Geologica Romana 37 (2003-2004), 165-173
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: [email protected]
** Dipartimento di Scienze della Terra, Università degli Studi di Siena, e-mail: [email protected]
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 antiapenninic 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 unconformably 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.
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 investigated 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 sediments, 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 environment (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 estensional 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 sedimentary 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 performed 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, following 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 formations are not described in the text, while they are distinguished in the geological map. The ages of these forma-
166
Geologica Romana 37 (2003-2004), 165-173
tions, as presented in the legend, are taken from the literature (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 consists mainly of massive and grey clays. These, are frequently 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 maximum 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. agrigentina, 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. socolovskiensis, 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).
BOSSIO et al.
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 recognized 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 conglomerates 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 lowermost 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 conglomerates with sub-rounded clasts (from a few centimetres 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 lithofacies 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 represented 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
GEOLOGY AND STRATIGRAPHY OF THE SOUTHERN ...
the marginal environment sometime emerged (sub-aerial 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 travertine 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 formation. 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 bulloides, Stilostomella spp.). Planktonic foraminifers
(common or abundant are Globigerina bulloides, G.
decoraperta, G. falconensis, Globigerinoides extremus,
G. obliquus, Neogoboquadrina acostaensis and
Geologica Romana 37 (2003-2004), 165-173
167
Orbulina universa; rare Globorotalia margaritae) and
calcareous nannofossils (common Calcidiscus macintyrei, 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 considered area where it unconformably overlies the late
Messinian formations and conformably rests on the
Sabbie di Podere Poggio Delci; it is also in heteropic relationship with these sands. The unit is composed of massive 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 pseudoungerianus, 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 paleoenvironment. The planktonic assemblages, mainly composed of Globigerina bulloides, G. falconensis,
Globigerinoides elongatus, G. extremus, Globorotalia
margaritae, G. puncticulata, Globoturborotalita apertura, G. decoraperta, Neogloboquadrina acostaensis,
Orbulina suturalis, O. universa and Turborotalita quinqueloba among the foraminifers and by Calcidiscus macintyrei, Coccolithus pelagicus, Discoaster asymmetricus,
D. brouweri, D. pentaradiatus, D. surculus, D. tamalis,
D. variabilis s.l., Gephyrocapsa spp., Helicosphaera carteri, H. sellii, Reticulofenestra pseudoumbilica, Pseudoemiliania lacunosa, Sphenolithus abies, Syracosphaera
histrica, Umbilicosphaera sibogae among the nannofossils, 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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Geologica Romana 37 (2003-2004), 165-173
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 formazione di Podere Melosella. The Calcareniti di
Magliano are mainly composed by well lithified medium-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 fossil 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 gibbosa, 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 (Globigerina falconensis, Globigerinita glutinata, Globigerinoides elongatus, G. extremus, Globorotalia bononiensis,
Globoturborotalita apertura and Neogloboquadrina
planispira were recorded among the others). The occurrence 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 conformably 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 heteropic 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 maximum thickness of about 30 m.
Remains of molluscs (Anomia, Cardium, Donax,
Flabellipecten, Ostrea and Dentalium), corals (Scleractinia), balanids and algae were observed. Benthic foraminifers (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, Semicytherura acuticostata and S. inversa) were recognised.
They indicate an inner shelf environment. An interesting
BOSSIO et al.
Fig. 1 - Calcareniti di Magliano. Interlayered bio-calcacrenites and clayey sands. South of Magliano.
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 brackish lagoon. Calcareous plankton is very rare end mainly
composed of Globigerina bulloides, Globigerinoides
elongatus, G. extremus, G. quadrilobatus, Globorotalia
aemiliana, G. bononiensis, G. crassaformis, Globoturborotalita apertura, Neogloboquadrina planispira
and Turborotalita quinqueloba among the foraminifers,
Calcidiscus macintyrei, Coccolithus pelagicus, Crenalithus 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 studied 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 frequent 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 additionally found. The maximum thickness of the formation
is about 45 m.
Undeterminable bivalves and specimens of Ostrea
GEOLOGY AND STRATIGRAPHY OF THE SOUTHERN ...
were observed. The samples collected within the sandstone have furnished benthic foraminifer assemblages
(with Ammonia beccarii, Aubignyna perlucida, Cribroelphidium decipiens, Florilus boueanus, and Protelphidium granosum) only. These assemblages indicate a
coastal marine environment.
A Piacenzian age of the unit is inferred for its heteropic 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 second is characterised by dark grey clays and marls, with a
variable amount of sands and with thin layers of conglomerates. 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 gastropods are present in some levels of the limestone.
Several shells of Paludina, opercula of Bithynia, of
Characeae and common ostracods (Cyprioogoniadeis
Geologica Romana 37 (2003-2004), 165-173
169
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 environment with shallow water. The calcare a Planorbis
represents a deposit of a low energy zone, without detrital influx, where the precipitation of the carbonate
occurred; the Argille di Valloria indicates a more proximal 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 formations. 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 subrounded 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, probably 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 overlies 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)
Fig. 2 - Formazione di Poggio Sassineri. Well stratified and laminated deposits of the calcare a Planorbis lithofacies. Outcrop of Case
Mercatale.
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 represented 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 deriving 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
170
Geologica Romana 37 (2003-2004), 165-173
were formed locally and testify to a subacqueous deposition, while in other places a sub-aerial deposition
occurred. The maximum thickness of these piroclastic
deposits is about 10 m. The third lithotype is represented 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 subrimata, and some specimens of Helicidae; terrestrial igrophilous gastropods: Carychium minimum, C. tridentatum 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 investigated area where it unconformably rests on the formazione 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 centimetres thick. The conglomerates are clast supported, well
BOSSIO et al.
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 definition 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 recognizable on the basis of relevant and abrupt sedimentological 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 lithostratigraphic 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.
GEOLOGY AND STRATIGRAPHY OF THE SOUTHERN ...
Geologica Romana 37 (2003-2004), 165-173
Fig. 4 - Depositional units and hyatus of the Neoautoctonous Succession of the Albegna River Basin.
171
172
Geologica Romana 37 (2003-2004), 165-173
-Sabbie e conglomerati di Poggio del Molinaccio
Q2 – Pleistocene II Unit (Middle Pleistocene)
-Formazione Botro le Stiacciole
Q1 - Chiani-Tevere-Montescudaio Unit (Early Pleistocene)
-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 considerations 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 characterises several Tuscany basins (Bossio et al., 1993). A significant 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 fluvial 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, during 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 testifies to a gradual transition from a lacustrine to a brackish-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
BOSSIO et al.
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 environment. The strong similarity between the Tortonian lithologic types of the Albegna and Ombrone areas allows to
suppose that these formed a single basin with an apenninic trend (see also Bossio et al., 1991, 2000 and Foresi
et al., 1997). The presence of sandstones and conglomerates 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 sandstones and conglomerates in the eastern outcrops, suggests 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 ostracod 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, sedimentation 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 EarlyMiddle 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 Pleistocene. Two sedimentary events (Q1 and Q2 units) separated by a positive tectonic input were recorded. Both
GEOLOGY AND STRATIGRAPHY OF THE SOUTHERN ...
Q1 (Early Pleistocene) and Q2 (Middle Pleistocene)
were deposited in a continental fluvio-lacustrine environment and the sedimentation of the Q2 Unit is certainly 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.
Geologica Romana 37 (2003-2004), 165-173
173
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.
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Accettato per la stampa: Marzo 2004