Page 76 - Sedimentology and Stratigraphy

Final Proof page 63
Nichols/Sedimentology and Stratigraphy 9781405193795_4_004
26.2.2009 8:16pm Compositor Name: ARaju
Mass Flows 63
Low density turbidite T c Cross-laminated medium to fine sand, some-
times with climbing ripple lamination, form the
Scale Lithology MUD SAND GRAVEL Structures etc Notes middle division of the Bouma sequence: these
characteristics indicate moderate flow velocities
clay silt vf m vc gran pebb cobb boul within the ripple bedform stability field (4.3.6)
f
c
and high sedimentation rates. Convolute lami-
nation (18.1.2) can also occur in this division.
Fine sand and silt in this layer are the products
'e' - hemipelagic T d
mud
of waning flow in the turbidity current: horizon-
tal laminae may occur but the lamination is
'd' - laminated silt
commonly less well defined than in the ‘b’ layer.
The top part of the turbidite consists of fine-
T e
grained sediment of silt and clay grade: it is
deposited from suspension after the turbidity
'c' - cross-laminated,
lower flow regime current has come to rest and is therefore a hemi-
ripples
10s cm pelagic deposit (16.5.3).
Turbidity currents are waning flows, that is, they
decrease velocity through time as they deposit mate-
'b' - laminated sand, rial, but this means that they also decrease velocity
upper flow regime
plane beds with distance from the source. There is therefore a
decrease in the grain size deposited with distance
'a' - massive, rapid (Stow 1994). The lower parts of the Bouma sequence
deposition (upper
flow regime) are only present in the more proximal parts of the
flow. With distance the lower divisions are progres-
Scoured base
sively lost as the flow carries only finer sediment
(Fig. 4.30) and only the ‘c’ to ‘e’ or perhaps just ‘d’
and ‘e’ parts of the Bouma sequence are deposited. In
Fig. 4.29 The ‘Bouma sequence’ in a turbidite deposit.
the more proximal regions the flow turbulence may
be strong enough to cause scouring and completely
remove the upper parts of a previously deposited bed.
The ‘d’ and ‘e’ divisions may therefore be absent due
100s km
Proximal: T a to T c present
Medial: T a to T e may be present

Distal: T c to T e present

T d and T e divisions
eroded by next flow

Fig. 4.30 Proximal to distal changes in
the deposits formed by turbidity currents.
The lower, coarser parts of the Bouma T a and T b divisions
not deposited distally
sequence are only deposited in the more
proximal regions where the flow also has
a greater tendency to scour into the
underlying beds.