The phoenix of beachrocks: Simultaneous breakdown and formation of an unusual facies on a high energy coastline (Mission Rocks Beach, 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 or when studying shoreline evolution over the centurial to millennial scales. Surprisingly however, the facies variability of beachrocks remains understudied as they are almost exclusively described as seaward dipping, slab-shaped outcrop forming in low energy dissipative beach environments. The Mission Rocks coastline of north-eastern South Africa is in stark contrast. Here the coast comprises an up to 3 m thick raised shore platform of beachrock, where a variety of sedimentological facies are observed. These comprise seaward-dipping planar bedded sandstones and conglomeratic units, often interbedded with bimodally-orientated trough cross bedded sandstones. In our study we aim to use sedimentological facies analysis, petrography and cathodoluminescence to unravel the deposition- and cementation processes of this beachrock facies.

In particular, an unusual beachrock breccia interposed amongst the breakdown remnants of the platform forms the basis of this paper. The breccia documents a cycle of simultaneous erosional breakdown and depositional buildup of the beachrock platform, a yet undescribed process for the development of beachrock.  Since it forms as a thin veneer (< 0.10 m), with a slightly thicker infill (≤ 0.5 m) amidst erosional hollows and gullies of the + 2 m high rocky platform, it raises into question the necessity of a thick sedimentary overburden, that is typically considered the requirement for beachrock cementation in the mixing zone.  Timing of beachrock formation is constrained by recent anthropogenic activities, as the underlaying platform was mined for building purposes during WWII and it is in these quarry slots and crack that the beachrock is found. While it is generally suspected that beachrocks may form at the centennial scale, evidence for this remains weak. Not only can the interpretation of this facies contribute to our understanding of the long term processes that form and break down beachrocks on high energetic coastlines, it provides insight into rapid beachrock formation and as such its utility as a sea level index point.