210
Narrow ridges and banktops. Deposition ontop of
narrow topographie highs is controlled by the factors
mentioned above. The distance from siliciclastic sediment sources and local topographie features also
play a significant role. In the example of Figure
5.l0b, the climate is semi-arid and sediment supply
by rivers is fairly high. Oceanic circulation is characterized by southward-directed cold surface currents
and moderate coastal upwelling fed by northward
flowing intermediate waters. Under these conditions
one can distinguish between several depositional
zones:
- Rocky bottoms with some sessil organisms, practically devoid of sediment. This situation is observed
on hills rising above the level of island shelves and
large, flat bank tops, at the outer edges of island
shelves and large bank tops, on the upper parts of
steep basin slopes, and on the walls of submarine
canyons.
- Mainland continental shelves and island shelves.
Terrigenous, and mixed terrigenous and biogenie
sand and mud, decreasing grain size and increasing
carbonate content with depth (for further details see
Sect. 3.4).
- Large bank tops at medium depths (about 100 to
300 m). Biogenie sands and muds, relic sediment and
residual sediment from underlying rocks, abundant
phosphorite and glauconitic sand. Depressions on the
flat ridges are filled with sediment.
- Large bank tops at greater depths (<:300 m), far
away from the coastline. Thick cover of biogenie
pelagic sediment, in this example mainly siliceous.
Layers of small nodules of phosphorite and other
authigenic minerals are, if present in certain horizons, diluted and buried.
The phosphorite occurs in small grains, nodules, and slabs,
or it forms thin coatings on rocky grounds and gravel. It is
usually covered by a film of manganese oxide. Frequently,
older nodules are cemented together by younger
phosphorite to form a nodular conglomerate. The phosphorite is often associated with glauconitic foraminiferal sand
(banktop glauco-phosphorites), which is found either
within or surrounding the nodules. In the Califomian borderland, the age of the phosphorite dredged from the surface of large banks is Late Miocene to Quatemary. Hence
the net sedimentation rate at these locations was extremely
low during the last 10 to 15 Ma (cf. Sect. 5.3.7).
Narrow basins. Most ofthe sediments filling the narrow, elongate basins between the ridges are delivered
via gravity mass movements (cf. Sect. 5.4). The inner, landward basins are fed primarily by submarine
canyons collecting sand and mud from the continent
and inner shelf; they are filled mainly by deep-sea
fans and display high sedimentation rates in the order
of 20 to 50 cmlka. The outer basins are cut off from
direct terrestrial supply. They accumulate pelagic to
Chapter 5 Oceanic Sediments
hemipelagic sediments derived from their slopes and
produced in the water column; minor amounts of material mayaiso come from island shelves. The sedimentation rates in these distal basins drop to moderate values (5 to 10 cmlka).
Summary (Plateaus, Ridges, and Associated
Basins)
On marginal and isolated plateaus of some
extent, ± complete pelagic carbonate and
biosiliceous sediments can accumulate under
weak current regimes and above the CCD.
Narrow ridges, banktops, and current-swept
highs commonly display strongly reduced,
condensed, or irregular deposition (inciuding
phosphorite and glauconite).
Adjacent basins may collect some of the plateau sediments, but can also be controlled by
different and more effective sediment sourees.
5.4 Gravity Mass Flow Deposits
and Turbidites
5.4.1 Introduction
Gravity mass movements and mass flows are very
common both in mountainous regions on the continents and on the floor of water-filled basins with a
high-relief subaquatic topography, such as along the
margins of deep ocean basins. The mechanics of
mass flows on land and below the sea differs in several respects, but it is of great importance here that
the preservation potential of marine mass flow deposits is much higher than that of continental environments. Mass flow deposits on land are mostly
removed by subsequent erosion, apart from those
which are rapidly buried in strongly subsiding areas.
Their marine and lacustrine counterparts are commonly buried by younger sediments and therefore
preserved in the geological record. On the other
hand, the various types of mass movements on land
can be weIl observed and have in fact been studied
intensively in many countries. Mass flows below the
sea are more difficult to study and have rarely been
observed in operation. This may be one of the reasons why so much has been written and speculated
about these processes. The deposits of marine mass
flows often reach much larger dimensions than those
on land, and they can make up a great portion of the
total sediment body of many marine basins. In addition, lacustrine and marine sediment gravity flows
Narrow ridges and banktops. Deposition ontop of
narrow topographie highs is controlled by the factors
mentioned above. The distance from siliciclastic sediment sources and local topographie features also
play a significant role. In the example of Figure
5.l0b, the climate is semi-arid and sediment supply
by rivers is fairly high. Oceanic circulation is characterized by southward-directed cold surface currents
and moderate coastal upwelling fed by northward
flowing intermediate waters. Under these conditions
one can distinguish between several depositional
zones:
- Rocky bottoms with some sessil organisms, practically devoid of sediment. This situation is observed
on hills rising above the level of island shelves and
large, flat bank tops, at the outer edges of island
shelves and large bank tops, on the upper parts of
steep basin slopes, and on the walls of submarine
canyons.
- Mainland continental shelves and island shelves.
Terrigenous, and mixed terrigenous and biogenie
sand and mud, decreasing grain size and increasing
carbonate content with depth (for further details see
Sect. 3.4).
- Large bank tops at medium depths (about 100 to
300 m). Biogenie sands and muds, relic sediment and
residual sediment from underlying rocks, abundant
phosphorite and glauconitic sand. Depressions on the
flat ridges are filled with sediment.
- Large bank tops at greater depths (<:300 m), far
away from the coastline. Thick cover of biogenie
pelagic sediment, in this example mainly siliceous.
Layers of small nodules of phosphorite and other
authigenic minerals are, if present in certain horizons, diluted and buried.
The phosphorite occurs in small grains, nodules, and slabs,
or it forms thin coatings on rocky grounds and gravel. It is
usually covered by a film of manganese oxide. Frequently,
older nodules are cemented together by younger
phosphorite to form a nodular conglomerate. The phosphorite is often associated with glauconitic foraminiferal sand
(banktop glauco-phosphorites), which is found either
within or surrounding the nodules. In the Califomian borderland, the age of the phosphorite dredged from the surface of large banks is Late Miocene to Quatemary. Hence
the net sedimentation rate at these locations was extremely
low during the last 10 to 15 Ma (cf. Sect. 5.3.7).
Narrow basins. Most ofthe sediments filling the narrow, elongate basins between the ridges are delivered
via gravity mass movements (cf. Sect. 5.4). The inner, landward basins are fed primarily by submarine
canyons collecting sand and mud from the continent
and inner shelf; they are filled mainly by deep-sea
fans and display high sedimentation rates in the order
of 20 to 50 cmlka. The outer basins are cut off from
direct terrestrial supply. They accumulate pelagic to
Chapter 5 Oceanic Sediments
hemipelagic sediments derived from their slopes and
produced in the water column; minor amounts of material mayaiso come from island shelves. The sedimentation rates in these distal basins drop to moderate values (5 to 10 cmlka).
Summary (Plateaus, Ridges, and Associated
Basins)
On marginal and isolated plateaus of some
extent, ± complete pelagic carbonate and
biosiliceous sediments can accumulate under
weak current regimes and above the CCD.
Narrow ridges, banktops, and current-swept
highs commonly display strongly reduced,
condensed, or irregular deposition (inciuding
phosphorite and glauconite).
Adjacent basins may collect some of the plateau sediments, but can also be controlled by
different and more effective sediment sourees.
5.4 Gravity Mass Flow Deposits
and Turbidites
5.4.1 Introduction
Gravity mass movements and mass flows are very
common both in mountainous regions on the continents and on the floor of water-filled basins with a
high-relief subaquatic topography, such as along the
margins of deep ocean basins. The mechanics of
mass flows on land and below the sea differs in several respects, but it is of great importance here that
the preservation potential of marine mass flow deposits is much higher than that of continental environments. Mass flow deposits on land are mostly
removed by subsequent erosion, apart from those
which are rapidly buried in strongly subsiding areas.
Their marine and lacustrine counterparts are commonly buried by younger sediments and therefore
preserved in the geological record. On the other
hand, the various types of mass movements on land
can be weIl observed and have in fact been studied
intensively in many countries. Mass flows below the
sea are more difficult to study and have rarely been
observed in operation. This may be one of the reasons why so much has been written and speculated
about these processes. The deposits of marine mass
flows often reach much larger dimensions than those
on land, and they can make up a great portion of the
total sediment body of many marine basins. In addition, lacustrine and marine sediment gravity flows