610
Foredeep of the Apennines, Italy. In the Miocene
(Marnoso-arenacea Formation) of the northern Apenninic
foredeep, sedimentological and petrofacies studies, using
lithoclasts, light and heavy minerals, have allowed the
identification of the various sediment sources (Fig.
12.33b,c; Ricci-Lucchi 1975 and 1986; Gandolfi et al.
1983). The basin received sediments not only from the
northeastward advancing overthrust belt and the eastern
foreland, but also from the neighboring Alpine mountain
range to the north which had already reached high altitudes. This sediment source was particularly important during an early stage of basin evolution. Large amounts of
sediment were transported southeast along the strike of the
basin. The megafacies of the basin changes in this direction
from proximal (not exposed) to deep-sea midfan, outer fan
and basin plain sediments. Outer fan sediments interfinger
with turbidites of the basin plain and hemipelagic slope
sediments (Fig. 12.33b; cf Sect. 5.4.2). In addition, slump
deposits and olistostromes are intercalated which are derived from the Apenninic thrust front. The composition of
individual turbidites also records different sediment
sources (Fig. 12.33c); sume turbidites consist of reworked
intra-basinal material. A striking feature of this basin is
axial sediment transport in both directions which indicates
the absence of a significant gradient along the basin floor.
Some spectacular beds (Contessa and Colombine key beds,
up to about 15 m in thickness) can be traced over about
100 km along the basin axis. They resulted from northward
directed catastrophic flows and contain material from several sources, including shelf carbonate. During the subsequent evolution of the Appenines, the depositional basin
migrated toward the foreland to the northeast and narrowed. Its shallow-marine sediments partially consist of
reworked material of the previous basin stage. The basin
became continental in the late Miocene (Messinian).
Changing transport directions in flysch basins of the
Alps have been reported, e.g. by Hesse and Butt (1976),
Ackermann (1986), and others. In addition to changing
sediment provenance, the facies evolution ofthese basins is
frequently controlled by synsedimentary tectonic basin deformation along the front of the overthrust belt (Homewood and Caron 1983; Ricci-Lucci 1990).
The transition from a deep flysch basin to a shallowmarine or continental molasse basin in front of the
advancing thrust belt is documented by many exampIes all over the world. A basin in this transitional
stage rnay show the following facies units (Sinclair
1997b):
- Carbonat deposition on the cratonic basin margin
(cf. Fig. 12.36c).
- Hemipelagic mud offshore from the cratonic rnargin.
- Deep-water siliciclastic turbidites (flysch) toward
the orogenie basin margin.
With the prograding front of the thrust belt, the basin
migrates cratonward; it is filled and transfonned into
a molasse basin. The foreland basin stage is commonly characterized by the formation of a forebulge
which rnay cause an erosional unconformity in the
older sediment cover of the passive margin sequence
(Fig. 12.34c; see below).
Chapter 12 Basin Evolution and Sediments
12.6.2 Foreland Basins with Molasse
General Aspects
The late stage in the evolution of an orogenie belt,
after plate collision, is characterized by the formation
of elongate depressions on continental crust between
the contractional belt and the adjacent craton(s).
Such foreland basins can develop on both sides of
the orogenie belt. Basins evolving along the former
subduction zone in front of the fold-thrust belt are
referred to as peripheral foreland basins; subsiding
zones in the hinterland, associated with a retroarc
fold-thrust belt, are retroarc foreland basins or retroforeland basins (Fig. 12.34a,b). Both types of foreland basins are asymmetrie in cross-sections and follow similar rules in their subsidence history and sedimentary filling. In detail, however, foreland basins
and their facies architeeture are very complex and
vary greatly from region to region.
The classical model of fore land basin evolution, as
derived from the Alps, cannot be applied everywhere. It had to be supplemented by modified scenarios, including so-called "composite" foreland basins. The latter are bounded by thrust fronts on two or
more sides and therefore tend to become less asymmetrie than the "normal" foreland basins. Examples
of composite basins include the Po basin-Adriatic
Sea and Ebro basin (Figs. 12.32c and 12.37a, see
below), some basins in the Rocky Mountains, in the
sub-Andean foredeep, and in central Asia (e.g. the
Tarim basin). After an introduction to basin-forrning
processes, we first discuss here the classical model
using peripheral fore land basins as examples.
Ancient and young fore land basins have been described
from numerous 10cations all over the world (e.g. Allen et
al. 1986; Mcqueen and Leckie 1992; Massari et al. (1993);
Dorobek et al. (1995); Van Wagoner and Bertram 1995).
General principles in the formation and filling of foreland
basins have been discussed, e.g., by Angevine et al. (1990),
Sinclair et al. (1991), Stockmal et al. (1992), Dorobek
(1995), DeCelles and Giles (1996). Ofthe many case studies, only a few can be mentioned here.
Basin-Forming Processes
The evolution of a peripheral foreland basin begins,
when the former ocean is closed and the front of the
overthrust belt has reached the stretched crust of the
pre-existing continental margin (cf. Fig. 1.3a; Fig.
12.31c). Then the active subduction zone evolves
into a collisional over-thrust belt as one continent
overrides another.
The most important processes creating subsidence
of foreland basins are the emplacement of tectonic
loads and the subsequently accumulating sediment
loads. Tectonic loads become particularly effective if
they reach high elevations above sea level. However,
Foredeep of the Apennines, Italy. In the Miocene
(Marnoso-arenacea Formation) of the northern Apenninic
foredeep, sedimentological and petrofacies studies, using
lithoclasts, light and heavy minerals, have allowed the
identification of the various sediment sources (Fig.
12.33b,c; Ricci-Lucchi 1975 and 1986; Gandolfi et al.
1983). The basin received sediments not only from the
northeastward advancing overthrust belt and the eastern
foreland, but also from the neighboring Alpine mountain
range to the north which had already reached high altitudes. This sediment source was particularly important during an early stage of basin evolution. Large amounts of
sediment were transported southeast along the strike of the
basin. The megafacies of the basin changes in this direction
from proximal (not exposed) to deep-sea midfan, outer fan
and basin plain sediments. Outer fan sediments interfinger
with turbidites of the basin plain and hemipelagic slope
sediments (Fig. 12.33b; cf Sect. 5.4.2). In addition, slump
deposits and olistostromes are intercalated which are derived from the Apenninic thrust front. The composition of
individual turbidites also records different sediment
sources (Fig. 12.33c); sume turbidites consist of reworked
intra-basinal material. A striking feature of this basin is
axial sediment transport in both directions which indicates
the absence of a significant gradient along the basin floor.
Some spectacular beds (Contessa and Colombine key beds,
up to about 15 m in thickness) can be traced over about
100 km along the basin axis. They resulted from northward
directed catastrophic flows and contain material from several sources, including shelf carbonate. During the subsequent evolution of the Appenines, the depositional basin
migrated toward the foreland to the northeast and narrowed. Its shallow-marine sediments partially consist of
reworked material of the previous basin stage. The basin
became continental in the late Miocene (Messinian).
Changing transport directions in flysch basins of the
Alps have been reported, e.g. by Hesse and Butt (1976),
Ackermann (1986), and others. In addition to changing
sediment provenance, the facies evolution ofthese basins is
frequently controlled by synsedimentary tectonic basin deformation along the front of the overthrust belt (Homewood and Caron 1983; Ricci-Lucci 1990).
The transition from a deep flysch basin to a shallowmarine or continental molasse basin in front of the
advancing thrust belt is documented by many exampIes all over the world. A basin in this transitional
stage rnay show the following facies units (Sinclair
1997b):
- Carbonat deposition on the cratonic basin margin
(cf. Fig. 12.36c).
- Hemipelagic mud offshore from the cratonic rnargin.
- Deep-water siliciclastic turbidites (flysch) toward
the orogenie basin margin.
With the prograding front of the thrust belt, the basin
migrates cratonward; it is filled and transfonned into
a molasse basin. The foreland basin stage is commonly characterized by the formation of a forebulge
which rnay cause an erosional unconformity in the
older sediment cover of the passive margin sequence
(Fig. 12.34c; see below).
Chapter 12 Basin Evolution and Sediments
12.6.2 Foreland Basins with Molasse
General Aspects
The late stage in the evolution of an orogenie belt,
after plate collision, is characterized by the formation
of elongate depressions on continental crust between
the contractional belt and the adjacent craton(s).
Such foreland basins can develop on both sides of
the orogenie belt. Basins evolving along the former
subduction zone in front of the fold-thrust belt are
referred to as peripheral foreland basins; subsiding
zones in the hinterland, associated with a retroarc
fold-thrust belt, are retroarc foreland basins or retroforeland basins (Fig. 12.34a,b). Both types of foreland basins are asymmetrie in cross-sections and follow similar rules in their subsidence history and sedimentary filling. In detail, however, foreland basins
and their facies architeeture are very complex and
vary greatly from region to region.
The classical model of fore land basin evolution, as
derived from the Alps, cannot be applied everywhere. It had to be supplemented by modified scenarios, including so-called "composite" foreland basins. The latter are bounded by thrust fronts on two or
more sides and therefore tend to become less asymmetrie than the "normal" foreland basins. Examples
of composite basins include the Po basin-Adriatic
Sea and Ebro basin (Figs. 12.32c and 12.37a, see
below), some basins in the Rocky Mountains, in the
sub-Andean foredeep, and in central Asia (e.g. the
Tarim basin). After an introduction to basin-forrning
processes, we first discuss here the classical model
using peripheral fore land basins as examples.
Ancient and young fore land basins have been described
from numerous 10cations all over the world (e.g. Allen et
al. 1986; Mcqueen and Leckie 1992; Massari et al. (1993);
Dorobek et al. (1995); Van Wagoner and Bertram 1995).
General principles in the formation and filling of foreland
basins have been discussed, e.g., by Angevine et al. (1990),
Sinclair et al. (1991), Stockmal et al. (1992), Dorobek
(1995), DeCelles and Giles (1996). Ofthe many case studies, only a few can be mentioned here.
Basin-Forming Processes
The evolution of a peripheral foreland basin begins,
when the former ocean is closed and the front of the
overthrust belt has reached the stretched crust of the
pre-existing continental margin (cf. Fig. 1.3a; Fig.
12.31c). Then the active subduction zone evolves
into a collisional over-thrust belt as one continent
overrides another.
The most important processes creating subsidence
of foreland basins are the emplacement of tectonic
loads and the subsequently accumulating sediment
loads. Tectonic loads become particularly effective if
they reach high elevations above sea level. However,