INTRODUCTION

The Neogene Albegna River Basin is located west ofthe Middle Tuscany Ridge (its southern extensionstretches as far as the Argentario Mount), east of theMontalcino Castell’Azzara Ridge, south of the Scansanohigh and north of the Argentario-Manciano high. Theneoautoctonous 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 thatthese formations were deposited in a continental envi-ronment (with fresh or brackish water) after theMessinian salinity crisis of the Mediterranean Basin.According to Pasquarè et al. (1983) the basin recordedthe last Apennines compressive phase of the lateMessinian. During this phase, the tilting of the Miocenestrata, the morphological renewal and the conglomeratedeposition 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-Mancianohighs. All the Pliocene sediments are referred to a singleZanclean sedimentary cycle, including the most recentunits (“Calcareniti di Magliano”, “Sabbie giallo ocra”,“Arenarie di Samprugnano”, Ghiaie e Sabbie di ColleLupo”, in Bonazzi et al., 1992) retained to be depositedduring a regressive trend. Barberi et al. (1994) regardthese formations as pertaining to a second Pliocene sed-imentary cycle of Piacenzian age. Finally, all the Authorsagree that the area emerged during the Late Pliocene.The sedimentation in the Albegna Valley starts again inthe Early Pleistocene with the break-up of the Scansanoand Argentario-Manciano structural highs which formedhorsts and graben.

The stratigraphical and micropaleontological studiesof the southern part of the Albegna River Basin was per-formed in 1997-1999 and a geological map as well (scale1:10.000) has been realised. In this work, a brief note ofstratigraphy and sedimentary evolution and a geologicalmap at reduced scale is proposed. Additionally, follow-ing Bossio et al. (1998, 2000), the formations aregrouped in depositional units. New depositional units arealso proposed.

Since the purpose of this work concerns strictly theNeogene-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-

Geologica Romana 37 (2003-2004), 165-173

ABSTRACT - The Neogene Basin of the Albegna River is located to east of the Middle Tuscany Ridge and itssouthern 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 thelate Tortonian; they have been interpreted to be deposited into a shallow lacustrine environment. During the earlyMessinian, the lacustrine facies evolves to brackish condition (Acquabona-Spicchiaiola Unit - M1). This event canbe correlated with a similar one known in the Southern Tuscany basins located to the west of the Middle TuscanyRidge. The third Miocene unit (Lago-Mare Unit - M3), late Messinian in age, unconformably rests on T and M1units. 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 aremoinly 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 arefollowed by continental-lacustrine deposits (Pliocene III Unit - P3), closing the Neogene sedimentation of theAlbegna 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 (PleistoceneII Unit - Q2) consist of marsh lithofacies and tuffite layers derived from the Latera volcanic apparatus. The age of320-240 Ky of the tuffite layers constrains Q2 to the Middle Pleistocene. Marine Pleistocene deposits (PleistoceneIII 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 recentalluvial 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 NEOGENEALBEGNA 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

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 iscomposed from the bottom by: Canetolo Unit and TolfaFlysch (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 formationsbelong to the Late Miocene-Pleistocene interval. Theywere alternatively deposited in marine and continentalenvironments. Foresi et al. (2001) zonal scheme isadopted for the bio- chronostratigraphic references. Therecognised lithostratigraphic units are from bottom:

Argille del Torrente Fosci (Ma)

The formation crops out only in the eastern sector ofthe mapped area, having a thickness of about 200 m. Itsunconformable contact with the Pre-Neogene units doesnot crops out and the visible contact (with the Argille apalombini) is always tectonic, by normal faults. It con-sists mainly of massive and grey clays. These, are fre-quently laminated and locally show intercalations ofmarls, peat, lignite or sands. The sands are medium orcoarse grained and show ripple cross-stratification.Lenses of sandstones well cemented, grey-brown incolour (Arenarie della Caprareccia-Mar) and layers ofconglomerates with small pebbles (up to 6-7 cm) poorlyrounded and deriving from the Ligurian Succession(limestones, siltstones, cherts) and from MancianoSandstones often occur. The sandstones have a maxi-mum thickness of 10 m and show a fining-upward trend.

Rare bivalves (Lymnocardium, Dreisseina) and capsof gastropods (Bithynia opercula) are present. Themicrofossil assemblages are composed by oogonia ofCharaceae, vegetal remains and ostracods. Within thelower part of the unit Candona sp., Amnicytherepalimpsesta, Bullocypris robusta, Tyrrenocythere sp.,and some Cyprideis (C. cf. heterostigma, C. aff. agri-gentina, C. cf. tuberculata among the others) have beenrecovered. The upper levels are characterised by morerich assemblages, with Amnicythere sp., Candona(Fabaeformiscandona) sp., Candona (Lineocypris) sp.,Candona (Sinegubiella) sp., Candona milinkae,Cyclocypris aff. laevis, Darwinula sp., Leptocytherelarga, L. stanchaevae, Limnocythere aff. socolovskien-sis, Loxoconcha aff. pseudovelata; Potamocypris sp.,Tavanicythere spp., and by the previously mentionedspecies of Cyprideis.

The lower part of the formation has been deposited ina shallow fresh water basin. The interbedded sandstonesand conglomerates testify to the presence of a fluvialand/or deltaic influence. The upper part of the formationshows characters indicative of a sedimentation in abrackish water basin. With regard to the age, thesedeposits are correlated with those of others well knownTuscany basins (Bossio et al., 1991, 1996a, b, 1998).

Therefore, it is possible to refer the lower part of the unitto the late Tortonian and the upper one to the earlyMessinian.

The Argille del Torrente Fosci and the Arenarie dellaCaprareccia (the names appear in the legend of the NewGeological Map of Italy, scale 1:50,000, MassaMarittima, Pomarance and Volterra Sud maps) were rec-ognized in the Volterra Basin (Bossio et al., 1996a;Pascucci & Sandrelli, 1999). The strong similarity hasinduced us to use the same formational names also forthe deposits of the Albegna River Basin.

Argille di Poggio Cafaggio (Mag)

The unit crops out in the eastern sector of the mappedarea. It unconformably overlays the Argille del TorrenteFosci and laterally passes to the Conglomerati di PoggioCampana. The Argille di Poggio Cafaggio formationcomprises light brown sandy clays without evidence ofstratification and layers of conglomerates with wellrounded clasts composed by the same lithologies of theConglomerati di Poggio Campana (Mcg). The maximumthickness 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 freshwaters. The layers and lenses of sands and conglomer-ates testify the probable presence of fluvial and deltaicdepositional system. The Messinian age of the formationis 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 themapped area. It unconformably overlies the Argille delTorrente 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 theLigurian and Tuscan successions, the matrix is sandyand abundant. The peculiar feature of this lithofacies isthe red film of hematite which coats the pebbles. Thelithofacies 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 clastsare imbricated and show a coarsening upward trend. TheConglomerati di Poggio Campana have a maximumthickness 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 acoarse-grained lacustrine delta, where Mcg-a represents

BOSSIO et al.166 Geologica Romana 37 (2003-2004), 165-173

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

Travertini della Marsiliana (Mtr)

The formation crops out south of Marsiliana, in thesouthernmost part of the studied area, where it overliesthe Argille di Poggio Cafaggio. The unit consists of astratified calcareous succession dipping up to 35° towardW and with a maximum thickness of about 70 m. Thelimestone strata, about 50 cm thick, are separated by thinlevels of yellowish sands. Only rare imprints dubiouslyreferred to vegetal remains were observed. A marsh wasprobably the depositional environment of this unit. Asconcerns 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 OmbroneBasin (Bossio et al., 1991). In this basin these limestonesdirectly underlie the lowermost Pliocene deposits.

Sabbie di Podere Poggio Delci (Ps)

The unit is widespread in the northernmost part of themapped area. Locally, the Sabbie di Podere Poggio Delcicrops out at the base of the Argille azzurre, but they canbe also in heteropic relationship with the latter forma-tion. Moreover, they overlie the Macigno (Poggio dellaSerpe), the Argille del Torrente Fosci (C. Granatelle) andthe Conglomerati di Poggio Campana (Poggio Lepraio).The formation consists of yellowish sands and clayeysands containing sandstone clasts derived from theMacigno and/or limestone clasts derived from theArgille a palombini. The pebbles of limestone are oftenbored by lithodomuos organisms. Locally the sands arereplaced by well cemented sandstones with levels ofconglomerate. The maximum thickness of the unit isabout 30 m.

This formation contain macrofossils; Cardium,Chlamys and Ostrea are the most common in the sands,Amusium, Flabellipecten, Neopycnodonte, Pecten, andVenus in the clayey sands; shells of Isognomon werefound in the sandstones. Microfossils are abundant andparticularly represented by foraminifers. The benthicassemblages suggested a rapid deepening; in fact, thebase of the formation is characterised by a coastal fauna(Ammonia beccarii, Asterigerinata planorbis, Cibicideslobatulus, Elphidium crispum and Florilus boueanus)and the middle-upper part by an outer shelf fauna (themost common are Bolivina spp., Bulimina spp.,Cibicidoides pseudoungerianus, Dentalina communis,Gyroidina soldanii, Heterolepa bellincionii, Lenticulinaspp., 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) andcalcareous nannofossils (common Calcidiscus macin-tyrei, Coccolithus pelagicus, Helicosphaera carteri,Reticulofenestra pseudoumbilica and rare Discoastervariabilis s.l., Syracosphaera histrica) also occur.Calcareous plankton assemblages allows us to refer theunit to the intervals Sphaeroidinellopsis seminulina s.l.Zone - Globorotalia margaritae Zone and Discoastervariabilis 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 lateMessinian formations and conformably rests on theSabbie 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. Itsmaximum thickness should be 200 m at least. Within theArgille azzurre remains of molluscs were observed; themost common are Neopycnodonte, Amusium andDentalium. The foraminifer assemblages are always rich,with abundant planktonic and benthic species. The mostcommon benthic foraminifers are Bigenerina nodosaria,Bolivina spp. Bulimina spp., Cibicidoides pseudoungeri-anus, Dentalina communis, Glandulina laevigata,Globobulimina affinis, Gyroidina soldanii, Heterolepabellincionii, Karreriella bradyi, Lenticulina spp.,Marginulina costata, Martinottiella communis, Melonispadanus, M. soldanii, Oridorsalis stellatus, Planulinaariminensis, Pullenia bulloides, Sphaeroidina bulloides,Stilostomella spp., Uvigerina peregrina, U. rutila. Theostracod assemblages are poor and characterised byAcanthocythereis histrix, Argilloecia acuminata,Bythocypris obtusata, Cytherella russoi, C. terquemi,Henryhowella asperrima, Krithe frutex, Parakrithedactylomorpha, Ruggieria tetraptera. These benthicassemblages allow the recognition of an outer shelf pale-oenvironment. The planktonic assemblages, mainly com-posed of Globigerina bulloides, G. falconensis,Globigerinoides elongatus, G. extremus, Globorotaliamargaritae, 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, Syracosphaerahistrica, Umbilicosphaera sibogae among the nannofos-sils, testify to a Zanclean age of the Argille azzurre (fromthe Sphaeroidinellopsis seminulina s.l. Zone to theGloborotalia puncticulata Zone, from the Discoastervariabilis s.l. Zone to the D. tamalis Zone).

Calcareniti di Magliano (Pc)

The unit crops out near Magliano in Toscana and in the

GEOLOGY AND STRATIGRAPHY OF THE SOUTHERN ... 167Geologica Romana 37 (2003-2004), 165-173

westernmost side of the mapped area. It unconformablyoverlies the Argille azzurre at Magliano and the Flyschcalcareo at Poggio Capitana (western boundary of themap) and it is in heteropic relationship with the for-mazione di Podere Melosella. The Calcareniti diMagliano are mainly composed by well lithified medi-um-coarse detrital limestones rich of fossil remains.They are well exposed to south-east of Magliano, alongan artificial cut (Fig. 1). Here, decimetric alternations ofyellow sands and calcarenites with horizontal traces fos-sil and red algae fragments were observed. This faciesrepresents the transition to the formazione di PodereMelosella. The maximum thickness of the unit is about50 m. Macrofossil remains are abundant and composedof gastropods, bivalves, anellids (Ditrupa), echinoids.Benthic foraminifers (Ammonia beccarii, Amphisteginagibbosa, Asterigerinata planorbis, Bolivina aenariensis,Bolivina sp., Buliminella gr. aldrovandii, Cassidulinaneocarinata, Cribroelphidium decipiens, Dorothia gib-bosa, Elphidium crispum, Florilus boueanus,Fursenkoina schreibersiana, Globobulimina affinis,Hanzawaia boueana, Heterolepa bellincionii, Melonissoldanii, Textularia aciculata, T. jugosa and Valvulineriabradyana among the others) are very common. Theyindicate that this unit was deposited in an inner shelfenvironment. Planktonic foraminifers are rare (Globige-rina falconensis, Globigerinita glutinata, Globigerinoi-des elongatus, G. extremus, Globorotalia bononiensis,Globoturborotalita apertura and Neogloboquadrinaplanispira were recorded among the others). The occur-rence of Globorotalia bononiensis allows to refer theunit to the Globorotalia aemiliana Zone, Piacenzian inage.

Formazione di Podere Melosella (Psa)

The formation crops out in the central-western side ofthe investigated area. It lies unconformably on theFlysch calcareo and on the Argille azzurre, and con-formably rests on the Calcareniti di Magliano. The unitshows heteropic relationships with the Arenarie eConglomerati di Poggio Volpaio and the Calcareniti diMagliano. 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, massiveand yellow in colour and show, locally, decimetric levelsof conglomerates; their maximum thickness is about 60m. 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, Asterigerinataplanorbis, Cibicides lobatulus, Elphidium crispum andFlorilus boueanus) and ostracods (with common Aurilaaff. 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, justbelow the contact with the formazione di PoggioSassineri, the microfossil assemblages are characterisedby Ammonia tepida only. This testifies a change of thedepositional environment from coastal marine to brac-kish lagoon. Calcareous plankton is very rare end mainlycomposed of Globigerina bulloides, Globigerinoideselongatus, G. extremus, G. quadrilobatus, Globorotaliaaemiliana, G. bononiensis, G. crassaformis, Globo-turborotalita apertura, Neogloboquadrina planispiraand Turborotalita quinqueloba among the foraminifers,Calcidiscus macintyrei, Coccolithus pelagicus, Crena-lithus doronicoides, Discoaster tamalis, Gephyrocapsaspp. Helicosphaera sellii, Pseudoemiliania lacunosa,Syracosphaera histrica and Umbilicosphaera sibogaeamong the nannofossils. The formazione di PodereMelosella belongs to the Globorotalia aemiliana andDiscoaster 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 diPodere Melosella and unconformably on the Argilleazzurre. The unit is composed by well lithified coarsesand with interbedded polygenic conglomerates. Thesandstones prevail on the conglomerates in the lowerpart; 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 apalombini) are sub-rounded with maximum size of about10 cm. Clasts of siltstones and travertine have been addi-tionally found. The maximum thickness of the formationis about 45 m.

Undeterminable bivalves and specimens of Ostrea

BOSSIO et al.168 Geologica Romana 37 (2003-2004), 165-173

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

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 acoastal 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 themapped area where it unconformably overlies thePliocene units. Only at Poggio Sacchetto the formationconformably lies on the Formazione di PodereMelosella. The unit consists of two heteropic lithofacies:the calcare a Planorbis (Pca-a) and the Argille diValloria (Pca-b). The first lithofacies is always wellstratified (the maximum thickness of the strata is 4-5cm), or laminated and whitish in colour (Fig. 2); the sec-ond is characterised by dark grey clays and marls, with avariable amount of sands and with thin layers of con-glomerates. These latter are matrix supported and showpebbles (limestones, sandstones and cherts of the localPre-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, ofCharaceae and common ostracods (Cyprioogoniadeis

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

The reconstructed stratigraphy of the PoggioSacchetto area allows to refer the unit to the Piacenzian.Hurzeler & Engesser (1976), Masini & Torre (1987) andBossio et al. (1992) confirm this dating. In fact, theyrefer to the early Villafranchian (portion correspondingto the Middle Pliocene) the deposits of the OmbroneBasin which shown the same lithology and stratigraphicposition of the formazione di Poggio Sassineri.

Conglomerati di Colle Lupo (Plc)

The unit crops out in the northern part of the mappedarea and unconformably overlies all the Pliocene forma-tions. It consists of clast supported, disorganised and notwell 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). Thecolour of the conglomerates is grey or red for oxidationprocesses. The maximum thickness of the unit is about30 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 theage, the unit is certainly more recent than the formazionedi Poggio Sassineri (Middle Pliocene in age) and moreancient than the formazione di Botro le Stiacciole(Middle Pleistocene in age), which unconformably over-lies it. Its strong similarity with the Conglomerati diPodere Menefrego, which crops out in the near OmbroneBasin 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 Neogeneunits and consists of three heteropic lithotypes: blackishclays with pebbles (Pla-a) and tuffites (Pla-b), whichcrop out only in the eastern part of the mapped area, andlacustrine limestones (Pla-c), which are mainly repre-sented in the northern part. The first lithotype is formedby blackish clays with scattered small size pebbles (2-3cm) composed of limestone, cherts and quartzites deriv-ing from the Pre-Neogene substratum. Decimetric levelsof peat and volcanoclastites are interlayered in the clays.These reach a maximum thickness of about 30 m. Thesecond lithotype consists of fine to coarse piroclasticdeposits. Bright grey ashes or layers with small bombsand pumices were observed. Sometime these depositsare well sorted and normally graded; pisolitic structures

GEOLOGY AND STRATIGRAPHY OF THE SOUTHERN ... 169Geologica Romana 37 (2003-2004), 165-173

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

were formed locally and testify to a subacqueous depo-sition, while in other places a sub-aerial depositionoccurred. The maximum thickness of these piroclasticdeposits is about 10 m. The third lithotype is represent-ed by pink-beige limestones, which might be massive orstratified, and sometime it is characterized by highporosity. The maximum thickness of the limestones isabout 40 m.

Macrofossils were recorded within the matrix of theblackish 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, Bithynialeachii thermalis, Planorbis planorbis, Radix sp.,Stagnicola sp., Valvata piscinalis. Only two species ofbivalves were recovered: Pisidium casertanum and P.personatum. Concerning the microfossils, the analyseshave furnished assemblages with oogonia of Characeaeand ostracods (Candona sp., Caspiocypris sp. Cyprideistorosa, Ilyocypris gibba, Lymnocythere sp.). Generally,the deposition of the unit occurred in a brackish marshenvironment. Locally, the sedimentological features ofthe pyroclastic deposits testify to sub-aerial conditions.The volcanic materials can be referred to the Lateraapparatus of the Magmatic Roman Province and to thetime interval 240 Ky - 320 Ky (Barberi et al., 1994).Therefore, the formazione di Botro le Stiacciole isMiddle 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 ofinterlayered sandstones and conglomerates (Fig. 3). Thesandstones are well cemented, coarse grained, calcareousand contains scarce matrix. The strata are a few centime-tres thick. The conglomerates are clast supported, well

cemented and with scarce sandy matrix. Pebbles verysmall (the average size is about 1 cm, max 5 cm) are wellrounded while the larger ones are flat. Some pebbles werebored by lithodomous organisms. The unit is a few metersthick and crops out at a maximum altitude of 15 m abovesea level.

Macrofossils are very rare, only remains of Cardiumand balanids were recovered. Benthic foraminifers(Ammonia beccarii tepida, Protoelphidium anglicum andAubignyna perlucida) and ostracods (Cyprideis gr.torosa) were recognized within the sandy facies. Fossilassemblages indicate a depositional environment withbrackish water. A Tyrrhenian age seems to be correct forthis formation because the height of the highest outcrops(15 m) is compatible with that of the Tyrrhenian marineterrace.

Alluvial deposits

These deposits, mainly composed by conglomerates,widely cover the western-central part of the consideredarea. They represent the depositional activity of theAlbegna and Osa rivers. The following four principalorders of terraces were recognized and distinguished inthe map legend: at1 (from 80 m to 115 m a.s.l.), at2 (from45 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 weregrouped in regional depositional units, following the def-inition and names of Bossio et al. (1998, pg. 514): “Aregional depositional unit is a sedimentary body boundedby two type of unconformity surfaces: 1) unconformitysurfaces everywhere present in the considered Apenninesector (US1); 2) unconformity surfaces detected only insome 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 addedto the Authors’s previous list. The firstone (Pleistocene II Unit - Q2), MiddlePleistocene in age, is still of uncertaindefinition. The second one(Pleistocene III Unit - Q3), LatePleistocene in age, corresponds to themarine Tyrrhenian terrace (15 ma.s.l.).

The depositional units which arepresent in the Albegna River Basin(Fig. 4) are indicated with their lithos-tratigraphic units as follows:Q3 - Pleistocene III Unit (LatePleistocene)

BOSSIO et al.170 Geologica Romana 37 (2003-2004), 165-173

Fig. 3 - Sabbie e conglomerati di Poggio delMolinaccio. Well stratified coarse sandstoneswith thin conglomerates. Type locality.

GEOLOGY AND STRATIGRAPHY OF THE SOUTHERN ... 171Geologica Romana 37 (2003-2004), 165-173

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

-Sabbie e conglomerati di Poggio del MolinaccioQ2 – Pleistocene II Unit (Middle Pleistocene)

-Formazione Botro le StiaccioleQ1 - Chiani-Tevere-Montescudaio Unit (Early Pleisto-cene)

-Conglomerati di Colle LupoP3 - Pliocene III Unit (Piacenzian)

-Formazione di Poggio SassineriP2 - 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 depositionalunits is reported in Bossio et al. (1998, 2000). Some con-siderations are here exposed in order to understand themeaning of the new Pleistocene units and the absence ofM2 unit in the Albegna River Basin.

1) - Pleistocene II Unit - Q2. It consists of continentaldeposits (marine sediments are present only along thecoast) unconformably resting on the previous units. Aswell 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-lacustrineUpper 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 area few meters thick and characterise only the coastalareas. They were formed about 124.000 years ago, dur-ing the highstand sea-level phase, corresponding to 5eoxygen isotopic stage.

3) - According to Bossio et al. (1995 and 2002), thestarting rising Middle Tuscany Ridge was not yet anobstacle for the connections between the basins locatedto its western and eastern side during the deposition ofthe Argille del T. Fosci. Therefore, the first Messinianmarine transgression, well recorded in the westernmostbasins, reaches also the eastern basins. This is showed inthe Albegna River Basin by the microfaunal content ofthe 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 theMiddle Tuscany Ridge, documented by the mentionedAuthors, implies the closure of the connections betweenAlbegna River Basin and western basins. Consequently,the Messinian marine formations (M2 Unit), which arelinked 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, thepost-collisional sedimentation started at the lateTortonian and occurred in a fresh lacustrine environ-ment. The strong similarity between the Tortonian litho-logic types of the Albegna and Ombrone areas allows tosuppose that these formed a single basin with an apen-ninic trend (see also Bossio et al., 1991, 2000 and Foresiet al., 1997). The presence of sandstones and conglom-erates in the eastern outcrops, suggests fluvial influxfrom east.

Outcrops of Miocene “epiligure” sandstones have notbeen recognised in the studied area. However, clastsderiving from the Arenaria di Manciano, Burdigalian -early Serravallian in age (Martini et al., 1995), occur inthe Tortonian conglomerates. The presence of sand-stones and conglomerates in the eastern outcrops, sug-gests fluvial influx from east; as a consequence, theArenaria di Manciano outcrops existed east of the basinand they covered the Pre-Neogene substratum ofCastellazzara Ridge.

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

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

The reopening of the Mediterranean-Atlantic Oceancommunication allowed a wide marine transgression inthe basin that, probably, had acquired an antiapenninicstructural feature. A generalised uplift occurred at the topof the Early Pliocene and the whole area emerged. Theemersion period was brief and encompassed the Early-Middle Pliocene boundary. A new sedimentary phasecharacterised the base of the Middle Pliocene (P2 Unit)but it is less extended than the previous one. During theMiddle Pliocene, climatic and tectonic factors caused theclosure of the marine communication. Thus, some areasof the basin were filled up and others changed from amarine sedimentary environment to a continental one(P3 Unit). During the Late Pliocene, a strong upliftoccurred in Tuscany and the Albegna area definitivelyemerged.

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

BOSSIO et al.172 Geologica Romana 37 (2003-2004), 165-173

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 volcanicapparatus. The last sedimentary event (Q3 Unit)occurred about 124,000 years ago and it is related to theTyrrhenian high stand.

ACKNOWLEDGEMENTS - The authors are especiallyindebted to the Professor Giuseppe Manganelli (SienaUniversity) and the Professor Raffaello Cioni (PisaUniversity). They have respectively studied the continentalgastropods assemblages and volcanic products of the PoggioSassineri Formation.

GEOLOGY AND STRATIGRAPHY OF THE SOUTHERN ... 173Geologica Romana 37 (2003-2004), 165-173

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Accettato per la stampa: Marzo 2004