Page 63 - Sedimentology and Stratigraphy

Processes of Transport and Sedimentary Structures
50 Nichols/Sedimentology and Stratigraphy 9781405193795_4_004 Final Proof page 50 26.2.2009 8:16pm Compositor Name: ARaju

at the velocities recorded in rivers, but even at the
very high wind strengths of storms the largest rock
and mineral particles carried are likely to be around a
millimetre. This limitation to the particle size carried
by air is one of the criteria that may be used to
distinguish material deposited by water from that
transported and deposited by wind. Higher viscosity
4 *
/ / fluids such as ice and debris flows (dense slurries of
sediment and water) can transport boulders metres or
tens of metres across.

4.3 FLOWS, SEDIMENT AND
BEDFORMS

A bedform is a morphological feature formed by the
interaction between a flow and cohesionless sediment
on a bed. Ripples in sand in a flowing stream and sand
dunes in deserts are both examples of bedforms, the
former resulting from flow in water, the latter by air-
flow. The patterns of ripples and dunes are products of
the action of the flow and the formation of bedforms
( 7
. 7
creates distinctive layering and structures within the
/ / sediment that can be preserved in strata. Recognition
of sedimentary structures generated by bedforms pro-
Fig. 4.6 Normal and reverse grading within indivi- vides information about the strength of the current,
dual beds and fining-up and coarsening-up patterns in a
series of beds. the flow depth and the direction of sediment transport.
To explain how bedforms are generated some
further consideration of fluid dynamics is required (a
comprehensive account can be found in Leeder
highest is considered to be fining-upward. The 1999). A fluid flowing over a surface can be divided
reverse pattern with the coarsest bed at the top is a into a free stream, which is the portion of the flow
coarsening-upward succession (Fig. 4.6). Note that unaffected by boundary effects, a boundary layer,
there can be circumstances where individual beds are within which the velocity starts to decrease due to
normally graded but are in a coarsening-up succes- friction with the bed, and a viscous sublayer,a
sion of beds. region of reduced turbulence that is typically less
than a millimetre thick (Fig. 4.7). The thickness of
the viscous sublayer decreases with increasing flow
4.2.6 Fluid density and particle size velocity but is independent of the flow depth. The
relationship between the thickness of the viscous sub-
A second important implication of Stokes Law is that layer and the size of grains on the bed of flow defines
the forces acting on a grain are a function of the an important property of the flow. If all the particles
viscosity and density of the fluid medium as well as are contained within the viscous sublayer the surface
the mass of the particle. A clast falling through air will is considered to be hydraulically smooth, and if
travel faster than if it was falling through water there are particles that project up through this layer
because the density contrast between particle and then the flow surface is hydraulically rough.As
fluid is greater and the fluid viscosity is lower. will be seen in the following sections, processes within
Furthermore, higher viscosity fluids exert greater the viscous sublayer and the effects of rough and
drag and lift forces for a given flow velocity. Water smooth surfaces are fundamental to the formation of
flows are able to transport clasts as large as boulders different bedforms.