106
4 TRANSPORTATION AND SEDIMENTATION
Fig. 4.17. Turbidite unit showing the complete Bouma sequence and its interpretation in terms of flow
regimes.
This is succeeded by a cross-laminated C unit, which often shows convolute deformational structures due to penecontemporaneous dewatering (see Section 5.3.4.1). The
cross-laminated zone reflects sedimentation from a lower flow regime. This C unit is
overlain by a second laminated zone, the D unit, which grades up into the pelagic muds
of the E unit, which settle from suspension.
Detailed statistical studies of whole turbidite formations show that sometimes the directional changes are predictable (e.g., Walker, 1970; Pett and Walker, 1971; Parkash,
1970). Moving downcurrent or down-section, the following trends are commonly found:
Grain size and sand bed thickness diminish; bottom structures tend to change from channelling to flute marks, groove marks, and finally tool marks. The internal structures of
the turbidite appear to undergo progressive truncation from the base upward. First, the
massive A unit is overstepped by the laminated B unit. This is in turn overstepped by
the cross-laminated C unit, and so on (Fig. 4.18).
Walker (1967a) proposed a statistical coefficient, the P index, as a measure of the
proximal (i.e., near source) or distal position of beds within a turbidite formation. Recognition that turbidite formations change their sedimentologic parameters upward and
sourceward throws considerable light on their genesis. Field geologists (e.g., Bailey,
1930) are impressed by the great difference between the deposits of traction currents
and those of turbidites. The former are cross-bedded, clean, and laterally restricted.
Turbidites, on the other hand, are flat-bedded, graded, argillaceous, and often laterally
extensive. One tends to contrast the sideways sedimentation of the traction current with
the essentially vertical sedimentation of the turbidite. This is an oversimplification and,
as further sections in this chapter show, turbidites form part of a continuous spectrum
of sedimentary processes that range from a landslide to a cloud of suspended clay.
4.2.3 Sedimentation from Low-Density Turbidity Currents
Fine-grained clay and silt are seldom if ever deposited from traction currents because
they tend to be transported in suspension rather than as bed load. A certain amount of
4 TRANSPORTATION AND SEDIMENTATION
Fig. 4.17. Turbidite unit showing the complete Bouma sequence and its interpretation in terms of flow
regimes.
This is succeeded by a cross-laminated C unit, which often shows convolute deformational structures due to penecontemporaneous dewatering (see Section 5.3.4.1). The
cross-laminated zone reflects sedimentation from a lower flow regime. This C unit is
overlain by a second laminated zone, the D unit, which grades up into the pelagic muds
of the E unit, which settle from suspension.
Detailed statistical studies of whole turbidite formations show that sometimes the directional changes are predictable (e.g., Walker, 1970; Pett and Walker, 1971; Parkash,
1970). Moving downcurrent or down-section, the following trends are commonly found:
Grain size and sand bed thickness diminish; bottom structures tend to change from channelling to flute marks, groove marks, and finally tool marks. The internal structures of
the turbidite appear to undergo progressive truncation from the base upward. First, the
massive A unit is overstepped by the laminated B unit. This is in turn overstepped by
the cross-laminated C unit, and so on (Fig. 4.18).
Walker (1967a) proposed a statistical coefficient, the P index, as a measure of the
proximal (i.e., near source) or distal position of beds within a turbidite formation. Recognition that turbidite formations change their sedimentologic parameters upward and
sourceward throws considerable light on their genesis. Field geologists (e.g., Bailey,
1930) are impressed by the great difference between the deposits of traction currents
and those of turbidites. The former are cross-bedded, clean, and laterally restricted.
Turbidites, on the other hand, are flat-bedded, graded, argillaceous, and often laterally
extensive. One tends to contrast the sideways sedimentation of the traction current with
the essentially vertical sedimentation of the turbidite. This is an oversimplification and,
as further sections in this chapter show, turbidites form part of a continuous spectrum
of sedimentary processes that range from a landslide to a cloud of suspended clay.
4.2.3 Sedimentation from Low-Density Turbidity Currents
Fine-grained clay and silt are seldom if ever deposited from traction currents because
they tend to be transported in suspension rather than as bed load. A certain amount of