Academic Article
Stratigrafia del neogene e quaternario del salento sud-orientale (con rilevamento geologico alla scala 1:25.000)
- Title
- Stratigrafia del neogene e quaternario del salento sud-orientale (con rilevamento geologico alla scala 1:25.000)
- Date
- 2005
- Is Part Of
- Geologica Romana
- Volume
- 38
- Pages
- 31-60
- Language
- ita
- Abstract
- The geologic map (scale 1:25.000) related to Neogene-Pleistocene sediments outcropping in the wide area along the Adriatic coast, which has Otranto-Cànnole and Marina Porto Tricase as northern and southern boundaries, is here presented. This map is very different with respect to the official 2th edition of the Geologic Map (scale 1:100.000) of Italy, particularly regarding the interpretation of the formations and their boundaries. The lithostratigraphic units have been framed in an updated biostratigraphic and chronostratigraphic context based on calcareous plankton (foraminifera and nannofossils). On the contrary, benthonic foraminifera and ostracods have been used for paleoenvironmental reconstructions. These reconstructions have concerned also particular phenomena as, for example, processes of phosphatization and glauconization in the Neogene sediments. This work completes previous researches of the writers on sectors of the same area. Its aim consists mainly to outline the Neogene-Lower Pleistocene sedimentary and paleogeographic evolution of the considered area. From the late Burdigalian to the late Calabrian (Sicilian), this area was subject to four sedimentary cycles which include six lithostratigraphic units (one of these recently instituted). The first cycle is constituted by the Pietra leccese formation and everywhere overlying Calcareniti di Andrano formation. The Pietra leccese is typically represented by biomicrites with prevalent calcareous plankton, generally straw-coloured, green in colour only in the upper part due to the abundance of glauconite. The formation, badly stratified in thick beds, through a conglomeratic level with phosphatic nodules or pebbles and macrofossils is transgressive on different pre-Neogene units. Occasionally, this level is replaced by a phosphatic film which covers surfaces and cavities of the substratum. The Pietra leccese formation belongs to the Globigerinoides trilobus Zone - Globorotalia conomiozea Zone and Sphenolithus heteromorphus Zone (lower part) - Amaurolithus delicatus - A. amplificus Zone intervals of the Mediterranean zonal scheme used here. In chronostratigraphic terms, it reaches from the upper Burdigalian to the basal Messinian; then, its deposition lasted about 11 M.A.. In spite of that, the Pietra leccese formation shows generally a small thickness (maximum 17 meters). The scanty thickness is found to be due to hiatuses linked to the erosive-dispersive action of deep sea currents. These hiatuses have been above all recorded in the uppermost part of the glauconitic interval (this dynamic environment suits well to the occurrence of glauconite), but they characterize also the bottom and top of the unit. The base of the Pietra leccese formation already contains benthonic assemblages of the outer neritic zone; therefore, during the initial phases of the subsidence, erosive processes alternated with active processes of phosphatization in upwelling regime. The subsidence produced rapidly a deepening up to a probable total submersion of the area (benthonic microfaunas indicate constantly the deeper part of the outer neritic zone). A rapid regression has been recorded at the top of the glauconitic Pietra leccese; the overlying Calcareniti di Andrano are a direct consequency of it. The Calcareniti di Andrano formation is represented by different carbonatic deposits which are well stratified and very fossiliferous. Its total thickness reaches 50 meters. The unit belongs to the G. conomiozea Zone - barren Zone (lowermost part) and A. delicatus - A. amplificus Zone-barren Zone (lowermost part) intervals; then, it can be entirely referred to the pre-evaporitic Messinian. The basal part of the Calcareniti di Andrano formation shows relatively diversified benthonic assemblages which indicate depositional environments near to the inner/outer boundary of the neritic zone. On the contrary, the unit becames increasingly poor in microfaunas when going upwards, which gives evidence of a progressive decrease of the depth. As regards its upper part, the benthonic assemblages testify a deteriorated chemical-physical condition of the sea, which precede the "salinity crisis" and, then, the precipitation of evaporites. The terminal part of the Calcareniti di Andrano formation is characterized by hypohaline assemblages. The Miocene sedimentary cycle ended owing to the emersion of the area (and Salento) which prevented the deposition of evaporites. The second cycle is represented by the Formazione di Lèuca. This unit is mainly constituted by breccias and conglomerates, which show generally heterogeneous carbonatic pebbles of different size (from few millimeters up to 60 centimeters) in a more or less abundant carbonatic, marly and sandy matrix. The macrofossils are rare and are mainly represented by Ostrea. The maximum thickness is about 30 meters. The breccias and conglomerates have been referred to the Sphaeroidinellopsis seminulina seminulina and Discoaster variabilis s.l. zones, which characterize the basal part of the Zanclean (Lower Pliocene). Sometimes, whitish marls (similar to those of the Sicilian "trubi" formation), sandy marls and yellowish calcarenites follow upwards the breccias and conglomerates. These deposits, which constitute the Palmariggi Member, encompass the S. seminulina seminulina Zone (pars) - Globorotalia puncticulata Zone (pars) and D. variabilis s.l. Zone (pars) - Discoaster tamalis Zone (lower part) intervals and, then, greater part of the Zanclean stage. The benthonic microfaunas indicate that already in the earliest Pliocene the subsidence produced rapidly a deepening of the area up to depths of the outer neritic zone. This deepening was probably associated to sudden tectonic negative activity because more or less coarse clastic material of the formation occurs within the lower part of the marls. Later (from the G. puncticulata - G. margaritae and Sphenolithus abies chronozones at least), the dynamic activity was completed by the erosive-dispersive action of deep currents and, locally, abundant glauconite was forming. The glauconitic biomicrites belong to the G. puncticulata - G. margaritae Zone - G. puncticulata Zone (pars) and S. abies Zone - D. tamalis Zone (lower part) intervals; consequently, they are partially heteropic to the Palmariggi Member. The glauconitic biomicrites are of scarce thickness and their outcroppings are not very extensive; therefore, they never have been mapped. The deposition of the Formazione di Lèuca lasted about 1.5 MA. The Formazione di Uggiano la Chiesa constitutes the third sedimentary cycle of the studied area. This unit, essentially represented by biodetritical limestones and yellowish calcareous sands, is well stratified and very fossiliferous, and shows a maximum thickeness of about 60 meters. The transgression of the formation is often emphasized by a conglomeratic level (more rarely by breccias) with phosphatic pebbles and macrofossils. Probably, the process of phosphatization alternated with the erosive-dispersive action of currents as it was during the Miocene transgression. In some areas the base of the Formazione di Uggiano la Chiesa belongs to the Piacenzian (upper part of the Globorotalia aemiliana and Discoaster pentaradiatus zones), in others it has been referred to the Gelasian (Globorotalia inflata and Discoaster brouweri zones). Therefore, it's very diachronous. The benthonic assemblages indicate typical depths of the inner neritic zone. Occasionally, lightly larger depths have been recorded. In the area near Otranto, this cycle is continued until the initial part of the Early Pleistocene (Santernian). The Santernian sediments contain Arctica islandica and calcareous plankton of the G. inflata Zone (uppermost part) and Crenalithus doronicoides Zone (upper part). Elsewhere, the uppermost part of the formation belongs to the Gelasian (Upper Pliocene); probably this suggests a diachronous regression. The deposition of the Formazione di Uggiano la Chiesa lasted about 1.0 M.A. The fourth cycle is represented by the Calcareniti del Salento formation which is Early Pleistocene in age. In fact, this unit has been referred to the Helicosphaera sellii Zone (uppermost part) - "small" Gephyrocapsa Zone and Globigerina cariacoensis Zone (upper part) - Globorotalia truncatulinoides excelsa Zone intervals and these intervals characterize the upper part of the Calabrian (highest Emilian - Sicilian). The Calcareniti del Salento formation, which is basically constituted by very fossiliferous (with A. islandica) biodetritical carbonatic sediments and shows commonly both bioturbations and cross stratification, lies in discordance on pre-Neogene, Miocene and Pliocene units. Sometimes, its base is emphasized by a conglomeratic level of scanty thickness. The Calcareniti del Salento formation reaches a maximum thickness of about 50 meters. The deposition of this unit is generally realized within limited depths of the inner neritic zone. Probably, during the Sicilian the sea, which lapped the escarpment of a considerable stretch of coast, managed to penetrate widely towards the inner areas only at south. This Pleistocene cycle encompasses at least 0.3 M.A.
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