The Origin of the Mediterranean Evaporite

Abstract

Prior to our cruise, one of us was asked by his colleagues to find in the Mediterranean the cause of the Pontian regression, which was one of the significant events in the Neogene history of Europe. Another of us was looking for a chance to continuously core a Neogene pelagic sequence, and thereby resolve the difficult Miocene-Pliocene boundary problem, and to establish a reference marine section for Neogene stratigraphers. The third had been puzzled by the almost ubiquitous presence of a strong acoustic reflector in the Mediterranean; he was hopeful by drilling, to determine its age, nature, and relation to the evolution of the Mediterranean basins. These three divergent lines of interest provided a basis for our eventual agreement through a shared belief that the Mediterranean evaporites owed their origin to the desiccation of a deep basin that had been isolated from the Atlantic. The presence of a salt deposit under the Balearic Basin had been suspected since 1961 when diapiric structures resembling salt domes were identified on the seismic reflection profiles of Chain cruise 21 (see Hersey, 1965). Similar diapiric structures were known from the eastern Mediterranean, but there had been doubts whether these are indeed salt domes, or whether they are mud diapirs instead (H. Closs and D. Neev, personal communication). The age of the Balearic salt had been a rather controversial problem; it had been postulated as Triassic (e.g., Glangeaud et al., 1966), or as Tertiary (e.g., Cornet, 1968; Mauffret, 1969; Montadert et al., 1970). Its genesis was even less certain. Comparing the Balearic deposits with those of the Rhine Graben and Rift valleys, the salt was thought to have been deposited in a rapidly subsiding basin, initially opened by rifting (Ryan, 1969; Schneider, 1970; Pautotef al, 1970). The presence of Upper Miocene evaporites in circum- Mediterranean countries has been well known, after their having been described from Spain, Italy, Crete, Turkey, Cyprus, Israel, North Africa, and other places, (see Kozary, et al, 1968; Ogniben, 1957; Rios, 1968; Freudenthal, 1969; Tortochaux, 1968). A Late Miocene Mediterranean "salinity crisis" had been recognized (e.g., Gentil, 1918; Trevisan, 1958; Gignoux, 1960; Ruggieri, 1967), and even the isolation of the Mediterranean from the Atlantic had been postulated (Ogniben, 1957). However, none had envisioned desiccated basins thousands of meters below the worldwide sea level, nor had anyone ventured to correlate the Mediterranean reflector with an extensive evaporite formation covering the whole Mediterranean basin. The presence of salt, not only halite, but also gypsum, anhydrite, and dolomite, in the Mediterranean was proven by DSDP drilling. Combining the drilling results with evidence furnished by geophysical records, the existence of an extensive evaporite unit of Upper Miocene age has been established. We propose to refer to this unit (a formation, a group, or a super-group as one may wish to view it) as the Mediterranean evaporite. Significance of the drilling results was not only to confirm the presence of halite under the Balearic Abyssal Plain, but to confirm that a major "salinity crisis" took place during the Late Miocene. It became apparent that the circum-Mediterranean evaporites of this age are not local deposits, but are uplifted fragments of a unit that once covered the whole Mediterranean Basin. The origin of the Mediterranean Evaporite could be accounted for by three different models: 1) Evaporation of a deep-water Mediterranean basin, which received constant inflow from the Atlantic and Maintained its water level at or only slightly below the worldwide sea level. 2) Evaporation of a shallow-water Mediterranean basin, which received inflows from the Atlantic and maintained its water level at about the sea level. 3) Desiccation of a deep Mediterranean basin isolated from the Atlantic; evaporites were precipitated from play as or salt lakes whose water levels were dropped down to thousands of meters below the Atlantic sea level. The first may be called a "deep-water, deep-basin model" (e.g., Schmalz, 1969), the second a "shallow-water, shallow-basin model" (e.g., Ogniben, 1957), and the third, a "desiccated, deep-basin model". The last is the one we prefer.