Page 74 - Sedimentology and Stratigraphy

Final Proof page 61
26.2.2009 8:16pm Compositor Name: ARaju
Nichols/Sedimentology and Stratigraphy 9781405193795_4_004
Mass Flows 61
gravity flows or density currents (Middleton &
Hampton 1973). A number of different mecha-
nisms are involved and all require a slope to provide
the potential energy to drive the flow. This slope may
be the surface over which the flow occurs, but a
gravity flow will also move on a horizontal surface if
it thins downflow, in which case the potential energy
is provided by the difference in height between the
tops of the upstream and the downstream parts of
the flow.

4.5.1 Debris flows
Fig. 4.26 A muddy debris flow in a desert wadi.
Debris flows are dense, viscous mixtures of sediment
and water in which the volume and mass of sediment
exceeds that of water (Major 2003). A dense, viscous
mixture of this sort will typically have a low Reynolds
number so the flow is likely to be laminar (4.2.1). In
the absence of turbulence no dynamic sorting of
material into different sizes occurs during flow and
the resulting deposit is very poorly sorted. Some sort-
ing may develop by slow settling and locally there
may be reverse grading produced by shear at the
bed boundary. Material of any size from clay to large
boulders may be present.
Debris flows occur on land, principally in arid
environments where water supply is sparse (such as
some alluvial fans, 9.5) and in submarine environ-
ments where they transport material down continen-
tal slopes (16.1.2) and locally on some coarse-grained
delta slopes (12.4.4). Deposition occurs when internal
friction becomes too great and the flow ‘freezes’
(Fig. 4.26). There may be little change in the thick-
ness of the deposit in a proximal to distal direction and
the clast size distribution may be the same throughout Fig. 4.27 A debris-flow deposit is characteristically poorly
the deposit. The deposits of debris flows on land are sorted, matrix-supported conglomerate.
typically matrix-supported conglomerates although
clast-supported deposits also occur if the relative
proportion of large clasts is high in the sediment 4.5.2 Turbidity currents
mixture. They are poorly sorted and show a chaotic
fabric, i.e. there is usually no preferred orientation to Turbidity currents are gravity-driven turbid mix-
the clasts (Fig. 4.27), except within zones of shearing tures of sediment temporarily suspended in water.
that may form at the base of the flow. When a debris They are less dense mixtures than debris flows and
flow travels through water it may partly mix with it with a relatively high Reynolds number are usually
and the top part of the flow may become dilute. The turbulent flows (4.2.1). The name is derived from
tops of subaqueous debris flows are therefore charac- their characteristics of being opaque mixtures of sedi-
terised by a gradation up into better sorted, graded ment and water (turbid) and not the turbulent flow.
sediment, which may have the characteristics of a They flow down slopes or over a horizontal surface
turbidite (see below). provided that the thickness of the flow is greater