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Processes of Transport and Sedimentary Structures
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to this erosion and the eroded sediment may be incor-
High density turbidite
porated into the overlying deposit as mud clasts. The
complete T a to T e sequence is therefore only likely to
occur in certain parts of the deposit, and even there Scale Lithology Structures etc Notes
intermediate divisions may be absent due, for exam- MUD SAND GRAVEL
ple, to rapid deposition preventing ripple forma- clay silt vf m vc gran pebb cobb boul
f
c
tion in T c . Complete T a–e Bouma sequences are in
fact rather rare.
High-density turbidity currents
water escape
structures
Under conditions where there is a higher density of
material in the mixture the processes in the flow and
hence of the characteristics of the deposit are different
from those described above. High-density turbidity laminated
currents have a bulk density of at least 1.1 g cm 3
(Pickering et al. 1989). The turbidites deposited by 10s cm
these flows have a thicker coarse unit at their base,
which can be divided into three divisions (Fig. 4.31).
Divisions S 1 and S 2 are traction deposits of coarse
structureless
material, with the upper part, S 2 , representing the
‘freezing’ of the traction flow. Overlying this is a
unit, S 3 , that is characterised by fluid-escape struc-
inverse grading
tures indicating rapid deposition of sediment. The
upper part of the succession is more similar to the
Bouma Sequence, with T t equivalent to T b and T c and
overlain by T d and T e : this upper part therefore
reflects deposition from a lower density flow once
most of the sediment had already been deposited in
the ‘S’ division. The characteristics of high-density Fig. 4.31 A high-density turbidite deposited from a flow
with a high proportion of entrained sediment.
turbidites were described by Lowe (1982), after
whom the succession is sometimes named.
elly material in a steep subaqueous setting such as the
foreset of a Gilbert-type delta (12.4.4).
4.5.3 Grain flows

Avalanches are mechanisms of mass transport down 4.6 MUDCRACKS
a steep slope, which are also known as grain flows.
Particles in a grain flow are kept apart in the fluid Clay-rich sediment is cohesive and the individual par-
medium by repeated grain to grain collisions and ticles tend to stick to each other as the sediment dries
grain flows rapidly ‘freeze’ as soon as the kinetic out. As water is lost the volume reduces and clusters
energy of the particles falls below a critical value. of clay minerals pull apart developing cracks in the
This mechanism is most effective in well-sorted mate- surface. Under subaerial conditions a polygonal pat-
rial falling under gravity down a steep slope such as tern of cracks develops when muddy sediment dries
the slip face of an aeolian dune. When the particles in out completely: these are desiccation cracks
the flow are in temporary suspension there is a ten- (Fig. 4.32). The spacing of desiccation cracks depends
dency for the finer grains to fall between the coarser upon the thickness of the layer of wet mud, with a
ones, a process known as kinetic sieving, which broader spacing occurring in thicker deposits. In
results in a slight reverse grading in the layer once it cross-section desiccation cracks taper downwards
is deposited. Although most common on a small scale and the upper edges may roll up if all of the moisture
in sands, grain flows may also occur in coarser, grav- in the mud is driven off. The edges of desiccation