5.5 Erosion and Redeposition ofDeep-Sea Sediments
235
5.4.7 Summary (Gravity Mass Flows, Turbidites, Deep,-Sea Fans)
Subaqueous gravity mass movements and turbidity currents ean episodieally displaee large volumes of
sediment and generate a family of related strata and bed types:
Debris flows deposits eontain elasts supported
by sandy and muddy matrix. Calcareous
megabreeeias are commonly poor in finegrained matrix. Large-seale olistostromes typieally earry elasts from sources outside the basin
(e.g. olistoliths).
Mud flows may display a limited number of
floating elasts or pebbles.
Sandy, ealcareous, and muddy turbidites eommonly result from gravity mass flows whieh
were transformed by uptake of water into turbulent suspension currents. Turbidites show grading (textural, ehemieal-mineralogical), speeifie
5.5 Erosion and Redeposition of
Deep-Sea Sediments
5.5.1 Deep Bottom-Current Erosion
Some decades ago it was largely assumed that sediment aeeumulation in the deep sea is a continuous
proeess and that deep-sea sediments represent a complete record of past environmental changes. This is
not true. Intensive investigations of the sea floor,
aided by underwater photography, eurrent measurements, sampling and drilling, bio- and magnetostratigraphy, and improved radiometrie dating have
revealed that the sediments of all large oeean basins
are subjeeted to winnowing and reworking by bottom
currents (cf. Fig. 5.3).
In the North Atlantic, for example, large erosional furrows
(several meters deep, 100 to 1000 m long) were discovered,
which also display smaller-scale sand dunes and current
ripples. The current directions indicated by both furrows
and ripples run largely parallel to the measured bottom
currents.
A speeifie type of bottom currents, generating socalled sediment drifts, are contour eurrents. They are
assoeiated with changes in the sea-floor topography
and preferentially oceur along submarine slopes influencing the paths and veloeities of deep oeean eurrents. Due to Coriolis forces, equator-directed bottom
eurrents are defleeted to the western margin of oeeanie basins in both the northern and southern hemispheres. They follow the contours of the continental
slope and rise (Sect. 5.2) or the foot of slopes of oeeanic plateaus and ridges. Conversely, poleward flowing bottom eurrents eonverge along the eastern
boundaries of such basins. These so-called contour
sole marks, internal sedimentary structures, displaced shallow-water biota, pre- and post-event
bioturbation, and a distinct proximal-distal
trend. All primary features of an individual
turbidite bed ean be explained by one single
sedimentologieal event.
Massflow deposits and turbidites form deepwater fans of varying size (up to ~ 1000 km in
length) , granulometry and architeeture (e.g. migrating channel-levee systems, overbank deposits, and often sand-rieh distal fan lobes).
Active fan growth may alternate with periods of
hemipelagic draping (during sea-level rise).
currents sometimes reaeh veloeities (probably ~30
cmls) suffieiently strong to erode and transport finegrained sediments. The erosional and depositional
features of these eurrents (see below) are therefore
partieularly weH developed along the paths of such
contour eurrents. Contour eurrents eontrolled by minor topographie features on the sea floor, such as
oceanie gateways, seamounts, or gaps in oeeanie
ridges, deviate in their direetions from the pattern
governed by the eontinental margins. Bottom currents of deep bays elose to land masses ean be quite
irregular in direetion.
In times of strong elimatie contrast between the
poles and the equator, eontour eurrents are intensified and able to undereut eontinental slopes and trigger large submarine slides and slumps in eertain regions. Even silts and sands of deep-sea fans and
slope aprons ean be partially reworked by these eurrents.
Deep-sea erosion and sediment redeposition is documented
in numerous publications (e.g. Kennett 1982; Tucholke and
Embley 1984; Sarnthein and Mienert 1986; Stein et al.
1986; Okada and Ohta 1993). Undercutting of slopes and
triggering of mass failures has been quoted, e.g., by
Sheridan (1981) and von Rad and Wissmann (1982). Contour currents and contourites have been described, e.g., by
Heezen and Hollister (1971) and later in more detail by
additional authors (e.g. Stanley 1988a; Locker and Laine
1992; Faugeres and Stow 1993; Faugeres et al. 1993;
Jansen and Raymo 1996; summary in Stowet al. 1996).
5.5.2 Contourites and Sediment Drifts
The sediment eroded by bottom eurrents is redeposited on the sea floor where it forms speeifie
depositional ridges, sheets, terraces, or fans. A neu-
235
5.4.7 Summary (Gravity Mass Flows, Turbidites, Deep,-Sea Fans)
Subaqueous gravity mass movements and turbidity currents ean episodieally displaee large volumes of
sediment and generate a family of related strata and bed types:
Debris flows deposits eontain elasts supported
by sandy and muddy matrix. Calcareous
megabreeeias are commonly poor in finegrained matrix. Large-seale olistostromes typieally earry elasts from sources outside the basin
(e.g. olistoliths).
Mud flows may display a limited number of
floating elasts or pebbles.
Sandy, ealcareous, and muddy turbidites eommonly result from gravity mass flows whieh
were transformed by uptake of water into turbulent suspension currents. Turbidites show grading (textural, ehemieal-mineralogical), speeifie
5.5 Erosion and Redeposition of
Deep-Sea Sediments
5.5.1 Deep Bottom-Current Erosion
Some decades ago it was largely assumed that sediment aeeumulation in the deep sea is a continuous
proeess and that deep-sea sediments represent a complete record of past environmental changes. This is
not true. Intensive investigations of the sea floor,
aided by underwater photography, eurrent measurements, sampling and drilling, bio- and magnetostratigraphy, and improved radiometrie dating have
revealed that the sediments of all large oeean basins
are subjeeted to winnowing and reworking by bottom
currents (cf. Fig. 5.3).
In the North Atlantic, for example, large erosional furrows
(several meters deep, 100 to 1000 m long) were discovered,
which also display smaller-scale sand dunes and current
ripples. The current directions indicated by both furrows
and ripples run largely parallel to the measured bottom
currents.
A speeifie type of bottom currents, generating socalled sediment drifts, are contour eurrents. They are
assoeiated with changes in the sea-floor topography
and preferentially oceur along submarine slopes influencing the paths and veloeities of deep oeean eurrents. Due to Coriolis forces, equator-directed bottom
eurrents are defleeted to the western margin of oeeanie basins in both the northern and southern hemispheres. They follow the contours of the continental
slope and rise (Sect. 5.2) or the foot of slopes of oeeanic plateaus and ridges. Conversely, poleward flowing bottom eurrents eonverge along the eastern
boundaries of such basins. These so-called contour
sole marks, internal sedimentary structures, displaced shallow-water biota, pre- and post-event
bioturbation, and a distinct proximal-distal
trend. All primary features of an individual
turbidite bed ean be explained by one single
sedimentologieal event.
Massflow deposits and turbidites form deepwater fans of varying size (up to ~ 1000 km in
length) , granulometry and architeeture (e.g. migrating channel-levee systems, overbank deposits, and often sand-rieh distal fan lobes).
Active fan growth may alternate with periods of
hemipelagic draping (during sea-level rise).
currents sometimes reaeh veloeities (probably ~30
cmls) suffieiently strong to erode and transport finegrained sediments. The erosional and depositional
features of these eurrents (see below) are therefore
partieularly weH developed along the paths of such
contour eurrents. Contour eurrents eontrolled by minor topographie features on the sea floor, such as
oceanie gateways, seamounts, or gaps in oeeanie
ridges, deviate in their direetions from the pattern
governed by the eontinental margins. Bottom currents of deep bays elose to land masses ean be quite
irregular in direetion.
In times of strong elimatie contrast between the
poles and the equator, eontour eurrents are intensified and able to undereut eontinental slopes and trigger large submarine slides and slumps in eertain regions. Even silts and sands of deep-sea fans and
slope aprons ean be partially reworked by these eurrents.
Deep-sea erosion and sediment redeposition is documented
in numerous publications (e.g. Kennett 1982; Tucholke and
Embley 1984; Sarnthein and Mienert 1986; Stein et al.
1986; Okada and Ohta 1993). Undercutting of slopes and
triggering of mass failures has been quoted, e.g., by
Sheridan (1981) and von Rad and Wissmann (1982). Contour currents and contourites have been described, e.g., by
Heezen and Hollister (1971) and later in more detail by
additional authors (e.g. Stanley 1988a; Locker and Laine
1992; Faugeres and Stow 1993; Faugeres et al. 1993;
Jansen and Raymo 1996; summary in Stowet al. 1996).
5.5.2 Contourites and Sediment Drifts
The sediment eroded by bottom eurrents is redeposited on the sea floor where it forms speeifie
depositional ridges, sheets, terraces, or fans. A neu-