216
less laminar, cohesive flows of comparatively dense,
sediment-fluid mixtures of plastic behavior.
Debris flow deposits (debrites) and olistostromes
(very thick, extensive debrites) consist of a medium
to fine-grained matrix and a varying proportion of
matrix-supported elasts. The typical debrite is rich in
elasts of different sizes; the elasts may be derived
from older sediments and rocks within the basin
(intraelasts ) or from sources outside the basin
(extraelasts, typical for olistostromes). Single elasts
or blocks (olistoliths) in olistostromes can reach the
size of a house and more.
The base of thick and coarse-grained debrites may
be scoured. The basal sediments often displaya thin
sheared zone and inverse grading caused by
prograding frictional freezing. The higher portion
may exhibit indistinct normal grading. Internally,
most debrites lack any bedding phenomena or
imbrication of elasts; in some examples only, elongate elasts are aligned horizontally, indicating the
direction of flow. The top of the bed is either sharp
or grades into an overlying turbidite, thus forming a
compound debrite-turbidite couplet (e.g. Stanley
1982; Mutti et a1. 1984). In places, the top of a
debrite may be current-winnowed and therefore
transformed to a elast-supported lag deposit. Then,
traces of burrowing organisms are missing. In some
cases, adebrite or mud flow deposit is directly overlain by a second debrite or an overlapping lobe of the
same mudflow (Fig. 5.13).
Calcareous debrites resulting from Iarge-scale
slope collapse of semi-lithified carbonate buildups
often form sheet-like megabreccia beds (Fig. 5.12c)
which contain little fine-grained matrix material and
are therefore primarily elast-supported.
For further details see, e.g., Mullins and Cook (1986),
Surlyk and Ineson (1992), Spence and Tucker (1997).
Brecciation of serni-lithified platform carbonates mayaIso
result from tectonic deformation, for example during rifting, and thus generate "internal breccias" which can be
incorporated into debris flows (Füchtbauer and Richter
1983).
Mud flow deposits have much in common with
debrites; in fact there is no sharp boundary between
these two end members of the same group (Fig.
5.13b). Mud flow deposits have a muddy matrix with
a high silt (or micro-fossil) content and contain no or
only a small amount of elasts, mostly intraelasts
which are frequently deformed by the preceding processes of slumping and mass flow. The admixture of
gravel or other coarse material from submarine canyons rnay locally generate pebbly mud or mudstone.
There are also couplets of debrites (or mud flow
deposits) with sandy and muddy turbidites (Fig.
5.13). The latter sit on top ofthe debrite or replace it
basinward.
Chapter 5 Oceanic Sediments
Such couplets have been observed in several ancient sedimentary sequences (e.g., Stanley 1982; Mutti et al. 1984;
Bourrouilh 1987; Souquet et al. 1987).
Elmore et al. (1979) described a modem example from
the Hatteras abyssal plain in the western Atlantic. The redeposited bed is 500 km long, more than 100 km wide, up to
4 m thick, and of upper Pleistocene in age. It consists predominantly of fluvially derived sand and shelf mud with a
large proportion of mollusc shell fragments. In proximal
regions, the poody sorted lower part of the bed (~20%
mud) may have been deposited as a sandy debris flow,
whereas its upper part and more distal portions reflect deposition from a turbidity current.
Another compound, but carbonate-bearing, debriteturbidite was observed in the Exuma Sound, Bahamas
(Crevello and Schlager 1980). This bed is 2 to 3 m thick
and covers an area ofmore than 6000 km 2.
Hieke (1984) reported a Holocene example from the
Ionian abyssal plain (water depth -4000 m) in the Mediterranean Sea. Here, a 12 m thick homogenized mud layer
containing around 50% carbonate, partially from intermediate and possibly even from shallow waters, covers an
area of 1100 km 2 • Locally, the layer has a sandy base composed of shell fragments.
Some workers have introduced additional terms for
some mass flow deposits which they ascribed to specific
triggering mechanisms: "unifites" (Feldhausen et al. 1981),
"homogenites" (Cita and Ricci Lucchi 1984), megaturbidites or "seismo-turbidites" (e.g. Mutti et al. 1984). As
mentioned above, it is norrnally difficult to infer the triggering mechanism from the character of the final bed.
5.4.3 Turbidity Currents and Turbidites
Suspension (Turbidity) Currents
The turbidity current hypothesis as a mechanism for
producing graded, sheet-like beds (sandy, silty or
muddy turbidites) in marine and lake environments
was inferred from the study of ancient rhythmic bed
successions, the internal structures of the sand beds,
and their allochthonous shallow water fauna (Kuenen
and Migliorini 1950).
Turbidity (or suspension) currents commonly
evolve from slope failures by uptake of water (Fig.
5.15a). As long as the density of the suspension is
greater than that of the surrounding water body, it
tends to move downslope, gain in speed, and form a
turbulent undelcurrent transporting its load into
deeper water. The density of suspensions caused by
river floods, however, is usually not high enough to
produce such undercurrents (density of sea water
1.027 g/cm 3 ). Because of the rare occurrence of turbidity current events in relation to the human life
span, such currents of some size could never be directly observed in operation, apart from some measurements in lakes, water reservoirs, or artificial
flumes. Indirect evidence for the high transport capacity of turbidity currents was gained from reports
on the breakage of submarine telegraph cables on
continental slopes as well as at the foot of some sub-
less laminar, cohesive flows of comparatively dense,
sediment-fluid mixtures of plastic behavior.
Debris flow deposits (debrites) and olistostromes
(very thick, extensive debrites) consist of a medium
to fine-grained matrix and a varying proportion of
matrix-supported elasts. The typical debrite is rich in
elasts of different sizes; the elasts may be derived
from older sediments and rocks within the basin
(intraelasts ) or from sources outside the basin
(extraelasts, typical for olistostromes). Single elasts
or blocks (olistoliths) in olistostromes can reach the
size of a house and more.
The base of thick and coarse-grained debrites may
be scoured. The basal sediments often displaya thin
sheared zone and inverse grading caused by
prograding frictional freezing. The higher portion
may exhibit indistinct normal grading. Internally,
most debrites lack any bedding phenomena or
imbrication of elasts; in some examples only, elongate elasts are aligned horizontally, indicating the
direction of flow. The top of the bed is either sharp
or grades into an overlying turbidite, thus forming a
compound debrite-turbidite couplet (e.g. Stanley
1982; Mutti et a1. 1984). In places, the top of a
debrite may be current-winnowed and therefore
transformed to a elast-supported lag deposit. Then,
traces of burrowing organisms are missing. In some
cases, adebrite or mud flow deposit is directly overlain by a second debrite or an overlapping lobe of the
same mudflow (Fig. 5.13).
Calcareous debrites resulting from Iarge-scale
slope collapse of semi-lithified carbonate buildups
often form sheet-like megabreccia beds (Fig. 5.12c)
which contain little fine-grained matrix material and
are therefore primarily elast-supported.
For further details see, e.g., Mullins and Cook (1986),
Surlyk and Ineson (1992), Spence and Tucker (1997).
Brecciation of serni-lithified platform carbonates mayaIso
result from tectonic deformation, for example during rifting, and thus generate "internal breccias" which can be
incorporated into debris flows (Füchtbauer and Richter
1983).
Mud flow deposits have much in common with
debrites; in fact there is no sharp boundary between
these two end members of the same group (Fig.
5.13b). Mud flow deposits have a muddy matrix with
a high silt (or micro-fossil) content and contain no or
only a small amount of elasts, mostly intraelasts
which are frequently deformed by the preceding processes of slumping and mass flow. The admixture of
gravel or other coarse material from submarine canyons rnay locally generate pebbly mud or mudstone.
There are also couplets of debrites (or mud flow
deposits) with sandy and muddy turbidites (Fig.
5.13). The latter sit on top ofthe debrite or replace it
basinward.
Chapter 5 Oceanic Sediments
Such couplets have been observed in several ancient sedimentary sequences (e.g., Stanley 1982; Mutti et al. 1984;
Bourrouilh 1987; Souquet et al. 1987).
Elmore et al. (1979) described a modem example from
the Hatteras abyssal plain in the western Atlantic. The redeposited bed is 500 km long, more than 100 km wide, up to
4 m thick, and of upper Pleistocene in age. It consists predominantly of fluvially derived sand and shelf mud with a
large proportion of mollusc shell fragments. In proximal
regions, the poody sorted lower part of the bed (~20%
mud) may have been deposited as a sandy debris flow,
whereas its upper part and more distal portions reflect deposition from a turbidity current.
Another compound, but carbonate-bearing, debriteturbidite was observed in the Exuma Sound, Bahamas
(Crevello and Schlager 1980). This bed is 2 to 3 m thick
and covers an area ofmore than 6000 km 2.
Hieke (1984) reported a Holocene example from the
Ionian abyssal plain (water depth -4000 m) in the Mediterranean Sea. Here, a 12 m thick homogenized mud layer
containing around 50% carbonate, partially from intermediate and possibly even from shallow waters, covers an
area of 1100 km 2 • Locally, the layer has a sandy base composed of shell fragments.
Some workers have introduced additional terms for
some mass flow deposits which they ascribed to specific
triggering mechanisms: "unifites" (Feldhausen et al. 1981),
"homogenites" (Cita and Ricci Lucchi 1984), megaturbidites or "seismo-turbidites" (e.g. Mutti et al. 1984). As
mentioned above, it is norrnally difficult to infer the triggering mechanism from the character of the final bed.
5.4.3 Turbidity Currents and Turbidites
Suspension (Turbidity) Currents
The turbidity current hypothesis as a mechanism for
producing graded, sheet-like beds (sandy, silty or
muddy turbidites) in marine and lake environments
was inferred from the study of ancient rhythmic bed
successions, the internal structures of the sand beds,
and their allochthonous shallow water fauna (Kuenen
and Migliorini 1950).
Turbidity (or suspension) currents commonly
evolve from slope failures by uptake of water (Fig.
5.15a). As long as the density of the suspension is
greater than that of the surrounding water body, it
tends to move downslope, gain in speed, and form a
turbulent undelcurrent transporting its load into
deeper water. The density of suspensions caused by
river floods, however, is usually not high enough to
produce such undercurrents (density of sea water
1.027 g/cm 3 ). Because of the rare occurrence of turbidity current events in relation to the human life
span, such currents of some size could never be directly observed in operation, apart from some measurements in lakes, water reservoirs, or artificial
flumes. Indirect evidence for the high transport capacity of turbidity currents was gained from reports
on the breakage of submarine telegraph cables on
continental slopes as well as at the foot of some sub-