Tectono-sedimentary analysis of a complex, extensional, Neogene basin formed on thrust-faulted, Northern Apennines hinterland: Radicofani Basin, Italy

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Sedimentary Geology 1

Tectono-sedimentary analysis of a complex, extensional, Neogene

basin formed on thrust-faulted, Northern Apennines hinterland:

Radicofani Basin, Italy

Vincenzo Pascucci a,*, Armando Costantini b, I. Peter Martini c, Riccardo Dringoli b

a Istituto di Scienze Geologico-Mineralogiche, Universita di Sassari, Corso Angioy 10, 07100 Sassari, Italyb Dipartimento di Scienze della Terra, Universita di Siena, Via Laterina 8, 53100 Siena, Italy

c Department of Land Resource Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1

Received 14 July 2004; received in revised form 8 September 2005; accepted 13 September 2005

Abstract

Large NW–SE oriented, Neogene–Quaternary structural depressions, up to about 200 km long and 25 km wide, have developed

on the western side (hinterland) of the Northern Apennines over thrust substrate. The depressions are now, for the most part,

laterally bounded by normal faults and are longitudinally separated into basins by transfer zones. A debate exists in the literature as

to whether these basins have developed as half-graben under a predominantly extensional regime since late Miocene, or as thrust-

top basins under a predominantly compressional regime that has continued until the Pleistocene. The Radicofani Basin is one of the

best-preserved basins. It developed mainly during the late Miocene–Early Pliocene in the southern half of the Siena–Radicofani

structural depression, and is now bounded on the east by normal faults that transect a thrust anticline bnoseb in the substrate, to the

north by a substrate high or transfer zone, and to the south and west by Quaternary igneous/volcanic edifices. The basin

experienced variable differential tectonic and associated sedimentation along linking, normal boundary faults. Along its eastern

margin it shows the development of thick (~600 m) alluvial fans that developed in relay areas between boundary faults and

transverse faults and transfer zones. Well-exposed sections generally show upward transitions from conglomeratic alluvial fans, to

shoreface sandstone, to offshore mudstones. Locally, the transition is marked by deltas primarily characterised by thick gravelly,

sandy, stacked cross-sets The thicker, sandy-gravel to gravelly-sand cross-sets (5–8 m thick) are interpreted as Gilbert-type deltas;

interstratified thinner (0.5–1 m thick), generally openwork gravelly strata are part of delta topset assemblages and probably

represent prograding fluvial bars. Tectonic movements provided the accommodation space for the total, ~2700 m thick basin fill.

Sea level fluctuations that led to the repeated development of the cross-sets may also have been influenced by climatic or eustatic

changes, possibly related to the effects of early Antarctic glaciations.

Some features of the Radicofani Basin can be found in both extensional and compressional basins. However, the position of the

basin in the mountain chain and the development of alluvial fans, fandeltas and associated deposits along the main boundary fault,

combined with structural evidence from seismic lines, show that during the early Pliocene this basin best conforms to existing

models of half-graben.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Gilbert-type delta; Neogene; Seismic stratigraphy; Extension; Northern Apennines; Normal fault linkage; Half-graben; Thrust-top basin

0037-0738/$ - s

doi:10.1016/j.se

* Correspondi

E-mail addr

83 (2006) 71–97

ee front matter D 2005 Elsevier B.V. All rights reserved.

dgeo.2005.09.009

ng author. Tel.: +39 0792006627; fax: +39 079231250.

ess: [email protected] (V. Pascucci).

V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–9772

1. Introduction

The goal of the study is to understand the formative

processes of Neogene–Quaternary basins of the western

(inner; hinterland) flank of the Northern Apennines

(Tuscany region, Italy) utilizing field and seismic in-

formation and detailed sedimentary facies analysis.

Specific objectives of this paper are to examine the

Radicofani Basin, one of the largest, well developed,

best-preserved basins (RA, Fig. 1), reconstruct its tec-

tono-sedimentary evolution, and establish whether the

architecture of its sedimentary fill can be considered

unique to extensional basins (such as half-graben; Lee-

der and Gawthorpe, 1987; Gawthorpe and Leeder,

2000) or whether it could also develop in compressive

depo-centres such as thrust-top, piggyback, satellite or

perched basins (Ori and Friend, 1984; Ricci Lucchi,

1986; Butler and Grasso, 1993). In this endeavour, the

type of sedimentation along the basin margins and the

localization and anatomies of major alluvial fans are of

particular interest.

This study has benefited from published and unpub-

lished information and ideas gathered over a century,

from our own field mapping and field-based facies

analyses, and from the interpretation of recently re-

leased industrial seismic profiles. First, the paper will

present a brief review of the geology of the Northern

Apennines and of the various hypotheses on the origin

of the hinterland basins. This will be followed by the

description of the sediments and structures of the Radi-

cofani Basin as observed in the field and in seismic

profiles. Finally, an analysis will be made of the possi-

ble tectonic and climatic significance of characteristic

sequences observed.

2. Geological setting

The Northern Apennines are characterised by an

active orogenic thrust wedge that has been moving

eastward since the late Oligocene leaving in its inner

part, Tuscany, a trail of NW–SE oriented, normal-fault

bounded basins (Fig. 1; Martini and Sagri, 1993, 1994;

Patacca et al., 1993; Sagri et al., 2004). These basins

become progressively younger from the western Tyr-

rhenian Sea shelf toward the eastern active orogenic

thrust wedge (Bartole, 1995; Pascucci et al., 1999). A

few embryonic, normal-fault bounded basin are deve-

loping now on the active orogenic thrust wedge itself

east of the divide, and many disastrous earthquakes are

tied to movements along their normal faults (Barchi et

al., 2001). The tectonic edifice is crossed diagonally by

morphostratigrapic lineaments that have been variously

interpreted as transcurrent faults, strike-slip faults, la-

teral ramps of thrust faults, normal faults, and transfer

zones/faults. These features have been variously active

at different times generating transtensive and transpres-

sive conditions (Liotta, 1991; Pascucci et al., in press).

Furthermore, plutonism and volcanism has occurred. It

becomes younger eastward and southward indicating

that the hinterland of the northern Apennines is part of a

magmatic arc (Fig. 1; Boccaletti and Guazzone, 1974;

Boccaletti and Dainelli, 1982; Serri et al., 2001). The

arc is a failed one to the north of the Livorno–Sillaro

transverse lineament (ls on Fig. 1), where constraints

limited the eastward migration of the thrust orogenic

wedge. In contrast it is a fully developed volcanic arc

farther south where the constraints were minor. Geody-

namically, the hinterland of the Northern Apennines has

a thin (~20–25 km) crust related to a doming of the

asthenosphere; the orogenic thrust wedge has a thicker

crust (~35 Km) (Fig. 2).

The geology of the study area — western (inner)

side of the Northern Apennines in Tuscany — is pri-

marily characterised by long (up to 200 km) and rela-

tively narrow (up to 25 km in width), NW–SE oriented,

structural depressions (Fig. 1; Martini and Sagri, 1993;

Vai and Martini, 2001). These depressions are now

laterally bounded by normal faults and subdivided

longitudinally into basins by transverse highs related

to transfer zone/faults (Liotta, 1991). The structural

depressions/basins have developed on thrust faulted,

pre-Neogene substrate, and their Neogene deposits

have been locally deformed during the Plio–Pleistocene

uplift (up to 900 m; Bartolini et al., 1983). This was in

part associated with emplacement of granitic plutons to

shallow depth such as at Larderello, and with intrusive

and volcanic edifices such as Mt. Amiata and Radico-

fani (Figs. 1 and 3; Franceschini, 1994, 1988; Baldi et

al., 1994; Acocella et al., 2002).

The close relationship of the basins to both thrust

and normal faults has led to the formulation of various

genetic hypotheses.

a. One of the early hypotheses is the bcuneo compostoQ(composite wedge) of Migliorini (1949), whereby

thrusted anticlinal bnosesQ in the substrate were dis-

sected by normal faults both in the rear and some-

time also in their frontal part. Due to differential

tectonic movements, the uplifted nose-wedges

would have formed NW–SE oriented ridges that

still separate major tectonic depressions. The Neo-

gene–Quaternary basins formed within these struc-

tural depressions, being separated longitudinally by

transverse highs.

Fig. 1. Schematic structural map of the Northern Apennines with location of detailed study areas in the Radicofani Basin (RA). (The two squares

indicate: Pr: Pietraporciana; C: Celle sul Rigo; r: Radicofani volcanic neck; CH: Val di Chiana Basin; SI: Siena Basin, VO: Volterra Basin; ls:

Livorno–Sillaro lineament; MTR: Middle Tuscany Ridge; 1.3 etc: age of magma in Ma).

V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–97 73


b. A modern spin-off of the composite wedge hypo-

thesis considers the basins to have developed during

a continuous or punctuated compressional regime

(Bernini et al., 1990; Boccaletti et al., 1997; Bocca-

letti and Sani, 1998; Bonini et al., 1999; Finetti et al.,

2001; Bonini and Sani, 2002). According to this

Fig. 3. Panoramic views of the Radicofani Basin. a. Western flank of the Cetona Ridge with steep scarps (arrows). b. Panoramic view showing the

Cetona Ridge in the background, the volcanic neck (1.3 Ma) of Radicofani in the middle ground, and the lower Pliocene clay succession (P1) in the

foreground.


hypothesis pre-Pleistocene normal faults would ge-

nerally be secondary accommodation features due to

pressure release in the thrust blocks (Fig. 2a). The

thrust-top basins are believed to have developed

through an interplay of substrate thrust and back-

thrust movements. The normal faults that now bound

the basins would have primarily formed during the

Pleistocene–Recent.

c. A more conservative, long-held hypothesis suggests

that compressive conditions existed until the late

Miocene (Tortonian) when the substrate thrust

blocks were emplaced in their present position. As

the compressive tectonic front migrated eastward,

Fig. 2. Geodynamic models for the evolution of the North Apennines. a. 3

Quaternary (8–1 Ma) (after Finetti et al., 2001). Thrust-top basins would ha

interplay of substrate thrust and back-thrust movements. The normal faults t

Recent. b. Coexistence of compressive and extensional conditions with narro

the orogenic thrust wedge (after Boccaletti and Sani, 1998). c. Emplacement o

of the Northern Apennines and thrust imbrications on the Adriatic side (late

Carmignani et al., 1994, 1995, 2001). Subsequent narrow rift extension w

(OW=Orogenic Wedge; FB=Foreland Basin).

starting in late Tortonian, the area west of the Nor-

thern Apennines divide was subjected to relatively

limited extension, and normal-fault bounded, rela-

tively narrow basins (graben to half-graben) deve-

loped (Fig. 2b; Giannini et al., 1971; Elter et al.,

1975; Bartolini et al., 1983; Bossio et al., 1993,

Martini and Sagri, 1993, 1994; Martini et al., 2001).

d. An extreme variant of the extension hypothesis

accepts the formation of the narrow, extensional,

Neogene–Quaternary basins as indicated above.

However, it also envisages a prior period of wide-

spread extension during the late Oligocene–middle

Miocene whereby much of the Northern Apennines

-D reconstruction of the North Apennines during the late Miocene–

ve developed in the hinterland and active orogenic wedge through an

hat now bound the basins would have formed during the Pleistocene–

w rift basins in the hinterland and piggyback and thrust-top basins on

f the Apuane core complex with wide extension in the Tyrrhenian side

Burdigalian–early Tortonian) (after Carmignani and Kligfield, 1990;

ould have occurred in the hinterland from late Tortonian to Present


mass slid eastward due to emplacement of core

complexes such as the Apuane (Figs. 1 and 2c);

Carmignani and Kligfield, 1990; Carmignani et al.,

1994, 1995, 2001). This would have caused wide

extension on the Tyrrhenian side of the Northern

Apennines and thrust imbrications on the Adriatic

side. The wide extensional basins are not now recog-

nised except as some remnant upper Burdigalian–

lower Tortonian deposits, preserved in a few of the

narrow basins that were dissected during the subse-

quent narrow-extension events.

3. Radicofani basin—sedimentary fill

The Radicofani Basin began forming during the mid-

dle Miocene, and a thick sedimentary pile accumulated

mainly during the early Pliocene (Signorini, 1966; Iac-

carino et al., 1994; Bossio et al., 1993; Pascucci, 2004).

The basin emerged toward the end of early Pliocene

(Fig. 4), and only its easternmost border area was af-

fected locally by a short-lived, middle Pliocene trans-

gression. Afterwards a general uplift occurred and no

younger sedimentary record is present. Magmatism has

affected the southern part of the basin since early Plio-

cene and volcanic eruptions occurred during the Pleis-

tocene (Fig. 1; Franceschini, 1998; Conticelli, 2004).

Pre-Neogene substrate rocks are composed primarily

of two superimposed thrust units: Tuscan unit and

Ligurides (Passerini, 1965; Costantini et al., 1977).

The lower Tuscan unit ranges in age from Triassic to

Oligocene, and the exposed rocks consist primarily of

shelf carbonates and minor, turbiditic, poorly cemented

turbiditic sandstones (Macigno) (Costantini et al., 1977,

1993). The Ligurides range in age from Cretaceous to

Eocene, and consist primarily of basinal siliceous lime-

stone and argillaceous limestone (marlstone). The

Macigno was the primary source of sand for the Neo-

gene basin fill. The other substrate rocks contributed

mostly pebbles and cobbles of limestone and metamor-

phic detritus, as well as some sand.

The overall stratigraphic architecture of the Neogene

basin fill can be determined from recently released

industrial seismic profiles calibrated by few stratigra-

phic wells (Liotta, 1996; Bonini and Sani, 2002). De-

tailed information can be obtained from locally well-

exposed type areas such as Celle sul Rigo to the SE (C

in Figs. 1 and 5) and Pietraporciana to the NE (Pr in

Figs. 1 and 6a).

The most complete Neogene succession, about 2700

m thick (Figs. 5 and 7), occurs in the Celle sul Rigo

area. The seismic profiles crossing the area show strong

reflectors at depths of about 0.5–0.7 s TWT, which

represent a major unconformity (C on Fig. 7). The

unconformity divides the Neogene sedimentary fill

into two major parts that display somewhat different

seismic responses and structures. The lower part of the

sedimentary fill consists of two seismically well--

defined sequences Seq 1 and Seq 2; the upper part

consists of the sedimentologically and structurally com-

plex Seq 3 (Fig. 4).

3.1. Seq 1

This sequence is present only in the central southern

part of the basin. It is characterised by horizontal and

discontinuous short wavy reflectors (Fig. 7). It rests

unconformably on bedrock. It is not exposed anywhere

in the basin, but it was penetrated by the S4 well (Fig.

5). It comprises middle Miocene (Langhian) shallow

marine deposits (Liotta, 1996; Foresi et al., 1997).

3.2. Seq 2

This sequence is best developed in the southern part

of the basin. In seismic profile 12 it is bounded by two

well-defined unconformities (B, C; Fig. 7). Laterally it

is delimited by a normal fault to the east. To the west,

reflectors onlap unconformity B and bedrock. Seq 2

shows various seismic characteristics with local strong

continuous, sub-horizontal reflectors (mainly at the bot-

tom), possibly representing sandy and gravel layers,

alternating with, and laterally equivalent to, weak dis-

continuous reflectors, possibly representing clay-rich

layers. This sequence does not outcrop in the basin

but it has been drilled by well S3 (Fig. 5; Barberi et

al., 1994; Pascucci et al., in press) and correlated with

the uppermost Miocene (Tortonian–early Messinian),

lacustrine to alluvial deposits (lignite-bearing sequence

locally called bSerie lignitiferaQ). However, in contrast

to Neogene basins developed west of the MTR ridge,

like Volterra (VO on Fig. 1), there is no evidence in the

Radicofani Basin of upper Messinian evaporite beds

that represent the regional Mediterranean bsalinitycrisisQ (Cita, 1982).

3.3. Seq 3

This lower Pliocene sequence constitutes most of the

basin fill (about 1.8 s TWT). It has variable seismic and

lithological characteristics reflecting sedimentation pat-

terns that changed through time in different parts of the

basin.

In the southern part of the basin, on seismic profile

12 (Fig. 7) Seq 3 shows variable seismic response and

Fig. 4. Stratigraphic scheme of the various units recognised in the Radicofani Basin. Subsurface sequences (1 2 and 3) have been defined on the

basis of seismic and well data. The outcropping units are dated according planktonic foraminifera (Bossio et al., 1993), and correlated with the

biostratigraphic scheme for the Mediterranean Deep Sea proposed by Cita (1975) for the planktonic foraminifera, and with the scheme by Rio et al.

(1990) for the calcareous nannofossils distribution in the western Mediterranean.


Fig. 5. Geological map of the central-southern part of the Radicofani Basin (after Liotta, 1996) (m: Globorotalia margaritae; pm: G. margaritae/

punticulata; p: G. puncticulata, recognised in measured sections).


is vertically subdivided into two parts by strong, gently

concave-downward, quasi continuous reflectors at

depths ~0.2–0.4 TWT, and by apparent angular discor-

dant relationships and/or facies changes. In the eastern

part of the profile, the overall seismic response is

characterised by well-marked, continuous, eastward

dipping reflectors, possibly associated with sandstones

and conglomerates interbedded with clays. These are

overlain by a zone of short reflectors (mostly clays)

with few sub-horizontal, strong wavy reflectors (con-

glomeratic beds; shotpoints ~830–900, depth ~�0.2).

In the central part of the basin (shotpoints ~410–520;

Fig. 7), the lower part has discontinuous short reflec-

tors, and the upper part has relatively well-defined, sub-

parallel, discontinuous reflectors. West of shotpoint

400, the superposition of the sedimentary facies

reverses, with the well-marked quasi-continuous reflec-

tors in the lower part and the short, locally chaotic ones

in the upper part. The well-marked reflectors can be

correlated with conglomerates found at the base of

Pliocene strata in the wells S4 and S3 (Liotta, 1996).

These conglomerates have been referred to the basal

Sphaeroidinellopsis seminulina s.l. Zone of the early

Pliocene (S. Merlini — AGIP, personal communica-

tion). The chaotic pattern can be correlated with the

olistostromes that characterise the western side of the

basin (P2, Fig. 5).

A similar but simpler response is seen to the north

where Seq 3 is thinner (about 1.0 s TWT; Fig. 8), as

illustrated on seismic profile 10. There it shows a quasi-

transparent seismic facies (clay) with some discontinu-

ous, locally chaotic reflectors (possibly sand) in the

western part of the basin (west of shotpoint 400). To

the east, well marked, quasi-continuous, gently inclined

reflectors (clay sandstone interbeds) are stratigraphi-

cally overlain by and somewhat chaotic, short reflectors

Fig. 6. Geological map and cross-section of the main study area near Pietraporciana in the northern part of the basin (after Costantini and Dringoli,

2003).


(clays). Note that the inclination of the reflectors

decreases toward the top and east from shotpoint 600

to 800 where reflectors become quasi-horizontal. A

lenticular fan-like body, with westward-inclined discon-

tinuous and poorly defined short reflectors (conglo-

merate), is identifiable at depth (0.3–0.7 TWT) in the

eastern part of the Radicofani Basin between shotpoints

~700 and 850 (Fig. 8).

In the field, Seq 3 can be further subdivided litho-

logically and paleontologically into various units (P1

P2 and P3) in the south, and their equivalents the

Lucciola bella and La Foce units in the north (Figs.

4–6 and 9; Liotta, 1996; Dringoli, 1996; Costantini and

Dringoli, 2003). We present first those units recognised

in the southern part of the basin, starting from those that

characterise the western flank (P2) of the basin, and the

outermost part of the eastern flank (P3). Then we

analyze the deposits of the central part that form the

bulk of the basin fill (P1). Finally, the units along the

eastern flank in the Pietraporciana area to the north are

analyzed in details because they provide important

insight about the evolution of the basin.

3.3.1. P2

The deposits of the western flank consist of lower

Pliocene, Globorotalia margaritae bearing, slightly

cemented, massive, gray clays with lenses of chaotic,

polymictic, poorly sorted clay, sandy and gravelly depo-

sits with clasts ranging in size from pebble to boulders

(Figs. 5 and 9). The clasts are predominantly of carbo-

Fig. 7. Seismic profile 12 across the central-south part of the Radicofani Basin (see Fig. 5 for location). a. Original data. b. Interpreted line drawing.

A, B, C: major unconformities recognisable in seismic profiles.


nates, derived from various units of the Ligurides. The

lenticular bodies are olistostromes emplaced as slumps

derived from the Ligurides substrate, sliding into marine

offshore clays. The olistostromes are evidence of unroo-

fing of the uplifting western margin of the basin. This

uplift may be partly related to early plutonic intrusions

that later led to the Pleistocene volcano of Mt. Amiata.

3.3.2. P3

This unit consists of sandstones and conglomerates

with calcareous cement and clasts bored by lithodoms

(Lithophaga), as well as carbonates (Amphistegina

limestone). It occurs in isolated areas and rests uncon-

formably on substrate units of the Cetona Ridge to the

east of the main boundary fault of the Radicofani Basin

(Figs. 5 and 9; Liotta and Salvatorini, 1994; Liotta,

1996). The unit, which contains the foraminifera Glo-

borotalia puncticulata and G. aemiliana is considered

to be middle Pliocene in age (Figs. 4 and 9). It is not

recognised on seismic profiles, because too thin, too

close to the surface, and too patchy.

The unit was deposited in a shallow, warm sea, with

the calcareous materials reworked by waves, locally

forming subaqueous bars. These deposits do not occur

in other parts of the Radicofani Basin. They indicate

local marine inundation from the south and southeast,

which flooded the uppermost and major lower-Pliocene

unconformity that extends throughout most of western

Tuscany (Boccaletti and Sani, 1998; Bossio et al.,

1998).

3.3.3. P1

This unit is exposed in central part of the basin and

is characterised primarily by massive, weakly cemen-

ted, marine clays to silty clays locally interbedded with

sandstone and conglomerate. It contains lower Pliocene

foraminifera, and, in the Celle sul Rigo area, it becomes

younger from the west to the east as the measured

sections contain successively G. margaritae (section

m on Fig. 5), G. punticulata/margaritae (section pm

on Fig. 5), and finally, to the east, G. puncticulata

(section p on Fig. 5) (Liotta, 1996).

In the Celle sul Rigo area, P1 can be further sub-

divided on the basis of the various coarse clastic inter-

beds (P1a P1b and P1c; Liotta and Salvatorini, 1994;

Liotta, 1996).

Fig. 8. Seismic profile 10 crossing the northern study area, the Cetona Ridge, and the Chiana Basin (see Fig. 6 for location). a. Original data. b.

Interpreted line drawing (the opposed arrows in (a) indicate the Cetona Ridge substrate thrust fault transected by the normal faults now delimiting

the Neogene basins). A, B, C: major unconformities recognisable in seismic profiles.


3.3.3.1. P1a. Thick (up to 25 m) lenses of sandy

conglomerate occur interstratified within the fossili-

ferous, marine clays toward the centre of the basin

(Figs. 5 and 9). The conglomerates are poorly sorted

with granules to small cobbles composed primarily of

carbonate, calcareous sandstone, and minor ophiolites

and other materials derived from the Ligurides. Some of

the calcareous clasts were perforated by lithofages. The

larger, flatter clasts show preferred imbrication indica-

ting a northward paleoflow (Fig. 10a). These conglo-

merates are interpreted to have been emplaced into

offshore clays by subaqueous gravity flow.

3.3.3.2. P1b. Well-stratified, locally graded sandstone

beds up to 50 m thick are present toward the centre of

the basin, within the fossiliferous marine clays (Figs. 5

9 and 10b). The bases of these beds exhibit interstrat-

ification of clays, but their upper contacts are sharp.

Where well exposed, the sandstone beds show flute

casts. Within the thick sandy intervals, the sandstone

layers can be separated one form the others by clay

laminae or thin layers with fine-grained clay chips.

Locally channels filled with conglomerate occur within

the sandstone units (Fig. 10c). The conglomerates have

clasts derived form the Tuscan unit exposed on the

Cetona Ridge to the east. Some clasts were perforated

by lithofages. These turbidite-like sandstones are inter-

preted as deposits of westward-directed hyperpycnal

flows, whereas the conglomerates were emplaced off-

shore by sediment gravity flows.

3.3.3.3. P1c. Toward the eastern margin of the basin,

local deposits of poorly sorted, calcareous pebble

breccia occur within marine clay containing neritic

microfaunal assemblages (Figs. 5 and 9; Liotta,

1996). The clasts are angular and composed of mate-

rials derived from Tuscan units outcropping on the

Cetona Ridge. Some of the clasts were perforated by

lithofages. These breccias are interpreted as rockfall

from coastal scarps that were rapid reworked into an

offshore setting.

In the Pietraporciana area the central basin unit P1 is

subdivided in two main sub-units locally called Luc-

ciola bella and La Foce (Figs. 4 and 6a).

3.3.4. The Lucciola bella

This unit is coeval with and environmentally equiv-

alent to the P1 unit of the Celle sul Rigo (Fig. 4).

However, in the Pietraporciana area, this unit consists

mostly of poorly-cemented, grey clays with few thick

Fig. 9. Stratigraphic units of the Radicofani Basin. (P1–P3) (P1: clay-predominant unit with interlayers of a: sandy conglomerate, b: sandstone, c:

calcareous breccia; ol=olistotromes; t=pre-Neogene substrate) (after Liotta and Salvatorini, 1994; Liotta, 1996).


(up to 5 m) interbeds of resedimented sandstone rich in

clay clasts and disseminated shallow water bivalves.

Locally, towards the top, thick interbeds (up to 20 m)

of resedimented, granule conglomerate occur.

3.3.5. La Foce

The La Foce unit consists of a large, coarse-

grained conglomeratic to sandy wedge developed

along the northeastern flank of the basin. Part of it

is reasonably well exposed, and part (about 1 km long

and ~500 m high) has been downfaulted and is repre-

sented by the lenticular body recognised at depth

along seismic profile 10 (Fig. 8). This coarse-grained

sedimentary wedge is interpreted as an alluvial fan/

delta system. It is laterally equivalent to part of the

Lucciola bella/P1 units, but has no equivalent in the

Celle sul Rigo area (Fig. 4). Its characteristics provide

valuable information on the tectono-sedimentary pro-

cesses along the eastern margin of the basin, so detailed

analysis is warranted.

La Foce unit can be subdivided into three parts: a

lower conglomeratic alluvial fan; a middle gravelly

delta deposit, only locally developed; and upper shal-

low marine sandstones (Fig. 11).

3.3.5.1. Lower alluvial fan. The lower alluvial fan is

composed of conglomerate and sandstone interlayers.

The conglomerates are clast-supported, poorly orga-

nized, coarse-grained, matrix-rich and generally moder-

ately to poorly sorted, composed of pebbles to cobbles,

with a few boulders up to 70 cm in diameter. In the

lowermost part, they contain a few argillaceous sand-

stone lenses with broken ostracod shells, indicating a

freshwater environment. Higher in the succession, some

reworked clasts are perforated by Lithophaga. Con-

glomerate beds vary in thickness from 1 to 4 m. The

sandstones are coarse-grained, massive to horizontally

laminated, forming beds from 0.30 to 1 m thick. They

do not contain fossils, except in locally occurring mud

pebbles with Ammonia beccari, indicating a brackish

environment. Going northwestwards and up-section, the

conglomerate layers alternate with progressively thicker

sandstone bodies eventually grading, by interlaying into

marine shoreface sandstones (Figs. 6 and 11).

Fig. 11. Composite stratigraphic section of La Foce unit.

Fig. 10. Deposits of the P1 unit in the Celle sul Rigo area. a. Thick

conglomerate lens in clay showing a-axis parallel imbrication.

b. Gradual transition between mudstone and turbidite sandstone.

c. Conglomeratic channel fills within turbidite sandstone layers.


3.3.5.2. Middle delta body. This 115 m thick delta

unit forms the transition between continental parts of

the lower alluvial fan and the upper marine sandstones

(Fig. 6a, near the locality Ca al Vento). Some large

nested channels, a few metres deep and tens of metres

wide, are visible in SW–NE oriented exposures, per-

pendicular to paleocurrent. In large outcrops cut parallel

to paleocurrent, the delta unit is characterised primarily

by stacked, multiple, medium- to large-scale cross-sets

predominantly conglomeratic foreset beds, (facies A).

This facies alternates with massive conglomerates (fa-

cies B) and massive to laminated sandstones (facies C),

and, toward the top, minor siltstones–clays (facies D)

(Figs. 12 and 13).

Facies A (conglomeratic cross-sets) is the most cha-

racteristic of this stratigraphic interval. Some beds are

openwork conglomerates with 0–5% sandy matrix; most

consist of sandy conglomerates to coarse-grained, con-

glomeratic sandstones (Fig. 13). The openwork con-

glomerates have an average coarse clast size ranging

between 2 and 20 cm, with occasional clasts up to 30 cm

in diameter. The foresets generally have sharp, angular

contacts with the underlying beds. The sandy conglo-

merates generally have 1 to 4 cm clasts, with a few up to

15 cm in diameter. These foresets have angle-of-repose

slopes, generally becoming asymptotic at the base. The

beds generally range from 0.5 to about 5 m in thickness

(Figs. 12–14), to a maximum of ~8 m in the lowermost

part of the unit. Reactivation surfaces (R in Fig. 14)

present in some beds, are characterised by single, len-

sing foresets composed of fairly well sorted, clast-free,

medium- to coarse-grained sandstone. The foresets with

open-framework conglomerates become thinner and

lens out in the down-paleocurrent direction.

Facies B (massive conglomerates) is characterised

by conglomerates with highly variable clast size (from

1 to 40 cm in diameter) and matrix content, occurring in

layers 0.40 to 1 m thick (Fig. 13). Several types are

present. (a) Moderately to poorly sorted conglomerates

with subrounded and subspherical clasts and abundant

sandy matrix (varying between 5–20%) occur in lenti-

cular beds. These generally lie on flat erosion surfaces,

or occasionally occur in cuts-and-fills ranging from

small (50 cm in width and 25 cm in depth) to large

Fig. 12. Upper part of the foreset-bedded part of the La Foce unit. a. Diagram with locations and the major layers (1–10) and of the vertical

section shown in Fig. 13; note basinwards (westwards) lensing out of openwork gravel foreset beds. b. Photograph of the upper part of the deltaic

body.


size (5 m in width and 1 m in depth). (b) Cobble

conglomerates with matrix composed of small pebbles

and coarse-grained sandstone, or, locally, isolated

boulders (discontinuous boulder pavement) rest on

flat erosional surfaces. The upper surfaces of the large

clasts frequently show borings by sponges and a few by

Lithophaga. (c) Near the top of the foreset beds, cob-

bles are partly encrusted with marine barnacles (Fig.

15a) and oysters indicating marine influence. Near the

top, thin conglomerate to conglomeratic sandstone

layers show concentrations of well imbricated flat peb-

bles in the upper part of beds.

Facies C (sandstone) is characterised by coarse- to

very coarse-grained, massive to laminated sandstone to

sandy granule conglomerates in beds 60 to 120 cm thick.

In some layers there are dispersed lignite fragments.

Some layers have disseminated clasts (pebbles and cob-

bles), some perforated by sponges and Lithophaga, and,

in the upper part of the succession, showing encrustation

by oysters, some still in living position (Fig. 15b). Other

beds show bioturbation (Fig. 13).

Facies D consists of clay (laminated siltstone to

mudstone) occurring in lensing beds up to 40 cm

thick, frequently with fine lignite fragments (Fig. 13).

Fig. 13. Sedimentological log of the uppermost 30 m of the foreset-bedded body of the La Foce unit (see Fig. 12 for location).


These three facies are vertically stacked in various

ways. The most common, characteristic succession is

composed of basal sandstone overlain by sandy con-

glomerate cross-sets, capped by massive conglomerates

resting on an erosion surface (Fig. 13). Locally, the

massive conglomerate facies (facies B) overlying the

foreset-bedded facies (facies A) is represented by a dis-

continuous boulder pavement.

Fig. 14. Photograph of the uppermost part of the deltaic succession in

the La Foce unit showing foreset-bedded (facies A) and non-foreset-

bedded (facies B) conglomerates, alternating with sandstone (facies

C) (R=sandy reactivation surfaces).


The thicker cross-sets of this unit are interpreted as

Gilbert-type deltas (Gilbert, 1890), where the basal

sandstones (facies C) represent the bottomsets of

steep foresets (facies A) of distributary mouth bars.

Massive conglomerates (facies B), or equivalent dis-

continuous boulder pavements, represent the topsets

(Figs. 12 and 13). It is possible that some of the thinner

foreset beds devoid of fossils could represent fluvial

transverse bars prograding over the thicker, Gilbert-type

deltas (see Appendix A for more extensive analysis).

3.3.5.3. Upper marine sandstones. The upper marine

sandstones of the La Foce unit are fossiliferous, bio-

turbated (Skolithos assemblage) and poorly cemented

(Fig. 11). They are interpreted to have formed in shore-

face to transitional setting, gradually passing basinward

to the offshore clay (mudstone) of the Lucciola Bella

unit.

4. Radicofani basin—tectonic structures

The structures of the basins are reasonably well

defined by surface mapping and seismic reflection

profiles.

In the field and seismic profiles, the western margin

is characterised mostly by onlap of the Neogene depoits

onto the substrate rocks, although local, eastward-dip-

ping normal faults occur as well (Figs. 7, 8 and 16).

In the field, the present-day main eastern boundary is

delimited by westward-dipping normal faults, although

thin Pliocene deposits locally extend beyond the boun-

dary and occur as isolated patches resting unconfor-

mably on the substrate rocks (Figs. 5 and 6). Borings by

marine lithodoms occur on some fault planes (A. Laz-

zarotto, personal communication) indicating their Plio-

cene age, the basin having begun its final uplift at the

end of the early Pliocene. Some faults, though, may

have been variously reactivated in post-Pliocene times.

In the Pietraporciana area to the northeast, mapping has

shown that the boundary normal fault splits into two or

more branches (Fig. 6). Furthermore, the eastern faulted

boundary has numerous indentations perhaps related to

diagonal linkage of propagating boundary normal faults

(Gupta et al., 1999) and/or to several orthogonal faults

with a strike-slip component (Fig. 5; Liotta, 1996;

Pascucci et al., in press). In the field a basin wide gentle

anticline has been mapped in the southern part of the

basin (Fig. 5; Liotta, 1996) and various small-scale

folds, reverse faults, and carbonate pebbles imprinted

with stylolitic pits have been observed in outcrops

(Bonini and Sani, 2002).

Furthermore, subsurface structural information is

provided by seismic profiles as follows.

a. On seismic profile 12, the sequences Seq 1 and Seq 2

and the lowermost part of Seq 3 are sharply delimited

by a westward dipping fault (Fig. 7). The bowl-

shaped basin fill of Seq 1 indicates a sedimentation

phase prior to the faulting (Pascucci, 2004; Pascucci

et al., 1999). Instead, the faulting has affected Seq 2

and part of Seq 3, indicating the start of the narrow rift

phase in late Miocene–early Pliocene. Concave-up

seismic reflectors, tangential to an irregular, possibly

faulted, substrate surface characterise the remainder

of Seq 3 to the east (Fig. 7, shotpoints 750–850).

Furthermore, the basin-wide, gentle folding of the

basin is shown to affect Seq 3 (Figs. 5 and 7).

Liotta (1996) interpreted these structures as formed

by progressive rotation of strata (roll-over) caused

by movement along an array of syn-sedimentary,

listric boundary faults (Figs. 5 and 7). Bonini and

Sani (2002) interpreted them, instead, to be part of a

growth fold mainly associated with blind thrust and

back-thrust faults. Neither solution is considered

satisfactory by Acocella et al. (2002) for the basin

wide fold. They suggested that magmatic laccolithic

intrusions at Radicofani, in the central-south part of

the basin, may have been determining factors for

Fig. 15. Fossils in horizontal sandy conglomeratic layers in the uppermost part of the foreset-bedded strata in the La Foce unit. a. Conglomerate

cobbles with encrustation by marine barnacles on the upper surfaces. b. Cobbles with oyster growths on the upper surfaces.


anticline formation. However, the available seismics

are unable to define clearly these magmatic bodies.

We conclude that the early intrusions and later de-

velopment of the volcanic edifice of Mt. Amiata may

have partly affected the basin geometry, the reacti-

vation of faults and folds, and perhaps led to lateral

foreshortening of the basin.

b. Seismic profile 10 crosses the Radicofani Basin, the

narrow Cetona Ridge and enters the adjacent Chiana

Basin to the east (Figs. 8 and 16). The Cetona Ridge

is part of the anticline (nose) of a large substrate

thrust, the back and frontal parts of which have been

cut by normal faults. These faults bound the two

adjacent Radicofani and Chiana Neogene basins.

This structural edifice is a good example of what

was called the bcuneo compostoQ (structural wedge)by Migliorini (1949).

c. The major faults active during the late Miocene and

early Pliocene can be mapped throughout the basin

by seismic reflection (Fig. 16). During late Miocene,

isolated fault-bounded basins developed in the cen-

tral-south part of the Radicofani Basin. One to the

NW has a western boundary fault; the other to the

SE has an eastern boundary fault. During the early

Pliocene the whole basin was inundated by a major

marine transgression. Various depocenters deve-

loped in association with discontinuous, en echelon

boundary faults (Fig. 16). To the NE, a small half

graben with a western boundary fault developed

south of Pienza. The main eastern boundary of the

basin was characterised by an array of offset, south-

western dipping faults that may have had large

differences in throw along their axes. Later, during

the Pliocene–Pleistocene the isolated faults linked to

Fig. 16. Schematic structural map of the Radicofani Basin showing Miocene and Pliocene faults mapped from seismic data and field observations

(SI: Siena Basin; RA: Radicofani Basin).


produce the modern, more continuous, eastern

boundary system (Fig. 16).

5. Syntheses and discussion

The following two questions will be examined in

this discussion: (Section 5.1) what is the sedimentation

pattern along the eastern master faulted-margin of the

Radicofani Basin indicate about the paleogeomorpho-

logy of the area and about the local and global paleo-

climate; and (Section 5.2) whether the observed

deformation and sedimentation patterns seen in the

basin are exclusive to half-graben basins or can be

applied to compressional thrust-top (or satellite) basins

as well, or to basins with a hybrid extensional–com-

pressional evolution.


5.1. Sedimentation patterns

The paleogeographic conditions of the basin are

dependent in great measure on the tectonics of the

area, and are indicated by the overall geometry of the

deposits, their coarseness, and paleocurrent directions.

Furthermore the cyclicity of certain deposits, every-

thing else being equal, may provide information about

climatic modulations of an overall transgressive trend.

The sedimentation pattern varies along the eastern

basin margin of the Radicofani Basin. To the south, in

the Celle sul Rigo area, there are no outcrops of lower

Pliocene alluvial deposits, and local fine-grained brec-

cia layers occur within marine clays near the coast.

Toward the centre of the basin, resedimented conglo-

merates occur in channels within offshore clays and

sandstones, and sandy turbidites alternate with silty

sandy clays. The paleocurrent directions of the

coarse-grained deposits indicate a source from the

southeast during the early stages and from the east in

later stages (Liotta, 1996). To the north, in the Pietra-

porciana area, the sediment provenance is consistently

from the east-southeast; and a complete transgressive

sequence occurs near the eastern border from a large,

gravelly alluvial fan, to deltaic conglomerates and sand-

stones, to massive, highly bioturbated, shallow marine

sandstone, to offshore silty clays with some resedimen-

ted sandstone and gravel layers (Fig. 11). This suggests

that to the south a relatively steep coast with a narrow

platform characterised the eastern border of the basin,

whereas to the north a more extensive platform existed.

Liotta (1996) visualised the southeastern part of the

basin as being fed by quasi-longitudinal flows during

the earliest Pliocene (Fig. 17a), and later by short-

headed streams leading directly to deep-water hyper-

pycnal flows, or by costal cliff collapses supplying

angular clasts that were rapidly reworked into deeper-

water clay settings (Fig. 17b). To the north the large

alluvial fan grading up into an impressive delta body

required (a) a relatively wide catchment that probably

developed at a relay zone between boundary faults; or

between boundary faults and transverse faults or a

transfer zone, such as the Pienza high (Fig. 16; Pascucci

et al., in press), and (b) a sufficiently wide platform

where marine sandstones formed and were locally

transected by delta-feeding streams (Fig. 18).

The deltaic to coastal sedimentation pattern of the

northeastern Pietraporciana area provides further infor-

mation on the tectonic/climatic influences.

Of particular interest is the transition from alluvial fan

to shoreface fossiliferous sandstone, locally via a thick

deltaic body. As previously indicated, we interpret this

deltaic body to be formed mostly by numerous (~15–20)

sets of relatively thin Gilbert-type delta-lobes (max

thickness about 8 m). The cross-sets show a quasi-uni-

directional paleocurrent direction, with materials

sourced from a persistent sediment injection point (con-

fined-fan phase, Muto, 1993). Similar Gilbert-type delta

cross-sets have been reported in the literature (Kazanci,

1988; Gupta et al., 1999), but never so numerous a series

of them. Twomain considerations support the hypothesis

that most of these cross-sets represent Gilbert-type deltas

rather than fluvial bars (see Appendix A for an expanded

analysis). One is that the overall setting of the area

would not justify the large, deep fluvial channels that

would be necessary to construct some of the thickest

cross-sets; nor is there independent evidence for them.

The area probably had highly competent but relatively

shallow, perhaps seasonal, streams. The other conside-

ration is the presence of oyster fragments in some

foresets, bioturbation in some sandy bottomsets, and

borings by sponges and mollusc encrustations on cob-

bles and boulders in some topsets.

Thus, the problem becomes how to explain the

repetitive occurrence of cross-sets, mostly tied to repet-

itive variation in sea level, and partly to variations in

flood strength of the injecting streams. On the whole,

the accommodation space for the thick Neogene suc-

cession of the Radicofani Basin was determined by

tectonics and by the sea level changes related to the

latest Miocene (Messinian) regression (Hsu et al., 1972;

Cita, 1982) and the lower Pliocene transgression that

characterised the whole Mediterranean area (Cita, 1975;

Mckenzie and Sprovieri, 1990; Mckenzie et al., 1991).

Small-scale, local tectonic movements may have had

some influence on the repetitive occurrence of the

Gilbert-type cross-sets of the Pietraporciana area, but

their modulation may have also been strongly influ-

enced by repetitive climatic events, both local (such as

multiyear variations in precipitation and therefore in

sediment supply and flood strength) and global (gla-

cially induced, high-frequency, repetitive eustatic

changes). In the global context, Neogene glacial events

have been recorded in Antarctica and other parts of the

globe since the late Miocene (Shackleton and Kennett,

1975; Woodruff et al., 1981; Mercer and Sutter, 1982;

Frakes et al., 1994). Glacial expansions in the Southern

Hemisphere were recorded in the late Miocene between

7 and 5.2 Ma (Mercer, 1983; Denton et al., 1984), and

between 5.6–5.4 Ma, followed by warmer conditions

during the earliest Pliocene (Keany, 1978; Ciesielski

and Weaver, 1974; Ciesielski et al., 1982). A new

glaciation may have developed in Antarctica later in

the early Pliocene by 4.4 Ma. A similar early glaciation

Fig. 17. Tectono-sedimentary models for the southern part of the Radicofani Basin (after Liotta, 1996). a. Lowermost Pliocene resedimented

conglomerates derived from the southeastern boundary, and olistostromes derived from the west of the basin. b. Lower Pliocene resedimented

sandstones and conglomerates and pebbly (angular clasts) sandstone derived from the eastern boundary of the basin; olistotromes may have

continued to develop along the western flank, but they are no longer present probably because of erosion (diagrams not to scale; symbols as in Figs.

5 and 9).


may have occurred in the Northern Hemisphere, at

about 4.3 Ma (Eldholm et al., 1986). Independently

of the accuracy of these dates, the important point is

that, starting in late Miocene and continuing through

the early Pliocene, climatic changes and the formation

and retreat of large glaciers have been recorded. Thus,

significant eustatic sea level changes occurred that

indirectly affected the whole Mediterranean area (Haq

et al., 1987; Kastens, 1992) and, hence, the Radicofani

Basin as well.

5.2. Tectono-sedimentary conditions

One of the lingering questions related to the Radi-

cofani Basin and the other hinterland basins of Northern

Apennines in Tuscany, and perhaps to other similar

complex folded mountains belts is whether their fills

retain diagnostic sedimentological characteristics that

objectively discriminate between half-graben, thrust-

top basins, and basins that have a hybrid developmental

history.

The processes and the sedimentary fills of rifts and

half-graben have been extensively studied and several

models are available both from cratonic areas and from

fold mountain chains (Leeder and Gawthorpe, 1987;

Colella, 1988; Leeder et al., 1988, Cipollari et al., 1998;

Gupta et al., 1999; Gawthorpe and Leeder, 2000).

Thrust-top (piggyback, satellite, or perched) basins

have been mainly described from foreland basin sys-

tems (Ori and Friend, 1984; Ricci Lucchi, 1986; Ori et

Fig. 18. Tectono-sedimentary model for the La Foce unit along the northeastern border of the Radicofani Basin. a. Alluvial–coastal fan developed

on a fault relay zone in the Cetona Ridge area, south of a transfer fault/zone (dash and dot line). The model calls for a distant source for the sand,

and proximal coastal hill and fault scarp sources for the coarse clasts perforated by Lithophaga. b. Model of formation of Gilbert-type deltas at and

near the mouth of a shallow channel (valley) incised within the fan (confined-fan phase).


al., 1986; Clevis et al., 2004a,b). Relatively few thrust-

top basins are well exposed in uplifted orogenic

wedges, such as those in Sicily and in the eastern

Pyrenees, where good structural, stratigraphic–sedi-

mentological information has been provided for them

(Butler and Grasso, 1993; Mellere, 1993).

Frostick and Steel (1993) stressed the complexity of

the problem and suggested that, assuming other varia-

bles, such a climate, remained constant, the various

basins would develop the following tectono-sedimen-

tary characteristics.

a. The accommodation space in half-graben is gene-

rated by basin subsidence. A coarse-grained alluvial

fan succession would most commonly develop ad-

jacent to a relatively permanent boundary fault or, in

the case of a back-faulting basin margin, a fining

upward succession. Some half-graben basins do,

however, develop coarsening upward successions

from basal open marine turbidites and fine deposits

to coarse-grained, proximal deltaic deposits (Gupta

et al., 1999).

b. The foreland basin system, whether its accommoda-

tion space is generated by plate subduction and/or

loading of the fold-thrust belt (DeCelles and Giles,

1996), experiences a basinward migration of thrust

faults and would commonly develop a coarsening-

upward succession. Eventually a continental se-

quence forms, separated by progressive unconformi-

ties (Riba, 1976; Anadon et al., 1986). The major

source of coarse clastics would be the advancing

fold-thrust belt, and the depocenter would in time

migrate outward.

Things, however, are not always clear-cut.

a. Mellere (1993) has shown that, in the outer part of

the orogenic thrust wedge of the southern Pyrenees,


the interplay between thrust and back-thrust faults

determines both the size and the variation in time

and space of sediment source areas, and the deve-

lopment of alluvial fans prograding into the basin

from various directions, similarly to what occurs in

rifts and strike slip basins. However, Blair and

McPherson (1994) have indicated that piedmont-

type alluvial fans are best developed and have a

higher probability of geological preservation in ex-

tensional and transtensional basins associated with

long persisting, high angle normal and strike-slip

faults. Alluvial fans that develop in other settings,

such as in thrust-bounded basins, have a lower pro-

bability of preservation because of b lateral instabil-

ity of the basin margin, and recycling of the thrust-

front deposits through timeQ (Blair and McPherson,

1994, p. 481).

b. Progressive unconformities are most likely to occur

in, but are not exclusive to continental deposits of

thrust-top basins. Gupta et al. (1999) have shown

that they can also develop in half-graben basins due

to vertical progressive development of blind normal

faults in the substrate and the formation of growth

folds and tilting of the basinal deposits.

The conclusion is that similar sedimentary

sequences can develop in both compressive and exten-

sional basins, but the prevalent character of the basins

can be discriminated by their close relationship with

tectonic structures (whether thrust or normal faults

prevail), and, in places, by the presence of various

types of igneous intrusions and volcanism. The location

of the basins within the orogen can also help because

half-graben occur more frequently, but not exclusively,

in the hinterland; thrust-top basins predominate in the

orogenic thrust wedge and foreland basin system. Fur-

thermore the behaviour of the basins changes through

time. The Northern Apennines show a gradual eastward

migration of the active orogenic wedge and a trail of

Neogene–Quaternary basins becoming increasingly

older westward. The older western basins, such as

those developed on the northern Tyrrhenian Sea shelf

(Pascucci et al., 1999) show a more persistent exten-

sional behaviour than do the younger ones near the

divide of the mountain chain, which may have been

affected by alternating extensional and compressive

regimes (Bonini, 1998). On the active orogenic thrust

wedge, the compressive regime predominates although

embryonic extensional basins appear. Furthermore, in

the hinterland of the Northern Apennines, Neogene–

Quaternary plutonism and volcanism occur, becoming

younger from the west to east and from north to south.

This affects several basins, complicating both their

development and the deformation of their deposits.

6. Conclusions

Structural depressions up to about 200 km long and

25 km wide characterise the western (inner) side of the

Northern Apennines in Tuscany. They are delimited by

anticlines (noses) of substrate thrust faults. During the

Neogene–Quaternary these depressions have been sub-

divided longitudinally into basins (on the order of 20 to

50 km long) by substrate highs, possibly related to

transfer zones. At present, the basins are bounded by

normal faults. The lower Pliocene Radicofani Basin is

one of the largest, well developed and best preserved of

these basins, although it has been affected at some

stages of its development by plutonic intrusions. Also

large extinct Pleistocene volcanoes (Mt. Amiata and

Bolsena) occur along part of its western and southern

flank and, a small volcano (Radicofani) is located in the

middle of the basin. Finally, the Tuscany area has

experienced considerable uplift (of the order of

hundreds of meters) since the end of early Pliocene.

To the south, the uppermost Miocene–lower Pliocene

sedimentary fill of the basin consists of offshore clays

and turbidite sandstones with lenses of resedimented

conglomerates and pebbly sandstones with angular

clasts. No large, nearshore sedimentary succession

crops out, and most sediment was funnelled directly

into deeper parts of the basin. However, to the north,

in the relay area between eastern bounding faults and a

transfer fault/zone south of the Pienza high, a complete

succession has developed from a thick, conglomeratic,

alluvial fan and delta, to shoreface sandstone, to off-

shore clay. Part of the succession crops out, and its

downfaulted counterpart has been imaged on seismic

profiles. Of particular interest is a 115 m thick, sandy

conglomeratic body containing numerous, 1 to 8 m

thick, stacked cross-sets, which characterises the con-

fined-fan phase at the transition between the primarily

continental and the marine deposits. These bodies have

been interpreted as Gilbert-type deltas formed in front of

shallow, gravelly, distributary channels within a valley

that was cut into a coastal fan (confined-fan phase); the

thinner cross-sets may be part of the delta-topsets as-

semblage and may represent fluvial prograding bars.

The accommodation space for the up to ~2700 m-

thick Neogene succession of the Radicofani Basin was

provided by tectonic and eustatic movements. The fine

modulation of sea level variations recorded by the

repetitive occurrence of Gilbert-type cross-sets may

have been partly related to climatic fluctuations. This


include the possible influence of Antarctic glaciations

when the Mediterranean Sea was cut off during the

upper Miocene bsalinity-crisisQ, and then reopened to

the Atlantic Ocean during the uppermost Miocene–

lower Pliocene regional transgression.

Due to the presence of compressive basinwide gentle

folds and small scale, local thrust faults and imprinted

pebbles, the presence of progressive unconformities in

some hinterland basins, and the regional reinterpretation

of deep seismic across the Northern Apennines (Finetti

et al., 2001), the long-held, purely post-Tortonian, half-

graben origin has been challenged, and the Neogene

basins of Tuscany have been reinterpreted by some

researchers as thrust-top basins. The literature indicates

that neither the sedimentary fill architecture nor the

structures just mentioned are exclusive to either type.

For example, offshore, to delta-fan, to alluvial fan sedi-

mentary successions can develop in both half-graben

and in thrust-top basins, and progressive unconformities

can occur in the former although they are more frequent

in the latter. The close relationship between the basin

bounding faults and the synsedimentary architecture of

the basin fill, and the position of the basins in the

mountain chains can indicate the most likely genetic

regime(s) that have influenced the formation and evo-

lution of the basins. The Radicofani Basin, like others of

Tuscany, shows a complex, progressive development

since the late Miocene through linkage of normal faults,

shifting of depocenters, expansion of the basin during

the early Pliocene, followed finally by an overall con-

siderable uplift. Some structures, such as the basinwide

gentle anticline, may have been related to regional

compressive pulses, as suggested by Bonini and Sani

(2002), but, it is equally likely that they may be associa-

ted with emplacement of plutons and the development

of volcanoes.

Everything considered, the uppermost Miocene–

lower Pliocene, tectono-sedimentary evidence from

the Radicofani Basin conforms best with existing mo-

dels of half-graben type evolution, and the possible

deformation of part of the sedimentary fills by mag-

matic intrusions.

Acknowledgements

We thank Prof. Lazzarotto, A. and Prof. Gibling, M.

for critically reading an early version of this paper and

for their comments to improve it. Prof. M. Roveri and

Prof. C. Viseras are kindly acknowledged for the con-

structive final review of the paper. Financial support was

provided by Italian Centro Nazionale della Ricerca

(CNR), the University of Siena (MURST to Costantini),

the University of Sassari (COFIN 2003 to Pascucci), and

the Natural Science and Engineering Research Council

of Canada (NSERC, Grant 0GP0007371 to Martini).

Appendix A

A question requiring some detailed analysis is

whether numerous, stacked cross-sets like those ob-

served in the Radicofani Basin represent prograding

fluvial bars or Gilbert-type deltas or a combination of

both. Whichever is the case, they indicate frequent

fluctuation in relative sea level, and episodic strong

flows funnelled through a local entrenched paleovalley

leading to a persistent sediment injection point into the

basin: confined-fan phase of Muto (1993).

Isolated Gilbert-type deltas clinoforms tens to hun-

dred metres thick have been reported along lakes and

seas coasts (Stanley and Surdam, 1978; Clemmensen

and Houmark-Nielsen, 1981; Postma and Roep, 1985;

Colella et al., 1987; Sohn et al., 1997). Some gravelly

successions composed of cross-sets meters to tens of

meters thick have been described as Gilbert-type deltas

(Ori and Roveri, 1987; Kazanci, 1988; Flores, 1990;

Boorsma, 1992; Dam and Surlyck, 1993). Thinner

cross-sets up ten meters thick, characterise gravelly

bars in braided and meandering streams, and occasio-

nally stacks of a few of them are interpreted as Gilbert-

type deltas. Stacks of multiple, usually thin and sandy

planar cross-beds a few metres thick are most usually

interpreted as subaqueous dunes, and rarely as small

Gilbert-type deltas (Flores, 1990).

If the cross-sets (facies A) of La Foce unit were

prograding fluvial bars, their residual thickness (ca.

5–8 m) and their coarse sized clasts, would imply fast

flows, more than 10 m deep (Steel and Thompson,

1983). The boulder pavements at the top of many

foresets would indicate partial reworking by supercri-

tical unidirectional flows. The massive conglomerates

(facies B), particularly those found in cuts-and-fills,

would be part of topsets. This channel-bar genesis

would be consistent with the presence of a few large

channels seen in outcrops perpendicular to the foresets,

although no direct relationship could be established

between the two features. However, special conditions,

such as an alluvial valley fill and restriction of powerful

flows, would have had to exist to maintain the persis-

tence of quasi-unidirectional sedimentation for repeated

floods, as is indicated by the quasi-regular orientation

of the superimposed cross-sets throughout a thickness

of 115 m. Less persistence of paleocurrent directions,

and more frequent channelisation would be expected

under fluvial conditions.


If the foreset beds were Gilbert-type deltas, they

would still require flow restriction to develop (Muto,

1993). However, the river depth requirement would not

be critical to explain the thickness of the cross-sets, as

that would depend only on the depth of the receiving

basin. That is, relatively shallow, gravelly-sandy rivers

could have fed material to developing delta lobes in a

deeper standing body of water (Fig. 18). Under these

circumstances, part of the sandy facies (facies C) would

still represent bottomsets. The massive conglomerates

would represent slightly channelised topsets. The boul-

der pavements would still be related to strong flows,

possibly fluvial, sweeping and leading to erosion and

local planation of topsets and parts of foresets. There is

no specific evidence to indicate whether waves were

effective in washing the surfaces clear of fines, but this

is a possibility as well. In such a case, some of the upper,

flat erosional surfaces of the cross-sets would represent

ravinement surfaces. The foresets would have been

formed by modified grain flow. Open framework con-

glomerate foresets alternating with sandy conglomerate

ones within the same layer would indicate the arrival of

different material supplied by the feeding streams during

single or multiple floods (Martini, 1990; Mastalerz,

1990). Recurring, multiple floods are well defined, in

places, by reactivation surfaces (Fig. 14). The oyster

fragments in some foresets, bioturbation in some sandy

bottomsets, and borings by sponges, and molluscan

encrustation on cobbles and boulders in some topsets,

favour the deltaic interpretation over the fluvial one.

That said it is also true that in the poorly accessible

part of the steep outcrop, there is no overwhelming

occurrence of fossils that would provide evidence for

continuous, open marine conditions in the main part of

the foreset-bedded body. Furthermore, the residual

thickness and the internal features of the cross-sets

differ, albeit not in a regular fashion. For instance,

thick, sandy conglomeratic cross-sets are occasionally

overlain unconformably by thinner conglomeratic cross-

sets b1 m that show openwork texture. Possibly the

thicker, sandier layers represent Gilbert-type deltas,

and some thinner, openwork ones were prograding flu-

vial bars (Fig. 18b). All this suggests that the cross-sets

prograded into brackish waters, within and at the head of

the distributary channels of a confined-fan.

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