12.6 Foreland Basins, Molasse
overview of cross-sections indicates the repeated basinward
prograding of nonmarine and coastal sand facies during
relative sea-levellowstands while carbonates or calcareous
shales accumulated in more distal regions during times of
high relative sea level (Fig. 12.38c). The facies architecture
differed, however, from subbasin to subbasin due to variations in accommodation space and sediment supply. The
depocenters tended to migrate southward.
During the Upper Cretaceous relatively thin strandplain
sands prograded rapidly cratonward during times of slow
subsidence whereas thick shales accumulated in periods of
rapid subsidence (Plint et al. 1993). Coastal plain strata,
thickening toward the thrust front, were deposited during
initial downwarping of a near-horizontal strandplain prior
to marine transgression. Uplift and migration of the
forebulge seems to have lagged 0.25-0.5 Ma behind the
onset of (repeated) tectonic loading which was responsible
for the long-term transgression-regression cycles. Erosional
unconformities on top of the forebulge are covered locally
by oolitic ironstone indicating shallow-marine sedimentstarved conditions.
South Pyreneen foreland basin, Spain. The overthrust
northem part of the early Ebro basin was segmented by
12.6.3 Summary (Remnant and Foreland Basins)
Remnant basins are narrowing oeean basins
shortly before c1osure. They eolleet sediments
(mainly turbidites) from the neighboring magmatie are, from the approaehing passive continental margin, and from their underlying oeeanie erust.
Foreland basins evolve either in the former foreare region (peripheral settings) or in the bakkare region (retroare settings). Their differential
subsidenee is eontrolled mainly by the loads of
the advaneing overthrust belt and the inereasing
sediment mass in the basin.
The overthrust belt of peripheral fore land basins does not always form a topographieally
high mountain front (c1assieal model), but ean
also advanee into areas of low elevation and
beeome synteetonieally buried (wedge-top satellite basins, modified foreland basin system).
Phases of aetive overthrusting are aeeompanied
by inereasing rates of subsidenee and sediment
supply; after this phase, subsidenee eontinues at
deereasing rates due to relaxation of the flexed
erust; finally, erosional unloading eauses isostatie rebound (uplift).
The geometry and facies assoeiations of eratonward prograding and thinning elastie wedges
are eontrolled by both stepwise overthrusting
621
synsedimentary structural highs transverse to the basin
axis. These structures created several subbasins of complex
morphology and sediment dispersal systems which have
been recently studied in some detail (e.g. Marzo et al.
1998).
Intraplate thrusts (mostly reverse faults) develop in
many foreland basins either during or after sediment
filling. They affeet, in contrast to the eommon "shallow" thrust faults, the underlaying eontinental erust
and contribute to erustal shortening (cf. Figs. l2.37b,
western Ebro basin, and Fig. l2.36e, southeastern
Franee). They normally run ± parallel to the axis of
the basin. In eertain eases they may eause basin segmentation.
Growth faults and growth strata in foreland basins
develop in sediments at the front of migrating thrust
sheets (Figs. 12.34d and 12.36d; e.g. Verges et al.
1996). They reeord both the time intervals and
amounts of aetive thrusting.
and the migration of the distal forebulge. Variations in sediment supply mayaiso be eontrolled by processes within the orogen. This
ean lead to overfilling or underfilling out-ofphase in relation to the aetivity of the thrust
front.
- For these reasons, the general trend in the filling of foreland basins from deep marine to
eontinental and from proximal eoarse- to distal
fine-grained sediments is frequently modified.
- Laterally shifting sediment wedges are separated by diaehronous surfaees, stratigraphie breaks, and partially by angular uneonformities.
Axial sediment transport generates lateral faeies ehanges (e.g. from eoarse-grained alluvial
fans to fine-grained lake deposits, or from eontinental to shallow-marine sediments).
- Distal foreland basin sediments are eharaeterized by onlap onto older basement rocks, floodplain, laeustrine and shallow-marine sediments
(often carbonates) of limited thiekness whieh
may rceord eustatie sea-Ieve1 ehanges.
- Foreland basin fills rieh in marine sediments,
inc1uding blaek shales, are interesting targets
in hydroearbon exploration (e.g. Van Wagoner
and Bertram 1995).
overview of cross-sections indicates the repeated basinward
prograding of nonmarine and coastal sand facies during
relative sea-levellowstands while carbonates or calcareous
shales accumulated in more distal regions during times of
high relative sea level (Fig. 12.38c). The facies architecture
differed, however, from subbasin to subbasin due to variations in accommodation space and sediment supply. The
depocenters tended to migrate southward.
During the Upper Cretaceous relatively thin strandplain
sands prograded rapidly cratonward during times of slow
subsidence whereas thick shales accumulated in periods of
rapid subsidence (Plint et al. 1993). Coastal plain strata,
thickening toward the thrust front, were deposited during
initial downwarping of a near-horizontal strandplain prior
to marine transgression. Uplift and migration of the
forebulge seems to have lagged 0.25-0.5 Ma behind the
onset of (repeated) tectonic loading which was responsible
for the long-term transgression-regression cycles. Erosional
unconformities on top of the forebulge are covered locally
by oolitic ironstone indicating shallow-marine sedimentstarved conditions.
South Pyreneen foreland basin, Spain. The overthrust
northem part of the early Ebro basin was segmented by
12.6.3 Summary (Remnant and Foreland Basins)
Remnant basins are narrowing oeean basins
shortly before c1osure. They eolleet sediments
(mainly turbidites) from the neighboring magmatie are, from the approaehing passive continental margin, and from their underlying oeeanie erust.
Foreland basins evolve either in the former foreare region (peripheral settings) or in the bakkare region (retroare settings). Their differential
subsidenee is eontrolled mainly by the loads of
the advaneing overthrust belt and the inereasing
sediment mass in the basin.
The overthrust belt of peripheral fore land basins does not always form a topographieally
high mountain front (c1assieal model), but ean
also advanee into areas of low elevation and
beeome synteetonieally buried (wedge-top satellite basins, modified foreland basin system).
Phases of aetive overthrusting are aeeompanied
by inereasing rates of subsidenee and sediment
supply; after this phase, subsidenee eontinues at
deereasing rates due to relaxation of the flexed
erust; finally, erosional unloading eauses isostatie rebound (uplift).
The geometry and facies assoeiations of eratonward prograding and thinning elastie wedges
are eontrolled by both stepwise overthrusting
621
synsedimentary structural highs transverse to the basin
axis. These structures created several subbasins of complex
morphology and sediment dispersal systems which have
been recently studied in some detail (e.g. Marzo et al.
1998).
Intraplate thrusts (mostly reverse faults) develop in
many foreland basins either during or after sediment
filling. They affeet, in contrast to the eommon "shallow" thrust faults, the underlaying eontinental erust
and contribute to erustal shortening (cf. Figs. l2.37b,
western Ebro basin, and Fig. l2.36e, southeastern
Franee). They normally run ± parallel to the axis of
the basin. In eertain eases they may eause basin segmentation.
Growth faults and growth strata in foreland basins
develop in sediments at the front of migrating thrust
sheets (Figs. 12.34d and 12.36d; e.g. Verges et al.
1996). They reeord both the time intervals and
amounts of aetive thrusting.
and the migration of the distal forebulge. Variations in sediment supply mayaiso be eontrolled by processes within the orogen. This
ean lead to overfilling or underfilling out-ofphase in relation to the aetivity of the thrust
front.
- For these reasons, the general trend in the filling of foreland basins from deep marine to
eontinental and from proximal eoarse- to distal
fine-grained sediments is frequently modified.
- Laterally shifting sediment wedges are separated by diaehronous surfaees, stratigraphie breaks, and partially by angular uneonformities.
Axial sediment transport generates lateral faeies ehanges (e.g. from eoarse-grained alluvial
fans to fine-grained lake deposits, or from eontinental to shallow-marine sediments).
- Distal foreland basin sediments are eharaeterized by onlap onto older basement rocks, floodplain, laeustrine and shallow-marine sediments
(often carbonates) of limited thiekness whieh
may rceord eustatie sea-Ieve1 ehanges.
- Foreland basin fills rieh in marine sediments,
inc1uding blaek shales, are interesting targets
in hydroearbon exploration (e.g. Van Wagoner
and Bertram 1995).