Biostratigraphy and chronostratigraphy of the Miocene Pietra leccese in the type area of Lecce (Apulia, southern Italy)

The Miocene Pietra Leccese formation characterizes the Salentine Peninsula in the southern Italian Apulia Region, where it crops out extensively from north of the city of Lecce to Lèuca. A biostratigraphical (planktonic foraminifera and calcareous nannofossils) and chronostratigraphical study of the Pietra Leccese formation in the Lecce type area is reported. The work is based on 186 samples collected from 19 sections and 5 boreholes. As elsewhere in the Salento area, the sedimentation of the Pietra Leccese spans about 11 Ma, from the late Burdigalian (Globigerinoides trilobus Zone of the planktonic foraminifera, Helicosphaera ampliaperta Zone of the calcareous nannofossils) to the early Messinian (Globorotalia miotumida Zone and Amaurolithus delicatus-A. amplificus Zone of the two groups of organisms respectively). The deposition was interrupted by the repeated action of marine currents, which inhibited the accommodation of the sediments and/or eroded those previously formed, resulting in hiatuses (generally testified to by the occurrence of glauconite mineralization) of different durations even in successions that are located very close each other. The Lecce area was studied by considering four geographical sectors (north-western, north-eastern, south-western, and south-eastern) and three hiatuses were recognized, which in stratigraphical order are: - first hiatus, between the typical Pietra Leccese and the overlying weakly glauconitic Pietra Leccese. This hiatus separates the upper Burdigalian sediments of the G. trilobus Zone and H. ampliaperta Zone (Sphenolithus heteromorphus-Helicosphaera ampliaperta Subzone) from the Langhian ones of the Orbulina suturalis-Globorotalia peripheroronda Zone (O. suturalis Subzone) and S. heteromorphus Zone (S. heteromorphus-Helicosphaera waltrans Subzone). The gap is shorter in the succession of the Seminario Borehole (south-eastern sector). In fact, the first sediments above the hiatus pertain to the upper part of the Praeorbulina glomerosa s.l. Zone (Paragloborotalia siakensis-Praeorbulina glomerosa circularis Subzone) and the lower part of the S. heteromorphus Zone (S. heteromorphus-H. waltrans Subzone); - second hiatus, between the weakly glauconitic and the intensely glauconitic Pietra Leccese. This hiatus separates the upper Langhian sediments of the Orbulina universa (O. suturalis-G. peripheroronda Zone) and the Helicosphaera walbersdorfensis-S. heteromorphus (S. heteromorphus Zone) subzones from the lower Tortonian ones of the Paragloborotalia siakensis and Discoaster bollii (lower part) zones. The gap is shorter in the north-eastern sector (succession of the Morello Borehole) as the sedimentation restarts in the Paragloborotalia partimlabiata (P. mayeri Subzone) and Calcidiscus macintyrei zones. In this sector, further hiatus cannot be ruled out, and it should embrace the interval P. mayeri Subzone (pars) - P. siakensis Zone (pars) of the foraminifera and C. macintyrei Zone (pars) - D. bollii Zone (pars) of nannofossils. - third hiatus, at the top of the glauconite-rich interval. This hiatus occurs between the lowermost Tortonian sediments and the middle Tortonian ones of the Neogloboquadrina acostaensis Zone (lower part) and the Discoaster bellus Zone. The gap is longer in the north-eastern sector since the deposits immediately above the intensely glauconitic interval pertain to the Globigerinoides obliquus extremus and Discoaster surculus zones. A fourth hiatus was also recognized, but only in the area north of Lecce. In the north-western sector, the uppermost Tortonian sediments of Globorotalia suterae and Amaurolithus primus zones directly overlie the middle Tortonian deposits of N. acostaensis and D. bellus zones. The gap is shorter in the north-eastern sector because the lowermost Messinian deposits of G. miotumida and A. delicatus-A. amplificus zones lie on those of the upper Tortonian belonging to the G. obliquus extremus and D. surculus zones. In the south-eastern sector, the deposition was continuous from the lower Tortonian (N. acostaensis and D. bellus zones) to the lower Messinian (G. miotumida and A. delicatus-A amplificus zones). This resulted in a greater thickness of the formation (about 90 m). However, it should be noted that in this interval the Pietra Leccese formation presents lithological features that differ from the typical ones. These features are the result of a progressive bathymetric decrease in the marine environment. The decreasing depth and lithological change led to the deposition of the Calcareniti di Andrano formation, the final Miocene unit on the Salentine Peninsula.