Page 120 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Low-density turbidity currents may be generated particles with minor granules or pebbles. Particles are
independently, for example, by rivers discharging into the supported mainly by turbulence and hindered settling.
sea, or develop from subaqueous debris flows and high- Gravelly high-density currents carry up to 15% pebble
density turbidity currents. They are a very important and cobble-size particles, together with clay to coarse
means by which sediment is transported or resedimented sand, supported by a combination of dispersive
from shallow to deep-water settings. pressure and matrix buoyant lift.

High-density turbidity currents Deposition from sandy high-density turbidity currents
occurs progressively from the base up, in stages that
High-density turbidity currents have greater reflect increasing flow unsteadiness and decreasing flow
concentrations of particles, and can transport coarser competence (Fig. 49B; 29.3). Traction structures are
particle sizes than low-density turbidity currents. generated by interaction between deposited sediment
High-density turbidity currents have widths and and the current. An ideal sequence comprises: a lower
lengths estimated to be up to tens of kilometers, and division of coarse sand to gravel with plane lamination
thicknesses of hundreds of meters. Recorded examples and cross stratification generated by traction
traveled at tens of kilometers per hour (e.g. 70 km/hr sedimentation (S 1); an intermediate division of reverse
maximum velocity for the turbidity current generated by graded (sand to granule), horizontal, thin beds,
the Grand Banks earthquake in 1929 — Stow, 1986). representing traction-carpet deposits (S 2); and an upper
Runout distances depend on sediment supply and division of grain-supported, massive or normally
probably reach several hundred kilometers from graded sand, commonly with water-escape structures,
source. deposited directly and rapidly from dense suspensions
(S 3) (Lowe, 1982). The residual turbulent suspension
Grain support and depositional processes in high- of fine sediment may be deposited above the S 1-3
density turbidity currents are more complex than in sequence as a low-density turbidite or else progress
low-density turbidity currents, and depend strongly on downslope, bypassing the sandy units.
grain size and concentration (Lowe, 1982). Sandy high-
density turbidity currents are dominated by sand-size

Fig. 48—Summary of the principal transport and depositional processes and deposits in deep subaqueous settings.
Modified from Stow (1986).

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