12.2 Continental Margin and Siope Basins
basins strongly reflect the characteristics of their hinterlands, the climate, and relative sea-Ievel changes.
Sediment-Starved Marginal Basins and
Submarine Plateaus
All the basin types discussed so far have in common
the tendency for being more or less continuously
fi1led with sediments up to the water depth of the
continental shelves and slopes. A surplus in sediment
supply will bypass the shelf and settle on the slope or
in deeper water (see below). This case is common
during late basin evolution, when subsidence slows.
In contrast, periods of rapid subsidence (during early
drifting) or insufficient sediment supply (e.g. to basins far away from terrigenous sources ) lead to
sediment-starved basins. This situation is demonstrated by a two examples from young ocean basins
and one example from the older Atlantic continental
margin (Figs. 12.14 and 12.15a,b).
Baja California passive margin. Sea-tloor spreading at
the southem end ofthe GulfofCalifomia began 4 Ma B.P.
and caused the passive, rifted continental margin adjacent
to the tip of Baja Califomia to subside by about 2 km (Fig.
12.13c,d; subsidence curve cf. Fig. 8.12a). The present,
post-rift sediment cover on top ofthe stretchedand sunken
continental crnst is only 100-200 m thick, although the
sedimentation rate for diatomaceous ooze on this slope was
relatively high (30-80 mlMa; Curray et al. 1982b).
East Sardinia continental margin. Deeply subsided, sediment-starved continental margins also occur in the Mediterranean (Burollet et al. 1978). The young, recently drilled
passive margin of Sardinia subsided rapidly with the onset
of sea-tloor spreading in the late Miocene or early Pliocene
(Fig. 12.14a,b; Kasten, Mascle et al. 1988; Savelli 1988;
Argnani and Savelli 1999). It borders the Tyrrhenian Sea,
which is interpreted as a backarc basin with young oceanic
crnst generated during two spreading episodes (7 Ma and
1.9 to 1.3 Ma). The center of magmatism migrated trom
northwest to southeast and formed the presently active
Eolian Islands, Strombo1i, and the volcanoes of central
Italy.
The syn-rift sediments of the sunken Sardinian continental margin (Site 654, Fig. 12.14a) partially show a typical transgressive sequence beginning with subaerial conglomerates, overlain by oyster-bearing sands and marine
marls. During the Messinian (latest Miocene) desiccation
of the Mediterranean, the landward margin accumulated
nannoplankton-bearing clays interbedded with larninated
gypsum, while the seaward, more rapidly subsiding margin
apparentlY stood higher, as indicated by subaerial and lacustrine facies. The post-rift sediments are about 200 m
thick and consist of nannofossil ooze with minor proportions of calcareous mud, volcanic ash, and sapropel.
Goban Spur southwest of Ireland. Parts of the old continental outer margin of the eastem Atlantic as, e.g., the
Goban Spur, subsided to bathyal depths without sufficient
sedimentary cover (Fig. 12.13a,b; de Graciansky and Poag
1985). Seismic profiles and drilling of this sediment567
starved margin have revealed a half-graben within the continental crnst subjected to rifting since the Triassic. Continental breakup occurred in the early Cretaceous prior to the
Albian (Colin et al. 1992). The sediment succession displays two major hiatuses, one in the late Jurassic and another one in the Turonian. The late syn-rift sediments of
early Cretaceous age consist of shallow-water marls and
limestones. The post-rift development began in the Albian
above an unconformity. Bathyal to abyssal chalks indicate
rapid subsidence subsequent to the onset of sea tloor
spreading. The further development is characterized by minor modifications due to a tluctuating CCD and periodic
intlux of terrigenous detritus, especially in the Cenozoic.
The total thickness of the post-rift sediments ranges trom
0.5 to 1.5 km and is thus in striking contrast to the huge
depositional prism ofthe opposing North American margin
(e.g. ~ 10 km in the Baltimore Canyon Trough).
The Galicia Bank off the Iberian peninsula shows a
similar situation, with a very thin, incomplete post-rift sediment cover.
Submarine plateaus, -1000-3000 m below sea level,
occur in all large ocean basins. They are more or less
separated from terrigenous sediment sources and
their sediments show characteristics similar to those
of sediment-starved outer margins mentioned above
(cf. Sect. 5.3.8).
Submarine plateaus of the southwest Pacific. The
Ontong-Java Plateau at the equator, several rises between
Melanesia and Australia, and some plateaus east of New
Zealand occupy broad areas of the sea tloor (Kennett and
von der Borch 1986). The tops ofmost ofthese plateaus lie
well above the CCD. They are usually covered by relatively
thick Paleogene to Neogene calcareous foraminiferal
nannofossil oozes, which are little to not affected by carbonate dissolution. Deeper portions of the plateaus may
have been affected by CCD tluctuations and therefore show
distinct layering. They indicate the changing climatic and
paleoceancgraphic conditions in the late Mesozoic and Cenozoic (e.g., Berger et al. 1991).
Exmouth Plateau off northwest Australia. This submarine plateau represents an old passive continental margin of
the Indian Ocean. It lost part of its syn-rift Jurassic sediment cover due to uplift in conjunction with block faulting.
After breakup of the Gondwana continent and the onset of
sea-tloor spreading in the early Cretaceous, the entire
Exmouth Plateau subsided below sea level. Outer parts of
the marginal basin (e.g., the Wombat Plateau) became
sediment-starved and therefore display only a thin post-rift
sequence, including condensed horizons, pelagic limestones and black shales at the Cenomanian-Turonian
boundary (Leg 122 Shipboard Scientific Party 1989; von
Rad et al. 1989).
basins strongly reflect the characteristics of their hinterlands, the climate, and relative sea-Ievel changes.
Sediment-Starved Marginal Basins and
Submarine Plateaus
All the basin types discussed so far have in common
the tendency for being more or less continuously
fi1led with sediments up to the water depth of the
continental shelves and slopes. A surplus in sediment
supply will bypass the shelf and settle on the slope or
in deeper water (see below). This case is common
during late basin evolution, when subsidence slows.
In contrast, periods of rapid subsidence (during early
drifting) or insufficient sediment supply (e.g. to basins far away from terrigenous sources ) lead to
sediment-starved basins. This situation is demonstrated by a two examples from young ocean basins
and one example from the older Atlantic continental
margin (Figs. 12.14 and 12.15a,b).
Baja California passive margin. Sea-tloor spreading at
the southem end ofthe GulfofCalifomia began 4 Ma B.P.
and caused the passive, rifted continental margin adjacent
to the tip of Baja Califomia to subside by about 2 km (Fig.
12.13c,d; subsidence curve cf. Fig. 8.12a). The present,
post-rift sediment cover on top ofthe stretchedand sunken
continental crnst is only 100-200 m thick, although the
sedimentation rate for diatomaceous ooze on this slope was
relatively high (30-80 mlMa; Curray et al. 1982b).
East Sardinia continental margin. Deeply subsided, sediment-starved continental margins also occur in the Mediterranean (Burollet et al. 1978). The young, recently drilled
passive margin of Sardinia subsided rapidly with the onset
of sea-tloor spreading in the late Miocene or early Pliocene
(Fig. 12.14a,b; Kasten, Mascle et al. 1988; Savelli 1988;
Argnani and Savelli 1999). It borders the Tyrrhenian Sea,
which is interpreted as a backarc basin with young oceanic
crnst generated during two spreading episodes (7 Ma and
1.9 to 1.3 Ma). The center of magmatism migrated trom
northwest to southeast and formed the presently active
Eolian Islands, Strombo1i, and the volcanoes of central
Italy.
The syn-rift sediments of the sunken Sardinian continental margin (Site 654, Fig. 12.14a) partially show a typical transgressive sequence beginning with subaerial conglomerates, overlain by oyster-bearing sands and marine
marls. During the Messinian (latest Miocene) desiccation
of the Mediterranean, the landward margin accumulated
nannoplankton-bearing clays interbedded with larninated
gypsum, while the seaward, more rapidly subsiding margin
apparentlY stood higher, as indicated by subaerial and lacustrine facies. The post-rift sediments are about 200 m
thick and consist of nannofossil ooze with minor proportions of calcareous mud, volcanic ash, and sapropel.
Goban Spur southwest of Ireland. Parts of the old continental outer margin of the eastem Atlantic as, e.g., the
Goban Spur, subsided to bathyal depths without sufficient
sedimentary cover (Fig. 12.13a,b; de Graciansky and Poag
1985). Seismic profiles and drilling of this sediment567
starved margin have revealed a half-graben within the continental crnst subjected to rifting since the Triassic. Continental breakup occurred in the early Cretaceous prior to the
Albian (Colin et al. 1992). The sediment succession displays two major hiatuses, one in the late Jurassic and another one in the Turonian. The late syn-rift sediments of
early Cretaceous age consist of shallow-water marls and
limestones. The post-rift development began in the Albian
above an unconformity. Bathyal to abyssal chalks indicate
rapid subsidence subsequent to the onset of sea tloor
spreading. The further development is characterized by minor modifications due to a tluctuating CCD and periodic
intlux of terrigenous detritus, especially in the Cenozoic.
The total thickness of the post-rift sediments ranges trom
0.5 to 1.5 km and is thus in striking contrast to the huge
depositional prism ofthe opposing North American margin
(e.g. ~ 10 km in the Baltimore Canyon Trough).
The Galicia Bank off the Iberian peninsula shows a
similar situation, with a very thin, incomplete post-rift sediment cover.
Submarine plateaus, -1000-3000 m below sea level,
occur in all large ocean basins. They are more or less
separated from terrigenous sediment sources and
their sediments show characteristics similar to those
of sediment-starved outer margins mentioned above
(cf. Sect. 5.3.8).
Submarine plateaus of the southwest Pacific. The
Ontong-Java Plateau at the equator, several rises between
Melanesia and Australia, and some plateaus east of New
Zealand occupy broad areas of the sea tloor (Kennett and
von der Borch 1986). The tops ofmost ofthese plateaus lie
well above the CCD. They are usually covered by relatively
thick Paleogene to Neogene calcareous foraminiferal
nannofossil oozes, which are little to not affected by carbonate dissolution. Deeper portions of the plateaus may
have been affected by CCD tluctuations and therefore show
distinct layering. They indicate the changing climatic and
paleoceancgraphic conditions in the late Mesozoic and Cenozoic (e.g., Berger et al. 1991).
Exmouth Plateau off northwest Australia. This submarine plateau represents an old passive continental margin of
the Indian Ocean. It lost part of its syn-rift Jurassic sediment cover due to uplift in conjunction with block faulting.
After breakup of the Gondwana continent and the onset of
sea-tloor spreading in the early Cretaceous, the entire
Exmouth Plateau subsided below sea level. Outer parts of
the marginal basin (e.g., the Wombat Plateau) became
sediment-starved and therefore display only a thin post-rift
sequence, including condensed horizons, pelagic limestones and black shales at the Cenomanian-Turonian
boundary (Leg 122 Shipboard Scientific Party 1989; von
Rad et al. 1989).