Middle Jurassic isolated reefal platform in an argillaceous contourite drift system in Northern Switzerland

This study investigates the influence of currents on the sedimentary dynamics and geological processes within the Laurasian Seaway during the Middle Jurassic in Northern Switzerland. Using seismic, core, and log data, we identify and characterize a previously undocumented coral reef bearing platform, termed the «Herrenwis Unit», within the region’s predominantly siliciclastic strata. Contrary to earlier interpretations that attributed sediment transport primarily to storm activity and thickness variations to tectonic activity, our findings reveal that marine currents were the dominant force shaping the sedimentary architecture and reef development in this area. Strong evidence for current activity is provided by several depositional features. First, an elongated, mounded body within the underlying silty claystone unit forms the foundation of the oval-shaped Herrenwis reefal platform. Second, during the growth of the platform a moat establishes along the platform, separating the platform from the mounded basinal sediment. Finally, moat and mounded drift geometries are observed in the overlying strata, indicating continuous current influence during the Late Jurassic. The pronounced presence of contourite drifts, and the distinct oval morphology of the reef, aligned with prevailing currents, underscore the significant influence of these currents on sediment deposition and reef ecology. The transition from aggradation to progradation within the reef and the presence of an iron-rich hardground at its top further highlight the role of shifting hydrodynamic conditions in the reef's eventual demise. This study offers new insights into the complex interplay between marine currents and sedimentary processes in the Jurassic epicontinental seas, challenging existing paradigms on coral reef formation in low-relief, current-dominated environments.