19 G07 Sandrelli
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Ofioliti, 2001, 26 (2a), 371-380 371 C - EASTERN SECTOR OF THE VOLTERRA BASIN Fabio Sandrelli Dipartimento di Scienze della Terra, Università di Siena, via Laterina 8, 53100 Siena, Italy (e-mail: [email protected]). Keywords: field trip, Neogene sediments, Unconformity bounded units, Volterra Basin. Italy. INTRODUCTION A different hypothesis on the geodynamic evolution of Southern Tuscany, considers that the first compressional The geological setting of Southern Tuscany, where the phase acted until the end of the Neogene. In such a case, the Volterra Basin is located, is due to two different tectonic sedimentary basins of western Tuscany would have formed phases. The first one, compressional, is linked to the conver- as piggy-back or thrust-top basins (Bonini et al., 1994; Boc- gence and collision between the Adriatic Microplate and the caletti et al., 1994; Bonini and Moratti, 1995; Boccaletti et European Plate and occurred from the Late Cretaceous to al., 1997). the Early Miocene. The second one is consequence of the During the field trip, we will visit the Volterra Basin be- extensional tectonic regime active since the Early-Middle tween Spicchiaiola and Pignano. This area is particularly Miocene (Carmignani and Kligfield, 1990; Jolivet et al., important because it lies on the western margin of the Mid- 1990; Bertini et al., 1992; Carmignani et al., 1994; 1995; El- dle Tuscany Ridge and recorded most of the depositional ter and Sandrelli, 1995a; Baldi et al., 1995; Dal Mayer et al., events which occurred in Southern Tuscany from Late 1996). According to Elter and Sandrelli (1995a; 1995b), the Miocene to Pliocene. The four stops will illustrate some of extensional phase included two main events. During the first the relationships between tectonics and sedimentation: one, late Burdigalian to early Tortonian in age, there was a Stop 1 - Transition from Turolian (upper Tortonian) la- rapid uplift of the lithosphere, thickened during both the custrine deposits to lower Messinian marine deposits in dis- Alpine and Apenninic orogeneses. The lithosphere never tal areas of the basin. A transgression affected different for- reached the temperature necessary to generate calc-alkaline mations, highlighting discontinuities and disconformities. magmas(600°-650°, Sonder et. al., 1987). This event, was Stop 2 - Gypsum deposits formed during the salinity cri- accompanied by “II phase folds” with both east and west sis which affected the Mediterranean Sea at the end of early vergence, by the emplacement of the Cretaceous Ligurian Messinian. A debris flow interrupts the continuity of these Units directly on the Triassic evaporites (“Serie ridotta”: deposits, suggesting synsedimentary tectonic activity. Signorini, 1949; Trevisan, 1955; Giannini et al., 1971; Stop 3 - Limestone deposits tilted down near a fault Bertini et al., 1992; Decandia et al., 1993) and by the depo- which formed before the deposition of Lower Pliocene ma- sition of the Epiligurian Units. rine sediments. This fact documents activity of the border During the second extensional event, which started in the fault between Early Pliocene and late Messinian. This por- late Tortonian, a “thermal re-equilibration” associated to tion of the basin was uplifted during late Messinian and sub- generation of anatectic magmas initiated in a thinned conti- sided again during Early Pliocene. nental crust characterized by a high thermal flow (Table 1 in Stop 4 - The marine deposits of the Early-Middle Elter and Sandrelli, 1995a). The extensional regime led also Pliocene, very well developed in the Mazzolla-Volterra to the formation of graben and half-graben basins where area, where they reach a thickness of about 1200 m. In the thick sedimentary successions were deposited. These basins western area of Spicchiaiola-Pignano the Pliocene succes- are laterally separated by WSW-ESE transverse lineaments sion is, instead, thin and discontinuous. This difference in (transfer zones Fig. 1). thickness is due to a synsedimentary normal fault. I I I I I I I I I I I I I I I I II I I I I I I I I I I I I I I I I FIRENZE I I I I I I I I I I PISA I I I I I I I I I I I I I THE VOLTERRA BASIN I Volterra BasinMIDDLE I I I I I I I I I I I II I I I I I I I I I I I I I I I I I I I I I I II I I I I I I I I I I I I I I I I I I I I I I I I The Volterra Basin is located in the central part of South- I I I I I I I I I I I I I VOLTERRA I I I I I I I I I II I TUSCANY I I I ern Tuscany and is filled by Upper Miocene to Pleistocene I I I I I I I AREZZO I I I I I I I I I I I I I I I I I I I I I sediments (Figs. 1 and 2). I I I I I I SIENA I I I I I I I I I I I I I I I III I I I I I I I I I I I I I I I The excursion will deal with the area between Mazzolla I I I I I I I I I I I I I CASTELNUOVOI I I I I I I I I I I I V. DI CECINA I I I I and Pignano (E of Volterra), located in the eastern side of I I I I 0 20 Km MONTICIANO I I I I I RIDGE I I I I I I I I I I I I I I I I I I II I the basin. I I I I I I I I I I I I I I I I I I I I I I I I I The basin formed since the late Serravallian through a I II I I I I I I I I I I I I I I I I I I I III I I I I complex series of events related to the extensional tectonic I I I I I I I I I I I I ELBA I. II I I I I I regime. During the late Serravallian-early Tortonian, most I I I I I I I I I I I I I I I I I I of the present Volterra Basin was part of a wide marine I I GROSSETOI I basin in which sediments of the Epiligurian Ponsano Sand- I I I stones were deposited (Bossio et al., 1996b). The bound- I I GRABEN I NORMAL FAULTS TRANSFER ZONE I aries of such early marine basin extended as far as the Chi- Fig. 1 - Structural sketch map of Southern Tuscany. White areas represent anti Ridge (Foresi et al., 1997a; Fig. 2A). the main Pliocene and Quaternary basins and dotted areas the structural After the early Tortonian a general uplift occurred in this highs. The square rappresents the field trip area. part of the Apennines and, during the late Tortonian, tecton- 372 Fig. 2 - Sketch of the paleogeographic evolution of Tuscany during Late Miocene-Pliocene time. (Modified after: Bossio et al., 1992; Foresi et al., 1997). A. Serravallian - early Tortonian: 1- emerged areas; 2- marine environment. B. Turolian (late Tortonian): 1- emerged areas; 2- lacustrine environment. C. Base of the early Messinian: 1-emerged areas; 2- marine environment (reef facies of the Acquabona Limestones Member of the Rosignano Limestones); 3-brackish environment (locally the first evaporite sediments occur); 4- la- custrine environment. D. early Messinian: 1- emerged areas; 2- marine envi- ronment (reef facies of the Castelnuovo Limestones Member and Pycn- odonte Clays Fm.); 3- lacustrine environment. E. Top of the early Messinian: 1- emerged areas; 2- second evaporitic episode; 3- lacustrine environment. F. late Messinian: 1- emerged areas; 2- lacustrine environment (“lago-mare”); 3- lacustrine environment. G. Early Pliocene (Sphaeroidinellopsis seminuli- na s.l. Zone): 1 - emerged areas; 2 -marine environment. H. Early Pliocene (Globorotalia margaritae Zone - lower part of the G. puncticulata Zone): 1- emerged areas; 2- marine environment. I. Early Pliocene (upper part of the G. puncticulata Zone): 1- emerged areas; 2- marine environment. L. Middle Pliocene (G. aemiliania Zone): 1- emerged areas; 2- marine environment. ic depressions separated by structural highs (ridges) formed. The Volterra Basin was part of one of such tectonic depres- sions, located between the Middle Tuscany Ridge to the west and the Peri-Tyrrhenian Ridge to the east (Bossio et al., 1996b; Fig. 2B). Recently, Bossio et al. (1997) studying the southern part of the Volterra Basin demonstrated that during the late Tor- tonian-late Messinian, the basin was a half-graben with the master fault located on the eastern side near Pignano, adja- cent to the Middle Tuscany Ridge. The same half-graben geometry was still present in the Pliocene, but the master fault shifted westward (Mazzolla Fault). It has been mapped near locality Mazzolla (Bossio et al., 1996a; 1996b; 1997; Fig. 3). According to Bossio et al. (1992), during the Late Miocene a thick succession of fluvio-lacustrine, lacustrine to brackish, marine and finally lacustrine (“lago-mare”) sed- iments was deposited in the depression (Fig. 2B-F). During the Early Pliocene a marine transgression oc- curred. It was caused by the rise of sea-level due to renewed communication of the Mediterranean Sea with the Atlantic Fig. 3 - Geological sketch of the Volterra Basin; in the square the studied Ocean, and by the contemporaneous subsidence of the area. area: 1- Alluvial deposits; 2- Magmatic deposits; 3- Pliocene sediments; 4- In some parts of the Volterra Basin (Volterra - Mazzolla Miocene sediments 5- Ponsano Sandstones; 6- Ligurian Units; 7- Tuscan area), the basal Pliocene sediments lie conformably on the Unit.