Il substrato del gruppo del Cilento tra il M. Ves
详细信息      
摘要

The outcropping sections within the Cilento (Southern Apennines) appear, from a preliminary approximation based upon their lithology, sedimentary characteristics and structural position, to be referable to two distinct wholes deposited in as many distinct paleaogeographic domains. The upper whole, from top to bottom, have a turbiditic origin; it is generally arenaceous-pelitic and appears strongly deformed. The Authors have considered it to be an allochtonous nappe and have correlated if to the Liguride Complex or "Argille Scagliose" of the Northern Apennines (Ogniben, 1969). The lower whole is a succession of predominantly platform carbonates, generally considered autochtonous (Southern Apennines or Alburni-Cervati platform carbonates Auctt.). Locally this relationship is inverted, probably from recent tectonics, after the placement of the allochthonous complex. Within the existing maps, for example sheet no. 209 Vallo della Lucania from the second edition of the geological map of Italy at a scale of 1:100,000, there exists a detailed stratigraphic subdivision of the geometrically superior part of the arenaceous-pelitic turbiditic allochthonous nappe, noted in the literature as the Cilento Group, and of the underlying carbonate succession. On the contrary, there has been no subdivision of the lower portion of the allochthonous complex, generically noted as "substratum" of the Cilento Group or as "Chaotic Complex", "Ascea Formation" or "S. Venere Formation", even so the succession consists of various lithozones with varying degrees of tectonisation. Successively it was noted that the substratum of the Cilento Group is differentiable, based essentially upon lithology, in part referable to the North-Calabrian unit (Saraceno Formation and Crete Nere Formation) and in part to informal units related by Bonardi et alii (1988a and 1988b) to Sicilian units ("Affinita Sicilide" Complex). Anyway dose not exist neither map or systematic description of these units and even the above correlations are of an informal nature. Even the geometric relationship between the North-Calabrian and the "Affinita Sicilide" Complex, within Cilento, has a difficult and uncertain significance. Thanks to a detailed geological survey (1:10,000) of the pelitic-arenaceous successions which outcrop between the Monte Vesalo and Monte Sacro, in-between the Cilento Group and the carbonate succession, noted as the allochthonous substratum of the Cilento Group, it was possible to recognize and to describe at least 5 litho-stratigraphic units belonging to 2 tectonic units (fig. 1). From the bottom it has been recognized the following geometric succession (figs. 2 and 3): 1) the lower unit (Castelnuovo Cilento tectonic unit) consisting of 3 lithozones; the lower lithozone consists of predominant variegated shales with siliciclastic thin bedded turbidites, the central litho-zone consists of predominant marly turbidites with calcareous arenites at the base and the upper lithozone consists of predominant thin bedded siliciclastic turbidites with variegated shales; in the opinion of Bonardi et alii (1988a and 1988b) the lithotypes of this succession are related to the Sicilian units (Affinita Sicilide Complex); 2) the upper tectonic unit consisting of turbiditic succession of shales and sandstones and of marlstones and calcareous arenites, sometimes with chert nodules, usually strongly deformed, which locally passes, stratigraphically, at its base into a predominant variegated shales lithozone; this unit is equitable to the higher stratotype of the Saraceno Formation whilst the lower shales lithozone can be related to the lower part of the Saraceno Formation or the higher part of the Crete Nere Formation; this tectonic unit is equitable to the upper part of the North-Calabrian unit of Bonardi et alii (1988a and 1988b). Regarding the 3 lithostratigraphic units of the Castelnuovo Cilento tectonic unit, it is considered that everyone have the required requisites (lithologic properties, stratigraphical position, thickness, areal extension) to make up a formation. But waiting other results, from other investigations in course, it is preferable to consider these units as informal and out of rank with the local following proposed nomination (from the bottom): Genesio shales lithozone, T. Trenico marlstones lithozone, Pianelli arenaceous-pelitic lithozone. From the survey it has emerged that the contact between the Cilento Group and its substratum (originally stratigraphic) actually shows evidence of mechanical remobilization. Also the boundary between the Bifurto Formation and the underlying carbonate platform succession, originally stratigraphic, is tectonic at the present (fig. 3). This tectonic horizon, made up by Bifurto Formation and Piaggine Sandstone, takes here the same significance of the "basal melange" outcropping in the Pollino Mountain Range (Calabro-Lucanian Apennine) (Monaco et alii, 1995). On the basis of the geometrical relationships (fig. 3) and the age of the lithostratigraphic units (fig. 4), probably the 2 tectonic units are developed during the Burdigalian tectonic phase and the thrust on the carbonatic succession is not older than Upper Tortonian. Finally, lithology (shales and calcareous arenites wich passes to siliciclastic thin bedded turbidites), structural position and the middle Eocene-late Oligocene age (biostratigraphic data based on nannoplancton analysis from Bonardi et alii, 1988b) (fig. 4) suggest that the Saraceno Formation and the "Affinita Sicilide" Complex outcropping in Cilento can be correlated to Subligurian Units instead of the Ligurian Complex of the Northern Apennines (Ogniben, 1969; Mostardini & Merlini, 1986; Bigi et alii, 1992).

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