208
The modem 800 to 1000 m deep Blake Plateau offthe eastern Florida shelf belongs to this type of deep plateau
(Sheridan and Enos 1979). Its older sediments represent
shallow-water carbonates, but at the beginning of the
Paleocene deep water environments were established. The
sediments deposited since that time contain considerable
quantities of phosphorite and some chert. The phosphorite
occurs in the form of phosphatized carbonate debris, pellets, pebbles and conglomeratic layers, as weil as continuous pavements (Manheim et al. 1980, cf. Sect. 5.3.6). It is a
lag deposit derived from Miocene strata which accumulated in an oceanographic regime quite different from the
present situation under the Gulf Stream.
Another example for a subsiding, sediment-starved,
Jurassic-Cretaceous continental margin at low latitudes is
the Mazagan Plateau off Morocco (Bemoulli and Kälin
1984; Jansa et al. 1984; Winterer and Hinz 1984). Here,
the constructional phase of carbonate buildup ended in the
early Cretaceous. Hence, greatly condensed, reworked,
middle to upper Jurassie reddish limestones are overlain by
hemipelagic Tertiary beds.
An ancient example is the widespread occurrence of
condensed nodular red limestones of the Ammonitico
Rosso-type in the Jurassie Tethys Ocean (Bemoulli and
Jenkyns 1974; cf. Sect. 3.4.2).
In cooler, temperate and subpolar waters with less
Current influence but greater influx of terrigenous
material, the pelagic to hemipelagic sediments of a
marginal plateau display mixtures of terrigenous
mud, and siliceous and calcareous ooze (Fig. 5.l0a).
Depending on climate change and oceanic circulation, one of these three main components may dominate over the others.
The example in Fig. 5.lOa is based on drilling results from
the approximately 1000 m deep Voering Plateau off the
Norwegian coast (Eldholm et al. 1987). Here, pelagic sedimentation began in the Miocene, after the plateau had subsided from a position near the sea level down to a considerable water depth. Since that time, the composition of the
sediment cover (200-300 m in thickness) varied between
dominantly siliceous, mixed siliceous and calcareous, to
dominantly siliciclastic deposits. Carbonate-dominated layers rarely accumulated. With the onset of glaciation in the
northern hemisphere in the Late Pliocene, alternating dark
and light mud layers were deposited. The dark ones represent glacial periods, are poor in carbonate, and contain icerafted debris. The light layers are richer in carbonate and
sand deposited during interglacials. Two groups of benthic
organisms, sponges and foraminifers, only are abundant
throughout the entire sequence, which is more or less
bioturbated. The mean sedimentation rate in the Neogene,
including the Pleistocene, was around 2 crnlka and thus
Fig. 5.10. a Sediments on deep submarine plateaus
under different environmental conditions, for exampie temperature, currents, nutrient supply. (Based on
several sources, e.g., Sheridan and Enos 1979; Stow
1986; Eldholm et al. 1987). b Sediments of complex,
segmented shelf (Califomia borderland-type) with
basins and ridges of various depths below sea level;
structural elements of wrench tectonics omitted.
(Based on Howell et al. 1980; Teng and Gorsline
Chapter 5 Oceanic Sediments
considerably higher than for the Blake Plateau. Nevertheless, reworking and hiatuses indicate the activity of bottom
currents. During certain time intervals, the biogenie carbonate settling on the sea floor was entirely dissolved
within the sediment.
Siopes and Basins Close to Submarine Plateaus
The slopes and basins adjacent to submarine plateaus
may be considerably affected by sediments shed from
the plateaus (see also Sects. 12.2.2 and 12.2.3). This
occurs, as long as the plateaus are within the reach of
oceanic currents strong enough to move sediment.
The sediments of slope aprons and peri-platform
oozes reflect to some extent the environmental conditions ofthe submarine plateaus.
Such sediments have been described from the present
oceans and ancient examples (e.g., Mullins and Cook 1986;
Schlager and Camber 1986; Schlanger and Premoli Silva
1986; Eberli 1989; and others, cf. Sect. 7.5). On shallow
carbonate platforms, aprons and peri-platform sediments
are markedly controlled by sea-1eve1 variations, e.g. by
"highstand shedding". In contrast, deeper plateaus may increase carbonate production during sea-Ieve110wstands.
Submarine Ridge and Basin Topography
Some continental margins display a series of topographic basins and ridges with markedly diversified
sediments. A very simplified, generalized view of
such a borderland scenario is presented in Fig. 5.l0b.
The narrow elongate ridges and basins have various
depths below sea level.
Such settings may originate from listric faulting a10ng divergent p1ate boundaries (provided that sediment accumu1ation is too 10w to smooth out the submarine relief), from
strike-slip motions, or from the accretion ofterranes.
A prominent modem example is the Califomia continental borderland which has a complicate tectonic history
(e.g. Ingersoll and Ernst 1987; Legg 1991; .Bohannon and
Geist 1998). Hs ridge-and-basin topography started to form
in the early Miocene as a result of oblique extension, rightlateral strike-slip in conjunction with clockwise rotation of
the coastal ranges, and uplift of parts of the structure. The
young sediments of this continental borderland have been
described, e.g., by Emery (1960), Garrison et al. (1987),
Gorsline and Douglas (1987), Teng and Gorsline (1989),
Gorsline (1992) and others.
1989). Note that only the youngest sediments are
shown, which may still be influenced by lower sealevel stands and temporal variations in the intensity
of coastal upweUing. (Based mainly on Emery 1960;
Garrison et al. 1987). Older sediments may be
strongly affected by block faulting, causing unconforrnities, pinch outs of certain layers, and slump
aprons of varying size and position along the foot of
the slopes. For further explanations see text
The modem 800 to 1000 m deep Blake Plateau offthe eastern Florida shelf belongs to this type of deep plateau
(Sheridan and Enos 1979). Its older sediments represent
shallow-water carbonates, but at the beginning of the
Paleocene deep water environments were established. The
sediments deposited since that time contain considerable
quantities of phosphorite and some chert. The phosphorite
occurs in the form of phosphatized carbonate debris, pellets, pebbles and conglomeratic layers, as weil as continuous pavements (Manheim et al. 1980, cf. Sect. 5.3.6). It is a
lag deposit derived from Miocene strata which accumulated in an oceanographic regime quite different from the
present situation under the Gulf Stream.
Another example for a subsiding, sediment-starved,
Jurassic-Cretaceous continental margin at low latitudes is
the Mazagan Plateau off Morocco (Bemoulli and Kälin
1984; Jansa et al. 1984; Winterer and Hinz 1984). Here,
the constructional phase of carbonate buildup ended in the
early Cretaceous. Hence, greatly condensed, reworked,
middle to upper Jurassie reddish limestones are overlain by
hemipelagic Tertiary beds.
An ancient example is the widespread occurrence of
condensed nodular red limestones of the Ammonitico
Rosso-type in the Jurassie Tethys Ocean (Bemoulli and
Jenkyns 1974; cf. Sect. 3.4.2).
In cooler, temperate and subpolar waters with less
Current influence but greater influx of terrigenous
material, the pelagic to hemipelagic sediments of a
marginal plateau display mixtures of terrigenous
mud, and siliceous and calcareous ooze (Fig. 5.l0a).
Depending on climate change and oceanic circulation, one of these three main components may dominate over the others.
The example in Fig. 5.lOa is based on drilling results from
the approximately 1000 m deep Voering Plateau off the
Norwegian coast (Eldholm et al. 1987). Here, pelagic sedimentation began in the Miocene, after the plateau had subsided from a position near the sea level down to a considerable water depth. Since that time, the composition of the
sediment cover (200-300 m in thickness) varied between
dominantly siliceous, mixed siliceous and calcareous, to
dominantly siliciclastic deposits. Carbonate-dominated layers rarely accumulated. With the onset of glaciation in the
northern hemisphere in the Late Pliocene, alternating dark
and light mud layers were deposited. The dark ones represent glacial periods, are poor in carbonate, and contain icerafted debris. The light layers are richer in carbonate and
sand deposited during interglacials. Two groups of benthic
organisms, sponges and foraminifers, only are abundant
throughout the entire sequence, which is more or less
bioturbated. The mean sedimentation rate in the Neogene,
including the Pleistocene, was around 2 crnlka and thus
Fig. 5.10. a Sediments on deep submarine plateaus
under different environmental conditions, for exampie temperature, currents, nutrient supply. (Based on
several sources, e.g., Sheridan and Enos 1979; Stow
1986; Eldholm et al. 1987). b Sediments of complex,
segmented shelf (Califomia borderland-type) with
basins and ridges of various depths below sea level;
structural elements of wrench tectonics omitted.
(Based on Howell et al. 1980; Teng and Gorsline
Chapter 5 Oceanic Sediments
considerably higher than for the Blake Plateau. Nevertheless, reworking and hiatuses indicate the activity of bottom
currents. During certain time intervals, the biogenie carbonate settling on the sea floor was entirely dissolved
within the sediment.
Siopes and Basins Close to Submarine Plateaus
The slopes and basins adjacent to submarine plateaus
may be considerably affected by sediments shed from
the plateaus (see also Sects. 12.2.2 and 12.2.3). This
occurs, as long as the plateaus are within the reach of
oceanic currents strong enough to move sediment.
The sediments of slope aprons and peri-platform
oozes reflect to some extent the environmental conditions ofthe submarine plateaus.
Such sediments have been described from the present
oceans and ancient examples (e.g., Mullins and Cook 1986;
Schlager and Camber 1986; Schlanger and Premoli Silva
1986; Eberli 1989; and others, cf. Sect. 7.5). On shallow
carbonate platforms, aprons and peri-platform sediments
are markedly controlled by sea-1eve1 variations, e.g. by
"highstand shedding". In contrast, deeper plateaus may increase carbonate production during sea-Ieve110wstands.
Submarine Ridge and Basin Topography
Some continental margins display a series of topographic basins and ridges with markedly diversified
sediments. A very simplified, generalized view of
such a borderland scenario is presented in Fig. 5.l0b.
The narrow elongate ridges and basins have various
depths below sea level.
Such settings may originate from listric faulting a10ng divergent p1ate boundaries (provided that sediment accumu1ation is too 10w to smooth out the submarine relief), from
strike-slip motions, or from the accretion ofterranes.
A prominent modem example is the Califomia continental borderland which has a complicate tectonic history
(e.g. Ingersoll and Ernst 1987; Legg 1991; .Bohannon and
Geist 1998). Hs ridge-and-basin topography started to form
in the early Miocene as a result of oblique extension, rightlateral strike-slip in conjunction with clockwise rotation of
the coastal ranges, and uplift of parts of the structure. The
young sediments of this continental borderland have been
described, e.g., by Emery (1960), Garrison et al. (1987),
Gorsline and Douglas (1987), Teng and Gorsline (1989),
Gorsline (1992) and others.
1989). Note that only the youngest sediments are
shown, which may still be influenced by lower sealevel stands and temporal variations in the intensity
of coastal upweUing. (Based mainly on Emery 1960;
Garrison et al. 1987). Older sediments may be
strongly affected by block faulting, causing unconforrnities, pinch outs of certain layers, and slump
aprons of varying size and position along the foot of
the slopes. For further explanations see text