12 Basin Evolution and Sediments
12.2 Continental Margin and
Slope Basins
12.2.1 Structural Types ofContinental Margins
12.2.2 Basin Geometry and Sediment Accumulation
(Overview)
12.2.3 Sediment Successions ofCoritinental Margin
Basins
Sediments of Atlantic-Type Margins
Sediment-Starved Marginal Basins and
Submarine Plateaus
12.2.4 Sediment Successions on Continental Slopes
Modern Slope Deposits
Ancient Slope Deposits
12.2.5 Summary (Continental Margin and Slope Basins)
12.2.1 Structural Types of Continental Margins
A rift system evolves into a eontinental margin basin
when two divergent plates are separated by a spreading center, ereating new oeeanie ernst. Crnstal extension and rifting (syn-rift stage) are then followed by
drifting (post-rift stage), but subsidenee of the
thinned, wedge-like eontinental ernst eontinues due
to eooling of the lithosphere and inereasing sediment
load. Syn-rift and post-rift sediments are frequently
separated by a more or less distinet breakup unconformity eaused by differential movements of the
ernstal blocks at the onset of drifting. Higher elevated blocks may undergo subaerial or submarine
erosion. Three different classes of passive margins
have been identified (Fig. 12.10; e.g. Roberts et al.
1984; Hinz et al. 1987):
- Nonvolcanic passive continental margins (Fig.
12.10a). Lithospherie deformation is dominated by
block faulting over a broad zone (100-300 km) and
volcanie aetivity in the upper ernst is of minor importanee. Examples: Red Sea, eastem United States,
nortbwestem Afriea, Galieia Bank off Iberia, Goban
Spur to the southwest of Ireland, northwestem Australia (see also below). These margins show all transitions from being sediment-starved (;;;1-2 km postrift sediments) or heavily sedimented (up to <:10 km
post-rift sediments).
- Volcanic passive margins (Fig. 12.l0b). The
boundary between eontinental and normal oeeanie
ernst is eharaeterized by a thiek zone of seaward dipping volcanie units overlying eontinental ernst. Margins of this type are narrower than nonvolcanie margins, and some of them appear to be assoeiated with
marginal plateaus. Examples include the NorwegianGreenland Sea (Rockall Bank, V",ring Plateau, Fig.
12.9a,b; e.g. Eldholm and Grne 1994; Larsen and
Dunean 1996) and parts of the South Atlantie (off
southem Brazil and Namibia; e.g. Gladezenko et al.
1997).
- Riß-transform margins (Fig. 12.l0e), whieh originate from extensional deformation, including a signifieant strike-slip eomponent. It is assumed, for example, that the early development of the South
Ameriean and southwest Afriean margins was affected by transtension. A younger example is the borderland off southem Califomia (cf. Fig. 5.1 Ob).
The following examples of basin evolution deal predominantly with the first and most eommon eategory
of eontinental margins. The fact that most margins
are segmented along their length on ascale of several
hundred to more than a thousand kilometers (cf. Fig.
12.7e, Canadian shelf) is not further dealt with here.
While the strueture and subsidenee his tory within
individual segments appear to be relatively uniform,
they may differ signifieantly from one segment to
another.
12.2.2 Basin Geometry and Sediment
Accumulation (Overview)
The width of eontinental margin basins is mainly a
funetion of the eontinental ernst, whieh was subjeeted to rifting prior to drifting. The eireum-Atlantie
basins are typieally 200 to 300 km wide, but some
are mueh narrower or eonsiderably wider (see overview in Fig. 12.17a). In many eases, the present shelf
edge ot base of slope eorresponds approximately
with the boundary between subsided eontinental
erust and oeeanie ernst. Narrower shelves may result
from large bounding faults (<:4 km throw) near the
shelf edge and a rapid transition from normally thiek
eontinental ernst to substantially thinned ernst
(Davison 1997). In other eases, strong oeean eurrents
preventing sediment aeeumulation (e.g. the Gulf
Stream off Florida) or insuffieient sediment supply
and heavy storms (e.g. along the margins of islands
exposed to the open sea) ean keep the shelves narrow. Then the subsided ernst forms marginal pla-
12.2 Continental Margin and
Slope Basins
12.2.1 Structural Types ofContinental Margins
12.2.2 Basin Geometry and Sediment Accumulation
(Overview)
12.2.3 Sediment Successions ofCoritinental Margin
Basins
Sediments of Atlantic-Type Margins
Sediment-Starved Marginal Basins and
Submarine Plateaus
12.2.4 Sediment Successions on Continental Slopes
Modern Slope Deposits
Ancient Slope Deposits
12.2.5 Summary (Continental Margin and Slope Basins)
12.2.1 Structural Types of Continental Margins
A rift system evolves into a eontinental margin basin
when two divergent plates are separated by a spreading center, ereating new oeeanie ernst. Crnstal extension and rifting (syn-rift stage) are then followed by
drifting (post-rift stage), but subsidenee of the
thinned, wedge-like eontinental ernst eontinues due
to eooling of the lithosphere and inereasing sediment
load. Syn-rift and post-rift sediments are frequently
separated by a more or less distinet breakup unconformity eaused by differential movements of the
ernstal blocks at the onset of drifting. Higher elevated blocks may undergo subaerial or submarine
erosion. Three different classes of passive margins
have been identified (Fig. 12.10; e.g. Roberts et al.
1984; Hinz et al. 1987):
- Nonvolcanic passive continental margins (Fig.
12.10a). Lithospherie deformation is dominated by
block faulting over a broad zone (100-300 km) and
volcanie aetivity in the upper ernst is of minor importanee. Examples: Red Sea, eastem United States,
nortbwestem Afriea, Galieia Bank off Iberia, Goban
Spur to the southwest of Ireland, northwestem Australia (see also below). These margins show all transitions from being sediment-starved (;;;1-2 km postrift sediments) or heavily sedimented (up to <:10 km
post-rift sediments).
- Volcanic passive margins (Fig. 12.l0b). The
boundary between eontinental and normal oeeanie
ernst is eharaeterized by a thiek zone of seaward dipping volcanie units overlying eontinental ernst. Margins of this type are narrower than nonvolcanie margins, and some of them appear to be assoeiated with
marginal plateaus. Examples include the NorwegianGreenland Sea (Rockall Bank, V",ring Plateau, Fig.
12.9a,b; e.g. Eldholm and Grne 1994; Larsen and
Dunean 1996) and parts of the South Atlantie (off
southem Brazil and Namibia; e.g. Gladezenko et al.
1997).
- Riß-transform margins (Fig. 12.l0e), whieh originate from extensional deformation, including a signifieant strike-slip eomponent. It is assumed, for example, that the early development of the South
Ameriean and southwest Afriean margins was affected by transtension. A younger example is the borderland off southem Califomia (cf. Fig. 5.1 Ob).
The following examples of basin evolution deal predominantly with the first and most eommon eategory
of eontinental margins. The fact that most margins
are segmented along their length on ascale of several
hundred to more than a thousand kilometers (cf. Fig.
12.7e, Canadian shelf) is not further dealt with here.
While the strueture and subsidenee his tory within
individual segments appear to be relatively uniform,
they may differ signifieantly from one segment to
another.
12.2.2 Basin Geometry and Sediment
Accumulation (Overview)
The width of eontinental margin basins is mainly a
funetion of the eontinental ernst, whieh was subjeeted to rifting prior to drifting. The eireum-Atlantie
basins are typieally 200 to 300 km wide, but some
are mueh narrower or eonsiderably wider (see overview in Fig. 12.17a). In many eases, the present shelf
edge ot base of slope eorresponds approximately
with the boundary between subsided eontinental
erust and oeeanie ernst. Narrower shelves may result
from large bounding faults (<:4 km throw) near the
shelf edge and a rapid transition from normally thiek
eontinental ernst to substantially thinned ernst
(Davison 1997). In other eases, strong oeean eurrents
preventing sediment aeeumulation (e.g. the Gulf
Stream off Florida) or insuffieient sediment supply
and heavy storms (e.g. along the margins of islands
exposed to the open sea) ean keep the shelves narrow. Then the subsided ernst forms marginal pla-