Paleoenvironmental reconstruction of a coastal barrier system guided by sedimentary facies and diatom assemblages

Coastal barriers are complex depositional systems where high-energy front-barrier zones coexist with low-energy back-barrier subenvironments. This dynamic setting results in a range of wave- and tide-influenced processes that govern their evolution. Understanding such evolution requires high-resolution studies that integrate sedimentological, paleohydrodynamic, and paleoecological data. In this context, diatom analysis is a valuable and widely used tool for reconstructing Holocene paleoenvironmental conditions, distinguishing marine, brackish, and freshwater influences. The Oriental Barrier, on Argentina’s Atlantic coast, exemplifies one of these dynamic systems, shaped by the interaction of front-, and back-barrier subenvironments. A facies and diatom assemblage analysis of Holocene deposits in its northern sector, based on sediment cores, defined six facies associations (FA1–FA6) representing both environments. The vertical and lateral distribution of these facies allowed for the reconstruction of mid-term barrier evolution. As the sandy spit prograded northward, forming a beach-ridge plain, the adjacent back-barrier area filled with sediments from sinuous tidal creeks, tidal flats, and marshes. The study area exhibits a unique depositional model where sandy spits and associated back-barrier tidal flats migrated northward in tandem, parallel to the shoreline, departing from classical models where back-barrier systems migrate perpendicularly. Sedimentological and paleoenvironmental data reveal a complex interaction zone between front- and back-barrier environments, with varying degrees of facies interdigitation and vertical successions influenced by tidal dynamics and sediment supply. This system, situated at the terminus of a large-scale littoral cell, supports the idea that laterally migrating barrier–tidal flat systems might be more common in such settings, though further case studies are needed to refine coastal sediment routing models.