At ODP Holes 975B and 975C (base of the Menorca Rise), sediment cores record facies changes across the transition from evaporitic to open marine conditions in the western Mediterranean Sea at the end of the Miocene (Messinian). Preliminary interpretations of depositional environments are based on detailed petrographic observations and bulk mineralogical analyses (XRD). Two broad evaporite cycles are recognized, starting at the base with clay or micrite-rich clay, followed by laminated to thinly-bedded gypsiferous chalk, finely laminated gypsum, and topped by nodular gypsum. Laminar to nodular morphologies may be the product of diagenetic overgrowth of detrital gypsum at and just below the sediment-water interface. These cycles could be explained by periods of fresher water influx and clay deposition, followed by evaporation, increasing salinity, and precipitation of carbonate, then gypsum. Sedimentary structures within the gypsum indicate that it was likely deposited below wave base. Foraminifers are present throughout, implying continuous seawater influx into the basin, or alternatively eolian contamination. The evaporite section is capped by a laminated foraminifer/terrigenous sandstone probably representing a catastrophic flooding event. After sand deposition and influx of fresher (meteoric or marine) water, there was a shift from gypsum to calcite precipitation in the basin, and laminated to thinly-bedded micrite and micritic silty clay accumulated. The micrite consists of inorganic(?) calcite, lacks bioturbation, and contains brackish water fauna. Overlying beds of nannofossil clay, silty clay and ooze indicate that open marine conditions were established in the Pliocene.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah