12.6 Foreland Basins, Molasse
shal~ow-marine sedimentation, and again a longer period of
fluVlal deposition (Galewsky and Silver 1997).
In contrast to the classieal model, fore land basins frequently evolve in a markedly different way. In these
eases, so-ealled sate/lite foreland basins develop on
top of deeper lying thrust sheets in addition to the
main f~reland trough. The basins may be split by
progradlllg thrust sheets or earried "piggybaek" toward the foreland (Fig. 12.36d,e).
Examples of such basins have been described from the
s?uthern Alpine foredeep (Po basin), the northeastern margm . of the Apennines, Italy, where young thrust sheets are
buned underneath ~he Po alluvial plain (Fig. 12.32c), and
th~ Pyrenean margm of the Ebro basin in Spain (Ori and
Fnend 1984; Massari et al. 1986· Ricci Lucchi 1986·
~occaletti. et al. .1990). ~odern, water-covered representa:
tlves of thls modlfied basm type are known from Indonesia
(retroarc ~oreland basin on the Sunda sheIt), the southwestern mar~n of the Zagros fold-thrust belt (peripheral foreland basm beiow the eastern Persian Gult) and off the
northwestern coastline of New Zealand (DeCeIIes and
Giles (1996).
These observations have led to a revised eoneept of a
foreland basin system which ean also be used for ex~ples where the front of the orogenie wedge is burled under younger sediments (Fig. 12.34d; DeCelles
and Giles 1996). Here, the topographieally visible
front of the thrust belt, delineating the inner boundary of the basin system, lies behind the true front of
the orogenie wedge. The entire foreland basin system
ean be subdivided into four discrete depozones: the
wedge-top zone, the main foredeep, the forebulge
and the back-bulge zones.
The sediments of the wedge-top depozone aeeumulate on. top of the frontal part of the orogenie
we~ge. ThlS zone rnay include speeific piggy-baek
baSlllS and thrust-top basins. It is eharacterized be
relatively eoarse-grained sediments, rnany tectonic
uneonformities, and progressive deformation. The
sediments of the foredeep depozone display the features known from the classieal foreland basin model:
inereasing thiekness toward the front of the (buried)
thrust ?elt an~ p~oximal-distal faeies change with
~ee~easlllg .gralll-sizes. The forebulge is a zone of
hrmted sediment aceumulation or potential flexural
uplift and erosion. The back-bulge zone comprises a
further broad region of potential flexural subsidence
and sediment aeeumulation of limited thiekness.
Other variations in foreland basin evolution result
from an irregular shape of the continental margin
subjected .to collision (e.g. Lash 1988). In this case,
the tectomc load of the thrust belt is emplaeed at different times along strike of the basin and thus eauses
diachronous pulses of subsidenee and sediment supply.
613
Subsidence History and Basin Filling
A~ the onset of ov~rtIu:tsting the orogenie belt may
s.t11l form a submanne ndge or a ridge of low elevation above sea level and thus induce only slow subsidence and provide limited amounts of sediment. The
further advance of thrust sheets into the basin results
in an. increasing rate of subsidence of the flexing
crust III front of the thrust belt (eonvex-up subsidence eurves, cf. Sect. 8.4). In partieular thick thrust
sheets advancing over thinned continental crust and
reac~ing high elevations above sea level, accelerate
subsldence. Therefore, many aneient foreland basins
became fairly deep during this early stage whieh is
reflected by submarine fans and flysch-type sediments near. the rising mountain range (cf. Fig.
12.32a). In distal reaches near the bordering platform
shallow-marine (marine molasse) or fluvio-deltaic
sediments (freshwater molasse) predominate. As an
alternative, thrust sheets can also advanee over continental settings and generate thick conglomerates deposited as alluvial fans along the front of the mountain belt .which undergoes rapid erosion (e.g. in the
Ebro baslll, see below). As long as sediment supply
eompensates for subsidence, the continental depositional environment is rnaintained. When uplift of
t~e orogenie belt is balanced by erosion, the elevation of the mountain belt and its denudation rate have
reached their maximum. Cratonward, the conglomerates grade into fluvial and frequently also into lake
deposits. Shallow-water carbonates (mostly carbonate ramps ~ are usually restricted to the distal margin
of the basm. In some fore land basins reefs developed
on topographie highs in more central parts and even
at proxi~l sites of the basin (Dorobek 1995).
The lllterplay between subsidence and sediment
supply (or sedimentation rates) can be best seen from
a geohistory analysis (Fig. 12.36a). This diagram
shows th~ basin evolution during one tectonic eycle
from actIve overthrusting to isostatic rebound. The
s~diments, how:ever, record two transgressive-regresSIV~ eyeles whleh result from two periods in which
sediment supply surpassed subsidence, i.e. the sediment accommodation space, and thus the basin became overfilled.
~ort~ern Alp~ne foredeep, Germany. Fig. 12.36a is a
slmphfied version ofbasin evolution in the German part of
the northern Alpi~e foreiand basin elose to the present-day
thrust front (modlfied after Zweigel 1998). Thrusting occurred between about 38 and 22 Ma and then ceased because subsequent crustal convergence was accommodated
?y lateral ex~sio~ of parts of the Eastern Alps. The strikmg decrease m sediment supply during the relaxation phase
w~s .probably c~used by intermittent sediment storage
wlthm the Alps m an area of deIayed uplift. Effects of sealevel change are neglected in this diagram.
. S~b-Himalayan foredeep. This peripheral foreland basm lS a~ exampl~ of a. basin receiving very large volumes
of cIastlc matenal smce the Miocene. After an early
shal~ow-marine sedimentation, and again a longer period of
fluVlal deposition (Galewsky and Silver 1997).
In contrast to the classieal model, fore land basins frequently evolve in a markedly different way. In these
eases, so-ealled sate/lite foreland basins develop on
top of deeper lying thrust sheets in addition to the
main f~reland trough. The basins may be split by
progradlllg thrust sheets or earried "piggybaek" toward the foreland (Fig. 12.36d,e).
Examples of such basins have been described from the
s?uthern Alpine foredeep (Po basin), the northeastern margm . of the Apennines, Italy, where young thrust sheets are
buned underneath ~he Po alluvial plain (Fig. 12.32c), and
th~ Pyrenean margm of the Ebro basin in Spain (Ori and
Fnend 1984; Massari et al. 1986· Ricci Lucchi 1986·
~occaletti. et al. .1990). ~odern, water-covered representa:
tlves of thls modlfied basm type are known from Indonesia
(retroarc ~oreland basin on the Sunda sheIt), the southwestern mar~n of the Zagros fold-thrust belt (peripheral foreland basm beiow the eastern Persian Gult) and off the
northwestern coastline of New Zealand (DeCeIIes and
Giles (1996).
These observations have led to a revised eoneept of a
foreland basin system which ean also be used for ex~ples where the front of the orogenie wedge is burled under younger sediments (Fig. 12.34d; DeCelles
and Giles 1996). Here, the topographieally visible
front of the thrust belt, delineating the inner boundary of the basin system, lies behind the true front of
the orogenie wedge. The entire foreland basin system
ean be subdivided into four discrete depozones: the
wedge-top zone, the main foredeep, the forebulge
and the back-bulge zones.
The sediments of the wedge-top depozone aeeumulate on. top of the frontal part of the orogenie
we~ge. ThlS zone rnay include speeific piggy-baek
baSlllS and thrust-top basins. It is eharacterized be
relatively eoarse-grained sediments, rnany tectonic
uneonformities, and progressive deformation. The
sediments of the foredeep depozone display the features known from the classieal foreland basin model:
inereasing thiekness toward the front of the (buried)
thrust ?elt an~ p~oximal-distal faeies change with
~ee~easlllg .gralll-sizes. The forebulge is a zone of
hrmted sediment aceumulation or potential flexural
uplift and erosion. The back-bulge zone comprises a
further broad region of potential flexural subsidence
and sediment aeeumulation of limited thiekness.
Other variations in foreland basin evolution result
from an irregular shape of the continental margin
subjected .to collision (e.g. Lash 1988). In this case,
the tectomc load of the thrust belt is emplaeed at different times along strike of the basin and thus eauses
diachronous pulses of subsidenee and sediment supply.
613
Subsidence History and Basin Filling
A~ the onset of ov~rtIu:tsting the orogenie belt may
s.t11l form a submanne ndge or a ridge of low elevation above sea level and thus induce only slow subsidence and provide limited amounts of sediment. The
further advance of thrust sheets into the basin results
in an. increasing rate of subsidence of the flexing
crust III front of the thrust belt (eonvex-up subsidence eurves, cf. Sect. 8.4). In partieular thick thrust
sheets advancing over thinned continental crust and
reac~ing high elevations above sea level, accelerate
subsldence. Therefore, many aneient foreland basins
became fairly deep during this early stage whieh is
reflected by submarine fans and flysch-type sediments near. the rising mountain range (cf. Fig.
12.32a). In distal reaches near the bordering platform
shallow-marine (marine molasse) or fluvio-deltaic
sediments (freshwater molasse) predominate. As an
alternative, thrust sheets can also advanee over continental settings and generate thick conglomerates deposited as alluvial fans along the front of the mountain belt .which undergoes rapid erosion (e.g. in the
Ebro baslll, see below). As long as sediment supply
eompensates for subsidence, the continental depositional environment is rnaintained. When uplift of
t~e orogenie belt is balanced by erosion, the elevation of the mountain belt and its denudation rate have
reached their maximum. Cratonward, the conglomerates grade into fluvial and frequently also into lake
deposits. Shallow-water carbonates (mostly carbonate ramps ~ are usually restricted to the distal margin
of the basm. In some fore land basins reefs developed
on topographie highs in more central parts and even
at proxi~l sites of the basin (Dorobek 1995).
The lllterplay between subsidence and sediment
supply (or sedimentation rates) can be best seen from
a geohistory analysis (Fig. 12.36a). This diagram
shows th~ basin evolution during one tectonic eycle
from actIve overthrusting to isostatic rebound. The
s~diments, how:ever, record two transgressive-regresSIV~ eyeles whleh result from two periods in which
sediment supply surpassed subsidence, i.e. the sediment accommodation space, and thus the basin became overfilled.
~ort~ern Alp~ne foredeep, Germany. Fig. 12.36a is a
slmphfied version ofbasin evolution in the German part of
the northern Alpi~e foreiand basin elose to the present-day
thrust front (modlfied after Zweigel 1998). Thrusting occurred between about 38 and 22 Ma and then ceased because subsequent crustal convergence was accommodated
?y lateral ex~sio~ of parts of the Eastern Alps. The strikmg decrease m sediment supply during the relaxation phase
w~s .probably c~used by intermittent sediment storage
wlthm the Alps m an area of deIayed uplift. Effects of sealevel change are neglected in this diagram.
. S~b-Himalayan foredeep. This peripheral foreland basm lS a~ exampl~ of a. basin receiving very large volumes
of cIastlc matenal smce the Miocene. After an early