Il plancton calcareo (Foraminiferi e nannofossili) del Miocene delle Isole Tremiti

The p1anktonic foraminifers and calcareous nannofossils assemblages of ten Miocene stratigraphic sections of the Tremiti Islands have been studied. The investigated succession includes the Cretaccio Formation (about 165 m thick) and the basal part (about 3 m thick) of the overling S. Nicola Formation. The first unit lies with angular unconformity on the S.Domino Formation (Oligocene in age) and its base (3-3.80 m) is composed of green (red when wheathered) glauconitic doloarenites which are often thinly stratified or laminitic and with Elasmobranch teeth and apatite grains. Carbonatic fraction is absent in the glauconitic doloarenites because of dissolution. They are followed by about 22 m of stratified ocraceous-yellow dolomitic limestones (the thickness of the strata is from decimetric to beyond 2 m), more arenaceous in the lower part and more marly in the upper part. The carbonates dissolution characterizes the lower part of this interval and decreases progressively upwards; so the calcareous plankton becomes abundant and well preserved from the upper part of the interval. However the major lithology of the Cretaccio Formation is represented by decimetric-metric light marly layers (in particular whitish and reddish bands are alterned) (about 140 m thick); the calcareous plankton is constantly abundan. In the uppermost part of the formation, an interval 3-5 m thick, rich in glauconite and fossils (especially Flabellipecten and Neopycnodonte) was observed. In the studied sections, the S.Nicola Formation conformably lies on the glauconitic calcarenites; previously, the Authors reported an unconformable boundary between the two formations. Only the lowermost part (3 m thick) of S.Nicola Formation has been studied; it is composed of yellowish calcarenites and the whitish limestones with Cardium and Serpulids assemb1ages (calcareous plankton is rare or absent, while microbenthos is abundant). In the succession all the planktonic foraminifers and nannofossils gones and subzones of the Langhian-Messinian interval were recognized; from the biostratigraphic point of view the succession encompasses the interval from Praeorbulina glomerosa s.l Zone (P. glomerosa sicana Subzone) and Discoaster exilis-Sphenolithus heteromorphus Zone (D. exilis-Helicosphaera ampliaperta Subzone) to ’Xon distinctive” Zone and Amaurolithus delicatus-A.amplificus Zone, respectively. For each biostratigraphic units we provide: the lower and upper boun- dary, the thickness, the reliability and the succession of the bioevents and a short description of the associations. In the S.Domino, Caprara and Cretaccio islands, the Cretaccio Formation is Langhian. A Burdigalian age (G. trilobus Zone and S. heteromorphus Zone of the foraminifers and calcareous nannofossi1s, respectively) for the lower part of the formation, glauconitic and barren of calcareous plankton could not be excluded. In the S. Nicola Island the Cretaccio Formation includes also the Serravallian, Tortonian and Messinian stages. In particular, the glauconite rich levels of the upper part of the unit belong to the last two stages representing a condensed and perhaps also discontinuous sedimentation, probahly due to strong currents effects. For the first time, a Messinian age for the upper part of the Cretaccio Formation and for the lowermost part of the S. Nicola Formation has been documented on the basis of the calcareous plankton. In the past, the S. Nicola Formation has been considered of Middle Pliocene age. The Messinian age of these sediments is also confirmed by the presence of the benthonic forarniniferal assernblages with Bulimina echinata, Bolivina dentellata and Rectuvigerina gaudrynoides. Through the new bio-chronostratigraphic framing, the Miocene sedirnentary succession of the Tremiti Islands is perfectly correlable to that of central-southern Salento Peninsula. Infact, the two areas are very similar for theirs lythologic, chronological and dynamic features. Also the closure of Miocene cycle sedimentation is synchronous in both localities and realized during Messinian. In appendix, the cornplete list of the taxa and their original references are added. Systematic, evolutionary, stratigraphic and bibliographic remarks have been carried out for many planktonic foraminifers.