Isotope geochemistry, petrology and depositional environments of the diatomite‐dominated Tripoli Formation (Lower Messinian), Sicily

Abstract

The Tripoli Formation (Lower Messinian) in Sicily includes diatomites irregularly alternating with marl and carbonate beds and lies, stratigraphically, between the Tortonian pelagic marls and the evaporitic Calcare di base. The relationships between mineralogy, textural features and oxygen‐carbon isotopic compositions of carbonate components point to a wide variability of depositional conditions and suggest that Tripoli sedimentation occurred in small basins characterized by periodic and marked restriction from the open sea. The isotopic values of calcite and dolomite in the diatomites suggest an evolution from normal marine towards more restricted environments. Evaporating conditions are also indicated by the occurrence of anhydrite, length‐slow chalcedonic quartz and moulds of gypsum. In a more advanced stage, the precipitation of heavy δ180 dolomite in the interstitial pores of fossil‐poor diatomites denotes an environment with highly evaporated water. Mixing of meteoric and marine waters, on the other hand, might have favoured the precipitation of a dolomite characterized by relatively low δ180 and δ13C values. The deposition of marl and carbonate beds alternating with or overlying the diatomites took place in an environment with highly evaporated marine waters on the basis of δl18O values of dolomite (up to + 9.10‰) and aragonite (up to + 5.83‰), occurrence of evaporitic minerals and lack of fossils. The presence at these levels of calcite with extremely negative δ13C values (down to ‐ 38.40‰), filling gypsum moulds, suggests activity of sulphate‐reducing bacteria. Some aragonitic marls, however, bear evidence of deposition in relatively normal marine conditions.

Authigenic carbonates in Upper Miocene sediments of the Tertiary Piedmont Basin (NW Italy): Vestiges of an ancient gas hydrate stability zone?

“…9) are comparable to those reported in coeval sediments of other Mediterranean sites and are characteristic of formation in open marine conditions (e.g. Bellanca et al, 1986;Pierre and Rouchy, 2004). The moderate 13 C depletion is related to a very minor amount of diagenetic carbonate sourced by decaying organic matter, actually very abundant in these sediments and commonly represented by land plant remains.…”

Carbonate stromatolites from a Messinian hypersaline setting in the Caltanissetta Basin, Sicily: petrographic evidence of microbial activity and related stable isotope and rare earth element signatures

“…Evaporite deposition was preceded in many Mediterranean peripheral basins by formation of cyclic, thinly bedded diatom‐rich deposits (e.g. the Tripoli Formation in Sicily) (Rouchy, 1982; Bellanca et al. , 1986; Martín & Braga, 1994; Clauzon et al.…”

“…On the basis of very negative carbon isotope signatures, McKenzie et al. (1979–1980) and Bellanca et al. (1983, 1986) ascribed precipitation of early diagenetic carbonate, in diatomites of the Messinian Tripoli Formation in Sicily, to the action of SRB; however, at that time, no petrographic evidence of bacterial biomineralization was provided.…”

Geochemical and petrologic evidence of the origin and diagenieis of a Late Mississippian, supratidal dolostone

“…5). This microfabric is strikingly differentfrom the granular, rounded to subhedral grain shape, and high porosity commonto modem evaporitic dolomite (Clayton et al 1968;Botz and Von der Borch 1984;Pierre et al 1984;Bellanca et al 1986;DeDeckker and Last 1989;Last 1990;Anad6nandUtrilla1993). However, thetextureis identical to recrystallization fabrics produced both by mechanical defonnation of minerals (Hobbs et al 1976;Milller et al 1981;Vernon 1981,esp.…”