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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
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–9774
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.
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–97 75
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
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–9776
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.
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–97 77
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).
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–9778
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).
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–97 79
(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.
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–9780
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.
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–97 81
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).
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–9782
(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.
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–97 83
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.
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–9784
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).
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–97 85
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).
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–9786
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.
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–97 87
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).
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–9788
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.
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–97 89
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).
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–9790
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).
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–97 91
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,
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–9792
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
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–97 93
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.
V. Pascucci et al. / Sedimentary Geology 183 (2006) 71–9794
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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