12.2 Continental Margin and Siope Basins
12.2.3 Sediment Successions of
Continental Margin Basins
Sediments of Atlantic-Type Margins
The post-rift sediments of continental margins overlie the older rift basin fills (in places characterized by
a breakup unconformity) and cover wider areas than
individual rift structures. The rates of post-rift subsidence decay exponentially with time and are commonly lower than those of syn-rift subsidence of the
former rift basins (Sects.8.3 and 8.4).
The sediments of Atlantic-type mature continental
margins, which are fi1led with sediments up to near
sea level (present water depth 100 to 200 m), can be
subdivided into four groups (Fig. 12.11; Kingston et
al. 1983):
(1) Silicic1astic sediments formed by upbuilding and
outbuilding. The subsiding continental margin accumulates predorninantly silicic1astic sediments at moderate, seaward increasing rates. The isochrones show
wedge-like sediment bodies resulting from both
upbuilding and outbuilding of sediment. The deepest
basin floor (basement top) is commonly found below
the continental slope or rise. The zone of maximum
thickness (depocenter) of a certain stratigraphic unit
tends to migrate basinward with time (cf. Fig.
l2.1la).
The modem margins of southwest Africa, southern Morocco, and the northem Gulf of Mexico provide examples
of this basin type. High-latitude continental margins have
accumulated Quaternary glacio-marine deposits of considerable thickness as known from the Barents shelf off northern Scandinavia (cf. Sect. 9.5).
(2) Carbonate buildups. Large portions of the modem
and ancient continental margins are built up of
shallow-marine carbonates (cf. Sect. 3.4). If the shelf
edge is dammed by reefs (thereby forrning a rimmed
carbonate shelt), the slope can become very steep.
Basinward of the reef front, the slope can prograde
due to the accumulation of reef talus (Fig. 12.l1b)
and debris flows, or she1f and terrigenous material
may spill over the shelf edge. Fine-grained, muddy
carbonates produce gentle prograding slopes (2-4°),
frequently characterized by erosional gullies, while
coarser grained debris forms steep slopes (10-12°, up
to more than 30° in the upper part; e.g. Kenter 1990).
The latter commonly exhibit increased slumping, debris flows, and bypassing of sediment via canyons
(Fig. 12.12a).
Modem examples of carbonate shelves include the continental margin off Florida, Yucatan, and the Senegal coast.
The upward growth of the Blake Plateau in the western
Atlantic was largely terminated by the Gulf Stream, however (cf Sect. 5.3.7, Fig. 5.10). The marginal Mazagan pla563
teau off Morocco shows an early, syn-rift phase of Jurassic
carbonate buildup which was terminated due to insufficient
subsidence in the early Cretaceous and then followed by
post-rift hemipelagic and pelagic deposition (cf Sect.
5.3.7).
Isolated carbonate platforms on continental or intermediate crust as, for example, the present-day Bahamas Archipelago, result entirely from carbonate accumulation. However, the previously held view that carbonate buildup was
accomplished predominantly by stationary upward growth
has to be modified (Schlager and Camper 1986; Eberli and
Ginsburg 1987 and 1989; Leg 101 Scientific Party 1988).
Seismic records and drilling have shown that this bankand-basin archipe1ago was characterized by periods of lateral expansion and retreat as weil as by phases of deeper
water carbonate deposition (Fig. 12.12b-d). Upbuilding in
the 1ate Jurassic and early Cretaceous was followed by
drowning and retreat during the mid-Cretaceous and by
renewed lateral expansion in the Cenozoic.
Lateral, discontinuous prograding of carbonate p1atforms (500-1000 m in thickness) has also been described
from the Triassic of the Dolomites in northern Italy
(Bosellini 1984). The effects of sea-level changes on carbonate buildups and carbonate slopes are described in Sect.
7.5 (cf. Figs. 7.24 and 7.35).
(3) Prograding deltaic sediments. High sediment influx from a major river delta causes continuous filling of the subsiding marginal basin and rapid
pro gradation of the slope beyond the continentalloceanic crust boundary (Fig. 12.llc). As a result,
the zone of maximum sediment thickness of a modem delta may lie landward of the present coast line.
The low strength of muddy, underconsolidated sediments leads to the formation of growth faults, rollover structures, and mud diapirs.
Prominent examp1es are the she1f off the Niger delta in
West Africa (Doust and Ornatsola 1990, cf. Fig. 14.9), and
the northern margin ofthe Gu1f ofMexico with the Mississippi delta (cf Sect. 5.4.5; Figs. 5.21 and 13.12) which is
also rnodified by sa1t diapirism.
(4) Continental margins affected by salt structures.
The growing sedimentary wedge of a marginal basin
can be strongly affected by halokinesis of underlying
salt (Fig. l2.l1d; cf. Sect. 6.4.4). Salt diapirs act as
darns for shelf sediments which in turn are deformed
by subsidence in special troughs and uplift over the
rising salt structures.
This type of continental margin basin is very common
around the Atlantic. It is known, for example, from the
Gu1f of Mexico, the she1f of Nova Scotia (Scotian basin)
and the conjugate margin ofWest Africa (Essaouira-Agadir
basin and Aaiun-Tarfaya basin, Fig. 12.9c; e.g., Jansa and
Wiedmann 1982) or the Gabon basin (Fig. 12.9d, Kingston
et al. 1983; Teisserene and Villernin 1990).
Many basins on passive continental margins exhibit a
combination of two or three of these end-member
basin types. In the marginal basins of northwest Af-
12.2.3 Sediment Successions of
Continental Margin Basins
Sediments of Atlantic-Type Margins
The post-rift sediments of continental margins overlie the older rift basin fills (in places characterized by
a breakup unconformity) and cover wider areas than
individual rift structures. The rates of post-rift subsidence decay exponentially with time and are commonly lower than those of syn-rift subsidence of the
former rift basins (Sects.8.3 and 8.4).
The sediments of Atlantic-type mature continental
margins, which are fi1led with sediments up to near
sea level (present water depth 100 to 200 m), can be
subdivided into four groups (Fig. 12.11; Kingston et
al. 1983):
(1) Silicic1astic sediments formed by upbuilding and
outbuilding. The subsiding continental margin accumulates predorninantly silicic1astic sediments at moderate, seaward increasing rates. The isochrones show
wedge-like sediment bodies resulting from both
upbuilding and outbuilding of sediment. The deepest
basin floor (basement top) is commonly found below
the continental slope or rise. The zone of maximum
thickness (depocenter) of a certain stratigraphic unit
tends to migrate basinward with time (cf. Fig.
l2.1la).
The modem margins of southwest Africa, southern Morocco, and the northem Gulf of Mexico provide examples
of this basin type. High-latitude continental margins have
accumulated Quaternary glacio-marine deposits of considerable thickness as known from the Barents shelf off northern Scandinavia (cf. Sect. 9.5).
(2) Carbonate buildups. Large portions of the modem
and ancient continental margins are built up of
shallow-marine carbonates (cf. Sect. 3.4). If the shelf
edge is dammed by reefs (thereby forrning a rimmed
carbonate shelt), the slope can become very steep.
Basinward of the reef front, the slope can prograde
due to the accumulation of reef talus (Fig. 12.l1b)
and debris flows, or she1f and terrigenous material
may spill over the shelf edge. Fine-grained, muddy
carbonates produce gentle prograding slopes (2-4°),
frequently characterized by erosional gullies, while
coarser grained debris forms steep slopes (10-12°, up
to more than 30° in the upper part; e.g. Kenter 1990).
The latter commonly exhibit increased slumping, debris flows, and bypassing of sediment via canyons
(Fig. 12.12a).
Modem examples of carbonate shelves include the continental margin off Florida, Yucatan, and the Senegal coast.
The upward growth of the Blake Plateau in the western
Atlantic was largely terminated by the Gulf Stream, however (cf Sect. 5.3.7, Fig. 5.10). The marginal Mazagan pla563
teau off Morocco shows an early, syn-rift phase of Jurassic
carbonate buildup which was terminated due to insufficient
subsidence in the early Cretaceous and then followed by
post-rift hemipelagic and pelagic deposition (cf Sect.
5.3.7).
Isolated carbonate platforms on continental or intermediate crust as, for example, the present-day Bahamas Archipelago, result entirely from carbonate accumulation. However, the previously held view that carbonate buildup was
accomplished predominantly by stationary upward growth
has to be modified (Schlager and Camper 1986; Eberli and
Ginsburg 1987 and 1989; Leg 101 Scientific Party 1988).
Seismic records and drilling have shown that this bankand-basin archipe1ago was characterized by periods of lateral expansion and retreat as weil as by phases of deeper
water carbonate deposition (Fig. 12.12b-d). Upbuilding in
the 1ate Jurassic and early Cretaceous was followed by
drowning and retreat during the mid-Cretaceous and by
renewed lateral expansion in the Cenozoic.
Lateral, discontinuous prograding of carbonate p1atforms (500-1000 m in thickness) has also been described
from the Triassic of the Dolomites in northern Italy
(Bosellini 1984). The effects of sea-level changes on carbonate buildups and carbonate slopes are described in Sect.
7.5 (cf. Figs. 7.24 and 7.35).
(3) Prograding deltaic sediments. High sediment influx from a major river delta causes continuous filling of the subsiding marginal basin and rapid
pro gradation of the slope beyond the continentalloceanic crust boundary (Fig. 12.llc). As a result,
the zone of maximum sediment thickness of a modem delta may lie landward of the present coast line.
The low strength of muddy, underconsolidated sediments leads to the formation of growth faults, rollover structures, and mud diapirs.
Prominent examp1es are the she1f off the Niger delta in
West Africa (Doust and Ornatsola 1990, cf. Fig. 14.9), and
the northern margin ofthe Gu1f ofMexico with the Mississippi delta (cf Sect. 5.4.5; Figs. 5.21 and 13.12) which is
also rnodified by sa1t diapirism.
(4) Continental margins affected by salt structures.
The growing sedimentary wedge of a marginal basin
can be strongly affected by halokinesis of underlying
salt (Fig. l2.l1d; cf. Sect. 6.4.4). Salt diapirs act as
darns for shelf sediments which in turn are deformed
by subsidence in special troughs and uplift over the
rising salt structures.
This type of continental margin basin is very common
around the Atlantic. It is known, for example, from the
Gu1f of Mexico, the she1f of Nova Scotia (Scotian basin)
and the conjugate margin ofWest Africa (Essaouira-Agadir
basin and Aaiun-Tarfaya basin, Fig. 12.9c; e.g., Jansa and
Wiedmann 1982) or the Gabon basin (Fig. 12.9d, Kingston
et al. 1983; Teisserene and Villernin 1990).
Many basins on passive continental margins exhibit a
combination of two or three of these end-member
basin types. In the marginal basins of northwest Af-