Advantages of integrated geophysical, sedimentary, and dating approaches for reconstructing Holocene tidal inlet evolution: A case study from offshore the Belgian coastal plain

Relict tidal inlets preserved on continental shelves provide key evidence for past shoreline positions and coastal evolution, but their recognition is often hindered by erosion, reworking and fragmentary preservation. Here we integrate high-resolution sub-bottom profiling, sedimentological facies analysis, radiocarbon dating, and an innovative application of optically stimulated luminescence (OSL) profiling to reconstruct the mid- to late Holocene evolution of a tidal inlet complex located offshore the Belgian coastal plain. Six major channels (A–F) were acoustically mapped and linked to sediment cores, revealing channel fills, lateral-accretion packages, and prograding platform deposits. Luminescence profiling of cores established a relative chronostratigraphy that, once anchored to radiocarbon dates and stratigraphic relationships, resolved anomalous ages and refined the chronology of channel activity. Results indicate that all of these mapped channels correspond to repeated phases of tidal inlet incision, migration, and reoccupation within a confined coastal sector over a period of at least four millennia. The preserved stratigraphy demonstrates that the inlet complex remained fixed in broadly the same position between 5.6 ka and 1.7 ka BP. This implies that the coastline and its associated barrier system persisted at approximately 2 km seaward of the present-day coastline for about 4000 years, despite sea-level rise and transgressive erosion. These findings provide the first offshore evidence complementing and challenging reconstructions from land-based studies of the Belgian coastal plain. More broadly, they demonstrate how integrating geophysical imaging, sedimentary facies, radiocarbon dating and OSL profiling can overcome the limitations of classic approaches by resolving reworked ages, constraining inlet mobility, and delivering more robust palaeogeographic reconstructions of Holocene coastal change. This approach also allows to reconstruct former coastline positions. • Luminescence profiling improves chronology of relic tidal inlets offshore Belgium. • Repeated incision and infill phases over four millennia are preserved offshore. • Offshore records complement and challenge onshore reconstructions. • A barrier system constrained inlet position despite sea-level rise and erosion. • Coastline was at least 2 km seaward of the present shore for most of the Holocene.