Abstract

The Macigno Fm. (Late Oligocene-Early Miocene) represents one of the most important turbidite foredeep wedges of the Northern Apennines. It extends from the Tuscan coast to the Tuscan-Emilian Apennines. It has been well studied from sedimentological, stratigraphic and petrographic points of view, mainly in the "chain" outcrops; i.e. along the backbone of the Apennines. In contrast, the westernmost outcrops of southern Tuscany are not well constrained, because of the high fragmentation due to the Neogene extensional tectonics that affected this region. A careful examination of these outcrops, using modern analytical methods, allows new data to be added, relevant to the Late Oligocene depositional evolution of the northern Apennines foredeep. This paper gives new petrographic data on the Late Oligocene turbidite deposits of the Macigno Fm. cropping out in the Piombino-Punta Ala and Caldana-Buriano areas. The former outcrops are called "Macigno costiero Auctt." and the latter are called "Macigno appenninico" (Gasperi, 1968; 1970; Gandolfi & Paganelli, 1992; Costa et alii, 1997; Cornamusini, 1998). This area has been regarded as very important for the understanding of the Late Oligocene foredeep sedimentation, because it could represent the connection between the two turbidite systems. The turbidite bodies have been identified through detailed geological survey, facies analysis and petrographical analysis; this last is reported here. Petrographical modal analysis of the sandstones, performed at two levels of point-counting (framework and finegrained lithic compositions) allowed two petrofacies to be identified, which are Petrofacies A, richer in volcanic and carbonate lithic grains than the Petrofacies B. These two petrofacies are not related to the evolution of the same turbidite system and associated source area, but belong to two distinct, partially interacting, turbidite systems. The data from the westernmost area of Baratti-Piombino (A belt) all show Petrofacies A and dispersal palaeocurrent directions (20 degrees -160 degrees ). The data coming from the more eastern areas of Punta Ala, Castiglione della Pescaia and Caldana are more complex. The Punta Ala outcrop shows prevalent sandstone beds of Petrofacies A interlayered with minor sandstone beds of Petrofacies B (B belt), in contrast to the more eastern sector where the latter petrofacies prevails (C belt). The easternmost area of Caldana-P.gio Ballone is characterized by Petrofacies B sandstone beds (D belt). The palaeocurrents obtained in this area are practically unimodal with longitudinal-basin directions (140 degrees -160 degrees ). This distribution is interpreted as the interaction between the transverse-basin feed of the "Macigno costiero" represented by Petrofacies A and the longitudinal basin feed of the "Macigno appenninico" represented by Petrofacies B. Such an interaction developed with alternating turbidite beds of different source that gradually moved from the former system to the second, as detected in the B and C belts.

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