Geology of the Coldigioco-Castellaro Basin in the northern Apennine
fold and thrust belt, Italy
Department of Geology, Whitman College, Walla Walla, WA 99362
The 1995 Keck Geology Consortium summer research project in the northern
Apennines of east-central Italy was based at the Osservatorio Geologico
di Coldigioco (OGC). Located in the hills of the Apennine fold and
thrust belt (see figure 1), it is ideally situated for studying geology.
Previous geologic field work in the area includes study of exhumed
Jurassic carbonate seamounts (Bice and Stewart, 1990), the K-T boundary
(Alvarez, et al., 196?? ), foreland basins (Ricci-Lucchi, 1986), and
numerous other projects. The 1995 Keck project focused on the tectonics
and sedimentology of the region. The location of the OGC provided
an ideal opportunity to study the sedimentation and structure of the
foreland basin environment. The specific goal of the study described
herein is to better understand the timing of the growth of the basin-bounding
anticlines by mapping and studying the sedimentary rocks of the basin.
The northern Apennines thrust belt is a result of complex and incompletely
understood Mediterranean microplate dynamics. Previous work has provided
a good background of the geologic history of the area. The oldest
exposed rocks are Jurassic carbonates which crop out at high elevations
in the cores of anticlines. The broad regionally extensive carbonate
platform on which they were deposited foundered in the Jurassic and
was replaced by pelagic sedimentation which remained dominant until
Early Miocene time (Calamita, et al., 1994). Miocene to Pliocene pelagic
to terrigenous sediments are exposed primarily in the basins of the
area. Tectonically, the Apennines are an east-northeast vergent fold
and thrust belt. The structures are a result of northeast-southwest
compression and shortening of the African-Adriatic continental margin
which began in the Late Oligocene and continued to migrate toward
the northeast until present (Calamita, et al., 1994; Ricci-Lucchi,
1986). Hinterland extension began at approximately the same time and
has followed the eastward migration of the compressional fronts (Lavecchia
and Brozzetti, 1994). The current model for this thrust belt is deep
seated west-dipping thrusting of multiple slabs of basement which
has resulted in crustal thickening to the east (Apennines) and thinning
to the west (Tyrrhenian) all accompanied by a counterclockwise rotation
of 60¡ to 90¡ from east to northeast (Oldow, et al., 1993;
Ghisetti and Vezzani, 1991). Foreland directed tectonic activity has
produced a series of piggy-back basins that are carried on active
thrust sheets which develop in the footwalls of older thrusts. These
basin rocks record the migration of depocenters which in turn reflect
the deep rooted thrust activity (Zoetemeijer, et al., 1992). Post-orogenic
extensional faulting following the compression has reactivated many
older structures and created intramontane basins (Calamita, et al.,
1994). The Coldigioco-Castellaro foreland basin provides an example
of a piggy-back basin to study.
The OGC is located within the basin of study. This proximity facilitated
detailed exploration of the surrounding area in search of outcrops.
Transportation was primarily by motorcycle which allowed access to
many otherwise inaccessible outcrops. Approximately 50 outcrops within
the basin were examined. At each outcrop geologic data including lithologic
descriptions, orientation of bedding, and paleocurrent indicators
was obtained. Three stratigraphic sections were measured to further
describe units and potential mechanisms of deposition.
The field area shown in Figure 2 is dominated by two northwest-southeast
trending anticlines composed of uplifted Jurassic to Early Miocene
limestone which bound the interior synclinal basin. The basin is filled
with Miocene to Pliocene sandstones, marls, and other marine sediments.
The Messinian stage of the Late Miocene is marked in the surrounding
area by gypsum-rich evaporites from a Mediterranean salinity crisis,
though is suspiciously absent from the area of study. Hence, the Miocene
may be further subdivided into pre or post-evaporitic (Odin, et al.,
1995). Much of the area is covered by Pleistocene gravels, Quaternary
alluvium, and is thickly vegetated.
The oldest unit in the research area is exposed along the flanks
of the bounding anticlines. These exposures consist of white to pink
pelagic limestone of Paleocene to Miocene age. This unit is commonly
exposed in hillside roadcuts. The outcrops are primarily comprised
of the Biscaro, Scaglia Cinerea, and Scaglia Variegata Formations
which for this study have been lumped into the unit "Paleocene to
At the southern extent of the research area, a weathered tan sandstone
unit was mapped. This unit ranged from weakly to well-cemented and
was predominantly massive with occasional concretion-rich layers.
This sandstone is of middle Miocene age and is pre-evaporitic (Odin,
et al., 1995).
A unit of clay-rich marls with occasional fine sand interbeds was
mapped. This unit has been named the Colomabacci Formation, and is
the regional marker for the uppermost Messinian. It contains a biotite-rich
tuff which yielded an age of approximately 5.5 Ma (Odin, et al., 1995).
Odin, et al., (1995) have suggested that the Colombacci Formation
be further subdivided into the Tetto Formation (clay-rich marls) and
Formation with Colombacci (clay-rich marls with thin limestone layers)
on the basis of their study of the Maccarone section (see Fig. 2),
and those of basins to the south. For the purpose of this study the
Late Miocene (Messinian) clay-rich marls have been combined into one
Pliocene rocks in the region were previously mapped as one unit by
the Italian Geologic Survey. Detailed mapping of the basin allows
the subdivision of the Pliocene into two units. The older unit consists
primarily of interbedded gray marls and fine-to medium-grained sandstones
that represent the transgressive sequence which occurs just above
the Miocene-Pliocene boundary (Odin, et al., 1995). This unit is spectacularly
exposed in the cliff south of Castellaro. Above the transgressive
unit is a resistant yellow/tan sandstone. This caprock-forming unit
forms the plateau on which Castellaro lies. The unit also comprises
the caprock on which Cupramontana is situated, as well as several
ridges in the area . In some areas, such as La Aqua, the resistant
sandstone unit lies unconformably over Miocene marls. The youngest
unit mapped consists of Pleistocene gravels and Quaternary alluvium
which have been grouped together for this study.
OBSERVATIONS AND INTERPRETATIONS
The rocks within the basin generally young to the north due to the
northwest plunge of the regional structures. Paleocurrent indicators
show an approximate south/southeast direction of deposition into the
basin. Many of the basin rocks are easily erodable, of which the badlands-forming
marls of Messinian age provide the best example.
The trend of the bounding anticlines is primarily northwest-southeast.
The southern extent of the basin forms a syncline also trending northwest-southeast,
as is indicated by a diagramatic cross-section of the basin (Figure
Generally, the dip of the rocks is steeper in the southern part of
the basin. This relationship corresponds with the southwest to northeast
direction of thrusting (Calamita, et al., 1994), as the older rocks
within the southern part of the basin have been more extensively deformed.
Perhaps this deformation is a result of the bounding thrust extending
into the basin sediments in the southern part of the basin. The basin
sedimentary rocks have a similar attitude to the flanks of the bounding
anticlines, and the older rocks at the edges of the syncline have
steeper dips than the younger rocks in the core because they have
experienced a longer period of deformation. These relationships indicate
that the anticlines experienced growth during and since the Late Miocene-Pliocene.
The oldest sandstones of the basin, exposed just north of Coldigioco,
and just south of Moscosi are locally folded and have steep dips.
Debris flows, interpreted to have resulted from the over-steepening
of slopes on the flanks of the growing anticlines, are present within
the vicinity of these exposed folds. Younger rocks in the basin are
not folded. The folds and associated debris flows within the southeastern
rocks of the basin suggest that the anticline to the east experienced
events of rapid growth and thrusting during the Miocene. The dips
of the Pliocene sandstone shallow in the northern part of the basin,
perhaps in response to the slowing or cessation of growth of the anticlines,
or as a result of being carried piggy-back without further deformation.
A northeast-trending normal fault was observed cutting the Messinian
marls. This fault offsets the biotite-rich tuff of the Colombacci
Formation. Normal faulting in this area is presumed to be a result
of extensional tectonics which followed compression.
During the Miocene and Pliocene, the basin remained submerged as
the two bounding anticlines, which were seamounts at this time, were
emerging. Deposition was from a northern source, as indicated by paleocurrent
measurements. The current Bosnian coastline (Dalmation coast), which
consists of a series of elongate northwest-trending seamount islands,
provides a modern analog. Perhaps studying the sedimentation of this
area would provide insight into the depositional environment of the
foreland basins of the Apennines.
The field area has rugged topography and though the area is inhabited
and farmed, extensive vegetation caused difficulty in reaching outcrops.
Many of the best outcrops are virtually inaccessible due to rivers,
dense vegetation, and steep and often muddy slopes. In these conditions,
even well-exposed outcrops may be out of reach. Pleistocene gravel,
predominantly flanking the anticlines, obscures many contacts, as
well as possible faults.
Perhaps the biggest unsolved question concerns the provenance of
the sandstones. Of the dominant lithologies mapped in the basin, the
sandstone has no local source. The Alps have been identified as a
possible source area (Stewart, 1987; Ricci-Lucchi, 1986), but no suitable
method of transport has been determined. Perhaps reworking of older
turbidites shed from the growing anticlines would best fit this situation
(Ricci Lucci, 1986; Bice, personal communication, 1996). CONCLUSIONS
The Coldigioco-Castellaro basin contains a sequence of sedimentary
rocks reflecting the evolution of the area from Late Miocene to Pliocene
time. Structural relationships indicate that both of the bounding
anticlines were actively growing during deposition of these rocks,
and perhaps after. The shallowing dips of younger rocks in the northern
portion of the basin indicate that the western anticline experienced
a quiescence or slowing of growth during the Pliocene, or perhaps
more likely that they were carried piggyback without becoming deformed
as the anticlines continued to grow.
I would like to thank the Keck foundation and Whitman college for
their support of this research, all those at OGC for everything, as
well as Dave Bice, Julie Maxson, and Paul Myrow for field support.
Charlotte Briggs made life enjoyable, thank you! Thanks goes to Maya
Del Margo and Colleen Dunlevy for their input. Many thanks to Kevin
Pogue and to all my teachers.
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