122
4 TRANSPORTATION AND SEDIMENTATION
flows are muddy. Debris flows occur in a wide range of environments, ranging from
deserts to continental slopes (Coussot and Meunier, 1996). In the former situation torrential rainfall is generally required to initiate movement; in the latter, earthquakes,
tide, or storm surges are required. In all situations a slope is a necessary prerequisite.
In desert environments the debris flow was first described by Blackwelder (1928). This
is a mass of rock, sand, and mud which, liquefied by heavy rain, moves down mountain
sides. Movement may initially be slow, but with increasing water content, can accelerate
to a fast-flowing flood of debris that carries all before it. Large debris flows are a truly
catastrophic process destroying animals, trees, and houses (e.g., Scott, 1971). Blackwelder (1928) lists the four prerequisites of a mud flow as an abundance of unconsolidated detritus, steep gradients, a lack of vegetation, and heavy rainfall.
The deposits of debris flows range from boulders to gravel, sand, silt, and clay. Where
sediment of only one grade is available at source, the resultant deposit will be of that
grade and well sorted. Characteristically, however, debris flow deposits are poorly sorted
and massive (Fig. 4.30A). They are variously named debrites, pebbly mudstone (Crowell, 1957), diamictites (Flint et al., 1960), or fluxoturbidites (Kuenen, 1958). The last
of these, though widely used, is a naughty genetic word for a little-known process, as
Walker (1967a) has enthusiastically pointed out. Debris flows may occur both on land
and under water. On land, debris flows grade with increasing water content into sheet
floods, which are midway between grain flows and traction currents. Sheet floods, or
flash floods, deposit subhorizontally bedded coarse sands and gravels with intermittent
channelling (Ives, 1936; Hooke, 1967). These characteristically occur on alluvial fans
and desert pediments. Subaqueous debris flows are typically found in submarine canyons on delta fronts and continental margins (Stanley and Unrug, 1972). Modern and
ancient diamictites from these settings are described in Chapter 6 (see Sections 6.3.2.1
and 6.3.2.9.1, respectively).
Fig. 4.30. Diagrams of the sedimentary sequences produced by gravitational and gravity-related processes.
Vertical scales are varied. Debris flows may be tens of meters thick, whereas turbidites are normally less than
1 m thick.
4 TRANSPORTATION AND SEDIMENTATION
flows are muddy. Debris flows occur in a wide range of environments, ranging from
deserts to continental slopes (Coussot and Meunier, 1996). In the former situation torrential rainfall is generally required to initiate movement; in the latter, earthquakes,
tide, or storm surges are required. In all situations a slope is a necessary prerequisite.
In desert environments the debris flow was first described by Blackwelder (1928). This
is a mass of rock, sand, and mud which, liquefied by heavy rain, moves down mountain
sides. Movement may initially be slow, but with increasing water content, can accelerate
to a fast-flowing flood of debris that carries all before it. Large debris flows are a truly
catastrophic process destroying animals, trees, and houses (e.g., Scott, 1971). Blackwelder (1928) lists the four prerequisites of a mud flow as an abundance of unconsolidated detritus, steep gradients, a lack of vegetation, and heavy rainfall.
The deposits of debris flows range from boulders to gravel, sand, silt, and clay. Where
sediment of only one grade is available at source, the resultant deposit will be of that
grade and well sorted. Characteristically, however, debris flow deposits are poorly sorted
and massive (Fig. 4.30A). They are variously named debrites, pebbly mudstone (Crowell, 1957), diamictites (Flint et al., 1960), or fluxoturbidites (Kuenen, 1958). The last
of these, though widely used, is a naughty genetic word for a little-known process, as
Walker (1967a) has enthusiastically pointed out. Debris flows may occur both on land
and under water. On land, debris flows grade with increasing water content into sheet
floods, which are midway between grain flows and traction currents. Sheet floods, or
flash floods, deposit subhorizontally bedded coarse sands and gravels with intermittent
channelling (Ives, 1936; Hooke, 1967). These characteristically occur on alluvial fans
and desert pediments. Subaqueous debris flows are typically found in submarine canyons on delta fronts and continental margins (Stanley and Unrug, 1972). Modern and
ancient diamictites from these settings are described in Chapter 6 (see Sections 6.3.2.1
and 6.3.2.9.1, respectively).
Fig. 4.30. Diagrams of the sedimentary sequences produced by gravitational and gravity-related processes.
Vertical scales are varied. Debris flows may be tens of meters thick, whereas turbidites are normally less than
1 m thick.