606
12.6
Remnant and Foreland Basins
12.6.1 Remnant Basins with Flysch
General Aspects
Basin Evolution and Sediments
12.6.2 Foreland Basins with Molasse
General Aspects
Basin-Forming Processes
Subsidence History and Basin Filling
Sedimentation Rates and Sediment Composition
Special Features ofForeland Basins
12.6.3 Summary (Remnant and Foreland Basins)
12.6.1 Remnant Basins with Flysch
General Aspects
The late phase of a closing ocean basin, associated
with subduction of oceanic crust and the formation
of an orogenie belt, is commonly characterized by the
transition from remnant basins to peripheral foreland
basins (cf. Fig. 1.3a). While remnant basins are
largely filled with deep-water flysch-type sediments,
foreland basins accumulate mainly shallow-marine
and continental molasse deposits. Flysch is typically
a thick marine sequence of turbidite beds and gravity
mass flows; it contains an autochthonous deep-water
fauna (apart from displaced shallow-water organisms) and is incorporated into the thrust and fold belt
of a growing orogenie belt. Molasse includes deposits of large alluvial fans, fan deltas, fluvial, lacustrine, and shallow-marine environments. The close
relationship of both types of basin fills to the
geodynamic development of an orogen is shown by
the simplified model in Fig. 12.31), which imitates to
some extent the situation in the Swiss Alps.
Remnant basins resulting from subduction are underlain by oceanic crust and therefore represent deep,
narrowing basins (cf. Sect. 1.2). In Figure 12.31a, the
remnant basin is confined on one side by a pre-existing passive continental margin with a wedge of older
clastics and carbonate sediments, and on the other
side by an approaching overthrust belt (accretionary
wedge) shedding relatively large volumes of various
clastics in the form of turbidites and mass flow deposits into the basin.
Principally, thick turbidite sequences exposed in
mountain belts can originate from three sourees:
Chapter 12 Basin Evolution and Sediments
- Remnant basins.
- Deep-sea trenches and their slope basins.
- Forearce basins.
The sediments of deep-sea trenches are norrnally incorporated into accretionary prisms (tectonic
melange zones), underplated or subducted (cf. Sect.
11.7.3). Accreted and underplated sediments become
highly deformed and more or less metamorphosed.
Foreare basin sediments are frequently uplifted and
eroded. The deposits of remnant basins can, at least
partially, be better preserved when they are taken up
by younger thrust sheets of an evolving orogenie belt
(cf. Fig. 12.47b).
This problem has been discussed, e.g., by Moss (1998) for
an example in Bomeo (Kalimantan and Sarawak). He interprets the late Cretaceous to Paleogene thick turbidite sequence of the Rajang-Embaluh Group, forming an extensive mountain range, as deposits of a remnant basin and
not, as suggested previously, as sediments of an arc-trench
system. The latter is older, and a direct strnctural relationship between the melange zone and the flysch sequence
obviously does not exist.
The simple model of a closing ocean basin with flysch
deposits cannot always applied. The Cretaceous Rhenodanubian flysch ofthe Eastern Alps, for example, has probably not accumulated in a "normal" remnant basin, but in a
narrow and deep intra-orogenie trough on thinned continental ernst (mid-Penninic) which resulted from left-Iateral
transtension and possibly from the onset of oblique ocean
spreading (Mattem 1999). According to heavy mineral
analyses and paleocurrent measurements, the flysch basin
received sediment first from northern and later from southem sourees; ophiolitic material is absent. The Rhenodanubian flysch is now present in three thrust sheets along
the northem front of the Alps.
Further references include, e.g., Matter et al. (1980),
Stockmal and Beaumont (1987), Caron et al. (1989). The
flysch to molasse transition in peripheral foreland basins is
discussed by Sinclair (l997a).
Basin Evolution and Sediments
Flysch sediments markedly vary in grain size distribution and composition in relation to the changing
influence of intra-basinal and extra-basinal sediment
sources (Fig. 12.32a). The wide spectrum of flysch
deposits may include proximal conglomeratic beds,
coarse-grained debris flows ("wildflysch" , but cf.
Sect. 12.5.2) and olistostromes (Sect. 5.4.1), as well
12.6
Remnant and Foreland Basins
12.6.1 Remnant Basins with Flysch
General Aspects
Basin Evolution and Sediments
12.6.2 Foreland Basins with Molasse
General Aspects
Basin-Forming Processes
Subsidence History and Basin Filling
Sedimentation Rates and Sediment Composition
Special Features ofForeland Basins
12.6.3 Summary (Remnant and Foreland Basins)
12.6.1 Remnant Basins with Flysch
General Aspects
The late phase of a closing ocean basin, associated
with subduction of oceanic crust and the formation
of an orogenie belt, is commonly characterized by the
transition from remnant basins to peripheral foreland
basins (cf. Fig. 1.3a). While remnant basins are
largely filled with deep-water flysch-type sediments,
foreland basins accumulate mainly shallow-marine
and continental molasse deposits. Flysch is typically
a thick marine sequence of turbidite beds and gravity
mass flows; it contains an autochthonous deep-water
fauna (apart from displaced shallow-water organisms) and is incorporated into the thrust and fold belt
of a growing orogenie belt. Molasse includes deposits of large alluvial fans, fan deltas, fluvial, lacustrine, and shallow-marine environments. The close
relationship of both types of basin fills to the
geodynamic development of an orogen is shown by
the simplified model in Fig. 12.31), which imitates to
some extent the situation in the Swiss Alps.
Remnant basins resulting from subduction are underlain by oceanic crust and therefore represent deep,
narrowing basins (cf. Sect. 1.2). In Figure 12.31a, the
remnant basin is confined on one side by a pre-existing passive continental margin with a wedge of older
clastics and carbonate sediments, and on the other
side by an approaching overthrust belt (accretionary
wedge) shedding relatively large volumes of various
clastics in the form of turbidites and mass flow deposits into the basin.
Principally, thick turbidite sequences exposed in
mountain belts can originate from three sourees:
Chapter 12 Basin Evolution and Sediments
- Remnant basins.
- Deep-sea trenches and their slope basins.
- Forearce basins.
The sediments of deep-sea trenches are norrnally incorporated into accretionary prisms (tectonic
melange zones), underplated or subducted (cf. Sect.
11.7.3). Accreted and underplated sediments become
highly deformed and more or less metamorphosed.
Foreare basin sediments are frequently uplifted and
eroded. The deposits of remnant basins can, at least
partially, be better preserved when they are taken up
by younger thrust sheets of an evolving orogenie belt
(cf. Fig. 12.47b).
This problem has been discussed, e.g., by Moss (1998) for
an example in Bomeo (Kalimantan and Sarawak). He interprets the late Cretaceous to Paleogene thick turbidite sequence of the Rajang-Embaluh Group, forming an extensive mountain range, as deposits of a remnant basin and
not, as suggested previously, as sediments of an arc-trench
system. The latter is older, and a direct strnctural relationship between the melange zone and the flysch sequence
obviously does not exist.
The simple model of a closing ocean basin with flysch
deposits cannot always applied. The Cretaceous Rhenodanubian flysch ofthe Eastern Alps, for example, has probably not accumulated in a "normal" remnant basin, but in a
narrow and deep intra-orogenie trough on thinned continental ernst (mid-Penninic) which resulted from left-Iateral
transtension and possibly from the onset of oblique ocean
spreading (Mattem 1999). According to heavy mineral
analyses and paleocurrent measurements, the flysch basin
received sediment first from northern and later from southem sourees; ophiolitic material is absent. The Rhenodanubian flysch is now present in three thrust sheets along
the northem front of the Alps.
Further references include, e.g., Matter et al. (1980),
Stockmal and Beaumont (1987), Caron et al. (1989). The
flysch to molasse transition in peripheral foreland basins is
discussed by Sinclair (l997a).
Basin Evolution and Sediments
Flysch sediments markedly vary in grain size distribution and composition in relation to the changing
influence of intra-basinal and extra-basinal sediment
sources (Fig. 12.32a). The wide spectrum of flysch
deposits may include proximal conglomeratic beds,
coarse-grained debris flows ("wildflysch" , but cf.
Sect. 12.5.2) and olistostromes (Sect. 5.4.1), as well