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Geology and stratigraphy of the southern sector of the Neogene Albegna River Basin (Grosseto, Tuscany, Italy)

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INTRODUCTION

The Neogene Albegna River Basin is located west of the Middle Tuscany Ridge (its southern extension stretches as far as the Argentario Mount), east of the Montalcino Castell’Azzara Ridge, south of the Scansano high and north of the Argentario-Manciano high. The neoautoctonous sediments of this basin were investigat- ed by many Authors (among the others: Mancini, 1960;

Dessau & De Stefanis, 1969; Dessau et al., 1972;

Bonazzi et al., 1980; Pasquarè et al., 1983; Bettelli, 1985; Tozzi & Zanchi, 1987; Bonazzi et al., 1992;

Barberi et al., 1994). Concerning the Late Miocene sed- iments, the Authors recognize one (Bonazzi et al., 1980), two (Bettelli, 1985; Bonazzi et al., 1992) or three (Tozzi

& Zanchi, 1987) formations. However, all retain that these formations were deposited in a continental envi- ronment (with fresh or brackish water) after the Messinian salinity crisis of the Mediterranean Basin.

According to Pasquarè et al. (1983) the basin recorded the last Apennines compressive phase of the late Messinian. During this phase, the tilting of the Miocene strata, the morphological renewal and the conglomerate deposition at the top of the succession occurred.

Moreover, these Authors distinguish that a new esten- sional phase affects the whole area during the Pliocene, but the basin assumed an antiapenninie arrangement due

to the uplift of the Scansano and Argentario-Manciano highs. All the Pliocene sediments are referred to a single Zanclean sedimentary cycle, including the most recent units (“Calcareniti di Magliano”, “Sabbie giallo ocra”,

“Arenarie di Samprugnano”, Ghiaie e Sabbie di Colle Lupo”, in Bonazzi et al., 1992) retained to be deposited during a regressive trend. Barberi et al. (1994) regard these formations as pertaining to a second Pliocene sed- imentary cycle of Piacenzian age. Finally, all the Authors agree that the area emerged during the Late Pliocene.

The sedimentation in the Albegna Valley starts again in the Early Pleistocene with the break-up of the Scansano and Argentario-Manciano structural highs which formed horsts and graben.

The stratigraphical and micropaleontological studies of the southern part of the Albegna River Basin was per- formed in 1997-1999 and a geological map as well (scale 1:10.000) has been realised. In this work, a brief note of stratigraphy and sedimentary evolution and a geological map at reduced scale is proposed. Additionally, follow- ing Bossio et al. (1998, 2000), the formations are grouped in depositional units. New depositional units are also proposed.

Since the purpose of this work concerns strictly the Neogene-Quaternary interval, the pre-Neogene forma- tions are not described in the text, while they are distin- guished in the geological map. The ages of these forma- ABSTRACT - The Neogene Basin of the Albegna River is located to east of the Middle Tuscany Ridge and its southern extension ends close to the M. Argentario. Moreover, it is disposed along a significant structural antia- penninic lineament.

The oldest Neogene sediments cropping out in the Albegna Basin (Lignitiferous Unit - T Unit) are related to the late Tortonian; they have been interpreted to be deposited into a shallow lacustrine environment. During the early Messinian, the lacustrine facies evolves to brackish condition (Acquabona-Spicchiaiola Unit - M1). This event can be correlated with a similar one known in the Southern Tuscany basins located to the west of the Middle Tuscany Ridge. The third Miocene unit (Lago-Mare Unit - M3), late Messinian in age, unconformably rests on T and M1 units. It was deposited in a fluvio-lacustrine system.

Early Pliocene deposits (Pliocene I Unit - P1) rest on the Miocene with a basal transgressive facies and are moinly represented by blue clays of outer marine shelf environment.

The Middle Pliocene (Pliocene II Unit - P2), is mainly composed of inner shelf terrigenous sediments uncon- formably overlying different biozones of the Early Pliocene. Such sediments mark a second marine event and are followed by continental-lacustrine deposits (Pliocene III Unit - P3), closing the Neogene sedimentation of the Albegna Basin. The Pleistocene deposits are widely spread; the most ancient (Chiani-Tevere-Montescudaio Unit - Q1) deposited in a fluvio-lacustrine system during the Early Pleistocene. The subsequent sediments (Pleistocene II Unit - Q2) consist of marsh lithofacies and tuffite layers derived from the Latera volcanic apparatus. The age of 320-240 Ky of the tuffite layers constrains Q2 to the Middle Pleistocene. Marine Pleistocene deposits (Pleistocene III Unit - Q3) are present in the westernmost part of the basin; they can be referred to the Late Pleistocene (Tyrrhenian) both for their altitude (not more than 15 m) and close distance to the coast. The ancient and recent alluvial deposits, with an estimated age younger than 240 ky, are separated into four orders of terraces.

KEY WORDS: Tuscany, Albegna Basin, Neogene, Stratigraphy.

GEOLOGY AND STRATIGRAPHY OF THE SOUTHERN SECTOR OF THE NEOGENE ALBEGNA RIVER BASIN (GROSSETO, TUSCANY, ITALY)

Alessandro Bossio*, Luca Maria Foresi**, Roberto Mazzei**, Gianfranco Salvatorini**, Fabio Sandrelli**, Massimo Bilotti**, Alice Colli*, Rudy Rossetto**

* Dipartimento di Scienze della Terra, Università degli Studi di Pisa, e-mail: bossio@dst.unipi.it

** Dipartimento di Scienze della Terra, Università degli Studi di Siena, e-mail: sandrelli@unisi.it

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tions, as presented in the legend, are taken from the lit- erature (Bonazzi et al., 1992 and reference therein).

According to the Authors, the Pre-Neogene succession is composed from the bottom by: Canetolo Unit and Tolfa Flysch (CF-Flysch calcareo and EC-Argille e calcari);

Pietraforte Unit (CP-Pietraforte) and Ofiolitic Unit (Cap-Argille a palombini).

Stratigraphy

In the Albegna River Basin outcropping formations belong to the Late Miocene-Pleistocene interval. They were alternatively deposited in marine and continental environments. Foresi et al. (2001) zonal scheme is adopted for the bio- chronostratigraphic references. The recognised lithostratigraphic units are from bottom:

Argille del Torrente Fosci (Ma)

The formation crops out only in the eastern sector of the mapped area, having a thickness of about 200 m. Its unconformable contact with the Pre-Neogene units does not crops out and the visible contact (with the Argille a palombini) is always tectonic, by normal faults. It con- sists mainly of massive and grey clays. These, are fre- quently laminated and locally show intercalations of marls, peat, lignite or sands. The sands are medium or coarse grained and show ripple cross-stratification.

Lenses of sandstones well cemented, grey-brown in colour (Arenarie della Caprareccia-Mar) and layers of conglomerates with small pebbles (up to 6-7 cm) poorly rounded and deriving from the Ligurian Succession (limestones, siltstones, cherts) and from Manciano Sandstones often occur. The sandstones have a maxi- mum thickness of 10 m and show a fining-upward trend.

Rare bivalves (Lymnocardium, Dreisseina) and caps of gastropods (Bithynia opercula) are present. The microfossil assemblages are composed by oogonia of Characeae, vegetal remains and ostracods. Within the lower part of the unit Candona sp., Amnicythere palimpsesta, Bullocypris robusta, Tyrrenocythere sp., and some Cyprideis (C. cf. heterostigma, C. aff. agri- gentina, C. cf. tuberculata among the others) have been recovered. The upper levels are characterised by more rich assemblages, with Amnicythere sp., Candona (Fabaeformiscandona) sp., Candona (Lineocypris) sp., Candona (Sinegubiella) sp., Candona milinkae, Cyclocypris aff. laevis, Darwinula sp., Leptocythere larga, L. stanchaevae, Limnocythere aff. socolovskien- sis, Loxoconcha aff. pseudovelata; Potamocypris sp., Tavanicythere spp., and by the previously mentioned species of Cyprideis.

The lower part of the formation has been deposited in a shallow fresh water basin. The interbedded sandstones and conglomerates testify to the presence of a fluvial and/or deltaic influence. The upper part of the formation shows characters indicative of a sedimentation in a brackish water basin. With regard to the age, these deposits are correlated with those of others well known Tuscany basins (Bossio et al., 1991, 1996a, b, 1998).

Therefore, it is possible to refer the lower part of the unit to the late Tortonian and the upper one to the early Messinian.

The Argille del Torrente Fosci and the Arenarie della Caprareccia (the names appear in the legend of the New Geological Map of Italy, scale 1:50,000, Massa Marittima, Pomarance and Volterra Sud maps) were rec- ognized in the Volterra Basin (Bossio et al., 1996a;

Pascucci & Sandrelli, 1999). The strong similarity has induced us to use the same formational names also for the deposits of the Albegna River Basin.

Argille di Poggio Cafaggio (Mag)

The unit crops out in the eastern sector of the mapped area. It unconformably overlays the Argille del Torrente Fosci and laterally passes to the Conglomerati di Poggio Campana. The Argille di Poggio Cafaggio formation comprises light brown sandy clays without evidence of stratification and layers of conglomerates with well rounded clasts composed by the same lithologies of the Conglomerati di Poggio Campana (Mcg). The maximum thickness is about 70 m.

Rare fragments of not determined bivalves, gastropods (Bithynia), oogonia of Characeae and ostracods (Amnicythere sp., Candona sp., Cyprideis gr. torosa, Cyprideis sp., Cypridopsis cucuroni, Cypridopsis sp., Ilyocypris gibba, Typhlocypris sp.) were recovered.

The depositional environment is lacustrine with fresh waters. The layers and lenses of sands and conglomer- ates testify the probable presence of fluvial and deltaic depositional system. The Messinian age of the formation is determined on the basis of its stratigraphic position, because it lies between the lower Messinian and the low- ermost Pliocene deposits.

Conglomerati di Poggio Campana (Mcg) The formation crops out in the eastern sector of the mapped area. It unconformably overlies the Argille del Torrente Fosci and the Pietraforte (Ligurian Succession), and laterally passes to the Argille di Poggio Campana.

Two lithofacies were recognized. The lithofacies a (Mcg-a) is composed by polygenic disorganised con- glomerates with sub-rounded clasts (from a few cen- timetres to one meter in size). Clasts derive from the Ligurian and Tuscan successions, the matrix is sandy and abundant. The peculiar feature of this lithofacies is the red film of hematite which coats the pebbles. The lithofacies b (Mcg-b), grey in colour, is more lithified, clasts are more rounded and small than those of lithofa- cies a, but the composition is the same. Frequently clasts are imbricated and show a coarsening upward trend. The Conglomerati di Poggio Campana have a maximum thickness of about 100 m.

The microfossil assemblages are very poor and repre- sented only by ostracods (Candona sp., Cyprideis gr.

torosa, Cyprideis sp.).

The formation can be considered the expression of a coarse-grained lacustrine delta, where Mcg-a represents

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the marginal environment sometime emerged (sub-aeri- al exposure favoured the oxidation with formation of hematite). Instead, Mcg-b probably represents the more distal portion of the delta, always submerged. The age is late Messinian like the Argille di Poggio Cafaggio.

Travertini della Marsiliana (Mtr)

The formation crops out south of Marsiliana, in the southernmost part of the studied area, where it overlies the Argille di Poggio Cafaggio. The unit consists of a stratified calcareous succession dipping up to 35° toward W and with a maximum thickness of about 70 m. The limestone strata, about 50 cm thick, are separated by thin levels of yellowish sands. Only rare imprints dubiously referred to vegetal remains were observed. A marsh was probably the depositional environment of this unit. As concerns the age, it is probably Messinian (Bosi et al., 1996). This dating is improved by the presence of traver- tine of certain Messinian age in the adjacent Ombrone Basin (Bossio et al., 1991). In this basin these limestones directly underlie the lowermost Pliocene deposits.

Sabbie di Podere Poggio Delci (Ps)

The unit is widespread in the northernmost part of the mapped area. Locally, the Sabbie di Podere Poggio Delci crops out at the base of the Argille azzurre, but they can be also in heteropic relationship with the latter forma- tion. Moreover, they overlie the Macigno (Poggio della Serpe), the Argille del Torrente Fosci (C. Granatelle) and the Conglomerati di Poggio Campana (Poggio Lepraio).

The formation consists of yellowish sands and clayey sands containing sandstone clasts derived from the Macigno and/or limestone clasts derived from the Argille a palombini. The pebbles of limestone are often bored by lithodomuos organisms. Locally the sands are replaced by well cemented sandstones with levels of conglomerate. The maximum thickness of the unit is about 30 m.

This formation contain macrofossils; Cardium, Chlamys and Ostrea are the most common in the sands, Amusium, Flabellipecten, Neopycnodonte, Pecten, and Venus in the clayey sands; shells of Isognomon were found in the sandstones. Microfossils are abundant and particularly represented by foraminifers. The benthic assemblages suggested a rapid deepening; in fact, the base of the formation is characterised by a coastal fauna (Ammonia beccarii, Asterigerinata planorbis, Cibicides lobatulus, Elphidium crispum and Florilus boueanus) and the middle-upper part by an outer shelf fauna (the most common are Bolivina spp., Bulimina spp., Cibicidoides pseudoungerianus, Dentalina communis, Gyroidina soldanii, Heterolepa bellincionii, Lenticulina spp., Marginulina costata, Martinottiella communis, Melonis soldanii, Pullenia bulloides, Sphaeroidina bul- loides, Stilostomella spp.). Planktonic foraminifers (common or abundant are Globigerina bulloides, G.

decoraperta, G. falconensis, Globigerinoides extremus, G. obliquus, Neogoboquadrina acostaensis and

Orbulina universa; rare Globorotalia margaritae) and calcareous nannofossils (common Calcidiscus macin- tyrei, Coccolithus pelagicus, Helicosphaera carteri, Reticulofenestra pseudoumbilica and rare Discoaster variabilis s.l., Syracosphaera histrica) also occur.

Calcareous plankton assemblages allows us to refer the unit to the intervals Sphaeroidinellopsis seminulina s.l.

Zone - Globorotalia margaritae Zone and Discoaster variabilis s.l. Zone - Amaurolithus tricorniculatus Zone, Zanclean in age.

Argille Azzurre (Pag)

The formation crops out in the central part of the con- sidered area where it unconformably overlies the late Messinian formations and conformably rests on the Sabbie di Podere Poggio Delci; it is also in heteropic rela- tionship with these sands. The unit is composed of mas- sive clays and silty or marly clays, grey-blue in colour. Its maximum thickness should be 200 m at least. Within the Argille azzurre remains of molluscs were observed; the most common are Neopycnodonte, Amusium and Dentalium. The foraminifer assemblages are always rich, with abundant planktonic and benthic species. The most common benthic foraminifers are Bigenerina nodosaria, Bolivina spp. Bulimina spp., Cibicidoides pseudoungeri- anus, Dentalina communis, Glandulina laevigata, Globobulimina affinis, Gyroidina soldanii, Heterolepa bellincionii, Karreriella bradyi, Lenticulina spp., Marginulina costata, Martinottiella communis, Melonis padanus, M. soldanii, Oridorsalis stellatus, Planulina ariminensis, Pullenia bulloides, Sphaeroidina bulloides, Stilostomella spp., Uvigerina peregrina, U. rutila. The ostracod assemblages are poor and characterised by Acanthocythereis histrix, Argilloecia acuminata, Bythocypris obtusata, Cytherella russoi, C. terquemi, Henryhowella asperrima, Krithe frutex, Parakrithe dactylomorpha, Ruggieria tetraptera. These benthic assemblages allow the recognition of an outer shelf pale- oenvironment. The planktonic assemblages, mainly com- posed of Globigerina bulloides, G. falconensis, Globigerinoides elongatus, G. extremus, Globorotalia margaritae, G. puncticulata, Globoturborotalita apertu- ra, G. decoraperta, Neogloboquadrina acostaensis, Orbulina suturalis, O. universa and Turborotalita quin- queloba among the foraminifers and by Calcidiscus mac- intyrei, Coccolithus pelagicus, Discoaster asymmetricus, D. brouweri, D. pentaradiatus, D. surculus, D. tamalis, D. variabilis s.l., Gephyrocapsa spp., Helicosphaera car- teri, H. sellii, Reticulofenestra pseudoumbilica, Pseudo- emiliania lacunosa, Sphenolithus abies, Syracosphaera histrica, Umbilicosphaera sibogae among the nannofos- sils, testify to a Zanclean age of the Argille azzurre (from the Sphaeroidinellopsis seminulina s.l. Zone to the Globorotalia puncticulata Zone, from the Discoaster variabilis s.l. Zone to the D. tamalis Zone).

Calcareniti di Magliano (Pc)

The unit crops out near Magliano in Toscana and in the

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westernmost side of the mapped area. It unconformably overlies the Argille azzurre at Magliano and the Flysch calcareo at Poggio Capitana (western boundary of the map) and it is in heteropic relationship with the for- mazione di Podere Melosella. The Calcareniti di Magliano are mainly composed by well lithified medi- um-coarse detrital limestones rich of fossil remains.

They are well exposed to south-east of Magliano, along an artificial cut (Fig. 1). Here, decimetric alternations of yellow sands and calcarenites with horizontal traces fos- sil and red algae fragments were observed. This facies represents the transition to the formazione di Podere Melosella. The maximum thickness of the unit is about 50 m. Macrofossil remains are abundant and composed of gastropods, bivalves, anellids (Ditrupa), echinoids.

Benthic foraminifers (Ammonia beccarii, Amphistegina gibbosa, Asterigerinata planorbis, Bolivina aenariensis, Bolivina sp., Buliminella gr. aldrovandii, Cassidulina neocarinata, Cribroelphidium decipiens, Dorothia gib- bosa, Elphidium crispum, Florilus boueanus, Fursenkoina schreibersiana, Globobulimina affinis, Hanzawaia boueana, Heterolepa bellincionii, Melonis soldanii, Textularia aciculata, T. jugosa and Valvulineria bradyana among the others) are very common. They indicate that this unit was deposited in an inner shelf environment. Planktonic foraminifers are rare (Globige- rina falconensis, Globigerinita glutinata, Globigerinoi- des elongatus, G. extremus, Globorotalia bononiensis, Globoturborotalita apertura and Neogloboquadrina planispira were recorded among the others). The occur- rence of Globorotalia bononiensis allows to refer the unit to the Globorotalia aemiliana Zone, Piacenzian in age.

Formazione di Podere Melosella (Psa) The formation crops out in the central-western side of the investigated area. It lies unconformably on the Flysch calcareo and on the Argille azzurre, and con- formably rests on the Calcareniti di Magliano. The unit shows heteropic relationships with the Arenarie e Conglomerati di Poggio Volpaio and the Calcareniti di Magliano. This formation is characterised by two het- eropic lithofacies: the sands (Psa-a) and the clayey sands (Psa-b). The sands are medium-fine grained, massive and yellow in colour and show, locally, decimetric levels of conglomerates; their maximum thickness is about 60 m. The clayey sands, light brown colour, reach a maxi- mum thickness of about 30 m.

Remains of molluscs (Anomia, Cardium, Donax, Flabellipecten, Ostrea and Dentalium), corals (Sclerac- tinia), balanids and algae were observed. Benthic forami- nifers (with common Ammonia beccarii, Asterigerinata planorbis, Cibicides lobatulus, Elphidium crispum and Florilus boueanus) and ostracods (with common Aurila aff. hesperiae, Costa runcinata, Cytheridea neapolitana, Ruggieria tetraptera, Miocyprideis italiana, Semi- cytherura acuticostata and S. inversa) were recognised.

They indicate an inner shelf environment. An interesting

outcrop was studied at Poggio Sacchetto, where, just below the contact with the formazione di Poggio Sassineri, the microfossil assemblages are characterised by Ammonia tepida only. This testifies a change of the depositional environment from coastal marine to brac- kish lagoon. Calcareous plankton is very rare end mainly composed of Globigerina bulloides, Globigerinoides elongatus, G. extremus, G. quadrilobatus, Globorotalia aemiliana, G. bononiensis, G. crassaformis, Globo- turborotalita apertura, Neogloboquadrina planispira and Turborotalita quinqueloba among the foraminifers, Calcidiscus macintyrei, Coccolithus pelagicus, Crena- lithus doronicoides, Discoaster tamalis, Gephyrocapsa spp. Helicosphaera sellii, Pseudoemiliania lacunosa, Syracosphaera histrica and Umbilicosphaera sibogae among the nannofossils. The formazione di Podere Melosella belongs to the Globorotalia aemiliana and Discoaster tamalis zones, Piacenzian in age.

Arenarie e conglomerati di Poggio Volpaio (Pcg) The formation crops out in the central part of the stud- ied area, where it conformably lies on the formazione di Podere Melosella and unconformably on the Argille azzurre. The unit is composed by well lithified coarse sand with interbedded polygenic conglomerates. The sandstones prevail on the conglomerates in the lower part; on the contrary, the conglomerates are more fre- quent in the upper part, with layers of about 1 m thick.

Clasts (mainly deriving from Macigno and Argille a palombini) are sub-rounded with maximum size of about 10 cm. Clasts of siltstones and travertine have been addi- tionally found. The maximum thickness of the formation is about 45 m.

Undeterminable bivalves and specimens of Ostrea

Fig. 1 - Calcareniti di Magliano. Interlayered bio-calcacrenites and cla- yey sands. South of Magliano.

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were observed. The samples collected within the sand- stone have furnished benthic foraminifer assemblages (with Ammonia beccarii, Aubignyna perlucida, Cribro- elphidium decipiens, Florilus boueanus, and Protelphi- dium granosum) only. These assemblages indicate a coastal marine environment.

A Piacenzian age of the unit is inferred for its heterop- ic relationship with the formazione di Podere Melosella.

Formazione di poggio Sassineri (Pca) This unit crops out in the central-western part of the mapped area where it unconformably overlies the Pliocene units. Only at Poggio Sacchetto the formation conformably lies on the Formazione di Podere Melosella. The unit consists of two heteropic lithofacies:

the calcare a Planorbis (Pca-a) and the Argille di Valloria (Pca-b). The first lithofacies is always well stratified (the maximum thickness of the strata is 4-5 cm), or laminated and whitish in colour (Fig. 2); the sec- ond is characterised by dark grey clays and marls, with a variable amount of sands and with thin layers of con- glomerates. These latter are matrix supported and show pebbles (limestones, sandstones and cherts of the local Pre-Neogene substratum) with maximum size of 3 cm.

The maximum thickness of the formation is about 50 m.

Macrofossils were not recovered through the clays, but many molds of Planorbis and others small gas- tropods are present in some levels of the limestone.

Several shells of Paludina, opercula of Bithynia, of Characeae and common ostracods (Cyprioogoniadeis

torosa, Candona candida, C. compressa, Candona sp., Candonopsis sp., Darwinula stevensoni, Ilyocypris bradyi) were recorded during the micropaleontologcal analysis. The formation deposited in a lacustrine envi- ronment with shallow water. The calcare a Planorbis represents a deposit of a low energy zone, without detri- tal influx, where the precipitation of the carbonate occurred; the Argille di Valloria indicates a more proxi- mal environment probably affected by a fluvial input.

The reconstructed stratigraphy of the Poggio Sacchetto area allows to refer the unit to the Piacenzian.

Hurzeler & Engesser (1976), Masini & Torre (1987) and Bossio et al. (1992) confirm this dating. In fact, they refer to the early Villafranchian (portion corresponding to the Middle Pliocene) the deposits of the Ombrone Basin which shown the same lithology and stratigraphic position of the formazione di Poggio Sassineri.

Conglomerati di Colle Lupo (Plc)

The unit crops out in the northern part of the mapped area and unconformably overlies all the Pliocene forma- tions. It consists of clast supported, disorganised and not well cemented polygenic conglomerates. The clasts, deriving from Calcareniti di Magliano, Macigno, Verrucano quartzite and Argille a palombini, are sub- rounded and show a wide size range (80 cm max). The colour of the conglomerates is grey or red for oxidation processes. The maximum thickness of the unit is about 30 m.

Poor ostracod assemblages (Ciprideis torosa, Ciprinotus sp. and Ilyocypris gibba) were recovered.

The depositional environment is fluvio-lacustrine, prob- ably related to a coarse-grained delta apparatus. As to the age, the unit is certainly more recent than the formazione di Poggio Sassineri (Middle Pliocene in age) and more ancient than the formazione di Botro le Stiacciole (Middle Pleistocene in age), which unconformably over- lies it. Its strong similarity with the Conglomerati di Podere Menefrego, which crops out in the near Ombrone Basin and which belong to the Late Villafranchian (Bossio et al., 1991), indicates an Early Pleistocene age.

Formazione di Botro le Stiacciole (Pla) This formation unconformably rests on the Neogene units and consists of three heteropic lithotypes: blackish clays with pebbles (Pla-a) and tuffites (Pla-b), which crop out only in the eastern part of the mapped area, and lacustrine limestones (Pla-c), which are mainly repre- sented in the northern part. The first lithotype is formed by blackish clays with scattered small size pebbles (2-3 cm) composed of limestone, cherts and quartzites deriv- ing from the Pre-Neogene substratum. Decimetric levels of peat and volcanoclastites are interlayered in the clays.

These reach a maximum thickness of about 30 m. The second lithotype consists of fine to coarse piroclastic deposits. Bright grey ashes or layers with small bombs and pumices were observed. Sometime these deposits are well sorted and normally graded; pisolitic structures

Fig. 2 - Formazione di Poggio Sassineri. Well stratified and laminat- ed deposits of the calcare a Planorbis lithofacies. Outcrop of Case Mercatale.

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were formed locally and testify to a subacqueous depo- sition, while in other places a sub-aerial deposition occurred. The maximum thickness of these piroclastic deposits is about 10 m. The third lithotype is represent- ed by pink-beige limestones, which might be massive or stratified, and sometime it is characterized by high porosity. The maximum thickness of the limestones is about 40 m.

Macrofossils were recorded within the matrix of the blackish clays. They consist of terrestrial gastropods:

Discus rotundatus, Monacha sp., Pomatis elegans, Truncatellina cylindrica, Vertigo angustior, Vitrea subri- mata, and some specimens of Helicidae; terrestrial igro- philous gastropods: Carychium minimum, C. tridenta- tum fresh water gastropods: Ancylus fluviatilis, Bithynia leachii thermalis, Planorbis planorbis, Radix sp., Stagnicola sp., Valvata piscinalis. Only two species of bivalves were recovered: Pisidium casertanum and P.

personatum. Concerning the microfossils, the analyses have furnished assemblages with oogonia of Characeae and ostracods (Candona sp., Caspiocypris sp. Cyprideis torosa, Ilyocypris gibba, Lymnocythere sp.). Generally, the deposition of the unit occurred in a brackish marsh environment. Locally, the sedimentological features of the pyroclastic deposits testify to sub-aerial conditions.

The volcanic materials can be referred to the Latera apparatus of the Magmatic Roman Province and to the time interval 240 Ky - 320 Ky (Barberi et al., 1994).

Therefore, the formazione di Botro le Stiacciole is Middle Pleistocene in age.

Sabbie e conglomerati di Poggio del Molinaccio (Pls) The unit crops out in the westernmost part of the inves- tigated area where it unconformably rests on the for- mazione di Podere Melosella and Macigno. It consists of interlayered sandstones and conglomerates (Fig. 3). The sandstones are well cemented, coarse grained, calcareous and contains scarce matrix. The strata are a few centime- tres thick. The conglomerates are clast supported, well

cemented and with scarce sandy matrix. Pebbles very small (the average size is about 1 cm, max 5 cm) are well rounded while the larger ones are flat. Some pebbles were bored by lithodomous organisms. The unit is a few meters thick and crops out at a maximum altitude of 15 m above sea level.

Macrofossils are very rare, only remains of Cardium and balanids were recovered. Benthic foraminifers (Ammonia beccarii tepida, Protoelphidium anglicum and Aubignyna perlucida) and ostracods (Cyprideis gr.

torosa) were recognized within the sandy facies. Fossil assemblages indicate a depositional environment with brackish water. A Tyrrhenian age seems to be correct for this formation because the height of the highest outcrops (15 m) is compatible with that of the Tyrrhenian marine terrace.

Alluvial deposits

These deposits, mainly composed by conglomerates, widely cover the western-central part of the considered area. They represent the depositional activity of the Albegna and Osa rivers. The following four principal orders of terraces were recognized and distinguished in the map legend: at1 (from 80 m to 115 m a.s.l.), at2 (from 45 m to 85 m a.s.l.), at3 (from 15 m to 55 m a.s.l.) and at4 (from 10 m to 40 m a.s.l.)

REGIONAL DEPOSITIONAL UNITS The Neogene and Pleistocene mapped formations were grouped in regional depositional units, following the def- inition and names of Bossio et al. (1998, pg. 514): “A regional depositional unit is a sedimentary body bounded by two type of unconformity surfaces: 1) unconformity surfaces everywhere present in the considered Apennine sector (US1); 2) unconformity surfaces detected only in some areas with correlative conformities always recog- nizable on the basis of relevant and abrupt sedimentolog- ical changes (US2)”. In this work, two new Pleistocene regional depositional units are added to the Authors’s previous list. The first one (Pleistocene II Unit - Q2), Middle Pleistocene in age, is still of uncertain definition. The second one (Pleistocene III Unit - Q3), Late Pleistocene in age, corresponds to the marine Tyrrhenian terrace (15 m a.s.l.).

The depositional units which are present in the Albegna River Basin (Fig. 4) are indicated with their lithos- tratigraphic units as follows:

Q3 - Pleistocene III Unit (Late Pleistocene)

Fig. 3 - Sabbie e conglomerati di Poggio del Molinaccio. Well stratified coarse sandstones with thin conglomerates. Type locality.

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Fig. 4 - Depositional units and hyatus of the Neoautoctonous Succession of the Albegna River Basin.

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-Sabbie e conglomerati di Poggio del Molinaccio Q2 – Pleistocene II Unit (Middle Pleistocene)

-Formazione Botro le Stiacciole

Q1 - Chiani-Tevere-Montescudaio Unit (Early Pleisto- cene)

-Conglomerati di Colle Lupo P3 - Pliocene III Unit (Piacenzian)

-Formazione di Poggio Sassineri P2 - Pliocene II Unit (Zanclean-Piacenzian)

-Arenarie e conglomerati di Poggio Volpaio -Formazione di Podere Melosella

-Calcareniti di Magliano P1 - Pliocene I Unit (Zanclean)

-Argille azzurre

-Sabbie di Pod Poggio Delci M3 - Lago-Mare Unit (late Messinian)

-Travertini della Marsiliana -Conglomerati di Poggio Campana -Argille di Poggio Cafaggio

M1 - Acquabona-Spicchiaiola Unit (early Messinian) -Argille del Torrente Fosci (uppermost part) T - Lignitiferous Unit (late Tortonian)

-Argille del Torrente Fosci

A more complete discussion about the depositional units is reported in Bossio et al. (1998, 2000). Some con- siderations are here exposed in order to understand the meaning of the new Pleistocene units and the absence of M2 unit in the Albegna River Basin.

1) - Pleistocene II Unit - Q2. It consists of continental deposits (marine sediments are present only along the coast) unconformably resting on the previous units. As well as in the Albegna River Basin, the unit characteris- es several Tuscany basins (Bossio et al., 1993). A signif- icant example is observable in the fluvio-lacustrine Upper Valdarno Basin (Sagri & Magi, 1992 cum bibl.) where it is composed of sediments of the third local flu- vial phase (Monticello-Ciuffenna deposits).

2) - Pleistocene III Unit - Q3. Deposits of this unit are a few meters thick and characterise only the coastal areas. They were formed about 124.000 years ago, dur- ing the highstand sea-level phase, corresponding to 5e oxygen isotopic stage.

3) - According to Bossio et al. (1995 and 2002), the starting rising Middle Tuscany Ridge was not yet an obstacle for the connections between the basins located to its western and eastern side during the deposition of the Argille del T. Fosci. Therefore, the first Messinian marine transgression, well recorded in the westernmost basins, reaches also the eastern basins. This is showed in the Albegna River Basin by the microfaunal content of the uppermost part of the Argille del T. Fosci, which tes- tifies to a gradual transition from a lacustrine to a brack- ish-lagoon environment. The progressive rise of the Middle Tuscany Ridge, documented by the mentioned Authors, implies the closure of the connections between Albegna River Basin and western basins. Consequently, the Messinian marine formations (M2 Unit), which are linked to the second and more significant Messinian

transgression, are not present in the considered area.

Sedimentary evolution and tectonic

In the southern part of the Albegna River Basin, the post-collisional sedimentation started at the late Tortonian and occurred in a fresh lacustrine environ- ment. The strong similarity between the Tortonian litho- logic types of the Albegna and Ombrone areas allows to suppose that these formed a single basin with an apen- ninic trend (see also Bossio et al., 1991, 2000 and Foresi et al., 1997). The presence of sandstones and conglom- erates in the eastern outcrops, suggests fluvial influx from east.

Outcrops of Miocene “epiligure” sandstones have not been recognised in the studied area. However, clasts deriving from the Arenaria di Manciano, Burdigalian - early Serravallian in age (Martini et al., 1995), occur in the Tortonian conglomerates. The presence of sand- stones and conglomerates in the eastern outcrops, sug- gests fluvial influx from east; as a consequence, the Arenaria di Manciano outcrops existed east of the basin and they covered the Pre-Neogene substratum of Castellazzara Ridge.

The first Messinian marine transgression affected the Albegna River Basin. This event is testified by the ostra- cod assemblages of brackish water which characterize the upper part of the Argille del Torrente Fosci. The uplift of the Middle Tuscany Ridge, during the early Messinian, prevents the deposition of M2 Unit. In fact, the M3 Unit directly covers the M1 and a sedimentary hyatus of about 700 ky exists between them.

With the deposition of the continental M3 Unit, sedi- mentation started again in the basin. These deposits are included within the “lago-mare” facies, which was deposited during the last phase of the Mediterranean salinity crisis. A second uplift, causing the emergence of the area, occurred in the uppermost Messinian. In fact, Pliocene sediments (P1 Unit) unconformably rest on the Miocene succession.

The reopening of the Mediterranean-Atlantic Ocean communication allowed a wide marine transgression in the basin that, probably, had acquired an antiapenninic structural feature. A generalised uplift occurred at the top of the Early Pliocene and the whole area emerged. The emersion period was brief and encompassed the Early- Middle Pliocene boundary. A new sedimentary phase characterised the base of the Middle Pliocene (P2 Unit) but it is less extended than the previous one. During the Middle Pliocene, climatic and tectonic factors caused the closure of the marine communication. Thus, some areas of the basin were filled up and others changed from a marine sedimentary environment to a continental one (P3 Unit). During the Late Pliocene, a strong uplift occurred in Tuscany and the Albegna area definitively emerged.

A new deposition activity characterised the Pleisto- cene. Two sedimentary events (Q1 and Q2 units) sepa- rated by a positive tectonic input were recorded. Both

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Q1 (Early Pleistocene) and Q2 (Middle Pleistocene) were deposited in a continental fluvio-lacustrine envi- ronment and the sedimentation of the Q2 Unit is certain- ly connected with the activity of the Latera volcanic apparatus. The last sedimentary event (Q3 Unit) occurred about 124,000 years ago and it is related to the Tyrrhenian high stand.

ACKNOWLEDGEMENTS - The authors are especially indebted to the Professor Giuseppe Manganelli (Siena University) and the Professor Raffaello Cioni (Pisa University). They have respectively studied the continental gastropods assemblages and volcanic products of the Poggio Sassineri Formation.

REFERENCES Barberi F., Buonasorte G., Cioni R., Fiordelisi A., Foresi L.M.,

Iaccarino S., Laurenzi M.A., Sbrana A., Vernia L., Villa I.M.

(1994) - Plio-Pleistocene geological evolution of the geo- thermal area of Tuscany and Latium. Mem. Descr. Carta Geol. d’It., 49, 77-134.

Bettelli G. (1985) - Geologia delle valli dei fiumi Albegna e Fiora (Toscana meridionale). Geologica Romana, 24, 147- 188.

Bonazzi U., Fazzini P., Gasperi G. (1992) - Note alla carta geo- logica del bacino del fiume Albegna. Boll. Soc. Geol. It., 111, 341-354.

Bonazzi U., Gasperi G., Fregni P. (1980) - Il Bacino neoautoc- tono del Fiume Albegna. Mem. Soc. Geol. It., 21, 267-271.

Bosi C., Messina P., Rosati M., Sposato A. (1996) - Età dei tra- vertini della Toscana meridionale e relative implicazioni neotettoniche. Mem. Soc. Geol. It., 51, 293-304.

Bossio A., Cornamusini G., Ferrandini J., Ferrandini M., Foresi L.M., Mazzanti R., Mazzei R., Salvatorini G., Sandrelli F. (2000) - Dinamica dal Neogene al Quaternario della Corsica orientale e della Toscana. Progetto Interreg II Toscana-Corsica. L’attività scientifica delle Università di Pisa e Corte, 87-95, ETS Ed.

Bossio A., Cerri R., Mazzei R., Salvatorini G., Sandrelli F.

(1996a) - Geologia dell’Area Spicchiaiola-Pignano (Settore orientale del Bacino di Volterra). Boll. Soc. Geol. It., 115, 393-422.

Bossio A., Costantini A., Foresi L.M., Lazzarotto A., Mazzanti R., Mazzei R., Pascucci V., Salvatorini G., Sandrelli F., Terzuoli A. (1998) - Neogene-Quaternary evolution in the western side of the Northern Apennines (Italy). Mem. Soc.

Geol. It., 52, 513-525.

Bossio A., Costantini A., Foresi L.M., Mazzei R., Monteforti B., Radi L., Salvatorini G., Sandrelli F. (1992) - Notizie pre- liminari sul Pliocene del Bacino del Medio Ombrone e della Zona di Roccastrada. Atti Soc. Tosc. Sc. Nat., Mem., Ser. A, 98, 259-269.

Bossio A., Costantini A., Foresi L.M., Mazzei R., Monteforti B., Radi L., Salvatorini G., Sandrelli F. (1991) - Carta Geologica dell’Area del Medio Ombrone (provincie di Siena e Grosseto). Cartografia S.E.L.C.A., Firenze, 1994.

Bossio A., Costantini A., Lazzarotto A., Liotta D., Mazzanti R., Mazzei R., Salvatorini G., Sandrelli F. (1993) - Rassegna delle conoscenze sulla stratigrafia del Neoautoctono Toscano. Mem. Soc. Geol. It., 49 - “Scritti in onore di L.

Trevisan”, 17-98.

Bossio A., Esteban M., Mazzanti R., Mazzei R., Salvatorini G.

(1996b) - Rosignano reef complex (Messinian), Livornesi Mountains, Tuscany, Central Italy. In Franseen E.K., Esteban M., Rouchy J.M. Eds., Models for carbonate stratig- raphy from Miocene reef complex of mediterranean Region.

SEPM, Concepts in Sediment and Paleont., 5, 277-294.

Bossio A., Foresi L.M., Mazzei R., Salvatorini G., Sandrelli F.

(1995) - Evoluzione tettonico-sedimentaria neogenica lungo una trasversale ai Bacini di Volterra e della Val d’Elsa. Studi Geol. Camerti, 1995/1 (volume speciale), 93-104.

Bossio A., Mazzei R., Salvatorini G.,, Sandrelli F. (2002) - Geologia dell’area compresa tra Siena e Poggibonsi (“Bacino del Casino”). Atti Soc. Tosc. Sci. Nat., Mem, Ser. A, 107, 69-85.

Dessau G., De Stefanis A. (1969) - Studio geologico-minerario della zona mercurifera di Cerreto Piano (Scansano Prov. di Grosseto). Mem. Soc. GeoI. It., 8, 289-323.

Dessau G., Duchi G., Stea B. (1972) - Geologia e depositi minerari della zona Monti Romani-Moneti (comuni di Manciano e Capalbio (Grosseto) ed Ischia di Castro (Viterbo). Mem. Soc. Geol. It., 11, 217-260.

Foresi L.M., Pascucci V., Sandrelli F. (1997) - Sedimentary and ichnofacies analysis of the Epiligurian Ponsano sand- stone (Northern Apennines, Tuscany, Italy). Giornale di Geologia, 59, 301-314.

Foresi L.M., Mazzei R., Salvatorini G. (2001) - Appendice bio- cronostratigrafica. In: Bossio A., Mazzei R. Monteforti B., Salvatorini G. - Note illustrative alla Carta Geologica di S.

Maria di Leuca. Atti Soc. Tosc. Sci. Nat., Mem, Ser. A, 107, 145-163.

Hurzeler J., Engesser B. (1976) - Les faunes de mammifères néogenes du Bassin de baccinello (Grosseto - Italie). C. R.

Acad. Sc., 283 (D), 333-336.

Iaccarino S., Salvatorini G. (1982) - A framework of plankto- nic foraminiferal biostratigraphy for early Miocene to Late Pliocene Mediterranean area. Pal. strat. Evol., 2, 115-125.

Mancini F. (1960) - Sulla geologia della piana dell’Albegna (Grosseto). Boll. Soc. Geol. It., 79, 1-90

Martini I.P., Cascella, A., Rau, A. (1995) - The Manciano Sandstone: a shoreface deposit of Miocene basin of the Northern Apennines, Italy. Sedimentary Geology, 99, 37-59.

Masini F., Torre D. (1987) - Review of the Villafranchian Arvicolids in Italy. Geologica Romana, 26, 127-133.

Pascucci V., Sandrelli F. (1999) - The Torrente Sellate Formation (Late Tortoniana-earliest Messinian). Giornale di Geologia, 41, 81-86.

Pasquarè G., Chiesa S., Vezzoli L., Zanchi A. (1983) - Evoluzione paleogeografica e strutturale di parte della Toscana Meridionale a partire dal Miocene superiore. Mem.

Soc. Geol. It., 25, 145-157.

Sagri M., Magi M. (1992) - Il bacino fluvio-lacustre del Valdarno Superiore (Pliocene superiore-Pleistocene). In:

B4- I Bacini distensivi neogenici e quaternari della Toscana.

76° Congresso S.G.I. Firenze, 21-23 Settembre 1992, Guide alle escursioni post-congresso, 201-226.

Tozzi M., Zanchi A. (1987) - Evoluzione paleogeografica e strutturale recente del bacino del Fiume Albegna. Geologica Romana, 26, 305-325.

Accettato per la stampa: Marzo 2004

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