The reconstruction of the tectono-stratigraphical framework of the oceanic units (Ligurian Units Auctt.: Helminthoid Flysch Units, Vara Units, Lanciaia Fm.) cropping out in southern Tuscany is difficult because of the complex tectonics which characterize this sector of the Northern Apennines. Moreover, the widespread occurrence of Miocene-Pliocene and Quaternary sedimentary covers interrupt the areal continuity of the outcrops of the structural pile. To reconstruct the stratigraphic-structural evolution of the Ligurian Units (LU) of southern Tuscany and to compare their sedimentary and compositional features in areas crossed by the CROP-18 profile, the authors performed sedimentological, biostratigraphical, petrographical and structural studies on some key outcrops (Gambassi-Montaione, S. Donato in Poggio, Northern Monti delta Gherardesca, Micciano-Libbiano and Serrazzano, Lanciaia-Montecastelli Pisano). Data from geothermal wells were also used in the regional correlation of units. The Cretaceous helminthoid flysches have been grouped into a single formation, the Monteverdi Marittimo Fm, subdivided into three members: (i) the Larderello Member; (ii) the Montaione Member; (iii) the San Donato Member. These represent different parts of the oceanic basin. The Larderello Member (Flysch Calcareo-Marnoso, Flysch di Monteverdi M.mo Auctt.) represents normal sedimentation in the middle part of the basin; the Montaione Member (Flysch di Montaione, Flysch di Chianni, Flysch di Castelluccio Auctt.) was deposited in the inner (western?) part and close to an ophiolitic ridge because of the occurrence of ophiolitic olistoliths and breccias; while the San Donato Member (Formazione di S. Donato: BORTOLOTTI, 1962) represents the distal eastern part of the turbiditic system. Even though the LU of southern Tuscany were affected by a complex polyphasic evolution (including out-of-sequence thrusting), we can recognize the sequence of five main tectonic units (from bottom to top of the pile): Morello Unit, Castelnuovo Val di Cecina Unit (e.g. Marly-Calcareous Flysch Unit, Aucct.), Vara Unit (Lower Ophiolitiferous Unit Auctt.) and Lanciaia Fm., Castelluccio Unit (Montaione Flysch Unit Auctt.) and Montignoso Unit (Upper Ophiolitiferous Unit Auctt.). The Lupicaia Creek Unit (Montecatini Sandstones Unit Auctt.) is also present locally at the top of the pile (Montecatini Val di Cecina and south of the Montaione-Gambassi areas). In the Early to Late Paleocene, the thrusting of the Vara Unit onto the Monteverdi M.mo Fm. occurred and was sealed by the sedimentation, in a piggyback basin, of the Lanciaia Fm., which can be considered an earlier Epiligurian unit. From the Late Eocene the Lanciaia Fm. was deformed by the out-of-sequence thrusting of the Castelluccio Unit and, finally, of the Montignoso Unit. The main structural features of the LU have an,<<anti-Apenninic>>, trend (N80 to N110) superimposed by a late <<Apenninic>> (N150 to N180) trend. The former trend involved the LU at least until Middle-Late Eocene, while is not recognizable in the Tuscan Units, and the Neogene deposits which preserve only the Apenninic orientation. Moreover, the regional distribution of the Cretaceous-Eocene ophiolitic debris in the flysch sequences reflects the anti-Apenninic trend. The above said data lead us to hypothesise, at least for the Ligurian Units of this sector of the Northern Apennines, a structural evolution probably linked, until the Middle-Upper Eocene, to a transpressive tectonic context.

The Ligurian Units of Southern Tuscany / G.NIRTA; E.PANDELI; G.PRINCIPI; G.BERTINI; N.CIPRIANI. - In: BOLLETTINO DELLA SOCIETÀ GEOLOGICA ITALIANA. - ISSN 0037-8763. - STAMPA. - VOL. SPEC. N.3 (2005):(2005), pp. 29-54.

The Ligurian Units of Southern Tuscany

PANDELI, ENRICO;PRINCIPI, GIANFRANCO;
2005

Abstract

The reconstruction of the tectono-stratigraphical framework of the oceanic units (Ligurian Units Auctt.: Helminthoid Flysch Units, Vara Units, Lanciaia Fm.) cropping out in southern Tuscany is difficult because of the complex tectonics which characterize this sector of the Northern Apennines. Moreover, the widespread occurrence of Miocene-Pliocene and Quaternary sedimentary covers interrupt the areal continuity of the outcrops of the structural pile. To reconstruct the stratigraphic-structural evolution of the Ligurian Units (LU) of southern Tuscany and to compare their sedimentary and compositional features in areas crossed by the CROP-18 profile, the authors performed sedimentological, biostratigraphical, petrographical and structural studies on some key outcrops (Gambassi-Montaione, S. Donato in Poggio, Northern Monti delta Gherardesca, Micciano-Libbiano and Serrazzano, Lanciaia-Montecastelli Pisano). Data from geothermal wells were also used in the regional correlation of units. The Cretaceous helminthoid flysches have been grouped into a single formation, the Monteverdi Marittimo Fm, subdivided into three members: (i) the Larderello Member; (ii) the Montaione Member; (iii) the San Donato Member. These represent different parts of the oceanic basin. The Larderello Member (Flysch Calcareo-Marnoso, Flysch di Monteverdi M.mo Auctt.) represents normal sedimentation in the middle part of the basin; the Montaione Member (Flysch di Montaione, Flysch di Chianni, Flysch di Castelluccio Auctt.) was deposited in the inner (western?) part and close to an ophiolitic ridge because of the occurrence of ophiolitic olistoliths and breccias; while the San Donato Member (Formazione di S. Donato: BORTOLOTTI, 1962) represents the distal eastern part of the turbiditic system. Even though the LU of southern Tuscany were affected by a complex polyphasic evolution (including out-of-sequence thrusting), we can recognize the sequence of five main tectonic units (from bottom to top of the pile): Morello Unit, Castelnuovo Val di Cecina Unit (e.g. Marly-Calcareous Flysch Unit, Aucct.), Vara Unit (Lower Ophiolitiferous Unit Auctt.) and Lanciaia Fm., Castelluccio Unit (Montaione Flysch Unit Auctt.) and Montignoso Unit (Upper Ophiolitiferous Unit Auctt.). The Lupicaia Creek Unit (Montecatini Sandstones Unit Auctt.) is also present locally at the top of the pile (Montecatini Val di Cecina and south of the Montaione-Gambassi areas). In the Early to Late Paleocene, the thrusting of the Vara Unit onto the Monteverdi M.mo Fm. occurred and was sealed by the sedimentation, in a piggyback basin, of the Lanciaia Fm., which can be considered an earlier Epiligurian unit. From the Late Eocene the Lanciaia Fm. was deformed by the out-of-sequence thrusting of the Castelluccio Unit and, finally, of the Montignoso Unit. The main structural features of the LU have an,<>, trend (N80 to N110) superimposed by a late <> (N150 to N180) trend. The former trend involved the LU at least until Middle-Late Eocene, while is not recognizable in the Tuscan Units, and the Neogene deposits which preserve only the Apenninic orientation. Moreover, the regional distribution of the Cretaceous-Eocene ophiolitic debris in the flysch sequences reflects the anti-Apenninic trend. The above said data lead us to hypothesise, at least for the Ligurian Units of this sector of the Northern Apennines, a structural evolution probably linked, until the Middle-Upper Eocene, to a transpressive tectonic context.
2005
VOL. SPEC. N.3 (2005)
29
54
G.NIRTA; E.PANDELI; G.PRINCIPI; G.BERTINI; N.CIPRIANI
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