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.

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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. “lago-mare” ones. In other parts (Spicchiaiola - Pignano 373 area) they lie with an angular unconformity on different Cecina River. This occurrences are considered local re- Miocene units and/or on the pre-Miocene substratum. Ac- sponses to the uplifting of southern areas (Fig. 4). cordingly, this area was uplifted at the end of the Messinian In the Mazzolla area, the presence of sandstones (L inter- and the transgression occurred during the G. margaritae val fig. 4b), interlayered with deep marine clays, suggests Zone (Early Pliocene) in response to renewed subsidence). that during the Pliocene this area represented the depocentre The different evolution between the two areas has been re- of the basin and that the Mazzolla Fault was the active mas- lated to the activation of the Mazzolla Fault that continued ter fault of the Volterra half graben (Bossio et al., 1996a). its activity during all the Pliocene. The continuous movement of this fault created the necessary The Lower-Middle Pliocene sediments, deposited north accomodation space for thick deposits. of the Cecina River, differ from those deposited south of the river (Figs. 2H-L, and 4). In the first area sedimentation was continuous until the Globorotalia aemiliana Zone of the Description of the formations Middle Pliocene. In the second area, instead, sedimentation Before dealing with the detailed tectonic and sedimentary evo- was discontinuous and included two sedimentary cycles lution of the Mazzolla-Pignano area, we will briefly describe the separated by a hiatus. The first cycle ranges from S. semi- Mio-Pliocene formations (Fig. 5). nulina Zone s.l to G. margaritae-G. puncticulata Zone (Ear- Ponsano Sandstones (late Serravallian-early Tortonian) ly Pliocene). The second one is comprised within the G. ae- The formation is characterized by interbedded sandstones and miliana Zone (Middle Pliocene). In this area therefore, an marly-claystones. The sandstones are fine to medium grained, well uplift event occurred during the G. puncticulata Zone. sorted, yellow to grey-yellow and well cemented. The marly-clay- A section trough the Cecina River - Volterra area (Fig. stones are massive, grey and contain variable amounts of sand. Both lithologies are highly bioturbated. This formation was de- 4b) shows the Pliocene succession. This is characterized in posited in an inner shelf environment. the lower portion by some conglomeratic levels (S. semi- Podere Luppiano Conglomerates (Turolian, late Tortonian) nulina Zone pp. - G. puncticulata (G. margaritae Zone pp.) This formation is a poorly organised conglomerate with a ma- and in the middle portion by three sandstone levels (G. trix of poorly sorted coarse sand, granules and small pebbles. puncticulata Zone), all deriving from areas south of the Clasts range from pebbles to boulders and most derive from the

Fig. 4 - a. Paleogeographic sketch showing the uplifted area from where mass flow started during Early Pliocene(after Bossio et al. 1994b). 1- Pliocene out- crops of the southern-central part of the Volterra Basin; 2- uplifting areas during Early Pliocene; 3- preferential paths for mass flow. b. Stratigraphic section of Casa al Pazzo - Volterra (after Bossio et al. 1994b). 374 Pycnodonte Sandy Marlstones, Ma- lenses and levels of pebbly sand- Bithynia Fig. 5 - Geological map (after Cerri and Sandrelli, 1994). Substratum of the Mio-Pliocene formations: Ta- Burano Anhydrite; D- Basalt; Cp- Palombini Shales; CM- Montaione Flysch. Miocene formations: Fig. 5 - Geological map (after Cerri and Sandrelli, 1994). Substratum of the Mio-Pliocene formations: Ta- Burano Anhydrite; D- of MaP- Ponsano Sandstones; McL- Podere Luppiano Conglomerates; MaF- Fosci Creek Claystones (ol- olistostrome, Mm- lenses and levels Member, McC- Castelnuovo Limestones Member); MP- stones; Mc- lenses and levels of conglomerates): Ms- Spicchiaiola Formation; Rosignano Limestones (McV- Villa Mirabella Conglomerates and levels, MtP - limestone lenses “Pignano Travertine”); McS- Clays (g- gypsum lenses and levels); MaE- Era Morta River Gypsums levels, NsN- sandstone Azzurre” Fm. (Pp- Il Poggino Conglomerates Member). Recent sedi- Borro Sassicaia Conglomerates (g- gypsum lenses and levels). Pliocene formations: PcV- Bosco delle Volpaie Conglomerates; Pa- “Argille ments: a- alluvial deposits. 375

Ligurian Units. The conglomerate is a debris flow deposited in an Borro Sassicaia Conglomerates (late Turolian, late Messinian) alluvial fan complex. These conglomerates are constituted by heterometric (up to 12 Fosci Creek Claystones (Turolian, late Tortonian - early cm in diameter) and polygenic cobbles (“Palombini” limestone and Messinian) porphyric basalt of the Ligurian Units are dominant) in a silty- This formation, well exposed in badlands, is made up of massive, sandy matrix. Often the conglomerates are matrix supported. This grey claystones and marly claystones with local small (about 10 cm) formation represents a lacustrine delta. peat lenses. It contains lenses and layers of sandy marlstones with Bosco delle Volpaie Conglomerates (Early Pliocene) Bithynia (Bithynia Sandy Marlstones), pebbly sandstones and con- These conglomerates mark the base of the Pliocene transgres- glomerates. It was deposited in a lacustrine environment. sion in the eastern part of the Spicchiaiola - Pignano area. They Spicchiaiola Formation (early Messinian) rest unconformably on the Palombini Shales and Era Morta River It rests conformably on the Fosci Creek Claystones and it con- Claystones and Gypsums. They consist of heterogeneous (mostly sists of grey to brown-grey claystones and sandy claystones which dolomitic) and generally well rounded pebbles in an abundant contain intercalations of laminated calcareous sandstones. These sandy matrix. sandstone intercalations increase in thickness and frequency up- The Bosco delle Volpaie Conglomerates is interfingered with wards. In the upper part of the formation, rare conglomerate layers the “Argille Azzurre” Fm. also occur. “Argille Azzurre” Formation (Early-Middle Pliocene) The macrofossil content is poor and it is almost entirely made up It consists of massive blue-grey clays or, near the base, sandy of fragments of lamellibranchs and gastropods, serpulides and vege- clays. tal remains. These fossils indicate a lagoonal-brackish environment. In the Spicchiaiola-Pignano area, this formation conformably Villa Mirabella Conglomerates Member (member of Rosig- rests on the Bosco delle Volpaie Conglomerates, or directly uncon- nano Limestones, early Messinian) formably transgresses onto the Era Morta River Claystones and It consists of massive conglomerates and lies at the base of, or Gypsums (western part of the investigated area). North of Spicchi- is eteropic with the Castelnuovo Limestones Member (eastern side aiola, at the base of the clays, there are conglomerate and breccia of the studied area). Pebbles are mostly calcareous, generally well (Il Poggino Conglomerates Member) intercalations, interpreted as rounded, variable in size, and show perforations by lithophagous mass-flow deposits. In the Mazzolla-Volterra area the “Argille Az- organisms. The arenitic-calcareous (or marly-clayey) matrix is zurre” Fm. rests conformably on Era Morta River Claystones and generally abundant. Gypsums. In the same locality conglomerates (Serrazzano Con- In the Spicchiaiola-Pignano area, these conglomerates represent glomerates, syn. of Zambra Creek Conglomerates) and sands the base of the marine transgression and is early Messinian in age. (Mazzolla Sands) occur interlayered with the clays. Both intercala- In the Middle Tuscany Ridge, the conglomerates directly overlie a tions have been referred to gravity flow deposits. pre-Neogene substrate (Palombini Shales); in the westernmost ar- As for the environment, benthonic Foraminifera indicate a eas theyit lie above several Miocene units. maximum depth corresponding to the deeper part of the outer Castelnuovo Limestones Member (member of Rosignano neritic zone. Limestones, early Messinian) This member is made up of different calcareous facies and is Unconformity bounded units partly interfingered with the Villa Mirabella Conglomerates Mem- ber and with the Pycnodonte Clays; the latter unit overlies the The Miocene formations of the Volterra Basin have been Castelnuovo Limestones Member. This member mostly consists of grouped in the unconformity bounded units (Bossio et al., 1997) bioclastic limestones, whitish to yellowish in colour and rich in ter- defined as “mappable stratiform body of sedimentary rock that is rigenous material. Bioclastic deposits, containing molluscs, echi- defined and identified on the basis of its bounding discontinuities noids, bryozoans, and Ditrupa, are associated with mounds of and their correlative conformities” (NASC, 1983, p. 865, modified rhodolites and/or Porites (stock-like and over 1 m long). Beds by Walker, 1992). More recently, Bossio et al. (1998) defined the (several cm to 1 m thick) of fine conglomerates are sometimes pre- Neogene unconformity bounded units of the western sector of the sent in the limestones. The limestones represent a patch reef de- Northern Apennines. The correlation between the Neogene region- posit while the conglomerates are a delta deposit. The occurrence al depositional units, the Miocene depositional units of the Volter- of these conglomerates suggests that the patch reefs indicate re- ra Basin and the sedimentary cycles recognised in the Spicchiaiola- peated attempts to colonize the delta, occasionally interrupted by Pignano area are shown in Fig. 6. The latter sedimentary cycles are the flow of clastic material. always separated by unconformities related to synsedimentary tec- Associations of benthonic Foraminifera and of Ostracoda indi- tonic activity (uplifting and/or subsidence). cate a marine environment (inner neritic zone). From late Serravallian to Middle Pliocene the following deposi- Pycnodonte Clays (early Messinian) tional units have been recognised in the Volterra Basin. From the bottom up, the formation is made up of a gypsum Unit 1. (Unit 1 of Bossio et al., 1997) It corresponds to the ma- unit, overlain by a clayey interval locally containing marine fossils. rine deposits of the Ponsano Sandstones (Giannini and Tongiorgi, At the bottom, the clay deposits reflect a brackish environment, at 1959; Mazzanti et al., 1981; Mazzei et al., 1981; Foresi et al., the top a marine environment. 1997a,b). It crops out near Volterra (Ponsano locality; Fig. 2), in the Era Morta River Clays and Gypsums (late Turolian, late eastern side of the studied area, and near Siena (Rencine). It lies un- Messinian) conformably on bedrocks belonging to the Ligurian Units. Accord- This formation is made up of a gypsum unit at the bottom, ing to Elter and Sandrelli (1995a), Unit 1 is autochthonous and not overlain by a thick clayey and clayey-sandy succession interbed- semiallochthonous as reported in the preceding literature (Baldacci ded with layers and lenses of gypsum, sandstones and travertines. et al., 1967; Decandia et al., 1993). The maximum thickness of the The gypsum unit was deposited in a marine environment, while the unit, measured in the Ponsano outcrop is 550 m. The age of this unit clayey succession in a lacustrine one (“lago-mare”). is late Serravallian-early Tortonian. The gypsum is present in primary as well as in diagenetic fa- Unit 2. (Unit 2 of Bossio et al., 1997, Unit T+M1, of Bossio et cies. The former comprises “balatino” gypsum, gypsarenite or gyp- al., 1998) It crops out extensively east of Volterra. It includes con- sum sandstone and selenitic gypsum. The latter is alabastrine gyp- glomerates of the Podere Luppiano Conglomerates and clays of the sum. Selenitic gypsum develops in proximal areas of the basin, Argille Fosci Creek Claystones in the lower part, and clays and whereas “balatino” gypsum and gypsarenite or gypsum sandstone sands of the Spicchiaiola Fm. and clays of the Pycnodonte Clays characterise distal areas of the basin. p.p. in the upper part. The lower part of this unit is known in the Travertine occurs in lenses from several to about 1000 m long literature as “Serie Lignitifera” (Trevisan, 1952; Mazzanti, 1961; and, up to about 60 m thick, it is bedded and generally shows plane 1966; Lazzarotto and Mazzanti, 1978; Bossio et al., 1996a). Unit 2 laminations. The presence of travertine indicates that the shallow lies unconformably on both Unit 1 and bedrock. The thickness of lacustrine basin was periodically and locally emerged. the unit, in the Spicchiaiola area, is 500 m. Its age is late Torton- 376 ian-early Messinian. paie Conglomerates. Its age is Early-Middle Pliocene. Unit 3. (Unit 3 of Bossio et al., 1997; Unit M2 of Bossio et al., In the Spicchiaiola-Pignano area the marine Pliocene transgres- 1998) It is well exposed in the Spicchiaiola area and lies uncon- sion on the Miocene units occurred in the Globorotalia margaritae formably (angular unconformity) on bedrock and on Unit 2, or, in Zone, while in the western Mazzolla area it occurred only on Unit places, conformably (correlative conformity) on Unit 2. Unit 3 is 4 but in the Sphaerodinellopsis seminulina Zone, that is the begin- characterized by conglomerates of the Villa Mirabella Conglomer- ning of Early Pliocene. ates Member, limestones of the Castelnuovo Limestones Member, As mentioned before, the Pliocene succession in the Volterra and clays of the Pycnodonte Clays p.p. (Cerri and Sandrelli, 1994; Basin, is continuous north of the Cecina River, whereas south of Bossio et al., 1996a). These three lithologies show interfingering re- such a river it is discontinuous. In this latter area, Unit 5 could be lationships. Conglomerates and limestones were deposited near the subdivided into two sub-units, due to the presence of an angular margins of the sedimentary basin, whereas clays occurred mainly unconformity. North of the Cecina River, instead, the two sub- towards the center of the basin. The unit ranges from 30 m to 50 m units have correlative conformity type boundary and, only in in thickness. Its age is early Messinian. places, they could be distinguished thanks to the presence of a sand Unit 4. (Unit 4 of Bossio et al., 1997; Unit M3 of Bossio et al., layer (III sandy layer of the Sabbie di Mazzolla, Bossio et al. 1998) It is present in the Spicchiaiola and Cecina River areas. Its 1994b; Bossio et al., 1998). lower contact is indicated as an unconformity on the basis of seis- mic data, but such an unconformity is not recognisable in outcrop. The upper boundary of the unit grades conformably (correlative FIELD TRIP conformity) into the Pliocene clays. Unit 4 is characterized by clays with interbedded sands, gypsum and reworked gypsum of the The four selected stops (Fig. 7) illustrate the main fea- Era Morta River Claystones and Gypsums p.p. and conglomerates of the Borro Sassicaia Conglomerates (Cerri and Sandrelli, 1994). tures of different stratigraphic horizons. For Stops 1, 2 and 3 The thickness of the unit measured in the Spicchiaiola area is 500 see Fig 8, sections 9, 3 and 2 respectively. m (Fig. 5). Its age is late Messinian. We take the road Pomarance - Saline di Volterra - Unit 5. (Unit P1+P2 of Bossio et al., 1998) includes marine clays of the “Argille Azzurre” Fm, sands of the Mazzolla Sands Volterra; the first part of the road is moslty in Miocene de- (Bossio et al., 1994b) and conglomerates of the Bosco delle Vol- posits, while the second part crosses Pliocene deposits.

NEOGENE UNITS OF WESTERN APENNINE

P2 SEDIMENTARY ENVIRONMENT RELATIONSHIPS BETWEEN THE NEOGENE FORMATIONS MIOCENE UNITS CYCLES OF VOLTERRA P1 BASIN IV cycle Marine "Argille Azzurre" Bosco delle Volpaie Conglomerates

Borro Sassicaia Lacustrine M3 Unit 4 Era Morta River Clays and Gypsums Conglomerates III cycle ("Lago-Mare ")

M2 Unit 3 Marine Pycnodonte Clays Castelnuovo Limestones Member V. Mirabella Conglomerates Member

M1 Brackish Spicchiaiola Formation Fosci Creek Claystones Unit 2 II cycle T Lacustrine pre-Neogene substratum Podere Luppiano Conglomerates a) Unit 1 I cycle Marine Ponsano Sandstones b) c)

Fig. 6 - a- Neogenic Units of the western Apennine (after Bossio et al., 1998); b- Miocene Units of the Volterra Basin (after Bossio et al., 1997); c- Strati- graphic relationships between the neoautochthonous formations which crop out in the Spicchiaiola-Pignano area. Wavy lines indicate transgression bound- aries (after Bossio et al., 1996a)

S. Gimignano

STOP 3

Pignano Colle di Volterra Ripaiola Val d'Elsa

STOP 2 STOP 1 Monte- Mazzolla riggioni

Casole STOP 4 d'Elsa SIENA

0 7 14 km Fig. 7 - Field trip itinerary along the eastern margin of the Volterra Basin. 377

Passed Volterra, we continue driving along the road S.S. n. 1 2 68, heading towards Colle Val d’Elsa, crossing the youngest deposits of the Pliocene marine succession. Passed the inter- section with the secondary road for Mazzolla (where later STOP 3 on we stop for a panoramic view of the area), the road crosses Miocene deposits. At km 52 of the S.S. n. 68 we are be able to observe the Messinian cliffs and their overlapping relationship with the lacustrine clays.

Stop 1. Transition from Tortonian lacustrine to Messin- 3 4 5 7 8 9 ian marine deposits. NON DISTINCTIVE ZONE Argille e gessi del fiume Era Morta Formation

NON DISTINCTIVE ZONE The Castelnuovo Limestones Member “patch reef” lies on STOP 2 lacustrine sediments. Generally the Messinian marine sedi- ments are in continuous on the lower brackish-lacustrine sedi- Calcari di Castelnuovo Member STOP 1 ments (Spicchiaiola Fm.). In the westernmost areas they are MESSINIAN

Argille a

Pycnod. F. transgressive onto the older Turolian (late Tortonian) lacus- 6 trine clays (Fosci Creek Claystones). In the easternmost areas

Formation of the Volterra Basin the transgression surface occurs directly Spicchiaiola on the pre-Miocene substrate. These relationships (Fig. 9) can G. CONOMIOZEA be explained by an uplift of the easternmost part of the lacus- trine basin, located near the Middle-Tuscany Ridge, before the general Messinian marine expansion (Bossio et al., 1996a;

1996b). AMPLIFICUS A. A. PRIMUS -

G. SUTERAE

The stop is along an escarpment where the Castelnuovo G. O. EXTREMUS 100 m

Argille del T. Fosci Formation Argille del T. Limestones Member and its relationships with the Fosci ??? 50 Creek Claystones are visible (Fig. 9). The latter consist com- TORTONIAN posed of grey clays rich in lacustrine Ostracods. 0 A layer (one meter thick) of the Villa Mirabella Con- glomerates Member is present at the base of the Castelnuo- a b c d e f g h i vo Limestones Member. The former is composed of rela- tively rounded, mostly carbonatic clasts (“Palombini” lime- Fig. 8 - Stratigraphic sections in their biochronostratigraphic context: 1 - stones of the Ligurian Unit and dolomites from the Burano synthesis of the succession in the Pignano-Spicchiaiola area; 2- Strati- Anhydrite Fm. of the Tuscan Nappe), sometimes perforated graphic section from Stalle to Pignano; 3- Gesseri stratigraphic section; 4- by lithophagous organisms. The overlying Castelnuovo Pod. Ripaiola stratigraphic section; 5- Era Morta stratigraphic section; 6- Limestones Member is a yellowish, detrital organogenic Spicchiaiola stratigraphic section; 7- Podere Spicchiaiolona 2 stratigraphic section; 8- Podere Spicchiaiolona 1 stratigraphic section; 9- le Bandite limestone rich in terrigenous material which locally forms stratigraphic section. Key to symbols: a- travertine; b-gypsum; c- clay; d- conglomerate lenses. This conglomerate contains unsorted, biogenic limestone; e- sandstone; f- conglomerate; g- pre-Neogenic sub- generally small clasts, with some clay pebbles from the un- stratum; h- line of lithostratigraphic correlation; i- marine transgression derlying lacustrine unit. A coarse bioclastic (Mollusc, Echi- (after Bossio et al., 1996a). The arrows indicate the stratigraphic level of noid, Briozoid and Rodolite fragments) and terrigenous de- the stops. trital sediment is associated with mounds made up of red al- gae (encrusting Rodolites) and/or “Porites” coral. Laterally Mediterranean Sea. In the Spicchiaiola-Pignano area the level to the mounds intermound sediments with frequent tractive is variable in thickness, texture and structure; “balatino” gyp- structures are present. They are composed of the same but sum, gypsarenite, selenitic gypsum (characteristic of the distal finer bioclastic material of the mounds. areas of the basin), microcrystalline gypsum and alabaster are The outcrop represents repeated attempts to establish a all present. Numerous quarries now mostly abandoned, were reef complex in a deltaic system. The deposition of terrige- used to extract alabaster (Volterra Alabaster). nous sediments, derived from eastern areas, temporarily in- Opposite to the entrance of the quarry, on the right side terrupted the growth of the reef. At the top of the outcrop, of the main front, gypsum and clay strata are interbedded there are clayey-sandy deposits which indicate the end of over massive marine clays. Different types of gypsum are the carbonatic sequence. observable: “balatino” gypsum, gypsarenite and alabaster (this lower portion of the stratigraphic-section is currently We drive on the road S.S. n. 68 in direction of Volterra; not well exposed). right before km 47 we will make a right on a dirt road that The lowermost portion of the outcrop exposes gypsum goes to Pod. Ripaiola. At this latter locality, we will visit a beds with alabaster nodules and localized carbonatic en- gypsum quarry where the stratigraphic contact between a crustations. It is covered by a paraconglomerate lens con- few meter thick gypsum layer and Messinian marine clays is taining clasts of dark dolomite and white limestone de- well exposed. rived from the pre-Miocene basin (of eastern provenance), reworked alabaster nodules (generally poorly rounded), Stop 2. Messinian evaporites and carbonatic (calcrete) concretions containing paludal In the quarry, the evaporitic gypsum level is interrupted by vegetal remains. The matrix is composed of clay, gypsum a debris flow deposit. Deposition of evaporites closes the and sand. Messinian marine domain. These evaporites constitute a con- The middle part of the quarry is characterized by 8 m of tinuous level in the whole Volterra Basin and can be correlat- laminated gypsum (Fig. 10) with thin intercalations of ed with sediments deposited during the “salinity crisis” of the chalky clay. 378

The upper part of the Era Morta River Claystones and Gypsums crops out in the Pignano area. This forma- tion is characterized by “lago-mare” clays interlayered with limestone lens- es (Pignano Travertine); these lenses have variable thicknesses and a 45° dip. Pliocene clays and conglomerates conformably overlie this succession. The stop is in a quarry 250 m to NNW of Pignano where the lime- stones are well exposed. They are up to 60 m thick and finely laminated. The planar to slightly undulated cen- timeter-thick laminae consist of three different facies which rhythmically al- ternate (Fig. 11): 1- Encrusting limestones consist of irregular sheets and small vegetal/mi- crobial domes encrusted by microcrys- talline calcite. The sedimentary and petrographic characteristics and the fa- Fig. 9 - Stratigraphic sketch of part of the Messinian succession which filled the cies association of these limestones are eastern margin of the Volterra Basin; the stratigraphic section shows the se- typical of travertine (Buccino et al., quence at Stop 1. Key to symbols: 1- clay and clayey sandstone; 2- conglomer- 1978; Chafetz and Folk, 1984). ate and paraconglomerate; 3- biogenic and detrital-organogenic limestone; 4- 2- Travertine-derived lime sands clayey sand. consist of sand-sized intraclasts of the encrusted facies, commonly showing In the left side of the quarry main front the monotonous calcretisation processes; sequence is interrupted by a channellised debris flow de- 3- Lime mudstones consist of homogeneous micrite with posit mainly consisting of clasts derived from the Palombini scattered Ostracods carapaces. Shales (Fig. 10). The channel cuts the laminated gypsum The depositional setting corresponds to a distal, flat flow- very deeply. The debris flow is correlated with active faults surface related to a thermal-spring system, where deposition located in the western areas, responsible for the subsidence of the encrusting facies was periodically interrupted by of the basin. stream-transported lime sand and temporary shallow lakes. The morphology of the flow surface appears to have been We return on road S.S. n. 68, and then we continue dri- controlled by the degree of water saturation and the mean- ving towards Colle Val d’Elsa. At km 51 we will make a dering of the streams fed by the spring. left in direction Gambassi, and then we will drive in direc- The position of the travertine above the “lago-mare” tion of Pignano. When in Pignano we drive on a dirt road clays testifies uplifting and emersion of some areas of the that goes for about 300 m in direction NW, and finally we basin. The deposition of travertine suggests that active nor- reach a quarry of Messinian travertine which has been used mal faults facilitated the upwelling of hydrothermal waters to build the town during the Middle Age. rich in calcium carbonate. The presence of several travertine beds in the Pignano area indicates synsedimentary fault ac- Stop 3. “Lago-mare” succession tivity in marginal areas of the basin. These faults caused This stop shows tilted limestones of the “lago-mare” suc- basin subsidence, and created the accomodation space nec- cession. Tilting occurred between the end of late Messinian essary for deposition of a thick Messinian succession. and the beginning of Early Pliocene. During the Miocene-Pliocene transition, the Pignano Fault was responsible for tilting of the travertine beds onto which Pliocene sediments transgressed. The Pignano Fault is considered the active master fault of the Volterra Basin half-graben (Bossio et al., 1996a,b; 1997); seismic data and the increased dip of nearby sediment sug- gest a listric geometry of this fault. The cross-section shown in Fig. 12 is a synthesis of the relationships be-

Fig. 10 -View of the main quarry front near Podere Ripaiola where the gypsum is deeply channelled. Key to symbols: 1- alabastrine gypsum; 2- laminated gypsum; 3- limestone; 4- debris flow. 379

Fig. 11 - Interpretation of the depositional set- ting of the Pignano Travertine. The square il- lustrates the typical travertine sequence as can be observed on the quarry wall. tween Upper Miocene and Pliocene sediments. Acknowledgments

Once back on S.S. n. 68, we drive towards Volterra, and I would like to thank Prof. I. Peter Martini (University of then we stop at the intersection with the road that goes to Guelph), Dr. Vincenzo Pascucci (University of Siena), and Mazzolla. On top of the hill located near the previous inter- reviewers for their useful suggestions. This work has been section, we have a general view of the Pliocene marine suc- financed by MURST 60% (Prof. F. Sandrelli). cession that from the area of the Cecina River extends to- wards Volterra. REFERENCES Stop 4. Lower-Middle Pliocene marine deposits. The lower portion of this succession, can be seen driving Baldacci F., Elter P., Giannini E., Giglia G., Lazzarotto A., Nardi R. and Tongiorgi M., 1967. Nuove osservazioni sul problema along the gravel road south of Mazzolla. The clay sedimen- della Falda Toscana e sulla interpretazione dei Flysch Arenacei tation, mainly referred to a deep marine environment, is in- tipo “Macigno” dell’Appennino Settentrionale. Mem. Soc. Ge- terrupted by the arrival of coarse material from southern ar- ol. It., 6: 213-244. eas. The deposition of coarse sediments is related to the Baldi P., Bertini G., Cameli G.M., Decandia F.A., Dini I., Laz- original half graben geometry of the basin. The Mazzolla zarotto A. and Liotta D., 1995. La tettonica distensiva post-col- Fault (Fig. 13) was active since basal Pliocene, dividing two lisionale nell’area geotermica di Larderello (Toscana Merid- areas having different evolution. The area west of the Maz- ionale). Studi Geol. Camerti, Vol. Spec. 1994/1: 139-150 zolla fault (Mazzolla-Volterra area) was characterized by Bertini G., Cameli G.M., Costantini A., Decandia F.A. Di Filippo strong subsidence and continuous deposition whereas the M., Dini I., Elter F.M., Lazzarotto A., Liotta D., Pandeli E., eastern area (Spicchiaiola - Pignano area) was affected by Sandrelli F. and Toro B., 1992. Struttura geologica fra i monti discontinuities and low sedimentation rate. In conclusion, di Campiglia e Rapolano Terme (Toscana Meridionale): stato attuale delle conoscenze e problematiche. Studi Geol. Camerti, during Early-Middle Pliocene this fault represented the most Vol. Spec. 1991/1: 155-178 important morphostructural element of the eastern sector of Boccaletti M., Cerrina Feroni A., Martinelli P., Moratti G., Plesi G. the Volterra Basin. and Sani F., 1994. L’area tosco-laziale come dominio di tran- sizione tra il bacino tirrenico e i thrust esterni: rassegna dei dati In a few minutes we reach the town of Volterra. mesostrutturali e possibili relazioni con le discontinuità del ci-

Pignan Pignano C Mazzolla F.Era Viva Pa PcV 50 F.Era Morta PcV Mt Pa Mc 40 40 Ma Pa Ma Mc Ma Ma 30 30 Mc 200 m m 200 Ta . . Fig. 12 - Geological section of the Pignano ...... g . . g area (after Cerri and Sandrelli, 1994); for ab- . M M M breviations see the geological map of Fig. 5. SW Ma N

Montecatini LU Val di Cecina Volterra 600 La Pa 400 MP 200 Pa m 0 MP m 0 Pa MS MaF MaF Ps Pc PF MaE McL 1000 LU 0 1 Km W E

Fig. 13 - Geological cross section through the Volterra Basin based on the new geological survey for the Geological Map of Italy (scale 1: 50.000, Sheet Po- marance). Symbols: La- Lamproite Formation; Pa- “Argille Azzurre” Fm. (Ps- Mazzolla Sands, Pc- Serrazzano Conglomerates); MaE- Era Morta River Clays and Gypsums; MP- Pycnodonte Clays; MS- Spicchiaiola Formation; MaF- Fosci Creek Claystones; McL- Podere Luppiano Conglomerates; PF- Ponsano Sandstones; LU- Ligurian Units. 380

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