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Processes of Transport and Sedimentary Structures
62 Nichols/Sedimentology and Stratigraphy 9781405193795_4_004 Final Proof page 62 26.2.2009 8:16pm Compositor Name: ARaju

upflow than it is downflow. The deposit of a turbidity bulent mixing of the current with the water dilutes
current is a turbidite. The sediment mixture may the turbidity current and also reduces the density
contain gravel, sand and mud in concentrations contrast. As more sediment is deposited from the
as little as a few parts per thousand or up to 10% decelerating flow a deposit accumulates and the flow
by weight: at the high concentrations the flows eventually comes to a halt when the flow has spread
may not be turbulent and are not always referred out as a thin, even sheet.
to as turbidity currents. The volumes of material
involved in a single flow event can be anything up Low- and medium-density turbidity currents
to tens of cubic kilometres, which is spread out by
the flow and deposited as a layer a few millime- The first material to be deposited from a turbidity
tres to tens of metres thick. Turbidity currents, and current will be the coarsest as this will fall out of
hence turbidites, can occur in water anywhere that suspension first. Therefore a turbidite is characteristi-
there is a supply of sediment and a slope. They are cally normally graded (4.2.9). Other sedimentary
common in deep lakes (10.2.3), and may occur on structures within the graded bed reflect the changing
continental shelves (14.1), but are most abundant in processes that occur during the flow and these vary
deep marine environments, where turbidites are the according to the density of the initial mixture. Low- to
dominant clastic deposit (16.1.2). The association medium-density turbidity currents will ideally form a
with deep marine environments may lead to the succession known as a Bouma sequence (Fig. 4.29),
assumption that all turbidites are deep marine depos- named after the geologist who first described them
its, but they are not an indicator of depth as turbidity (Bouma 1962). Five divisions are recognised within
currents are a process that can occur in shallow water the Bouma sequence, referred to as ‘a’ to ‘e’ divisions
as well. and annotated T a ,T b , and so on.
Sediment that is initially in suspension in the tur- T a This lowest part consists of poorly sorted, struc-
bidity current (Fig. 4.28) starts to come into contact tureless sand: on the scoured base deposition
with the underlying surface where it may come to a occurs rapidly from suspension with reduced
halt or move by rolling and suspension. In doing so it turbulence inhibiting the formation of bedforms.
comes out of suspension and the density of the flow is T b Laminated sand characterises this layer, the
reduced. Flow in a turbidity current is maintained by grain size is normally finer than in ‘a’ and the
the density contrast between the sediment–water mix material is better sorted: the parallel laminae are
and the water, and if this contrast is reduced, the flow generated by the separation of grains in upper
slows down. At the head of the flow (Fig. 4.28) tur- flow regime transport (4.3.4).

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Fig. 4.28 A turbidity current is a turbu-

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lent mixture of sediment and water that
deposits a graded bed – a turbidite.