Holocene marine hardground formation in the Arabian Gulf: Shoreline stabilisation, sea level and early diagenesis in the coastal sabkha of Abu Dhabi

This study provides the first comparison between a seaward and a landward section of the same diachronous hardground surface observed in the coastal sabkha of Abu Dhabi. This hardground is described here in terms of its mode of formation, its diagenetic environment and its impact on shoreline stabilisation during transgression. The hardground is exposed in the intertidal zone and buried by a late Holocene prograding succession of carbonates, evaporites and microbial sediments in the supratidal zone. The hardground itself is composed of bioclastic grains, primarily of aragonitic composition, that originate from intertidal depositional environments. Aeolian silt to sand-sized quartz grains are also observed. Lithification occurred through the precipitation of pore-filling aragonite, high-Mg calcite and dolomite cements from sea and interstitial water that was marked by high salinities and temperatures, as confirmed by stable isotope analyses. High-Mg calcite and non-stoichiometric dolomite are also observed as secondary recrystallisation products. The formation of these two mineral phases as recrystallisation products was possibly microbially-mediated. Lithification progressed in two phases, the older phase of which is marked by higher amounts of non-stoichiometric dolomite and high-magnesium calcite as compared to the younger phase. Transgressive reworking of precursor siliciclastic sands was inhibited by the development of transgressive pore-filling gypsum cements in the supratidal zone.