236
tral tenn for these sediment bodies is "sediment
drifts"; if they are related to contour currents, they
are referred to as "eontourites". Sediment drifts or
eontourites ean be divided into different groups:
- Giant sediment drifts. These are very large elongate
sediment bodies, tens to hundreds of kilometers long,
tens of kilometers wide, and 0.1 to > 1 km high (cf.
Fig. 4.3c and Fig. 5.23). Theyfonn along lower
slopes or at the foot of slopes at different water
depths. They consist mainly of silty, muddy and
biogenie skeletal material sorted out from older sediment. Many of these drift bodies are mantled with
flat mud waves a few tens of meters high.
- Contourite sheets are thinner than the sediment
drifts, but they can also cover large areas, e.g. on the
eontinental rise or on abyssal plains. They mayaiso
display large sediment waves on their tops.
- Channel-related sediment drifts (contourite fans).
They are associated with submarine channels or passages where bottom currents gain in velocity and beeome able to erode.
Many examples of sediment drifts have been deseribed
from the northem North Atlantie (e.g. Faugeres et al. 1993;
Wold 1994). Many of them were generated by strong
south-direeted bottom eurrents to the south of the
Greenland-Seotland Ridge (ef. Fig. 4.3e). These sediment
drifts were built up sinee Eoeene in three growth phases
whieh were related to episodie mantle plume aetivity (ef.
Seet. 5.6.5). Contourites also formed along the lower slope
of the Barents Sea off Norway (Fig. 5.23e; Yoon and
Chough 1994). Other examples inelude the western margin
of the North and Central Atlantie (Faugeres et al. 1993;
Sarnthein and Faugeres 1993), theWeddell Sea elose to
Antaretiea (Fig. 5.23f; Weber et al. 1994), and the SW Paeifie (Carter and MeCave 1994). In the eentral Mediterranean Sea, the Quaternary sediment drifts formed in shal10wer water (mostly between 500 and 2000 m waterdepth)
and are eommonly assoeiated with oeeanie ehanne1s foeussing the bottom eurrents between topographie highs (Fig.
5.23d; Marani et al. 1993).
Due to fluctuations in the velocity of bottom currents, sediment drifts and eontourites cannot be expeeted to aeeumulate eontinuously. Average sedimentation rates of large drifts are in the order of5
emlka. They display either small-seale cross-bedding
or ± even lamination, indieating eurrent traetion.
However, these struetures are often disturbed by
bioturbation. The identification of eontourites is faeilitated by the presenee of layers eonsisting entirely
of pelagie microfossils sorted out from mixed sediments (e.g. radiolarites).
Contour eurrents ean also affeet deep-sea fans and
their turbidite beds. Some of these may be eroded
and redeposited as eontourites displaying, in eontrast
to the turbidites, eurrent direetions parallel to the
strike of the slope.
In some eases it was observed that the basal, graded subdivision of an individual turbidite is preserved, while the
Chapter 5 Oceanic Sediments
higher portions are replaced by structures characteristic of
normal tractive transport. Such phenomena ean only be
found if subsequent bioturbation has not destroyed the evidence. For that reason, contourites are often masked or difficult to recognize.
On mature passive eontinental margins, the foot-ofslope or continental rise sediments fonn thiek
wedges whieh thin seaward (cf. Figs. 5.3 and 12.15).
These slope aprons often include deposits of gravity
mass flows, slope ehannels, and small deep-sea fans,
apart from nonnal hemipelagie sediment. In addition,
they may eomprise sediment drifts or eontourites
eonsisting of llldterial of different provenanee (Fig.
23a-e).
The diserimination of fine-grained turbidites,
contourites, and nonnal hemipelagie material is often
diffieult in such settings, partieularly in sediments
whieh are intensely bioturbated.
5.5.3 Stratigraphie Gaps in the Deep-Sea Reeord
Seismie reeords, numerous gravity eores, and hundreds of drill holes from deep sea drilling have
shown that erosional features are not restrieted to the
present sea floor.
This was pointed cut by several authors (e.g. Van Andel et
al. 1977; Moore et al. 1978; Berger 1981; Thiede 1981;
Ehrmann and Thiede 1985; Burckle and Abrams 1987;
Wright and Miller 1993; Ramsay et al. 1994: SpencerCervato 1998).
Time intervals of nondeposition, stratigraphie gaps
(hiatuses), and erosional uneonfonnities of both short
and long duration are ubiquitous in the deep oeean
and oeeur throughout the his tory of these large basins. In the South Atlantie, ab out one third of the aggregated time represented by all drill sites is oecupied by stratigraphie gaps. About 10% of all Cenozoie seetions drilled in the Indian Ocean do not contain hiatuses.
Here, the hiatus es have been correlated with periods of
decreased Antaretic glaciation in which, due to ice-free
shelves, more bottom water formed than during larger-extended ice cover (Ramsay et al. 1994). The flow paths of
the northward directed eroding bottom currents ehanged
with time and thus generated hiatuses whieh are not timeequivalent in this ocean basin.
Generally, more than half of the record is missing in
the Paleogene in which a few (worldwide ~ simultaneous) hiatuses represent several millions of years.
The Neogene is characterized by shorter but more
frequent hiatuses with the result that between one
tenth and one half of the sedimentary record are
missing.
tral tenn for these sediment bodies is "sediment
drifts"; if they are related to contour currents, they
are referred to as "eontourites". Sediment drifts or
eontourites ean be divided into different groups:
- Giant sediment drifts. These are very large elongate
sediment bodies, tens to hundreds of kilometers long,
tens of kilometers wide, and 0.1 to > 1 km high (cf.
Fig. 4.3c and Fig. 5.23). Theyfonn along lower
slopes or at the foot of slopes at different water
depths. They consist mainly of silty, muddy and
biogenie skeletal material sorted out from older sediment. Many of these drift bodies are mantled with
flat mud waves a few tens of meters high.
- Contourite sheets are thinner than the sediment
drifts, but they can also cover large areas, e.g. on the
eontinental rise or on abyssal plains. They mayaiso
display large sediment waves on their tops.
- Channel-related sediment drifts (contourite fans).
They are associated with submarine channels or passages where bottom currents gain in velocity and beeome able to erode.
Many examples of sediment drifts have been deseribed
from the northem North Atlantie (e.g. Faugeres et al. 1993;
Wold 1994). Many of them were generated by strong
south-direeted bottom eurrents to the south of the
Greenland-Seotland Ridge (ef. Fig. 4.3e). These sediment
drifts were built up sinee Eoeene in three growth phases
whieh were related to episodie mantle plume aetivity (ef.
Seet. 5.6.5). Contourites also formed along the lower slope
of the Barents Sea off Norway (Fig. 5.23e; Yoon and
Chough 1994). Other examples inelude the western margin
of the North and Central Atlantie (Faugeres et al. 1993;
Sarnthein and Faugeres 1993), theWeddell Sea elose to
Antaretiea (Fig. 5.23f; Weber et al. 1994), and the SW Paeifie (Carter and MeCave 1994). In the eentral Mediterranean Sea, the Quaternary sediment drifts formed in shal10wer water (mostly between 500 and 2000 m waterdepth)
and are eommonly assoeiated with oeeanie ehanne1s foeussing the bottom eurrents between topographie highs (Fig.
5.23d; Marani et al. 1993).
Due to fluctuations in the velocity of bottom currents, sediment drifts and eontourites cannot be expeeted to aeeumulate eontinuously. Average sedimentation rates of large drifts are in the order of5
emlka. They display either small-seale cross-bedding
or ± even lamination, indieating eurrent traetion.
However, these struetures are often disturbed by
bioturbation. The identification of eontourites is faeilitated by the presenee of layers eonsisting entirely
of pelagie microfossils sorted out from mixed sediments (e.g. radiolarites).
Contour eurrents ean also affeet deep-sea fans and
their turbidite beds. Some of these may be eroded
and redeposited as eontourites displaying, in eontrast
to the turbidites, eurrent direetions parallel to the
strike of the slope.
In some eases it was observed that the basal, graded subdivision of an individual turbidite is preserved, while the
Chapter 5 Oceanic Sediments
higher portions are replaced by structures characteristic of
normal tractive transport. Such phenomena ean only be
found if subsequent bioturbation has not destroyed the evidence. For that reason, contourites are often masked or difficult to recognize.
On mature passive eontinental margins, the foot-ofslope or continental rise sediments fonn thiek
wedges whieh thin seaward (cf. Figs. 5.3 and 12.15).
These slope aprons often include deposits of gravity
mass flows, slope ehannels, and small deep-sea fans,
apart from nonnal hemipelagie sediment. In addition,
they may eomprise sediment drifts or eontourites
eonsisting of llldterial of different provenanee (Fig.
23a-e).
The diserimination of fine-grained turbidites,
contourites, and nonnal hemipelagie material is often
diffieult in such settings, partieularly in sediments
whieh are intensely bioturbated.
5.5.3 Stratigraphie Gaps in the Deep-Sea Reeord
Seismie reeords, numerous gravity eores, and hundreds of drill holes from deep sea drilling have
shown that erosional features are not restrieted to the
present sea floor.
This was pointed cut by several authors (e.g. Van Andel et
al. 1977; Moore et al. 1978; Berger 1981; Thiede 1981;
Ehrmann and Thiede 1985; Burckle and Abrams 1987;
Wright and Miller 1993; Ramsay et al. 1994: SpencerCervato 1998).
Time intervals of nondeposition, stratigraphie gaps
(hiatuses), and erosional uneonfonnities of both short
and long duration are ubiquitous in the deep oeean
and oeeur throughout the his tory of these large basins. In the South Atlantie, ab out one third of the aggregated time represented by all drill sites is oecupied by stratigraphie gaps. About 10% of all Cenozoie seetions drilled in the Indian Ocean do not contain hiatuses.
Here, the hiatus es have been correlated with periods of
decreased Antaretic glaciation in which, due to ice-free
shelves, more bottom water formed than during larger-extended ice cover (Ramsay et al. 1994). The flow paths of
the northward directed eroding bottom currents ehanged
with time and thus generated hiatuses whieh are not timeequivalent in this ocean basin.
Generally, more than half of the record is missing in
the Paleogene in which a few (worldwide ~ simultaneous) hiatuses represent several millions of years.
The Neogene is characterized by shorter but more
frequent hiatuses with the result that between one
tenth and one half of the sedimentary record are
missing.