Geochemistry of evaporite in the Red Sea

One of the major shipboard findings during Leg 23 drilling in the Red Sea was the presence of late Miocene evaporites at Sites 225, 227, and 228. The top of the evaporite sequence correlates with a strong reflector (Reflector S) which has been mapped over much of the Red Sea (Ross et al., 1969, Phillips and Ross, 1970). This indicates that the Red Sea appears to be extent. Miocene sediments, including evaporites, are known from a few outcrops along the coastal plains of the Gulf of Suez to lat 14°N (Sadek, 1959, cited in Friedman, 1972; Heybroek, 1965; Friedman, 1972). Along the length of the Red Sea, the presence of Miocene salt is indicated by seismic reflection studies (Lowell and Genik, 1972) and confirmed by drilling. The recently published data from deep exploratory wells (Ahmed, 1972) demonstrate the great thickness of elastics and evaporites which were deposited in the Red Sea depression during Miocene time. The Red Sea evaporites are of the same age as the evaporites found by deep sea drilling (DSDP Leg 13) in the Mediterranean Sea. Therefore, Reflector S in the Red Sea is comparable to Reflector M in the Mediterranean. It is assumed that during Miocene time a connection between these two basins was established (Coleman, this volume) resulting in a similar origin for the evaporites deposited in the Red Sea and in the Mediterranean Sea. The origin of the Mediterranean evaporites has been discussed in great detail (Hsü et al., 1973; Nesteroff, 1973; Friedman, 1973). The formation of evaporites may be interpreted by three different hypotheses. 1) Evaporation of a shallow restricted shelf sea or lagoon which receives inflows from the open ocean. 2) Evaporation of a deep-water basin which is separated from the open ocean by a shallow sill (Schmalz, 1969). 3) Evaporation of playas or salt lakes which are situated in desiccated deep basins isolated from the open ocean (Hsü et al., 1973). The purpose of this study is to show whether one of these models might apply to the formation and deposition of the Red Sea evaporites. Therefore, a detailed petrographic and geochemical investigation was carried out.红海第23航次钻探期间的主要现场观测结果之一，是在225、227、228站位钻遇晚中新世蒸发岩（evaporites）。该蒸发岩序列的顶界与强反射界面（Reflector S）对应，该界面已在红海大部分区域完成填图（Ross等人，1969；Phillips与Ross，1970）。这表明该套蒸发岩在红海的分布范围较为广泛。 包括蒸发岩在内的中新世沉积，已在苏伊士湾沿岸平原的数处露头中被发现，分布纬度可达北纬14°（Sadek，1959，引自Friedman，1972；Heybroek，1965；Friedman，1972）。沿红海全域，地震反射研究已证实中新世盐岩的存在（Lowell与Genik，1972），且该结论经钻探得到验证。近期发表的深层探井数据（Ahmed，1972）表明，中新世时期红海坳陷内沉积了巨厚的碎屑岩与蒸发岩。 红海蒸发岩与地中海海域深海钻探计划（Deep Sea Drilling Project, DSDP）第13航次所发现的蒸发岩形成时代一致。因此，红海的反射界面S可与地中海的反射界面M（Reflector M）类比。学界普遍认为，中新世时期这两个盆地之间曾存在水道连通（Coleman，本卷），进而使得红海与地中海的蒸发岩具有相似的成因。地中海蒸发岩的成因已得到详尽讨论（Hsü等人，1973；Nesteroff，1973；Friedman，1973）。 蒸发岩的形成可通过三种不同假说进行解释： 1) 受限制的浅陆架海或泻湖蒸发，该体系接收来自开阔大洋的水体补给； 2) 被浅海槛与开阔大洋分隔的深水盆地蒸发（Schmalz，1969）； 3) 位于与开阔大洋隔绝的干涸深海盆地中的干盐湖与盐湖蒸发（Hsü等人，1973）。 本研究的目的在于验证上述三种模型中，哪一种可适用于红海蒸发岩的形成与沉积过程，据此开展了详细的岩石学与地球化学调查。