618
The cl asts of syntectonic conglomerates may exhibit either
"normal" or "reverse" vertical unroofing sequences (Colombo 1994). Normal sequences begin with elasts derived
from the uppermost part of a thrust sheet, which is eroded
first, and end with clasts from later eroded deeper parts of
the overthrust mountain front. Reversed elast sequences
may form when an older, normal elast sequence is incorporated into a younger thrust sheet, uplifted and eroded.
Then, in an idealized case, the new (reversed) elast sequence may resemble the sequence present in the first
thrust sheet. Besides these endmember types, various mixtures of elasts can be produced by further recyeling of
proximal conglomerates.
The efficiency of denudation and recyeling of overthrust
fore land sediments has been demonstrated for northern Pakistan (erosion rates of 1-15 mika; Burbank and Beck
1991). Uplift of proximal fans of the Siwalik Group has
caused a fundamental change in the drainage pattern and
depositional environment of this region (Pivnik and Khan
1996). Examples from the Swiss Alpine foreland have been
reported, e.g., by Schlunegger et al. (1997a).
Special Features of Foreland Basins
Some exarnples of different types of foreland basins
may further underline the above mentioned general
characteristics and, in addition, indicate special features observed in several basins:
- Axial sediment transport andfacies change (transition from flysch to molasse). Flysch deposition in
deep remnant basins may continue into the early
stage of fore land basin filling. Although the sediment
is commonly derived from the evolving orogenie belt
and initially shed from point sources along the thrust
front directly into the basin, the transport directions
of gravity mass flows and turbidites in deeper water
often change and follow the basin axis (Fig. 12.32a).
Later, these basins often displaya coarsening- and
shallowing-upward trend, i.e. the transition from
flysch to molasse.
Axial transport and facies change rnay be even more
pronounced at the transition from fluvial-Iacustrine
to brackish and shallow marine sediments. Coal
searns of limited thickness and lateral extent are common in such transitional regimes.
Excellent examples ofaxial foredeep fillings are known
from the northern Apennines (Fig. 12.33, see above) and
the Ebro basin (Fig. 12.37a). In the latter case, turbidites
Fig. 12.38. Thrust loading and flexural subsidence in
the retro-foreland basin of the Western Interior,
USA, during the Upper Cretaceous. a Simplified
scheme describing basin formation and subsidence of
the basement surface and the foreland basin. (Modified from Leckie and Smith 1992). b Plan view of
specific subbasins and exposed Precambrian rocks
within the present-day retro-foreland basin. c Overview ofUpper Cretaceous stratigraphy in W-E crossseetion of basin (southern Wyoming). Dashed line
Chapter 12 Basin Evolution and Sediments
were shed along the northern basin margin in an axial,
western direction; they also inelude carbonate turbidites
derived from shelves to the north and south of the deep
basin.
Axial facies change in the molasse stage is observed, for
example, in the Swiss and German sectors ofthe North Alpine foreland basin in certain time intervals (Fig. 12.35a,e;
e.g., Bachrnann et al. 1987; Zweiger 1998). Another exampIe is the Neuquen basin of western Argentina (Barrio
1990).
- Pre-molasse and intra-molasse paleo-valleys. The
North Alpine peripheral foreland basin, about 1200
km in length from southeastern France to Vienna,
Austria, displays a number of paleo-valleys which
were incised either prior to or during basin filling.
Principally, basal paleo-valleys are expected to develop on both sides of the peripheral forebulge running either toward the mountain front or the craton
(Fig. 12.36b,c).
Pre-molasse paleo-valleys have been described from the
Swiss and French sectors of the foreland basin (Crampton
and Allen 1995; Gupta 1997). In SE France, the valleys are
located at the base of the oldest foreland basin sediments
(Fig. 12.36c). These Eocene Nurnrnulitic limestones onlap
onto the ramp, formed by tilted Jurassic limestones of the
craton, and step back (cratonward) with time (see also
Dorobek 1995). The sediment-filled paleo-valleys and
younger molasse deposits were incorporated into the young
thrust belt of the sub-Alpine chains and partially exhumed
by subsequent erosion. The valleys run into almost all directions and are evidently associated with a struetural high
within the forebulge zone (Fig. 12.36e). This situation is
interpreted with reverse faults within the basement which
in turn appear to be assoeiated with convergenee of the
orogenie belt. The teetonic strueture of this region is eomplex and not diseussed further here.
A large intra-molasse valley incised parallel and elose to
the distal basin margin has been observed in South Germany (cf. Fig. 7.28e). Its formation is aseribed to rapid sealevel fall in the Miocene. Many examples of intra-molasse
paleo-valleys have been deseribed from the Western Interior basin ofNorth Ameriea (cf. Seet. 7.6).
- Transgression-regression megacycles. Cross-sections of the sub-Alpine molasse in Switzerland and
Germany normally show two megacycles grading
from marine deposits (Marine Molasse) into continental sediments (Freshwater Molasse; Fig.
12.35d,e).
indicates backstripped interval. d Backstripped subsidence profiles across basin (for locations see b)
showing subsidcnce during two time slices. Subsidence varies spatially and temporally along strike and
dip relative to the Sevier thrust beIt. This pattern is
thought to reflect variations in the nature of the underlying crust and in the emplacement of thrust
loads. e Tectonic and total subsidence histories at
locations B-F, 100-150 km east of thrust belt (cf. b).
(b-e Modified from Pang and Nummedal 1995)
The cl asts of syntectonic conglomerates may exhibit either
"normal" or "reverse" vertical unroofing sequences (Colombo 1994). Normal sequences begin with elasts derived
from the uppermost part of a thrust sheet, which is eroded
first, and end with clasts from later eroded deeper parts of
the overthrust mountain front. Reversed elast sequences
may form when an older, normal elast sequence is incorporated into a younger thrust sheet, uplifted and eroded.
Then, in an idealized case, the new (reversed) elast sequence may resemble the sequence present in the first
thrust sheet. Besides these endmember types, various mixtures of elasts can be produced by further recyeling of
proximal conglomerates.
The efficiency of denudation and recyeling of overthrust
fore land sediments has been demonstrated for northern Pakistan (erosion rates of 1-15 mika; Burbank and Beck
1991). Uplift of proximal fans of the Siwalik Group has
caused a fundamental change in the drainage pattern and
depositional environment of this region (Pivnik and Khan
1996). Examples from the Swiss Alpine foreland have been
reported, e.g., by Schlunegger et al. (1997a).
Special Features of Foreland Basins
Some exarnples of different types of foreland basins
may further underline the above mentioned general
characteristics and, in addition, indicate special features observed in several basins:
- Axial sediment transport andfacies change (transition from flysch to molasse). Flysch deposition in
deep remnant basins may continue into the early
stage of fore land basin filling. Although the sediment
is commonly derived from the evolving orogenie belt
and initially shed from point sources along the thrust
front directly into the basin, the transport directions
of gravity mass flows and turbidites in deeper water
often change and follow the basin axis (Fig. 12.32a).
Later, these basins often displaya coarsening- and
shallowing-upward trend, i.e. the transition from
flysch to molasse.
Axial transport and facies change rnay be even more
pronounced at the transition from fluvial-Iacustrine
to brackish and shallow marine sediments. Coal
searns of limited thickness and lateral extent are common in such transitional regimes.
Excellent examples ofaxial foredeep fillings are known
from the northern Apennines (Fig. 12.33, see above) and
the Ebro basin (Fig. 12.37a). In the latter case, turbidites
Fig. 12.38. Thrust loading and flexural subsidence in
the retro-foreland basin of the Western Interior,
USA, during the Upper Cretaceous. a Simplified
scheme describing basin formation and subsidence of
the basement surface and the foreland basin. (Modified from Leckie and Smith 1992). b Plan view of
specific subbasins and exposed Precambrian rocks
within the present-day retro-foreland basin. c Overview ofUpper Cretaceous stratigraphy in W-E crossseetion of basin (southern Wyoming). Dashed line
Chapter 12 Basin Evolution and Sediments
were shed along the northern basin margin in an axial,
western direction; they also inelude carbonate turbidites
derived from shelves to the north and south of the deep
basin.
Axial facies change in the molasse stage is observed, for
example, in the Swiss and German sectors ofthe North Alpine foreland basin in certain time intervals (Fig. 12.35a,e;
e.g., Bachrnann et al. 1987; Zweiger 1998). Another exampIe is the Neuquen basin of western Argentina (Barrio
1990).
- Pre-molasse and intra-molasse paleo-valleys. The
North Alpine peripheral foreland basin, about 1200
km in length from southeastern France to Vienna,
Austria, displays a number of paleo-valleys which
were incised either prior to or during basin filling.
Principally, basal paleo-valleys are expected to develop on both sides of the peripheral forebulge running either toward the mountain front or the craton
(Fig. 12.36b,c).
Pre-molasse paleo-valleys have been described from the
Swiss and French sectors of the foreland basin (Crampton
and Allen 1995; Gupta 1997). In SE France, the valleys are
located at the base of the oldest foreland basin sediments
(Fig. 12.36c). These Eocene Nurnrnulitic limestones onlap
onto the ramp, formed by tilted Jurassic limestones of the
craton, and step back (cratonward) with time (see also
Dorobek 1995). The sediment-filled paleo-valleys and
younger molasse deposits were incorporated into the young
thrust belt of the sub-Alpine chains and partially exhumed
by subsequent erosion. The valleys run into almost all directions and are evidently associated with a struetural high
within the forebulge zone (Fig. 12.36e). This situation is
interpreted with reverse faults within the basement which
in turn appear to be assoeiated with convergenee of the
orogenie belt. The teetonic strueture of this region is eomplex and not diseussed further here.
A large intra-molasse valley incised parallel and elose to
the distal basin margin has been observed in South Germany (cf. Fig. 7.28e). Its formation is aseribed to rapid sealevel fall in the Miocene. Many examples of intra-molasse
paleo-valleys have been deseribed from the Western Interior basin ofNorth Ameriea (cf. Seet. 7.6).
- Transgression-regression megacycles. Cross-sections of the sub-Alpine molasse in Switzerland and
Germany normally show two megacycles grading
from marine deposits (Marine Molasse) into continental sediments (Freshwater Molasse; Fig.
12.35d,e).
indicates backstripped interval. d Backstripped subsidence profiles across basin (for locations see b)
showing subsidcnce during two time slices. Subsidence varies spatially and temporally along strike and
dip relative to the Sevier thrust beIt. This pattern is
thought to reflect variations in the nature of the underlying crust and in the emplacement of thrust
loads. e Tectonic and total subsidence histories at
locations B-F, 100-150 km east of thrust belt (cf. b).
(b-e Modified from Pang and Nummedal 1995)