Annals of the South African MuseumAnnale van die Suid-Afrikaanse Museum . on & Maddocks Caudites knysnaensis Hartmann . Procythereis sp. Urocythereis sp. Loxoconcha parameridionalis Benson & MaddocksL. peterseni HartmannCytheromorpha sp. .Br ad ley a sp. Xestoleberis capensis G. W. MiillerCytherella punctata Brady .(Values expressed as percentages separately for Ostracoda and Foraminifera. :but not counted.) ddocks 6 6 < 1 X 1 . 4 11 < 1 XX 27 22 412 38 < 1 < 121 16 X 203 25 49 49 7 25 8 7 10 17 1 X 56 X 8 12 21 < 1 present Krumbein & Sloss (1963) discuss the origins and associa
Annals of the South African MuseumAnnale van die Suid-Afrikaanse Museum . on & Maddocks Caudites knysnaensis Hartmann . Procythereis sp. Urocythereis sp. Loxoconcha parameridionalis Benson & MaddocksL. peterseni HartmannCytheromorpha sp. .Br ad ley a sp. Xestoleberis capensis G. W. MiillerCytherella punctata Brady .(Values expressed as percentages separately for Ostracoda and Foraminifera. :but not counted.) ddocks 6 6 < 1 X 1 . 4 11 < 1 XX 27 22 412 38 < 1 < 121 16 X 203 25 49 49 7 25 8 7 10 17 1 X 56 X 8 12 21 < 1 present Krumbein & Sloss (1963) discuss the origins and associations of evaporitedeposits. Two conditions are necessary for the genesis of evaporites: a warmand arid climate with little freshwater run-off, and a restricted body of sea-water. Excessive evaporation at high temperature and little dilution by inflowof freshwater raises salinity above that of the open sea, and ultimately leadsto precipitation of salt. Calcium carbonate is the first to precipitate when 92 ANNALS OF THE SOUTH AFRICAN MUSEUM Bobbejaansberg. Modem dune sand Evaponte Calcareous quartzose sand Table Mountain sandstone Fig. 9. Summary of the history of evaporite accumulationat Cape Deseada. evaporation halves the volume of water. Gypsum is next to precipitate atabout 25 °C, followed by halite after 85 per cent of the gypsum has Cape Deseada a restricted lagoon has developed behind an emergedbreaker-bar in a warm and arid climate. The mollusc fauna of this transgressionsuggests a hydroclimate warmer than today (Tankard 1975). Present-dayrainfall is less than 200 mm/year. Microscopic examination of evaporite speci-mens shows that each cycle begins with carbonate precipitation, and is followedby gypsum and frequently by halite. But the absence of halite in some of theunits suggests inflow of more sea-water, probably tidal, before precipitationof gypsum from the previous brine had been completed. The precipitationcycle would be terminated at that stage an
Size: 1345px × 1858px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorsouthafr, bookcentury1800, bookdecade1890, bookyear1898
