24. Petrography and Geochemistry of Red Sea Dolomite

Examination of piston cores and, particularly, sediments collected by the Deep Sea Drilling Project has shown that authigenic dolomite occurs in a wide range of sedimentary associations. While the trace to a small percentage of dolomite often encountered in pelagic carbonate sequences may be a result of authigenesis under "normal" deep-sea conditions, the few studies to date seem to indicate that dolomite is abundant only in sediments presumably formed under conditions other than normal open marine. For example, certain dolomite sequences have been ascribed to magnesium enrichment associated with igneous activity (Bonatti, 1966; Riedel et al., 1961). In other cases dolomites are thought to have formed in association with hypersaline brines. A DSDP core (Site 12) from the African continental margin contained abundant dolomite and palygorskite. Peterson, Edgar, et al. (1970) feel that hypersaline brines, formed in near-shore lagoons, moved down through the deep-water sediment sequence by a process of refluxion. In other cases, hypersalinity is associated with desiccation. Miocene sediment sequences drilled in the Mediterranean contain abundant dolomite associated with halite and anhydrite. Ryan, Hsü, et al. (1973) postulate that these sediments formed during a "salinity crisis." Another compelling association is that of dolomite and inferred reducing conditions noted in cores recovered by DSDP from the African continental margin and the Cariaco Trench of the Caribbean Sea (Hayes, Pimm, et al., 1973; Edgar, Saunders, et al., 1973). Whether organic matter itself is a big factor, or whether hypersalinity associated with environmental extremes in these inferred barred basins is controlling, is still a problem.