4.5 GRAVITATIONAL PROCESSES
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4.5.2 Grain Flows
The concept of the grain flow was expounded by Bagnold (1954, 1966). Grain flows are
liquefied cohesionless particle flows in which the intergranular friction between sand
grains is reduced by their continuous agitation. This is believed by some to be nonturbulent and to involve considerable horizontal shearing (Lowe, 1976; Middleton and Hampton, 1976). Grain flows have been observed in modern submarine channels. They appear
to require a high gradient to initiate them, and a confined space (i.e., a channel) to retain
the high pore pressure required for their maintenance. Unlike most debris flows, grain
flows are typically well sorted and clay free, though they may contain scattered clasts
and astonished marine invertebrates. The Turbo-charged grain flows of offshore southern Africa are an example). Internally, grain flows are generally massive with no vertical size grading, though grain orientation may be parallel to flow. Bases are abrupt, often loaded, but seldom erosional. Tops are also sharp. Individual units may be 1 m or
so in thickness, but multistory sequences of grain flows may attain tens of meters (see
Fig. 6.71).
Hendry (1972) described graded massive marine breccias from the Alpine Jurassic,
known locally as Wild-flyseh, and interpreted them as grain flow deposits. Stauffer
(1967) defined the typical features of grain flow deposits and pointed out the differences
between them and true turbidity current deposits; namely, grain flow deposits are more
typical of the submarine channel, whereas turbidites occur more generally on the fan
or basin floor. Individual grain flow beds have erosional bases but lack the suite of sole
marks characteristic of turbidites. Grain flow beds are massive or faintly bedded with
clasts up to cobble size, scattered throughout them. They are not graded. Turbidites, by
contrast, are graded, laminated, and/or cross-laminated and their coarsest clasts are restricted to the base of the bed (Figs. 4.30C and D). Grain flows are commonly derived
from continental shelf sands, and are thus normally clean and well sorted and may make
excellent petroleum reservoirs (see Section 6.3.2.9.3).
4.5.3 Fluidized Flows
Fluidization of a sand bed occurs when the upward drag exerted by moving pore fluid
exceeds the effective weight of the grains. When this upward movement exceeds the
minimum fluidization velocity, the bed expands rapidly, porosity increases, and the bed
becomes liquefied and fluid supported, rather than grain supported.
The sediments produced by fluidization are similar in many ways to grain flows. They
occur in thick nongraded clean sands, with abrupt tops and bottoms. Because of their
high porosities, however, fluidized beds frequently contain sand pipes and dish structures due to postdepositional dewatering (Fig. 4.30B). Like grain flows, fluidized flows
appear to require a slope and trigger to initiate them, and a channel to retain pore pressure. Observations of ancient deep-sea sands show that, once initiated, both grain flows
and fluidized flows may move down channels far across basin floors with minimal gradient. For example, sands with typical grain flow characteristics occur in the Paleocene
reservoir of the Cod gas field of the North Sea. These sands are now some 200 km from
the delta slope from whence they came (Kessler et al., 1980).
123
4.5.2 Grain Flows
The concept of the grain flow was expounded by Bagnold (1954, 1966). Grain flows are
liquefied cohesionless particle flows in which the intergranular friction between sand
grains is reduced by their continuous agitation. This is believed by some to be nonturbulent and to involve considerable horizontal shearing (Lowe, 1976; Middleton and Hampton, 1976). Grain flows have been observed in modern submarine channels. They appear
to require a high gradient to initiate them, and a confined space (i.e., a channel) to retain
the high pore pressure required for their maintenance. Unlike most debris flows, grain
flows are typically well sorted and clay free, though they may contain scattered clasts
and astonished marine invertebrates. The Turbo-charged grain flows of offshore southern Africa are an example). Internally, grain flows are generally massive with no vertical size grading, though grain orientation may be parallel to flow. Bases are abrupt, often loaded, but seldom erosional. Tops are also sharp. Individual units may be 1 m or
so in thickness, but multistory sequences of grain flows may attain tens of meters (see
Fig. 6.71).
Hendry (1972) described graded massive marine breccias from the Alpine Jurassic,
known locally as Wild-flyseh, and interpreted them as grain flow deposits. Stauffer
(1967) defined the typical features of grain flow deposits and pointed out the differences
between them and true turbidity current deposits; namely, grain flow deposits are more
typical of the submarine channel, whereas turbidites occur more generally on the fan
or basin floor. Individual grain flow beds have erosional bases but lack the suite of sole
marks characteristic of turbidites. Grain flow beds are massive or faintly bedded with
clasts up to cobble size, scattered throughout them. They are not graded. Turbidites, by
contrast, are graded, laminated, and/or cross-laminated and their coarsest clasts are restricted to the base of the bed (Figs. 4.30C and D). Grain flows are commonly derived
from continental shelf sands, and are thus normally clean and well sorted and may make
excellent petroleum reservoirs (see Section 6.3.2.9.3).
4.5.3 Fluidized Flows
Fluidization of a sand bed occurs when the upward drag exerted by moving pore fluid
exceeds the effective weight of the grains. When this upward movement exceeds the
minimum fluidization velocity, the bed expands rapidly, porosity increases, and the bed
becomes liquefied and fluid supported, rather than grain supported.
The sediments produced by fluidization are similar in many ways to grain flows. They
occur in thick nongraded clean sands, with abrupt tops and bottoms. Because of their
high porosities, however, fluidized beds frequently contain sand pipes and dish structures due to postdepositional dewatering (Fig. 4.30B). Like grain flows, fluidized flows
appear to require a slope and trigger to initiate them, and a channel to retain pore pressure. Observations of ancient deep-sea sands show that, once initiated, both grain flows
and fluidized flows may move down channels far across basin floors with minimal gradient. For example, sands with typical grain flow characteristics occur in the Paleocene
reservoir of the Cod gas field of the North Sea. These sands are now some 200 km from
the delta slope from whence they came (Kessler et al., 1980).