12.6 Foreland Basins, Molasse
shallow-marine phase, the widely extended basin was filled
almost entirely with sediments of alluvial fans and fluvial
systems (Fig. 12.36e; Masele et al. 1986; Delcaillau 1986).
Subsequently, the major part ofthe incoming sediment was
transported through the foredeep and deposited in neighboring basins (cf. Sect. 11.5.5; Fig. 11.31). The early foreland basin development (Eocene to early Miocene) of the
Hirnalayas is addressed by DeCelles et al. (1998). The evolution of the frontal thrust sheets and their syntectonic erosional truncation is discussed by Delcaillau and Carozza
(1993).
A discontinuous movement of the thrust sheets and
erosional unloading of the thrust front create irregular subsidence curves and migration of the forebulge.
Active overthrusting is followed by prograding
wedges of coarse-grained clastics; phases of tectonic
quiescence lead to backstepping facies belts (toward
the thrust front) and decreasing rates of subsidence
(concave-up curves) l;lnd finally uplift.
Foreland basin sediments are predominantly
clastics derived in particular from highly elevated
parts of the folded overthrust belt. A frequently observed overall tendency is a shallowing-upward succession from deep-water sediments over shallow-marine and deltaic deposits to continental sediments
(Figs. 12.31a and 12.35b). In addition, coarseningand fining-upward megasequences may develop in
response to the discontinuous tectonic evolution of
the thrust be1t. It is, however, difficult to discriminate
between the effects of tectonism (subsidence),
changing sediment supply, and eustatic sea-Ievel
changes (cf. Sect. 7.2). The general trend in basin
filling may be modified significantly by spatial and
temporal changes in the supply (e.g. climate-induced)
and dispersal of sediment within the basin (Figs.
12.32a and 12.33c; see below). The depositional environment of many fore land basins therefore varied
several times between shallow-marine, brackish, and
continental.
Some distance away from the mountain front, the
distributary systems mainly follow the longitudinal
trend of the basin. Surplus fine-grained sediment can
be transported laterally out of the foreland basin into
adjacent ocean basins. The molasse sediments
pro grade onto the foreland platform with time (Fig.
12.35c); younger beds successively overlap older,
pinching-out strata (e.g. Homewood et al. 1986). By
contrast, distal sediments onlap onto the top of the
foreland platform, e.g. shallow-water carbonates, and
display backstepping towards the craton.
Finally, isostatic adjustrnent to the continuous erosion of the rising mountain range leads to exhumation of formerly deeply buried rocks and structures,
as well as to some uplift of the neighboring parts of
the foreland basin (cf. Sect. 8.4).
615
Sedimentation Rates and Sediment Composition
The average sedimentation rates in foreland basins
strongly differ between proximal (near the thrust
belt) and distal areas. Average rates at proximal sites
often range from 100 to > 1 000 m1Ma. These rates
can build up thick sedimentary successions in time
spans of a few millions of years. The sediment thicknesses of proximal fore land basin fins often surpass
5000 m.
In the foreland basin south of the High Hirnalayas 200-500
mlMa are common (Masele et al. 1986). The spectacular
Eocene elastic wedges at the southeastern margin of the
Ebro basin, Spain, including the conglomerates of
Montserrat, were deposited at mean rates of at least 330
mlMa (Marzo et al. 1998) in a time period of about 4 Ma.
In the western Ebro basin, in the Rioja trough in front of
the Iberian overthrust belt, the mean rate was 100 mlMa
(30 Ma). In other parts of the Ebro basin, elose to the Pyrenees, rapid subsidence led to the accumulation of turbidite
sequences ofup to 3.5 km in thickness. The Lower Freshwater Molasse ofthe Swiss foreland basin was deposited at
rates from 300 to 600 m1Ma, but locally values up to
> 1000 mlMa were reached (Schlunegger et al. 1997a). Sedimentation rates in the Upper Cretaceous of the Western
Interior basin, USA, were in the order of 100 mlMa.
The abundance of gravel in conglomerates commonly points to nearby source areas of high relief
which can be well identified. However, wet tropical
climate and increased weathering rates may largely
prevent the availability of gravel-sized material.
Changes in the gravel composition indicates the
unroofing of thrust sheets of different lithologies or
discontinuous overthrusting (Fig. 12.37).
The mineralogical composition of molasse sandstones varies from quartz-rich, feldspar-poor types
derived from intensely weathered continental blocks
to varieties which are less quartz-rich but rich in rock
fragments (Schwab 1986). The latter types constitute
the bulk of sand in many foreland basins. Material
derived from tectonically uplifted subduction complexes or magmatic arcs is found only in small quantities. Systematic variations in composition through
time, related to unroofing of deeply buried rocks, are
rarely observed. However, shifts in the principal
source areas providing sediment for particular segments of a foreland basin are common. They result
from differential rock uplift and exhumation within
the mountain belt and are reflected, for example, by
the temporally and spatially changing heavy mineral
associations derived from different rock types (e.g.
Füchtbauer 1967; Zuffa 1987; Schlunegger et al.
1998; Kempf et al. 1999).
In studies on the provenance of foreland basin
sediments, one has to bear in mind that the composition of middle- and late-stage sediments can be affected by the recycling of older portions of the foreland basin fill. These are frequently incorporated into
the thrust belt, uplifted, and subjected to erosion.
shallow-marine phase, the widely extended basin was filled
almost entirely with sediments of alluvial fans and fluvial
systems (Fig. 12.36e; Masele et al. 1986; Delcaillau 1986).
Subsequently, the major part ofthe incoming sediment was
transported through the foredeep and deposited in neighboring basins (cf. Sect. 11.5.5; Fig. 11.31). The early foreland basin development (Eocene to early Miocene) of the
Hirnalayas is addressed by DeCelles et al. (1998). The evolution of the frontal thrust sheets and their syntectonic erosional truncation is discussed by Delcaillau and Carozza
(1993).
A discontinuous movement of the thrust sheets and
erosional unloading of the thrust front create irregular subsidence curves and migration of the forebulge.
Active overthrusting is followed by prograding
wedges of coarse-grained clastics; phases of tectonic
quiescence lead to backstepping facies belts (toward
the thrust front) and decreasing rates of subsidence
(concave-up curves) l;lnd finally uplift.
Foreland basin sediments are predominantly
clastics derived in particular from highly elevated
parts of the folded overthrust belt. A frequently observed overall tendency is a shallowing-upward succession from deep-water sediments over shallow-marine and deltaic deposits to continental sediments
(Figs. 12.31a and 12.35b). In addition, coarseningand fining-upward megasequences may develop in
response to the discontinuous tectonic evolution of
the thrust be1t. It is, however, difficult to discriminate
between the effects of tectonism (subsidence),
changing sediment supply, and eustatic sea-Ievel
changes (cf. Sect. 7.2). The general trend in basin
filling may be modified significantly by spatial and
temporal changes in the supply (e.g. climate-induced)
and dispersal of sediment within the basin (Figs.
12.32a and 12.33c; see below). The depositional environment of many fore land basins therefore varied
several times between shallow-marine, brackish, and
continental.
Some distance away from the mountain front, the
distributary systems mainly follow the longitudinal
trend of the basin. Surplus fine-grained sediment can
be transported laterally out of the foreland basin into
adjacent ocean basins. The molasse sediments
pro grade onto the foreland platform with time (Fig.
12.35c); younger beds successively overlap older,
pinching-out strata (e.g. Homewood et al. 1986). By
contrast, distal sediments onlap onto the top of the
foreland platform, e.g. shallow-water carbonates, and
display backstepping towards the craton.
Finally, isostatic adjustrnent to the continuous erosion of the rising mountain range leads to exhumation of formerly deeply buried rocks and structures,
as well as to some uplift of the neighboring parts of
the foreland basin (cf. Sect. 8.4).
615
Sedimentation Rates and Sediment Composition
The average sedimentation rates in foreland basins
strongly differ between proximal (near the thrust
belt) and distal areas. Average rates at proximal sites
often range from 100 to > 1 000 m1Ma. These rates
can build up thick sedimentary successions in time
spans of a few millions of years. The sediment thicknesses of proximal fore land basin fins often surpass
5000 m.
In the foreland basin south of the High Hirnalayas 200-500
mlMa are common (Masele et al. 1986). The spectacular
Eocene elastic wedges at the southeastern margin of the
Ebro basin, Spain, including the conglomerates of
Montserrat, were deposited at mean rates of at least 330
mlMa (Marzo et al. 1998) in a time period of about 4 Ma.
In the western Ebro basin, in the Rioja trough in front of
the Iberian overthrust belt, the mean rate was 100 mlMa
(30 Ma). In other parts of the Ebro basin, elose to the Pyrenees, rapid subsidence led to the accumulation of turbidite
sequences ofup to 3.5 km in thickness. The Lower Freshwater Molasse ofthe Swiss foreland basin was deposited at
rates from 300 to 600 m1Ma, but locally values up to
> 1000 mlMa were reached (Schlunegger et al. 1997a). Sedimentation rates in the Upper Cretaceous of the Western
Interior basin, USA, were in the order of 100 mlMa.
The abundance of gravel in conglomerates commonly points to nearby source areas of high relief
which can be well identified. However, wet tropical
climate and increased weathering rates may largely
prevent the availability of gravel-sized material.
Changes in the gravel composition indicates the
unroofing of thrust sheets of different lithologies or
discontinuous overthrusting (Fig. 12.37).
The mineralogical composition of molasse sandstones varies from quartz-rich, feldspar-poor types
derived from intensely weathered continental blocks
to varieties which are less quartz-rich but rich in rock
fragments (Schwab 1986). The latter types constitute
the bulk of sand in many foreland basins. Material
derived from tectonically uplifted subduction complexes or magmatic arcs is found only in small quantities. Systematic variations in composition through
time, related to unroofing of deeply buried rocks, are
rarely observed. However, shifts in the principal
source areas providing sediment for particular segments of a foreland basin are common. They result
from differential rock uplift and exhumation within
the mountain belt and are reflected, for example, by
the temporally and spatially changing heavy mineral
associations derived from different rock types (e.g.
Füchtbauer 1967; Zuffa 1987; Schlunegger et al.
1998; Kempf et al. 1999).
In studies on the provenance of foreland basin
sediments, one has to bear in mind that the composition of middle- and late-stage sediments can be affected by the recycling of older portions of the foreland basin fill. These are frequently incorporated into
the thrust belt, uplifted, and subjected to erosion.