Deciphering megafan systems from the geological record: The classical fluvial Buntsandstein deposits revisited (NE France and SW Germany)

Fluvial successions are commonly interpreted as trunk river deposits. However, recent advances in fluvial sedimentology suggest fluvial fan systems may be more prevalent than previously recognised. Recognising megafans in the rock record remains challenging, requiring integration of multiple datasets across extensive areas to overcome inherent stratigraphic limitations. Using the Grès Vosgien Formation (Middle Buntsandstein, Early Triassic) as a case study, the present study applies a multi-approach method to challenge previous trunk river model and proposes a qualitative and quantitative methodological framework to recognise megafan systems in the geological record. The Grès Vosgien Formation in NE France and W Germany has been interpreted as braided trunk fluvial deposits interbedded with aeolian and playa-lake deposits. Sedimentological analysis of 52 outcrops across ~5000 km², and a geothermal borehole within the Upper Rhine Graben, identifies three facies associations: fluvial channel-fill, sheet flood, and wind- and water-laid. Their spatial distribution reveals three depositional zones: proximal, dominated by amalgamated fluvial deposits; medial, with increasing sheet flood and wind- and water-laid deposits preservation; and distal, characterised by predominant aeolian deposition. Four diagnostic criteria support a megafan reinterpretation: proximal-to-distal facies architecture, radial palaeocurrent pattern, palaeohydraulic trends, and subtle downstream grain-size reduction. Deposits extending >80 km in length define a megafan. Four successive evolutionary phases record retraction and expansion through fluvial-aeolian system interaction. Stratigraphic evolution reflects megafan migration, exposing deposits to aeolian reworking, rather than exclusively climatic control. Integrating architectural analysis, palaeohydraulic reconstruction and regional correlation, the present study proposes a methodological framework for megafans recognition in continental successions.