Page 262 - Sedimentology and Stratigraphy

Ocean Basins 249

deep. Where they occur adjacent to continental Denser mixtures result in high-density turbidites
margins (e.g. the Peru–Chile Trench west of South that have different characteristics to the ‘Bouma
America) they are filled with sediment supplied from Sequences’ seen in low- and medium-density turbi-
the continent, but mid-ocean trenches, such as the dites. Direct observation of turbidity currents on the
Mariana Trench in the west Pacific, are far from ocean floor is very difficult but their effects have been
any source of material and are unfilled, starved of monitored on a small number of occasions. In Novem-
sediment. ber 1929 an earthquake in the Grand Banks area off
the coast of Newfoundland initiated a turbidity cur-
rent. The passage of the current was recorded by the
16.1.2 Depositional processes in deep seas severing of telegraph cables on the sea floor, which
were cut at different times as the flow advanced.
Deposition of most clastic material in the deep seas is Interpretation of the data indicates that the turbidity
by mass-flow processes (4.5). The most common are current travelled at speeds of between 60 and
debris flows and turbidity currents, and these form 100 km h 1 (Fine et al. 2005). Also, the deposits of
part of a spectrum within which there can be flows recent turbidity flows have been mapped out, for
with intermediate characteristics. example, in the east Atlantic off the Canary Islands
a single turbidite deposit has been shown to have a
3
volume of 125 km (Masson 1994).
Debris-flow deposits
Remobilisation of a mass of poorly sorted, sediment-
High- and low-efficiency systems
rich mixture from the edge of the shelf or the top of
the slope results in a debris flow, which travels down A deep marine depositional system is considered
the slope and out onto the basin plain. Unlike a debris to be a low-efficiency system if sandy sediment is
flow on land an underwater flow has the opportunity carried only short distances (tens of kilometres) out
to mix with water and in doing so it becomes more onto the basin plain and a high-efficiency system if
dilute and this can lead to a change in the flow the transport distances for sandy material are hun-
mechanism and a transition to a turbidity current. dreds of kilometres (Mutti 1992). High-volume flows
The top surface of a submarine debris flow deposit will are more efficient than small-volume flows and the
typically grade up into finer deposits due to dilution of efficiency is also increased by the presence of fines
the upper part of the flow. Large debris flows of mate- that tend to increase the density of the flow and
rial are known from the Atlantic off northwest Africa hence the density contrast with the seawater. The
(Masson et al. 1992) and examples of thick, extensive deposits of low-efficiency systems are therefore con-
debris-flow deposits are also known from the strati- centrated near the edge of the basin, whereas mud-
graphic record (Johns et al. 1981; Pauley 1995). dier, more efficient flows carry sediment out on to the
Debris-flow deposits tens of metres thick and extend- basin plain. The high-efficiency systems will tend to
ing for tens of kilometres are often referred to as have an area near the basin margin called a
megabeds. bypass zone where sediment is not deposited, and
there may be scouring of the underlying surface,
with all the deposition concentrated further out in
Turbidites
the basin.
Dilute mixtures of sediment and water moving as
mass flows under gravity are the most important
Initiation of mass flows
mechanism for moving coarse clastic material in
deep marine environments. These turbidity currents Turbidity currents and mass flows require some form
(4.5.2) carry variable amounts of mud, sand and of trigger to start the mixture of sediment and water
gravel tens, hundreds and even over a thousand kilo- moving under gravity. This may be provided by an
metres out onto the basin plain. The turbidites depos- earthquake as the shaking generated by a seismic
ited can range in thickness from a few millimetres to shock can temporarily liquefy sediment and cause it
tens of metres and are carried by flows with sediment to move. The impact of large storm waves on shelf
concentrations of a few parts per thousand to 10%. sediments may also act as a trigger. Accumulation of