Significance of Red Sea in Problem of Evaporites and Basinal Limestones

Abstract

According to traditional concepts, one of the sites of accumulation of evaporite deposits is a barred basin having restricted access to the open sea. Evaporite minerals are thought to form in this basin as precipitates from seawater that accumulate on the basin floor. The Red Sea offers geologic settings where the barred-basin concept can be tested. The Red Sea, which is a partly enclosed body of marine water separated from the Indian Ocean by a shallow ( ~ 100 m at its shallowest depth) threshold, was isolated from the Indian Ocean periodically during the Pleistocene. If the barred-basin concept is correct the subbottom sediments of the Red Sea should contain evaporite minerals. Diligent search was made for gypsum and anhydrite in cores taken from the entire length of the Red Sea, including the Gulf of Aqaba (Elat), and which record approximately 70,000 years of Red Sea sedimentation. No sulfate minerals of evaporite origin were found. Yet the presence of lithified layers of aragonite, enrichment of the heavier carbon and oxygen isotopes of sediments and organic remains, abundance of lutite containing low-magnesium calcite, and faunal indications of high salinity suggest that in the late Pleistocene sulfate minerals, specifically gypsum, may have been precipitated from the surface waters across the entire Red Sea. The absence of sulfates in the bottom sediments is ascribed to bacterial reduction which resulted in the formation of low-magnesium calcite. A rise in pH accompanying the bacterial degradation of sulfate appears to have led to an additional precipitation of high-magnesium calcite with the low-magnesium calcite. Hence the Red Sea shows that in a deep barred basin evaporitic sulfate minerals may not accumulate even should they form. Instead they will be degraded to form low-magnesium calcite with high-magnesium calcite; high-magnesium calcite will alter diagenetically to low-magnesium (ordinary) calcite. This calcite which forms as a lutite is a classic basinal limestone. In the presence of an adequate concentration of calcium ions calcite may also form from the bacterial attack of the sulfate ion in solution followed by subsequent reaction of sulfide with bicarbonate; hence basinal limestones may form without the intermediate stage of solid gypsum. Basinal limestones in the rock record which result from this biochemical degradation process smell of H2S and have been blackened wit FeS as an accompaniment of the sulfate-replacement process. Such basinal limestones, analogous to the sediments in the southern Red Sea, are commonly rich in organic matter and may constitute source rocks for petroleum. By contrast to the deep basin in which calcitic lutites (i.e., basinal limestones) form, thick sequences of evaporitic sulfates form on sea-marginal flats (sabkhas), such as marginal to the Red Sea during the Miocene and even during the Pleistocene in the Danakil depression of Ethiopia. If calcitic (basinal) limestones instead of evaporites form in restricted basins and evaporitic sulfates are laid down on sea-marginal shelves, then the origin of salt deposits becomes an increasing mystery; after all, salts are classically considered to be the final products of evaporation in an isolated basin. However, the fabric of salts from several ancient basins, among them the Salina (Silurian), Zechstein (Permian), and Miocene in the Mediterranean, suggests an in-situ replacement of preexisting anhydrite nodules and stromatolites, and hence deposition of salts in sabkha-like brine-logged flats. Although these salts are now in deeper parts of their respective basins, and even in the centers of basins, the inference is that original deposition was in shallow flats prior to subsi ence.