of gradual abandonment of the system. It further highlights the need for caution in interpreting subsurface structure from surface geomorphology in fluvial systems.

New insights into the morphology, fill, and remarkable longevity (>0.2 m.y.) of modern deep-water erosional scours along the northeast Atlantic margin

A series of large-scale erosional scours are described from four modern deep-water canyon and/or channel systems along the northeast Atlantic continental margin. Regional-scale geophysical data indicate that most scours occur in zones of rapid flow expansion, such as canyon and/or channel termini and margins. High-resolution images of the scours cover ∼25 km2 at 2 × 2 m pixel size, and were obtained at depths of 4200–4900 m using Autosub6000, an autonomous underwater vehicle equipped with an EM2000 multibeam bathymetry system. Sedimentological and microfossil-based chronological data of scour fills and interscour areas were obtained via accurately located piston cores that targeted specific sites within imaged areas. These core data reveal a number of key findings. (1) Deep-water scours can be very long lived (>0.2 m.y.) and may undergo discrete phases of isolation, amalgamation, and infilling. (2) Deep-water scours can develop via a composite of cutting and filling events with periodicities of between tens of thousands and hundreds of thousands of years. (3) Immediately adjacent scours may have strikingly different sedimentological histories and do not necessarily evolve contemporaneously. (4) Scour infills are typically out of phase with sedimentation in intrascour areas, having thin sands internally and thick sands externally, or thick muds internally and thin muds externally. (5) Erosional hiatuses within scour fills may represent hundreds of thousands of years of time, and yet leave little visible record. Four distinct morphologies of scour are identified that range from 40 to 3170 m wide and 8 to 48 m deep: spoon shaped, heel shaped, crescent shaped, and oval shaped. Isolated scours are shown to coalesce laterally into broad regions of amalgamated scour that may be several kilometers across. The combined morphosedimentological data set is used to examine some of the putative formative mechanisms for scour genesis.

Development and flow structures of sand injectites: The Hind Sandstone Member injectite complex, Carboniferous, UK

The Hind Sandstone Member of the Carboniferous Craven Basin occurs within a thick succession of marine mudstone and is here interpreted as a sandstone injectite complex. Injectites take the form of (i) sills; (ii) upwards emplaced dykes; (iii) downwards emplaced dykes; (iv) a sedimentary laccolith; and (v) sandstone bodies remobilised in-situ. The remobilised and injected sand originated from small turbidite scour fills and potentially a more significant sand body such as a submarine slope gully fill. The sedimentary laccolith appears to jack-up the units it injects, and is filled by sandstone with a chaotic fabric suggestive of brecciation of partly cemented sand. The laccolith and sill-dominated nature are suggestive of relatively shallow remobilisation. Sills are commonly laterally gradational with upwards emplaced dykes, and in some cases these feed apparently downwards emplaced dykes. A series of flow structures are preserved within some of the injectites, and these provide clues to the emplacement history. Crack propagation direction is inferred from the orientation of undulating crests developed on the upper and lower surfaces of injectites. Peak-flow is indicated by scour marks, resembling flute and tool marks, which are superimposed onto these undulating surfaces. Internal flow structures highlighted by pyrite mineralistion record the waning stage of injection. The observed flow structures suggest that the apparent crack propagation direction was generally oblique to the peak flow, and nearly perpendicular to the direction of late-stage fluidised sediment flow. Crack propagation relates to the local stress field, whilst injection flow may be related to the local pore pressure distribution. Recognition of this suite of structures may provide a useful interpretational tool for other injectites in both core and outcrop. Pore fluid overpressure which drove injection developed through the migration of petroleum and mineralising fluids from the underlying Bowland Shales and older limestones. A complex brecciated fracture network within the mudstones underlying the injectites records fluid migration pathways. Sliced blocks and thin sections from injectites reveal flow structures defined by iron pyrite mineralisation. The trigger is here related to growth of compressional structures and associated syn-sedimentary slumps which occur at the same stratigraphic level as the injectites.

Archaic-style shell concentrations in brackish-water settings: Lower Cretaceous (Wealden) examples from southern England

Shell pavements, shelly gutter and scour fills, and coquinas are described from brackish-water mudrock facies of the southern English Lower Cretaceous (Wealden) successions. These shell concentrations are consistently thin, reflecting small shell sizes, conservative molluscan life-modes, high net deposition and low levels of bioturbation in lacustrine and lagoonal environments. In biostratinomic terms the shell concentrations are of archaic aspect and similar to other occurrences within Mesozoic mudrock formations ascribed to ‘stressed’, brackish-water settings. As such, Wealden mud-dominated brackish-water settings are identified as a biostratinomic ‘window’ for generation and preservation of archaic-style shell concentrations.

Geostatistical Simulation and Numerical Upscaling, to Model Ground-Water Flow in a Sandy-Gravel, Braided River, Aquifer Analogue

Abstract An integrated approach combining sedimentological, geostatistical, and numerical analyses has been applied to model groundwater flow through a glaciofluvial, braided-stream, gravel–sand aquifer of Pleistocene age, exposed in a gravel pit in the eastern Piedmont (northern Italy). The aquifer under investigation consists of a stack of braided bar and minor channel fills and scour fills, where different gravel–sand bedforms migrated. It represents an analogue of buried sediments, which host some of the most important aquifer complexes and water resources of northern Italy. The work has been performed with three main steps: (1) drawing and discretization of the geological models of two orthogonal exposures and estimate of conductivity of every facies; (2) geostatistical simulation of the 3D facies heterogeneity, based on semivariogram analysis of the geological models; (3) application of a 3D numerical model of flow to upscale the conductivity field. Semivariogram analysis developed from facies indicator functions has shown that: (a) the variability in the data set in the horizontal direction is a representation of the oriented distribution of the sedimentary facies, corresponding to the average N–S elongation of sediment units, according to the paleoflow direction; (b) the vertical correlation length approximates the average thickness of the bedsets bounded by fourth-order erosional surfaces (unit bars, scour fill, and small channel fill associations) and may represent the thickness of an individual hydrostratigraphic unit; (c) the conductivity and facies distributions are more variable in the vertical direction than in the horizontal one, with a ratio between vertical and horizontal ranges of about 1:8. The 3D conductivity distribution within the considered deposits has been determined using a sequential indicator simulation, whereas transition-probability analysis has been used to confirm the correlation model obtained with semivariogram analysis and to check the result of the simulation. The equivalent hydraulic-conductivity tensor has been computed with finite-difference modeling of groundwater flow for cubic blocks of the aquifer analogue with side length of about 13 m. At this scale the medium is almost homogeneous, but anisotropic, with the lowest values of hydraulic conductivity in the vertical direction and the greatest values along the elongation of the sedimentary units, i.e., almost parallel to average paleoflow.

Upper flow regime sheets, lenses and scour fills: Extending the range of architectural elements for fluvial sediment bodies

Fluvial strata dominated internally by sedimentary structures of interpreted upper flow regime origin are moderately common in the rock record, yet their abundance is not appreciated and many examples may go unnoticed. A spectrum of sedimentary structures is recognised, all of which occur over a wide range of scale: 1. cross-bedding with humpback, sigmoidal and ultimately low-angle cross-sectional foreset geometries (interpreted as recording the transition from dune to upper plane bed bedform stability field), 2. planar/flat lamination with parting lineation, characteristic of the upper plane bed phase, 3. flat and low-angle lamination with minor convex-upward elements, characteristic of the transition from upper plane bed to antidune stability fields, 4. convex-upward bedforms, down- and up-palaeocurrent-dipping, low-angle cross-bedding and symmetrical drapes, interpreted as the product of antidunes, and 5. backsets terminating updip against an upstream-dipping erosion surface, interpreted as recording chute and pool conditions. In some fluvial successions, the entirety or substantial portions of channel sandstone bodies may be made up of such structures. These Upper Flow Regime Sheets, Lenses and Scour Fills (UFR) are defined herein as an extension of Miall's [Miall, A.D., 1985. Architectural-element analysis: a new method of facies analysis applied to fluvial deposits. Earth Sci. Rev. 22: 261–308.] Laminated Sand Sheets architectural element. Given the conditions that favour preservation of upper flow regime structures (rapid changes in flow strength), it is suggested that the presence of UFR elements in ancient fluvial successions may indicate sediment accumulation under the influence of a strongly seasonal palaeoclimate that involves a pronounced seasonal peak in precipitation and runoff.

Vegetation‐induced sedimentary structures from fossil forests in the Pennsylvanian Joggins Formation, Nova Scotia

AbstractVegetation‐induced sedimentary structures (VISS) are primary structures formed by the interaction of detrital sediment within situplants. VISS provide physical evidence of vegetation's fundamental role in mediating sediment accumulation and erosion in clastic depositional systems. Despite the abundance of modern VISS, descriptions from the ancient record are surprisingly rare. Analysis of fossil forests in floodplain strata of the Pennsylvanian Joggins Formation has identified seven types of VISS. Five hydrodynamic types comprise simple forms developed around a single plant (centroclinal cross‐strata, scratch circles, upturned beds) and complex forms where structures have amalgamated between several plants (coalesced scour fills, scour‐and‐mound beds). Two types (mudstone‐filled hollows, downturned beds) form by the decay of an entombed plant and represent infilling of an empty hollow and/or soft‐sediment deformation. Hydrodynamic types are present in both dryland and wetland settings, whereas decay‐related types occur only in dryland settings where the water table was perennially or periodically low. VISS represent significant accumulations of sediment and are present in a high proportion of Joggins Formation strata. The paucity of descriptions from the ancient record suggests that these structures have been overlooked or misinterpreted as strictly hydrodynamic. Recognition of VISS in early to middle Palaeozoic strata may provide an important insight into the distribution of early land plants in oxidizing environments where organic material was not preserved.