Rapid beachrock cementation on a South African beach: Linking morphodynamics and cement style

摘要

Rapid cementation (5 years) of beachrock is reported from the wave-dominated, microtidal coastline of Durban, South Africa. The beachrock is developing at mean sea level on a steep (10°–30°), reflective beach prone to periodic erosion by high-energy wave action; the most recent erosional event resulted in retrogradation of the beach by up to 10 m landward of the beachrock outcrop discussed in this study. Transmitted light microscopy and scanning electron micrographs show the precipitation of two dominant cement types which are linked to the beachface morphodynamics. During the accreted beach phase, when the locus of beachrock formation lay well landward of the beachface in the undersaturated upper intertidal to supra tidal backbeach, meniscus cements were precipitated. Following a period of erosion, associated with a landward shift of the beachface, the incipient beachrock was then exposed to saturated conditions which promoted growth of acicular cements. Contemporary beachrock cementation occurred very rapidly (5 years) between significant erosional events. The observed cement textures record short-term beach progradation and retrogradation but are similar to those typically associated with longer-term regression and transgression.