Sedimentary Petrology and Structures of Messinian Evaporitic Sediments in the Mediterranean Sea, Leg 42A, Deep Sea Drilling Project

Abstract

Analysis of the petrology and sedimentary structures of Messin-ian evaporites recovered during Leg 42A yields tentative interpretations of the depositional environments at the various sites. At Site 371, on the southern edge of the Balearic Abyssal Plain, coring retrieved about 2 meters of nodular to laminated anhydrite overlying 2 meters of barren dolomitic mudstone. We interpret this sequence as part of a prograding sabkha cycle. Site 372, on the flank of the East Menorca Rise, contained a thin Messinian sequence of laminated and nodular gypsum with minor dolomitic marls; these rocks lie stratigraphically between pelagic nannofossil marls. The most prominent lithology at Site 372 is laminated gypsum which contains cross bedding, ripple marks, and other indications of current deposition. We believe deposition of this gypsum occurred in a very shallow water evaporite flat that may have been exposed subaerially from time to time. Evaporitic sediments at Site 374, in the Ionian Sea, comprise a sequence beginning with halite and nodular anhydrite, and progressing stratigraphically upward into cycles of gypsum and dolomitic mudstone and then into dolomitic marl. This succession records a progressive decrease in average salinity as well as an increase in average water depth. The gypsum-mudstone cycles were the result of shorter term fluctuations of water level that produced depositional environments ranging from shallow subaqueous to subaerial. Coring at Sites 375 and 376, west of Cyprus in the eastern Mediterranean, yielded a variety of Messinian evaporitic rocks including halite, anhydrite, and several kinds of gypsum. Most prominent among the latter are laminated gypsum, selenite, and resedimented gypsiferous deposits (gypsarenite and gypsrudite). Deposition of all of these evaporitic rocks occurred in either very shallow subaqueous or subaerial environments. Lying stratigraphi-cally above them is a sequence of Messinian dolomitic marlstones with interbedded turbiditic arenite and siltites, indicating markedly less saline conditions and deeper water. Coarse selenitic gypsum was the main deposit of Messinian age recovered at Site 378, in the North Cretan Basin. Precipitation of this gypsum occurred in shallow water. Periodic influxes of less saline water caused some dissolution of the selenite and produced dissolution breccias of slightly reworked selenite crystals. At least brief periods of subaerial exposure are indicated by the presence of small amounts of dolomitic caliche breccia.