Shoreline Trajectory Research Articles

Sediment dispersal and quantitative stratigraphic architecture across an ancient shelf

Two large (200 to 300 km), near-continuous outcrop transects and extensive well-log data (ca 2800 wells) allow analysis of sedimentological characteristics and stratigraphic architecture across a large area (ca 60 000 km2) of the latest Santonian to middle Campanian shelf along the western margin of the Western Interior Seaway in eastern Utah and western Colorado, USA. Genetically linked depositional systems are mapped at high chronostratigraphic resolution (ca 0·1 to 0·5 Ma) within their sequence stratigraphic context. In the lower part of the studied interval, sediment was dispersed via wave-dominated deltaic systems with a ‘compound clinoform’ geomorphology in which an inner, wave-dominated shoreface clinoform was separated by a muddy subaqueous topset from an outer clinoform containing sand-poor, gravity-flow deposits. These strata are characterized by relatively steep, net-regressive shoreline trajectories (>0·1°) with concave-landward geometries, narrow nearshore belts of storm-reworked sandstones (2 to 22 km), wide offshore mudstone belts (>250 km) and relatively high sediment accumulation rates (ca 0·27 mm year−1). The middle and upper parts of the studied interval also contain wave-dominated shorefaces, but coeval offshore mudstones enclose abundant ‘isolated’ tide-influenced sandstones that were transported sub-parallel to the regional palaeoshoreline by basinal hydrodynamic (tidal?) circulation. These strata are characterized by relatively shallow, net-regressive shoreline trajectories ( 190 km) and relatively low sediment accumulation rates (ca ≤0·11 mm year−1). The change in shelfal sediment dispersal and stratigraphic architecture, from: (i) ‘compound clinoform’ deltas characterized by across-shelf sediment transport; to (ii) wave-dominated shorelines with ‘isolated’ tide-influenced sandbodies characterized by along-shelf sediment transport, is interpreted as reflecting increased interaction with the hydrodynamic regime in the seaway as successive shelfal depositional systems advanced out of a sheltered embayment (‘Utah Bight’). This advance was driven by a decreasing tectonic subsidence rate, which also suppressed autogenic controls on stratigraphic architecture.

Shoreline trajectories on a glacially influenced stable margin – insight from the Barents Sea Shelf, NW Russia

ABSTRACTThis study describes shoreline migration paths for late Quaternary sediments on the inner Barents Sea shelf between Kola and the Pechora Sea. The depositional geometries provide an example of stratigraphical architecture in a glacially influenced basin prone to isostatic movements as well as rapid and high‐amplitude changes in eustatic sea level. The depositional geometries reflect asymmetrical relative sea level changes characterised by marine inundation upon deglaciation and prolonged forced regressions. Thus, all deposition occurs during the falling stage and lowstand systems tracts. The transgressive and highstand systems tracts are lacking and the maximum landward position of the shoreline is coinciding with the basal surface of forced regression. Shoreline migration is dominated by downward and seaward trajectories, but aggradation occurs on the falling limb of the relative sea level curve due to superimposed eustatic cycles of lower hierarchical order. Fluvial aggradation behind the shoreline takes place during the lowstand systems tract, but is also linked to high sediment supply and may also respond to superimposed lower order sea level fluctuations. Lateral variations in isostatic load due to asynchronous ice advances lead to regional variations in shoreline trajectories. Significant differences in sea level history exist across former ice margins leading to time‐transgressive and laterally discontinuous stratigraphical surfaces. Sequence boundaries are not only diachronous along the depositional profile, but also laterally, and basal surfaces of forced regression are strongly diachronous across former ice margins. Absolute age control allows for estimates of the time differences along significant stratigraphical surfaces.

Shelf edge and shoreline trajectories, a dynamic approach to stratigraphic analysis

ABSTRACTThe Shelf Edge and Shoreline Trajectories Conference, convened in Tromsø, Norway, during the autumn of 2007, was attended by a group of specialists working in the crossover between industry and academia. This paper introduces the concepts of shelf edge‐ and shoreline‐trajectory analysis, and discusses some of the advantages of applying such concepts in contrast to more traditional sequence stratigraphic analysis. This special issue of Basin Research focuses on how observations of outcrop and subsurface datasets, particularly three‐dimensional (3D) seismic data, may be used as an aid to identify palaeo‐shelf edges and shorelines. Moreover, the approach shows how linking the cross‐sectional path of a shoreline as it migrates (shoreline trajectory) and the pathway taken by the shelf‐edge during the development of a series of accreting clinoforms (shelf‐edge trajectory) to the analysis of sedimentological or seismic facies can improve predictions of lithology distribution. The following 15 papers present well‐documented case studies from a variety of shelf and shelf‐margin settings where these concepts have been applied to depositional systems ranging in age from Permian to Recent. A wide spectrum of data types and methods, including two dimensional and 3D seismic data, well logs and core material as well as high‐resolution biostratigraphy, outcrop studies and modern bathymetric data have been applied in the various papers. Despite the considerable age range of the deposits investigated and the data types used for the studies, all of the authors have converged towards the objective approach of trajectory analysis. However, any analytical method has some uncertainty attached to it, and a discussion of possible pitfalls and sources of error is also a part of this introductory paper. Although this special issue presents some recent advances in the way to conduct stratigraphic analysis, we realise that this is only a further step in an evolving discipline. Development of sequence stratigraphic concepts will continue, and new contributions will evaluate past work as they seek to develop the subject.

Quantitative analysis of net‐transgressive shoreline trajectories and stratigraphic architectures: mid‐to‐late Jurassic of the North Sea rift basin

ABSTRACTThis paper outlines the use of the shoreline trajectory concept to understand the controls on net‐transgressive reservoir distribution and architecture in the highly productive Middle and Late Jurassic plays in the North Sea. Two broad groups of regressive–transgressive sandstone tongue are identified, with distinctive geometries, architectures and values of net‐transgressive shoreline trajectory defined by the stacking arrangement of multiple tongues. Shoreface tongues were supplied by longshore‐transported, marine‐reworked sediment and are associated with low‐to‐moderate transgressive trajectories (typically <0.2°). These tongues have variable dip extents that decrease weakly as the angle of shoreline trajectory increases, relatively small thicknesses that increase weakly with the angle of transgressive trajectory, and partial or no overlap with underlying and overlying tongues down depositional dip. Deltaic‐to‐estuarine tongues were supplied directly by fluvial sediment and are associated with moderate‐to‐very high transgressive trajectories (typically >0.1°). These tongues have small dip extents, variable thicknesses that increase weakly with the angle of transgressive trajectory, and partial to full overlap with underlying and overlying tongues down depositional dip, although vertically stacked tongues are separated by thin mudstones over much of their extents. There is some overlap in geometry and stacking arrangement of these two groups of sandstone tongues. The temporal and spatial distribution of shoreface and deltaic‐to‐estuarine sandstone tongues reflects linked variations in tectonic subsidence and sediment routing within the evolving rift basin. Deltaic‐to‐estuarine tongues with moderate‐to‐very high transgressive trajectories were developed in rapidly subsiding fault‐bounded depocentres supplied directly by fluvial sediment, whereas shoreface tongues with low‐to‐moderate transgressive trajectories characterised slowly subsiding fault terraces and platforms that were distant from fluvial sediment supply routes. Thus, the evolving structural template of the mid‐to‐late Jurassic North Sea rift can be used to predict reservoir distribution and architecture via analysis of shoreline trajectory.

Genesis of an over‐thickened shoreface sandstone tongue: The Rannoch and Etive formations of the Middle Jurassic Brent delta, North Sea

ABSTRACTProgradational, wave‐dominated shoreface sandstones are typically reported to have thicknesses of less than 30 m. The shoreface sandstones of the Rannoch and Etive formations in the North Sea Brent Group, however, comprise far greater thicknesses (in places exceeding 100 m). The genesis of these successions have not been well understood, and the purpose of this study is to investigate within the framework of facies geometries in both modern and ancient depositional systems how the Rannoch‐Etive sandstones could have been formed, on a local‐regional scale.The facies analysis concludes that the Brent delta advance represents a single progradational event, not punctuated by transgressions. In such a scenario, the vertical sandstone thickness would be determined by: 1) shoreline trajectory angle, 2) the horizontal (dip‐directed) length of shoreface sand (typically 1–2 km in modern systems), and 3) shoreface sand pinch‐out depth (typically 5–12 m in modern systems). Within a framework of modern facies geometries, a 100 m thick vertical shoreface sandstone succession could result from a regression characterised by a shoreline trajectory of 2.6–5.4° (implying 80–95 m rise in relative sea level per 1–2 km of progradation), a 5–12 m deep shoreface sand pinch‐out depth, and a 1–2 km shoreface sand length.If, however, one applies such a model for the entire Brent delta progradation in the study area (200 km) this implies a rise in relative sea level of c. 5000 m, which clearly would be associated with large amounts of deposits accumulating behind and in front of the advancing shoreline. Such deposits are not present, which implies that modern facies geometries do not apply and that rising sea level was not of major importance in defining stratigraphic architecture. It is more likely that the regional overthickening of the Rannoch‐Etive sandstones resulted from progradation with a deep shoreface sand pinch‐out depth.

Trajectory analysis: concepts and applications

ABSTRACTShoreline and shelf‐edge trajectories describe the migration through time of sedimentary systems, using geomorphological breaks‐in‐slope that are associated with key changes in depositional processes and products. Analysis of these trajectories provides a simple descriptive tool that complements and extends conventional sequence stratigraphic methods and models. Trajectory analysis offers four advantages over a sequence stratigraphic interpretation based on systems tracts: (1) each genetically related advance or retreat of a shoreline or shelf edge is viewed in the context of a continuously evolving depositional system, rather than as several discrete systems tracts; (2) subtle changes in depositional response (e.g. within systems tracts) can be identified and honoured; (3) trajectory analysis does not anticipate the succession of depositional events implied by systems‐tract models; and (4) the descriptive emphasis of trajectory analysis does not involve any a priori assumptions about the type or nature of the mechanisms that drive sequence development. These four points allow the level of detail in a trajectory‐based interpretation to be directly tailored to the available data, such that the interpretation may be qualitative or quantitative in two or three dimensions. Four classes of shoreline trajectory are recognized: ascending regressive, descending regressive, transgressive and stationary (i.e. nonmigratory). Ascending regressive and high‐angle (accretionary) transgressive trajectories are associated with expanded facies belt thicknesses, the absence of laterally extensive erosional surfaces, and relatively high preservation of the shoreline depositional system. In contrast, descending regressive and low‐angle (nonaccretionary) transgressive trajectories are associated with foreshortened and/or missing facies belts, the presence of laterally extensive erosional surfaces, and relatively low preservation of the shoreline depositional system. Stationary trajectories record shorelines positioned at a steeply sloping shelf edge, with accompanying bypass of sediment to the basin floor. Shelf‐edge trajectories represent larger spatial and temporal scales than shoreline trajectories, and they can be subdivided into ascending, descending and stationary (i.e. nonmigratory) classes. Ascending trajectories are associated with a relatively large number and thickness of shoreline tongues (parasequences), the absence of laterally extensive erosional surfaces on the shelf, and relatively low sediment supply to the basin floor. Descending trajectories are associated with a few, thin shoreline tongues, the presence of laterally extensive erosional surfaces on the shelf, and high sediment supply to basin‐floor fan systems. Stationary trajectories record near‐total bypass of sediment across the shelf and mass transfer to the basin floor.

Vertically stacked Gilbert-type deltas of Ventimiglia (NW Italy): The Pliocene record of an overfilled Messinian incised valley

Overfilled incised valleys develop when the rate of sediment supply outpaces the rate of accommodation. An overfilled incised valley presents simple or compound valley-fill architecture, depending on the depth of the valley incision, compared with the height reached by the following sea-level rise. The Ventimiglia incised valley, exposed on the Ligurian coast, north-western Mediterranean margin, presents a spectacular example of compound incised-valley fill, developed in perennial “overfill” conditions. The valley was subaerially incised during the Messinian Salinity Crisis and rapidly flooded by the sea at the beginning of Pliocene, then filled by eleven coarse-grained Gilbert-type deltas during Early–Middle Pliocene time. The basal Messinian unconformity is locally paved with subaerial scree breccias and bioclastic shallow-marine sandstones, and blanketed by bathyal marls. These deposits record the lowstand, transgressive and early-highstand systems tracts of the first valley-fill sequence. The subsequent progradation of Gilbert-type deltas occurred in four stages, or depositional sequences, separated by transgressive marine-marl intervals. Within each depositional sequence, the deltaic bodies display offlapping architecture, recording falling shoreline trajectory, downward shifts in facies, and overall forced regression. The water depth and accommodation in the inundated coastal valley was gradually decreasing with time. The reduced accommodation allowed the youngest deltas to prograde out to the shelf edge, triggering mass collapses and subsequent filling into the newly created slump scars. Some of the deltas probably acted as “canyon-perched deltas” and supplied sediment to the deep-water slope and floor of the Ligurian Basin. The vertical stacking of Gilbert-type deltas is usually attributed, in tectonically active basins, to fault-related subsidence pulses. In Ventimiglia, the accommodation was created by high-frequency eustatic sea-level rises that, probably accompanied by climate controlled reductions in sediment supply, temporarily outpaced uplift, leading to the development of multiple cycles of infill.

A unifying framework for shoreline migration: 1. Multiscale shoreline evolution on sedimentary coasts

A unifying framework for shoreline migration is presented which synthesizes problems from coastal engineering, geomorphology, and stratigraphy into a common quantitative basis. Consideration of characteristic problems from these fields reveals that multiscale shoreline evolution problems naturally break into three classes on the basis of the morphologic unit of evolution: shoreface bed versus shoreline planform versus stratigraphic shoreline trajectory. Shoreline evolution models correspondingly break into morphodynamic, morphokinematic, and hybrid approaches. A generalized shoreline continuity equation is derived, the shoreline Exner equation, which subsumes equilibrium profile models commonly used in geomorphology and engineering and provides a natural framework for interpreting shoreline trajectories preserved in the sedimentary record. The shoreline Exner equation provides a rigorous basis for geometric and moving boundary models, which have significant untapped potential for shoreline migration problems. Recent advances in computational methods for evolving interface problems make hybrid models particularly attractive for problems of large‐scale planform shoreline dynamics, such as delta evolution.

Shoreline motion in nonlinear shallow water coastal models

Different shoreline boundary conditions for numerical models of the Non-Linear Shallow Water Equations based on Godunov-type schemes are compared. The study focuses on the Peregrine and Williams [Peregrine, D.H., Williams, S.M., 2001. Swash overtopping a truncated plane beach. Journal of Fluid Mechanics 440, 391–399.] problem of a single bore collapsing on a slope. This is considered the best test to assess performances of the shoreline boundary treatments in terms of all the parameters of interest in swash zone modelling. Emphasis is given to the shoreline trajectory and flow velocity modelling. A mismatch of the velocity at the early stage of the motion is highlighted. Most of the tested techniques perform similarly in terms of maximum run-up, the backwash phase is critical in all cases. Starting from the Brocchini et al. [Brocchini, M., Bernetti, R., Mancinelli, A., Albertini, G., 2001. An efficient solver for nearshore flows based on the WAF method. Coastal Engineering 43(2), 105–129.] shoreline boundary treatment, a simple technique that improves the accuracy of velocity predictions is also developed. A sensitivity analysis of the domain resolution and the threshold value of the water depth that defines a wet cell is also presented.

Latest Carboniferous–earliest Permian transgressive deposits in the Paganzo Basin of western Argentina: Lithofacies and sequence stratigraphy of a coastal-plain to bay succession

The upper Paleozoic rocks of Gondwana record a complex paleoclimatic history related to the migration of the supercontinent over high latitudes. Changes in climate and relative sea level can be traced through detailed sedimentologic and sequence-stratigraphic analysis. Our study focuses on transgressive deposits of Stephanian–Early Permian age in the lower member of the Tupe Formation with the objective of characterizing lithofacies and sedimentary environments within a sequence-stratigraphic framework in order to achieve a better understanding of the sedimentary history of the Paganzo Basin and the nature of transgressive deposits. Sixteen lithofacies grouped in eight assemblages were defined and arranged in two complete sequences. A sequence boundary (SB) is identified at the base of the Tupe Formation. Coastal-plain deposits capped by marine embayment lithofacies are included within sequence 1. A relative sea-level fall (SB) is recorded by an abrupt change into braided alluvial-plaine deposits (LST). The beginning of the TST is characterized by the appearance of coal and deltaic lithofacies. Late TST deposits occur above a ravinement surface and comprise bay-margin to distal-bay deposits forming a retrogradational stacking package. These lithofacies are replaced upwards by HST deposits. A relative sea-level fall (SB) is recorded by the presence of fluvial deposits overlying marine lithofacies. The Tupe Formation illustrates the transition of a coastal-plain to a marine embayment. The detection of a transgressive surface within the coastal-plain deposits of sequence 1 expanded significantly the volume of deposits now included as part of the latest Carboniferous–earliest Permian transgression, and underscores the importance of searching for transgressive signatures in non-marine environments. The presence of two sequences supports a punctuated shoreline trajectory with an overall retrogradational stacking pattern. An abrupt relative sea-level fall and increased aridity is recorded at the end of the transgressive event.

Sedimentological parameterization of shallow-marine reservoirs

The key causes of heterogeneity within progradational shallow-marine reservoirs have been defined as: shoreline type (wave vs. fluvial dominated); shoreline trajectory; the presence of permeability contrasts associated with dipping clinoform surfaces within the shoreface or delta front; the presence of cemented barriers between parasequences; and the progradation direction of the shoreline (described with respect to the main waterflood direction in the simulated reservoir). These parameters were recorded from a series of 56 modern and ancient depositional systems from a variety of climatic and tectonic settings. These data were then used to build the 408 synthetic sedimentological models that formed the basis for the SAIGUP study.

Structural, Sedimentologic, and Sea-Level Controls on Sand Distribution in a Steep-Clinoform Asymmetric Wave-Influenced Delta: Miocene Billund Sand, Eastern Danish North Sea and Jylland

Abstract The lower Miocene Billund sand of Jylland, Denmark, provides an excellent opportunity to study the distribution of sands and the types of clinoform generated by a wave-dominated delta prograding into a deep shelf environment. In particular seismic sections, the sand-rich parts correlate with parallel clinoformal seismic facies, in which the clinoforms dip 7° to 10°. The stacking pattern of this seismic facies also indicates whether the progradation occurred during rising or falling sea level by displaying an ascending or descending shoreline trajectory, respectively. The Billund delta was deposited in the eastern North Sea Basin during the early Miocene and is on the available data recognized as a NW to SE trending feature. It forms a wave-dominated delta with thick sand bodies deposited in relatively deep waters (> 100 m) at the river mouth and in the up-drift area of newly formed delta lobes. Sands and muds associated with a spit complex or a lagoonal complex dominated the down-drift area. In the distal delta-front area, alternating sands and muds were laid down, the sands associated mainly with storm events. Progradation of the studied part of the delta occurred during high sea level and a subsequent fall in sea level that was caused by a eustatic fall during the mid Aquitanian (early Miocene). Although sand intervals 10 to 20 m thick are common throughout the delta complex, the thickest (up to 100 m) and cleanest sand is found in units deposited in narrow bands within structural lows and during periods of forced regression. The Billund delta may be an excellent analogue for hydrocarbon-bearing, wave-dominated deltas, and seems to exhibit similarities to Jurassic deposits in the North Sea area.

The architectural variability of small-scale cycles in shelf and ramp clastic systems: The controlling factors

Many sedimentary successions are composed of metre- to decametre-scale cycles exhibiting various stratal architectures that depend on the complex interplay among several syndepositional controlling factors. These controlling factors may produce extremely different stratal and facies architectures along relatively brief distances in the same basin, but in contrast they also may form similar architectures in basins that experience a radically different tectonic setting. The recognition of factors that controlled the architecture of small-scale cycles, therefore, is very important to reconstruct the depositional history of sedimentary successions. These factors include: 1) the interplay between the rate of accommodation development and the rate of sediment supply (A/S); 2) the amplitude and shape of the relative sea-level curve; 3) the position in the shoreface–shelf system; 4) the basin physiography; 5) the shoreline trajectory; 6) the climate-driven environmental energy; 7) the siliciclastic input vs. carbonate production. Shoreface to shelf small-scale cycles can be grouped into three main categories: R cycles, showing a dominant regressive interval; T– R cycles, with transgressive and regressive intervals of similar thickness; T cycles, showing a dominant transgressive interval. Although some depositional settings may favour the deposition of a particular type of cycle, these cycle motifs appear to be mainly controlled by the interaction of local and wider-scale controlling factors. For example, T cycles may be present in both subsiding and uplifting settings, depending on the interplay between the A/S and the other factors cited above. As a result, R, T– R and T cycles may be recognized within the same depositional system. These evidences suggest that care must be taken to apply general models that predict the stratal architecture of cycles mainly in function of the long-term A/S. The correct prediction of the cycle architecture is important also for hydrocarbon researches, as the thickness of potential reservoir deposits is the result of the interplay among factors discussed here.

Cyclical Evolution of a Modern Transgressive Sand Barrier in Northwestern Spain Elucidated by GPR and Aerial Photos

Abstract The internal structure of a modern baymouth barrier is analyzed in detail, including its historical evolution and the factors controlling it. Historical evolution was reconstructed using available aerial photographs from 1956 to 2003. Detailed geophysical data were acquired using ground-penetrating radar (GPR) and analyzed along with airphotos to reconstruct the recent evolution of a barrier located in northwestern Spain. The GPR data reveal the internal architecture of the barrier, which consists of two main radar facies: (1) washover deposits associated with landward barrier migration and vertical aggradation, and (2) beach deposits associated with seaward barrier progradation. The reconstruction of the stratigraphic sequence indicates the occurrence of two erosive periods followed by an accretionary period. The erosion along the barrier is outlined by the presence of three well-defined bounding surfaces. The stratigraphic record of the subsequent recovery phase depends on the impact level of the erosive phase, which is controlled by the presence of ephemeral inlets along the barrier. This alternating behavior implies a discontinuous shoreline trajectory in spite of a net landward migration. This trend is consistent with regional sea-level rise albeit being intensified locally by the interplay of sediment starvation and storm regime. Landward barrier retreat occurs by overwash and inlet breaching, which permits barrier preservation of volume due to a continuous constructive-destructive cycle. The results acquired from this example expand our understanding of the internal structure and evolutionary trends of transgressive sand barriers.

Interplay between shoreline migration paths, architecture and pinchout distance for siliciclastic shoreline tongues: evidence from the rock record

AbstractFacies, geometry and key internal stratigraphic surfaces from eight Cretaceous and Eocene clastic shoreline tongues have been documented. The regressive parts of all the studied tongues represent storm‐wave influenced strandplains, deltas or fan‐deltas, and the regressive shoreline trajectories varied from descending to ascending. The transgressive parts of the tongues are dominated by either estuarine or coastal‐plain deposits. The distance from the coeval, up‐dip non‐marine deposits to the basinward pinchout of amalgamated shoreface sandstones, measured along depositional dip, is here termed the sand pinchout distance. The study shows that the angle of regressive‐to‐transgressive turnaround (defined by the angle between the regressive and subsequent transgressive shoreline trajectories) and the process regime during turnaround largely control the sand‐pinchout distance. The amount of transgressive erosion can also partly control the pinchout distance, but this parameter was comparable for the different examples presented here. If the type of depositional system at turnaround and the depth of transgressive erosion are constant, small angles of turnaround are associated with large pinchout distances, whereas larger angles of turnaround result in smaller pinchout distances. The model developed allows sand‐pinchout distance to be predicted, using data for the landward parts of shoreline tongues. The dataset also shows that steeply rising (aggrading) shoreline trajectories tend to produce more heterolithic sandstone tongues than those formed by lower‐angle trajectories.

Early Miocene lacustrine deposits and sequence stratigraphy of the Ermenek Basin, Central Taurides, Turkey

The lacustrine Ermenek Basin evolved as a SE-trending intramontane graben affected by strike–slip deformation, with the initial two lakes merging into one and receiving sediment mainly through fan deltas sourced from the basin's southern margin. The northern margin was a high-relief rocky coast with a wave-dominated shoreline. The Early Miocene lacustrine sedimentation was terminated by a late Burdigalian marine invasion that drowned the basin and its surroundings. The lacustrine basin-fill succession is up to 300 m thick and best exposed along the southern margin, where it consists of four sequences bounded by surfaces of forced regression. The offshore architecture of each sequence shows a thin lowstand tract of shoreface sandstones overlain by a thick transgressive systems tract of mudstones interbedded with sandy tempestites and delta-derived turbidites, which form a set of coarsening-upward parasequences representing minor normal regressions. The corresponding nearshore sequence architecture includes a thick lowstand tract of alluvial-fan deposits overlain by either a well-developed transgressive systems tract (backstepping parasequence set or single fan-deltaic parasequence) and poorly preserved highstand tract; or a thin transgressive tract (commonly limited to flooding surface) and a well-developed highstand tract (thick fan-deltaic parasequence). The sequences are poorly recognizable along the northern margin, where steep shoreline trajectory rendered the nearshore system little responsive to lake-level changes. The resolution of local stratigraphic record thus depends strongly upon coastal morphology and the character of the depositional systems involved. The sequential organization of the basin-fill succession reflects syndepositional tectonics and climate fluctuations, whereas the lateral variation in sequence architecture is due to the localized sediment supply (deltaic vs. nondeltaic shoreline), varied coastal topography and differential subsidence. The study points to important differences in the sequence stratigraphy of lacustrine and marine basins, related to the controlling factors. A crucial role in lacustrine basin is played by climate, which controls both the lake water volume and the catchment sediment yield. Consequently, the effects of tectonics and the dynamics of changes in accommodation and sediment supply in a lacustrine basin are different than in marine basins.

Role of autoretreat and A/S changes in the understanding of deltaic shoreline trajectory: a semi‐quantitative approach

ABSTRACT There is continued interest in how the rate of relative sea‐level rise [A ( > 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( > 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 (> 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 (> 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.

Predicting the pinchout distance of shoreline tongues

Sand‐rich shoreline tongues are common features of the stratigraphic record, and many of them are important petroleum reservoirs. The basinward extent of such shallow‐marine deposits is highly variable, from a few to a few hundred kilometres. Although indirect information exists, the question as to what parameters actually cause this variation and their relative importance has not been addressed. Here, it is argued that the sand pinchout distance of regressive‐to‐transgressive shoreline tongues is controlled by (1) the type of depositional system (e.g. fluvial‐, tide‐ or wave‐dominated); (2) the regressive and transgressive shoreline trajectories; and (3) the depth of transgressive erosion. In contrast to the shoreline trajectory, the angle of turnaround, defined by the pathway taken by the regressive and subsequent transgressive shorelines, is simple to measure and gives a first approximation of the pinchout distance. For otherwise equal systems, the pinchout distance is inversely related to the angle of turnaround.

Shoreline Autoretreat Substantiated in Flume Experiments

ABSTRACT Under conditions of constant sediment supply (S) to the basin and a steady rate of rise (A) in relative sea level, the autoretreat theory predicts that shorelines can retreat landwards at a relatively early stage of delta growth and attain an autobreak state, after which the existing subaqueous slope begins to be starved of sediment and thereby lose its clear delta-front configuration. A numerical model based on this theory further suggests that shoreline migration depends on the inclination of basin's basement slope () and subaqueous depositional slope () and, to a lesser degree, on the inclination of subaerial depositional slope (). Two-dimensional flume experiments in which a miniature delta is subject to steady forcing (A = const > 0, S = const > 0), were conducted to test the expectations of the autoretreat theory. The experiments consisted of two different series of runs: one was of associated with a narrow range of the ratio of water discharge to sediment discharge (q/S), the other was of with kept nearly constant. The q/S ratio was mostly responsible for changes in but also affected to some extent. All experimental runs substantiated the landward turnaround feature of shoreline accretion (i.e., autoretreat) and the geometrical break of the delta-front shape during its retreat phase (i.e., autobreak). The obtained shoreline trajectories hold their geometrical patterns even after they were nondimensionalized in terms of the S/A ratio. A key hypothesis confirmed in the experiments is that A and S can never balance each other. Both of the experimental series brought some systematic change of shoreline trajectories, but the variations of shoreline trajectory due to modulation were much more striking than those related to q/S. Even these contrasting results are consistent with the numerical predictions. The present experimental results support the applicability of the autoretreat concept to natural environments and help clarify the processes governing the regression and transgression of a shoreline on a constructional margin.

Bored pebbles and ravinement surface clusters in a transgressive systems tract, Sant Llorenç del Munt fan-delta complex, SE Ebro Basin, Spain

A 25-m thick transgressive systems tract in the Sant Llorenç del Munt, wave-influenced, fan-delta system (Eocene, SE Ebro Basin) has an internal framework consisting of a cluster of transgressive erosion surfaces, each of which has minor relief and which are collectively stacked vertically no more than two metres apart. Each erosion surface bounds a cycle containing a conglomeratic lag (up to 0.5-m thick) followed by a coarsening-upward sandstone to conglomeratic unit (the uppermost levels of which can be nonmarine). Individual cycles become entirely nonmarine landwards of the termination of the basal-bounding erosion surface, whereas they thin and eventually become entirely marine basinwards. The individual erosion surfaces within the transgressive tract, some 16 of them within a 20-m thick lithosome, are interpreted as wave-ravinement surfaces that repeatedly eroded into the conglomeratic shoreface during transgression. This interpretation, rather than one invoking nonmarine flooding or other marine erosion surface types, is consistent with the arrangement of bivalve and sponge borings on the top surfaces of clasts and with the associated lag pavements. Multiphase boring around the entire surface of clasts, as well as erosion of the clasts at some horizons, particularly in reaches of the tract where the ravinement trajectory is sub-horizontal, suggest repeated reworking of previously generated lag pavements in zones of minimal aggradation during transgression. Where the transgressive shoreline trajectory rises more steeply and there has been more rapid aggradation during transgression, the lag pavements show only single-phase borings, with the borings on the upper surface of the pebble-pavement only. The close spacing of erosion surfaces within the transgressive systems tract, together with estimates of time span in the tract, suggest that transgressive erosion occurred with a frequency of less than 500 years.