Shoreface Profile Research Articles

Assessing the effects of shoreface profile concavity on long-term shoreline changes: an exploratory study

Coastal erosion under the context of global sea-level rise has threatened societies living along the coast. The processes related to decadal- to centennial-scale shoreline changes are much more complex than those suggested by the Bruun effect. This paper derives a one-dimensional model to explore the relationships between shoreline change, profile concavity, and sediment fluxes in wave-dominated coastal systems where alongshore sediment transport dominates the shoreline changes. The model incorporates an exponential approximation of shoreface profiles into the shoreline Exner equation. We found an equilibrium relationship in which a stable morphology profile is maintained during synchronous changes in sediment loss (gain) and shoreline retreat (advance) distances. Complex shoreline retreat behaviours can be classified into two dynamic equilibrium cases that are either above or below this equilibrium relationship. For a given negative net sediment flux, the shoreline retreat slows when the shoreface morphology steepens (i.e. increased profile concavity), and vice versa. Likewise, for a given positive net sediment flux, the shoreline advance slows when the shoreface morphology becomes gentle (i.e. decreased profile concavity), and vice versa. Interestingly, the higher the net sediment flux is, the greater the influence of shoreline changes on shoreface morphology. The short- and long-term processes governing the shoreface morphological changes are also discussed. This study demonstrates the importance of shoreface morphological concavity on the shoreline changes. Furthermore, the presented first-order model might have the potential to be used as a quick assessment for future decadal- to centennial-scale shoreline changes in coastal management and engineering activities.

Rapid shoreline progradation followed by vertical foredune building at Pedro Beach, southeastern Australia

AbstractAt Pedro Beach on the southeastern coast of Australia a series of foredune ridges provides an opportunity to explore the morphodynamic paradigm as it applies to coastal barrier systems using optically stimulated luminescence (OSL) dating, ground penetrating radar (GPR) and airborne LiDAR topography. A series of sandy dune‐capped ridges, increasing in height seawards, formed fromc. 7000 years ago toc. 3900 years ago. During this time the shoreline straightened as the embayment filled and accommodation space for Holocene sediments diminished. Calculation of Holocene sediment accumulation above mean sea level utilising airborne LiDAR topography shows a decline in average sediment supply over this time period coupled with a decrease in shoreline progradation rate from 1.2 m/yr to 0.38 m/yr. The average ridge ‘exposure lifetime’ during this period increases resulting in higher ridges as dune‐forming processes have longer to operate. Increasing exposure to wave and wind energy also appears to have resulted in higher ridges as the sheltering effect of marginal headlands was diminished. An inherited disequilibrium shoreface profile will drive onshore accumulation of sandy sediments forming a prograded barrier; however, if there is no longer ‘accommodation space’ for sediment, this will be an overriding factor causing the cessation of progradation, as occurredc. 3900 years ago at Pedro Beach. Excess sediment in the nearshore zone after 3900 years ago may have been moved northward to nourish downdrift beaches in the compartment. A high outer foredune has formed through vertical accretion after 500 years ago, evidenced by GPR subsurface structures and OSL ages, with a distinct period of vertical and lee slope accretion and dated to the period 1890–1930 AD. The increased dune sediment transport resulting in foredune building is attributed to recent human disturbance. © 2018 John Wiley & Sons, Ltd.

The Ichnological and Sedimentological Signature of a Late Paleozoic, Postglacial Marginal-Marine and Shallow-Marine, Tidally Influenced Setting: The Wudayhi Member of the Nuayyim Formation (Unayzah Group) in the Subsurface of Central and Eastern Saudi Arabia

Abstract Sedimentation in the Nuayyim Formation across subsurface central and eastern Saudi Arabia has been chiefly ascribed to continental sedimentary environments. However, a number of levels in this formation show evidence for marginal-marine and shallow-marine sedimentation. This paper documents trace-fossil faunas and sedimentological characteristics from these facies and presents a regional depositional model for the Wudayhi Member that confirms the presence of shallow-marine sedimentary environments. Marginal-marine and shallow-marine sedimentation in the Wudayhi Member includes estuarine deposits and upper-shoreface through lower-shoreface deposits. They are reflective of several transgressive–regressive (T-R) cycles in an overall shoaling-upwards, tidally influenced progradational succession that exhibit mixed-process (wave-, tide-, and fluvially-influenced) depositional environments. The succession displays an overall upward decrease in bioturbation intensity with elements of an impoverished proximal expression of the Cruziana Ichnofacies at the base in proximal- and distal- lower-shoreface deposits (Facies 5–6), passing into an impoverished expression of the Skolithos Ichnofacies in upper-shoreface deposits (Facies 4). Therein, Facies 5 records storm-influenced sedimentation and includes hummocky cross-stratification (HCS), combined-flow ripples, cross-lamination, and ichnofossils that alternate between storm-related and fair-weather assemblages. An overall reduction in both the diversity of ichnogenera and the intensity of burrowing across the shoreface profile, an apparent lack of middle-shoreface deposits, and the occurrence of storm-wave-generated structures interspersed with tidally generated structures in the center and southeast of the study area are interpreted as tidal modulation, suggesting macrotidal conditions at the time of sediment deposition. Estuarine sedimentation includes coal deposits (Facies 1), tidal flats (Facies 2), and fluvio-tidal deposits (Facies 3) that overlie the succession, and display a stressed, impoverished mixture of traces with facies-crossing elements of the Skolithos and Cruziana Ichnofacies in the north and northwest of the study area. Sedimentation took place on a broad and sandy, partially restricted shelf, chiefly influenced by tidal currents that was variably fluvially influenced. The interplay of the aforementioned processes results in a complex architecture that is traceable, based on the sedimentological and ichnological content. Colonization by bioturbating infauna exhibiting elements of an overall impoverished Skolithos and Cruziana Ichnofacies in the Wudayhi Member is interpreted to be related to sea-level rise, with regional transgression induced by the melting of glacier ice following the Late Carboniferous to Early Permian glaciation that affected southern Gondwana. Subsequently, as the Arabian Plate migrated northwards, isostatic rebound concomitant with increased tectonism associated with the Neotethys propagating into the Arabian Peninsula caused regional regression responsible for the progradational sequence presented herein in the Early to Middle Permian. Trace-fossil associations and ichnofacies presented in this paper are significant, because they provide evidence for marginal-marine and shallow-marine tide- and storm-influenced processes in the Unayzah Group during deposition of the Wudayhi Member of the Nuayyim Formation. Furthermore, it establishes a framework than can be linked to fully marine coeval deposits southeastern of the Arabian Peninsula into Oman.

Episodic river flooding events revealed by palynological assemblages in Jurassic deposits of the Brent Group, North Sea

Spore and pollen (sporomorph) assemblages from Middle Jurassic marine deposits of the Brent Group in the northern North Sea are investigated to assess temporal and spatial variations in vegetation and depositional processes. Four wells were sampled for palynology from the Penguins Cluster and the Don North East fields through the Rannoch Formation shoreface succession. Hyperpycnite deposits occur throughout, but are concentrated within the lower part of the section. These are expressed by sand-prone beds displaying waxing and waning current motifs, normally graded muddy beds and structureless mudstones. Hyperpycnal/hypopycnal deposits resulting from episodic river flooding represent important sedimentary features as they may be preserved below fair weather wave base in more offshore settings and potentially be the only record of the former presence of a nearby river mouth. The hyperpycnites typically contain abundant Botryoccocus spp., Amorphous Organic Matter (AOM) and hinterland sporomorph taxa with relatively few marine components compared to associated marine shoreface facies. Variations in palynofacies assemblages and Botryococcus spp. abundances indicate frequent river mouth avulsion. Ordination of samples using non-metric multidimensional scaling (NMDS) indicates that shoreface samples of the sampled wells are relatively distinct, but hyperpycnite samples are highly similar regardless of their sampled well. This suggests that depositional processes and spore/pollen sources (i.e. catchment zones) were similar among hyperpycnite events across different wells. Abundant bisaccate pollen, Botryococcus spp. and AOM within interpreted hyperpycnites suggest sediment mixing along the fluvial drainage path during flooding events. The terrestrial signature of hyperpycnite sporomorph assemblages demonstrates that underflows remained coherent as they descended the shoreface profile with little turbulent mixing with ambient marine waters. Sporomorph assemblages display few large changes through time suggesting vegetation on the adjacent coastal plain was relatively static through the studied interval.

Equilibrium shoreface profiles: A sediment transport approach

Large-scale coastal behaviour models use the shoreface profile of equilibrium as a fundamental morphological unit that is translated in space to simulate coastal response to, for example, sea level oscillations and variability in sediment supply. Despite a longstanding focus on the shoreface profile and its relevance to predicting coastal response to changing environmental conditions, the processes and dynamics involved in shoreface equilibrium are still not fully understood. Here, we apply a process-based empirical sediment transport model, combined with morphodynamic principles to provide new insight into equilibrium shoreface profile development and shape. The quantitative model is based on field measurements and shows that a balance between onshore sediment transport due to oscillatory wave motion, and offshore sediment transport due to gravity, produces profile shapes that are similar to natural shoreface profiles from the Danish North Sea coast and the southeast Australian coast. Model predictions of equilibrium profile response to the effects of climate change, such as rising sea level, or changing wave climates, are examined. Implementation of the model is straightforward; there is no tuning or calibration and computation times are short. It is therefore easily implemented with repeated iterations to manage uncertainty.

High-Resolution Stratigraphy of the lower portion of the Green River Formation at Raven Ridge and Red Wash Field, NE Uinta Basin, Utah, Colorado, USA: Facies and Stratigraphic Patterns in a High-Gradient, High-Energy Lacustrine System

Lacustrine strata of a portion of the Green River Formation studied in outcrops along Raven Ridge and in the subsurface around Red Wash Field in the northeastern Uinta Basin represent cyclic storm-dominated shoreface to deep lake deposition within a syntectonic embayment on the margin of Eocene Lake Uinta. The study interval consists of the lower Green River Formation, including the Douglas Creek, Garden Gulch, and lower Parachute Creek members, from the top of the underlying Wasatch to the Mahogany Zone of the Parachute Creek Member. Data consists of 11,900 ft (3627 m) of section measured at 23 locations across Raven Ridge, including 7,800 ft (2377 m) of hand-held gamma-ray scintillometer measurements, and over 500 wells in the greater Red Wash Field area, including pay zone analysis correlated to stratigraphy in the field. Facies analysis, as the basis for an integrated stratigraphic approach, reveals a seven-fold hierarchy of stratigraphic cycles ranging from two orders of large-scale cycles to five orders of progressively higher-frequency (smaller scale) shoreface-lake cycles across an 18-mile (29 km) long and 1900-ft (579 m) thick dip-oriented stratigraphic transect. This study recognized twenty-two facies grouped into eight facies tracts using Walther’s Law for two composite shoreface successions: one for a siliciclastic storm-graded shoreface profile that was dominant during times of regression and one for a carbonate profile that dominated in times of transgression. Two important regional facies trends across Raven Ridge include: 1) greater proportions of mudstone facies present in the southern, upper portion of the Green River Formation; and 2) significantly higher proportion of bioturbated sandstone in the northern, lower portion of the formation. The long-term 2nd-order transgression of Lake Uinta from base to top of the study interval results in an evolution from a low-gradient shoreline with marshes, ponds, and sand/mud flats to a high-gradient high-energy profile composed of spits and shorefaces that grew southward away from the emergent highlands. A composite storm-graded shelf profile shows how trough, hummocky, and swaley cross stratification type and amalgamation style change offshore proportionally to contain mud-dominated tempestites, erosional storm furrows, and oil shale. In the most offshore positions, diastasis cracks caused by differential loading are common. The lacustrine shoreface profile is compressed in the Green River Fm. in the study area with narrow facies tracts and large local gradient changes as a result of different responses to sediment supply. As a lake grows and shorelines migrate, the increase in accommodation is balanced, or in-phase, by a corresponding increase in sediment supply resulting in shoreface progradation keeping up with lake-level rise. As the shoreface stacks vertically during the rise, over steepening and failure of the profile generates gravity-flow sandstone facies. Little reworking of hummocky cross stratification high on the profile was observed, probably because wave power was limited by a shallow fair-weather wave-base. These differences also result in more symmetrical lacustrine shoreface cycles, with a large proportion of sediment partitioned into rise hemicycles, as opposed to the classical fall-asymmetric marine para-sequence which tends to have little to no strata preserved in the rise hemicycle along most of the shoreface profile. Landward-stepping lacustrine shoreface cycles are more common during the early rise portions of larger-scale 3rd-order megacycles for similar reasons. Strata at Raven Ridge support the concept that Eocene Lake Uinta was chemically stratified, or meromictic, at least during certain periods. The equable subtropical Eocene paleoclimate is interpreted to be the most important control on meromixis. Chemical stratification played a critical role in the development and preservation of organic matter, as evidenced by oil shale facies. The equable climate, however, might also have made the lake prone to thermal stratification. A paradox exists in the storm dominance of the lacustrine shorefaces and the coeval lake stratification: wave energy apparently was insufficient to break through the strong chemocline. Red Wash Field, directly downdip from the Raven Ridge outcrop belt, is an example of an oil field in a setting where the lake margin is not coincident with a structural feature: a “non-coincident” margin. Reservoirs mostly are present in the 6th-order aggradational shoreface cycles that are interpreted to have accumulated in the rise portions of 3rd-order megacycles. The best reservoir facies are trough, hummocky, and swaley cross-stratified sandstone deposited by storm processes and structureless sandstone probably derived from over steepening and failure of the shoreface during transgression. A petroleum accumulation model encompassing the Red Wash-Raven Ridge area proposes that lacustrine-sourced petroleum originated from an over-pressured mature cell in the Altamont-Bluebell field region. Oil migrated updip through leaky seals and became trapped in reservoirs within the non-coincident lake margin strata. An irregular shoreline configuration and compaction folds at Red Wash Field trapped petroleum. After reaching spill point at Red Wash Field, oil migrated farther updip to Raven Ridge and Asphalt Ridge, forming tar sand accumulations.

Rollover, drowning, and discontinuous retreat: Distinct modes of barrier response to sea‐level rise arising from a simple morphodynamic model

AbstractWe construct a simple morphodynamic model to investigate the long‐term dynamic evolution of a coastal barrier system experiencing sea‐level rise. Using a simplified barrier geometry, the model includes a dynamic shoreface profile that can be out of equilibrium and explicitly treats barrier sediment overwash as a flux. With barrier behavior primarily controlled by the maximum potential overwash flux and the rate of shoreface response, the modeled barrier system demonstrates four primary behaviors: height drowning, width drowning, constant landward retreat, and a periodic retreat. Height drowning occurs when overwash fluxes are insufficient to maintain the landward migration rate required to keep pace with sea‐level rise. On the other hand, width drowning occurs when the shoreface response rate is insufficient to maintain the barrier geometry during overwash‐driven landward migration. During periodic barrier retreat, the barrier experiences oscillating periods of rapid overwash followed by periods of relative stability as the shoreface resteepens. This periodic retreat, which occurs even with a constant sea‐level rise rate, arises when overwash rates and shoreface response rates are large and of similar magnitude. We explore the occurrence of these behaviors across a wide range of parameter values and find that in addition to the maximum overwash flux and the shoreface response rate, barrier response can be particularly sensitive to the sea‐level rise rate and back‐barrier lagoon slope. Overall, our findings contrast with previous research which has primarily associated complex barrier behavior with changes in external forcing such as sea‐level rise rate, sediment supply, or back‐barrier geometry.

SHOREWARD SAND TRANSPORT OUTSIDE THE SURFZONE, NORTHERN GOLD COAST, AUSTRALIA

To date, no suitable theoretical basis has been derived to predict with reliable accuracy the shoreward sand transport under waves in the deeper water outside the surf zone. This is important for understanding the rate of recovery of beaches after major storm erosion and, in some circumstances, to quantify net shoreward supply of sand to the shoreline from the active lower shore-face below the depth of storm erosion bar development. Even a relatively low rate of long term shoreward net supply may contribute to shoreline stability where it offsets a gradient in the longshore sand transport that would otherwise lead to recession. This paper outlines the results of analysis of a 41 year dataset of beach and nearshore profile surveys to quantify annual average rates of shoreward net sand transport in 6-20m water in an area where the profiles are not in equilibrium due to the existence of a residual river mouth ebb delta bar lobe. Additionally, an empirical adaptation of the sheet flow relationship of Ribberink and Al-Salem (1990) to provide for the effects of ripples has been derived from large wave flume data and correlates well with the measured Gold Coast transport rates. These have been applied to a new coastline modelling system developed as part of research into the long term evolution of Australia’s central east coast region in response to sea level change and longshore sand transport processes, which combines the one-line concept of shoreline profile translation within the zone of littoral sand transport with cross-shore profile evolution across the deeper shore-face profile below that zone. It demonstrates the importance of providing for both the shoreward supply from the continental shelf and the varying profile response time-scale across the shore-face in predicting shoreline evolution.

Can beach dune ridges of the Texas Gulf Coast preserve climate signals?

A study of the evolution of North Padre Island (southern Texas Gulf Coast) dunes was carried out using LIDAR topographic data, dune vibracores through the center of the dunes, and grab samples of shoreface sand at four locations along a cross-shore profile. Grain-size analyses of the vibracores show vertical variations in shoreface sand deposition over decimeter depth intervals. A dune ridge growth model is introduced that describes the dune vertical accretion rate as a function of island progradation and freshwater lens expansion. This model allows indirect dating of the dune core samples based on a known island progradation rate (1 m/year), and height and spacing of the dunes calculated from the topographic data. A sand provenance model is also proposed that links the sand deposition in the dunes with sand sourced from various depths along the shoreface profile, depending on storm activity. We present evidence linking the changes in storm-sand deposition in the dune cores with yearly climatic fluctuations in the Gulf of Mexico associated with landfalling tropical storm activity in the period from 1942- 1965. This record of storm-induced sand variability is negatively correlated with El Nino-Southern Oscillation (Pacific) sea surface temperature variability, and positively correlated with North Atlantic decadal sea surface temper- ature variability.

Tidal effects on the shoreface: Towards a conceptual framework

Tidal processes can have a significant impact on the sedimentological and ichnological character of wave-dominated shoreface deposits. As the influence of tides increases, the resulting shoreface successions begin to depart markedly from those postulated by the conventional, wave-dominated shoreface model, which was built upon essentially non-tidal shoreline settings. In shoreface settings subject to stronger tidal flux, tides can be manifest either directly or indirectly. Direct tidal effects refer to those characteristics imparted by tidal energy (e.g., tidal currents) per se, and are best expressed in offshore and lower shoreface positions. Key evidence of direct tidal control includes uniform sediment calibres from the upper shoreface to the offshore, and little or no mud preserved in the lower shoreface. Additionally, sands in the lower shoreface and offshore tend to be intensely bioturbated. Where primary stratification is preserved, it largely comprises current-generated structures. Such shoreface deposits are referred to herein as “tide-influenced shorefaces”, and are expected in settings with low storm-wave input coupled with strong tidal currents (e.g., straits). Indirect tidal influences are manifest by the lateral translation of wave zones across the shoreface profile owing to changes in water depth during the tidal cycle. This is best developed in macrotidal to megatidal settings. Indirect tidal influences are more pronounced in the upper and lower shoreface, and are recorded through the interbedding of sedimentary structures produced by shoaling waves, breakers and surf, swash–backwash, and surface runoff. The boundaries between shoreface subenvironments are correspondingly poorly defined. The foreshore in settings of elevated tidal range is also generally much thicker (typically 4 to 5m). Bioturbation tends to be patchy in distribution across the shoreface, and dominated by vertical structures. Such systems are defined as “tidally modulated shorefaces”.Using well-established sedimentological and ichnological criteria for recognizing wave-dominated (non-tidal) shorefaces — wherein sediment deposition is nearly wholly controlled by fair-weather wave and storm-wave processes — a conceptual model is developed for discriminating fair-weather (non-tidal) shorefaces, storm-influenced (non-tidal) shorefaces, and tidally influenced shorefaces. Five shoreface archetypes are defined: storm-affected, storm-influenced, storm-dominated, tide-influenced, and tidally modulated.

Tidally Modulated Shorefaces

Abstract Tidally modulated shorefaces (TMS) are wave-dominated, but differ from conventional shorefaces in that sediments deposited in water depths equivalent to the upper, middle, and lower shoreface (depending upon the tidal range) are regularly subjected to variable wave processes, including swash–backwash, breaking-wave and surf processes, and shoaling waves during the tidal cycle. In upper macrotidal and megatidal settings, it is also possible that these shoreface deposits are subaerially exposed during low tide. TMS exhibit the morphology, seaward decrease in sediment caliber, and dominance of wave-generated sedimentary structures consistent with beach–shoreface settings. However, the sedimentological and ichnological structures of deposits exposed in the laterally extensive intertidal zone are more akin to those of the upper and lower shoreface, and not the beach. Four major differences permit the ready differentiation of tidally modulated shoreface successions from conventional shorefaces. (1) Sedimentary structures generated by swash–backwash (plane beds), surf and breaking waves (current ripples and trough cross-beds), shoaling waves (oscillation ripples), and storm waves (hummocky and swaly cross-stratification) are interbedded with one another across the shoreface. (2) Ebb-oriented tidal currents and surface runoff during the falling tide and at low tide deposit offshore-directed current ripples and combined-flow ripples in sandy sediments, or trough cross beds in gravel-dominated sediments. (3) Ichnologically, TMS exhibit a reduction in both the diversity of ichnogenera and density of burrowing across the entire shoreface profile. However, the incipient-trace associations are most similar to the Skolithos Ichnofacies in the upper shoreface–equivalent zone, and to the Cruziana Ichnofacies in the lower shoreface–equivalent zone. (4) The sedimentological and ichnological criteria commonly employed to identify the middle shoreface are spread out across the upper and lower shoreface, making this subenvironment difficult to differentiate.

The Influence of Cryogenic Processes on the Erosional Arctic Shoreface

Coastal dynamics and shoreface relief in ice-free seas are a function of hydrodynamic interactions between the sea and bottom sediments. In the Arctic, additional, cryogenic factors such as permafrost and the action of sea ice influence coastal processes. The goal of our paper is to assess this influence, mainly on the profile shape. Mathematical analyses of the shape of 63 shoreface profiles from the Laptev, Beaufort, and Chukchi Seas were carried out. The shapes of Arctic shoreface profiles and those of ice-free seas are compared. We found that large ice and silt content in perennially frozen sediments composing Arctic coasts favor their erosion. Sea ice plays an important role in sediment transport on the shoreface and correspondingly changes shoreface relief significantly. Some effects of ice intensify coastal erosion considerably, but others play a protective role. The overall influence of cryogenic processes on Arctic coasts composed of loose sediments is seen in that the average rate of coastal retreat is larger than in the temperate environments, even though Arctic coasts are protected by a continuous ice cover most of the year. The shape of the shoreface profile in the Arctic does not differ from that in ice-free seas, and is satisfactorily described by the Bruun/Dean equilibrium profile equation. The explanation of this fact is that all changes of the profile shape, caused by cryogenic processes, are short lived and quickly eliminated by wave action.

TheAandmCoefficients in the Bruun/Dean Equilibrium Profile Equation Seen from the Arctic

Abstract The Bruun/Dean relation between water depth and distance from the shore with a constant profile shape factor is widely used to describe shoreface profiles in temperate environments. However, it has been shown that the sediment scale parameter (A) and the profile shape factor (m) are interrelated variables. An analysis of 63 Arctic erosional shoreface profiles shows that both coefficients are highly variable. Relative frequency of the average m value is only 16% by the class width 0.1. No other m value frequency exceeds 21%. Therefore, there is insufficient reason to use average m to characterize Arctic shoreface profile shape. The shape of each profile has a definite combination of A and m values. Coefficients A and m show a distinct inverse relationship, as in temperate climate. A dependence of m values on coastal sediment grain size is seen, and m decreases with increasing grain size. With constant m = 0.67, parameter A obtains a dimension unit m1/3. But A equals the water depth in meters 1 m f...

High-ResoIution Stratigraphy of the Green River Formation, NE Uinta Basin: Implications for Red Wash Reservoir Compartmentalization: ABSTRACT

An extensive subsurface data base from the Greater Red Wash area and outcrops 4-8 miles from main producing areas are used to construct a high-resolution correlation framework for the lowerpart of the Eocene lacustrine Green River Formation. An orthogonal grid of 29 cross-sections and 11 cores comprise the subsurface database. The outcrop database consists of 12 measured sections (7,500 ft) along an oblique depositional dip profile at Raven Ridge. Outcrop gamma ray profiles collected for 4600 ft of the measured sections allow for a good outcrop to subsurface tie and provide a calibration set for interpreting subsurface log patterns. The correlation framework is based on the identification of a hierarchical set (4 scales) of base-level cycles (changes in accommodation to sediment supply ratio). Cycle stacking patterns (lakeward-stepping, vertically-stacked and landward-stepping) provide the chronostratigraphic framework to properly evaluate depositional facies. For mapping reservoir zones, medium-scale maximum flooding surfaces (base-level rise-to-fall turnarounds) are the most useful correlation surfaces. To better understand depositional facies and heterogeneity within cycles or reservoir zones, base-level fall-to-rise surfaces are correlated in outcrop sections. This allows for the evaluation of sediment volume partitioning and facies differentiation between base-level fall and base-level rise parts of cycles at all scales.more » Facies indicate a steep-gradient (high-subsidence), high-energy, storm-dominated, lacustrine shoreface profile. Best reservoirs are stacked shoreface cycles (25-35 ft thick) and occur during long-term base-level fall. Reservoir facies are restricted to upper and middle shoreface, through and hummocky cross-stratified sandstones. Another important reservoir facies is massive dispersed-pebble sandstone interpreted as high-density sediment gravity flows.« less

Modelling shoreface profile evolution

Current knowledge of hydro-, sediment and morpho-dynamics in the shoreface environment is insufficient to undertake shoreface-profile evolution modelling on the basis of first physical principles. We propose a simple, panel-type model to map observed behaviour. The internal dynamics are determined by slope-dependent, wave-induced cross-shoreface transports, while the external driving factors are lateral sediment supply and sea-level rise. This model concept is tested with reasonable success against the observed behaviour of the Central Holland Coast, considering two hindcast periods, one covering the evolution over the last century, the other the Subboreal/Subatlantic evolution.A limitation of this model is that the cross-shoreface dynamics are solely steered by the variations of shoaling, short waves. Since a variety of other wave and current dynamics may be expected to be present in the coastal boundary layer, it may well be that the effects of the mechanisms and conditions which are not represented are hidden in the coefficients of the sediment-transport formula. This limits the accuracy of the coefficients as used, and our findings should be considered as an-order-of-magnitude estimate only. Indeed, behaviour-oriented modelling implies that generalization of results to arbitrary situations and conditions is not straightforward. Yet, we expect that some of the conclusions are more generally applicable.This concerns the substantiation of the assumption that the upper shoreface responds on a much smaller time scale than the lower shoreface, and the idea that the shoreface profile is not always and everywhere in equilibrium with its forcing. A worthwhile observation from the Holland Coast application is, that the bottom slope effect on the transport is only important at geological time scales. The profile evolution at the engineering time scales (say 10 to 100 years) is effectively quasi-static, in that there is no feedback between the long-term averaged transport and the state of the profile. This implies that at these smaller scales the profile changes can be predicted on the basis of a static sediment balance. This does not mean that the gravitational downslope transport is unimportant as a physical phenomenon in coastal profile evolution: It is only unimportant if a highly aggregated model like this is applied at relatively short time scales.

Facies architecture and sequentiality of nearshore and ‘shelf’sandbodies; Haystack Mountains Formation, Wyoming, USA

ABSTRACTThe Haystack Mountains Formation (Campanian, Mesaverde Group, US Western Interior Basin, Wyoming) contains a series of shallow‐marine sandbodies, extending tens of kilometres out from a basin margin. The study succession (around 200 m thick) is composed of eight major sandstone tongues (Bolten Ranch, O'Brien Spring, Seminoe 1–2–3–4, Hatfield 1 and 2 members), each partially encased within marine shale intervals. The Formation is ‘sequential’at several scales. At the largest scale, the whole succession presents an aggradational to basinward‐stepping stacking pattern of the sandstone tongues. At a lower level, each tongue (member) is characterized internally by two different types of lithosome: the first represents shoreface progradation with hummocky cross‐strata passing up to swaley and trough cross‐stratified sandstones. This lithosome is erosively truncated at its top in most cases, and has a general sheet‐like geometry along strike, whereas down dip it displays a series of sharp‐bounded clinothems. The latter sometimes indicate a downward as well as a basinward shift through time, as suggested by the occurrence of coarser and/or shallower facies at a lower level in the shoreface profile. The second type of lithosome is sheet‐ or wedge‐like and sharply overlies the shoreface deposits. The lithosome consists of laterally widespread units of planar tabular to trough cross‐bedded medium sandstones passing laterally (in a dip direction) into bioturbated sandstones. The lower part of this lithosome is progradational, becoming retrogradational into the overlying shales. The facies within the cross‐bedded lithosome suggest a tidally dominated delta front to estuarine depositional setting.The two types of lithosome are not related genetically. The erosion surface separating the two lithosomes is a sequence boundary separating forced‐regressive (relative sea‐level fall) shoreface deposits from lowstand to transgressive (early relative sea‐level rise), cross‐bedded deposits. The uppermost part of the cross‐stratified lithosome shows a landward‐stepping of component parasequences and is abruptly blanketed by open‐marine shales.The most widespread cross‐bedded lithosomes are apparently best developed in the lowermost members of the Haystack Mountains Formation, i.e. in the aggradational part of the large‐scale progradational succession. In the uppermost, highly progradational sandstone tongues, the shoaling‐upward shoreface lithosome dominates, whereas the cross‐bedded lithosome occurs in narrow, lensoid belts, or is absent. The middle portion of the succession shows intermediate characteristics. The vertical variation in geometry, thickness and progradational extent of successive cross‐bedded lithosomes results from greater confinement of the incised nearshore systems both in space (landward direction) and in time (from the aggradation to the progradation architecture). The latter is a consequence of a decreasing rate of accommodation creation through time.

Geology of the Wrightsville Beach, North Carolina shoreface: Implications for the concept of shoreface profile of equilibrium

Nearly 300 km of 3.5 kHz subbottom profile and 100 kHz sidescan-sonar data, a suite of over 100 short (~2 m) percussion cores and vibracores have been collected on the shoreface and inner continental shelf off Wrightsville Beach, North Carolina. Sidescan-sonar images were analyzed for acoustic backscatter to delineate the surface sediment distribution. Groundtruth data for the sidescan-sonar interpretations were provided by surface grab samples.Cross-shore sediment transport by combined waves and currents is the predominant sedimentologic signature on this shoreface. The shoreface is dominated by a shore-normal system of rippled scour depressions that begin in 3–4 m water depth and extend to the base of the shoreface about 1 km offshore, at 10 m depth. The depressions are 40–100 m wide, and up to 1 m deep. They are floored by coarse, rippled shell hash and gravel; some are separated by rock-underlain fine sand ridges. On the inner shelf, the bathymetric and sedimentary fabrics become shore-oblique, due to a series of relict ridges with 1–2 m of relief. The ridges are coarse on their landward sides and covered on their seaward flanks by thin veneers of fine sand.Field evidence from the Wrightsville Beach shoreface demonstrates that a shoreface equilibrium profile as defined by Dean (1991) and others does not exist here. For example: (1) the grain size varies widely and inconsistently over the profile; (2) shoreface profile shape is controlled predominantly by underlying geology, including Tertiary limestone outcrops and Oligocene silts; and (3) sediment transport patterns cannot be explained by simple diffusion due to wave energy gradients, and that transport occurs seaward of the assumed engineering “closure depth” of 8.5 m. This has several implications for the application of equilibrium profile-based numerical models used to investigate coastal processes and design coastal engineering projects at Wrightsville Beach. The most important practical implication is that a number of assumptions required by existing analytical and numerical models (e.g., Dean, 1991; genesis; sbeach) used for the design of shore protection projects and large-scale coastal modeling over decadal time scales cannot be met.

Sequential Development of an Valley Fill: the Twowells Tongue of the Dakota Sandstone, Acoma Basin, New Mexico

ABSTRACT The late Cenomanian Twowells Tongue of the Dakota Sandstone in the Acoma Basin (northwest New Mexico, USA) is a basinwide sandbody, 30 m thick, deposited during a period of relative sea-level fall and subsequent rise. For most of its extent, the Twowells Tongue consists of two markedly different sandstone units: shoreface and estuarine, respectively. The shoreface sediments were deposited during a period of relative sea-level highstand. The shoreface profile is incomplete, having its uppermost part (upper shoreface and foreshore) removed during subaerial valley incision. In places, the lower shoreface deposits are overlain by paleosols. The erosional surface is of regional extent; it depicts a complex valley morphology, in places 30 m deep, and is locally overlain by pebbles and shell lags. The valley, created during a relative sea-level lowstand, was filled by the second sandbody, consisting of a thick package (20-35 m) of transgressive, cross-bedded medium sandstones probably deposited in a middle to outer estuarine setting. The transgressive deposits of the Twowells Tongue are capped by a horizon of Pycnodonte oysters underlying black shales interpreted to represent abrupt marine deepening. The cross-bedded unit, sharply overlying shales or lower shoreface sediments, resembles other sandbodies of the Cretaceous Western Interior Seaway, traditionally interpreted as offshore ridges, or more recently as lowstand shorelines. However, the investigations in the Acoma Basin show that both the shelf-ridge and the lowstand shoreline models are inadequate explanations, because the cross-bedded lithosome of the Twowells Tongue has the internal characteristics of tidally dominated estuarine deposits.

Coastal retreat and shoreface profile variations in the Canadian Beaufort Sea

The coastline of the southern Canadian Beaufort Sea consists primarily of unconsolidated bluffs. Although the sea is ice-free for 3 months of the year and wave energy is restricted by pack ice, the coast is undergoing regional retreat with erosion rates as high as 10 m a −1 in some locations. Simple and multiple regression analyses were carried out to determine the degree of correlation between the mean retreat rate measured at various locations and the different parameters that may control shoreline recession. Sediment texture, ground-ice content, cliff height, wave energy and shoreface gradient revealed medium to poor correlation with erosion rates, showing that the recessive evolution of the coastline can not be explained solely by wave-induced and subaerial processes. The comparison of nearshore echo-sounding records from 1987 with bathymetry from 1971 showed substantial erosion (up to 1 m) of the submarine profile between 12 and 15 m of water. There is strong evidence that this erosion has been caused by sea ice gouging on the seafloor. From depths of 5 to 9 m, accretion has taken place, possibly induced by ice-push processes, and inshore of the 5 m isobath wave and current erosion of the shoreface has occurred. These results suggest that the erosion of the inner shelf by ice gouging drives the erosion observed inshore on the coastal bluffs and nearshore zone as the shoreface profile strives for a state of dynamic equilibrium.

The Importance of Sea-Level Fluctuations in the Formation of Linear Conglomerate Bodies; Carrot Creek Member of Cardium Formation, Cretaceous Western Interior Seaway, Alberta, Canada

ABSTRACT The Raven River Member of the Cardium Formation (Turonian, Upper Cretaceous) in the Carrot Creek area of Alberta contains two coarsening-upward sequences of marine mudstones and sandstones. The lower sequence coarsens from black mudstones into bioturbated, fine-grained sandstones, and is capped by a gritty siderite horizon which probably indicates a pause in deposition. The sandstones of the upper sequence contain hummocky cross stratification, suggesting open-marine deposition below fairweather wave base. The two sequences are scoured to variable depths by a major erosion surface which has a relative relief of up to 20 m. A structure map and three-dimensional mesh diagrams of this surface suggest four topographic elements--a relatively high terrace, a bevel at which und rlying beds are truncated, and a series of bumps and hollows. These are interpreted to represent a remnant erosional topography, which gradually fades away into the final topographic element, the smooth basin plain. The erosion surface is covered with conglomerate, assigned to the Carrot Creek Member of the Cardium. The conglomerate thickness reaches about 20 m against the bevel and in some of the hollows, but usually thins to a veneer on the terrace, on top of the bumps, and on the distal basin floor. The conglomerate bodies are relatively long and narrow and trend northwest-southeast. They are overlain by marine mudstones of the Dismal Rat Member. The elongate conglomerate bodies appear at first sight to be another case of Western Interior Seaway offshore bars encased in marine mudstones many tens of kilometers seaward of time-equivalent shoreline deposits. However, the erosion surface indicates that the conglomerates are not genetically part of the underlying coarsening-upward sequence. We propose a rapid relative lowering of sea level following deposition of the hummocky cross-stratified sandstones. The shoreline advanced many kilometers northeastward into the basin, and during stillstand, wave erosion cut a new shoreface profile (the bevel). During this lowstand, incised rivers supplied gravel to the shoreface. Waves worked the gravel alongshore against the bevel, and into the erosional hollows. Subsequent transgres ion spread some gravel southwestward back across the terrace. Storms transported gravel stringers into the transgressive muds accumulating in the hollows. Other erosional surfaces of this type exist in Alberta at the Cardium, Viking, and Bad Heart horizons, and in New Mexico at the Gallup-Tocito horizon, suggesting that sea-level changes and shoreface incision may be an alternative explanation for other long narrow offshore bars encased in marine shales.