5.4 Gravity Mass Flow
Coarse-grained, gravel-rich deep-water fans (and
fans on submarine ramps) develop on slopes in front
of alluvial fans. They can be classified as fan deltas
(cf. Sect. 2.2.2) or "short-headed delta front" fans.
They are restricted to relatively shallow water and
therefore do not correspond to the "normal" type of
deep-sea fan. Part of these wedge-shaped bodies consists of gravity flow deposits; muddy interbeds are
more or less missing.
Sand-rich deep-water fans with a sand to mud ratio
>70:30 often develop either in front ofthe mouths of
small mountainous rivers or at the lower ends of
shelf-slope canyons collecting sand from longshore
transport on the shelf. The sand is distributed by radially diverging braided channels on the fan; levees
and individual depositional lobes are absent. The radii of fans evolving in small basins is commonly 5-10
km in length; sand-rich fans on Atlantic-type passive
margins can be larger. Multi-source systems form
channelized coalescing lobes.
Sand-rich fans have been described, e.g., from the Californian borderland (deep-sea fans), the North Sea, and the
Atlantic margin of South America (Campos Basin, off
Brazil, ramp settings).
Mudlsand-rich deep-water fans are fed by mixedload rivers (30-70% sand) or shelves cut by submarine canyons. Their size is mostly intermediate (some
tens to some hundreds of kilometers in length), and
their shape tends to be lobate rather than elongate.
Their slope gradients are in the order of 10%0
(10m/km). Due to their mud content, these fans develop sinuous channel-Ievee systems which divide in
the mid-fan region into distributaries which feed individual outer fan lobes.
Modem examples are the Rhöne fan in the Mediterranean
and some ofthe deep-sea fans offthe Pacific coast ofNorth
America (Delgada, La Jolla, and Navy fans). Of 26 small
submarine fans (mainly less than 10000 km 2 in area) identified in the Tertiary of the North Sea Basin, one half has
sand contents >30%, the other half is mud-rich (Reynolds
1994).
Mud-rich deep-sea fans are commonly associated
with individual major rivers and therefore fed by
point sourees. These deliver sediment either direct1y
by highly concentrated suspension currents from the
river, or by far-travelling large sediment gravity
flows originating in the prodelta region. Most of the
modem examples are very large, elongate sediment
bodies dominated by mud turbidites. The slope gradients of these fans, ranging from about 1 %0 to 50/00 in
the mid-fan and distal fan regions, are greater than
those of comparable subaerial rivers. The upper and
mid-fan regions are characterized by sinuous meandering channel-Ievee systems which can be up to several kilometers wide. The levees may rise by tens up
227
to 100 m above the surrounding sea floor. Large
slump and debris flow deposits occasionally bury
parts of the pre-existing channel-Ievee system and
thus cause changes in the sediment distributary system.
The modem Bengal, lndus, Laurentian, Amazon, Mississippi (see below), Monterey, Congo and Nile fans are 250
to 2500 km long and display wide channels (up to 5-25 km;
Mutti and Normark 1987; Wetzel1993; Reading and Richards 1994; cf. Fig.l1.30). In addition to large mass flows
originating from the slopes, smaller mass flows down the
backside ofthe levees have been observed (e.g. on the Amazon fan; Damuth et al. 1988).
All these fan types, c1assified by grain size, can also
evolve from linear sources and then show laterally
coalescing composite fans (Fig. 5.19). During longterm basin evolution, e.g. from a continental rift zone
to a wide ocean basin, the nature of deep-water fans
may change. In an early stage ofrifting, when a deep,
narrow basin and high relief are established, a number of small gravel- and sand-rich fans are likely to
develop. With increasing size of the ocean basin and
enlarged drainage systems of the entering rivers,
sand/mud-rich and finally mud-rich deep-water fans
tend to form.
Slope aprons develop at the foot of continental
slopes or along the flanks of submarine ridges,
seamounts and plateaus (cf. Fig. 5.12a). They commonly result from linear sources and consist of redeposited material from the upper slope or outer shelf.
Slope aprons dominated by silicic1astics are mostly
fme-grained, whereas those on the foot of carbonate
buildups are frequently coarser in grain size. The
deposits of slope aprons are mostly nonchannelized.
The Sediment Distribution System
of Deep-Sea Fans
The sediment distribution in a deep-sea fan system is
described here in more detail for the type 11 turbidite
deposits (Mutti and Normark 1987) or the deposits of
mud/sand-rich deep-water fans (Reading and Richards 1994).
(I) Channel fills. Moving down the feeder channel,
one may encounter deposits of cohesive mass flows,
coarse grained sands and conglomeratic sandstones.
The latter represent traction carpets, partially displaying inverse grading, and normally graded beds
(Bouma divisions Ta,b' cf. Fig. 5.18 A). The channel
fills cut into or pass laterally into thick or thin-bedded turbidites of the more c1assical types on the levees and fan lobes (cf. Fig. 5.18 Band C).
(2) Challnel-Iobe transition. In the transition zone
between relatively high-gradient channelized flow
Coarse-grained, gravel-rich deep-water fans (and
fans on submarine ramps) develop on slopes in front
of alluvial fans. They can be classified as fan deltas
(cf. Sect. 2.2.2) or "short-headed delta front" fans.
They are restricted to relatively shallow water and
therefore do not correspond to the "normal" type of
deep-sea fan. Part of these wedge-shaped bodies consists of gravity flow deposits; muddy interbeds are
more or less missing.
Sand-rich deep-water fans with a sand to mud ratio
>70:30 often develop either in front ofthe mouths of
small mountainous rivers or at the lower ends of
shelf-slope canyons collecting sand from longshore
transport on the shelf. The sand is distributed by radially diverging braided channels on the fan; levees
and individual depositional lobes are absent. The radii of fans evolving in small basins is commonly 5-10
km in length; sand-rich fans on Atlantic-type passive
margins can be larger. Multi-source systems form
channelized coalescing lobes.
Sand-rich fans have been described, e.g., from the Californian borderland (deep-sea fans), the North Sea, and the
Atlantic margin of South America (Campos Basin, off
Brazil, ramp settings).
Mudlsand-rich deep-water fans are fed by mixedload rivers (30-70% sand) or shelves cut by submarine canyons. Their size is mostly intermediate (some
tens to some hundreds of kilometers in length), and
their shape tends to be lobate rather than elongate.
Their slope gradients are in the order of 10%0
(10m/km). Due to their mud content, these fans develop sinuous channel-Ievee systems which divide in
the mid-fan region into distributaries which feed individual outer fan lobes.
Modem examples are the Rhöne fan in the Mediterranean
and some ofthe deep-sea fans offthe Pacific coast ofNorth
America (Delgada, La Jolla, and Navy fans). Of 26 small
submarine fans (mainly less than 10000 km 2 in area) identified in the Tertiary of the North Sea Basin, one half has
sand contents >30%, the other half is mud-rich (Reynolds
1994).
Mud-rich deep-sea fans are commonly associated
with individual major rivers and therefore fed by
point sourees. These deliver sediment either direct1y
by highly concentrated suspension currents from the
river, or by far-travelling large sediment gravity
flows originating in the prodelta region. Most of the
modem examples are very large, elongate sediment
bodies dominated by mud turbidites. The slope gradients of these fans, ranging from about 1 %0 to 50/00 in
the mid-fan and distal fan regions, are greater than
those of comparable subaerial rivers. The upper and
mid-fan regions are characterized by sinuous meandering channel-Ievee systems which can be up to several kilometers wide. The levees may rise by tens up
227
to 100 m above the surrounding sea floor. Large
slump and debris flow deposits occasionally bury
parts of the pre-existing channel-Ievee system and
thus cause changes in the sediment distributary system.
The modem Bengal, lndus, Laurentian, Amazon, Mississippi (see below), Monterey, Congo and Nile fans are 250
to 2500 km long and display wide channels (up to 5-25 km;
Mutti and Normark 1987; Wetzel1993; Reading and Richards 1994; cf. Fig.l1.30). In addition to large mass flows
originating from the slopes, smaller mass flows down the
backside ofthe levees have been observed (e.g. on the Amazon fan; Damuth et al. 1988).
All these fan types, c1assified by grain size, can also
evolve from linear sources and then show laterally
coalescing composite fans (Fig. 5.19). During longterm basin evolution, e.g. from a continental rift zone
to a wide ocean basin, the nature of deep-water fans
may change. In an early stage ofrifting, when a deep,
narrow basin and high relief are established, a number of small gravel- and sand-rich fans are likely to
develop. With increasing size of the ocean basin and
enlarged drainage systems of the entering rivers,
sand/mud-rich and finally mud-rich deep-water fans
tend to form.
Slope aprons develop at the foot of continental
slopes or along the flanks of submarine ridges,
seamounts and plateaus (cf. Fig. 5.12a). They commonly result from linear sources and consist of redeposited material from the upper slope or outer shelf.
Slope aprons dominated by silicic1astics are mostly
fme-grained, whereas those on the foot of carbonate
buildups are frequently coarser in grain size. The
deposits of slope aprons are mostly nonchannelized.
The Sediment Distribution System
of Deep-Sea Fans
The sediment distribution in a deep-sea fan system is
described here in more detail for the type 11 turbidite
deposits (Mutti and Normark 1987) or the deposits of
mud/sand-rich deep-water fans (Reading and Richards 1994).
(I) Channel fills. Moving down the feeder channel,
one may encounter deposits of cohesive mass flows,
coarse grained sands and conglomeratic sandstones.
The latter represent traction carpets, partially displaying inverse grading, and normally graded beds
(Bouma divisions Ta,b' cf. Fig. 5.18 A). The channel
fills cut into or pass laterally into thick or thin-bedded turbidites of the more c1assical types on the levees and fan lobes (cf. Fig. 5.18 Band C).
(2) Challnel-Iobe transition. In the transition zone
between relatively high-gradient channelized flow