12.5 Foreare Basins
ments in subduetion eomplexes. Thiek foreare fills
on top of deeply subsided basin floors have a relatively high preservation potential.
For further diseussion see, e.g., Diekinson and Seely
(1979), Maedonald (1993), Busby and Ingersoll (1995).
Examples of Foreare Basins
Modern Foreare Basins on Continental Margins
The above mentioned general trends and some other
speeifie features are illustrated by a few examples
from modem foreare basins whieh still aeeumulate
sediment. The examples demortstrate different teetonie and sedimentary aspeets of these basins.
Foreare basins along eontinental margins are mostly
well supplied with terrigenous material from the hinterland of the magmatie are, apart from are-derived
volcanic1astie material. Intraoeeanie basins are normally eut off from signifieant terrigenous sediment
sourees.
South Ameriean foreare basins along the eoast and offshore Ecuador, Peru, and Chile eontain Cretaeeous and
Tertiary sediments with thieknesses of several km up to 10
km (Moberly et al. 1982). Their facies range from deep
marine sediments, eontaining large proportions of
turbidites and deposits of gravity mass flows and slides, to
paralie and eontinental deposits. They evolved on eontinental erust (intra-massifbasins, see above) and frequently
show landward dipping strata in their lower seetions, indieating syndepositional uplift of a seaward basement high.
Syndepositional uplift of landward or seaward parts of the
basins also eaused the eutting of deep submarine ehannels.
As a result of eoastal upwelling, some of these basins aeeumulated substantial amounts of hydroearbon souree rocks.
The foreare basins off Colombia belong to the eonstrueted basin type resting on both the eontinental erust of
the magrnatie are and the aeeretionary prism. The presentday tectonic setting (eonvergenee rate 50-65 m/Ma) was
established in the early Mioeene. The seaward-dipping
edge of the eontinental erust aets as backstop, and the progressive growth ofthe aeeretionary wedge leads to uplift of
the outer-are high. This eauses tilting of the older sedimentary sequenees and sueeessive landward migration of the
depoeenters of the foreare basins (Mountney and Westbrook 1997; cf. Fig. 12.25e). The maximum thiekness of
the basin fills is almost 5 km.
Foreare basin off Guatemala. This sediment-filled basin is mentioned here as an example of late PleistoeeneHoloeene eontinental foreare basin sediments. It has aeeumulated non-marine, predominantly voleaniclastie material
during the past 20 to 30 ka (Vessell and Davies 1981).
These sediments refleet major voleanie eruptions, whieh
have demonstrated a eertain degree of eyelieity expressed
by (1) a long quieseent phase with ineision of meandering
rivers and delta reworking, (2) an eruptive phase with
airfall ash and pyroclastie flows, (3) a fan building phase
dominated by debris flows and eoarse grained fluvial deposits, and (4) a braided stream phase delivering large sediment volumes into the fluvial system and eausing delta
pro gradation on the eoast.
595
Modern Intraoceanic Foreare Basins
Sunda are. The foreare basins of the Sunda are briefly
deseribed in Seet. 12.5.2 (cf. Fig. 12.23). They represent
basins whieh are unequally filled in relation to their sediment sourees.
Boso Peninsula, Japan. This Plio-Pleistoeene foreare
basin (Kazusa Group) is loeated to the east of Tokyo and
assoeiated with the subduction eomplex of the southem
Japan treneh (cf. Fig. 12.24b; Ito and Katsura 1992, 1993;
Makino et al. 1992). The western, landward part ofthe basin fill is uplifted and forms the Boso Peninsula. The Tokyo Bay farther west is a relie of this basin whieh demonstrates the shallow-water, coastal and deltaie stage of the
foreare (Ito and O'Hara 1994). East of the peninsula, the
foreare basin is still below the sea and being filled. The
sediments exposed on the peninsula refleet a shelf-slopedeep basin setting and reaeh up to 3 km in thiekness. The
prograding, mainly volcaniclastie pebbly sands and muds
of the narrow shelf-steep slope configuration display many
mass flow deposits and form relatively small submarine
fans. Ash fall deposits have been radiometrieally dated and
serve as marker beds for the identifieation of about twenty,
sea-Ievel eontrolled depositional sequenees.
Tonga Ridge, western Paeifie. This is an example of an
intraoeeanie are-treneh system whieh is far away from any
signifieant eontinental sediment souree (cf. Fig. 12.30a).
The subduetion zone is non-aeeretionary and eharaeterized
by tectonic erosion. A special feature of this are is its eollision with a seamount and an oeeanie ridge whieh have
shortened and steepened the foreare zone (Clift et al.
1998). The uplifted foreare region forrns a plateau (Tonga
platform) whieh is affeeted by aeross-strike fault zones and
dissected trenehward by submarine eanyons. The relatively
thin fore are sediments eonsist mainly of redeposited material from the volcanie are, ash fall, some pelagie sediment.
Minor amounts of sediment are delivered by erosion of
older foreare strata and the Eoeene igneous basement of the
foreare.
Mariana are system (cf. Figs. 12.28 and 12.30). Similarly, the youngest part of this system displays only a thin
sediment cover. It is an example of repeated are and
baekare formation with a very eomplex interrelationship
between teetonism and magrnatism (Fryer 1996).
Less Antilles. In contrast to the Boso peninsula exampie, the late Pleistoeene sediments of this are system eontain high proportions of pelagie, hemipelagie, and even
eolian sediments besides voleaniclasties provided by episodie volcanie aetivity.
Ancient Foreare Basins on Land
The identifieation of aneient foreare basins on land is
not always easy. The fills of these basins are deformed and uplifted by post-depositional teetonism
and more or less truneated by erosion. One needs
good exposures and a c1ear understanding of the general teetonie setting of the region investigated. Relies
of a former foreare basin ean be expeeted between
the roots of a magmatie are and the thrust sheets
(nappes) and melange zones of an aneient aeeretionuy prism now representing part of an
orogenie belt. The presenee of ophiolites in the
ments in subduetion eomplexes. Thiek foreare fills
on top of deeply subsided basin floors have a relatively high preservation potential.
For further diseussion see, e.g., Diekinson and Seely
(1979), Maedonald (1993), Busby and Ingersoll (1995).
Examples of Foreare Basins
Modern Foreare Basins on Continental Margins
The above mentioned general trends and some other
speeifie features are illustrated by a few examples
from modem foreare basins whieh still aeeumulate
sediment. The examples demortstrate different teetonie and sedimentary aspeets of these basins.
Foreare basins along eontinental margins are mostly
well supplied with terrigenous material from the hinterland of the magmatie are, apart from are-derived
volcanic1astie material. Intraoeeanie basins are normally eut off from signifieant terrigenous sediment
sourees.
South Ameriean foreare basins along the eoast and offshore Ecuador, Peru, and Chile eontain Cretaeeous and
Tertiary sediments with thieknesses of several km up to 10
km (Moberly et al. 1982). Their facies range from deep
marine sediments, eontaining large proportions of
turbidites and deposits of gravity mass flows and slides, to
paralie and eontinental deposits. They evolved on eontinental erust (intra-massifbasins, see above) and frequently
show landward dipping strata in their lower seetions, indieating syndepositional uplift of a seaward basement high.
Syndepositional uplift of landward or seaward parts of the
basins also eaused the eutting of deep submarine ehannels.
As a result of eoastal upwelling, some of these basins aeeumulated substantial amounts of hydroearbon souree rocks.
The foreare basins off Colombia belong to the eonstrueted basin type resting on both the eontinental erust of
the magrnatie are and the aeeretionary prism. The presentday tectonic setting (eonvergenee rate 50-65 m/Ma) was
established in the early Mioeene. The seaward-dipping
edge of the eontinental erust aets as backstop, and the progressive growth ofthe aeeretionary wedge leads to uplift of
the outer-are high. This eauses tilting of the older sedimentary sequenees and sueeessive landward migration of the
depoeenters of the foreare basins (Mountney and Westbrook 1997; cf. Fig. 12.25e). The maximum thiekness of
the basin fills is almost 5 km.
Foreare basin off Guatemala. This sediment-filled basin is mentioned here as an example of late PleistoeeneHoloeene eontinental foreare basin sediments. It has aeeumulated non-marine, predominantly voleaniclastie material
during the past 20 to 30 ka (Vessell and Davies 1981).
These sediments refleet major voleanie eruptions, whieh
have demonstrated a eertain degree of eyelieity expressed
by (1) a long quieseent phase with ineision of meandering
rivers and delta reworking, (2) an eruptive phase with
airfall ash and pyroclastie flows, (3) a fan building phase
dominated by debris flows and eoarse grained fluvial deposits, and (4) a braided stream phase delivering large sediment volumes into the fluvial system and eausing delta
pro gradation on the eoast.
595
Modern Intraoceanic Foreare Basins
Sunda are. The foreare basins of the Sunda are briefly
deseribed in Seet. 12.5.2 (cf. Fig. 12.23). They represent
basins whieh are unequally filled in relation to their sediment sourees.
Boso Peninsula, Japan. This Plio-Pleistoeene foreare
basin (Kazusa Group) is loeated to the east of Tokyo and
assoeiated with the subduction eomplex of the southem
Japan treneh (cf. Fig. 12.24b; Ito and Katsura 1992, 1993;
Makino et al. 1992). The western, landward part ofthe basin fill is uplifted and forms the Boso Peninsula. The Tokyo Bay farther west is a relie of this basin whieh demonstrates the shallow-water, coastal and deltaie stage of the
foreare (Ito and O'Hara 1994). East of the peninsula, the
foreare basin is still below the sea and being filled. The
sediments exposed on the peninsula refleet a shelf-slopedeep basin setting and reaeh up to 3 km in thiekness. The
prograding, mainly volcaniclastie pebbly sands and muds
of the narrow shelf-steep slope configuration display many
mass flow deposits and form relatively small submarine
fans. Ash fall deposits have been radiometrieally dated and
serve as marker beds for the identifieation of about twenty,
sea-Ievel eontrolled depositional sequenees.
Tonga Ridge, western Paeifie. This is an example of an
intraoeeanie are-treneh system whieh is far away from any
signifieant eontinental sediment souree (cf. Fig. 12.30a).
The subduetion zone is non-aeeretionary and eharaeterized
by tectonic erosion. A special feature of this are is its eollision with a seamount and an oeeanie ridge whieh have
shortened and steepened the foreare zone (Clift et al.
1998). The uplifted foreare region forrns a plateau (Tonga
platform) whieh is affeeted by aeross-strike fault zones and
dissected trenehward by submarine eanyons. The relatively
thin fore are sediments eonsist mainly of redeposited material from the volcanie are, ash fall, some pelagie sediment.
Minor amounts of sediment are delivered by erosion of
older foreare strata and the Eoeene igneous basement of the
foreare.
Mariana are system (cf. Figs. 12.28 and 12.30). Similarly, the youngest part of this system displays only a thin
sediment cover. It is an example of repeated are and
baekare formation with a very eomplex interrelationship
between teetonism and magrnatism (Fryer 1996).
Less Antilles. In contrast to the Boso peninsula exampie, the late Pleistoeene sediments of this are system eontain high proportions of pelagie, hemipelagie, and even
eolian sediments besides voleaniclasties provided by episodie volcanie aetivity.
Ancient Foreare Basins on Land
The identifieation of aneient foreare basins on land is
not always easy. The fills of these basins are deformed and uplifted by post-depositional teetonism
and more or less truneated by erosion. One needs
good exposures and a c1ear understanding of the general teetonie setting of the region investigated. Relies
of a former foreare basin ean be expeeted between
the roots of a magmatie are and the thrust sheets
(nappes) and melange zones of an aneient aeeretionuy prism now representing part of an
orogenie belt. The presenee of ophiolites in the