Composition of Messinian sediments from the Tyrrhenian Sea

The data base for this study is represented by essentially nonevaporitic Messinian sediments recovered at ODP Sites 654, 653, 652, and 656 along the eastern Sardinian margin, and of the overlying early Pliocene oozes. Grain-size distribution, carbonate content, and microscopic observation of the sand size fractions were investigated. Messinian paleoenvironments, documented in the western Tyrrhenian Sea (ODP Sites 654 and 653), provide additional evidence supporting the deep basin desiccation model. A sharp lithologic contrast between early Pliocene pelagic oozes and latest Messinian conformable gypsiferous silts supports this model. The "lago-mare" biofacies was only occasionally observed in the shallowest site and is limited to the topmost part of the Messinian. Sites 652 and 656, lying in the deeper part of the Tyrrhenian and located on the downthrown side of an important eastward dipping fault system known as "Faglia centrale" are characterized by terrigenous sedimentation, with partly recycled minor evaporites. Of special interest is Site 652, where the thickness of the (probable) Messinian is 530 m. Sedimentary characters indicate a permanently subaqueous but nonmarine environment, with turbidites accumulating in a rapidly subsiding basin. According to the model proposed, this basin was fed by continental waters during times of maximum evaporitic draw-down, with temporary marine incursions from the west or southwest when the water level was higher. A basement ridge separated the evaporating pond from this endoreic lake located on the opposite (eastern) margin of the Tyrrhenian Basin, which was then limited to its western part. Post-Messinian reactivation of the "Faglia centrale" is necessary to account for the inversion of the relief.

Geochemistry of basal dolomitic and Fe-Mn sediments from the Tyrrhenian Sea

In the Tyrrhenian Sea (Western Mediterranean), unusual reddish, soft to lithified, dolomitic sediments up to 45 m thick overlie igneous crust at the base of thick Pliocene-Quaternary deep-sea sediment successions in the Marsili (Site 650) and Vavilov (Site 651) basins. These sediments also overlie the Gortani Ridge, a basaltic Seamount near the base of the Sardinian continental margin (Site 655). At both basinal sites (650, 651), the lowest sediments are dolomitic, with manganese oxide (MnO) segregations. Whole-rock X-ray diffraction indicates abundant dolomite and quartz, with subordinate calcite, illite (authigenic), feldspar and minor kaolinite, chlorite, and anhydrite. Chemical analyses show strong enrichment in magnesium oxide (MgO) and MnO relative to shale or deep-sea clay. Mg and Mn correlate positively and exhibit decreasing concentrations up the succession in the Marsili Basin (Site 650). The following scenario is proposed: peridotites were exposed on the seafloor in the Vavilov Basin (Site 651) and then eroded, depositing talc in local fine-grained dolomitic sediments within the igneous basement. After local magmatism ended, the igneous basement at each site subsided rapidly (about 800 m/m.y.) and was blanketed with calcareous and clay-rich oozes. During early diagenesis (from isotopic evidence; McKenzie et al., this volume) tepid fluids, of modified seawater composition, reacted with and dolomitized the overlying deep-sea sediments. At Site 651 additional Mg may have been extracted from asthenosphere peridotite cored at shallow depths (about 100 m). One can hypothesize that fluids rich in Mg and Mn were flushed from the igneous basement, triggered by extensional faulting and local tilting during subsidence of the basement, and that these fluids then dolomitized the base of the overlying sediment succession. Late tectonic movements in the Vavilov Basin (Site 651) fractured already lithified dolomitic sediments and more reducing (? hydrothermal) fluids locally remobilized Fe and Mn and corroded dolomite crystals.

Heavy mineral content of ODP Leg 107 holes

Analyses of rock clasts and of heavy minerals in upper Miocene coarse detrital units drilled along the East Sardinia passive-type continental margin (Sites 654, 653, 652, and 656) reveal that the stretched basement contains quite complex rock suites. Taking also into account previous sampling data, in moving from west to east across the margin, the nature of the basement changes drastically. To the west there are mostly Hercynian basement rocks with their cover, referable to the alpine foreland of the Corsica-Sardinia block. To the east, along the lower margin, where crustal thinning is quite severe, the basement contains rock suites referable to a pre-upper Tortonian orogenized zone with units constituting parts of the Alpine and Apenninic chains (presumably with thickened continental crust prior to stretching). Largest thinning and ocean forming occurred then, in a rather short time, mostly at the expense of unstable crust just thickened by orogenetic/tectogenetic processes.