What beachrock can and can not do as a sea level indicator: setting out to refine the model of beachrock facies interpretation based on examples from Oman and South Africa

Beachrocks are coastal sediments that are lithified through the precipitation of carbonate cements. It is widely acknowledged that lithofacies in beachrocks are variable and their interpretation is useful when using beachrock as a sea-level indicator. Surprisingly, the facies variability of beachrocks remains understudied as they are almost exclusively described as seaward dipping, sandy, slab-shaped outcrop forming in low energy dissipative beach environments. The Mission Rocks coastline of South Africa is in stark contrast. Here the coast comprises an up to 3 m thick platform of beachrock, where a variety of sedimentological facies are observed. The northeastern coastline of Oman is rich in unusually coarse beachrock conglomerates and also beachrocks showing a diverse ichnological community. In our studies we aim to use sedimentological facies analysis, petrography and SEM to unravel the deposition- and cementation processes of several beachrock facies. Beachrock lithofacies that can be linked to a certain sea-level are then utilised to refine the current model of beachrock facies interpretation used in sea-level research. However, some of the newly described facies raise questions about the accuracy of beachrock facies interpretation. In particular, an unusual beachrock breccia interposed amongst the breakdown remnants of the platform at Mission Rocks beach comes to mind. The breccia documents a cycle of simultaneous erosional breakdown and depositional build-up of the beachrock platform, a yet undescribed process for the development of beachrock, and inhibits a cement mineralogy indicative of marine phreatic cementation while forming in the supratidal zone.