Mesoscopic structural styles of deformation within

Boll. Soc. Geol. It., Volume speciale n. 1 (2002), 641-648, 2 ff., 3 tabb.
Mesoscopic structural styles of deformation
within the Frosolone unit multilayer (Molise Region, Central Italy)
A. ANTONUCCI (*), E. DI LUZIO (**), F. LENCI (***), D. SCROCCA (**) & M. TOZZI (**)
ABSTRACT
The analysis of the mesoscopic structures observed within the
Frosolone unit multilayer (Molise region, Central Italy) has led to
the distinction of three main «structural groups», each one characterised by different styles of deformation as a response to their lithological features. In this scenario, the Oligo-Miocene Macchiagodena
Formation clearly represents the upper weakness level of the multilayer, which is mainly made up of Mesozoic and Cenozoic formations affected by less pervasive brittle deformations. Moreover, mesoscopic structures likely related to break-back thrusting sequences
have been observed, and might represent an indication of larger
scale out-of-sequence thrust propagation occurred in the area.
KEY WORDS: mesoscopic structures, deformational style,
Frosolone unit, Molise Apennine.
RIASSUNTO
Stili deformativi mesoscopici all’interno dell’unità di Frosolone (Molise, Italia centrale).
L’analisi delle strutture mesoscopiche osservate all’interno della
successione stratigrafica dell’Unità Frosolone (Molise, Italia centromeridionale) ha portato alla distinzione di tre principali «gruppi
strutturali». In questo contesto la Formazione di Macchiagodena,
dell’Oligocene-Miocene, rappresenta il livello di debolezza strutturale più alto dell’intero multilayer, costituito principalmente da formazioni meso-cenozoiche caratterizzate da deformazioni fragili non
pervasive. Inoltre, strutture mesoscopiche osservate in alcuni affioramenti sono probabilmente legate a processi di break-back thrusting e potrebbero essere un indizio di fenomeni di fuori-sequenza
avvenuti nell’area a più grande scala.
TERMINI CHIAVE: strutture mesoscopiche, stile deformativo, Unità Frosolone, Appennino molisano.
INTRODUCTION
The central-southern part of the Molise region (Central Italy) has been the topic of several geological surveys
led by the CNR during the last five years. Starting from a
relative shortage of structural and tectonic data, in spite
of an accomplished geological and stratigraphic literature, we have focused our attention on the deformational
(*) Dipartimento di Scienze della Terra – Università degli
Studi di Roma «La Sapienza».
(**) C.N.R. – Centro di Studio per il Quaternario e l’Evoluzione Ambientale, Roma. C/o Dip. Scienze della Terra – Piazzale Aldo
Moro, 5 – 00185 Roma.
(***) Dottorato di ricerca in Scienze della Terra – Università
degli Studi di Roma «La Sapienza».
processes that took place during the Late Miocene-Early
Pliocene, responsible for the building up of the Molise
Apennines. Our previous efforts (DE CORSO et alii, 1998;
DI LUZIO et alii, 1999; SCROCCA & TOZZI, 1999; ANTONUCCI et alii, 2000; TOZZI et alii, 2000a, b) have particularly focused on the structural and tectonic setting of an
area in the Molise region, commonly known as the Montagnola di Frosolone (fig. 1).
As part of a larger regional research project (CARG),
in this paper we describe the mesoscopic deformational
styles observed within the Frosolone multilayer. We have
divided the pre-siliciclastic deposits of the multilayer into
three main «structural groups» (sensu RAMSAY & HUBER,
1987), each one characterised by a nearly homogeneous
mechanical behaviour. Deformational styles are strongly
controlled by the lithological features.
GEOLOGICAL FRAMEWORK
The Montagnola di Frosolone is a wide dome-shaped
mountain ridge lying east of Isernia (fig. 1), where the
transitional facies of the Frosolone Unit, i.e. the inner
Molise geological unit according to PATACCA et alii
(1992), are exposed. These facies represent the result of
the Mesozoic and Cenozoic sedimentation that took
place along the northern margin of the Matese carbonatic platform, nowadays located few tens of kilometres
to the south.
On the western and northern edges, the Montagnola
di Frosolone is completely surrounded by terrigenous
facies that were deposited within the Early Messinian
Molise foredeep: the contact has almost everywhere a tectonic nature, as already pointed out in NASO et alii (1995),
DI LUZIO et alii (1999), and SCROCCA & TOZZI (1999). The
deeper basinal facies of the Molise geological domain outcrop in the northern Molise area.
The geological and stratigraphical aspects of the Montagnola di Frosolone have been properly investigated by
several authors in the last four decades (SELLI, 1957; SIGNORINI, 1961; SIGNORINI & DEVOTO, 1962; PESCATORE,
1965; STEFFENS, 1968; CLERMONTE` , 1977; CLERMONTE` &
PIRONON, 1979; PIRONON, 1980; RENAUD et alii, 1990;
PATACCA et alii, 1992). Basically, in this area a Late Jurassic-Middle Miocene transitional geological sequence outcrops above a buried late Triassic-Early Liassic dolomitic
substratum found in the Frosolone 1 and 2 wells. This
sequence underlies synorogenic foredeep deposits mainly
made up of Messinian clays, with rare interbedded sandstones.
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A. ANTONUCCI ET ALII
Fig. 1 - Geological and structural sketch of the Central-Southern Apenines. The frame shows the area of research (after SCROCCA et alii,
1995, modified). LEGEND: 1: Marine and continental deposits (Pleistocene-Pliocene); 2: Sannio Unit (Miocene-Oligocene); 3: syntectonic
siliciclastic deposits (Early Pliocene-Late Messinian); 4: syntectonic siliciclastic deposits (Early Messinian-Late Tortonian); 5: carbonatic
platform and shelf-edge sedimentary deposits (Middle Miocene-Late Triassic); 6: transitional and basin sedimentary deposits (Middle
Miocene-Late Triassic); 7a: main thrusts (the arrow points out the average transport direction as inferred by structural analysis); 7b: second
order thrusts and inverse faults; 8a: anticlinal axial surface; 8b: synclinal axial surface; 9a: strike-slip faults; 9b: normal faults; 10: wells.
– Schema geologico-strutturale dell’Appennino centro.meridionale. Il riquadro mostra l’area di ricerca (da SCROCCA et alii, 1995, modificato).
LEGENDA: 1: depositi marini e continentali (Pleistocene-Pliocene); 2: Unità del Sannio (Miocene-Oligocene); 3: depositi silicoclastici sintettonici
(Pliocene inferiore-Messiniano superiore); 4: depositi silicoclastici sintettonici (Messiniano inferiore-Tortoniano superiore); 5: successioni
sedimentarie carbonatiche di piattaforma e di soglia (Miocene medio-Triassico superiore); 6: successioni sedimentarie di transizione e di bacino
(Miocene medio-Triassico superiore); 7a: thrust principali (la freccia indica il verso del trasporto tettonico, come evidenziato dall’analisi
strutturale); 7b: thrust di secondo ordine e faglie inverse; 8a: superficie assiale di anticlinale; 8b: superficie assiale di sinclinale; 9a: faglie
trascorrenti; 9b: faglie dirette; 10: pozzi di esplorazione.
MESOSCOPIC STRUCTURAL STYLES OF DEFORMATION WITHIN THE FROSOLONE
643
Fig. 2 - Structural groups within the Frosolone unit multilayer. From the bottom to the top the calcareous-dolomitic substratum, the
calcareous-marly, and the marly-calcareous groups are distinguished. Formation thicknesses are the average thicknesses surveyed in the
whole outcropping area (Montagnola di Frosolone).
– Gruppi strutturali individuati all’interno dell’Unità di Frosolone. Dal basso verso l’alto, il substrato calcareo-dolomitico, il gruppo calcareomarnoso e il gruppo marnoso-calcareo. Gli spessori riportati sono gli spessori medi delle formazioni, calcolati su tutta l’area di affioramento
(Montagnola di Frosolone).
The transitional sequence shows evident re-sedimentation processes of debris, probably derived from the
Matese carbonatic platform to the south. Therefore, detritic calcareous layers, often rich in broken benthonic fossils, are interbedded to thinner layers of calcareous marls,
marls and clays, that are characterised, instead, by pelagic foraminifera.
Consequently, an heterogeneous multilayer results,
whose deformational styles clearly rely on the competent/incompetent material ratio within each geological
interval.
STRUCTURAL GROUPS WITHIN
THE FROSOLONE MULTILAYER
Three main «structural groups» have been recognised
within the pre-siliciclastic deposits of the Frosolone multilayer (fig. 2), subdivided on the basis of their deformational
styles strongly determined by lithology. From the bottom to
the top, we distinguish a calcareous-dolomitic substratum, a
calcareous-marly group, and a marly-calcareous group.
In this section the mesoscopic structures commonly
observed within the outcrops are described. Most of them
644
A. ANTONUCCI ET ALII
were formed in a thrust tectonic environment, even
though structural features related to normal and strikeslip faults have also been observed.
CALCAREOUS-DOLOMITIC
SUBSTRATUM
(tab. 1)
Above the Fontegreca Formation (Early Liassic-Late
Triassic), found in the Frosolone 1 and 2 wells, lies the
Indiprete Formation, the oldest formation outcropping in
the whole area. It is splitted into a lower Albian-Dogger
dolomitic member (600-700 m thick) and in an upper
member, which consist of 180-270 metres of detritic dolomitic limestones and clays dated from the Albian to the
Early Cenomanian.
The formation is mainly characterised by brittle behaviour: the deformational style mostly consists of fractures
and faults (tab. 1, fig. 1a). Major thrust surfaces (tab. 1,
fig. 1b) are responsible for the genesis of wide cataclastic
zones which often obscure the bedding. Duplex structures
have been found where very thin clayey layers are interbedded with the dolomitic massive body (tab. 1, fig. 1c).
Mesoscopic folds have rarely been found, probably due to
the high competence of the whole formation.
layers. As a consequence, duplexes result to be the dominant thrust structure, with calcareous elongate horses
between roof and floor thrusts along the marly layers
(tab. 2, fig. 2a).
As concerns the folding mechanism, the competent
layers usually shows a nearly perfect parallel fold form
(class 1B of RAMSAY & HUBER, 1987), whereas the thin
interbedded calcilutites, marly limestones and marls
layers are usually thickened in the hinge zone and thinned in the limbs (class 3); the combination of the two different folding styles gives folds in the calcareous-marly
group which are often recognisable as 1C class folds.
As well as for the thrusting styles, the folding styles are
clearly controlled by the different mechanical behaviour of
the layers. For instance, fault-propagation folds grow at
the tips of thrusts propagating through the calcareous
layers (tab. 2, fig. 2c): as the fault continues to cut the
rigid packstones layers, the covering marl layer is folded.
Detachment folds have also been observed (tab. 2, fig. 2b):
the detachment is required to allow a different style and
amount of deformation between thick and strong calciruditic layers and thin micritic or marly-calcareous strata.
MARLY-CALCAREOUS
CALCAREOUS-MARLY
GROUP
(tab. 2)
It is a group of geological formations strongly affected
by re-sedimentation processes. These formations seem basically to have the same deformational style and this evidence
led us to consider them as a single group with a peculiar
mechanical behaviour. The brief following lithological
description of these formations does not consider the
details about their biostratigraphic features and geological
meaning; a more extensive account can be found in TOZZI
et alii (2000a, b). From the bottom to the top we recognised:
– Monte Coppe Formation (Early Turonian-Early Cenomanian): thinly layered white calcilutites with nodules
and lenses of red and black chert; layers of calcarenites,
green marls, and varicoloured clays are often interbedded. Average thickness: 60-80 m.
– Coste Chiavarine Formation (Senonian-Early Turonian): thickly layered grey calcarenites and calcirudites,
with nodules and lens of grey or brown chert; layers of
calcilutites. Average thickness: 100-150 m.
– Monte Calvello Formation (Maastrichtian-Campanian): white calcarenites, rich in debris of benthonic fossils; thin layers of green or red marls and marly clays are
often interbedden. Average thickness: 160-200 m.
– Monaci Formation (Early Oligocene-Early Eocene):
it is possible to distinguish two heteropic members: a calcareous member, which mainly consists of brown and
well-layered calcarenites, rich in debris of Eocenic benthonic organisms, interbedded to calcirudites and calcilutites; and a marly-calcareous member, which is made up
of red marly limestones and marls, with a reduced
amount of calcarenitic layers. Boh members present nodules and lenses of chert. Average thickness: 80-100 m.
Thrust styles within the calcareous-marly group is evidently influenced by the different mechanical behaviour
of the calcarenitic and calciruditic strata with respect to
the thinner and weaker layers of calcilutites, marly limestones and marls. In these formations thrusts develop as
bedding-parallel flats in the less competent layers, whereas as bedding-oblique ramps in the more competent
GROUP
(tab. 3)
The Frosolone multilayer ends upwards with two Oligocene-Miocene formations mainly made up of marls and
calcareous marls belonging to the marly-calcareous group.
The 50-80 metres of marls and calcareous marls of
the Macchiagodena Formation, dated from the Early Oligocene to the Early Miocene (Langhian), represent a
meaningful variation in the sedimentary features of the
Frosolone multilayer that leads also to a structural style
of deformation which is unique among the other geological formations.
First of all, thrust structures appear here to be pervasive at the outcrop scale, as clearly highlighted by the
wavelength of ramps in the duplex structures, that is reduced with respect to the calcareous-marly group (compare
tab. 3, fig. 3a, to fig. 2a in tab. 2). Moreover, as a result of
pressure solution processes, foliations and S-C structures
can be easily detected in the Macchiagodena Formation.
Considering the fold style, the Macchiagodena Formation shows in some outcrops a first deformation, represented by an early foliation (S1) that overprints the original bedding (S0), and a late foliation producing kink
bands (tab. 3, fig. 3b and see also NASO et alii, 1995).
Taking into account all these field evidences, we consider the Macchiagodena Formation as a fault gathering
zone, the uppermost weakness level within the Frosolone
multilayer.
The Macchiagodena Formation is followed upwards
by the Longano Formation, which consists of 40-80
metres thick hemipelagic marly limestones, marls and
calcarenites rich in pelagic foraminifera as Orbulinidi
(Later Tortoniano-Langhiano), locally characterised, at
the bottom, by interbedded calcareous coarse clastic horizons. Such formation, similar to others outcropping in
the Central-Southern Apennines, represents the depositional response to the flexural bending of the foreland. Its
deformational style shows low wavelength thrusting features, foliations and S-C structures (tab. 3, fig. 3c); faultpropagation folds, similar to those found in the calcareous-marly group, have been also locally observed in the
lower part of this formation.
Deformational styles within the calcareous-dolomitic substratum.
TABLE 1
MESOSCOPIC STRUCTURAL STYLES OF DEFORMATION WITHIN THE FROSOLONE
645
Deformational styles within the calcareous-marly structural group.
TABLE 2
646
A. ANTONUCCI ET ALII
Deformational styles within the marly-calcareous group.
TABLE 3
MESOSCOPIC STRUCTURAL STYLES OF DEFORMATION WITHIN THE FROSOLONE
647
648
A. ANTONUCCI ET ALII
OUT-OF-SEQUENCE FEATURES
Our interest has been also focused on some thrust
structures related to out-of-sequence thrust propagation
(sensu MORLEY, 1988) that have been observed in several
outcrops in the Montagnola di Frosolone area. The duplextype structures, as represented in tab. 2, fig. 2a, are often
affected by out of sequence thrust phenomena displacing
previously formed carbonatic horses and partly developing along their roof thrust.
CONCLUSIONS
The analysis of the mesoscopic deformational styles
represent a precious tool to better understand the geological features of an area. In this study, the evidence collected on the outcrop scale have given us a sounder idea
of how the Frosolone multilayer could have been deformed during the Apenninic orogenesis.
A careful observation of the mesoscopic structures
inside the Frosolone multilayer, mainly related to a thrust
tectonic environment, has allowed us to distinguish three
main structural groups, each one characterised by a peculiar type of deformational style, strongly controlled by the
primary lithological features. In this scenario, an OligoMiocene marly formation placed at the bottom of the
third group, the Macchiagodena Formation, has been
identified as the uppermost weak structural level of the
Frosolone multilayer, a fault gathering zone that has
played a major role in the geometric configuration of the
whole area (ANTONUCCI et alii, 2000). It is worthwhile to
note that also the Monte Coppe Formation (NASO et alii,
1995; DI LUZIO et alii, 1999) and the marly-calcareous
member of the Monaci Formation (TOZZI et alii, 2000b)
are both strongly deformed on the mesoscale. Nevertheless, as indicated by the geological section in ANTONUCCI
et alii (2000), only the Macchiagodena Formation seems to
play a critical role in the macroscopic geometric configuration of the Frosolone multilayer. The Monte Coppe Formation and the marly-calcareous member of the Monaci
Formation represent internal minor weakness levels of the
multilayer.
Moreover, assuming that the mesoscopic structures
reflect the macroscopic deformation environment of an
area, the out-of-sequence thrusting features found in
some outcrops within the Frosolone multilayer might
well be incorporated to the geological framework outlined
by SCROCCA & TOZZI (1999), as other evidence for the
occurrence of such thrusting process in the centralsouthern Molise area.
ACKNOWLEDGEMENTS
Authors wish to thank Stefano De Corso for his help during
land survey.
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