Knickpoints often form in bedrock rivers in response to base-level lowering. These knickpoints ca... more Knickpoints often form in bedrock rivers in response to base-level lowering. These knickpoints can migrate upstream without dissipating. In the case of alluvial rivers, an impulsive lowering of base level due to, for example, a fault associated with an earthquake or dam removal commonly produces smooth, upstreamprogressing degradation; the knickpoint associated with suddenly lowered base level quickly dissipates. Here, however, we use experiments to demonstrate that under conditions of Froude-supercritical flow over an alluvial bed, an instantaneous drop in base level can lead to the formation of upstream-migrating knickpoints that do not dissipate. The base-level fall can generate a single knickpoint, or multiple knickpoints. Multiple knickpoints take the form of cyclic steps, that is, trains of upstream-migrating bedforms, each bounded by a hydraulic jump upstream and downstream. In our experiments, trains of knickpoints were transient, eventually migrating out of the alluvial reach as the bed evolved to a new equilibrium state regulated with lowered base level. Thus the allogenic perturbation of base-level fall can trigger the autogenic generation of multiple knickpoints which are sustained until the alluvial reach recovers a graded state.
The dynamics of delta distributary channels can be intensely affected by basin water depth in fro... more The dynamics of delta distributary channels can be intensely affected by basin water depth in front of the delta, particularly in terms of a long time scale. The previous experiments conducted in use of tank facilities suggest that with deeper basin water, delta distributary channels have lower rates of alluvial aggradation and lateral migration. If the delta faces very deep water, the channels attain alluvial grade and are stabilized in a particular position in the delta plain. This effect of basin water depth can be numerically expressed with grade index (Gindex), which ranges between 0 (perfect alluvial aggradation) and 1 (alluvial grade). Given basin water depth (h), delta plain radius (x) and alluvial slope (Sa), we define dimensionless basin water depth as h*=h/S ax. Assuming a set of particular geometrical conditions that (1) the basin floor is flat and horizontal, (2) the delta is always attached with a vertical wall on the back, and (3) base level is stationary, grade index...
Physical and numerical modeling of fluviodeltaic depositional systems, conducted under constant c... more Physical and numerical modeling of fluviodeltaic depositional systems, conducted under constant conditions of relative sea-level fall, sediment discharge, and water discharge, demonstrates that ''sustained'' alluvial aggradation is the inherent stratigraphic response of alluvial rivers that are steeper than the receiving basin. High alluvial gradients are primarily a consequence of high sediment supply relative to available water discharge (low rate of terrestrial diffusion). During the course of the experiments, fluvial grade was never reached, fluvial incision was never observed, and aggradation occurred independently of shoreline position. The experiments revealed detachment of the shoreline from the alluvial river as an autogenic response of certain fluviodeltaic systems to steady relative sea-level fall, creating a ''non-deltaic'' alluvial-river system that continues to aggrade beyond shoreline detachment. We term this behavior ''autodetachment.'' Autodetachment is a consequence of sustained, extensive alluvial aggradation during relative sea-level fall. Sediment supply was increasingly partitioned into the ever-enlarging alluvial profile to the detriment of the delta front prior to shoreline detachment. Thus, as relative sea level fell, the length of the delta front decreased to zero. Simultaneously, alluvial aggradation also led to decreasing rates of delta progradation. Therefore, as the delta front disappeared, the rate of shoreline regression exceeded the rate of progradation, producing a ''bedrock'' river between the detached alluvial river and the shoreline. The Canterbury Plains of New Zealand is a natural-world system with alluvial gradient greater than shelf gradient. Geometric modeling suggests that autodetachment could have occurred at the Canterbury Plains within the timescale of Quaternary glacio-eustatic sea-level falls, and therefore, is a realistic stratigraphic response at the scale of natural-world systems. Our model demonstrates that deltas with these geometric parameters become poorly supplied during regression, and large lowstand deltas should not be expected for these types of systems. The results of this study complement existing autostratigraphic theory, and provide an alternative to allogenic drivers for geomorphic and architectural evolution of fluviodeltaic wedges.
With each lowering of relative sea level, "fan valleys" of coastal alluvial fan systems... more With each lowering of relative sea level, "fan valleys" of coastal alluvial fan systems adjacent to high-gradient submarine slopes become entrenched or resume deepening at sites where the coastal slope is steepest. Tectonic tilting events that changed the maximum slope direction in a fan system should thus be identifiable in the geologic record by noting temporal changes in position and/or trend of fan valley deposits. This method of detecting ancient fan valley trends, and thus tilting events, is illustrated with Quaternary fans on the Pacific coast of central Honshu, Japan.
The most recent deglaciation resulted in a global sea level rise of some 120 m over approximately... more The most recent deglaciation resulted in a global sea level rise of some 120 m over approximately 12000 years. A moving boundary numerical model is developed to predict the response of rivers to this rise. The model was motivated by experiments at small-scale, which have identified two modes describing the transgression of a river mouth: autoretreat without abandonment of the
Grade index (G index) is a dimensionless number given as the volume-in-unit-time ratio of subaeri... more Grade index (G index) is a dimensionless number given as the volume-in-unit-time ratio of subaerial allocation to both subaerial and subaqueous allocations of sediment supplied to a delta from upstream. It was originally proposed for understanding the effect of basin water depth on the morphodynamics of delta distributary channels under stationary relative sea level. We here examine how rising relative sea level modulates the G index , using geometrical reasoning and numerical simulations. We find that the grade index model can account for autoretreat of the deltaic shoreline, autodrowning of the whole system, and autobreak of the deltaic sedimentation, all of which are the consequences of autogenic nonequilibrium responses to steadily rising relative sea level. The regressive-to-transgressive threshold (i.e. the onset of autoretreat) is crossed when the delta plain's dimensionless basal area (A t*) encounters a critical value that is expressed in terms of G index : regression and transgression are sustained when A t* is below and above the threshold, respectively. The mode of transgression depends on the slope conditions. If the hinterland slope (γ) is steeper than the foreset slope (β), both A t* and G index decrease as the relative sea-level rises. Eventually, the depositional system experiences autodrowning when A t* = G index = 0. If γ < β; on the other hand, both A t* and G index increase. This latter slope condition eventually causes autobreak of the deltaic sedimentation, afterward of which A t* = G index = 1. The grade index model is useful for interpreting and predicting the stratigraphic responses of natural deltaic clinoforms in conditions of rising relative sea level.
A debate is provoked here as to what is meant exactly by the term 'accommodation' and how well th... more A debate is provoked here as to what is meant exactly by the term 'accommodation' and how well this concept, as it presently stands, serves the conceptual and quantitative framework of sequence stratigraphy. 'Accommodation', in terms of its original definition, contains some conceptual flaws. It represents a space that has three-dimensional extent associated with a certain availability of sediment and thus is not really independent of sedimentation. However, this space is fairly unspecifiable because of the inherent difficulty of distinguishing real accommodation from the space which is very unlikely to be filled by sediment (anti-accommodation). The accommodation concept cannot be extended to subaerial environments because the equilibrium profile of streams, which is sometimes used for this purpose, is difficult to recognize even if it existed. A possible redefinition of accommodation is suggested as: 'the thickness, measured at a specified site and time, of a space which becomes filled with sediments during a specified time interval'. This accommodation is not a space but the thickness of a space, not something potential but an objective result, not specifiable from the graded profile or geomorphic base level but from stratigraphic base level, and not separated from sedimentation but partly dependent from it. 'Potential accommodation' may be used to describe the maximum of possible accommodation (at a specified site and time), which can substantially coincide with the height of a water column at a given site and time.
Knickpoints often form in bedrock rivers in response to base level lowering. These knickpoints ca... more Knickpoints often form in bedrock rivers in response to base level lowering. These knickpoints can migrate upstream without dissipating. In the case of alluvial rivers, an impulsive lowering of base level due to, for example, a fault associated with an earthquake or dam removal commonly produces smooth, upstream-progressing degradation. The knickpoint associated with suddenly lowered base level quickly dissipates. Here, however, we use experiments to demonstrate that, under conditions of Froudesupercritical flow over an alluvial bed, suddenly lowered base level can lead to the formation of upstream-migrating knickpoints that do not dissipate. The base level fall can generate a single knickpoint or multiple knickpoints. Multiple knickpoints take the form of cyclic steps (i.e., trains of upstream-migrating bedforms, each bounded by a hydraulic jump upstream and downstream). In our experiments, trains of knickpoints were transient, eventually migrating out of the alluvial reach as the bed evolved to a new equilibrium state regulated with lowered base level. Thus the allogenic perturbation of lowered base level can trigger the autogenic generation of multiple knickpoints, which are sustained until the alluvial reach recovers a graded state.
ABSTRACT There is continued interest in how the rate of relative sea‐level rise [A ( &gt; 0)]... more ABSTRACT There is continued interest in how the rate of relative sea‐level rise [A ( &gt; 0)] and the rate of sediment supply [S] function during the growth and evolution of deltaic shorelines. The theory of shoreline autoretreat, recently corroborated in flume experiments, claims that (1) A( &gt; 0) and S can never be in equilibrium, and (2) shoreline or shelf‐edge progradation inevitably turns to retrogradation, when relative sea level is rising even modestly and even if A/S = const (&gt; 0). Autoretreat arises because the area of the clinoform surface of the delta (or shelf edge) per kilometer of shoreline must increase as the relative sea level rises, and the delta (or shelf edge) progrades into deeper water. A finite sediment supply rate is thus liable to become inadequate to sustain progradation. The problem increases further as a rising sea level also greatly increases the delta‐plain volume that needs to be filled, further limiting the progradation of the system. The fundamental trajectory of shoreline migration is thus one characterized by a concave‐landward shape, even under the steady forcing of the basin. The magnitudes of A (&gt; 0) and S, or A/S do not determine whether the landward turnaround of the shoreline is realized or not, but affect merely the length and height of the fundamental trajectory curve. Thus, any attempt to detect and interpret temporal changes in A and S from the observed stratigraphic record of shoreline trajectory needs first to take full account of the inbuilt autoretreat mechanism.We develop here a simple, semi‐quantitative method of reconstructing the basin conditions (A and S) from the stratigraphic record of prograding deltaic shorelines (or prograding shelf‐margin clinoforms) on the basis of the theory of shoreline autoretreat. The deterministic nature of the autoretreat theory is advantageous in managing this latter issue, because any expected or unexpected change emerges as some discrepancy from a trajectory that was predicted for the initial conditions. The autoretreat theory also provides a convenient graphical method of dealing with the uncertainty of the field data, and with evaluating the accuracy of any reconstruction. Our methodology has been developed to deal with the behaviour of deltaic shorelines, but is basically applicable to any clinoform system, the development of which is affected by relative sea level.The suggested method is applied to an Early Eocene (Ypresian) regressive shoreline succession in the Central Tertiary Basin on Spitsbergen. The studied regressive wedge developed as a delta‐driven, progradational shelf‐margin system under a regime of overall (i.e. long‐term) rise of relative sea level, but also suffered short‐term sea‐level falls associated with valley incisions on the coastal plain and shelf. On the assumption that S was constant or was steadily decreasing, the analysis of field data obtained from three sites within the basin suggests that the initial water depth in the basin was around 0.45 km, and that the overall relative sea‐level rise (c. 0.80 km) happened largely during an early time period and was followed by a longer period of much lower rate of rise. This pattern of relative sea‐level rise is consistent with the Palaeogene tectonic subsidence trend of the basin which was determined independently through a geohistory analysis. The uncertainty of the field data does not negate our reconstruction.The combined effects of autoretreat and A/S changes on a deltaic shoreline trajectory are confirmed through the development of an autoretreat‐based methodology. Conventional sequence stratigraphic models that assume a possible equilibrium condition between A and S are both conceptually misleading and insufficient to analyse basin conditions quantitatively. Sequence stratigraphic analyses of shorelines need to incorporate the autoretreat concept.
We report the experimental discovery of autogenic cyclicity in delta foreset bedding that arises ... more We report the experimental discovery of autogenic cyclicity in delta foreset bedding that arises simply from steady water and sediment input under particular hydraulic characteristics. This autogenesis is intrinsic to Froude-supercritical flow associated with a train of upstream-migrating hydraulic jumps that delineate cyclic steps. Upstream migration of hydraulic jumps is associated with discrete packages of sediment accumulation on the foreset slope. This synchronism originates from the periodic alternation between supercritical and subcritical flow states on the delta topset just upstream of the shoreline, i.e., the topset-foreset break of the delta. The alternation in turn depends on the distance from the topset-foreset break to the nearest hydraulic jump immediately upstream. When the hydraulic jump is still in close proximity to the river mouth (delta shore), it reworks existing topset deposits (and perhaps upper foreset deposits as well) as it migrates upstream. Reworked sediment is then entrained into the subcritical flow between the shore and the hydraulic jump, which then emplaces this fine suspended sediment onto the foreset slope. As the hydraulic jump moves sufficiently far upstream of the mouth, the topset flow becomes supercritical, with a flow velocity sufficient to transport fine sediment offshore beyond the foreset. Under these conditions, the sediment emplaced on the foreset tends to be coarser material deposited via avalanching or proximal deposition from suspension. The details of this selective sedimentation vary according to the material in the topset bed near the mouth that is available for entrainment into the subcritical flow. Regardless of the details, however, distinct cyclic sedimentation of the delta foreset can be sustained as long as the inflow conditions of water and sediment dictate the formation of cyclic steps on the alluvial bed of the topset. Such hydraulic autogenesis accounts for a set of stratigraphic features that are common in ancient Gilbert deltas.
Knickpoints often form in bedrock rivers in response to base-level lowering. These knickpoints ca... more Knickpoints often form in bedrock rivers in response to base-level lowering. These knickpoints can migrate upstream without dissipating. In the case of alluvial rivers, an impulsive lowering of base level due to, for example, a fault associated with an earthquake or dam removal commonly produces smooth, upstreamprogressing degradation; the knickpoint associated with suddenly lowered base level quickly dissipates. Here, however, we use experiments to demonstrate that under conditions of Froude-supercritical flow over an alluvial bed, an instantaneous drop in base level can lead to the formation of upstream-migrating knickpoints that do not dissipate. The base-level fall can generate a single knickpoint, or multiple knickpoints. Multiple knickpoints take the form of cyclic steps, that is, trains of upstream-migrating bedforms, each bounded by a hydraulic jump upstream and downstream. In our experiments, trains of knickpoints were transient, eventually migrating out of the alluvial reach as the bed evolved to a new equilibrium state regulated with lowered base level. Thus the allogenic perturbation of base-level fall can trigger the autogenic generation of multiple knickpoints which are sustained until the alluvial reach recovers a graded state.
The dynamics of delta distributary channels can be intensely affected by basin water depth in fro... more The dynamics of delta distributary channels can be intensely affected by basin water depth in front of the delta, particularly in terms of a long time scale. The previous experiments conducted in use of tank facilities suggest that with deeper basin water, delta distributary channels have lower rates of alluvial aggradation and lateral migration. If the delta faces very deep water, the channels attain alluvial grade and are stabilized in a particular position in the delta plain. This effect of basin water depth can be numerically expressed with grade index (Gindex), which ranges between 0 (perfect alluvial aggradation) and 1 (alluvial grade). Given basin water depth (h), delta plain radius (x) and alluvial slope (Sa), we define dimensionless basin water depth as h*=h/S ax. Assuming a set of particular geometrical conditions that (1) the basin floor is flat and horizontal, (2) the delta is always attached with a vertical wall on the back, and (3) base level is stationary, grade index...
Physical and numerical modeling of fluviodeltaic depositional systems, conducted under constant c... more Physical and numerical modeling of fluviodeltaic depositional systems, conducted under constant conditions of relative sea-level fall, sediment discharge, and water discharge, demonstrates that ''sustained'' alluvial aggradation is the inherent stratigraphic response of alluvial rivers that are steeper than the receiving basin. High alluvial gradients are primarily a consequence of high sediment supply relative to available water discharge (low rate of terrestrial diffusion). During the course of the experiments, fluvial grade was never reached, fluvial incision was never observed, and aggradation occurred independently of shoreline position. The experiments revealed detachment of the shoreline from the alluvial river as an autogenic response of certain fluviodeltaic systems to steady relative sea-level fall, creating a ''non-deltaic'' alluvial-river system that continues to aggrade beyond shoreline detachment. We term this behavior ''autodetachment.'' Autodetachment is a consequence of sustained, extensive alluvial aggradation during relative sea-level fall. Sediment supply was increasingly partitioned into the ever-enlarging alluvial profile to the detriment of the delta front prior to shoreline detachment. Thus, as relative sea level fell, the length of the delta front decreased to zero. Simultaneously, alluvial aggradation also led to decreasing rates of delta progradation. Therefore, as the delta front disappeared, the rate of shoreline regression exceeded the rate of progradation, producing a ''bedrock'' river between the detached alluvial river and the shoreline. The Canterbury Plains of New Zealand is a natural-world system with alluvial gradient greater than shelf gradient. Geometric modeling suggests that autodetachment could have occurred at the Canterbury Plains within the timescale of Quaternary glacio-eustatic sea-level falls, and therefore, is a realistic stratigraphic response at the scale of natural-world systems. Our model demonstrates that deltas with these geometric parameters become poorly supplied during regression, and large lowstand deltas should not be expected for these types of systems. The results of this study complement existing autostratigraphic theory, and provide an alternative to allogenic drivers for geomorphic and architectural evolution of fluviodeltaic wedges.
With each lowering of relative sea level, "fan valleys" of coastal alluvial fan systems... more With each lowering of relative sea level, "fan valleys" of coastal alluvial fan systems adjacent to high-gradient submarine slopes become entrenched or resume deepening at sites where the coastal slope is steepest. Tectonic tilting events that changed the maximum slope direction in a fan system should thus be identifiable in the geologic record by noting temporal changes in position and/or trend of fan valley deposits. This method of detecting ancient fan valley trends, and thus tilting events, is illustrated with Quaternary fans on the Pacific coast of central Honshu, Japan.
The most recent deglaciation resulted in a global sea level rise of some 120 m over approximately... more The most recent deglaciation resulted in a global sea level rise of some 120 m over approximately 12000 years. A moving boundary numerical model is developed to predict the response of rivers to this rise. The model was motivated by experiments at small-scale, which have identified two modes describing the transgression of a river mouth: autoretreat without abandonment of the
Grade index (G index) is a dimensionless number given as the volume-in-unit-time ratio of subaeri... more Grade index (G index) is a dimensionless number given as the volume-in-unit-time ratio of subaerial allocation to both subaerial and subaqueous allocations of sediment supplied to a delta from upstream. It was originally proposed for understanding the effect of basin water depth on the morphodynamics of delta distributary channels under stationary relative sea level. We here examine how rising relative sea level modulates the G index , using geometrical reasoning and numerical simulations. We find that the grade index model can account for autoretreat of the deltaic shoreline, autodrowning of the whole system, and autobreak of the deltaic sedimentation, all of which are the consequences of autogenic nonequilibrium responses to steadily rising relative sea level. The regressive-to-transgressive threshold (i.e. the onset of autoretreat) is crossed when the delta plain's dimensionless basal area (A t*) encounters a critical value that is expressed in terms of G index : regression and transgression are sustained when A t* is below and above the threshold, respectively. The mode of transgression depends on the slope conditions. If the hinterland slope (γ) is steeper than the foreset slope (β), both A t* and G index decrease as the relative sea-level rises. Eventually, the depositional system experiences autodrowning when A t* = G index = 0. If γ < β; on the other hand, both A t* and G index increase. This latter slope condition eventually causes autobreak of the deltaic sedimentation, afterward of which A t* = G index = 1. The grade index model is useful for interpreting and predicting the stratigraphic responses of natural deltaic clinoforms in conditions of rising relative sea level.
A debate is provoked here as to what is meant exactly by the term 'accommodation' and how well th... more A debate is provoked here as to what is meant exactly by the term 'accommodation' and how well this concept, as it presently stands, serves the conceptual and quantitative framework of sequence stratigraphy. 'Accommodation', in terms of its original definition, contains some conceptual flaws. It represents a space that has three-dimensional extent associated with a certain availability of sediment and thus is not really independent of sedimentation. However, this space is fairly unspecifiable because of the inherent difficulty of distinguishing real accommodation from the space which is very unlikely to be filled by sediment (anti-accommodation). The accommodation concept cannot be extended to subaerial environments because the equilibrium profile of streams, which is sometimes used for this purpose, is difficult to recognize even if it existed. A possible redefinition of accommodation is suggested as: 'the thickness, measured at a specified site and time, of a space which becomes filled with sediments during a specified time interval'. This accommodation is not a space but the thickness of a space, not something potential but an objective result, not specifiable from the graded profile or geomorphic base level but from stratigraphic base level, and not separated from sedimentation but partly dependent from it. 'Potential accommodation' may be used to describe the maximum of possible accommodation (at a specified site and time), which can substantially coincide with the height of a water column at a given site and time.
Knickpoints often form in bedrock rivers in response to base level lowering. These knickpoints ca... more Knickpoints often form in bedrock rivers in response to base level lowering. These knickpoints can migrate upstream without dissipating. In the case of alluvial rivers, an impulsive lowering of base level due to, for example, a fault associated with an earthquake or dam removal commonly produces smooth, upstream-progressing degradation. The knickpoint associated with suddenly lowered base level quickly dissipates. Here, however, we use experiments to demonstrate that, under conditions of Froudesupercritical flow over an alluvial bed, suddenly lowered base level can lead to the formation of upstream-migrating knickpoints that do not dissipate. The base level fall can generate a single knickpoint or multiple knickpoints. Multiple knickpoints take the form of cyclic steps (i.e., trains of upstream-migrating bedforms, each bounded by a hydraulic jump upstream and downstream). In our experiments, trains of knickpoints were transient, eventually migrating out of the alluvial reach as the bed evolved to a new equilibrium state regulated with lowered base level. Thus the allogenic perturbation of lowered base level can trigger the autogenic generation of multiple knickpoints, which are sustained until the alluvial reach recovers a graded state.
ABSTRACT There is continued interest in how the rate of relative sea‐level rise [A ( &gt; 0)]... more ABSTRACT There is continued interest in how the rate of relative sea‐level rise [A ( &gt; 0)] and the rate of sediment supply [S] function during the growth and evolution of deltaic shorelines. The theory of shoreline autoretreat, recently corroborated in flume experiments, claims that (1) A( &gt; 0) and S can never be in equilibrium, and (2) shoreline or shelf‐edge progradation inevitably turns to retrogradation, when relative sea level is rising even modestly and even if A/S = const (&gt; 0). Autoretreat arises because the area of the clinoform surface of the delta (or shelf edge) per kilometer of shoreline must increase as the relative sea level rises, and the delta (or shelf edge) progrades into deeper water. A finite sediment supply rate is thus liable to become inadequate to sustain progradation. The problem increases further as a rising sea level also greatly increases the delta‐plain volume that needs to be filled, further limiting the progradation of the system. The fundamental trajectory of shoreline migration is thus one characterized by a concave‐landward shape, even under the steady forcing of the basin. The magnitudes of A (&gt; 0) and S, or A/S do not determine whether the landward turnaround of the shoreline is realized or not, but affect merely the length and height of the fundamental trajectory curve. Thus, any attempt to detect and interpret temporal changes in A and S from the observed stratigraphic record of shoreline trajectory needs first to take full account of the inbuilt autoretreat mechanism.We develop here a simple, semi‐quantitative method of reconstructing the basin conditions (A and S) from the stratigraphic record of prograding deltaic shorelines (or prograding shelf‐margin clinoforms) on the basis of the theory of shoreline autoretreat. The deterministic nature of the autoretreat theory is advantageous in managing this latter issue, because any expected or unexpected change emerges as some discrepancy from a trajectory that was predicted for the initial conditions. The autoretreat theory also provides a convenient graphical method of dealing with the uncertainty of the field data, and with evaluating the accuracy of any reconstruction. Our methodology has been developed to deal with the behaviour of deltaic shorelines, but is basically applicable to any clinoform system, the development of which is affected by relative sea level.The suggested method is applied to an Early Eocene (Ypresian) regressive shoreline succession in the Central Tertiary Basin on Spitsbergen. The studied regressive wedge developed as a delta‐driven, progradational shelf‐margin system under a regime of overall (i.e. long‐term) rise of relative sea level, but also suffered short‐term sea‐level falls associated with valley incisions on the coastal plain and shelf. On the assumption that S was constant or was steadily decreasing, the analysis of field data obtained from three sites within the basin suggests that the initial water depth in the basin was around 0.45 km, and that the overall relative sea‐level rise (c. 0.80 km) happened largely during an early time period and was followed by a longer period of much lower rate of rise. This pattern of relative sea‐level rise is consistent with the Palaeogene tectonic subsidence trend of the basin which was determined independently through a geohistory analysis. The uncertainty of the field data does not negate our reconstruction.The combined effects of autoretreat and A/S changes on a deltaic shoreline trajectory are confirmed through the development of an autoretreat‐based methodology. Conventional sequence stratigraphic models that assume a possible equilibrium condition between A and S are both conceptually misleading and insufficient to analyse basin conditions quantitatively. Sequence stratigraphic analyses of shorelines need to incorporate the autoretreat concept.
We report the experimental discovery of autogenic cyclicity in delta foreset bedding that arises ... more We report the experimental discovery of autogenic cyclicity in delta foreset bedding that arises simply from steady water and sediment input under particular hydraulic characteristics. This autogenesis is intrinsic to Froude-supercritical flow associated with a train of upstream-migrating hydraulic jumps that delineate cyclic steps. Upstream migration of hydraulic jumps is associated with discrete packages of sediment accumulation on the foreset slope. This synchronism originates from the periodic alternation between supercritical and subcritical flow states on the delta topset just upstream of the shoreline, i.e., the topset-foreset break of the delta. The alternation in turn depends on the distance from the topset-foreset break to the nearest hydraulic jump immediately upstream. When the hydraulic jump is still in close proximity to the river mouth (delta shore), it reworks existing topset deposits (and perhaps upper foreset deposits as well) as it migrates upstream. Reworked sediment is then entrained into the subcritical flow between the shore and the hydraulic jump, which then emplaces this fine suspended sediment onto the foreset slope. As the hydraulic jump moves sufficiently far upstream of the mouth, the topset flow becomes supercritical, with a flow velocity sufficient to transport fine sediment offshore beyond the foreset. Under these conditions, the sediment emplaced on the foreset tends to be coarser material deposited via avalanching or proximal deposition from suspension. The details of this selective sedimentation vary according to the material in the topset bed near the mouth that is available for entrainment into the subcritical flow. Regardless of the details, however, distinct cyclic sedimentation of the delta foreset can be sustained as long as the inflow conditions of water and sediment dictate the formation of cyclic steps on the alluvial bed of the topset. Such hydraulic autogenesis accounts for a set of stratigraphic features that are common in ancient Gilbert deltas.
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