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Mouth bar formation is critical for channel avulsions and progradation of river deltas. The morphology of mouth bars results from different hydrodynamic forcings such as river jets, tides, and wind waves. Here we study the asymmetry of mouth bars due to alongshore currents. Adopting a numerical model, we study how alongshore propagating tides and net alongshore currents cause asymmetric mouth bar formation. The results indicate that alongshore propagating tides shift the depocenter of the mouth bar in the direction of the alongshore currents during peak ebb. Net alongshore currents shift the depocenter to its down‐current side. The main channels are oriented in the opposite direction of peak flood flows (with tides) or in the direction of the net alongshore currents (without tides). Systems highly influenced by alongshore tidal flows tend to form more and wider distributary channels which are oriented toward the direction of the alongshore ebb flows. With increased river discharge and sediment influx, the number of bifurcations and channels increases while the mouth bar is less asymmetric. We developed a predictor showing that the mouth bar asymmetry is directly proportional to alongshore currents divided by river jet velocities and the width of the river mouth. Our findings provide insights into the evolution of river deltas and contribute to the management of mouth bars and channels. Plain Language Summary: River deltas evolve due to external forces such as river flows, tides, and waves. A key feature in this process is the formation of a mouth bar, which influences how the delta grows outward into the sea and how channels develop. Despite widespread research on how these external forces shape river deltas, we lack an understanding of how the asymmetry of mouth bar responds to coastal currents that are directed alongshore. Using a computer model, we found that tidal currents moving back and forth along the shore shift the mouth bar's center in the direction of ebb flows, and they cause the channel to align in the opposite direction of flood flows. Additionally, there are steady currents that flow consistently along the shore, shifting both the channel and the mouth bar in the direction of the down current. We developed a simple model to predict how asymmetric a mouth bar may form. This prediction is based on the relative strength of coastal currents, river flows, and the width of the river channel. This research helps us understand how river deltas evolve and can inform efforts to manage river mouth channels. Key Points: Alongshore currents shift the depocenter of the mouth bar in the direction of peak ebb flows at seaChannel formation mainly occurs during flood tides and the channel is oriented in the opposite direction of peak flood currentsThe asymmetry scales with alongshore flow magnitude divided by the square root of the product of cross‐shore flow magnitude and inlet width [ABSTRACT FROM AUTHOR]

Integrated sedimentological and ichnological studies in deltaic systems are key to determine whether a delta is river‐dominated, tide‐dominated, wave‐dominated or hybrid, and thus predict the distribution of their geometries and main reservoirs. The Austral Basin is one of the most important oil and gas basins from Argentina, and the Upper Cretaceous La Anita Formation has shown excellent attributes as a hydrocarbon reservoir. Analysis of eleven cores, supplemented by a large volume of well logging data and two‐dimensional and three‐dimensional seismic, allowed to recognize a river‐dominated deltaic system affected by hyperpycnal discharges and storm events. Four main facies associations were recognized (offshore, prodelta, delta front and interdistributary bay), being the delta front facies associations, the most recurrent facies and constituting the major reservoir of the La Anita Formation. Three types of deltaic mouth bars have been characterized in the delta front. Type I corresponds to active mouth bars with preservation of sedimentary structures and bioturbated by suspension and deposit feeding organisms. Type II represents the stopped progradation and the abandonment in the construction of mouth bars, which are completely reworked by suspension and deposit feeding organisms and, in some cases, affected by fair‐weather or storm‐wave processes at the top. Type III comprises hyperpycnal mouth bars characterized by transitional and recurrent intervals of sedimentary structures, high concentration of phytodetritus, and paucity of bioturbation. The amalgamation and coalescence of these three types of sandy mouth bars produce lobe‐like geometries and are the main processes of delta front construction and progradation. This proposed model allows to explain the principal mechanism of construction of the delta front and can be applied to the analysis of other reservoirs where mouth bar architectural elements are dominant. [ABSTRACT FROM AUTHOR]

Abstract The sedimentary architecture of fan deltas, which commonly constitute reservoirs, is a primary control on distribution and recovery efficiency of oil and gas. Deep-water fan deltas have been extensively discussed in the literature, whereas the sedimentary architecture of shallow-water fan-delta fronts, especially the characterization of mouth bar, quantification of architectural elements, and factors controlling the various architectural elements, remain poorly understood. Here, based on the integration of field survey and unmanned aerial vehicle (UAV) observation, the sedimentary architecture of Sangyuan section of the Lower Cretaceous Xiguayuan Formation in the Luanping Basin to is qualitatively and quantitatively characterized. Furthermore, factors controlling the architecture of the shallow-water fan-delta front are highlighted. Five facies associations, which differ in both lithofacies and dimensions, were interpreted in gravel-sand deposits of Sangyuan section, and indicate variations in flow conditions during deposition. The width/thickness ratio of facies associations generally increases rapidly as the flow energy and flow concentration decreases from debris flow to traction currents during the evolution of shallow-water fan delta. Both distributary channels and mouth bars dominate facies associations in the shallow-water fan-delta front, accounting for 53.42% and 36.88% of gravel-sand deposits respectively in Sangyuan section. Mouth bar consists of gravel accretions and sand accretions. Grain size of sediments influences the relative strength of the inertial force and bed friction, which determine the effluent behavior in a river-mouth system and the vertical grain-size trend of mouth bar accretions. Gravel accretions are characterized by normal grading, and these are interpreted as the products of inertia-dominated effluent. Sand accretions exhibit coarsening-upward trends in the river-mouth systems where friction-dominated outflow occurred. The overall vertical grain-size trend of mouth bar depends on the stacking pattern of accretions rather than the grading of one accretion.

The sedimentary architecture of fan deltas, which commonly constitute reservoirs, is a primary control on distribution and recovery efficiency of oil and gas. Deep-water fan deltas have been extensively discussed in the literature, whereas the sedimentary architecture of shallow-water fan-delta fronts, especially the characterization of mouth bar, quantification of architectural elements, and factors controlling the various architectural elements, remain poorly understood. Here, based on the integration of field survey and unmanned aerial vehicle (UAV) observation, the sedimentary architecture of Sangyuan section of the Lower Cretaceous Xiguayuan Formation in the Luanping Basin to is qualitatively and quantitatively characterized. Furthermore, factors controlling the architecture of the shallow-water fan-delta front are highlighted. Five facies associations, which differ in both lithofacies and dimensions, were interpreted in gravel-sand deposits of Sangyuan section, and indicate variations in flow conditions during deposition. The width/thickness ratio of facies associations generally increases rapidly as the flow energy and flow concentration decreases from debris flow to traction currents during the evolution of shallow-water fan delta. Both distributary channels and mouth bars dominate facies associations in the shallow-water fan-delta front, accounting for 53.42% and 36.88% of gravel-sand deposits respectively in Sangyuan section. Mouth bar consists of gravel accretions and sand accretions. Grain size of sediments influences the relative strength of the inertial force and bed friction, which determine the effluent behavior in a river-mouth system and the vertical grain-size trend of mouth bar accretions. Gravel accretions are characterized by normal grading, and these are interpreted as the products of inertia-dominated effluent. Sand accretions exhibit coarsening-upward trends in the river-mouth systems where friction-dominated outflow occurred. The overall vertical grain-size trend of mouth bar depends on the stacking pattern of accretions rather than the grading of one accretion. [ABSTRACT FROM AUTHOR]

Copyright of Journal of the Sedimentological Society of Japan / Taisekigaku Kenkyu is the property of Sedimentological Society of Japan and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

An estuarine turbidity maximum (ETM) functions as a filter and plays a key role in transporting suspended sediment. The features of ETM and their formation mechanisms vary with different types of estuaries. Their responses to morphological factors require further study. In this study, the Modaomen Estuary of the Pearl River was selected to examine the formation mechanisms of the ETM and roles of the mouth bar. Hydrodynamics, suspended sediment concentration, and sediment size distribution were measured via cruise shipboard in the mouth bar zone during the dry season of 2020, which indicated that the ETM occurred in the central and western bars. Furthermore, numerical modeling of an idealized estuary based on Delft3D was performed to explore the effects of the existence of a mouth bar on the dynamic structures causing the ETM, including saltwater intrusion, residual longitudinal circulation, and bed sediment resuspension. The results indicated that the existence of a mouth bar contributed to the occurrence of double residual circulations and enhanced bed shear stress on the mouth bar, which favor the ETM formation. In general, two different dynamic regimes were responsible for the ETM formation, that is, sediment trapping by saltwater intrusion and sediment resuspension induced by tidal currents. The mouth bar played a key role in these two mechanisms, including altering the estuarine circulation and bed shear stress. The results contribute to a better elucidation of the ETM formation in estuaries and provide scientific guidelines for estuarine management and engineering. Plain Language Summary: In estuaries, sediment can accumulate forming a sediment‐rich zone with high suspended sediment concentration. The occurrence and distribution of sediment‐rich zones affect water quality and ecological systems in estuary, as well as dredging and shipping needs, which has attracted significant interests of scientists and engineers. We have conducted a field survey to investigate the locations of the sediment‐rich zone and their causes. And we find that high sediment concentration mainly occurs on a shallow depth zone in estuary, which is caused by saltwater intrusion and sediment resuspension. Furthermore, we use a computer model to explore the roles of shallow topography on the accumulation of massive sediment. The results indicate that the shallow topography enhances the impacts of saltwater intrusion and sediment resuspension on sediment accumulation, favoring the occurrence of sediment‐rich zones. These findings have important scientific implications for estuarine management and engineering efforts. Key Points: Formation of turbidity maximum in different mouth bar zones was caused by estuarine circulation and sediment resuspension varying with timeSill topography favored the formation of double estuarine circulation and subsequently trapped sediment on the mouth barExistence of mouth bar enlarged the bed shear stress and then strengthened sediment resuspension caused by tidal current scouring [ABSTRACT FROM AUTHOR]

In the past two decades, the dynamic sedimentation process of the Yangtze Estuary has been seriously disturbed by coupled human interventions from the river basin to the estuary, especially the impoundment of the Three Gorges Dam in 2003 and the large-scale Deep-water Navigational Channel (DNC) regulation project in 1998–2010. This study investigated the changes in sedimentary dynamic and geomorphological processes in the turbidity maximum zone (TMZ) by analyzing the historical and present data for current, salinity, suspended sediment, and bathymetry. The results show that the decreased riverine sediment input caused a lagging decrease in suspended sediment concentration in the TMZ during the flood seasons. The DNC caused changes in the flow structure, sediment transport, and geometry of the TMZ in the North Passage (NP) and the South Passage (SP). In the NP, decreased ebb transport in the upper reaches led to landward migration of the TMZ during low discharges, while increased ebb transport in the middle and lower reaches caused the seaward migration of the TMZ during high discharges. As the associated topography of the TMZ, the mouth bar in the NP was mostly removed by channel dredging. However, rapid deposition at the location of the previous mouth bar indicates the formation of an incipient bar. In the SP, increased ebb transport after the DNC-induced disappearance of the TMZ and the mouth bar in the upper reaches and the seaward migration of the TMZ in the middle and lower reaches. Therefore, we found that the construction of dams and large-scale estuarine projects changed the sediment dynamics and geomorphological processes of the TMZ and even affected the long-term evolution of the estuary. Construction regulation projects in the TMZ, intended to narrow the cross-section and enhance seaward sediment transport, may produce the opposite effect. Before and after engineering projects, their impacts on estuarine processes need to be carefully estimated. [ABSTRACT FROM AUTHOR]

Abstract Mouth bars are the fundamental architectural elements of proximal deltaic successions. Understanding their internal architecture and complex interaction with coastal processes (fluvial, tide and wave‐dominated) is paramount to the interpretation of ancient deltaic successions. This is particularly challenging in low‐accommodation systems, because they are commonly characterized by thin, condensed and top‐truncated sections. This study analyses the exhumed Cenomanian Mesa Rica Sandstone (Dakota Group, Western Interior Seaway, USA), a fluvio‐deltaic system covering a ca 450 km depositional dip‐parallel profile. The study targets the proximal deltaic expression of the system, using 22 sedimentary logs (total of 390 m) spatially correlated within a ca 25 km2 study area at the rim of the Tucumcari Basin. Analysis of facies distributions, depositional architecture and spatial extent of stratigraphic surfaces reveals a 6–10 m thick, sharp‐based and sand‐prone deltaic package, comprising several laterally extensive (>1.4 km width) mouth bars. Composite erosional surfaces infilled with multi‐storey fluvial and marine‐influenced channel deposits (12–20 m thick, 100–250 m wide) scour locally into the deltaic package. Based on differences in sedimentary structures, bed thicknesses, occurrence of interflood beds and bioturbation indexes, four different sub‐environments within single mouth bars were distinguished. These range from mouth‐bar axis, off‐axis, fringe to distal‐fringe deposits, which reflect waning depositional energy with increasing distance from the distributary channel mouth. The interpreted mouth‐bar components also show internal variability in dominant process regime, with overall river dominance but local preservation of tide influence in the fringe and distal fringe components. Mouth‐bar deposits amalgamate to form an extensive sand‐rich sheet body throughout the study area, in which interflood mudstone to very‐fine grained sandstone beds are nearly absent. These features reflect successive coalescence of mouth bars in a low accommodation/supply (A/S) setting. These conditions promoted recurrent channel avulsion/bifurcation and thus the potential reworking of previously deposited mouth‐bar fringe and distal‐fringe sediments, where time and background processes are better recorded. Results of this study evidence internal process‐regime variability within mouth‐bar components. They also caution against the possible loss of preservation of subordinate coastal processes (e.g. tidal indicators), and consequent underestimation of the true mixed influence in low‐accommodation deltaic settings.

Abstract Modern and ancient analogues are often consulted by geologists to help understand subsurface systems. While modern analogues provide information on the areal relationship between facies, ancient systems provide detailed data on the vertical facies variations, typically along a two‐dimensional outcrop. Combining data from modern and ancient systems effectively requires translating areal morphology, which is often still evolving, to the related sediments preserved in three dimensions. Process‐based models simulate both depositional processes while preserving stratigraphy. These models can be employed to unravel the relationship between sediment supply and preserved deposits in natural systems and to help integrate field data. Four synthetic deltas were modelled using different sediment supply compositions, from coarse to very fine sand systems. The resultant sedimentary deposits are classified into architectural elements, and the grain size composition of each architectural element is studied over time. Facies that are extensive in their horizontal dimensions are often less abundant in three‐dimensional preserved deposits. Between deltas, grain size compositions of a specific architectural element type (e.g. mouth bars) are more similar than their corresponding sediment supply compositions. This is due to selective deposition of grain size classes across each architectural element type. This selective deposition causes overrepresentation of the same range of grain sizes, even for systems with different sediment supply compositions. When a particular supply composition does not contain enough of the overrepresented grain size class for a particular architectural element, that element will be under‐supplied and constitute a smaller proportion of the overall delta deposits. It is imperative to account for over‐representation of grain size classes in particular architectural elements when estimating palaeo‐sediment supply, delta architecture and morphology from field data. Even when data availability/accessibility does not allow the inclusion of distal deposits in field studies, process‐based simulations can contribute valuable information on sediment sorting patterns in three dimensions.

Investigation of the process sedimentology of mouth bar deposition demonstrates a link to the seasonality associated with abrupt climate warming events during the early Eocene. While a relationship between fluvial sedimentology and climate events has been established previously, such documentation within mouth bar deposits is missing. Deltaic mouth bars of the Douglas Creek Member of the Green River Formation in the Uinta Basin record a fluvial–lacustrine response to abrupt warming events associated with the Early Eocene Climatic Optimum. This study ties the observed sedimentology of these mouth bars and their deposition via hyperpycnal currents to these climatic events. Detailed measured sections and analysis of stratigraphic architecture of outcrops in the eastern Uinta Basin document the sedimentology and stratigraphy of delta mouth bar complexes. Internal structures include climbing ripple cross‐lamination, convolute bedding, and those associated with transitional to upper‐flow regime bedforms such as planar laminations with normal grading and convex‐upward stratifications. The dominance of upper‐flow regime bedforms as well as interpreted long run out distances suggests that density currents deposited mouth bars tens of kilometres from the shoreline under Froude supercritical flow conditions. The highly seasonal climate regime of the early Eocene, capable of producing high‐sediment yield flooding events, likely provided the mechanism for hyperpycnal current formation within a relatively freshwater lake and controlled the timing of mouth bar deposition. Discrete lacustrine carbonate‐dominated packages separate periods of episodic fluvial discharge, recording reduced fluvial deposition between warming events. The established link between the occurrence of upper‐flow regime bedforms and the timing of climate warming events suggests that these climate conditions may be important for their formation and preservation. The study has implications for expanding existing mouth bar depositional models to include climatically controlled upper‐flow regime bedforms. [ABSTRACT FROM AUTHOR]

Improved understanding of mouth bar morphodynamics, and the resulting stratigraphic architectures, is important for predicting the loci of deposition of different sediment fractions, coastal geomorphic change and heterogeneity in mouth bar reservoirs. Facies and architectural analysis of exceptionally well‐exposed shallow water (ca 5 m depth) mouth bars and associated distributaries, from the Xert Formation (Lower Cretaceous), of the Maestrat Basin (east‐central Spain), reveal that they grew via a succession of repeated autogenic cycles. An initial mouth bar accretion element forms after avulsion of a distributary into shallow standing water. Turbulent expansion of the fluvial jet and high bed friction results in rapid flow deceleration, and deposition of sediment in an aggradational to expansional bar‐form. Vertical bar growth causes flattening and acceleration of the jet. The accelerated flow scours channels on the bar top, which focuses further expansion of the mouth bar at individual loci where the channels break through the front of the mouth bar. Here, new mouth bar accretion elements form, downlapping and onlapping against a readily recognizable surface of mouth bar reorganization. Vertical growth of the new mouth bar accretion elements causes flattening and re‐acceleration of the jet, leading to channelization, and initiation of the next generation of mouth bar accretion elements. Thus the mouth bar grows, until bed‐friction effects cause backwater deceleration and super‐elevation of flow in the feeding distributary. Within‐channel sedimentation, choking and upstream avulsion of the feeding channel, results in mouth bar abandonment. In this study, mouth bars are formed of at least two to three accretion elements, before abandonment happened. The results of this study contrast with the notion that mouth bars form by simple vertical aggradation and radial expansion. However the architecture and facies distributions of shallow water mouth bars are a predictable product of intrinsic processes that operate to deposit them. [ABSTRACT FROM AUTHOR]

In the past two decades, the dynamic sedimentation process of the Yangtze Estuary has been seriously disturbed by coupled human interventions from the river basin to the estuary, especially the impoundment of the Three Gorges Dam in 2003 and the large-scale Deep-water Navigational Channel (DNC) regulation project in 1998–2010. This study investigated the changes in sedimentary dynamic and geomorphological processes in the turbidity maximum zone (TMZ) by analyzing the historical and present data for current, salinity, suspended sediment, and bathymetry. The results show that the decreased riverine sediment input caused a lagging decrease in suspended sediment concentration in the TMZ during the flood seasons. The DNC caused changes in the flow structure, sediment transport, and geometry of the TMZ in the North Passage (NP) and the South Passage (SP). In the NP, decreased ebb transport in the upper reaches led to landward migration of the TMZ during low discharges, while increased ebb transport in the middle and lower reaches caused the seaward migration of the TMZ during high discharges. As the associated topography of the TMZ, the mouth bar in the NP was mostly removed by channel dredging. However, rapid deposition at the location of the previous mouth bar indicates the formation of an incipient bar. In the SP, increased ebb transport after the DNC-induced disappearance of the TMZ and the mouth bar in the upper reaches and the seaward migration of the TMZ in the middle and lower reaches. Therefore, we found that the construction of dams and large-scale estuarine projects changed the sediment dynamics and geomorphological processes of the TMZ and even affected the long-term evolution of the estuary. Construction regulation projects in the TMZ, intended to narrow the cross-section and enhance seaward sediment transport, may produce the opposite effect. Before and after engineering projects, their impacts on estuarine processes need to be carefully estimated.

River plumes are an important component of estuarine circulation and have a vital role in the transport of estuarine material and momentum balance. However, little information is available on the influence of mouth bars on estuarine plumes. To address this, momentum balance analysis was carried out using the ocean numerical model, to analyze the dynamic characteristics of the Pearl River plumes. The results show that the Pearl River plumes may be divided into three dynamic regions: near-, mid-, and far-field region. In the near-field region are dominated by pressure gradient force, bottom pressure forcing, and horizontal advection, which are obviously affected by river mouth bar. The mid-field momentum balance indicates seasonal differences, while the Coriolis force, pressure gradient force, and horizontal advection have great contribution to momentums in flood season, and in the dry season, momentum is balanced by the Coriolis force, pressure gradient force, and bottom pressure forcing. The momentum balance in the far-field region is determined by the Coriolis force, pressure gradient force, and surface pressure forcing. The internal hydraulic jump can be detected around the mouth bar when the hydrodynamic condition transitions from supercritical to subcritical, which has a close relationship with the local topographic structures. Internal estuarine waves may be further generated with interaction of tide and stratified flows around the river mouth bar. [ABSTRACT FROM AUTHOR]

Mouth bars are the fundamental architectural elements of proximal deltaic successions. Understanding their internal architecture and complex interaction with coastal processes (fluvial, tide and wave‐dominated) is paramount to the interpretation of ancient deltaic successions. This is particularly challenging in low‐accommodation systems, because they are commonly characterized by thin, condensed and top‐truncated sections. This study analyses the exhumed Cenomanian Mesa Rica Sandstone (Dakota Group, Western Interior Seaway, USA), a fluvio‐deltaic system covering a ca 450 km depositional dip‐parallel profile. The study targets the proximal deltaic expression of the system, using 22 sedimentary logs (total of 390 m) spatially correlated within a ca 25 km2 study area at the rim of the Tucumcari Basin. Analysis of facies distributions, depositional architecture and spatial extent of stratigraphic surfaces reveals a 6–10 m thick, sharp‐based and sand‐prone deltaic package, comprising several laterally extensive (>1.4 km width) mouth bars. Composite erosional surfaces infilled with multi‐storey fluvial and marine‐influenced channel deposits (12–20 m thick, 100–250 m wide) scour locally into the deltaic package. Based on differences in sedimentary structures, bed thicknesses, occurrence of interflood beds and bioturbation indexes, four different sub‐environments within single mouth bars were distinguished. These range from mouth‐bar axis, off‐axis, fringe to distal‐fringe deposits, which reflect waning depositional energy with increasing distance from the distributary channel mouth. The interpreted mouth‐bar components also show internal variability in dominant process regime, with overall river dominance but local preservation of tide influence in the fringe and distal fringe components. Mouth‐bar deposits amalgamate to form an extensive sand‐rich sheet body throughout the study area, in which interflood mudstone to very‐fine grained sandstone beds are nearly absent. These features reflect successive coalescence of mouth bars in a low accommodation/supply (A/S) setting. These conditions promoted recurrent channel avulsion/bifurcation and thus the potential reworking of previously deposited mouth‐bar fringe and distal‐fringe sediments, where time and background processes are better recorded. Results of this study evidence internal process‐regime variability within mouth‐bar components. They also caution against the possible loss of preservation of subordinate coastal processes (e.g. tidal indicators), and consequent underestimation of the true mixed influence in low‐accommodation deltaic settings. [ABSTRACT FROM AUTHOR]

Abstract Integration of the geological context, core analysis, and petrographic investigation, together with data from wireline logs, resulted in the recognition of petrofacies types and the representation of facies models characterizing the X–XII sand groups of the K2 qn 3 Formation in the Daqingzijing area of the Songliao Basin, China. The results show that the lithology of the reservoir’s X–XII sand groups includes shale calcium-bearing siltstone and sandstone, calcareous siltstone and silty sandstone. The petrofacies consist mainly of quartz, feldspar and rock fragments cemented by clay, and some samples include calcite. The studied interval was deposited onto a delta front. Four types of petrofacies were recognized: subaqueous distributary channel, mouth bar, distal bar and subaqueous interdistributary bay. According to background investigation, the study area’s X–XII sand groups have experienced a large-scale decline in lake level. There are many subaqueous distributary channels in layers 36–40, while other layers were mainly deposited in a subaqueous interdistributary bay.

Modern and ancient analogues are often consulted by geologists to help understand subsurface systems. While modern analogues provide information on the areal relationship between facies, ancient systems provide detailed data on the vertical facies variations, typically along a two‐dimensional outcrop. Combining data from modern and ancient systems effectively requires translating areal morphology, which is often still evolving, to the related sediments preserved in three dimensions. Process‐based models simulate both depositional processes while preserving stratigraphy. These models can be employed to unravel the relationship between sediment supply and preserved deposits in natural systems and to help integrate field data. Four synthetic deltas were modelled using different sediment supply compositions, from coarse to very fine sand systems. The resultant sedimentary deposits are classified into architectural elements, and the grain size composition of each architectural element is studied over time. Facies that are extensive in their horizontal dimensions are often less abundant in three‐dimensional preserved deposits. Between deltas, grain size compositions of a specific architectural element type (e.g. mouth bars) are more similar than their corresponding sediment supply compositions. This is due to selective deposition of grain size classes across each architectural element type. This selective deposition causes overrepresentation of the same range of grain sizes, even for systems with different sediment supply compositions. When a particular supply composition does not contain enough of the overrepresented grain size class for a particular architectural element, that element will be under‐supplied and constitute a smaller proportion of the overall delta deposits. It is imperative to account for over‐representation of grain size classes in particular architectural elements when estimating palaeo‐sediment supply, delta architecture and morphology from field data. Even when data availability/accessibility does not allow the inclusion of distal deposits in field studies, process‐based simulations can contribute valuable information on sediment sorting patterns in three dimensions. [ABSTRACT FROM AUTHOR]

Copyright of Special Oil & Gas Reservoirs is the property of Special Oil & Gas Reservoirs Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract Tidal asymmetry (i.e., unequal ebb and flood intensities and durations) is a dominant factor in the dynamic equilibrium of current-related sediment transport in estuaries. The evolution of estuarine tides and sediment transport is of great importance to the development and utilization of channels in estuaries. Based on field hydrological and sedimentological data collected over multiple years, the hydrodynamic characteristics, suspended sediment concentration (SSC) and grain size, and median grain size of seabed sediments were analyzed to investigate the effects of the construction of a water–sediment regulation scheme (WSRS) in the Sheyang Estuary in China. Our results showed that small grain sizes (fine sand, silt, and clay) dominated and that suspension was the dominant transport mechanism in the area where tidal asymmetry was greatest. Tidal distortion occurred due to the construction of the WSRS. In this flood-dominant system (i.e., before and after the construction of phase I of the WSRS), the flood current velocity was significantly greater than the ebb current velocity, while it also had a shorter duration that caused higher SSCs during the flood current. This likely induced net sediment import to the estuary, which was the main cause of estuary infilling. Furthermore, the net transport of sediment into the estuary caused by the high flood current velocities resulted in the gradual deposition of sediments in the shipping channel of Sheyang Port. Ebb-dominant conditions (i.e., after the construction of phase II of the WSRS) induced net sediment export from the estuary, and sedimentation was significantly reduced. Since the ebb–flood current velocity ratio (R v) and ebb–flood mean SSC ratio (R s) could reflect tidal asymmetry and the direction of residual sediment transport with and without the WSRSs, they were used as important and sensitive indices to investigate hydraulic and sedimentological conditions. Results from this study can be used to improve the monitoring and management of navigation channels by providing insights into new methods of preventing or reducing sediment deposition in estuarine channels (i.e., by employing R v and R s). Highlights • A water–sediment regulation scheme (WSRS) in the Sheyang Estuary was evaluated. • Tidal asymmetry and channel infilling worsened with phase I of WSRS construction. • Overtopping flow and backflow near jetties caused the enhanced asymmetry. • Lengthening and raising of the jetties (phase II) resolved channel infilling. • WSRSs are effective for controlling sediment deposition in shipping channels. [ABSTRACT FROM AUTHOR]

Abstract Mouth bars are important morphological units in deltaic and estuarine systems, which have encountered with significant topography variations driven by anthropogenic interferences and natural forcings. On the basis of seven selected maritime charts covering from 1960s to 2016, we focus on the morphological evolution of mouth bar with bifurcating channels in the Modaomen Estuary, the main fluvially-dominated estuary of the Pearl River Delta. The bar area is relatively in equilibrium with barely aggregation and degradation before 1990 while overall erosion was found thereafter. Sediment volume of the mouth bar between the −2 m and −7 m isobaths drastically decreased after 1990s. Correspondingly, the area of −4 m isobaths of the bar stepwise decreased from about 18 × 106 m2 to 2 × 106 m2, indicating the body of mouth bar above −4 m almost disappeared. The two side channels also suffered significantly morphological evolution. The West Channel (WC) was subjected to uninterrupted erosion. Especially during 2000–2005, the WC below the −2 m isobath experienced remarkable erosion with the increment of water volume reaching 6.7 × 106 m3. The East Channel (EC) had the potential to expand seaward between 2000 and 2011 yet encountered with accumulation after 2011. Meanwhile, the thalweg of the EC seemed hard to distinguish and the WC displayed more meandering thalweg. Furthermore, Empirical Orthogonal Function was applied to quantify the bathymetric changes with independent modes. The temporal variations of fluvial sediment input displayed significant decreasing trend at MK from 1960 to 2014, which presented a statistically significant relationship with the third mode. However, it only accounts for 13.4% of the overall evolution patterns. Although the remarkable reduction of sediment input generally associates with the degradation in bar area, the sediment load is probably not the primary mechanisms driving morphological changes in the Modaomen Estuary. Anthropogenic interferences including large-scale reclamation and intensive sand excavation should be responsible for the morphological changes. Highlights • Three major evolution patterns of mouth bar are distinguished by Empirical Orthogonal Function analysis. • The phase difference of flow between two distributaries relates with the erosion in the West Channel. • Morphological changes should be more attributed to anthropogenic disturbances rather than fluvial sediment source. [ABSTRACT FROM AUTHOR]

Integration of the geological context, core analysis, and petrographic investigation, together with data from wireline logs, resulted in the recognition of petrofacies types and the representation of facies models characterizing the X-XII sand groups of the K2qn3 Formation in the Daqingzijing area of the Songliao Basin, China. The results show that the lithology of the reservoir’s X-XII sand groups includes shale calcium-bearing siltstone and sandstone, calcareous siltstone and silty sandstone. The petrofacies consist mainly of quartz, feldspar and rock fragments cemented by clay, and some samples include calcite. The studied interval was deposited onto a delta front. Four types of petrofacies were recognized: subaqueous distributary channel, mouth bar, distal bar and subaqueous interdistributary bay. According to background investigation, the study area’s X-XII sand groups have experienced a large-scale decline in lake level. There are many subaqueous distributary channels in layers 36-40, while other layers were mainly deposited in a subaqueous interdistributary bay. [ABSTRACT FROM AUTHOR]

ABSTRACT Shim, W.J. and Kim, C.W., 2018. The necessity of coastal management in Busan Considering human and natural conditions: The case of barrier islands in Nakdong-estuary. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp.1501–1505. Coconut Creek (Florida), ISSN 0749-0208. The Busan coasts have complex natural and human conditions including dynamic coastal, river processes and human activities. These complex natural and human factors have had direct and indirect impacts on the coastal areas of Busan, so a spatially integrated approach for management is required. Especially, the barrier islands located in the Nakdong River estuary are proper examples that show the effect of the sediment budget as natural and human conditions. This research aims to identify the effects of natural and human conditions on the barrier islands. The area of the barrier islands, sediment budget from the watershed, the amount of dredging at Nakdong-Estuarine-Dam and land use coverage were compared using correlation analysis. The more sediment eroded from watershed and the less sediment was dredged at Nakdong-Estuarine-Dam, the larger area of the barrier islands. This indicates that boundary conditions such as the supply of sediment from the rivers are related to the barrier islands; thus river and watershed management are important. Regarding land use, used and barren areas are positively correlated with the barrier islands, but water, forests and agricultural lands are negatively correlated. Internal factors within Busan, such as coastal modifications and urban growth, are affecting coastal dynamics, which change the sediment budget. These results show that the barrier islands sensitively respond to boundary conditions from the river and watershed and to internal conditions such as shoreline management and development. Therefore, for the effective management of Busan's coasts, it is necessary to consider how natural and human conditions interact spatially. [ABSTRACT FROM AUTHOR]

The geomorphology of the Yangtze Estuary in the Changjiang River Delta in Eastern China has been the subject of extensive research. This study extends previous work to examine the influence of wind-waves on the mouth-bar, where about half of the river-borne material settles to the bed. The site is located just outside of Changjiang River mouth, which is meso-tidal and subject to seasonally varying river flows and wind-wave conditions. Modeling was performed with a coupled wave-current hydrodynamic model using TELEMAC and TOMAWAC and validated against observed data. Bottom Shear Stress (BSS) from river, tide and waves based on the numerical model output was used to infer the respective contribution to the evolution of the subaqueous delta. Our examination did not however extend to modeling the sediment transport or the morphological bed changes. The results suggest that (i) the dominance of river discharge is limited to an area inside the mouth, while outside, the mouth-bar is tide-wave dominant; (ii) considering just the tide, the currents on the shallow shoals are flood dominant and deep channels are ebb dominant, which induces continued accretion over the shallows and erodes the deeper parts of the mouth-bar until the tidal currents become too weak to transport sediment; (iii) whereas waves are very efficient at reshaping the shallow shoals, with the effect being subtly dependent on the depth distribution over the mouth-bar; (iv) the stability of shallow shoal morphology is highly dependent on the presence of seasonal wind-waves and characterized as “summer storing and winter erosion”, while deep channels perform like corridors of water and sediment, exporting sediment all year round. The nature of the mouth-bar response has important implications for coastal management, such as the ongoing deep water channel maintenance, reclamations and coastal defense measures. [ABSTRACT FROM AUTHOR]

Abstract: Island formation and distributary channel branching are important processes in prograding river deltas. We develop and test a new theory predicting the distance to islands and channel bifurcations based on fluid mass conservation and radially symmetric transport conditions. We analyze channelization and island formation using nine new and five existing delta experiments as well as four field deltas. The new experiments were designed to produce islands from initial deposition of a mouth bar. Before island formation, each bar evolved into a radially symmetric deposit with unchannelized flow over its top previously described as a topographic flow expansion. This morphology was stable to topographic perturbations, and its distal limit prograded basinward while maintaining a characteristic flow depth. Island formation and channel branching occurred on top of this deposit. We hypothesize that this distance (Ψ) is set by the location where boundary shear stress applied by expanding, radially averaged flow falls below the threshold of sediment motion. The model predicts that the distance to the first island scales with water discharge, scales inversely with flow depth, and scales with the inverse square root of median grain diameter. From experiment to field scales, distances to island locations are predicted within a factor of two. [ABSTRACT FROM AUTHOR]

This study evaluates the evolution of a small-scale growth fault within a deltaic parasequence of the Ferron Sandstone, Utah. Analysis is based on 4 stratigraphic logs, and a panorama covering 100 m of lateral exposure. Five growth-fault blocks are displayed in strike and dip views. The listric faults dip basinward, and sole out into bedding parallel decollement surfaces associated with deformed prodelta shales, which also form diapirs. Throw on individual faults averages 4 m and heave increases with depth from ∼2.5 to ∼5 m. Strikes on fault planes average 160°, and fault dip decreases with depth from 50° to 0°, where they become parallel to bedding. Pre-growth facies comprise heterolithics with abundant graded beds, current-ripples, wave-ripples, small-scale soft sediment deformation, and a low bioturbation index. This indicates deposition in a river-dominated, storm-influenced delta that was supplied primarily by storm-triggered flood events and prograded rapidly. Growth strata are primarily planar and quasi-planar fine sandstones deposited as upstream and downstream accreting mouth bars. Faulting initiates with the building of a “critical load” reflecting a low-angle delta front foreset, probably linked to deposition in a mouth bar. Mobile prodelta muds form diapirs that breached the sediment-water interface, providing accommodation for the fault blocks. Termination of faulting occurred when sandy growth strata contacted and locked into higher strength pre-growth heterolithics deeper in the section. Minor deformation of growth strata is demonstrated by synthetic microfaulting in growth sands. This study emphasizes a localized stress field (i.e., point sourced) associated with sandy mouth bar deposits, which acted as fault initiation points. This contrasts with the distributed stress regimes commonly associated with regional-scale growth faults along continental margins. This can lead to expanded, but localized, reservoir compartments within river-dominated deltaic systems, especially those in epeiric seas or other shallow-water sedimentary basins, such as Prudhoe Bay Field, Alaska. [ABSTRACT FROM AUTHOR]

After nearly 40 years exploration, the development of petroleum reservoir in Shangyi block of Shanghe oilfield, Huimin sag becomes much more difficult. In order to find the distribution of remaining oil, based on the abundant core and well logging data, the mouth bar reservoir in the third and the fourth sand groups of the second member of Shahejie Formation was analyzed for many aspects, including the grading system of architecture interfaces, the identification and characteristics of a single mouth bar and the vertical evolution law. The results show that the mouth bar reservoir can be classified into 5 grades architecture interfaces; the origin types of the fifth and fourth architecture interfaces are flood shale and argillaceous layers, respectively; the origin types of the third architecture interface are sedimentary with poor physical properties and diagenetic calcareous sandstone, and can be further subdivided into shale interlayer, calcareous interlayer and transitional interlayer; the shale interlayer has more lateral continuity than the calcareous and transitional interlayers in the third architecture interface, and can extend for 300-800 m; along the progradation direction of mouth bar, the dip angle of interface in the slope break belt can up to 8°, while the dip angle is less than 2°in the flat terrain; along the vertical provenance direction of sandbody, the dip angle of interface is generally below 1.5°; four identification marks recognizing lateral boundary of single mouth bar include that the top surface altitude difference of reservoir, the appearance of interval zone of mouth bar deposits, the fringe of two mouth bars overlapping each other and the existing difference from curves and deposition thickness of sandbody; the fluctuation of base level controls the superimposed relationship of sandbody, and determines the deposition thickness of mouth bar sandbody and the physical characteristics of reservoir. This results have an important significance in guiding prediction of the remaining oil’s distribution in Shanghe oilfield and other similar oilfields. [ABSTRACT FROM AUTHOR]

Shoal water deltaic sand bodies are widely deposited in lacustrine basins and serve as essential hydrocarbon reservoirs. The complex distribution patterns of such sand bodies present a challenge in the characterization and exploration of hydrocarbon reservoirs. Improving the understanding of jet plume deposits in ancient shoal water deltas could give new implication for reservoir characterization. Sedimentary facies of shoal water deltas were interpreted using high-resolution aerial photographs of the modern Ganjiang River delta and the ancient shoal water deltaic deposits in the Lower Cretaceous Quantou Formation, Songliao Basin. There are three jet plume-related sand bars between the multi-order distributary channel network in the Ganjiang River Delta. They are referred to as triangular-shaped bars, V-shaped bars, and diamond-shaped bars in this study. The V-shaped bar and diamond-shaped bar were also interpreted in the ancient deltaic deposits of the Songliao Basin. The jet plume related deposits experienced a river mouth stage under fully turbulent and expanding flow, followed by a fluvial-dominated stage during the progradation of the topset-dominated shoal-water deltas in geological history. The jet plume related sand bars in the lower delta plain were initially generated from the mouth bars in the delta front and were then incised by the fluvial processes during the delta progradation. Overall, knowledge of geometry and internal architecture of these jet plume related sand bars could provide new implication for characterization and prediction of ancient shoal-water deltaic reservoirs. • morphology of modern and ancient lacustrine shoal-water deltaic deposits. • ancient river mouth deposits influenced by jet plume. • deltaic sedimentary architecture controlled by jet plume and fluvial evolution. [ABSTRACT FROM AUTHOR]