5.4 Gravity Mass Flow
California were deposited in time intervals ranging from a
few hundred years up to 10 ka, depending mainly on the
thicknesses of the beds (Rupke and Stanley 1974; Einseie
and Kelts 1982). There is probably an inverse (logarithmic)
relation between bed thicknesses and frequency in ancient
turbidites (Piper and Normark 1983. Furthermore, it appears that frequent earthquakes, volcanic eruptions, or
rapid uplift in the source area cause relatively short recurrence intervals for redepositional events.
The shortest time interval (some tens to hundreds of
years) between (mostly amalgamated) turbidite events is to
be expected within submarine channels. Small suspensions
currents are not thick enough to spill over channel levees
and deposit their suspended load as overbank deposits (cf.
Fig. 5.18 C). Similarly, the frequency of preserved
turbidites in distal fan regions appears to be less than that
in more proximal channels and overbank settings.
Sediment redeposition and turbidite sequences of
some thickness are affected by relative sea-Ievel
changes which mayaiso result from "pulses" of tectonic activity. Lowering of sea level accentuates
terrigenous sediment input, gravity mass movements,
and rapid progradation andJor upbuilding of deep-sea
fans. The resulting sequences displaya coarsening or
thicking-upward trend. In contrast, sea-Ievel rise reduces lateral sediment supply from terrestrial
sourees, backstepping of the depocenter and a fining
or thinning-upward trend.
Finally, progress in basin filling will lead to a
long-term trend from distal to proximal turbidites and
ultimately to shallow-water and continental deposits,
as known from remnant basins, foreare and backare
basins, and foreland basins (cf. Chap. 12).
5.4.5 Deep-Sea Fan Associations
Deep-sea fans collect large volumes of sediment, the
maj or part of which is of terrestrial origin and deposited by mass flows and turbidity currents. These large
wedge-shaped sediment bodies and their sandy channels are important not only for the filling of several
types of sedimentary basins but also as sites of the
generation and storage of hydrocarbons. For these
reasons, deep-sea fans have attracted much interest
by academia and oil companies. Early studies on uplifted ancient examples were mainly based on exposures on land which are commonly not large enough
to reveal the architecture of such huge depositional
systems. One has to realize that many deep-sea fans
have lengths of some 100 km, and their channels may
be hundreds of meters wide. For these reasons, the
study of modem and ancient deep-sea fans with the
aid of geophysical methods only allows a better understanding of these sediment bodies.
A summary of earlier publications (including Barnes and
Normark 1985; Shanmugan and Moiola 1985, 1991; Mutti
and Normark 1987; Damuth et al. 1988; Nelson and
225
Maldonado 1988; Mutti 1992) is given by Reading and
Richards (1994) and Stowet al. (1996).
Types of Deep-Sea Fans
The size, granulometry and facies architecture of
deep-sea fans vary greatly. Several authors have proposed specific schemes for the various fan systems,
but it appears that up to date no universal classification system exists which is generally accepted. In the
past, many workers distinguished between three
types offan systems (e.g. Mutti and Normark 1987):
- Type I turbidite deposits. The turbidites originate
from a highly efficient sediment source and accumulate on the plain of elongate basins. There is a sediment bypass zone between the feeder channel, incised into the slope, and the detached, unchannelized
fan lobes (cf. Fig. 5.18, D). This is the "classical"
model explaining the proximal-distal trends of individual turbidite beds.
- Type 11 turbidite deposits. These fan systems are
strongly affected by a channelized sediment distribution system (Fig. 5.18 A through E). They are normally fed by sediment input from large rivers, i.e.
from efficient point sources and their depositional
lobes are attached to the slope channels. Most of the
modem large deep-sea. fans belong to this category.
They develop aggrading, rnigrating, sinuous channellevee complexes with crevasse splays similar to those
of subaerial meandering rivers (cf. Fig. 5.18 Band
C).
- Type III turbidite deposits. In these relatively small
fan systems, sand-filled channels are restricted to the
upper fan whereas the interchannel areas and the basin plain is dominated by fine-grained, thin-bedded
silt and mud turbidites.
These relatively simple fan models have been recently replaced by a wide spectrum of deep-water
clastic models which, in particular, take into account
the rate and grain size distribution of the terrestrial
sediment input, the type of feeder system (point
source versus linear, multi-source system), the morphology of the sea floor, and relative sea-Ievel
changes. The influence of the first two points is summarized in Fig. 5.19).
Some authors distinguish between deep-water fans, fed by
individual, deeply incised canyons, and fans aggrading and
prograding on ramps of shallower basins (e.g. the North
Sea). These ramps comprise the slope and base-of-slope of
such basins. The sediment accumulating on submarine
ramps is often delivered by multiple sourees. In this brief
overview, the differences between deep-water fans and
ramp fans are not discussed further. For more details about
the various architectural elements of deep-sea fans, their
California were deposited in time intervals ranging from a
few hundred years up to 10 ka, depending mainly on the
thicknesses of the beds (Rupke and Stanley 1974; Einseie
and Kelts 1982). There is probably an inverse (logarithmic)
relation between bed thicknesses and frequency in ancient
turbidites (Piper and Normark 1983. Furthermore, it appears that frequent earthquakes, volcanic eruptions, or
rapid uplift in the source area cause relatively short recurrence intervals for redepositional events.
The shortest time interval (some tens to hundreds of
years) between (mostly amalgamated) turbidite events is to
be expected within submarine channels. Small suspensions
currents are not thick enough to spill over channel levees
and deposit their suspended load as overbank deposits (cf.
Fig. 5.18 C). Similarly, the frequency of preserved
turbidites in distal fan regions appears to be less than that
in more proximal channels and overbank settings.
Sediment redeposition and turbidite sequences of
some thickness are affected by relative sea-Ievel
changes which mayaiso result from "pulses" of tectonic activity. Lowering of sea level accentuates
terrigenous sediment input, gravity mass movements,
and rapid progradation andJor upbuilding of deep-sea
fans. The resulting sequences displaya coarsening or
thicking-upward trend. In contrast, sea-Ievel rise reduces lateral sediment supply from terrestrial
sourees, backstepping of the depocenter and a fining
or thinning-upward trend.
Finally, progress in basin filling will lead to a
long-term trend from distal to proximal turbidites and
ultimately to shallow-water and continental deposits,
as known from remnant basins, foreare and backare
basins, and foreland basins (cf. Chap. 12).
5.4.5 Deep-Sea Fan Associations
Deep-sea fans collect large volumes of sediment, the
maj or part of which is of terrestrial origin and deposited by mass flows and turbidity currents. These large
wedge-shaped sediment bodies and their sandy channels are important not only for the filling of several
types of sedimentary basins but also as sites of the
generation and storage of hydrocarbons. For these
reasons, deep-sea fans have attracted much interest
by academia and oil companies. Early studies on uplifted ancient examples were mainly based on exposures on land which are commonly not large enough
to reveal the architecture of such huge depositional
systems. One has to realize that many deep-sea fans
have lengths of some 100 km, and their channels may
be hundreds of meters wide. For these reasons, the
study of modem and ancient deep-sea fans with the
aid of geophysical methods only allows a better understanding of these sediment bodies.
A summary of earlier publications (including Barnes and
Normark 1985; Shanmugan and Moiola 1985, 1991; Mutti
and Normark 1987; Damuth et al. 1988; Nelson and
225
Maldonado 1988; Mutti 1992) is given by Reading and
Richards (1994) and Stowet al. (1996).
Types of Deep-Sea Fans
The size, granulometry and facies architecture of
deep-sea fans vary greatly. Several authors have proposed specific schemes for the various fan systems,
but it appears that up to date no universal classification system exists which is generally accepted. In the
past, many workers distinguished between three
types offan systems (e.g. Mutti and Normark 1987):
- Type I turbidite deposits. The turbidites originate
from a highly efficient sediment source and accumulate on the plain of elongate basins. There is a sediment bypass zone between the feeder channel, incised into the slope, and the detached, unchannelized
fan lobes (cf. Fig. 5.18, D). This is the "classical"
model explaining the proximal-distal trends of individual turbidite beds.
- Type 11 turbidite deposits. These fan systems are
strongly affected by a channelized sediment distribution system (Fig. 5.18 A through E). They are normally fed by sediment input from large rivers, i.e.
from efficient point sources and their depositional
lobes are attached to the slope channels. Most of the
modem large deep-sea. fans belong to this category.
They develop aggrading, rnigrating, sinuous channellevee complexes with crevasse splays similar to those
of subaerial meandering rivers (cf. Fig. 5.18 Band
C).
- Type III turbidite deposits. In these relatively small
fan systems, sand-filled channels are restricted to the
upper fan whereas the interchannel areas and the basin plain is dominated by fine-grained, thin-bedded
silt and mud turbidites.
These relatively simple fan models have been recently replaced by a wide spectrum of deep-water
clastic models which, in particular, take into account
the rate and grain size distribution of the terrestrial
sediment input, the type of feeder system (point
source versus linear, multi-source system), the morphology of the sea floor, and relative sea-Ievel
changes. The influence of the first two points is summarized in Fig. 5.19).
Some authors distinguish between deep-water fans, fed by
individual, deeply incised canyons, and fans aggrading and
prograding on ramps of shallower basins (e.g. the North
Sea). These ramps comprise the slope and base-of-slope of
such basins. The sediment accumulating on submarine
ramps is often delivered by multiple sourees. In this brief
overview, the differences between deep-water fans and
ramp fans are not discussed further. For more details about
the various architectural elements of deep-sea fans, their