Recognition of cross-shore dynamics of longshore bars in upper-shoreface deposits of prograding sandy coastal barriers

The cross-shore migration of longshore bars on sandy coasts typically displays a cyclic behavior characteristic of a specific wave climate (e.g., Davidson-Arnott 2022). This study aims to establish connections between these behavioral variations and the sedimentary architecture of sandy prograding coastal barriers as revealed in cross-sectional ground-penetrating radar (GPR) data of subrecent coastal barrier deposits (Table 1). In these radargrams, a prominent feature often observed is large-scale seaward-inclined low-angle stratification (SIS), which encompasses the beach and upper shoreface (Fig. 1). A secondary objective is to identify evidence of SIS in ancient deposits of prograding coastal barriers, both in outcrop and in core.

Here, a (coastal) barrier is defined as proposed by Dillenburg and Hesp (2009) as a shore-parallel structure, formed by an accumulation of sand, gravel, shells, and small amounts of organic material due to the action of waves, tides, and winds. This definition has the advantage that it includes strandplains and therefore, when applied to ancient deposits, does not require evidence of backbarrier lagoons or marshes, which may be lacking due to erosion. The upper shoreface is defined according to Hamon-Kerivel et al. (2020) as the zone seaward of the beach (mean low-water line), characterized by significant morphologic variability over seasonal to annual timescales, in contrast to the adjacent lower shoreface. This definition acknowledges the importance of longshore bars and rip channels in the formation of sedimentary structures, as well as the erosional processes shaping the lower boundary of the upper shoreface. Given the steep nature of the beach face in gravelly barriers, which lack well-developed longshore bars (e.g., Wright and Short 1984; Davidson-Arnott 2022), this analysis focuses specifically on coastal barriers composed of sand, with, at most, some admixture of gravel.

SIS typically exhibits decimeter-spaced surfaces. While these surfaces generally appear smooth, it is not uncommon for individual low-angle surfaces to display irregular undulations and occasional erosion, sometimes even eroding into each other (e.g., Fig. 1). In many publications this stratification is labeled as clinoforms, representing timelines of the progradation of the coastal barrier (Fitzgerald et al. 1992; Smith et al. 1999; Goslin and Clemmensen 2017; Oliver et al. 2017; Van Heteren et al. 2018; Berton et al. 2019; Dougherty et al. 2019) because it resembles the coastal time-averaged equilibrium profile.

A well-documented case is the prograding barrier of Long Beach Peninsula, Washington, USA, located north of the mouth of the Columbia River, along the Pacific coast (Figs. 1, 2; Smith et al. 1999; Jol et al. 2002; Moore et al. 2004). At this location, information of GPR profiles of deposits less than a century old can be compared with morphodynamic data of the current upper shoreface (Cohn and Ruggiero 2016; Ruggiero et al. 2016). In the beach zone the presence of seaward-inclined low-angle surfaces is attributed to the well-known flat erosion profiles created during storms on exposed sandy coasts (e.g., Doeglas 1954; Hayes and Boothroyd 1969; Van den Berg 1977; Dougherty 2014). Notably, Moore et al. (2004) identified annual layering in GPR images of the beach and uppermost upper shoreface of a Long Beach Peninsula cross section, that they attributed to erosion during winter storm periods. However, the series of GPR tests performed by Jol et al. (2002) indicated that the SIS can be followed to much lower levels, down to 3–5 m in the upper shoreface. The radar information effectively covers most of the upper shoreface, which in the adjacent coasts contains 1–3 longshore bars with troughs extending to depths between 6.8 meters (Ruggiero et al. 2016) and 8 m (Cohn and Ruggiero 2016). The 1–3° seaward-dipping GPR reflectors in this zone cannot be attributed to erosion surfaces from storm events, as in the beach, simply because measurements over many years indicate that the longshore bars are permanent features that are not washed away during storms (Cohn and Ruggiero 2016; Ruggiero et al. 2016).

It is remarkable that longshore bars are not mentioned in many sedimentological studies focusing on the sedimentary architecture of barrier shorefaces (e.g., Olsen et al. 1999; Rodriguez et al. 2001; Fanget et al. 2014). Some barrier models disregard their existence (Reinson 1984; Oertel and Leatherman 1985; Goslin and Clemmensen 2017), despite their significant role in shaping the upper shoreface through interactions with flow and bedforms. To find the shape of the base of reworking by morphodynamic processes, Schwartz and Birkemeier (2004) and Tamura et al. (2007) considered the erosional depth reached by longshore bar troughs obtained from a series of measurements of coastal profiles from a nearby site. This approach can provide valuable insights into intra-shoreface erosional boundaries, particularly those resulting from rapid sea-level fluctuations (Tamura et al. 2007). However, while these bumpy surfaces of maximum erosion may resemble the equilibrium profile of the upper shoreface to some extent, they are composed of distinct erosional events and therefore cannot account for the continuity of SIS observed in many GPR radargrams.

In the past decades it has become clear that longshore bars along many barrier coastlines exist either as a permanent feature along coasts of marginal seas or lakes (Ruessink and Kroon 1994; Wijnberg and Terwindt 1995; Kuriyama 2002; Aagaard et al. 2010; Aleman et al. 2013; Ruggiero et al. 2016; Yuhi et al. 2016) or as temporal alternations with a non-barred shoreface along oceanic coasts (Shepard 1950; Birkemeier 1984; Kuriyama 2002; Tabajara et al. 2008; Short 2020; Ruiz de Alegría-Arzaburu et al. 2022). In most studies of permanent bars along coastal barriers, the longshore bars exhibit a cyclic behavior of seaward migration. This is known as net offshore migration, first described for the Dutch barrier by Ruessink and Kroon (1994) and Wijnberg (1995) and abbreviated as NOM (e.g., Shand et al. 1999; Aleman et al. 2013). Conversely, along many oceanic coasts where barred and non-barred conditions alternate over periods of months to years, bars tend to show a net landward movement (Shepard 1950; Komar 1967; Short 2020; Ruiz de Alegría-Arzaburu et al. 2022), here abbreviated as NLM. This paper will demonstrate that SIS observed in GPR cross sections of upper-shoreface deposits can convincingly be explained by NOM behavior.

So far, the behavior of longshore bars has been described separately for different wave climates. In this paper, we aim to integrate this knowledge with structures observed in radargrams. To accomplish this, first the available understanding of cross-shore dynamics of longshore bars is briefly summarized. Finally, the paper addresses how SIS and associated structures are possibly expressed in outcrop and core, highlighting the need for establishing diagnostic criteria for interpretation.

Superimposed on the concave-upward equilibrium profile of a sandy upper shoreface (Bruun 1954; Dean 1990; Hamon-Kervel et al. 2020; Davidson-Arnott 2022), generally a system of a single, non-permanent longshore bar, or a permanent multibar bar system, is found. When morphodynamics are not influenced by sand-supply-limited conditions, the presence and dynamics of these bars in sandy shorefaces of coastal barriers is determined by the wave climate. Three morphodynamic systems can be distinguished:

These systems of single, and sometimes double, upper-shoreface bars, characterized by fair-weather long, deep-reaching swell and occasional storm wave conditions, are found along oceanic coasts (e.g., Shepard 1950; Birkemeier 1984; Tabajara et al. 2008; Vidal-Ruiz et al. 2017; Ruiz de Alegría-Arzaburu et al. 2022). The presence of the bars ranges between seldom and of short duration to almost permanent. Their dynamics were first described by Shepard (1950) on the Pacific coast of the USA. During (severe) storm wave events, sand eroded from the beach accumulates in mostly single longshore bars (Shepard 1950; Komar 1967, Silvester 1974). The bar exhibits a NLM behavior and eventually welds on the beach recovering the storm sand loss of the beach and resulting in a temporary non-barred shoreface. During this process, the beach slope increases, or, in terms of the widely used classification scheme of Wright and Short (1984), passes from dissipative to reflective. The occurrence of the opposite process, the destruction of longshore bars during extreme storms, suggested by some researchers (e.g., Bristow and Pucillo 2006; Santos et al. 2022) is unlikely, as it contradicts this mechanism and has never been observed in nature for the sand-rich systems considered here.

The upper-shoreface bar that is formed in non-permanent systems during storm wave events is built from sand eroded from the beach until, at some point, so much wave energy is dissipated by waves breaking on the bar that beach erosion is retarded. Thus, the bar serves as a natural protection against beach erosion during a severe storm or series of storms, as long as the beach contains enough sand to feed the bar (Silvester 1974; Vousdoukas et al. 2012). If upper-shoreface bars are not formed, the coast will remain unprotected and storm erosion of the beach continues. Therefore the existence of prograding barrier deposits representing non-barred coasts, as suggested in reviews (Galloway and Hobday 1996; Clifton 2006; Tamura 2012; Howell 2020) and outcrop studies (Johnson 1975; Isla et al. 2020a, 2020b) seems incompatible with an assumed presence of storm waves. Isla et al. (2020a) argue that: 1) a very high sand supply rate can make the coastal profile so steep that bars are not formed and 2) longshore bars, if present, are destroyed during exceptional storm events. However, both processes have never been documented and their occurrence is theoretically unlikely: The supposed steepening is precluded by the very short time frame of adaptation of the beach to upper-shoreface profile to changing conditions, which is in the order of years or even less (Birkemeier 1984; Cowell and Thom 1995; Anthony and Aagaard 2020), following Bruun’s equilibrium rule of invariance of this profile relative to the mean sea level (Bruun 1954, 1962). As argued earlier in this paper, severe storms lead to activation of longshore bars instead of their destruction.

Existing facies models of non-barred coastal barrier coasts are based on descriptions by Clifton et al. (1971) and Howard and Reineck (1981). These studies were conducted in a pocket beach and on a location with a narrow shoreface adjacent to a steep offshore ramp. Both sites represent sand-supply-limited conditions, meaning that during severe storms not enough loose sand is available to build a temporary longshore bar. In the case of the south California site (Howard and Reineck 1981), beaches are retained by artificial structures. Most of the sand that is transported alongshore is trapped in the heads of submarine canyons (Griggs et al. 2020) and eventually flushed down canyon by turbidity currents, possibly during breach-failure events as described for a similar canyon along the California coast (Mastbergen and Van den Berg 2003). An indication that supply-limited conditions also apply to the location at the Oregon site comes from a remark that the study area is non-barred “at least within the area of observation” (Clifton et al. 1971, p. 652). Indeed, along the same Oregon coast, in the same wave climate, some kilometers south of the non-barred example, several, possibly non-permanent, barred sites were described by Hunter et al. (1979). They mention that bars are best developed along long, straight, sandy stretches of the Oregon coast. This additional evidence strengthens the hypothesis that the permanent absence of bars in examples studied by Clifton et al. (1971) and Howard and Reineck (1981) is due to sand supply-limited conditions. Theoretically, in a wave climate dominated by long swell and a complete absence of storm waves, bars would never form and a non-barred barrier shoreface might occur. Nevertheless, in his insightful review on the morphodynamics of nearshore bars, Davidson-Arnott (2022, and personal communication 2023) concluded, on the basis of Google Earth images and his diving experience at many locations in the Caribbean, the presence of non-barred shoreface systems along low-latitude barrier coasts dominated by swell waves and light trade winds, and occasional frequented by storms. For instance, he highlights Manzanilla Beach at the northern tip of the Cocos Bay barrier on the east coast of Trinidad as an illustrative example. Based on soundings, Ibrahim (2005) and Darsan et al. (2012) present different observation from the Caribbean region, indicating that during storms, temporary longshore bars do form on the shoreface, including within Cocos Bay. The apparent contradiction with Davidson-Arnott’s non-barred observations may be explained by a quick disappearance of storm-generated bars on these coasts due to rapid landward migration and welding on the beach, caused by strong swell.

So far, in the available literature, conclusive evidence of (almost) permanently non-barred barrier beaches seems to be restricted to the Bay of Aigues Mortes, NW Mediterranean Sea, an area sheltered by a spit (Aleman et al. 2015), where wave refraction produces low steepness waves leading to a dominance of onshore sediment transport, generally impeding the formation of a bar during storm wave events (Davidson-Arnott 2022).

In the case of a high storm frequency along oceanic coasts, such as the example north of the mouth of the Columbia River (Fig. 2), time in between storms is insufficient for the removal of upper-shoreface bars (e.g., Wright and Short 1984). Here, and in marginal seas and lakes where long, deep-reaching swell is absent, longshore bars are permanent features and usually more than one bar is present showing a NOM behavior (e.g., Ruessink and Kroon 1994; Wijnberg and Terwindt 1995; Ruggiero et al. 2016). This behavior involves a dynamic interplay characterized by the swift seaward movement of longshore bars during periods of storm waves, juxtaposed with a slower landward migration during extended periods of fair-weather wave conditions. Three distinct phases in the lifecycle of bars are distinguished: their emergence in shallow waters near the beach, subsequent growth and migration seaward, and eventual decline and disappearance at the transition to the lower shoreface (Fig. 3). This process has been described in detail for ocean coasts (Kuriyama 2002; Cohn and Ruggiero 2016; Ruggiero 2016) and sites of marginal seas (Ruessink and Kroon 1994; Wijnberg and Terwindt 1995; Van Rijn et al. 2002; Aagaard et al. 2010; Walstra et al. 2011; Aleman et al. 2013 2015, 2017; Walstra 2016; Yuhi et al. 2016; Walstra and Ruessink 2017; Yuhi and Uneda 2018). During this process, the inner bar displaces the outer bar, while a new inner bar forms in shallow water. The lifespan of a NOM cycle varies from 1 to 22 years, with migration rates reaching up to 70 m per year (Shand and Bailey 1999; Kuriyama 2002; Aagaard et al. 2010). Notably, along the Gold Coast in southeast Queensland, Australia, a unique bar system exhibits a balance between seaward and landward migration, with non-barred conditions encountered only exceptionally (Ruessink et al. 2009).

Interestingly the NOM of shoreface bars was extensively investigated at two locations near the GPR profile depicted in Figure 1, enabling a direct comparison between current cross-shore dynamics of longshore bars and preserved sedimentary structures derived from GPR data. The short-term morphodynamic changes were simulated by Cohn and Ruggiero (2016) for one of these coastal sites using detailed information obtained from two cross-shore profiles at Long Beach Peninsula (Cohn and Ruggiero 2016; Ruggiero 2016) indicated in Figure 2. Figure 4 illustrates the NOM dynamics of the longshore bars in this shoreface. Due to the high frequency of storms at this location, the NOM cycle exhibits a short duration, with a lifespan of 2–6 years. To enhance visualization of the evolution of low-angle erosional surfaces related to the bar–trough system, in Figure 4 a seaward migration rate of up to 40 meters per annum is adopted. This is approximately three times higher than the average seaward accretion rate of the deposits observed in the radargram, as inferred from 19th-century maps (Jol et al. 2002; Fig. 2), and can thus be considered a periodic maximum. It is plausible that along these erosional surfaces, heavy minerals such as magnetite accumulated, forming lag deposits that served as the primary reflectors of the GPR signals (Moore et al. 2004; Smith et al. 1999). The trough landward of the innermost bar starts at a depth of about 2 m below sea level. Unfortunately, the upper part of the shoreface was not included in the profile analysis. Thus, the initiation of the innermost bar is missing. Above this level, the well-known cycle of flat erosional storm beach succeeded by sand recovery through one or more landward migrating ridges or slip-face bars (cf. Masselink et al. 2006) is found. The GPR low-angle strata therefore have at least two origins: in the beach region, the reflectors refer to storm erosion, while in the shoreface the NOM of the longshore bars explains the seaward-dipping GPR reflectors. An important difference between the two erosion surfaces is that it is synchronous in the beach zone, representing a single storm event, while the offshore-facing low-angle strata in the upper shoreface are diachronous, with a duration equal to the lifespan of a longshore bar, emerging near the beach and ending at the transition to the lower shoreface. In some cases, the picture is further complicated by foreshore runnels that scour deeper than the flat storm profile (Van den Berg 1977). During severe onshore gales due to storm surge, coastal erosion may be concentrated in the dunes and backshore and may be minor or even absent on the lower parts of the beach (Van Rijn 2009). In such cases, the low-angle reflectors of the lower part of the beach represent the scour of a landward-migrating runnel (Fig. 5).

Figure 1 illustrates the longshore trough origin of the SIS in the radargram of Long Beach Peninsula: the concave-upward slope of the bar-trough erosional surface depicted in Figure 4 closely aligns with the GPR strata. The only discrepancy lies in the exclusion of irregularities present in the GPR low-angle strata, which result from irregular depth variations in the bar troughs due to periodic changes in wave conditions not accounted for in the analysis of bar dynamics.

The overall shape of SIS surfaces depicted in the publications listed in Table 1 is concave-upward to sigmoidal. As the SIS approaches the base of the upper shoreface, its slope gradually decreases, eventually becoming nearly horizontal (Van Heteren et al. 2018; Jol et al. 2002; Moore et al. 2004; Bristow and Pucillo 2006; Rodriguez and Meyer 2006; Hede et al. 2015; Dougherty et al. 2018; Da Rocha et al. 2017; Oliver et al. 2017; Santos et al. 2022). In the case of the Long Beach Peninsula, the GPR reflectors are attenuated as they approach a horizontal slope at a depth of about 6 meters, indicating that the base of the upper shoreface is not much deeper but lies beyond the range of the radar signal. This is supported by morphodynamic studies conducted at the site closest to the location of the radargram (marked 2 in Fig. 2), where the base of the deepest trough (equivalent to the base of the upper shoreface) was measured at 6.8 meters below mean sea level (MSL). At this depth, the seaward-migrating bar troughs erode into lower-shoreface deposits.

It is worth emphasizing that since the NOM-related low-angle bedding is produced by the migration of longshore-bar troughs it is limited to the upper shoreface. Note that this is often a minor part of the total thickness of the shoreface as defined by lithological change (Reineck and Singh 1973). For instance, in the case of the Long Beach Peninsula, well logs indicate that the base of the shoreface is found at depths ranging from 17 to 25 meters (Smith et al. 1999).

During prolonged fair-weather conditions the longshore bar of a non-permanent bar system may weld onto the beach. This process was documented in detail from a large series of cross-shore profile measurements at the field research facility near Duck, located on a coastal barrier facing the Atlantic Ocean (South Carolina, USA). The trough in front of the bar produces a lag deposit that reaches deeper than the flat beach profile immediately after a severe storm event, despite the large amount of sand being eroded from the beach (Schwartz and Birkemeier 2004). The envelope of the profiles or sweepzone of 155 surveys of a cross-shore profile collected between 1981 and 1984 is presented in Figure 6. Note that this envelope represents a stable coastline. In case of progradation, where series of low beach ridges are formed instead of a steep foredune (Hesp 2011), the backshore part of the curve would have a gentler slope. Therefore, the SIS shape in GPR sections of this barrier would resemble that of barriers with permanently barred shorefaces. This observation is corroborated by all radargrams of barriers with supposed non-permanent bar systems listed in Table 1. The large-scale low-angle strata of such barriers are also diachronous, except for the backshore. However, unlike barriers with permanently barred shorefaces, the seaward-most part of the SIS is deposited first in coastal barriers of non-permanent longshore-bar systems. It is worth noting that according to the widely used definition of a clinoform as a “frozen” paleo-bathymetric profile (Patruno and Helland-Hansen 2018), the partly diachronous SIS of sandy coastal barriers does not strictly meet this criterion.

In the upper-shoreface zone of certain GPR sections, located between the SIS, units of relatively short, low-angle strata are observed, exhibiting opposing, landward-directed slopes (Hede et al. 2015; Da Rocha et al. 2017; Oliver et al. 2017; Van Heteren et al. 2018; Santos et al. 2022). These strata are interpreted as representing the landward-facing slope of landward-migrating bars (Hede et al. 2015; Santos et al. 2022). In some radargrams of barriers with non-permanent longshore bar systems—and NLM behavior—they are overwhelmingly present (no. 4, 9, 10, and 12 in Table 1). Figure 7 shows an example of this from one of the barrier systems located in southeastern Australia studied by Oliver et al. (2017). The units with strata that climb and erode SIS in these cases were formed both on the upper shoreface and on the beach. On the shoreface they generally exhibit a somewhat steeper slope. To obtain the properties of a reflector, the landward inclined strata possibly represent erosion lags, as with SIS. Regarding the beach, it is speculated that these lags are formed by smoothing of the steep landward-facing slope of slip-face bars during minor storm events (Van Houwelingen 2008). In the case of the Australian examples (nos. 9 and 10 in Table 1), slip-face bars that emerge after a storm can reach a height of several decimeters (Thom Oliver, personal communication 2023), which aligns with the magnitude recorded in the radargram. With the return of fair-weather wave conditions, the ridges regain their steepness and continue their landward migration. In the upper shoreface, the low-angle landward directed strata have a steeper slope, because the bars retain a steep landward slope during their seaward movement, in both non-permanent (Birkemeier 1984) and permanent (Van Rijn 2002) bar systems. Figure 8 illustrates the change in bar shape after a series of storms for a profile across the Dutch coastal barrier shoreface. Successive low-angle strata are generated by alternating fair-weather and storm-wave conditions. This phenomenon is well documented in studies with high-time-resolution observations of Dutch coastal barrier longshore bars, using video images of foam streaks formed above bars by breaking waves (cf. Fig. 2). An example of such alternations is illustrated in Figure 9. In the radargrams presented by Oliver et al. (2017) and Santos et al. (2022), which frequently display landward-inclined reflectors, relatively short seaward-directed low-angle strata are also observed. Unlike the landward-facing strata, these seaward-directed strata typically do not form sets of numerous low-angle strata, but instead appear as solitary features or occur in short bundles of 2–3 seaward-directed strata. Santos et al. (2022) interpret these features as the seaward growth or migration of a longshore bar or slip-face bar.

Since the introduction of GPR as an aid in detecting sedimentary structures in coastal barriers (Jol et al. 1996), many studies reveal the cross-sectional radar reflector structure of subrecent coastal barrier deposits (Table 1). In the present analysis, this body of research is integrated with the record of the cross-shore dynamics of longshore bars along sandy, stable, or prograding coastal barriers (Table 2). Only radargrams that extend into the deposits of the upper shoreface and exclude complexities introduced by the influence of inlets and spits are considered. For locations in Table 1 typified by non-permanent bar systems, this cannot be determined with certainty because of lack of data. At some of these locations, complete onshore welding and disappearance of upper-shoreface bars may rarely be achieved. For instance, near the vicinity of the location of the radargram published by Santos et al. (2022; no. 12 in Table 1), Tabajara et al. (2008) found a permanent presence of one or two longshore bars over a period of eight months, while a cycle of storm erosion and fair-weather recovery affecting a large sediment volume was noted over a much longer timeframe. Therefore, over time the landward transport of sand by swell waves may ultimately lead to the development of a reflective beach face devoid of longshore bars (Short 2020). Thus, while the classification of bar systems as truly non-permanent may not always be conclusive, it is crucial to recognize that in a wave climate dominated by swell waves, there is a prevailing trend towards the landward migration of longshore bars.

The suggestion of Bristow and Pucillo (2006; no. 3 in Table 1) that a transition from the upper shoreface to a muddy substrate on the lower shoreface exists less than two meters below paleo–sea level seems implausible given the high-energy wave conditions that characterize this location. Furthermore, the attenuated reflectors in their radargrams do not exhibit discernible changes that would support the existence of such a transition. It should be noted that reference to estuarine deposits detected in the shallow subsurface south of the position of their radargrams, particularly behind the wave shelter of a protruding Tertiary limestone headland, as outlined by Sprigg (1979), do not substantiate the conclusion of a transition to muddy substrates in the area of interest. Instead of ending in a shallow muddy substrate the radargram shows that SIS continues several meters below this level, as interpreted by Isla et al. (2020a).

The sediment layer sandwiched between two successive low-angle reflectors in GPR sections encompassing the upper shoreface has a maximum duration of deposition equal to a NOM (or NLM) return period (Fig. 3). This duration represents the maximum time frame for net deposition unless the base of the upper stratum eroded previously deposited GPR strata. Given that the formative mechanisms of large-scale low-angle strata in the beach and the shoreface differ, one might anticipate a discontinuity between them. The kink observed in the slope of the strata at the base of the beach, transitioning to a smaller inclination in the upper shoreface, as found in some coastal cross sections listed in Table 1 (no. 6–8 and 13; Fig. 1) serves as a manifestation of this. A detail of a radargram featuring such kinks is presented in Figure 10. The discontinuity observed in the strata can be caused by: 1) the transition of NOM to NLM behavior in a landward direction, 2) the landward transition to a flat erosional storm beach profile, or 3) the development of a beach step (Bauer and Allen 1995). Beach steps are typically fair-weather features, forming through backwash just below the waterline and generally washed away during storms. However, these steep slopes, which reach heights of 20–40 cm with dip angles near the angle of repose, can be preserved near the low-water level, because erosion of the lower beach during storms is often minimal or absent. Their preservation serves as an indicator of sea level in microtidal regimes (Bardaji et al. 1987; Roep et al. 1998; Nielsen and Clemmensen 2009; Dabrio and Polo 2013; Hede et al. 2015). In the case presented in Figure 10, the strata suggest a beach-step origin for the kink (Hede et al. 2015), buried by swash lamination associated with storm waves.

So far, the distinction between ancient barrier depositional environments has been limited to low and high wave energy, as proposed by Galloway and Hobday (1996), later refined by Clifton (2006) and extended to a tripartite division by Dashtgard et al. (2021). This classification assumes that in less energetic environments, wave-induced cross-bedding extends less deeply. The fact that the cross-shore behavior of longshore bars is dependent on wave climate raises the question whether this also affects the sedimentary architecture of coastal barriers, potentially allowing for a more nuanced paleoenvironmental distinction. To investigate this idea, the inventory of Table 2 was used.

In non-permanent systems, the migration of bars tends to be quite irregular. Although eventually welding of the bar onto the beach may occur, this process is easily interrupted by recurring storm-wave conditions. For instance, in the case of two locations studied at Ensenada, Mexico (see Table 2, no. 2 and 3), a prolonged period of energetic waves resulted in a six-month delay in the welding process onto the beach. Therefore, statistics regarding NLM behavior of non-permanent bar systems, similar to those of NOM in Table 2, do not provide meaningful insights and have not been included. For conditions in lakes and bays with a fetch less than about 100–150 km (i.e., without long and deep-reaching swell) the available data (Table 2, no. 18–20) suggest that cyclic behavior of longshore bars (NOM or NLM) does not occur.

The presence of a kink in the SIS near the low-water mark, as observed in some radargrams in permanent bar systems, seems to signify the transition from NLM to NOM bar behavior. This is supported by the absence of such kinks in radargrams representing non-permanent bar systems, which are characterized by NLM for the entire beach and upper shoreface. Thus, the presence of this kink in GPR cross sections of ancient coastal barrier deposits serves as a criterion for identifying a multi-bar shoreface paleoenvironment with NOM in the upper shoreface. From Table 2, it is evident that longshore bar systems differ in their deepest trough depth (representing the transition from upper to lower shoreface) and, in the case of systems exhibiting NOM behavior, the bar return period, depending on the coastal setting. The absence of NOM in systems with a limited fetch may result in chaotic GPR reflectors in the upper shoreface part of barrier cross sections, potentially illustrating the disorganized behavior of longshore bars in these environments.

The combined information from Table 2 and the understanding of the cross-shore dynamics of longshore bars leads to a tentative differentiation of five types of sandy prograding barrier architecture, as summarized in Table 3. The GPR architecture of prograding barrier deposits along coasts with a limited fetch (Type D in Table 3) remains to be explored. The same holds for permanently non-barred barriers (Type E in Table 3) if such conditions occur. Note that in the latter case the coastal setting of an upper-shoreface deposit can be readily identified by the absence of storm layers in the lower shoreface. Consistent with the findings of Galloway and Hobday (1996), Clifton (2006), and Dashtgard (2021), the depth of transition from upper to lower shoreface increases with wave energy. Oceanic coasts experiencing frequent storm-wave events represent the highest-energy conditions and show the deepest transition to lower shoreface (Type B in Table 3). Oceanic coasts with occasional storms (Type A in Table 3) and water basins with a fetch < 100–150 km (Type D in Table 3) typically have the shallowest upper-to-lower-shoreface boundary. It is worth noting that in small lakes with a fetch on the order of 10 km and less, the troughs of the longshore bars extend even less deeply and the base of the upper shoreface is found at less than 2 meters below lake level (Prieto 1995; Eliot et al. 2006; Nordstrom and Jackson 2012; Ton et al. 2023). Knowledge of their existence and dynamics in coastal barriers, however, is insufficient to incorporate these as a separate class in Table 3. The proposed subdivision in Table 3 serves as a nuanced and extended variant of the models of low- and high-energy barrier facies originally proposed by Galloway and Hobday (1996).

Application of GPR in ancient rocks is hindered by the limited penetration of the required high-frequency electromagnetic signal. The method can only effectively examine deposits that are near a relatively horizontal ground surface. For such conditions GPR imaging technology can provide very good agreement with bedding structures seen in outcrop across a wide range of sedimentological conditions (Akinpelu 2010; Magalhães et al. 2017). One additional advantage of GPR is its capability to enable 3D studies, particularly when an accessible flat operational surface is available (cf. Coll et al. 2013). However, despite these capabilities, Akinpelu (2010) noted a strong bias towards Quaternary deposits in the application of GPR, a trend that has persisted over the past decades. To date, there has been no reported application of this method to ancient upper-shoreface deposits in outcrop.

This section begins with comparisons between radargrams and outcrop structures, followed by a discussion of potential SIS evidence found in published work. Subsequently, new evidence of SIS from a Miocene coastal barrier deposit in SE Spain is presented.

Dougherty and Nichol (2007) compared structures of beach and eolian dune deposits in a Pleistocene outcrop and a 200 MHz radargram collected close to the outcrop. Good similarity was found for the beach deposits, where up to several-mm-thick laminae of heavy-mineral concentrations are present between sets and strata. However, in the eolian deposits, these heavy-mineral laminae are absent, resulting in a poorer resemblance with only a few faint reflectors visible, despite the presence of detailed structures in the outcrop. Conversely, SIS may be more clearly delineated in GPR images due to their ability to capture subsurface differences in lithology and porosity, which may not be as discernible in outcrop settings (Jol et al. 1996; Akinpelu 2010). Moreover, given its large-scale nature, identifying SIS visually necessitates well-exposed cross sections transverse to the paleocoastline, covering the entire upper shoreface over extensive lengths, conditions that do not occur commonly.

In fact, SIS in upper-shoreface deposits has gone almost unnoticed in outcrop studies, partly due to the paucity of published detailed cross-shore architectural profiles. Consequently, criteria identifying SIS based on recent or sub-recent and outcrop studies are not documented in the literature. Only a few examples exist where the presence of individual large-scale seaward-inclined low-angle strata in shoreface deposits is mentioned. One such instance is documented by Hampson (2000), who identified some of these strata encompassing a 6-meter-thick deposit of a barrier parasequence from the Upper Cretaceous exposed in the Book Cliffs, Utah, USA. These low-angle discontinuities in the upper shoreface “cannot be traced farther paleoseaward.” This observation aligns with the understanding that bar-migration-related SIS is confined to the upper shoreface. Given the paleocoastal location of the Western Interior Seaway, it is plausible that SIS originated from NOM behavior of bars in a multibar system with a fetch greater than 100–150 km and maximum bar-trough depths exceeding 5 meters (setting C in Table 3), consistent with the upper-shoreface thickness of the Book Cliffs exposure.

Isla et al. (2020a) described SIS in sandstones of a supposed coastal barrier from the late Valanginian and early Hauterivian in the Neuquén Basin, a large depositional system in the mid-west of Argentina. They interpreted the presence of up to 11° surfaces as evidence of a non-barred shoreface. However, the identification of hummocky cross-stratified sandstone beds in the lower-shoreface, commonly associated with storm events (e.g., Clifton 2006), challenges the interpretation of a non-barred environment, as argued before. Typically, upper-shoreface deposits show multidirectional (swaly/hummocky/normal) cross-bedding and an erosional base cut into finer and more bioturbated beds of the lower-shoreface (Walker and Plint 1992; Galloway and Hobday 1996; Clifton 2006; Pemberton et al. 2012). In contrast, the upper-shoreface deposits discussed by Isla et al. (2020a) show a dominance of unidirectional seaward cross-bedding, that pass gradually into lower-shoreface deposits (Isla et al. 2020a, 2020b), suggesting rip currents as forming agents. Rip channels can erode below the level of the longshore-bar trough in the non-permanent, single bar system of oceanic coasts with occasional storm waves (Hunter et al. 1979), producing dominant seaward-directed cross-bedding at the base of preserved upper-shoreface deposits (Clifton 1981). Soundings and studies using video remote sensing of multibar systems show that rip flows are concentrated in narrow channels that cut through the innermost longshore bar but expand seaward and hardly affect the height of more seaward-located longshore bars (Van Rijn et al. 2002). This means that seaward-directed cross-bedding generated by rip currents may be restricted to the typical non-permanent single-bar systems of oceanic coasts. However, the channelized geometries of rip channels were not observed in the SIS described by Isla et al. (2020a, 2020b). Isla et al. (2020a, 2022) suggest that their barred examples refer to systems with maximum bar-trough depths of about 2 m below MSL, indicating deposition in a sheltered area with a small paleo-fetch. For such conditions, the possibility of bar destruction during severe storms cannot be excluded, given the limited understanding of bar dynamics in these systems. However, such an interpretation challenges their assumption of a coast exposed to high-energy waves.

One of the first detailed descriptions and explanations of sedimentary structures of prograding coastal barrier parasequences was made by Roep et al. (1979, 1998) based on Miocene rocks in the canyon walls below the town of Sorbas, SE Spain (Fig. 11). These barriers were formed in the Neogene Sorbas Basin, which is part of a series of intramontane basins that formed above the exhumed metamorphic zones of the internal Betics (Clauzon et al. 2015). Three superimposed barrier parasequences formed in a 10–15 km wide embayment of the late Messinian Mediterranean, under conditions of practically no infaunal activity (burrowing) due to raised salinities (Roep et al. 1979). Consistent with non-tidal settings, no channel deposits were found intersecting the barriers. Thus, the barriers possibly consisted of closed systems with a curved outline and one opening through which the small Messinian Río de Aguas paleoriver must have debouched, instead of a series of small barrier islands, as assumed by Martín et al. (1999). Within the scope of the present analysis, the site was revisited. The middle parasequence (parasequence II in Roep et al. 1998) is the best exposed and can be easily observed from a distance at several locations. The analysis presented here is therefore focused on this parasequence.

It is assumed that raised salinities were the result of evaporation in a marginal basin behind a very shallow and narrow connection with the Mediterranean (Braga et al. 2006). However, the transition of lower- to upper-shoreface deposits, which is located at 6 m below assumed paleosealevel (Roep et al. 1979), and the occurrence of fine gravel in deposits of the lower-shoreface indicate that the barrier of parasequence II was formed in an energetic wave climate (Table 3, Type C). This means that high storm waves could pass the sill without losing a significant amount of energy by dissipation and diffraction. More recently, Clauzon et al. (2015) argue that parasequence II was formed in a wedge-shaped depression formed by subaerial erosion during the peak of the Messinian salinity crisis, which contradicts the presence of a sill. Based on dip directions of swash lamination, Roep et al. (1998) concluded that the coastline trended NNW–SSE.

The shoreface deposits show the multidirectional cross-bedding characteristic of this environment. This cross-bedding is generally organized into one or several sets, delineated by horizontal to low-angle surfaces, which may be accentuated by a lag of very coarse sand, occasionally containing some fine gravel (Figs. 12C, D, 13B). As tides were negligible (Roep et al. 1979), the crossbedding was likely produced by dunes migrating in response to wave-generated currents. In an outcrop normal to the paleoshoreline, at the SE side of Sorbas (Fig. 11), low-angle strata encompassing the upper shoreface can be observed over a distance of 120 m (Fig. 12A). Roep et al. (1998) explained this inclination by synsedimentary folding implying a downward bending of the upper- to lower-shoreface boundary. Re-examination of the outcrop, however, shows that this is not the case. Instead, the lower boundary is straight and shows an overall paleoseaward tectonic dip of 2.5°. The strata are either parallel or slightly inclined paleoseaward relative to this dip, and therefore are interpreted as SIS (Fig. 12B). Note that the average spacing of 0.6 m of the bold lines in Figure 12D, interpreted as SIS, falls in the range of 0.2–0.6 m stratal thickness observed in the radargrams listed in Table 1.

The SIS shows a slightly sigmoidal shape, consistent with most radargrams. In the absence of significant tides, the water level remained relatively stable for extended periods, facilitating the formation of beach steps that can produce steep seaward-directed cross-bedding (Bardaji et al. 1987). Dabrio and Polo (2013) described this cross-bedding and associated swash lamination with concentrated heavy-mineral lamination from a nearby location (see Fig. 11). A similar beach-step structure is evident in the outcrop shown in Figure 12B. In this case of a microtidal sea, the top of the beach-step structure roughly corresponds to the base of the beach, indicating a thickness of approximately 7 m for the upper shoreface at this point. In radargrams, beach steps are manifest as kinks in the SIS. However, unlike sub-recent barriers studied with GPR, ancient barrier deposits usually show only partially preserved or completely eroded beach tops, as in the Sorbas case. Consequently, in radargrams of ancient barriers, a kink in the SIS may rarely be present.

These findings highlight the well-organized sedimentary architecture of the upper-shoreface deposits at the Sorbas site, characterized by cross-bedding organized between SIS. The cross-bedding, which is sometimes swaly, exhibits numerous discontinuity planes, multidirectionality, and a prevalence of short set lengths, indicative of the ever changing flow conditions in this environment (e.g., Walker and Plint 1992; Figs. 12C, D, 13B), which in this case are solely caused by varying wind and wave directions. Longshore-directed cross-bedding is dominant, leading to the frequent occurrence of herringbone cross-stratification in outcrops parallel to the paleoshoreline (Fig. 13B). Vertically, there is no discernible change in the style of cross-bedding throughout the upper shoreface. In outcrops aligned along the paleoshore, the (sub)horizontal strata representing SIS may be truncated by 10–20 m wide, shallow, channelized erosion surfaces, which are interpreted as rip channels (Fig. 13A).

The simple sedimentary architecture observed at the Sorbas site likely results from the cyclic behavior of longshore bars, particularly NOM in this case. Given the prevalence of SIS in most radargrams, it is plausible that the sedimentary architecture of other ancient deposits of prograding barriers show variations of a similar SIS framework. Exceptions to this general trend may arise in barrier deposits formed: 1) in seas or lakes with limited fetch, where cyclic behavior of longshore bars does not occur and 2) in permanently non-barred systems, as the absence of longshore bars in such systems, if they exist, impedes the generation of bar-trough-related SIS.

The core repository of the Middle Jurassic stratigraphic record (covering approximately 11 Myr) on the Norwegian Continental Shelf (NCS; Table 4) has been consulted to investigate if and how SIS and associated structures in the upper shoreface are preserved and expressed. To accurately identify SIS in a core dataset, some upfront considerations are necessary regarding three aspects of features that have the potential to enable its identification. Individually, these aspects are not unique to upper-shoreface deposits, and they need to be considered collectively. They include: 1) the potential of the upper-shoreface facies belt on a regressive coastline to become preserved in the subsurface, 2) lithofacies characteristics of upper shoreface including the unambiguous presence of discontinuity surfaces in the succession, and 3) the presence of heavy-mineral concentrations and/or shell–gravel lags, and their identification on seismic lines, and in gamma-ray and density logs. In addition, the consequences of sampling volumes and data resolution of the observational tools deployed need to be considered.

Upper-shoreface lithofacies in the core examples are typified by upper-fine- to lower-medium-grained homogeneous well-sorted sandstone commonly with darker and lighter laminae. In all cases, the darker laminae are rich in mica, and potentially heavy-mineral segregation may occur as well. Cross-stratification dominates in planar to wedge-shaped sets that often truncate each other at low angles, with minor variations in grain size across the set boundaries; characteristic phenomena of the upper shoreface, such as reactivation surfaces (cf. Olsen et al. 1999; Hampson 2000) and occasional Skolithos and Ophiomorpha burrows (Moslow and Pemberton 1988; Reynolds 1992; Cantalamessa et al. 2005; Mitchell et al. 2006; Schwarz et al. 2018) are present.

In recent environments, the inaccessibility of the upper shoreface coupled with a paucity of long (more than 3 m) vibracores (with the exception of the study by Smith et al. 1999) prevents the physical inspection of SIS facies. This hampers the definition of criteria that can be used to recognize discontinuity surfaces in core that can unambiguously be ascribed to SIS in upper-shoreface lithofacies successions. In the case of the selected and deeply buried NCS examples (Table 4), seismic resolution is insufficient to observe lithology contrasts formed by discrete heterogeneities less than 25 cm thick and embedded in well-sorted and homogeneous upper- to lower-shoreface sandstones more than 15 m thick. Lithology contrasts are insufficient, and consequently these architectural features will go unnoticed, including those formed by heavy-mineral concentrations on SIS surfaces or most carbonate-cemented layers. The gamma-ray (GR) borehole logging tool has a resolution of 25 to 30 cm and reacts only to potassium (K), thorium (Th), and uranium (U) elements that occur in the feldspar series and heavy minerals. In addition, there is always background radiation present. Heavy minerals have densities of 2.9 g cm⁻³ or greater (for example magnetite, ilmenite, zircon, rutile, staurolite, kyanite) and unique settling characteristics. In GPR studies of the upper shoreface, heavy-mineral concentrations (lags) are considered to be the main reflectors of the signals received (Moore et al. 2004; Smith et al. 1999; Frihy et al. 2008). Heavy minerals can occur as concentrations in beds of up to 90 cm thick (Smith et al. 1999) but also as dispersed components in shoreface sediment or in mud layers; the latter, however, generally are not preserved in upper-shoreface environments. Reynolds (1992), for example, reported an average heavy-mineral content in the total shoreface sand population of the Eocene Meridian Sand Formation (Mississippi, USA) of somewhat less than 2%. In most cases, the heavy-mineral suites are derived from fluvial sources of nearby rivers (Reynolds 1992; Frihy and Komar 1993; Smith et al. 1999). In the studied NCS core, some heavy-mineral concentrations are identified (up to 12%, for example well 34/8-1), but these are thin (a few centimeter thick) and not systematically mapped. In conclusion, in those cases where the GR log possibly records the presence of enhanced gamma-radiation levels, they have been found to be too ambiguous to interpret the presence of heavy-mineral concentrations.

Shell lags in the upper shoreface described in the literature are mostly associated with shoreface ravinement surfaces at temporal scales larger than those represented by a parasequence, but they can also occur at intra-parasequence scale (Fürsich and Aberhan 1990; Kidwell and Bosence 1991). They are identified as coarse transgressive deposits, accumulated as composite, time-averaged concentrations modified by numerous events (i.e., by mixing of skeletal elements of non-contemporaneous populations or communities, cf. Fürsich and Aberhan 1990). Only within-habitat time-averaged shell lags (assemblages composed of numerous generations from a single community, cf. Kidwell and Bosence 1991) in a regressive setting are valid indicators of SIS. The type of shell concentration leading to shell lags of relevance for this study (cf. Anderson and McBride 1996, their Table 3; see also their Fig. 15) are those formed by numerous storm events causing longshore bar migration and the formation of composite shell concentrations. They form due to winnowing or obrution (smothered by rapid burial), due to which finer terrigenous grain-size fractions may also be removed resulting in relative enrichment of coarser grain-size fractions (such as the inconclusive core example in Fig. 14). Such surfaces may become closely spaced, or even merged, towards the bottom section of a SIS, potentially locally increasing their amalgamated thickness. Characteristics include an erosional base and a gradational top, winnowed, thick, and laterally continuous beds that are often trough cross-stratified, and with poorly organized, imbricated nested fabrics, often with poorly preserved bioclasts. A high sedimentation rate is likely to be the cause of rapid burial and excellent preservation of bioclasts with little or no abrasion. An upper-shoreface shell lag is thus defined here as a concentration of coarse-grained well-preserved shelly material produced by physical sorting and reworking (cf. Anderson and McBride 1996) that is related to the time it took to form a regressive shoreline parasequence. An example of such a deposit of Pleistocene age in the paleo–Tokyo Bay is described by Okazaki and Masuda (1990). These authors identified granule or pumice grains and shell fragments contained in the basal part of longshore bar deposits. Cleveringa (2003) reported thick chaotic shell beds from Holocene upper-shoreface deposits along the Dutch coast, and Van der Valk (1991) found large amounts of bivalve shell material at 5 to 6 m depth in the upper shoreface. Norris (1986) described time-averaged fossil assemblages (coquinas) from upper-shoreface deposits of the Pliocene Purisma Formation (California, USA) interpreted to have been deposited just seaward of the breaker zone in 8–10 m of water. They are composed of amalgamated beds and rounded shell fragments, and may lack bioturbation.

Schwarz et al. (2018) describe preferential concentration of gravel-size and abundant sand-size bioclasts, and terrigenous sand and pebbles present at the base of longshore bar troughs from the Lower Cretaceous Pilmatué Member of the Neuquén Basin (Argentina). These deposits are interpreted as having formed in a storm-dominated regressive shoreface system. Thus, shell lags form in the upper shoreface and can potentially form a detectable surface at the base of a series of offshore-inclined low-angle surfaces associated with longshore bars (SIS). Most mollusk shells are composed of aragonite or high-Mg calcite, an unstable form of carbonate, not likely to leave any body fossils in the rock record after burial and subsequent complex diagenetic history (cf. Worden et al. 2020). It is assumed here that most of the calcite in core originated as bioclastic or micritic debris deposited as lags or scattered throughout the upper-shoreface sands. It then underwent progressive dissolution and reprecipitation during diagenesis, ultimately forming partly or wholly cemented patches or layers up to 1 m thick that can be identified on the density log. Some of the selected examples for this study (Table 4) show cemented intervals in upper-shoreface facies interpreted as originating from shell lags. However, these cases occur in a highly irregular fashion and are uncommon. They are in some cases identified on the density log, but they cannot be used in a conclusive manner. In conclusion, currently SIS cannot be identified with sufficient certainty in core, well logs, or seismic profiles of shoreface successions buried to depths greater than 1 km due to a lack of diagnostic criteria as well as limited tool resolution.

The variation in the maximum angle of SIS observed in the available radargrams, ranging from 2° to 8° (Table 1), raises an intriguing question about the underlying factors driving these differences. It is known that the long-term equilibrium slope of the beach upper-shoreface profile tends to decrease with increasing wave energy (e.g., D’Anna et al. 2021). Additionally, this slope is steeper for larger grain sizes (Dean 1987, 1990). The latter may explain the high maximum SIS inclination of 8° of SIS in the Sorbas barrier case, which is developed in medium to coarse sand with some gravel. Information on grain size and wave energy near the radargram locations listed in Table 1, however, is insufficient to investigate this further. The shape of SIS in the shoreface resembles the equilibrium profile. Nonetheless, the troughs between the longshore bars exhibit erosion below this profile. Consequently, SIS exhibits a more pronounced concavity, with steeper contours near the beach zone and gentler slopes transitioning to horizontal closer to the lower-shoreface, as clearly depicted in the radargrams listed in Table 1.

Sets of landward-sloping low-angle strata superimposed on SIS in radargrams possibly represent a non-permanent longshore bar system, characterized by the dominant landward bar migration (Type A in Table 3). However, lack of local information on longshore bar systems in the actual shoreface near the GPR profiles makes it impossible to find out why these bar-edge structures occur in some GPR profiles and not in others. It is worth noting that, in some radargrams, bar flank structures may not have been recognized due to low resolution or signal loss caused by attenuation. Examples of this are GPR sections of the Kujukuri barrier, eastern Japan (Tamura et al. 2008) showing some landward- and seaward-inclined low-angle reflectors. The signal frequency used was only 100 MHz, while in the Australian and Brazilian cases (no. 9, 10, and 12 in Table 1) it was 250 MHz, which may have reduced the detail in the Kujukuri barrier radargram.

Since coastal barrier radargrams generally focus on aspects of barrier progradation, very few are oriented parallel to the barrier. The documented examples show a succession of subhorizontal, slightly undulating strata (Jol et al. 2002; Rodriguez and Meyer 2006; Tamura et al. 2008). This is to be expected because shoreface and beach dynamics are dominated by offshore and onshore movement of bars paralleling the coast. Two radargrams, one from a Japanese coastal barrier facing the Pacific Ocean (Tamura et al. 2008) and the other from the Long Beach Peninsula (Jol et al. 2002), show some counter-oriented discontinuous low-angle strata. Two possible origins are conceivable, either by longshore shift of the 3D morphology of longshore bars or by lateral shift of rip channels. Longshore bars are not completely straight but show alongshore non-uniformities superimposed on the overall straight base pattern, yielding a three-dimensional morphological system that can vary in time and space between linear, crescentic, and sinuous. The three-dimensionality and variation is greatest for the inner bar (Wright and Short 1984; Van Rijn et al. 2002; Armaroli and Ciavola 2011; Davidson-Arnott 2011, 2022). Crescent-bar morphologies are observed to develop mainly during fair-weather conditions and disappear during storms (Van Enckevort et al. 2004). Another possibility is scour by rip channels. Both features could produce low-angle strata in barrier-parallel GPR sections and can explain their occurrence in the uppermost part of the upper shoreface. The presence of low-angle reflectors about a meter above the transition to the lower-shoreface in the Japanese case, therefore, remains inexplicable. A good example of scour by a rip channel was found in Sorbas parasequence II (Fig. 13A). The base of the channel consists of fine gravel, which could generate a reflector in GPR.

It is important to emphasize that Table 3 is provisional, because it is based on a small database. Further data collection and analysis may lead to alterations or refinements. For example, kinks have been only recorded thus far in radargrams of types B and C coastal settings (Table 1), whereas beach steps, which are one of the causes of kinks, have also been found in outcrops of a Type A setting (De Souza et al. 2012). As mentioned before, the development of beach steps is favored by the absence of tides, and therefore possibly their presence in the geological record points to a micropaleotidal coast.

Seaward-Inclined, low-angle stratification (SIS) observed in ground-penetration radar (GPR) radargrams of many studies of subrecent coastal barrier deposits, generally spanning the entirety of preserved upper-shoreface to beach deposits of a sandy prograding coastal barrier, has traditionally been interpreted as storm-erosion clinoforms. However, in sand-rich systems, like prograding coastal barriers, this interpretation is unlikely, because longshore bars under such conditions are not destroyed during storm events. In this paper, a reinterpretation of these surfaces is proposed, suggesting that SIS is formed through a combination of synchronous storm-related beach erosion surfaces and diachronous surfaces caused by cross-shore migration of longshore bar troughs. The latter surfaces result from cyclic processes of net landward migration (NLM) or net offshore migration (NOM), determined by the paleowave climate.

In oceanic coastal barriers, SIS can possibly also be formed by the base of rip channels, recognized by a dominance of seaward cross-bedding directions. Radargrams of coastal barriers with oceanic shores, characterized by long-swell conditions and occasional storm waves, exhibit sets of smaller landward-sloping low-angle strata superimposed on SIS. In marginal seas, such as the North Sea or the Mediterranean, along oceanic coasts with a high frequency of storms and in very large lakes, longshore bars show a NOM behavior. SIS formed under these conditions may show a kink around the low-water level marking the seaward transition from NLM to NOM. Notably, the depth of the upper-shoreface base varies depending on the wave climate and the frequency of storm occurrences, with oceanic coasts with occasional storms typically showing shallower depths compared to barriers formed along coasts of marginal seas. With limited fetch, the depth of the transition from upper to lower-shoreface is also relatively small. The differences observed suggest a classification of prograding barrier GPR architecture into five types based on wave climate.

Despite its dominant presence in radargrams, SIS has hardly been recognized in outcrop studies due to unfavorable orientation of 2D exposures, limited variation in lithology, and its low-angle nature. Re-evaluation of a Miocene coastal barrier deposit in SE Spain reveals the presence of characteristic multidirectional cross-bedding between regularly spaced low-angle erosion surfaces, containing coarse sand and sometimes some pea gravel. In cross sections normal to the paleoshore the overall shape of this SIS is sigmoidal. Similarity with SIS observed in virtually all published radargrams suggests that most prograding ancient coastal barrier deposits in outcrop likely exhibit a comparable basic sedimentary architecture, except for those formed in seas or lakes with limited fetches or permanently non-barred shorefaces. The latter systems, however, are rare in sandy barriers and are possibly restricted to coarse-grained gravelly barriers, which are not considered here. Further field studies are necessary to validate these conclusions.

Analysis of examples of subsurface shoreface systems using core, well logs, and seismic lines reveal challenges in recognizing SIS due to issues with sampling volumes and tool resolution, and the integration of different data types. Targeted research on exposures and core, using improved criteria for recognition, are essential to translate paleoenvironmental criteria derived from GPR data into the reliable characterization of SIS-related sedimentary structures in outcrop and subcrop.

The authors contributed in the following proportions to conception and design, data collection, analysis and conclusions, and manuscript preparation: JHB (100, 70, 90, 95%), AMM (0, 30, 10, 5%). We would like to thank Nicolas Aleman, Robin Davidson-Arnott, Sytze van Heteren, Kick Kleverlaan, Harry Jol, Thomas Oliver, Peter Ruggiero, Robert Schwartz, and Dirk-Jan Walstra for their help during various stages of this work. We are indebted to Manuel Isla, Toru Tamura, and Associate Editor Gary Hampson for their thoughtful and constructive reviews, which significantly improved the paper. Last but not least we very much appreciate the final edit by Editor John Southard.