Unusual Marine Evaporites with Salts of Calcium and Magnesium Chloride in Cretaceous Basins

Tachyhydrite (CaC12-2MgC12'12H20), a rare mineral in evaporite deposits, occurs in units of over 100 m thickness in the Lower Cretaceous (Aptian) evaporite sequences of Sergipe, Brazil, and has also been reported in Aptian evaporites of the coastal regions of the Congo and Gabon, Africa. At an advanced stage of evaporation, the brines which produced these deposits must have been abnormally enriched in CaCI • and depleted in SO4. Brines with these char-acteristics exist in the Red Sea and Dead Sea today, where the structural setting, in fault bounded troughs with large local relief, may have resulted in modification of marine brines by discharge of CaCl,rich ground waters. If South America separated from Africa, and if the beginnings of this separation are dated correctly as Jurassic, the Lower Cretaceous evaporites of the coastal strips of eastern Brazil and of west Africa could have formed in a structural setting for which the Red Sea provides a present-day model. The coastal evaporites of Brazil and West Africa lack basal sulfate deposits. It i