Quantitative forminiferal analysis and environmental interpretation of the Pliocene and topmost Miocene on the south coast of Sicily

Abstract

Benthonic and planktonic foraminiferal faunas have been investigated quantitatively from sediments of the Pliocene and topmost Miocene on the south coast of Sicily. Environmental and bathymetrical reconstructions have been established on the basis of the lithological and foraminiferal data. Two sections are studied in detail: the Lower Pliocene Trubi formation at Capo Rossello and the Middle and Upper Pliocene Monte Narbone formation at Punta Piccola. In both sections relations between benthonic and planktonic foraminiferal assemblages and lithology can be clearly established. Continuous marine sedimentation occurred across the ill-defined Miocene- Pliocene boundary, initially at depths of 50-100 m, but gradually the depth increased to 500-800 m for the middle part of the Trubi. At the onset of more clayey sedimentation at the base of the Narbone formation, depth of deposition decreased to some 100-400 m. For the relatively short biostratigraphic interval of the lower part of the G. puncticulata Interval-Zone the paleobathymetry was estimated at three localities revealing different depositional depths of the same type of Trubi sediment. A multi-depressional paleogeography for the entire Mediterranean during at least Late Messinian and Early Pliocene times is proposed to explain the seemingly conflicting implications of both the "desiccated, deep basin model" and the "shallow water, shallow basin model". This alternative model involves syn- and post-Messinian subsidence and may easily account for the presence of Trubi in deep basinal settings and elevated land-sections. The sections on the south coast of Sicily seem to have been located in an intermediate paleogeographical realm, i.e. on the slope from shoals to deeps. Periods of upwelling are thought to be responsible for increased quantities of planktonic foraminifera, diatoms and radiolarians in some laminated intervals within the Trubi formation. Oxygen depletion at the bottom may explain the low diversity and dwarfing of the benthonic foraminiferal associations in these laminated sediments. Precipitation of ferromanganese oxides during deposition of the Narbone formation determined the composition of foraminiferal assemblages. Oligotypical buliminidjbolivinid assemblages characterize the darker coloured, ferromanganese-rich intervals, whereas Globorotalia bononiensis dominates the planktonic associations in some of these horizons. Deposition of the Monte Narbone formation progressed in a shallowing environment. Superimposed on this process decreasing water temperatures are recorded in the Upper Pliocene Globorotalia inflata Assemblage Zone.