Detailed facies mapping of Jurassic carbonates in the area around
Monte Pietroso and Monte Scocione in the Apennine mountains of Italy
Maya T. Del Margo
Department of Geology, Whitman College, Walla Walla, WA 99362
INTRODUCTION
The Umbria-Marches Apennine Mountains form a northwest-trending segment
of an arcuate thrust belt that extends southward and westward through
Calabria and into Sicily. Thrusting occurred in response to the westward
subduction of the European plate beneath Adria (Italy) as the African
plate moved northward relative to the European plate. Thrusting began
in Late Oligocene to Early Miocene time, and is still active in the
northern part of the Apennines today. The belt began undergoing extension
shortly after it started to form. Extensional and compressional fronts
have both migrated to the northeast over time; the active thrust front
is now along the Adriatic coastline, and the active extensional front
runs through the center of the peninsula (Bice and Stewart, 1990).
The purpose of the project was to create a detailed facies map of
the area around Mount Pietroso and Monte Scocione, and to constrain
the geometry and timing of Jurassic extensional tectonics in this
area. Oligocene-Quaternary compressional and extensional tectonism
may have reactivated Mesozoic structures in the Umbria-Marches region.
A detailed study of the Monte Pietroso - Monte Scocione area may lead
to a better understanding of the influence of older extensional structures
on the younger compressional activity.
OVERVIEW OF GEOLOGIC HISTORY
During the Early Jurassic, a large carbonate platform - the Calcare
Massicio - developed on Italian continental crust. By the middle of
the early Jurassic, the Liguride Ocean basin began to open as the
Italian Peninsula moved away from southeastern Europe. The opening
of this small ocean basin produced a passive continental margin in
the area that is now the northern Apennine Mountains. This margin
underwent thinning and subsidence and persisted until the Oligocene
when it was reactivated as part of the fold-and-thrust belt (Alvarez
and Montanari, 1988).
The Calcare Massicio platform experienced normal faulting during
this episode of extension and was fragmented into small isolated seamounts
separated by intervening basins. This topography persisted until the
beginning of the Cretaceous, when it was drowned as eustatic sea level
rose by as much as 2000 meters (Bice, personal communication, 1996).
Paleomagnetic data suggests that the study area was 15-30û N
latitude at the time of drowning, and separated from any source of
significant siliclastic input (Channel et al., 1984, and Westphal
et al., 1986, cited in Bice and Stewart, 1990). The setting was therefore
favorable for the production of carbonates. The overlying sediments
of both the basins and the seamounts are predominantly limestone.
UNIT DESCRIPTIONS
Because the seamounts were at a significantly different water depth
than the basins, and because carbonate sediments are excellent depth
indicators, there is a clear facies difference between shallow-water
seamount sequences and deep-water basinal sequences. The deep-water
basinal facies have been represented as the ÔComplete SequencesÕ;
this includes the Corniola, Rosso Ammonitico, Marne a Posidonia, and
Calcari Diaspri units (Alvarez and Montanari, 1988). In general, the
basinal facies are characterized by thick deposits of radiolarian
cherts and may contain slumps, pebbly mudstones, megabreccias, or
turbidites. The ÔComplete SequenceÕ is about 300 meters
thick (Alvarez and Montanari, 1988).
In contrast, the shallow-water seamount facies have been characterized
as ÔCondensed Sequences,Õ which are thinly bedded, densely
fossiliferous limestones. These may be only 50 meters thick, and,
in some locations, represented by only one formation - the Bugarone
(Alvarez and Montanari, 1988). Both the basinal ÔComplete SequencesÕ
and the seamount ÔCondensed SequencesÕ are overlain by
the Maiolica Formation, a white, creamy, fine-grained pelagic limestone.
It is thinly bedded and may be dolomitized at the base where it overlies
a seamount, or thickly bedded and may contain slumps and gravity flows
where it overlies a basin (Alvarez and Montanari, 1988). The ÔbasinalÕ
Maiolica facies may be 200 meters thick; the seamount facies may be
only 50 meters. Deposition of the Maiolica Formation partially leveled
the seamount and basin topography; by the end of the Early Cretaceous
the basin was essentially flat.
DISCUSSION
The Calcare Massiccio platform is composed of massive, fine-grained,
thickly-bedded limestone. It is highly resistant, and makes up the
cores of many of the large anticlines in the Umbria-Marches region
of the Apennines. As the facies map (Figure 2) shows, the area around
Monte Pietroso and Monte Scocione is no exception. The outcrops found
in the part of the study area with the highest elevation are composed
of the Calcare Massiccio, dolomitized Maiolica, and Bugarone units.
The presence of the Bugarone Formation (a unit of the Condensed ÔseamountÕ
Sequences) in these locations suggests that the summits of Monte Pietroso
and Monte Scocione may have been the tops of one or more seamounts
in Jurassic and Early Cretaceous time. Furthermore, the dolomitization
of the outcrops of Maiolica Formation on the top of Monte Scocione
suggest that it might have been exposed alternately to air and water
as it was being deposited. This supports the hypothesis that the top
of Monte Scocione may have been the top of a seamount in Jurassic
and Early Cretaceous time.
There are also three locations in the study area that are possible
sites for the edges, or slopes, of one of these paleo-seamounts. The
first two are on the northern and southern sides of Monte Scocione.
In both cases, the outcrops consist of Calcare Massicio with small
pockets of unbedded Bugarone Formation on them. Since the Bugarone
is usually distinctly and thinly bedded on seamount tops, the lack
of bedding here might suggest deposition on a slope, with synsedimentary
deformation resulting in distorted bedding, or a post-depositional
slide or slump. There are large outcrops of Calcare Massiccio and
dolomitized Maiolica uphill from both of the ÔpocketÕ
outcrops. It is possible that the current topographical hillsides
in each of these locations mimic the original paleoslope of the Jurassic
seamounts. It is also possible that the pockets of Bugarone were tectonically
emplaced either during a later phase of normal faulting in the Jurassic,
or during the younger compressional orogeny.
The third possible location for a paleo-seamount slope is in the
valley east of Monte Scocione and Monte Pietroso. This outcrop is
located at an elevation of 550 meters, and is unique. The limestone
here lacks bedding and contains fossils usually associated with the
Bugarone Formation, which was deposited in the Early Cretaceous.Within
this limestone are embedded clasts of Early Jurassic Calcare Massicco,
which indicates that there was an exposed section of Calcare Massiccio
uphill from this location at the time of deposition. The presence
of both Early Cretaceous fossils and clasts of Jurassic-age limestone
in the same outcrop strongly suggests the presence of a Mesozoic fault
scarp with Calcare Massiccio exposed in the upthrown block.
CONCLUSIONS
The overall geography and relationship of the outcrops to one another
suggest that the area around Monte Scocione and Monte Pietroso was
once a Jurassic seamount. The outcrops on the highest portions of
these mountains suggest the top of such a seamount. Outcrops on the
hillsides below may represent the paleoslopes on the sides of this
seamount.
Seamounts were defined and bound by a complex system of intersecting
normal faults in Jurassic time. Because the Calcare Massiccio is such
a highly resistant unit, Oligocene-Quaternary tectonism may have reactivated
older extensional faults, as opposed to initiating new faults through
the thick seamount blocks. Preliminary measurements suggest that Monte
Pietroso and Monte Scocione are part of a large anticline that resulted
from the Oligocene-Quaternary folding and thrusting. This large anticline
may have deformed around a large seamount block in the area of Monte
Pietroso and Monte Scocione.
REFERENCES CITED
Alvarez, W., and Montanari, A ., 1988, The Scalia limestones in the
northeastern Apennines carbonate sequence: stratigraphic context and
geological significance in Premoli-Silva, I., Coccioni, R., and Montanari,
A., eds., The Eocene-Oligocene Boundary in the Marche-Umbria Baisn
(Italy), p.111-119.
Bice, D.M., and Stewart, K.G., 1990, The formation and drowning of
isolated carbonate seamounts: tectonic and ecological controls in
the northern Apennines: Spec. Publs. Int. Ass. Sediment, 9, p. 145-168.
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