5.4 Gravity Mass Flow
However, sea-Ievel changes contributed only to a small
degree to the cutting of many deep canyons on active continental margins. Particular intriguing are large canyons
without a major river, "headless" canyons, deep submarine
canyons in granitic rocks (e.g. the Monterey Canyon), and
canyons oriented obliqueiy to the coastline. Many exampies of this group occur along the Califomia continental
margin. They formed since the Miocene and are clearly
tectonically controlled. They commonly follow major fault
lines characterized by rock fracturing and strike-slip. Uplift
and subaerial erosion must have been the main process in
shaping these spectacular features.
(3) In contrast, sea-level rise usually leads to reduced
sediment supply from terrestrial sources. Most of the
incoming sediment is deposited on the shelf and, in
addition, may fill submarine valleys and canyons.
Gravity mass movements become rare or cease, and
pre-existing fan deposits are often draped by normal
hemipelagic to pelagic sediments (Fig. 5.20c, section
I). In particular the transition from the transgressive
to the highstand phase is characterized by reduced
sediment accumulation (cf. Sect. 7.2). The resulting
condensed section is frequently represented by pelagic oozes, limestones, or thin black shales.
The best modem examples showing the impact of
both changing sediment supply and high-frequency,
high-amplitude sea-level changes are the Mississippi
deep-sea fan in the Gulf of Mexico (Fig. 5.21) and
the Amazon fan in the western Atlantic (Fig. 5.22).
Both fans are mud-dominated, but the evolution of
the Mississipi fan is complicated further by the lateral migration of its feeder system from southwest to
northeast.
Only the youngest feeder canyon of the Mississippi fan is
preserved. This canyon was cut more than 500 m deep into
older shelf and slope sediments during the late Pleistocene
(prior to 30 ka; Goodwin and Prior 1989). Older canyons
are completely filled with sediments and incorporated into
the prograding slope, which also includes localized slope
fans. Since 19 ka B.P., even the youngest canyon was partially filled with the material of a prograding delta and
gravity mass movements along the originally steep canyon
flanks. Thus, the canyon widened by retrogressive slope
failure and mass movement processes. Erosional unconformities within the channel fill indicate two to three episodes of erosion and subsequent deposition. About 7.S ka
B.P. the delta moved northward away from the canyon
Fig. 5.20. Response of deep-sea fan to sea-level
change and vertical sediment sections within fan.
a Constant sea level and steady position of continental slope, permanently high sediment input via submarine canyon (point source); switching fan lobes
(1,2,3) and migrating fan valleys. Note that both fining (andJor thinning) upward (A and B) and coarsening (andJor thickening) upward sequences (C and D)
occur; the overall tendency is fining upward (E).
b Relative sea-level fall favors rapid prograding and
upbuilding of fan due to increased sediment input.
231
head (Fig. 5.21a). Since that time, hernipelagic muds drape
the fill of the feeder canyon.
In the upper and middle fan regions, the older, buried
channel systems, as revealed by multifold seismic
data, show a complicated pattern of superposed,
mostly branching channels (Fig. 5.21b) which generally young from west to east. The individual sequences consist of channel-levee systems, which may
be branched, and associated overbank deposits (Fig.
5.2Ic).
The Mississippi fan sediments are up to 4 km thick
and can be subdivided into 13 to 17 depositional sequences (Fig. 5.21c) reflecting the same number of
sea level oscillations. Most of the fan sediments accumulated during the last 2.4 Ma at rates up to more
than 1000 m1Ma.
Several sequences are bounded by erosional unconformities truncating the upper portion of the pre-existing fan
surface. Deposits of slides and gi-avity mass flows frequently form the base of a new depositional sequence.
They are interpreted to have formed during a 10wering sea
level, whereas the channel-Ievee systems were deposited
when the sea level was near its lowest position. Deposition
of turbidites continued into the period of early sea-Ievel
rise as a result of increased sediment supply during the last
deglaciation. Then, a thin layer of hemipelagic sediment
accumulated on the fan. F or more details see F eeley et al.
(1990), Weimer (1990), Wetzel and Kohl (1986).
Similar to the Mississippi fan, the channe1 system of the
Indus deep-sea fan (second largest fan, covering an area of
1.210 6 km 2 ) migrated from west to east (Kolla and Coumes
1987).
Hemipelagic draping of fan deposits of glacial age is
also known from the modem Amazon fan. This large,
cone-shaped fan reaches a thickness up to 7 km near
the shelf break and developed since the middle Miocene. In the mid-fan region (water depth ab out 3500
m), the late Pleistocene sediments display a characteristic alternation between thick, sandy to muddy
channel-levee complexes, deposited during glacial
lowstand periods, and thin pelagic to hemipelagic
calcareous oozes (interglacial highstand deposits,
Fig. 5.22a and b). The sand-filled channels and their
finer-grained levees often changed their course
which resulted from crevasse splays leading to bifurcation. The channel-levee complexes partially rest on
Most of the sections tend to coarsen (andJor thicken)
upward (F,G,H). c Relative sea-level rise and resulting large reduction in sediment supply may terminate
fan growth. Coarse-grained channel fills and
turbidites are overlain by fine-grained transgressive
deposits (TD) and possibly by a thin condensed section (CS; cf. Sect. 7.2). Highstand deposits (HSD)
may again increase in thickness and coarsen upward.
(Partially based on Walker 1978; Bally 1987, greatly
modified)
However, sea-Ievel changes contributed only to a small
degree to the cutting of many deep canyons on active continental margins. Particular intriguing are large canyons
without a major river, "headless" canyons, deep submarine
canyons in granitic rocks (e.g. the Monterey Canyon), and
canyons oriented obliqueiy to the coastline. Many exampies of this group occur along the Califomia continental
margin. They formed since the Miocene and are clearly
tectonically controlled. They commonly follow major fault
lines characterized by rock fracturing and strike-slip. Uplift
and subaerial erosion must have been the main process in
shaping these spectacular features.
(3) In contrast, sea-level rise usually leads to reduced
sediment supply from terrestrial sources. Most of the
incoming sediment is deposited on the shelf and, in
addition, may fill submarine valleys and canyons.
Gravity mass movements become rare or cease, and
pre-existing fan deposits are often draped by normal
hemipelagic to pelagic sediments (Fig. 5.20c, section
I). In particular the transition from the transgressive
to the highstand phase is characterized by reduced
sediment accumulation (cf. Sect. 7.2). The resulting
condensed section is frequently represented by pelagic oozes, limestones, or thin black shales.
The best modem examples showing the impact of
both changing sediment supply and high-frequency,
high-amplitude sea-level changes are the Mississippi
deep-sea fan in the Gulf of Mexico (Fig. 5.21) and
the Amazon fan in the western Atlantic (Fig. 5.22).
Both fans are mud-dominated, but the evolution of
the Mississipi fan is complicated further by the lateral migration of its feeder system from southwest to
northeast.
Only the youngest feeder canyon of the Mississippi fan is
preserved. This canyon was cut more than 500 m deep into
older shelf and slope sediments during the late Pleistocene
(prior to 30 ka; Goodwin and Prior 1989). Older canyons
are completely filled with sediments and incorporated into
the prograding slope, which also includes localized slope
fans. Since 19 ka B.P., even the youngest canyon was partially filled with the material of a prograding delta and
gravity mass movements along the originally steep canyon
flanks. Thus, the canyon widened by retrogressive slope
failure and mass movement processes. Erosional unconformities within the channel fill indicate two to three episodes of erosion and subsequent deposition. About 7.S ka
B.P. the delta moved northward away from the canyon
Fig. 5.20. Response of deep-sea fan to sea-level
change and vertical sediment sections within fan.
a Constant sea level and steady position of continental slope, permanently high sediment input via submarine canyon (point source); switching fan lobes
(1,2,3) and migrating fan valleys. Note that both fining (andJor thinning) upward (A and B) and coarsening (andJor thickening) upward sequences (C and D)
occur; the overall tendency is fining upward (E).
b Relative sea-level fall favors rapid prograding and
upbuilding of fan due to increased sediment input.
231
head (Fig. 5.21a). Since that time, hernipelagic muds drape
the fill of the feeder canyon.
In the upper and middle fan regions, the older, buried
channel systems, as revealed by multifold seismic
data, show a complicated pattern of superposed,
mostly branching channels (Fig. 5.21b) which generally young from west to east. The individual sequences consist of channel-levee systems, which may
be branched, and associated overbank deposits (Fig.
5.2Ic).
The Mississippi fan sediments are up to 4 km thick
and can be subdivided into 13 to 17 depositional sequences (Fig. 5.21c) reflecting the same number of
sea level oscillations. Most of the fan sediments accumulated during the last 2.4 Ma at rates up to more
than 1000 m1Ma.
Several sequences are bounded by erosional unconformities truncating the upper portion of the pre-existing fan
surface. Deposits of slides and gi-avity mass flows frequently form the base of a new depositional sequence.
They are interpreted to have formed during a 10wering sea
level, whereas the channel-Ievee systems were deposited
when the sea level was near its lowest position. Deposition
of turbidites continued into the period of early sea-Ievel
rise as a result of increased sediment supply during the last
deglaciation. Then, a thin layer of hemipelagic sediment
accumulated on the fan. F or more details see F eeley et al.
(1990), Weimer (1990), Wetzel and Kohl (1986).
Similar to the Mississippi fan, the channe1 system of the
Indus deep-sea fan (second largest fan, covering an area of
1.210 6 km 2 ) migrated from west to east (Kolla and Coumes
1987).
Hemipelagic draping of fan deposits of glacial age is
also known from the modem Amazon fan. This large,
cone-shaped fan reaches a thickness up to 7 km near
the shelf break and developed since the middle Miocene. In the mid-fan region (water depth ab out 3500
m), the late Pleistocene sediments display a characteristic alternation between thick, sandy to muddy
channel-levee complexes, deposited during glacial
lowstand periods, and thin pelagic to hemipelagic
calcareous oozes (interglacial highstand deposits,
Fig. 5.22a and b). The sand-filled channels and their
finer-grained levees often changed their course
which resulted from crevasse splays leading to bifurcation. The channel-levee complexes partially rest on
Most of the sections tend to coarsen (andJor thicken)
upward (F,G,H). c Relative sea-level rise and resulting large reduction in sediment supply may terminate
fan growth. Coarse-grained channel fills and
turbidites are overlain by fine-grained transgressive
deposits (TD) and possibly by a thin condensed section (CS; cf. Sect. 7.2). Highstand deposits (HSD)
may again increase in thickness and coarsen upward.
(Partially based on Walker 1978; Bally 1987, greatly
modified)