Can halite form progradational lowstand wedges? New insights into basin margin evaporites from the Permian Zechstein

Zechstein platforms in the Southern North Sea were thought to be fringed by kilometre-scale high-energy progradational carbonates with lowstand wedge geometries. Instead, a new well encountered a halite-rich evaporitic wedge that replicates the coveted progradational carbonates. This study integrates sedimentological observations with geochemistry, petrophysics and 3D seismic data to characterise the evaporitic replicas and then distinguish them from the genuine carbonate platform geometries. The evaporitic wedges generate laterally symmetrical rims around the platforms which are approximately 150m thick and 2000m wide. Wedge precipitation predated the well-characterised basin-wide Z2 drawdown event, and therefore the localised evaporitic progradational system was onlapped by basin-fill halite. The evaporitic wedges often feature domino slumping along weak progradational bedding planes, resulting in blocks that collapsed into the basin. The earliest progradational beds contained the highest concentration of microbialites which assisted stabilisation of the evaporitic wedges on the rims of carbonate platforms, whilst also indicating that the wedges were part of an environmental transition from marine highstand into basin desiccation. Microbial fragments were reworked downslope into brecciated debris flow deposits, demonstrating the relationship between wedge construction and syndepositional collapse on the outer slopes. Petrophysical and geochemical data show that the evaporitic wedges were closely associated with the early stages of drawdown, and a lack of post-anhydrite marine recharge resulted in the depletion of Ca²⁺ from the brine and accordingly limited anhydrite (CaSO₄) precipitation within the wedges. This depletion in brine Ca²⁺ led to dolomitic microbialites (CaMg(CO₃)₂) in the lower wedge; however, the upper wedge features Kutnohorite (Ca(Mn,Mg)(CO₃)₂) showing that Mg²⁺ also eventually became depleted and was replaced by Mn²⁺. This analysis characterises a rare evaporitic phenomenon which can hide in plain sight in seismic data and act as a depositional intermediary between platform construction and basin-fill halite precipitation.