CONTROLS ON FAUNAL AND SEDIMENT CYCLICITY WITHIN THE TRIPOLI AND CALCARE DI BASE BASINS (LATE MIOCENE) OF CENTRAL SICILY

Cyclicity within the Late Miocene Tripoli and Calcare di Base Formatio ns of Sicily was controlled by two processes: (a) partial evaporation of Palaeomediterranean waters but with periodic replenishment by Atlan tic derived inflow on several occasions during the Late Tortonian and Messinian stages; (b) deformation of a regional foreland basin and ass ociated satellite basins perched above south-verging sedimentary thrus t sheets. These were intermittently set in motion by active plate coll ision onto the passive African Plate margin. Although it is not possib le to discriminate fully the world eustatic signal under such a scenar io, nevertheless, sufficient recolonization by marine faunas in these Sicilian basins occurred to permit modelling of onlap/offlap events fo r the Late Miocene intervals. Basin-wide correlation of these variably fossiliferous formations hangs on the recognition of a late Tortonian -early Messinian onlap event marked by coral reef development at the t op of the Terravecchia Formation around the northern margins of the re gion. A further carbonate episode (Calcare di Base Formation) lies abo ve a transgressive surface and directly overlies fluvial conglomerates at the top of the Terravecchia Formation around the northern margins of the basin. The Calcare ii Base is partly diachronous with the top o f the Tripoli Formation in the foreland basin but further north it ove rsteps the coral reefs on the southern margins of the Madonie Mountain s. A variable number of internal Tripoli Formation cycles (confirmed o n both faunal and sedimentological grounds) and Calcare di Base cycles (demonstrated only by sedimentation patterns), have been recognized. This minor cyclicity is considered to have resulted from periodic tect onic uplift and isolation of foreland basin and associated perched bas ins. This isolation controlled both influx of siliciclastic sediment a nd access by marine biota. Prolonged basin isolation during the deposi tion of the Tripoli Formation lead to rapidly declining diversity patt erns in both benthonic and planktonic communities. Benthonic foraminif era disappeared early in many cycles with oligotypic planktonic forami nifera and radiolaria remaining longest. Under increasingly hostile co nditions only diatoms remained. Each new cycle is marked at its base b y a return to near normal conditions and a relatively diverse biota. A lthough faunas were stressed during the isolation events no clear evid ence of salinity increase is seen. Rather we consider that a combinati on of oxygen depletion (especially on basin floors receiving prolific algal bioproduction) and toxin build-up (generally as the by product o f algal metabolic processes) was the cause. The Calcare di Base cycles are more difficult to interpret on account of the virtual absence of biota. Here the cycles are clearly related to desiccation and salinity increase during times of restriction. The base of each cycle is marke d by brine dilution and basin flooding, with some evidence of low dive rsity microfauna input in the early cycles. We envisage a Late Miocene palaeogeography with thrust-fold belts well established to the north of Sicily and a NE-SW oriented seaway developed ahead of them. To the south of this seaway lay an en-echelon belt of NE-SW oriented submarin e ridges and localised basins which effected a sill, or at times a bar rier, between the seaway and the Tripoli basins. A broad diatom-domina ted perched basin complex extended along the southern margin of the th rust-related ridges and was intimately linked to a broad foreland basi n, also diatomitic, lying immediately to the south. The foreland basin was separated from a further open sea area in southern Sicily by a se cond NE-SW oriented sill now hidden by later overthrusting.