Neogene-Quaternary stratigraphy of the north-eastern sector of the Lecce Province (with geological surveying in scale 1:25,000) [Stratigrafia neogenico-quaternaria del settore nord - orientale della provincia di Lecce (con rilevamento geologico alla scala 1:25000)]

The lithological features and the stratigraphical and paleoenvironmental significance of the formations outcropping in the north-eastern sector of the Lecce Province, (the respective Geologic Map in scale 1:25.000 has been attached) are shown here. Starting from the oldest one, the units are: Calcari di Melissano formation, Galatone Formation, Lecce Formation, Pietra leccese formation, Calcareniti di Andrano formation, Lèuca Formation, Uggiano la Chiesa Formation and Calcareniti del Salento formation. They represent the results of 7 sedimentary cycles developed from the Late Cretaceous to the Early Pleistocene. The first cycle is represented by the Calcari di Melissano formation. This unit widely outcrops north of Lecce, in the surroundings of Surbo village, and it is constituted by compact limestones, dolomitic limestones and micritic limestones whitish to grey in colour. The Calcari di Melissano formation generally consists of centimetric to metric layers, but locally it can be laminated. The rare macrofossils are fragments of rudists as well as corals and bivalves. Its depositional environment corresponds to shallow water marine conditions with continental episodes. In literature the formation has been attributed to the Turonian-Senonian interval (Late Cretaceous). The second cycle is represented by the Galatone Formation, which outcrops south-west of Lecce , along NW-SE oriented strip. The unit is characterized by white–greyish micritic limestones, white-brownish limestones and yellowish marls. The litotypes are organized in layers from a few centimetres to several decimetres thick, commonly showing planar laminae. The abundant fossil assemblages are mainly constituted by gastropods (especially of the Potamididae family; inner moulds of Ampullinopsis crassatina also occur), ostracods and small bivalves (mainly of the Cardidae and Veneridae families). The sedimentological features and the macro-microfossil content indicate a lacustrine depositional environment; episodic connections with coastal restricted environments characterized by brackish waters are also recorded. The presence of A. crassatina allow to refer the Galatone Formation to the late Oligocene. The Lecce Formation, formalized in this paper, represents the third cycle. The unit consists of whitish massive calcarenites (often characterized by more or less distinct decimetric stratification) with intercalations of greyish marly and micritic limestones. Rare bivalves (particularly Cardium), echinoids (Scutella), gastropods and macroforaminifers (Operculina) occur. The planktonic foraminifer and calcareous nannofossil assemblages allow to attribute the upper part of the formation to the Paragloborotalia kugleri Zone and to the Sphenolithus delphix Zone respectively and, thus, to the basal part of the Aquitanian (Early Miocene). Although, the lower part of the unit has been doubtfully referred to the late Chattian. From a paleoenvironmental point of view, the lacking of sedimentary structures and the good preservation of the macroforaminifers led to hypothesize a sedimentary accumulation showing any transport evidences; furthermore, the common occurrence of Scutella indicates sandy backdrops and low hydrodynamism in shallow water marine conditions. The fourth cycle is formed by the Pietra leccese and Calcareniti di Andrano formations. The first unit consist of pale yellow biomicrite, very rich of planktonic foraminifers. Its base is characterized by a breccia with carbonatic heterometric elements and scarce matrix or by a thin brown level with small phosphatic elements. The features of this basal level and the paleoenvironmental evidence deriving from the benthonic assemblages, are in disagreement with a transgression, they rather suggest that the sedimentation started within depositional conditions characterized both by phases of phosphatization, probably in upwelling regime, and phases of erosion/dispersion linked to the action of deep sea currents. During the late Burdigalian (Globigerinoides trilobus and Sphenolithus heteromorphus cronozones) these particular conditions ended (shortly after the early stage of the formation deposition). In addition, a strong activity of deep sea currents responsible for erosion and partial redistribution of sediments, has also been hypothesized for the deposition of the glauconitic layers located in the upper part of the formation. In this contest, the local basal hiatus represents the first of a series which affected the Pietra leccese formation up to the early Messinian (Globorotalia conomiozea Zone and Amaurolithus delicatus - A. amplificus Zone); the presence of sedimentary hiatuses also justifies the relatively limited thickness of the formation itself (about 80 m) in spite of its long - lasting deposition (about 11 M.A.). The extension of the hiatuses changes in the different sectors of the considered area. In the northern sector, for example, it has been observed that the glauconitic layers immediately underlie the Messinian carbonatic deposits of the Calcareniti di Andrano formation; on the contrary, in the southern sector, these levels are overlaid by the Tortonian calcareous - detritic sediments of the Pietra leccese formation; these sediments, up to 30 m thick, show some differences in the granules size, colour and fossil content, in comparison with the typical Burdigalian Pietra leccese; this is probably due to the progressive raising of the area, suddenly increasing since the Messinian. The Calcareniti di Andrano formation represents the regressive term closing the Miocene cycle in the whole Salento area. The fifth sedimentary cycle is represented by the Lèuca Formation. This unit, over ten meters thick, is constituted by breccias, conglomerates and, subordinately, by glauconitic biomicrites. Breccias and conglomerates are formed by carbonatic heterometric pebbles included in a mainly sandy or marly sandy matrix. The matrix yields foraminifers and nannofossils of the Sphaeroidinellopsis seminulina s.l. and Discoaster variabilis s.l..zones (basal Zanclean, Early Pliocene) and benthonic assemblages (foraminifers and ostracods) indicating a shallow water marine environments. The glauconitic biomicrites, attributed to the Globorotalia margaritae and Amaurolithus tricorniculatus zones (Zanclean), are the sedimentary expression of an abrupt deepening (up to the deepest outer neritic zone) and of an intense erosive/dispersive action of the currents. The sixth cycle is represented by the Uggiano la Chiesa Formation consisting of stratified and fossiliferous biodetritical limestones and yellowish calcareous sands; about 50 meters thick. The calcareous plankton content is referable to the Globorotalia puncticulata and D. tamalis zones, Zanclean in age; in other areas of the Salento this formation has been referred to the Piacenzian and Gelasian; while, in its type area it has shown a wider stratigraphic range spanning from the Piacenzian to the Santernian time interval. In spite of these diachroneity, in the Lecce area, the deposition of the formation starts in a similar context, characterized by erosive/dispersive processes alternated to active processes of phosphatization. In fact, the local occurrence of a basal conglomerate with phosphatic pebbles, as well as the benthonic assemblages, indicating the outer neritic zone, are in contrast with the evidence of a trasgression. The last sedimentary cycle is represented by the Calcareniti del Salento formation. It consist of a fossiliferous (among the other, Arctica islandica, Mya truncata and Panopea norvegica) biodetritical carbonatic unit up to 20 meters thick. The calcareous micro- and nannofossil content allow to refer it to the Globorotalia truncatulinoides excelsa and “small” Gephyrocapsa zones and, then to the Sicilian (Early Pleistocene). The benthonic assemblages indicate a shallow water marine depositional environment.