Sedimentary Geology 139 (2001) 217±228
www.elsevier.nl/locate/sedgeo
The sedimentary record of the exhumation of a granitic intrusion
into a collisional setting: the lower Gonfolite Group,
Southern Alps, Italy
B. Carrapa a,1, A. Di Giulio b,*
a
Faculteit der Aardwetenschappen, De Boelelaan1085, 1081 HV Amsterdam, The Netherlands
b
Dipartimento di Scienze della Terra, UniversitaÁ di Pavia, via Ferrata 1, 27100 Pavia, Italy
Received 10 January 2000; accepted 30 August 2000
Abstract
The clastic wedge of the Gonfolite Lombarda Group (GLW) accumulated during Oligocene±Miocene times in the Southern
Alps foreland basin, formed on the southern, inner side of the Alpine belt. It represents the depositional counterpart of the
exhumation and erosion of the Central Alps metamorphic±magmatic units.
Among the Central Alps units, the Tertiary Bergell Intrusion (TBI) is one of the principal sources of pebbles occurring within
the GLW. Geochronologic data, both from intrusive pebbles and present-day outcrops of intrusive rocks, document the rapid
uplift history of the GLW source area.
The lower Gonfolite clastic wedge (Como Conglomerate and Val Grande Sandstone Formations, Oligocene±Early Miocene)
has been investigated through the study of sandstone and conglomerate petrology for detecting the effects in the sedimentary
record of this collision-related event.
The main results are: (i) sandstone petrology of the Como Conglomerate records an evolution from feldspatholithic to
feldspathic sandstones; (ii) the related Q/F±F/L ratios suggest an evolution from a mixed plutonic±metamorphic to a mainly
plutonic source; (iii) consistently, conglomerate petrology records a progressive increase of plutonic pebbles (from nearly
0±50% of the total), a corresponding decrease of metamorphic clasts (from nearly 80 to nearly 50%) and the disappearance of
cover rock fragments. Considering the high relief/short transport setting of the GLW clastic routing system, these values
probably resemble the real proportions of such rocks in the Gonfolite catchment area.
During the Aquitanian, the return to a metamorphic-rich source is recorded both by sandstones and conglomerates at the top
of the Como Conglomerate and in the Val Grande Sandstone. This last signal is interpreted as the result of the reorganisation of
the Gonfolite source area, possibly related to the northward shift of the main Alpine divide. q 2001 Elsevier Science B.V. All
rights reserved.
Keywords: Sandstone; Conglomerate; Petrography; Collision; Exhumation; Southern Alps
1. Introduction
* Corresponding author. Tel./fax: 139-382-505890.
E-mail addresses: carb@geo.vu.nl (B. Carrapa),
digiulio@unipv.it (A. Di Giulio).
1
Tel.: 131-20-444-7403; fax: 131-20-646-2457.
In most cases, the clastic ®ll of sedimentary basins
is the main source of information concerning the
changing paleogeology of an orogenic wedge during
its uplift and erosion. Provenance studies of sediments
0037-0738/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S 0037-073 8(00)00167-6
218
B. Carrapa, A. Di Giulio / Sedimentary Geology 139 (2001) 217±228
2
1a
4
3
a
A
6
5
b
b
PF
p
edee
e for
ss
Mola
Jura
NCA
TW
HE
A
AU
GO
HE
MB
SA
Ad
B
DB
IVR
SA
STUDIED AREA
BE
Po plain
PE
AP
EN
NIN
ES
AG
nce
Prove
100
0 Km
ADRIATIC
SEA
B
N
HE
MO
PF
PA
NCA
AU
VA
1
A
A
B 2
GO
A
3
4
B
5
PA
AU
SU
TA
AD
B
MA
PL
S
STUDIED AREA
SA
10
20
Km
B. Carrapa, A. Di Giulio / Sedimentary Geology 139 (2001) 217±228
fed by collisional belt generally successfully recognize the types of rocks eroded in different time spans,
but hardly succeed in evaluating their relative amount,
exact location and volume. This prevents them from
reaching reliable paleogeologic reconstructions of the
belt at different times.
In this paper, the provenance study of the lower part
of the Gonfolite Lombarda clastic wedge (GLW)
attempts to estimate the relative contributions of
various rock types eroded from a well constrained
source area, located in the rapidly uplifting Central
Alps. This is the starting point for improving the
reconstruction of the geological evolution of that
part of the belt, through the image re¯ected by its
erosion products.
2. Geological framework
The GLW constitutes the Oligocene±Miocene ®lling of the foredeep formed at the southern edge of the
Alpine belt, due to the backthrusting of the Southern
Alps tectonic units onto the Padan±Adriatic crust. It
crops out in an east±west trending zone, approximately 200 km long and 4 km wide, homoclinally
dipping southward under the Quaternary sediments
of the Po River Plain, between Lake Maggiore
(west) and Lake Como (east; Fig. 1).
The Oligocene±Miocene part of the wedge is
composed of deep-sea coarse-grained clastic deposits
interbedded with hemipelagic marls. Data from
benthic foraminifera point out a bath depositional
setting for the entire Oligocene±Miocene part of the
wedge, yal with depths ranging from 500±700 m to
1000±1300 m (Gelati et al., 1988).
This study focuses on the coarse-grained deposi-
219
tional bodies forming the bulk of the lower part of
the clastic wedge: the Como Conglomerate and Val
Grande Sandstone Formations (Fig. 2), which have a
combined maximum stratigraphic thickness of nearly
3 km.
The Chattian±Aquitanian Como Conglomerate is
the lowermost unit of the GLW. It represents a deep
sea, coarse-grained, turbidity complex, ®lling a largescale submarine erosion surface, incised in the underlying Chiasso Formation; as a result, the thickness of
the Como Conglomerate varies between 800 and
1500 m (Gelati et al., 1988). It is mainly composed
of coarse to medium grain-supported amalgamated
conglomerate beds, matrix-supported conglomerates,
massive sandstones and occasionally thin sandstone±
mudstone bed sets (Gelati et al., 1988, 1992).
In the Como area, a mudstone unit (Prestino Marl,
upper Chattian±Aquitanian), which pinches out westward, overlies the Como Conglomerate. This unit is
mainly composed of low-density mud-rich turbidite
beds interspersed with hemipelagic marls reaching a
maximum thickness of 450 m. The Prestino Marls, in
turn, are overlain and laterally substituted by the Val
Grande Sandstone, having a maximum thickness of
700 m (Gelati et al., 1992); it is mainly composed of
sandy turbidites and sandstone±mudstone turbidite
couplets, with subordinate gravelly sandstone facies,
clast supported conglomerates and pebbly mudstones.
As a whole, it is interpreted as a sandy lobe facies
association produced by low- to high-density turbidity
currents (Gelati et al., 1992).
Most authors have agreed for many years (e.g. Fiorentini Potenza, 1957; Giger and Hurford, 1989;
Bernoulli et al., 1993; Bersezio et al., 1993) that the
source area of the GLW was located in the Central
Alps, and speci®cally in the Valtellina region. This
Fig. 1. (A) Tectonic sketch map of the Alps including distribution of eoalpine metamorphism and (B) schematic cross-section of Central Alps
(both slightly modi®ed after Polino et al., 1990); location of the studied area in the box. Symbols of sketch (A): 1 ˆ Cretaceous blueschist (a)
and eclogitic (b) metamorphism in the continental units of Western and Central Alps, including the Eocene assemblages of Grand St. Bernard
System; 2 ˆ Cretaceous very low grade (a) and greenschist to amphibolite facies (b) metamorphism in Austroalpine cover and basement units;
3 ˆ Cretaceous±Eocene ¯ysch and Gosau beds; 4 ˆ undifferentiated ophiolite units; 5 ˆ main tertiary intrusions along the Periadriatic fault
system (B ˆ Bergell intrusion; Ad ˆ Adamello intrusion); 6 ˆ Perialpine and intramontane Oligocene±Miocene basins; AU ˆ eastern
Austroalpine cover and basement nappes; TW ˆ Tauern Window; HE ˆ Ultrahelvetic, Helvetic and Dauphinois units; AG/PE/BE/MB/A/
GO ˆ Argentera, Pelvoux, Belledonne, Mt. Bianco, Aar, Gottardo external crystalline massifs; NCA ˆ Northern Calcareous Alps; SA ˆ
Southern Alps; PF ˆ Penninic Thrust Front. Symbols of sketch (B): 1 ˆ Eastern Austroalpine system (AU) with Eoalpine very low grade
(a) and greenschist-amphibolite facies (b) metamorphism; 2 ˆ Platta-Arosa (PA) and Malenco-Avers (MA) Piedmont ophiolite units and
Valais (VA) ophiolite and ¯ysch units; 3 ˆ ¯ysch decollement units (mostly Cretaceous); 4 ˆ Tertiary European molasse (a; MO) and Po Plain
molasse (b; Gonfolite group); 5 ˆ Tertiary Bergell (B) Periadriatic intrusion; PF ˆ Penninic Thrust Front.
220
B. Carrapa, A. Di Giulio / Sedimentary Geology 139 (2001) 217±228
CHRONO
STRATIGRAPHY
Bio
strati
graphy
Ma
GELATI
et l.1988
HARLAND
et al. 1990
GONFOLITE GROUP
W
VARESE
CASTIGLIONE O.
STAGES
Burdigalian
N6/N5
21.5
Aquitanian
EARLY MIOCENE
Lurate-Caccivio Fm.
?
LUCINO CONGLOMERATE
VAL GRANDE
Val Grande SANDSTONE
San Fermo D. Battaglia
Prestino
Prestino
Mudstone
Mudstone
Belforte Mudst.
23.3
29.3
OLIGOCENE
Chattian
P20/19?
Montano M.
Erosive hiatus
COMO CONGLOMERATE
?P22
P21
COMO
LUCINO
CHIASSO
? N7
N4
E
Rupelian
St.Stefano
Baradello
SP 17
Rio Maiocca
Mt.Olimpino
Truncation
CHIASSO FM.
Villa Olmo Conglomerate
Villa Olmo
Fig. 2. Lithostratigraphic scheme of the Gonfolite Lombarda Wedge (redrawn after Gelati et al., 1988) and stratigraphic/geographic location of
the studied sections (vertical bars). Time scale after Harland et al. (1990).
conclusion is based primarily on the recognition in the
Como Conglomerate of unmistakable pebbles sourced
by the Bergell Intrusion; and, more recently, also in
the small conglomerate lenses (Villa Olmo Conglomerate) interbedded in the underlying Chiasso Formation (Giger and Hurford, 1989).
Therefore, radiometric data regarding the uplift
history of the Bergell Intrusion can provide fundamental constraints to aid in unravelling the paleogeologic evolution of the lower Gonfolite source area.
3. The uplift of the source area: the exhumation
history of the Bergell Intrusion
Among the Tertiary Alpine intrusions, the Bergell
igneous body is crucial for the understanding of
Central Alps tectonics. It exhibits 10 km of exposed
intrusion levels, which, coupled with its presence as
boulders in the Gonfolite clastic wedge, provides the
uncommon opportunity of studying an intrusive body
and its uplift/erosion products over a considerable
crustal thickness and a relatively long time span.
This is the reason why it has attracted numerous
geochronological studies in the last 40 years, concerning both its present-day outcrops and boulders from
the intrusion occurring in the Gonfolite succession
(e.g. see Giger and Hurford, 1989; von Blanckenburg,
1992; Bernoulli et al. 1993; Hansmann, 1996 for a
complete review).
Present-day outcrops of the intrusion provide
geochronological cooling ages (K/Ar hornblende
and biotite ages and apatite FT ages) signi®cantly
younger than those provided by the Bergell boulders
embedded in Oligocene Gonfolite deposits (Giger and
Hurford, 1989; Hansmann, 1996). This means that a
considerable vertical distance existed between the
present-day and the Oligocene erosion levels of the
intrusion, so that erosion of an intrusive vertical
column of up to 26 km has been reported since the
earliest deposition of Bergell boulders at about 29 Ma
(Hansmann, 1996).
The Oligocene exhumation rate of the Bergell
intrusion derived from the cooling history recorded
by boulders embedded in the Como Conglomerate is
very high, with a cooling rate .1008C/Ma in some
cases, corresponding to an uplift rate .3 mm/yr for a
geothermal gradient of 308C/km (Giger and Hurford,
B. Carrapa, A. Di Giulio / Sedimentary Geology 139 (2001) 217±228
1989). According to the cited authors, these rapidly
cooled boulders probably originated from the western,
more deeply eroded, part of the Bergell body, where
only the deepest roots of the intrusion are currently
exposed (Fig. 1).
In addition, both the currently exposed intrusive
rocks and the plutonic boulders record a very short
time interval between the emplacement of the intrusion and its exhumation. This has been interpreted as
the result of an Eocene slab breakoff event in the
Alpine evolution (von Blanckenburg and Davies,
1995).
In this framework, the interpretation of the data
concerning the oldest Oligocene coarse-grained clastic unit (Villa Olmo Conglomerate) is still troublesome. The K/Ar biotite cooling age of 31.7 ^ 0.5
obtained from a boulder by Giger and Hurford
(1989), obviously contrasts with the earliest Oligocene biostratigraphic age (32±33.5 Ma) recently
reported as preliminary results for the conglomerate
221
unit (Valdisturlo et al., 1998). At present, a re®nement
both of radiometric and biostratigraphic data is necessary to solve this problem. What seems certain is that
the exhumation of the Bergell intrusion was very rapid
during Oligocene time.
This work investigates how this rapid exhumation
is recorded in its depositional counter-part, i.e. in the
lower part of GLW, the only remaining record of
Oligo±Miocene geological history concerning the
source area.
4. The sedimentary counterpart: petrography of
the lower Gonfolite Group
4.1. Facies, material and methods
The study of sandstone and conglomerate detrital
mode evolution was undertaken in the Como area
along eight stratigraphic sections covering the whole
Table 1
Detrital modes of pebble/cobble-size fraction of the Villa Olmo and Como Conglomerate
Conglomerate detrital mode
Formation
Stratigraphic section
Base
V. Olmo CGL
V. Roscui
Como CGL
Mt. Olimp.
Como CGL
R. Maiocca
Como CGL
SP17
Como CGL
Baradello
Como CGL
S. Stefano
Top
Como CGL
S. Fermo B.
Pebble type
Sandstones
Dolostones
Mesozoic limestones
Nummulitic limestones
Mesozoic sedimentary rocks
Cainozoic sedimentary rocks
Total sedimentary rocks
Granitoid plutonic rocks
Total plutonic rocks
Leucocratic porphiric rocks
Mesocratic porphiric rocks
Aplites
Rhyolites
Total volcanic/shallow
intrusive rocks
Marbles
Quartzites
Slates
Fine-grained greenschists
Paragneiss
Anphibolites
Orthogneiss
Total metamorphic rocks
Total determined pebbles
(%)
0.0
0.0
11.0
6.0
11.0
6.0
17.0
0.0
0.0
2.0
3.0
1.0
0.0
6.0
(%)
0.0
3.0
6.0
5.0
9.0
5.0
14.0
0.0
0.0
2.0
0.0
0.0
2.0
4.0
(%)
1.0
4.0
19.8
4.0
24.8
4.0
28.7
20.8
20.8
0.0
0.0
2.0
1.0
3.0
(%)
0.0
0.0
0.0
1.1
0.0
1.1
1.1
24.5
24.5
3.2
0.0
0.0
0.0
3.2
(%)
0.0
0.0
1.2
1.2
1.2
1.2
2.4
23.2
23.2
3.7
1.2
0.0
0.0
4.9
(%)
0.0
0.0
0.0
0.0
0.0
0.0
0.0
47.0
47.0
0.0
6.0
0.0
0.0
6.0
(%)
0.0
0.0
1.0
5.0
1.0
5.0
6.0
30.0
30.0
2.0
1.0
0.0
0.0
3.0
0.0
11.0
2.0
11.0
0.0
0.0
53.0
77.0
100.0
0.0
12.0
0.0
13.0
0.0
1.0
56.0
82.0
100.0
4.0
8.9
8.9
7.9
1.0
0.0
16.8
47.5
100.0
0.0
6.4
16.0
19.1
0.0
1.1
28.7
71.3
100.0
0.0
8.5
6.1
26.8
0.0
1.2
26.8
69.5
100.0
0.0
3.0
2.0
4.0
1.0
2.0
35.0
47.0
100.0
0.0
9.0
16.0
7.0
0.0
28.0
1.0
61.0
100.0
222
B. Carrapa, A. Di Giulio / Sedimentary Geology 139 (2001) 217±228
Como Conglomerate and Val Grande Sandstone stratigraphic interval (seven sections in the conglomerate
and one 180 m thick section in the sandstone). A ninth
section was measured in the Rupelian Villa Olmo
Conglomerate, a small coarse-grained, lens-shaped
channel-®ll, interbedded in the underlying marly
Chiasso Formation (Valdisturlo et al., 1998; Fig. 2).
In each stratigraphic section, facies analysis as well as
conglomerate and sandstone petrology were studied.
This paper concentrates on sandstone and conglomerate petrology.
The Villa Olmo and Como Conglomerates are
mostly composed of amalgamated grain supported
conglomerate and pebbly sandstone beds deposited
by hyperconcentrated ¯ows and gravely high-density
turbidity currents in a channel ®ll depositional
context. Conglomerate beds are commonly composed
of pebbles ranging in diameter from 5 to 20 cm, with
regular occurrences of boulders up to 1 m in diameter.
Conversely, the Val Grande section is composed
mainly of alternating sandstone±mudstone turbidites
deposited by low-density turbidity currents or massive
sandstone beds and occasionally ®ne, grain-supported
conglomerates related to sandy and ®ne gravely highdensity turbidites. This facies association is interpreted
as the result of deposition in a sand-rich, low-ef®ciency
turbidite system (Gelati et al., 1992).
Pebble composition was studied in all sections
except for the Val Grande Sandstone, which was
omitted due to the lack of suitable coarse conglomerate. The composition was determined directly in the
®eld by counting pebbles of different lithology occurring within a 50 £ 50 cm 2 grid, having individual
10 £ 10 cm 2 square. The grid was moved laterally,
on the same bed, at least four times in order to have
a minimum count of 100 pebbles. One count was
made in each studied section.
Sandstone detrital modes were studied by thin
section point counts performed on 44 sandstone
samples according to the G±D method (Gazzi,
1966; Dickinson, 1970). In each thin section, a double
point count was performed. The ®rst count considered
Fig. 3. Histogram showing the evolution of the conglomerate detrital mode from the Villa Olmo Conglomerate to the lower Gonfolite
Lombarda clastic wedge (Como Conglomerate).
B. Carrapa, A. Di Giulio / Sedimentary Geology 139 (2001) 217±228
223
Table 2
Framework detrital modes and ®ne-grained rock fragment compositions of sand-size sediment fractions of Villa Olmo Conglomerate, Como
Conglomerate and Val Grande Sandstone Formations
Sandstone framework detrital
mode
Formation
Sections
Number of samples
Monocrystalline quartz (right
ext)
Monocrystalline quartz
(unduluse ext)
Coarse grained polycrystalline
quartz
Fine-grained polycrystalline
quartz
Chert
Qz in Plutonic RF
Qz in Gneissic RF
Qz in other metamorphic RF
Qz in volcanic RF
Base
VOCGL
Villa Olmo
Top
COCGL
Mt. Olimpino
COCGL
R. Maiocca
COCGL
SP 17
COCGL
Baradello
COCGL
St. Stefano
COCGL
S. Fermo
VGSD
V. Grande
3
3.2
3
0.5
6
0.7
5
1.2
7
1.3
5
0.5
3
0.3
12
1.7
9.3
1.3
3.9
4.9
6.1
3.9
3.5
5.0
8.2
4.2
6.3
3.4
2.9
5.3
3.4
3.7
2.1
0.4
0.8
0.2
1.1
0.4
0.4
0.6
0.8
2.2
2.8
4.8
0.0
0.7
0.1
3.0
7.6
0.0
0.2
0.0
3.2
7.9
0.0
0.1
0.3
3.5
7.8
0.2
0.0
1.2
6.9
3.6
0.3
0.0
0.0
6.9
4.7
0.0
0.0
0.2
4.0
6.8
0.0
0.1
0.3
5.0
5.7
0.0
Monocrystalline K-feldspar
K-feldspar in Plutonic RF
K-feldspar in Gneissic RF
K-feldspar in other
metamorphic RF
K-feldspar in volcanic RF
10.2
1.5
3.1
3.7
4.8
0.2
7.8
13.0
4.7
0.0
7.7
6.5
9.5
0.1
7.0
7.5
11.3
2.8
9.6
3.2
9.1
0.2
16.2
8.9
8.2
0.4
7.0
12.2
9.9
1.1
8.1
6.6
0.1
0.0
0.0
0.0
0.2
0.0
0.0
0.0
Monocrystalline Plagioclase
Plagioclase in Plutonic RF
Plagioclase in Gneissic RF
Plagioclase in other
metamorphic RF
Plagioclase in volcanic RF
3.3
0.4
0.1
0.4
0.2
0.0
0.0
0.1
0.5
0.0
0.7
0.2
2.4
0.1
1.4
0.7
3.8
0.7
1.4
0.5
1.6
0.1
2.0
0.7
1.4
0.0
1.0
0.5
2.9
0.4
1.2
0.8
0.2
0.0
0.0
0.0
0.0
0.0
0.1
0.0
Fine-grained metamorphic
rock fragment
Fine-grained volcanic
rockfragment
Fine-grained clastic rock
fragment
Calcareous rock fragment
Dolomitic rock fragment
Total essential grains
12.9
20.4
18.3
26.7
20.5
13.6
25.0
19.5
1.7
1.1
0.9
0.4
0.7
0.1
0.1
0.0
0.6
0.0
0.0
0.0
0.0
0.0
0.0
0.0
1.5
1.7
74.3
1.2
1.4
68.0
0.6
0.1
63.5
0.2
0.0
77.6
0.0
0.0
78.2
0.1
0.0
74.4
0.0
0.0
74.2
0.8
0.1
73.7
Biotite and Clorite
Muscovite
Phyllosilicates in RF
Heavy mineral
Heavy mineral in RF
Mud clast
Bioclast
Undetermined carbonate grain
Alterite
Total accessory grains
Total framework grains
4.0
1.9
1.4
0.4
0.1
0.0
0.0
0.1
0.7
8.5
82.8
2.9
2.3
1.9
0.6
0.4
0.1
0.0
0.1
1.4
9.6
77.7
5.9
4.0
0.9
0.8
0.2
0.0
0.0
0.0
1.6
13.5
77.0
2.4
1.7
0.9
0.4
1.0
0.0
0.0
0.0
1.0
7.3
84.9
2.7
0.8
1.5
0.7
1.2
0.0
0.0
0.0
0.8
7.7
85.8
1.9
1.1
1.6
0.5
0.8
0.0
0.0
0.0
0.9
6.7
81.1
2.2
1.6
1.4
2.2
0.3
0.0
0.0
0.0
1.0
8.7
82.9
2.6
1.2
1.1
0.5
0.5
0.1
0.1
0.0
1.0
7.0
80.7
224
B. Carrapa, A. Di Giulio / Sedimentary Geology 139 (2001) 217±228
Table 2 (continued)
Sandstone framework detrital
mode
Formation
Sections
Silty matrix
Muddy matrix
Qz overgrowth
Phyllosilicate cement
Calcite cement
Other authigenic minerals
Authigenic mineral on
undetermined grain
Intragranular secondary pore
Intergranular pore
Calcite patch
Oversized pore
Total rock
Detrital mode of lithic grains
Quartzite
Qz±Bi schist
Qz±Mu schist
Microcrystalline mica
Green schist
Slate
Vitric volcanic
Acid volcanic
Intermediate and basic
volcanic
Shale
Siltstone
Detritic limestone
Mudstone±Wackestone
Packstone±Grainstone
Sparitic limestone
Dolostone
Total ®ne-grained rock
fragment
Base
VOCGL
Villa Olmo
Top
COCGL
Mt. Olimpino
COCGL
R. Maiocca
COCGL
SP 17
COCGL
Baradello
COCGL
St. Stefano
COCGL
S. Fermo
VGSD
V. Grande
0.1
0.7
0.1
2.8
6.7
0.1
0.1
0.5
0.5
0.0
2.7
8.5
0.0
0.7
1.5
3.5
0.0
7.3
6.1
0.0
0.6
1.1
1.3
0.0
3.7
2.4
0.0
0.1
0.9
1.9
0.0
3.7
0.0
0.0
0.0
1.1
1.7
0.0
3.8
2.0
0.4
0.2
0.5
0.9
0.0
2.6
1.1
0.0
0.0
1.0
1.1
0.0
2.2
7.0
0.0
0.3
0.3
3.7
1.4
1.3
100.0
0.5
5.5
3.4
0.0
100.0
0.5
2.5
0.7
0.3
100.0
0.9
3.2
0.0
2.5
100.0
1.3
3.5
0.0
2.9
100.0
0.6
4.0
2.8
2.3
100.0
0.1
6.7
0.1
5.0
100.0
0.3
1.3
5.9
0.3
100.0
0.3
19.8
30.4
2.4
4.3
6.4
1.3
0.6
12.5
0.3
35.4
43.4
0.7
1.7
1.7
0.2
0.0
6.2
1.4
28.1
42.8
0.1
5.8
9.3
3.4
0.0
0.5
0.6
33.7
31.8
0.0
24.9
4.0
3.8
0.0
0.2
0.5
30.8
36.5
0.0
20.1
5.5
4.6
0.0
0.6
0.6
32.5
34.1
0.1
28.5
2.1
1.4
0.0
0.2
5.2
36.0
32.1
0.2
24.3
1.7
0.0
0.2
0.5
0.9
29.6
35.3
0.1
23.4
3.4
5.3
0.0
0.1
0.0
1.6
2.6
0.7
4.8
12.2
0.0
100.0
0.0
0.0
0.0
0.3
4.3
5.7
0.0
100.0
4.6
0.5
0.4
1.3
1.3
0.7
0.0
100.0
0.9
0.0
0.0
0.1
0.0
0.0
0.0
100.0
1.3
0.0
0.0
0.0
0.0
0.0
0.0
100.0
0.0
0.0
0.0
0.2
0.2
0.0
0.0
100.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
100.0
0.1
0.1
0.0
0.6
1.1
0.0
0.0
100.0
all constituents and at least 250 essential framework
grains were counted within each thin section. The
second count was focused only on ®ne-grained rock
fragments (i.e. with internal constituents ,0.062 mm,
according to the G±D method) and at least 200 of
such grains were determined in each sample.
4.2. Conglomerate petrology
The results of pebble counts are reported in Table 1;
as a whole they point to a source area composed
mostly of metamorphic and plutonic rocks. Among
the metamorphic rocks, orthogneiss, greenschists
and quartzites are the most recurrent rock types,
together with amphibolites, which occur in strongly
variable amounts. As a whole, their provenance from
the metamorphic units of Central Alps and Southern
Alps basement seems clear (Longo, 1968). Plutonic
rocks are mostly composed of granodiorites sourced
by the Bergell Intrusion. In the Villa Olmo Conglomerate and in the very base of the Como Conglomerate
they occur in very small amounts (Giger and Hurford,
1989; Bernoulli et al., 1993).
Volcanic/hyperbyssal rocks are present in small
B. Carrapa, A. Di Giulio / Sedimentary Geology 139 (2001) 217±228
225
Q
VGS
Lm
COC
Voc
VGS
VOC
Co2
Co5
Co4
Co1
Co3
Q/
F1
:1
Co6
F
L+C
Lv
Ls+C
Fig. 4. Evolution of the sandstone detrital modes from the Villa Olmo Conglomerate to the lower Gonfolite Lombarda formations (Como
Conglomerate and Val Grande Sandstones). VOC ˆ Villa Olmo Conglomerate; COC ˆ Como Conglomerate; Co1 ˆ M. Olimpino section;
Co2 ˆ Rio Maiocca section; Co3 ˆ SP 17 section; Co4 ˆ Baradello section; Co5 ˆ S.Stefano section; Co6 ˆ S. Fermo section; VGS ˆ Val
Grande Sandstone.
amounts; comprising both leucocratic and mesocratic
porphyrites, possibly sourced at least in part by
Tertiary dyke swarms cross cutting Southern Alps
units (Fantoni et al., 1999, and references therein).
Sedimentary clasts are mainly represented by limestone and dolostone pebbles sourced both by Mesozoic covers and Paleogene nummulitic limestones.
Pebble counts record an evolution of the relative
amount of these rock types in the Villa Olmo±
Como Conglomerate succession, re¯ecting the geologic evolution of the source area (Fig. 3). The pebbles
fed by the Bergell intrusion are very few (,1%) both
in the Villa Olmo Conglomerate and in the very base
of Como Conglomerate, where its erosion of the
underlying Chiasso Formation is greater (Mt. Olimpino section). In contrast, Bergell clasts abruptly
increase upsection, and they are also abundant in the
basal Como Conglomerate, where erosion of the
Chiasso Formation is less (Rio Maiocca section).
This increase of the Bergell contribution continues
up to the S. Stefano section, where it represents almost
half of the total pebble population; upward, it abruptly
decreases in the topmost analysed section of the Como
Conglomerate.
The increase of plutonic pebbles is counterbalanced
by a decrease of metamorphic clasts and by the
progressive decrease of pebbles from sedimentary
rocks, which progressively disappear up to the S.
Stefano section, but reoccur at the top of the formation
(Fig. 3).
These data re¯ect the geological evolution of the
GLW source area in the Oligocene±Early Miocene. It
quite rapidly evolved from a metamorphic dominated
high relief area, with remains of Mesozoic cover and
226
B. Carrapa, A. Di Giulio / Sedimentary Geology 139 (2001) 217±228
3,5
Q/F
3,0
Metamorphic vs. Granitic source
according to key petrologic ratios
Sourc e reorganisation
F/L+C
2,5
2,0
1,5
1,0
base
GLW Sandstones
top
Pure Granitic
Pure
Metamorphic
V.Grande
S.Fermo
St. Stefano
Baradello
SP 17
R.Maiocca
Mt. Olimpino
0,0
Villa Olmo
0,5
Calabrian Arc
Modern
Sands
Fig. 5. Comparison of key petrologic ratios of studied Oligocene±Early Miocene formations, and modern analogues provided by the Calabrian
Arc, for purely plutonic sourced sands and purely metamorphic sourced sands (Calabrian arc data, after Valloni in Ibbeken and Schleyer, 1991).
very few Tertiary plutonic rocks, to a plutonic±metamorphic massif composed largely of the exhumed
Bergell intrusion, virtually without meaningful
evidence of cover remains.
This trend is inverted in the topmost section in the
Como Conglomerate (S. Fermo section; Fig. 3), where
an increase of metamorphic pebbles is recorded, counterbalanced by a decrease of clasts fed by the Bergell
Intrusion. This event occurred in a poorly constrained
interval during the Early Miocene.
4.3. Sandstone petrology
According to Dickinson's (1970) classi®cation,
sandstones of the Lower GLW are feldspatholithic
sandstones. Monocrystalline and polycrystalline
quartz grains, coarse-grained crystalline rock fragments and K-feldspars form the bulk of framework
grains, with minor amounts of plagioclase and ®negrained crystalline rock fragments. Fine-grained
volcanic and sedimentary rock fragments occur
more rarely (Table 2). This average composition
re¯ects a mainly plutonic±metamorphic source, as
shown by the conglomerate detrital modes.
Within this general picture, the sandstone composition also reveals a relatively regular stratigraphic
variation from a lithic±feldspathic composition
(Villa Olmo Conglomerate) to feldspathic sandstones
in the middle±upper Como Conglomerate. The
topmost part of the Como Conglomerate and the
overlaying Val Grande Sandstones record a break
in the trend and a return to a more lithic composition
(Fig. 4).
This evolutionary pattern of sandstone composition
mirrors that in the conglomerates. It re¯ects the fast
exhumation of the Bergell plutonic massif during the
Chattian±Aquitanian, and an abrupt change in
the ratio between plutonic/metamorphic rocks in the
source area in the Early Miocene. Additional support
to this interpretation is provided by comparison
(Fig. 5) of the collected data with the information
on modern sands of the Calabrian arc, having a pure
plutonic or metamorphic source (Valloni in Ibbeken
and Schleyer, 1991).
B. Carrapa, A. Di Giulio / Sedimentary Geology 139 (2001) 217±228
227
Q
CRATON
INTERIOR
B
TRANSITIONAL
CONTINENTA L
RECYCLED
OROGEN
DISSECTED ARC
A
BA
SE
ME
NT
UP
LIF
T
C
TRANSITIONAL
ARC
UNDISSECTED ARC
F
L
Fig. 6. Compositional trends related to the exhumation of a granite crust in different geodynamic settings: (A) ˆ trend related to the dissection
of a volcanic arc in an arc-trench system (after Dickinson, 1985); (B) ˆ trend related to a basement uplift in a continental block setting (after
Dickinson, 1985); (C) ˆ trend related to the exhumation of syn-orogenic intrusions in a collisional setting (present paper).
5. Conclusions
Conglomerate and sandstone detrital modes of the
lower GLW consistently record an increasing input
from plutonic rocks during the Chattian±early Aquitanian, re¯ecting the very fast exhumation of the
Bergell intrusion during that time. Contribution from
plutonic rocks increased from nearly 0 to almost 50%,
counterbalanced by a decreasing contribution from
metamorphic rocks from nearly 80 to 50% and by a
progressive elimination of cover rock contribution.
In terms of source area geology, assuming a
comparable contribution from metamorphics and
plutonic rocks per unit area of exposure, this means
a plutonic/metamorphic ratio in the source area
increasing from nearly 0 to .1 in an approximately
6 Ma time span.
According to geochronology, it represents the sedimentary counterpart of an impressively fast uplift of
the Bergell intrusion immediately after its emplacement (up to 13 km in just a few Ma; Giger and
Hurford, 1989; Hansmann, 1996), probably following
a slab breakoff event (von Blanckenburg and Davies,
1995).
A break in this plutonic-enriching trend is recorded
in the uppermost part of the studied sequence,
re¯ecting a source area re-organization during the
Aquitanian. This was probably related to a regional
tectonic phase, and possibly to a shift in the main
Alpine drainage divide (Schlunegger et al., 1998;
Kuhlemann, 1999). It roughly correlates with sedimentary unconformities recorded in several other
Alpine-fed clastic wedges of the Southern Alps and
the Northern Apennines (Castellarin et al., 1992).
228
B. Carrapa, A. Di Giulio / Sedimentary Geology 139 (2001) 217±228
Thus, the GLW provides the record of the effects
of the exhumation and erosion of an orogenic intrusion in the related clastic sediments. Both the
intrusion and its fast exhumation were probably
linked to a slab breakoff event during the Alpine collisional evolution. Therefore, the lower Gonfolite
Lombarda succession provides the sedimentary record
of a geodynamic event of paramount importance in
collisional belts, and highlights a compositional trend
due to the uplift of granitic crust in a collisional
environment, which can be added to those considered
in Dickinsons' provenance model for dissected arc
and continental basement settings (Fig. 6) (Dickinson,
1985).
Acknowledgements
B. Messiga is acknowledged for the review of the
preliminary draft. We are also grateful to W. Cavazza
and F. Ricci Lucchi for their careful reading and
constructive reviews. Glen Murrell is kindly acknowledged for English revision. Financial grants by Italian
Ministry of University (MURST) and Pavia University (FAR funds).
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