570
Thus, we can distinguish between constructional
and destructional phases of slope deposition. Some
features of these two phases are summarized in Figs.
l2.l5c and 12.16 for mainly siliciclastic material (for
carbonate slopes see Sects. 12.2.3 and 7.5.5). Slopes
dammed by structural highs, volcanic structures,
reefs, salt or mud diapirs tend to remain sedimentstarved until the shelf basin behind the dam is filled.
Progradation of the slope may proceed either rapidly
or slowly in relation to the influx of terrigenous or
biogenic sediment. The upper slopes on modem continental margins are frequently characterized by sediment bypassing and local sediment ponding,
pro gradation, and the filling of channels and gullies.
The outbuilding slope sediments therefore frequently
reflect the composition of the shelf deposits and their
variation through time. The lower slope or slope
apron preferentially collects slides, debris flows, and
turbidites.
Seismic lines along the slopes of the western Atlantie off
the United States, for example, have revealed that
terrigenous material was transported aeross the early Cretaeeous eontinental shelf to form several large shelf edge
deltas (Poag et al. 1990). Eaeh delta supplied sediment
gravity flows to slope aprons and deep-sea fans on the
lower slope and eontinental rise. In the northern Hatteras
basin, one of these sediment distributory systems extended
500 km basinward.
Mid-Cretaeeous to Cenozoie multi-phase slope destruetion is demonstrated by two examples from the margin off
northwest Afriea (Fig. 12.15; von Rad and Wissmann
1982). Undereutting of the slope at various levels ereated
long hiatuses and angular uneonforrnities in the stratigraphie reeord. Mass wasting was partieularly aetive during
Chapter 12 Basin Evolution and Sediments
the mid-Mioeene and the late Pleistoeene. These proeesses
were aceompanied by the ineision of major slope eanyons.
At least part of the displaeed sediments is redeposited on
the eontinental rise. Here, debris flow deposits and huge
olistostromes interfinger with eontourite drifts.
The eross seetions in Figure 12.15 also display antithetie growth faults with giant rollover struetures, forming
a "slope antieline", and mud diapirs. These struetures are
eommon in rapidly aeeumulating thiek sedimentary sequenees, as represented here by lower Cretaeeous prodelta
deposits.
Ancient Slope Deposits
The identification of ancient slope sediments in exposures of limited extent can be difficult. They may
be characterized by predominantly siliciclastic, hemipelagic, or bioclastic carbonate sediments. Useful
criteria are slope scars and gullies on the upper slope,
canyons fills and mass flow deposits on the lower
slope. The foot of the slope may display features of
undercutting and erosion. Contourites may inter-finger with redeposited sediments (cf. Sect. 5.5). In any
case, the knowledge of the large-scale facies association of the basin fill is useful for the identification of
slope sediments. In addition, faunistic criteria can
indicate the paleo-waterdepth.
If slope successions are incorporated into subduction and collision-related deposits, their identification is even more problematic. Slope sediments
preserved due to rapid subsidence and burial under
thick younger sequences tend to be affected by regionallow-grade metamorphism.
12.2.5 Summary (Continental Margin and Slope Basins)
- Passive continental margins are either nonvolcanic or affected by extensive volcanism and/or
strike-slip. They are commonly segmented and
display various widths.
- The sediment successions of continental margins result from both upbuilding and outbuilding and their architecture is often modified by
outer margin highs (e.g. reefs) and salt structures.
- Atlantic-type mature margins have commonly
accumulated thick sedimentary piles which reflect long-term changes in climate, ocean circulation, and the evolution of marine organisms.
Parts of the outer margin are sediment-starved;
sunken submarine plateaus, and young margins
mostly exhibit a thin sediment cover.
Conjugate passive margins of the same ocean
basin often exhibit a striking similarity of their
syn-rift sediments, but differ in their post-rift
sedimentary evolution.
Margin successions dominated by siliciclastics
form wedge-like sediment bodies. The depocenters of certain time periods tend to migrate
basinward with time. Prograding deltas may
build shelf edges and slopes beyond the
continent-ocean crustal boundary. The slope
sediments often display growth faults, rollover
structures, and mud diapirs.
The sediments of gentle continental slopes can
cover wide areas; carbonate slopes are partially
steeper. Slopes are characterized by constructional and destructional processes, the fills of
slope gullies and canyons, slide scars, and redeposited material at their foot.
As a result of ocean closure, subduction and
collision, most of the sediments of old passive
continental margins were incorporated into
accretionary wedges and orogenic belts where
they underwent deformation and metamorphism.
Thus, we can distinguish between constructional
and destructional phases of slope deposition. Some
features of these two phases are summarized in Figs.
l2.l5c and 12.16 for mainly siliciclastic material (for
carbonate slopes see Sects. 12.2.3 and 7.5.5). Slopes
dammed by structural highs, volcanic structures,
reefs, salt or mud diapirs tend to remain sedimentstarved until the shelf basin behind the dam is filled.
Progradation of the slope may proceed either rapidly
or slowly in relation to the influx of terrigenous or
biogenic sediment. The upper slopes on modem continental margins are frequently characterized by sediment bypassing and local sediment ponding,
pro gradation, and the filling of channels and gullies.
The outbuilding slope sediments therefore frequently
reflect the composition of the shelf deposits and their
variation through time. The lower slope or slope
apron preferentially collects slides, debris flows, and
turbidites.
Seismic lines along the slopes of the western Atlantie off
the United States, for example, have revealed that
terrigenous material was transported aeross the early Cretaeeous eontinental shelf to form several large shelf edge
deltas (Poag et al. 1990). Eaeh delta supplied sediment
gravity flows to slope aprons and deep-sea fans on the
lower slope and eontinental rise. In the northern Hatteras
basin, one of these sediment distributory systems extended
500 km basinward.
Mid-Cretaeeous to Cenozoie multi-phase slope destruetion is demonstrated by two examples from the margin off
northwest Afriea (Fig. 12.15; von Rad and Wissmann
1982). Undereutting of the slope at various levels ereated
long hiatuses and angular uneonforrnities in the stratigraphie reeord. Mass wasting was partieularly aetive during
Chapter 12 Basin Evolution and Sediments
the mid-Mioeene and the late Pleistoeene. These proeesses
were aceompanied by the ineision of major slope eanyons.
At least part of the displaeed sediments is redeposited on
the eontinental rise. Here, debris flow deposits and huge
olistostromes interfinger with eontourite drifts.
The eross seetions in Figure 12.15 also display antithetie growth faults with giant rollover struetures, forming
a "slope antieline", and mud diapirs. These struetures are
eommon in rapidly aeeumulating thiek sedimentary sequenees, as represented here by lower Cretaeeous prodelta
deposits.
Ancient Slope Deposits
The identification of ancient slope sediments in exposures of limited extent can be difficult. They may
be characterized by predominantly siliciclastic, hemipelagic, or bioclastic carbonate sediments. Useful
criteria are slope scars and gullies on the upper slope,
canyons fills and mass flow deposits on the lower
slope. The foot of the slope may display features of
undercutting and erosion. Contourites may inter-finger with redeposited sediments (cf. Sect. 5.5). In any
case, the knowledge of the large-scale facies association of the basin fill is useful for the identification of
slope sediments. In addition, faunistic criteria can
indicate the paleo-waterdepth.
If slope successions are incorporated into subduction and collision-related deposits, their identification is even more problematic. Slope sediments
preserved due to rapid subsidence and burial under
thick younger sequences tend to be affected by regionallow-grade metamorphism.
12.2.5 Summary (Continental Margin and Slope Basins)
- Passive continental margins are either nonvolcanic or affected by extensive volcanism and/or
strike-slip. They are commonly segmented and
display various widths.
- The sediment successions of continental margins result from both upbuilding and outbuilding and their architecture is often modified by
outer margin highs (e.g. reefs) and salt structures.
- Atlantic-type mature margins have commonly
accumulated thick sedimentary piles which reflect long-term changes in climate, ocean circulation, and the evolution of marine organisms.
Parts of the outer margin are sediment-starved;
sunken submarine plateaus, and young margins
mostly exhibit a thin sediment cover.
Conjugate passive margins of the same ocean
basin often exhibit a striking similarity of their
syn-rift sediments, but differ in their post-rift
sedimentary evolution.
Margin successions dominated by siliciclastics
form wedge-like sediment bodies. The depocenters of certain time periods tend to migrate
basinward with time. Prograding deltas may
build shelf edges and slopes beyond the
continent-ocean crustal boundary. The slope
sediments often display growth faults, rollover
structures, and mud diapirs.
The sediments of gentle continental slopes can
cover wide areas; carbonate slopes are partially
steeper. Slopes are characterized by constructional and destructional processes, the fills of
slope gullies and canyons, slide scars, and redeposited material at their foot.
As a result of ocean closure, subduction and
collision, most of the sediments of old passive
continental margins were incorporated into
accretionary wedges and orogenic belts where
they underwent deformation and metamorphism.