4.2 AQUEOUS PROCESSES
105
Fig. 4.16. Graded bedding in Khreim Group (Silurian) sandstones. Southern Desert of Jordan.
bidity currents. The sediments termed turbidites show the following features: They are
generally thick sequences of regularly interbedded sandstones and shales. These typically occur in orogenic belts or in fault bounded marine basins. The sands have abrupt
basal contacts and show a variety of erosional and deformational structures that are
more fully described in Chapter 5. These include pear-shaped hollows, termed "flutes,"
which taper downcurrent, and various erosional grooves and tool marks caused by debris marking the soft mud beneath the turbidite sand. The mud often shows deformation caused by differential movement of the overlying sand. This generates load
structures, pseudonodules, slides, and slumps. Internally, the sands tend to show an
upward-fining of grain size, termed graded bedding (Fig. 4.16). There are several different types of graded bedding. Distribution grading shows a gradual vertical decrease
of grain size, while maintaining the same distribution, that is, sorting of the sediment.
Coarse tail grading shows a gradual vertical decrease in the maximum grain size. Hence
there is a vertical improvement in sorting. These textural differences may relate to different types of density flow (Allen, 1970, p. 194). Compound grading may be present
within a single sandstone bed. Reverse grading has been observed. An absence of grading in a turbidite may indicate a source of uniform sediment grade.
The internal structures of turbidite beds are few in number, and tend to be arranged
in a regular motif, termed a Bouma sequence (Bouma, 1962) (Fig. 4.17). In the ideal
model five zones can be recognized from A to E. These have been interpreted in terms
of the flow regime by Walker (1965), Harms and Fahnestock (1965), Hubert (1967), and
Shanmugam (1997). The scoured surface at the base of a turbidite bed is frequently succeeded by a conglomerate of extraneous pebbles and locally derived mud clasts. This
indicates the initial high-power erosive phase of the current. Typically this is overlain by
a massive sand, termed the A unit, attributed to sedimentation from antidunes in the
upper flow regime. Upcurrent dipping foresets in this unit have been discovered by
Walker (1967b) and Skipper (1971). The massive A unit is overlain by the laminated B
unit, attributable to a shooting flow regime with deposition from a planar bed form.
105
Fig. 4.16. Graded bedding in Khreim Group (Silurian) sandstones. Southern Desert of Jordan.
bidity currents. The sediments termed turbidites show the following features: They are
generally thick sequences of regularly interbedded sandstones and shales. These typically occur in orogenic belts or in fault bounded marine basins. The sands have abrupt
basal contacts and show a variety of erosional and deformational structures that are
more fully described in Chapter 5. These include pear-shaped hollows, termed "flutes,"
which taper downcurrent, and various erosional grooves and tool marks caused by debris marking the soft mud beneath the turbidite sand. The mud often shows deformation caused by differential movement of the overlying sand. This generates load
structures, pseudonodules, slides, and slumps. Internally, the sands tend to show an
upward-fining of grain size, termed graded bedding (Fig. 4.16). There are several different types of graded bedding. Distribution grading shows a gradual vertical decrease
of grain size, while maintaining the same distribution, that is, sorting of the sediment.
Coarse tail grading shows a gradual vertical decrease in the maximum grain size. Hence
there is a vertical improvement in sorting. These textural differences may relate to different types of density flow (Allen, 1970, p. 194). Compound grading may be present
within a single sandstone bed. Reverse grading has been observed. An absence of grading in a turbidite may indicate a source of uniform sediment grade.
The internal structures of turbidite beds are few in number, and tend to be arranged
in a regular motif, termed a Bouma sequence (Bouma, 1962) (Fig. 4.17). In the ideal
model five zones can be recognized from A to E. These have been interpreted in terms
of the flow regime by Walker (1965), Harms and Fahnestock (1965), Hubert (1967), and
Shanmugam (1997). The scoured surface at the base of a turbidite bed is frequently succeeded by a conglomerate of extraneous pebbles and locally derived mud clasts. This
indicates the initial high-power erosive phase of the current. Typically this is overlain by
a massive sand, termed the A unit, attributed to sedimentation from antidunes in the
upper flow regime. Upcurrent dipping foresets in this unit have been discovered by
Walker (1967b) and Skipper (1971). The massive A unit is overlain by the laminated B
unit, attributable to a shooting flow regime with deposition from a planar bed form.