5.4 Gravity Mass Flow
vertical succession of grain sizes andlor sediment composition (Fig. 5.16b).
As with silicic1astic turbidites, the maximum thickness of an individual bed is commonly not attained in
the neighborhood of the source area, but at some distance downcurrent (Fig. 5 .16b); then it decreases distally.
(4) Mud Turbidites. This common turbidite type has
been frequently overlooked, although the sediment
fills of many basins and the sediment bodies of
prominent deep-sea fans, ancient and modem, consist
to a large degree ofmud turbidites (see below).
Mud turbidites may be regarded either as an end
member of gravity mass flows of mixed granulometry which have lost their coarser grain size
fraction (Chough 1984; Stanley 1985), or they are
derived solely from muddy sediment sources such as
fine-grained slope sediments. In addition to gravity
movements, large river floods or muddy sediments
stirred up by storms in shallow seas can contribute to
the formation of mud turbidites. Because these beds
are deposited from low-density suspension currents,
they are usually thin.
However, in the transitional stage from mud flows to
muddy suspension currents (hyperconcentrated flow) they
also fonn thick, indistinctly graded beds (EinseIe and Kelts
1982). Thick mud turbidites mayaIso result from flow reversals of large suspension flows in a narrow basins
(Porebski et al. 1991).
The mode of mud turbidite fonnation has been discussed, e.g., by Piper and Stow (1991). The sediments at
the foot of the modern continental slopes, the fillings of
deep, relatively narrow basins (e.g. Mediterranean, Black
Sea, Gulf of California) and many deep-sea fans contain
high proportions of mud turbidites (see, e.g., summary by
EinseIe et al. 1996).
The most characteristic features of mud turbidites are
a sharp basal contact to the underlying bed, internal
normal grading (indistinct) corresponding to the
Bouma divisions T d and Tc. Proximal mud turbidites
may contain a thin laminated sand layer at their base
(Fig. 5.17b). Under oxic environments, the tops of
mud turbidites are bioturbated. Thin, distal mud
turbidites tend to become obscured by intense
bioturbation. Then they can only be recognized if
their material andlor fauna differ substantially from
the pelagic or hemipelagic background sediment.
Fig. 5.15. Models for the generation of proximal and
distal sandy turbidites and their internal structures.
a Transition from slope failures (slumps, debris and
mud flows, grain flows) on continental slopes and
canyon heads to turbidity currents. b "Classic", complete proximal turbidite showing the total succession
of Bouma divisions (in parentheses). See Figure
5.14a for further descriptive symbols. Note
autosuspension due to turbulence. c More distal
221
Taking into account the provenance of the muddy
material, one can distinguish between hemipelagic,
pelagic (fine-grained bioc1astics) and volcanic1astic
(ash) mud turbidites (Kelts and Arthur 1981; Fig.
5.17c and d). The redeposition of siliceous ooze,
mainly consisting of diatoms and radiolaria, is an
important process in the formation of rhythmically
bedded marine cherts.
Upward increasing contents of biogenic opal or carbonate
within aI'. individual bed can produce chemical grading
(Fig. 5.14b; see also Korsch et al. 1993). Internally, a laminated division (according to Piper EI) is often followed by
a structur~less, indistinctly graded division (E2). Turbidite
radiolarian beds alternating with fine-grained ash turbidites
have been observed in the Upper Jurassic of the western
Carpathians (Misik et al. 1991).
Other Characteristics of Turbidites
Many of the various types of turbidites display evidence of pre-event trace fossil associations of the
pelagic to hemipelagic host sediment and post-event
assemblages in the freshly deposited turbidite layer
(Fig.5.15t).
The burrows of the first group are exhumed by the erosive
force of the turbidity current and immediately afterwards
filled up with sediment settling out from the slowing suspension current. These "lebens spuren" are preserved on the
sole of the turbidite besides casts produced by current erosion (flute casts) or the imprints ofvarious objects dragged
by the current over the sea bottom (groove casts, tool
marks; Fig. 5.15d and e). The recolonizing trace fossil assemblage has to dig down into the turbidite from a new,
higher level. If the turbidite is thin, some of the burrowing
organisms may reach its base and feed on the background
sediments; but if the turbidite is thick, its top section only
can be burrowed (see also Seilacher 1962).
All types of turbidites can contain biogenic carbonate
or silica derived from shallow-water environments. If
deposited below the calcite compensation depth
(CCD) where the background sediments are poor in
or free of carbonate or opaline silica, these event deposits produce ahernating layers with and without
carbonate or biogenic silica. Thus, in particular mud
turbidites can form "banded" sequences. Similarly,
mud turbidites containing high amounts of organic
turbidite, no erosion, basal Bouma divisions are
missing. d Idealized proximal-distal development of
turbidite bed; proximal channel fills may show sedimentary structures due to traction by normal currents
(st), as wen as traction carpets (ig inversely graded).
e Different sizes of flute and groove casts in relation
to bed thicknesses. (Based on several sources). f Predepositional and post-depositional trace fossils. (After Seilacher 1962; Kern 1980)
vertical succession of grain sizes andlor sediment composition (Fig. 5.16b).
As with silicic1astic turbidites, the maximum thickness of an individual bed is commonly not attained in
the neighborhood of the source area, but at some distance downcurrent (Fig. 5 .16b); then it decreases distally.
(4) Mud Turbidites. This common turbidite type has
been frequently overlooked, although the sediment
fills of many basins and the sediment bodies of
prominent deep-sea fans, ancient and modem, consist
to a large degree ofmud turbidites (see below).
Mud turbidites may be regarded either as an end
member of gravity mass flows of mixed granulometry which have lost their coarser grain size
fraction (Chough 1984; Stanley 1985), or they are
derived solely from muddy sediment sources such as
fine-grained slope sediments. In addition to gravity
movements, large river floods or muddy sediments
stirred up by storms in shallow seas can contribute to
the formation of mud turbidites. Because these beds
are deposited from low-density suspension currents,
they are usually thin.
However, in the transitional stage from mud flows to
muddy suspension currents (hyperconcentrated flow) they
also fonn thick, indistinctly graded beds (EinseIe and Kelts
1982). Thick mud turbidites mayaIso result from flow reversals of large suspension flows in a narrow basins
(Porebski et al. 1991).
The mode of mud turbidite fonnation has been discussed, e.g., by Piper and Stow (1991). The sediments at
the foot of the modern continental slopes, the fillings of
deep, relatively narrow basins (e.g. Mediterranean, Black
Sea, Gulf of California) and many deep-sea fans contain
high proportions of mud turbidites (see, e.g., summary by
EinseIe et al. 1996).
The most characteristic features of mud turbidites are
a sharp basal contact to the underlying bed, internal
normal grading (indistinct) corresponding to the
Bouma divisions T d and Tc. Proximal mud turbidites
may contain a thin laminated sand layer at their base
(Fig. 5.17b). Under oxic environments, the tops of
mud turbidites are bioturbated. Thin, distal mud
turbidites tend to become obscured by intense
bioturbation. Then they can only be recognized if
their material andlor fauna differ substantially from
the pelagic or hemipelagic background sediment.
Fig. 5.15. Models for the generation of proximal and
distal sandy turbidites and their internal structures.
a Transition from slope failures (slumps, debris and
mud flows, grain flows) on continental slopes and
canyon heads to turbidity currents. b "Classic", complete proximal turbidite showing the total succession
of Bouma divisions (in parentheses). See Figure
5.14a for further descriptive symbols. Note
autosuspension due to turbulence. c More distal
221
Taking into account the provenance of the muddy
material, one can distinguish between hemipelagic,
pelagic (fine-grained bioc1astics) and volcanic1astic
(ash) mud turbidites (Kelts and Arthur 1981; Fig.
5.17c and d). The redeposition of siliceous ooze,
mainly consisting of diatoms and radiolaria, is an
important process in the formation of rhythmically
bedded marine cherts.
Upward increasing contents of biogenic opal or carbonate
within aI'. individual bed can produce chemical grading
(Fig. 5.14b; see also Korsch et al. 1993). Internally, a laminated division (according to Piper EI) is often followed by
a structur~less, indistinctly graded division (E2). Turbidite
radiolarian beds alternating with fine-grained ash turbidites
have been observed in the Upper Jurassic of the western
Carpathians (Misik et al. 1991).
Other Characteristics of Turbidites
Many of the various types of turbidites display evidence of pre-event trace fossil associations of the
pelagic to hemipelagic host sediment and post-event
assemblages in the freshly deposited turbidite layer
(Fig.5.15t).
The burrows of the first group are exhumed by the erosive
force of the turbidity current and immediately afterwards
filled up with sediment settling out from the slowing suspension current. These "lebens spuren" are preserved on the
sole of the turbidite besides casts produced by current erosion (flute casts) or the imprints ofvarious objects dragged
by the current over the sea bottom (groove casts, tool
marks; Fig. 5.15d and e). The recolonizing trace fossil assemblage has to dig down into the turbidite from a new,
higher level. If the turbidite is thin, some of the burrowing
organisms may reach its base and feed on the background
sediments; but if the turbidite is thick, its top section only
can be burrowed (see also Seilacher 1962).
All types of turbidites can contain biogenic carbonate
or silica derived from shallow-water environments. If
deposited below the calcite compensation depth
(CCD) where the background sediments are poor in
or free of carbonate or opaline silica, these event deposits produce ahernating layers with and without
carbonate or biogenic silica. Thus, in particular mud
turbidites can form "banded" sequences. Similarly,
mud turbidites containing high amounts of organic
turbidite, no erosion, basal Bouma divisions are
missing. d Idealized proximal-distal development of
turbidite bed; proximal channel fills may show sedimentary structures due to traction by normal currents
(st), as wen as traction carpets (ig inversely graded).
e Different sizes of flute and groove casts in relation
to bed thicknesses. (Based on several sources). f Predepositional and post-depositional trace fossils. (After Seilacher 1962; Kern 1980)