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Relict and Recent Beachrock from Southern Africa

1974; Geological Society of America; Volume: 85; Issue: 12 Linguagem: Inglês

10.1130/0016-7606(1974)85 2.0.co;2

1943-2674

William G. Siesser,

Archaeology and Rock Art Studies

Research Article| December 01, 1974 Relict and Recent Beachrock from Southern Africa WILLIAM G. SIESSER WILLIAM G. SIESSER 1Department of Geology, University of Cape Town, Rondebosch 7700, South Africa Search for other works by this author on: GSW Google Scholar GSA Bulletin (1974) 85 (12): 1849–1854. https://doi.org/10.1130/0016-7606(1974)85 2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation WILLIAM G. SIESSER; Relict and Recent Beachrock from Southern Africa. GSA Bulletin 1974;; 85 (12): 1849–1854. doi: https://doi.org/10.1130/0016-7606(1974)85 2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.