INTRODUCTION
Parasequences have been originally defined as shallowing-upward units limited by regional-scale flooding surfaces (Van Wagoner et al., 1990), and which represents the progradation of a single/ individual shallow-marine systems (delta-front and/ or shoreface deposits; Table 1). Despite the great advances made in their knowledge, there are still big debates related to their controlling factors, time span and their bounding surfaces (Zecchin and Catuneanu, 2013; Burgess et al., 2016; Hampson, 2016; Muto et al., 2016; Ridente, 2016; Zecchin et al., 2017; Catuneanu, 2019). On the contrary, the temporal variability of the depositional systems as they prograde to form the parasequence has been less explored by sequence stratigraphers and it has been typically assumed that the configuration of the given shallow-marine system remains relatively constant (Van Wagoner et al., 1988, 1990; Posamentier and Allen, 1999). This has been the case for most of the examples reported in the literature, for example in many outcropping parasequences in the Cretaceous of the Book Cliffs (Utah, USA) (Fig. 1a). Infrequently, the lateral and vertical juxtaposition of shoreface and delta front strata/bedsets has been documented within a single parasequence (e.g. Charvin et al., 2010; Fig 1b), and was attributed to the spatial-related co-existence of delta-front settings and wave-dominated shoreline systems. The apparent low variability of shallow-marine strata at intra-parasequence scale lead to classify these high-frequency units as deltaic, shoreface (linear coasts) or deltaic-shoreface based on the interpreted formative shoreline (Colombera and Mountney, 2020).
The record of wave-dominated clastic shorelines typically consists in deposits of the upper-shoreface and foreshore settings within the shoreface-shelf systems, which are highly susceptible to changes in wave climate (Clifton, 2006). Their morphological configuration is extremely dynamic and may vary from barred to non-barred conditions in short time spans (Davidson-Arnott, 2010). These different configurations are related to: (i) the presence or complete absence of nearshore bars separated by longshore troughs, which in turn are mostly linked to the gradient of coastal profiles and (ii) the dissipative versus reflective mechanisms developed under those gradients (Aagaard et al., 2013). Recent studies have demonstrated that the seaward migration (progradation) of barred and non-barred shorelines produce different architectural styles in the stratigraphic record (Isla, 2019). However, this short-term variability of shallow-marine systems during the progradation of an individual shoreline, particularly for wave-dominated shoreface-shorelines systems, has not been documented.
Parasequences comprising wave-dominated shoreface-shelf deposits are ubiquitous in the Early Cretaceous Pilmatué Member of the Agrio Formation in the Central Neuquén Basin (Argentina). The aims of this work are two-fold: 1) to describe and interpret barred and non-barred architectural elements and their vertical juxtaposition within two parasequences, 2) to discuss the possible processes controlling these temporal changes within the progradation of a wave-dominated clastic shoreline.
GEOLOGIC AND STRATIGRAPHIC BACKGROUND
The Neuquén Basin is located in the west-central sector of Argentina (Fig. 2a) and comprises a continuous stratigraphic record from the Late Triassic to the Early Cenozoic (Vergani et al., 1995; Howell et al., 2005). To the west, it is limited by the Andean Volcanic Arc, whereas the Sierra Pintada System and the Northpatagonian Massif constitute the northeast and southeast borders, respectively (Fig. 2a). It is characterized by a complex history of tectono-stratigraphic evolution which involves an initial rift phase during the Late Permian to the Early Triassic (Howell et al., 2005) which lasted until the Early Jurassic to Middle Jurassic, when a post-rift phase started and the basin was reconfigured as a unified back-arc basin. During this phase, sedimentation was governed by eustatic changes and the uplift of the magmatic arc, as both conditioned the connection with the proto-Pacific Ocean, and thus controlled the sea level in the semi-enclosed basin (Legarreta and Uliana, 1991). Since the Late Cretaceous the basin has behaved as a foreland basin (Howell et al., 2005).
The Pilmatué Member is the lowermost unit contained in the Agrio Formation and was accumulated between the late Valanginian and early Hauterivian (Fig. 2b), under a climatic regime characterized by arid conditions (Scotese, 2000). Ubiquitous throughout the Neuquén Basin, this unit overlies the Mulichinco Formation composed of continental or shallow-marine deposits (Schwarz and Howell, 2005), and is abruptly truncated by fluvial and aeolian deposits of the Avilé Member (Veiga et al., 2007). The Pilmatué Member comprises up to 650 m thick of dark shales and marls that reflect offshore/basinal conditions and subordinated sandstones, representing shallow-marine accumulation in a wave-dominated shoreface system (Lazo, 2005; Spalletti et al., 2011; Schwarz et al., 2018). The system was compositionally defined as siliciclastic-dominated whereas carbonate deposits may form pure packages or mixed successions combined with terrigenous sediments (Schwarz et al., 2018). However, towards the north of the basin the carbonate content of the deposits becomes more important even reaching to configure a carbonate ramp (Sagasti, 2005). Due to the development of regional-scale structural belts such as the Agrio fold-and-thrust belt, the Cretaceous stratigraphy (including the Pimatué Member) of the basin have great exposures (Fig. 3a).
Mixed (carbonate-siliciclastic) shallow-marine deposits dominate the succession of the Pilmatué Member in the Agrio Fold and Thrust Belt. These deposits have been interpreted as the deposition under a wave- and- storm-dominated offshore-shoreface depositional system (Isla et al., 2018; Schwarz et al., 2018). Five main facies associations were defined within the preserved stratigraphic record (Table 2) whose vertical/lateral arrangement defines a typical shallowing-upward succession. Each succession is characterized by basal offshore bioturbated mudstones that are vertically replaced by siltstones interbedded with hummocky cross-stratified sandstones interpreted as offshore-transition deposits. These deposits grade into very fine-grained sandstones with ripple lamination, hummocky or swaley cross-stratification accumulated in a lower-shoreface setting. At the top of each parasequence, upper-shoreface and foreshore deposits are recorded and are the focus of the present contribution. Capping the shallowing-upward successions, a sixth facies association consisting in shell beds was interpreted as wave-ravinement deposits (Isla et al., 2018). Reworking of previously accumulated shoreface deposits during minor transgressive episodes was responsible for the concentration of residual sediment such as large bioclasts, ooids and gravels, which formed the carbonate-dominated shell beds (Table 2).
Schwarz et al. (2018) defined these coarsening-upward cycles as parasequences and recognized up to 17 of them in the Pilmatué Member within the study area (Fig 3b). These shallowing-upward units are limited by regional-scale flooding surfaces (Van Wagoner et al., 1990). In general, parasequences or high-frequency sequences (according to Zecchin and Catuneanu, 2013), are 30 to 50 m thick and show good exposure of their internal architecture and excellent lateral continuity in a dip direction, which allowed to perform a high-resolution sedimentary analysis at intra-parasequence scale (Fig. 3b). The identification and correlation of vertical and lateral changes of facies associations allowed defining several bedsets and bounding surfaces within each parasequence. The lateral continuity of outcrops allowed tracking bounding surfaces and internal surfaces with confidence. The criterion used to define these minor cycles as bedsets is related to the relative deepening degree of the transgressive bounding surface, and hence the associated magnitude of shoreline shift (Isla et al., 2018). The deepening degree associated to bedset boundaries corresponds to a vertical change of one or two facies belt (e.g. offshore transition deposits on top of lower shoreface or upper shoreface deposits), which would represent a shoreline displacement up to 5 km (Schwarz et al., 2018). This magnitude of shoreline displacement is significantly minor than those related to parasequence boundaries of the Pilmatué Member, which had been estimated in the order of 20 km (Schwarz et al., 2018). Nevertheless, bedset boundaries are not exclusively related to shoreline shift and their recognition may be limited to relative short distances along depositional dip and strike (Zecchin et al., 2017).
DATASET AND METHODS
The dataset used here consists of two large (12 and 16 km) outcrop transects that trend WNW-ESE (Fig. 3a) and correspond to the strata of two individual parasequences: PS300 and PS400 (Fig. 3b). The full extent of these parasequences has been mapped and reported in detail in previous studies (Schwarz et al., 2018 for the PS300 and Isla et al., 2018 for PS400). The selected transect of each parasequence for the present study consists of near-continuous exposures, which allowed describing the shoreface and foreshore strata (facies and architectural elements) in great detail.
The parasequence PS300 was studied in the San Eduardo anticline, located near the town of Chos Malal, in northern Neuquén province (Argentina) (Figs. 3a, 4a). It was mapped for over 18 km along depositional dip, showing a northward gradual thickening from 20 to 50 m. The parasequence PS300 represents the progradation of the shoreline for at least 16 km in a roughly northward direction (Schwarz et al., 2018). The reconstructed shoreline trajectory using the boundary between upper-shoreface and lower-shoreface facies belts (Hampson, 2000), suggests a regressive trajectory with minimum aggradation. Internally the parasequence PS300 has been subdivided in 5 bedsets (BS300.1 to BS300.5; Fig. 4b), comprising small scale (<10 m thick) coarsening-upward successions. Bedsets are defined as concordant successions of genetically related beds within parasequences, limited by surfaces of no deposition or erosion, and their correlative conformities (Van Wagoner et al., 1990). A 600-m long outcrop where two-dimensional exposure of the PS300 deposits runs north-south was used for the architectural analysis (Fig. 4a). Fifteen sedimentary sections (1:10 scaled), were logged every 40 m and 2D sketches were drawn in the field. Dip and dip-direction of bed boundaries, and paleocurrents (total of measurements: 98) from trough cross-bedding and ripple crests were measured.
The parasequence PS400 was studied in the Loma Rayoso anticline located in the southern margin of the Neuquén River (Figs. 3a, 4c). The area is 12 by 5 km, and is part of the southwest flank of the anticline whose axial trace is oriented approximately N30°W. This parasequence is 30 to 40 m thick and it was mapped in detail for 12 km. Because the area is located in a more proximal section of the parasequence than in the previous unit (Fig. 3b), upper shoreface and foreshore deposits are relatively more abundant. Thirteen bedsets (BS400.1 to BS400.13; Fig. 4d) were defined within the parasequence PS400 (Isla et al., 2018). Most of the identified bedsets were recognized across the whole study area, but some of them are missing in the southern sector due to truncation (Fig. 4d).
During field work, 28 sedimentary logs at a scale of 1:50 were measured along, spaced ~500 m, for over 12 km (Fig. 4c). Also, 40 sedimentary logs at a scale of 1:10 were incorporated following a space between them of 15 to 40 m, for over 1 km. Paleocurrent data (total of measurements: 98) was obtained mostly from trough cross-stratified facies and large-scale inclined beds. The compilation of all outcrop information, added to thin section observations, led to carry out a facies analysis. Sedimentary facies were grouped in facies associations considering interpretation of depositional processes and hydrodynamic regimes within the depositional system.
The architectural analysis of upper-shoreface and foreshore deposits was based on the classical concepts and methods of architectural-element analysis (Miall, 1985, 1988). The nature of bounding surfaces (erosional or concordant), and their scale and geometry were described. This approach allowed identification of the extent and geometry of different rock bodies as well as the distribution of facies within them. These depositional units were nested in terms of their scale and the intervening sedimentary processes to establish different hierarchies. This work allowed identifying different architectural styles in both parasequences. Each architectural style is composed of a specific set of orders of bounding surfaces, representing different hierarchies of discontinuities. Four hierarchies were defined and numbered for each architectural style.
Vertical and lateral changes of facies associations allowed the identification and correlation of several bedsets and their bounding surfaces. Also the variability of different sedimentary attributes (number of units, thickness, and proportions of carbonate-siliciclastc grains) among successive bedsets was analyzed to compare the deposits of identified shoreline morphologies. Detailed logs were measured exclusively focusing on the upper-shoreface and foreshore facies associations (Figs. 4b and c). This high-resolution outcrop dataset involving grain-size distribution, carbonate-siliciclastic grains, paleocurrents was combined with 2D sketches and high-resolution photomosaic to enable analysis of the two-dimensional distribution of the preserved deposits. New elements as beds geometries, orientation and hierarchy of heterogeneities were incorporated to the previous facies analysis. The photomontages comprise a set of photos with 30% mutual overlap that allowed bed tracing and measurement. Paleocurrent data were obtained mostly from trough cross-stratified facies and seaward-dipping, inclined beds. These data were structurally restored and statistically analysed using the GEOrient Software, and presented as rose diagrams in order to establish sediment transport directions. Thin sections of representative facies were studied, focusing on grain-size distribution and contribution of siliciclastic versus carbonate grains.
ARCHITECTURAL STYLE 1
Facies distribution and depositional architecture
The most frequent architectural style (AE1) recognized in both parasequences is characterized by meter-scale, trough cross-bedded sandstones within the upper shoreface (Fig. 5a), and planar lamination in the foreshore deposits (Fig. 5b). Specific attributes of this architectural style have been already documented in a previous contribution (Isla, 2019).
The upper-shoreface deposits are up to 2 m thick and consist in mixed (carbonate-siliciclastic), pebbly sandstones and/or pure siliciclastic, fine-grained sandstones. Shell fragments (from fine- to medium-grained gravel) and ooids (up to coarse-grained sand) dominate at the base of the cross-bedding sets. Siliciclastic pebbles are dispersed and consist mainly of quartz and igneous rock fragments. When fragmentation is fairly low, bivalves such us Cucullaea sp. and Trigonia sp. are identified. Sedimentary structures consist in trough or planar/ tangential cross-stratification (Fig. 5a), forming beds up to 2 m thick. Thickness of cross-stratified sets varies from 0.2 to 1 m, with a mean of 0.3 m. Individual sets consist of concave up, erosive-based lenses that range between from 0.5 to 2 m wide. In some cases, laterally and vertically stacked sets may form 3-5 m wide co-sets, bounded by erosive concave-upward surfaces. Bioturbation intensity is low (BI 1-2) and is largely dominated by Ophiomorpha. The measured palaeocurrent data from these trough cross-bedded sandstones showed a consistent low-angle obliquity (0 - 30°) with respect to the reconstructed shoreline (Fig. 5c).
Overlying these cross-bedded facies, there are up to 2 m thick intervals consisting in fine-grained sandstones dominated by planar lamination (Fig. 5b). In some cases, soft-sediment deformation structures, with no evidence of vertical or lateral displacement, may also be present. Planar lamination form beds, within which lamination thickens-upward from 0.3 to 1 cm. This facies association is very conspicuous, dominating in the southern part of the outcrop (proximal section), where it always overlies upper-shoreface deposits. Bioturbation is completely absent; but wood fragments may be present.
Altogether, the upper-shoreface and foreshore deposits range in thickness from 0.5 to 3 m. Within these packages, the architectural analysis suggests the development of four hierarchies of bounding surfaces (S1b to S4b), based on their extension, geometry, truncation relationships and facies contrasts across the surface. Transgressive surfaces associated with boundaries of genetic units as bedsets (S0) were excluded from the analysis. Surfaces include: 1) first-order surfaces (Fig. 5d) correspond to the boundaries of upper-shoreface and foreshore deposits (S1b), 2) second-order low-angle surfaces (S2b), bounding large-scale lenticular bodies within the upper-shoreface deposits (Fig. 5d), 3) third-order, concave-upward surfaces were identified in both upper-shoreface and foreshore deposits (Fig. 5e; S3b), 4) fourth-order surfaces represented by the base of individual trough cross-bedding sets (S4b).
Interpreted shoreline morphology
These deposits were interpreted as having accumulated in a clastic shoreline, more specifically displaying a bar-trough configuration (Fig. 6a, b). The presence of trough or planar cross-stratification suggests the formation and migration of sinuous crested dunes under a current-dominated, upper-shoreface setting (Greenwood and Mittler, 1985; Schwartz and Birkemeier, 2004; Clifton, 2006). These dunes would have formed within troughs or rip channels generated between large-scale bars (Fig. 6a), where unidirectional-dominated, combined flows were generated by the interaction between asymmetric oscillatory waves and strong longshore or rip currents (Wright et al., 1991 Clifton, 2006). The lack of interpreted bar-related facies is due to their low potential of preservation within the fossil record (Davidson-Arnott and Greenwood, 1976; Hunter et al. 1979; Clifton, 2006; Isla, 2019). The constant obliquity of palaeocurrent directions inferred from trough cross-stratification relative to the orientation of the palaeoshoreline (Fig. 5c), suggests that dunes mostly migrated alongshore, under the influence of longshore transport processes (Fig. 6b).
Furthermore, the presence of planar lamination within the foreshore indicates upper-flow regime conditions, which take place within the intertidal section where swash and backwash processes associated with breaking waves occur (Masselink and Puleo, 2006). Also, the presence of soft-sediment deformation and the absence of bioturbation suggest that the foreshore was a high-energy, turbulent and stressful environment that inhibited organism colonization (MacEachern and Pemberton, 1992).
Concave-upward surfaces (S3b) associated with the base of planar-laminated foreshore deposits (Fig. 5e), are interpreted as the sediment fill of semi-permanent intertidal channels. This type of intertidal morphology is typical of ridge-runnel configurations (Greenwood and Davidson-Arnott, 1979; Wijnberg and Kroon, 2002), where nearshore sand bars are welded to the beach during their onshore migration (Wright and Short, 1984). The currents within the runnels were possibly connected with the associated longshore channels (Aagaard et al., 2006).
An additional criterion for interpreting a bar-trough configuration of the shoreline is the presence of a well-defined erosional surface at the boundary between the upper shoreface and lower shoreface (S1b). This surface termed surf diastem (Zhang et al., 1997; Swift et al., 2003; Clifton, 2006; Figs. 5c, 6a), marks an abrupt increase in grain-size between the very fine grained sandstones of the lower shoreface and the fine-grained or mixed pebbly sandstones of the upper shoreface (Fig. 5a). These erosional surfaces are commonly marked by the coarsest sediment available in the system (Hunter et al., 1979; Clifton, 2006). Finally, the slightly erosional, low-angle surfaces defining large-scale lenticular sand bodies (S2b; Fig. 6c) have been interpreted as the preservation of the shoreface profile related to the erosion of major storm events (Isla, 2019).
Facies distribution and depositional architecture
This architectural style (AE2) is characterized by large-scale inclined beds and its facies distribution is closely related to the geometry of these beds (Fig. 7a). In contrast with the AE1 where there is a clear separation between upper-shoreface and foreshore deposits, the tangential beds show a systematic distribution of different facies from foresets to bottomsets. Planar lamination is ubiquitous in the foreset segments (Fig. 7b), both in fine-grained siliciclastic sandstones, and in fine-to lower medium-grained bioclastic sandstones. The textural attributes of mixed deposits is quite similar to those previously descripted for barred morphologies, i.e. shell fragments and ooids are up to fine- to coarse-grained sand.
Concentrations of carbonate grains form bands that alternate with less concentrated bands, defining a lamination. Towards the bottomsets, small-scale trough and ripple cross-lamination becomes common (Figs. 7c and 7d), together with asymmetric ripples migrating onshore. Bioturbation varies from very low in the foresets to low in the bottomsets, and is represented by Ophiomorpha and subordinate Teichichnus. The inclined beds overlie and eventually, pass laterally to sub-horizontal beds mostly comprising lower-shoreface bioturbated sandstones (Fig. 7e; Isla et al., 2020).
Three hierarchies of surfaces (S1nb to S3nb) were defined in the architectural analysis of these upper-shoreface/foreshore deposits. The boundary between the underlying lower-shoreface deposits and the upper shoreface/foreshore is not represented by a sharp surface, and is instead transitional. Foresets are commonly grouped in packages and such packages form sets that may show different dip orientations. These sets of foreset packages are bounded by erosional surfaces (S1nb) that truncate the underlying packages (Fig. 7a). Truncation surfaces extend for over 300 to 1000 m (mean 600 m) and are associated with relatively abrupt changes in the dominant dip direction of foresets. Each set contain up to 6 packages of inclined beds oriented parallel to each other, but bounded by erosional surfaces with a scarp-like geometry (Fig. 7a; S2nb). Finally, the foresets packages contain between 3 and 20 inclined beds limited by individual foresets (S3 nb). Beds immediately on top of these truncation surfaces are enriched in coarse bioclasts and carbonate cement. The steep segment rapidly becomes sub-horizontal, but due the reddish distinct color of the associated deposits, these erosional surfaces can be traced down-dip into bioturbated sandstones. The downdip extension of erosional surfaces ranges from 30 to 150 m (mean 60 m) and they are spaced between them 20 to 40 m. Their dip direction is highly variable but always coincident with the quadrant towards which dips of foresets are oriented.
The palaeocurrent data were measured mostly in the individual foresets (S3nb) that dip dominantly from N to NW for the parasequence PS300 with a mean dip direction of N323.3°, while foresets measured within the parasequence PS400 dip from N to NE with a mean of 21°. In both cases, the average dipping direction of foresets is approximately shore-perpendicular.
Interpreted shoreline morphology
Much of the inclined beds are composed of planar-laminated facies, parallel to the beds related to swash and backwash processes (Fig. 8a; Reading and Collinson, 1996; Plint, 2010). Contrary to the Architectural Style 1, where upper shoreface was characterized by trough cross-stratification, these deposits lack of facies that could suggest the presence (and preservation) of dunes related to wave-generated currents. The widespread development of plane bed seems to be favored when the surf zone is narrow or even inexistent and the swash zone is wider than the surf zone (Aagaard et al., 2013). The sedimentation associated with the surf zone could be attributed to the small-scale trough cross-stratification characterizing the bottomsets of inclined beds.
The interpretation of a narrow, or even inexistent, surf zone as well as an extensive development of the swash zone would be directly linked to the steepness of the nearshore seabed profile, as the steeper profile, the narrower the surf zone. In this context, the large-scale inclined sandstone beds described have been interpreted to represent high-gradient, marine depositional profiles, of a non-barred configuration (Fig. 8b; Isla et al., 2020). Most of this non-barred profile was covered by planar-laminated facies produced under oscillatory or combined flows (Schwartz and Birkemeier, 2004), and without a distinct facies break between the foreshore and the proximal upper shoreface. Seaward of the breaker zone (i.e., in the distal upper shoreface), onshore-migrating asymmetric ripples and small dunes were commonly formed and preserved (Fig. 8b), most likely under oscillatory-dominant flows (Clifton, 2006; Cummings et al., 2009). A non-barred setting is also supported by the absence of a surf diastem and the consistent seaward accretion of foresets and foreset packages for hundreds of meters with no landward dipping surfaces (Fig. 8c).
Successive seaward-dipping foresets (S3nb) represent the accretion of upper-shoreface/foreshore deposits during shoreline progradation, regularly-spaced erosional surfaces recognized (Fig. 8c). At the same time, bounding foresets packages (S2nb) are interpreted to represent beach scarps generated during stages of beach retreat (Fig. 8b). These surfaces represent significant sediment erosion and export to the lower shoreface that can be triggered by exceptional storms or periods of high wave energy (Isla et al., 2020). The presence of first-order surfaces bounding sets of foreset packages (S1nb) and the abrupt changes in the dominant dip orientation would be related to beach rotation processes (Anthony, 2008; Davison-Arnott, 2010), associated with major-scale reconfigurations of the nearshore system, as seen in beach-ridges set boundaries (Otvos, 2000; Tamura, 2012).
INTRA-PARASEQUENCE ANATOMY
The detailed facies and architectural analysis of upper-shoreface and foreshore deposits allowed interpreting two different shoreline morphologies (Figs. 6b and 8b), recorded within the progradation of each parasequence (Fig. 9) and corresponding to a barred and non-barred nearshore configuration that succeeded though time. Both the number of bedsets with upper-shoreface and foreshore deposits and their proportion in a parasequence indicate an important difference between barred and non-barred shoreline deposits. The first were recorded in the upper-shoreface and foreshore deposits of 10 bedsets (out of 13) of the parasequence PS400 and in 4 (out of 5) of the parasequence PS300, in both cases involving around the 90% of the total parasequence thickness (Table 3). On the contrary, non-barred deposits were exclusively recorded in the uppermost bedset of each parasequence, representing just 10% their cumulative of the thickness (Table 3). Thicknesses of upper-shoreface and foreshore deposits also mark an important difference between the barred and non-barred record. In the first case, the average thickness is less than a meter (0.96 m), while for non-barred deposits they can reach 2.66 m (Table 3). When the thickness of lower shoreface is compared, values are quite similar (1.35 and 1.40 m). The shoreline trajectory for AE1 deposits commonly define an aggrading to low prograding evolution (high angle sensuHelland-Hansen and Martinsen, 1996), while for non-barred the shoreline trajectory has low angle corresponding to a moderate rate of progradation relative to aggradation.
The described transition from AE1 to AE2 architectural style is also correlated to changes in the proportion of carbonate and siliciclastic components (Table 3). The storm-dominated shoreface-offshore deposits interpreted for the Pilmatué Member are characterized by a limited abundance of carbonate grains such as bioclasts and ooids, which led to define it as a siliciclastic-dominated mixed system. However, the distribution of carbonate components is not homogeneous across the whole system but it is restricted to upper-shoreface and foreshore deposits. The abundance of carbonate particles within AE1 deposits (67%; Table 3) could be related to the effectiveness of longshore currents (Isla et al., 2018; Schwarz et al., 2018). However, AE2 deposits also have a high proportion of mixed deposits (60%; Table 3) suggesting that the influence of longshore currents was not the sole control. The abundance of carbonate grains within the proximal areas of the system was always constant during the evolution of the Pilmatué Member.
Changes in sediment transport dynamics for wave-dominated shorelines are intimately related to the wave behavior (Davidson-Arnott, 2010; Aagaard et al., 2013). Barred coasts have been considered as the resulting morphology of dominantly dissipative conditions where the wave energy is more effectively dissipated during their approach to the shore (Wright and Short, 1984). Dissipative coasts and nearshore bars are directly related to low gradients (Wijnberg and Kroon, 2002; Aagaard et al., 2013; Hughes et al., 2014), where sediment is continuously redistributed by longshore processes. On the contrary, non-barred coasts are closely related to more reflective conditions where weak longshore currents are not capable to redistribute sediment which tends to be accreted to the shore generating high-gradient and bar-less configurations (Isla et al., 2020). The identified transformation in shoreline morphology, reflected in the rock record (Table 3), is the long-term response to a change in wave behavior from dissipative to reflective dynamics.
The larger thicknesses of upper-shoreface/ foreshore deposits for non-barred conditions compared to barred conditions could be associated with a ‘relative excess’ of sediment budget within the proximal areas related to two aspects: the dominance of onshore sediment transport by swell waves typical of non-barred (reflective) conditions and the weakness of longshore currents that may redistribute the sediment alongstrike (Aaggard et al., 2013).
RESPONSES TOWARDS DISTAL-SHOREFACE SETTINGS
Changes in shoreline morphology are associated with the nearshore setting, which represents the most variable and dynamic part of the marine depositional system (Davidson-Arnott, 2010). The distal parts of the shoreface should be also influenced by the long-term prevailing conditions of shoreline dynamics (i.e. waves, currents, sediment budget; Fig. 10a). The analysis of lower-shoreface facies correlatable to the described barred and non-barred deposits were analyzed. Each lower-shoreface interval was evaluated in terms of their proportions of facies generated by fair-weather and storm conditions, showing significant differences. The obtained results indicate that those bedsets where barred configurations were interpreted, present both types of facies in variable proportions (Fig. 10b). These lower-shoreface deposits are almost entirely composed of interbedded fair-weather ripple cross-laminated or highly bioturbated sandstones, with storm-related beds with hummocky or swaley cross-stratification (HCS and SCS) (Fig. 10c). Instead, intervals where non-barred shoreline settings were interpreted are exclusively composed of ripple-laminated and/or bioturbated sandstones deposited during fair-weather conditions (Fig. 10d). These lower-shoreface deposits completely lack of sandstones with HCS.
During extended fair-weather conditions, the dominant onshore transport and the dominant swell waves generate a net sediment transport to the proximal areas, “starving” the distal parts where no bars are constructed (Fig. 10a). These prevailing transport dynamics characterize non-barred morphologies, where there are important gradient differences between the nearshore and the lower shoreface. Instead, during storm conditions, the waves and down-welling currents transport masses of sediment seaward, which brings the necessary budget for bar building (Fig. 10a; Davidson-Arnott, 2010). Also, during storms the erosion of proximal areas and redistribution towards the outer part of the shoreface smooth the coast gradient.
The shoreface is an exchange and buffering zone dominated by friction, constantly changing in order to balance the sediment budget of the system (Fig. 10a; Swift, 1975; Niedoroda et al., 1985; Cowell et al., 1995; Anthony, 2008). Wave-climate changes impact directly on the resulting shoreline morphology and they have a correlative expression towards the distal parts of the system, especially in those located above the depth of closure (Ortiz and Ashton, 2016). Even the sum of onshore-directed and offshore-directed flows for a coastal setting is balanced over short periods (Wright et al., 1991); imbalances may occur among larger scales of time, due to the dominant cross-shore transport conditions. During fair-weather conditions, wave-dominated shorelines are dominantly affected by swell waves, which transport much of the carrying sediment onshore, and hence produce the starvation of distal parts of the shoreface and the absence of bars (Fig. 10a; Davidson-Arnott, 2010). On the other hand, storm-waves tend to export sediment towards the distal areas favoring the building of bars. In that sense, because of their distinct wave properties and transport dynamics, fair-weather and storm processes generate two different lower-shoreface architectural styles and facies successions. During fair-weather, the shoaling zone is mostly covered by symmetric to asymmetric ripples produced by oscillatory-dominated combined flows and an intense organic activity favored by the well-oxygenated water column (Fig. 10a). On the contrary, the strong and high-amplitude storm waves produce hummocky bedforms that result in the preservation of sandstones with HCS. Storm processes also constitute stressful conditions of the environment that inhibit the colonization (Fig. 10a), limiting the benthic activity to escape traces of opportunistic fauna. There is a clear correlation between the morphology of proximal nearshore settings and the prevailing processes of sediment exportation, which are reflected in the preserved deposits of the lower shoreface.
Discussion: controls in the transition from barred to non-barred morphology
During the last two decades, several studies have analyzed the intra-parasequence stratigraphic architecture of deltaic and strandplain successions (Gani and Bhattacharya, 2007; Hampson et al., 2008; Sømme et al., 2008; Charvin et al., 2010; Forzoni et al., 2015). It is well known that high-frequency changes in depositional conditions triggered by autogenic or allogenic factors may occur resulting in the development of bedsets (Van Wagoner et al., 1990). However, the high-resolution analysis of such progradational wave-dominated successions has not documented significant changes in depositional facies and architectural elements as the ones described for the Pilmatué Member. The complex stratigraphic architecture reconstructed for the Pilmatué Member demonstrates other changes than the ones resulting from the development of besets and that result in different shoreline morphologies (Isla et al., 2018). Such complexity combines with changes in sediment transport dynamics operating for longer, that result in different preserved shoreline morphologies (Figs. 6b and 8b). However, the defined three different scales of changes need to be framed in a sequence-stratigraphic context. The main processes associated with both ends of defined scales and hence, with development of parasequences and bedsets have been already discussed previously (Schwarz et al., 2018; Isla et al., 2018). The change in architectural style between bedsets and within a parasequence records an intermediate high-frequency change that occurred during the progradation of a single parasequence, but of a larger scale than the one responsible for the development of successive besets. The transformation of the shoreline morphology due to changes in depositional conditions could be explained by determinate controlling factor or even a combination of different factors therefore, operating at different scales (Heller et al., 1993; Burgess et al., 2006).
High-gradient profiles, where bar generation is commonly inhibited, are characteristic of reflective conditions, whereas for low-gradient profiles, the energy of waves is dissipated and used for building nearshore bars (Davidson-Arnott, 2010; Aagaard et al., 2013). As a significant change in sediment grain-size -that may help explain the gradient change and therefore the transition to reflective conditions- has not been registered within the studied successions, this change must reflect the morphological responses to balance modifications in cross-shore and along-shore sediment budget. In that sense, an increase in the nearshore gradient could be related to a relative excess of sediment on the proximal parts of the shoreline system and different processes are proposed to explain the relative excess of sediment and consequent steepening of the nearshore, which result in the change from barred to non-barred conditions during the parasequence evolution.
The first possibility is that changes in the sediment budget of nearshore settings were associated with changes in the prevailing cross-shore and long-shore transport mechanisms (Fig. 11a). Low-gradient, sand-rich beaches are typically dissipative with a wide surf zone and an irregular cross-shore profile with nearshore bars and troughs. In this context, the influence of storms and longshore currents plays a key role in terms of sediment redistribution. Instead, high-gradient, sand-rich beaches tend to develop under reflective conditions where dominant swell waves result in an onshore sediment transport (Davidson-Arnott, 2010; Aagaard et al., 2013). In this regard, the change from low gradient (barred) to high-gradient (non-barred), could be related to climatic oscillations, more specifically to wave-climate variations. Wave climate may be relatively constant for timescales of 102 to 103 years (Li et al., 2015), but not necessarily for longer periods. Previous studies have demonstrated that wave-climate variations could generate imbalances in sediment budget at the scale of individual bedsets (Storms and Hampson, 2005; Isla et al., 2018).
A second possibility is that the steepening of the nearshore could be related to long-lived prograding conditions, which eventually should result in large amounts of available sediment over the proximal settings. The studied parasequences of the Pilmatué Member constitute dominantly aggrading to prograding stratigraphic units that need a constant increase in accommodation to build-up. In that sense, the maximum rates of progradation should be reached towards the upper part of the parasequence, if accommodation increases relatively constant and the rate of sediment supply do not change. If the rate of relative sea-level rise remains constant, a higher rate of sediment supply is necessary to balance the increasing accommodation and maintain the progradation. Therefore, the preferential occurrence of non-barred conditions towards the uppermost bedset within a parasequence (Fig. 9) could be associated with a long-lived high rate of sediment supply to the shore (Fig. 11b). These could be associated with large amounts of sediment that are accreted to the coast by onshore transport. If the mechanisms of exportation or along-strike redistribution are not efficient enough, the relative excess of available sediment within the proximal settings would result in the steepening of the nearshore, regarding the rest of the depositional profile. This increase of the nearshore gradient would trigger the formation of a prograding, high-gradient non-barred shoreline. Therefore, this model proposes the presence of high-gradient, non-barred conditions as a morphological response of the depositional profile due to long-lived normal regressive conditions.
Finally, the generation of non-barred conditions could also represent the prelude of depositional changes triggered during the deepening of the system, which eventually would result in the generation of a large-scale flooding surface (parasequence boundary). Under a protracted and slow relative sea level rise, without significant changes in sediment supply, the sediment budget may exceed the accommodation within proximal areas, but simultaneously be relatively smaller for the distal settings (Fig. 11c). Besides, the relative deepening generates the weakening of longshore currents, which are responsible of the sediment redistribution the system. If longshore transport, operating over the nearshore, is not efficient enough, it could trigger a sediment excess in the proximal area and a relative “sediment starvation” towards the outer parts of the shoreface-offshore system. If this situation remains, the adjustment of the system profile may result in a relative “autosteepening” or a passive increase in the gradient of the nearshore.
The proposed models explain a morphological response of the shoreline to ongoing changes on the prevailing transport conditions and particularly, to a relative increase in the available sediment over the nearshore settings. However, these changes in depositional conditions could be triggered by allogenic processes controlling the parasequence evolution, as relative changes in sea level or changes in sediment supply. The vertical stacking of successive bedsets in both studied parasequences show a progradational stacking pattern with seaward migration of the shoreline up to the top (Figs. 4b and 4d), which seems to exonerate the possibility that non-barred morphologies were generated during transgressive conditions. Hence, the steepening of the nearshore and the consequent installation of non-barred conditions would be related to wave climate changes and/or long-lived regressive conditions.
CONCLUSIONS
The present work documents changes in shoreline morphology interpreted from the preserved depositional architecture of each studied parasequence (104 to 105 y). The facies and architecture of barred and non-barred deposits represent the dominant processes that governed the hydrodynamic conditions of sedimentation, which prevailed over long-lived periods. The detailed analysis of the sedimentary facies and the depositional/ erosional surfaces within the upper-shoreface and foreshore deposits of successive bedsets composing a parasequence led to reconstruct how the shoreline morphology may have changed from persistent bar-trough configurations to the absence of nearshore bars through time. Those changes also represent the transformation from a relatively low-gradient beach profile to a high-gradient profile due to the adjustment to new conditions in sediment transport dynamics. The transition from barred to non-barred conditions implicates that the coastal sediment transport shifted from a longshore-dominated to an onshore-dominated dynamics due to a weakening of wave-induced currents and strengthen of swell conditions.
There is an increasing requirement to incorporate the analysis of upper-shoreface and foreshore deposits to the high-resolution depositional models of shallow-marine environments. Sequence stratigraphy either as a conceptual model or a methodology for understanding the sedimentary record is in constant metamorphosis of its concepts. Results led to propose a number of possible controls that could triggered the change from barred to non-barred conditions. Invariably, the proposed models are related to a mechanism of steepening of the nearshore profile due to a relative excess in the sediment budget compared to the rest of the system. The triggering mechanism for the steepening of nearshore settings may be related to climatic external changes (variations in wave climate), or to changes in the sequence-stratigraphic context (regressive or transgressive conditions). Based on the progradational stacking pattern of both studied parasequences, the transgressive model could be dismissed. Notwithstanding, more research in needed to be able to establish the dominant control. The interpretations made, suggest that mid-term evolution of clastic shorelines may result in a complex sequence-stratigraphic architecture. A better understanding of high-frequency processes operating and responsible from intra-parasequence changes is increasingly necessary.