CONCLUSIONS

On the basis of detailed petrographic observations, we propose that the gypsum-bearing sediments of Unit III at Site 975 were deposited in a subaqueous environment, below wavebase. The planar-laminated gypsum appears to be detrital in origin, and the laminations reflect episodic fluctuations in the balance of evaporation and influx of meteoric or marine waters. The wavy-planar and planar-cumulus laminated facies, and perhaps also the pinch-and-swell gypsum facies, are thought to have formed by early-diagenetic overgrowth of the detrital planar gypsum at, or just below, the sediment-water interface. The deposition and diagenetic overprinting of the gypsum facies was periodically punctuated by influx of matrix sediment. The gypsiferous chalk facies is thought to have formed by displacive, diagenetic growth of gypsum crystals from the gypsum-saturated pore waters of a carbonate sediment.

These gypsum facies, along with intervals of clay and micritic silty clay, are present at Site 975 in cycles.

A possible scenario of formation of these cycles, in the context of a deep (below wavebase) basin with periodic marine influx, is as follows (moving upsection):

1. Basal clay: high influx of marine water; deepest and least saline conditions; minor carbonate or gypsum precipitation;
2. Gypsiferous chalk: increasing evaporation; carbonate precipitation dominated with in situ diagenetic growth of gypsum; and
3. Laminated to pinch-and-swell gypsum: increasing salinity; deposition of detrital gypsum (precipitated at air-water interface?) followed by extensive diagenetic overgrowth. Isotopic and chemical analyses, currently in progress, will help clarify the changes in environmental conditions during formation of these gypsum cycles,

The gypsum-dominated sediments of Unit III are capped by the laminated calcareous/terrigenous sand at the base of Unit II, a thin interval of micrite-rich sediment, and a layer of graded, calcareous- volcaniclastic sand. These sand layers reflect influx of exotic sediment and mark significant depositional events in the history of this section. Deposition of the sands was accompanied by influx of water to the basin, either marine waters from the west, or perhaps meteoric waters from lacustrine environments to the east.

The influx of the waters from which the sands were deposited resulted in cessation of gypsum deposition, and a shift to calcite precipitation. The interlaminated and interbedded micrites and micritic silty clays of Unit II reflect periodic, variable magnitude, influx of terrigenous sediment to a calcite precipitating basin. The light and dark bands that characterize the Miocene/Pliocene boundary interval are perhaps just subtle versions of the micritic and clay-dominated layers downsection and may reflect the final sedimentological fluctuations before fully marine conditions of the Pliocene were established. Detailed paleontological work on samples from the sandy intervals of Unit II and isotopic analyses on overlying micritic sediments should shed light on their origins and the implications for changes in depositional environments during the terminal stages of the Messinian and the transition to the open-marine environment of the lower Pliocene at Site 975.