Page 57 - Sedimentology and Stratigraphy

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26.2.2009 8:15pm Compositor Name: ARaju

4

Processes of Transport and

Sedimentary Structures

Most sedimentary deposits are the result of transport of material as particles. Movement
of detritus may be purely due to gravity but more commonly it is the result of flow in water,
air, ice or dense mixtures of sediment and water. The interaction of the sedimentary
material with the transporting media results in the formation of bedforms, which may be
preserved as sedimentary structures in rocks and hence provide a record of the processes
occurring at the time of deposition. If the physical processes occurring in different modern
environments are known and if the sedimentary rocks are interpreted in terms of those
same processes it is possible to infer the probable environment of deposition. Under-
standing these processes and their products is therefore fundamental to sedimentology.
In this chapter the main physical processes occurring in depositional environments are
discussed. The nature of the deposits resulting from these processes and the main
sedimentary structures formed by the interaction of the flow medium and the detritus
are introduced. Many of these features occur in a number of different sedimentary envir-
onments and should be considered in the context of the environments in which they occur.

4.1 TRANSPORT MEDIA further down slope. The slope angle for loose debris
varies with the shape of the clasts and distribution of
clast sizes, ranging from just over 308 for well-sorted
Gravity The simplest mechanism of sediment trans-
sand to around 368 for angular gravel (Carson 1977;
port is the movement of particles under gravity down
Bovis 2003). Scree deposits are localised in mountain-
a slope. Rock falls generate piles of sediment at the
ous areas (6.5.1) and occasionally along coasts: they
base of slopes, typically consisting mainly of coarse
are rarely preserved in the stratigraphic record.
debris that is not subsequently reworked by other
processes. These accumulations are seen as scree Water Transport of material in water is by far the
along the sides of valleys in mountainous areas. most significant of all transport mechanisms. Water
They build up as talus cones with a surface at the flows on the land surface in channels and as overland
angle of rest of the gravel, the maximum angle at flow. Currents in seas are driven by wind, tides and
which the material is stable without clasts falling oceanic circulation. These flows may be strong enough