Abstract

Beachrock forms a broken pavement along part of the southwestern coast of South Africa. The rocks are found in the intertidal zone as well as slightly above high-tide and below low-tide level. The beachrock is composed chiefly of quartz grains, with lesser amounts of skeletal fragments, cemented by micrite. Carbonate mineralogy of the rocks consists of low-Mg calcite and aragonite, with low-Mg calcite predominating. Laminated calcrete (wholly low-Mg calcite) covers much of the beachrock exposures. The presence above spring-tide level, poor correspondence to the composition of the surrounding beach, and presence of abundant low-Mg calcite suggest that the beachrock is relict. A C14 date on the beachrock of 25,860−1,190+1,040 yr B.P. confirms this; the laminated calcrete layer overlying the beachrock formed 25,430−1,210+1,050 yr B.P. It is believed that this beachrock formed in an intertidal environment during withdrawal of the sea from its Würm I/II interstadial high. Subsequent subaerial exposure has resulted in diagenetic alteration of some of the original metastable carbonate minerals to the stable variety, low-Mg calcite.

Recent beachrock occurs in the intertidal zone at Vilanculos, Mozambique. The rocks consist of layers of coarse-grained skeletal fragments cemented by fibrous aragonite, alternating with layers of quartz grains cemented by micrite. The carbonate mineralogy is almost wholly aragonite. The components and mineralogy of the surrounding beach correspond closely to that of the beachrock. C14 dates of 910−140+120 and 920−150+140 yr B.P. were obtained for these rocks.

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