Depositional architecture of a wave-dominated clastic shoreline (Pilmatué Member, Argentina): Linking dynamics and stratigraphic record of bar-trough systems

Abstract

Bar-trough morphologies are a dominant feature of modern wave-dominated clastic shorelines. Despite thesebarred configurations being widely described from modern systems there is still some uncertainties associatedwith their interpretation in the ancient record. There is a lack of detailed outcrop-based studies describing thefacies distribution and architecture within bar trough systems which causes a disparity between observations ofmodern processes and the subsequent preserved rock record.This work presents a detailed characterization and interpretation of the sedimentary record of a bar-troughsetting. This was achieved through the analysis of nearshore (i.e. upper shoreface and foreshore) deposits withina parasequence in the Lower Cretaceous, Pilmatué Member of the Agrio Formation (Neuquén Basin, Argentina).A high-resolution facies and architectural analysis was developed, combining detailed sedimentary logs with themapping of bounding and internal surfaces within the nearshore deposits. The facies analysis suggests thattrough and planar cross-bedded facies are remnants of dunes in the upper shoreface due to a strong influence oflongshore currents, while planar laminated facies would be the result of swash processes in the foreshore.Additionally, the presence of a well-defined surf diastem, the persistent low-angle obliquity of palaeocurrentsand the preservation of concave-upward stratigraphic surfaces led to the interpretation of those deposits asrepresenting the production of a bar-trough configuration with longshore currents distributing large amounts ofsediment along the shore. This analysis allowed us to reconstruct the dominant processes for sediment transportand deposition within these barred nearshore settings and to identify different key architectural surfaces for theupper-shoreface and foreshore deposits. The architectural analysis of such surfaces led to define four differentorders of hierarchy, which respond to depositional and erosional processes operating at different scales. Thecomparison between different resulting architectures of wave-dominated barred shoreline offers the possibilityto improve in the palaeoenvironmental reconstructions for ancient or recent successions. The proposed model forbar-trough clastic shoreline successions relates an absence of bar-related deposits to major storms and shorelineerosional processes. This conceptual model offers a new interpretation for explaining the mid-scale evolution ofbar-trough settings and the relationship between the modern and ancient record.