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 these barred configurations being widely described from modern systems there is still some uncertainties associated with their interpretation in the ancient record. There is a lack of detailed outcrop-based studies describing the facies distribution and architecture within bar trough systems which causes a disparity between observations of modern processes and the subsequent preserved rock record.This work presents a detailed characterization and interpretation of the sedimentary record of a bar-trough setting. This was achieved through the analysis of nearshore (i.e. upper shoreface and foreshore) deposits within a 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 the mapping of bounding and internal surfaces within the nearshore deposits. The facies analysis suggests that trough and planar cross-bedded facies are remnants of dunes in the upper shoreface due to a strong influence of longshore 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 palaeocurrents and the preservation of concave-upward stratigraphic surfaces led to the interpretation of those deposits as representing the production of a bar-trough configuration with longshore currents distributing large amounts of sediment along the shore. This analysis allowed us to reconstruct the dominant processes for sediment transport and deposition within these barred nearshore settings and to identify different key architectural surfaces for the upper-shoreface and foreshore deposits. The architectural analysis of such surfaces led to define four different orders of hierarchy, which respond to depositional and erosional processes operating at different scales. The comparison between different resulting architectures of wave-dominated barred shoreline offers the possibility to improve in the palaeoenvironmental reconstructions for ancient or recent successions. The proposed model for bar-trough clastic shoreline successions relates an absence of bar-related deposits to major storms and shoreline erosional processes. This conceptual model offers a new interpretation for explaining the mid-scale evolution of bar-trough settings and the relationship between the modern and ancient record.