264
6 DEPOSITIONAL SYSTEMS
depths sand deposition by turbidity currents and other gravity processes is the norm,
with minimal reworking by traction currents, except for relatively low-velocity geostrophic ones. Deep-water sands occur principally in marine basins, but also in large
freshwater lake basins. Geomorphologically, deep-water sand deposition occurs in
channels cut into a slope and on fan-shaped lobes at the foot of the slope. Sedimentary
models for terrigenous and carbonate deep-sea sands are rather different, so they are
discussed separately.
6.3.2.9.1 Terrigenous deep-sea sands
Some of the best known modern continental margin turbidite systems occur off the Californian coast (Fig. 6.67). The submarine valley and fan systems of Monterey and La
Jolla are particularly well documented (Gorsline and Emery, 1959; Hand and Emery,
1964; Shepard et al., 1969; Wolf, 1970; Horn et al., 1971). The seaward progradation of
this type of submarine slope produces the following sequence: At the top is a facies of
sands, tending to be well sorted, often glauconitic and with a fraction of skeletal sand.
These sediments are cross bedded, cross laminated, and often burrowed. This facies is
deposited by traction currents on the continental shelf. It passes down, generally
abruptly, into the second facies composed of varying amounts of two distinct subfacies.
One consists of laminated clays and silts which were deposited out of suspension in the
quieter deeper water of the slope zone. These slope deposits tend to slump and slide because of their high water saturation and unstable situation. The Canary Island slide was
described and illustrated earlier (see Section 4.5). The slope shales are cut into by varying amounts of the second slope subfacies. These are the submarine canyon or valleyfill deposits. They consist principally of debris flow, fluidized flow, and grain flow deposits. Where the submarine channels reach the base of the slope they radiate like the
distributaries of a bird foot delta, the analogy even extends to the existence of raised
lev6es on the channel sides. This channel" lev6e system overlies fan-shaped lobes of interbedded turbidite sands and pelagic shales. Sand content and grain size increase toward the top of the fan and toward the apex.
Though the submarine channels of the Californian fans radiate, this pattern is by no
means universal. The submarine channels on the Bengal fan are braided, while those of
the Amazon and Indus fans are highly meandriform (Fig. 6.68).
Ancient terrigenous deep-sea sands are commonly found in linear troughs, sometimes of rifted origin, sometimes associated with zones of subduction. The latter areas,
once termed geosynclines, are now referred to as foredeep basins (see Section 10.2.3).
These sedimentary sequences consist of thick formations of interbedded shales and
graded beds, often interpreted as turbidites (Fig. 6.69). This type of facies has often
been termed "flysch" (Dzulinski and Walton, 1965; Hsu, 1970).
Terrigenous deep sea sands and their sedimentary models have been described by
Howell and Normark (1982), Stow (1985, 1991), Hartley and Prosser (1995), Pickering
et al. (1995), and Stow et al. (1996). There are two common settings. Deep-water sediments are found on continental margins where the slopes are cut in bedrock, and they
are found at the edges of many prograding deltas. The latter arrangement has already
been described as an integral part of the high-slope delta model and modern and ancient examples have been cited.
Reference has ~ready been made to the studies of the La Jolla, Monterey, and asso-
6 DEPOSITIONAL SYSTEMS
depths sand deposition by turbidity currents and other gravity processes is the norm,
with minimal reworking by traction currents, except for relatively low-velocity geostrophic ones. Deep-water sands occur principally in marine basins, but also in large
freshwater lake basins. Geomorphologically, deep-water sand deposition occurs in
channels cut into a slope and on fan-shaped lobes at the foot of the slope. Sedimentary
models for terrigenous and carbonate deep-sea sands are rather different, so they are
discussed separately.
6.3.2.9.1 Terrigenous deep-sea sands
Some of the best known modern continental margin turbidite systems occur off the Californian coast (Fig. 6.67). The submarine valley and fan systems of Monterey and La
Jolla are particularly well documented (Gorsline and Emery, 1959; Hand and Emery,
1964; Shepard et al., 1969; Wolf, 1970; Horn et al., 1971). The seaward progradation of
this type of submarine slope produces the following sequence: At the top is a facies of
sands, tending to be well sorted, often glauconitic and with a fraction of skeletal sand.
These sediments are cross bedded, cross laminated, and often burrowed. This facies is
deposited by traction currents on the continental shelf. It passes down, generally
abruptly, into the second facies composed of varying amounts of two distinct subfacies.
One consists of laminated clays and silts which were deposited out of suspension in the
quieter deeper water of the slope zone. These slope deposits tend to slump and slide because of their high water saturation and unstable situation. The Canary Island slide was
described and illustrated earlier (see Section 4.5). The slope shales are cut into by varying amounts of the second slope subfacies. These are the submarine canyon or valleyfill deposits. They consist principally of debris flow, fluidized flow, and grain flow deposits. Where the submarine channels reach the base of the slope they radiate like the
distributaries of a bird foot delta, the analogy even extends to the existence of raised
lev6es on the channel sides. This channel" lev6e system overlies fan-shaped lobes of interbedded turbidite sands and pelagic shales. Sand content and grain size increase toward the top of the fan and toward the apex.
Though the submarine channels of the Californian fans radiate, this pattern is by no
means universal. The submarine channels on the Bengal fan are braided, while those of
the Amazon and Indus fans are highly meandriform (Fig. 6.68).
Ancient terrigenous deep-sea sands are commonly found in linear troughs, sometimes of rifted origin, sometimes associated with zones of subduction. The latter areas,
once termed geosynclines, are now referred to as foredeep basins (see Section 10.2.3).
These sedimentary sequences consist of thick formations of interbedded shales and
graded beds, often interpreted as turbidites (Fig. 6.69). This type of facies has often
been termed "flysch" (Dzulinski and Walton, 1965; Hsu, 1970).
Terrigenous deep sea sands and their sedimentary models have been described by
Howell and Normark (1982), Stow (1985, 1991), Hartley and Prosser (1995), Pickering
et al. (1995), and Stow et al. (1996). There are two common settings. Deep-water sediments are found on continental margins where the slopes are cut in bedrock, and they
are found at the edges of many prograding deltas. The latter arrangement has already
been described as an integral part of the high-slope delta model and modern and ancient examples have been cited.
Reference has ~ready been made to the studies of the La Jolla, Monterey, and asso-