Rock formation and deformation - Fundamentals of the Physical Environment - page 352

measure sorting visually by the relative horizontal span of
each curve and statistically by standard deviation.
field and elongated particles tend to move either parallel or transverse (normal) to the
flow or slope. This is often preserved as particle orientation in sediments and, together
with imbrication - the stacking of particles dipping (sloping) towards or away from the
flow direction - provide palaeocurrent information of former intransit material. This
extends to include syndepositional sedimentary structures , formed during or
immediately after deposition. Parcels of sediment are laid down as a layer or stratum
determined by the underlying topographical surface, sediment properties and the
geometry of the transporting medium. Its lower surface or bedding plane usually
distinguishes it from the previous (older) parcel by subtle changes in texture or colour in
a conformable sequence , or dramatically so at an unconformity which marks a pause,
erosive event, etc. (Plate 12.1). Within each stratum, a series of subsidiary structures
reflect the direction and energy of transport (Figure 12.13 and Plate 12.2).
SEDIMENTARY FACIES, ENVIRONMENTS AND TECTONIC
BASINS
So far we have reviewed some general, descriptive aspects of sediments. We also
appreciate that each parcel is the
Figure 12.13 Relations between stream power, particle size
and bed forms: (a) the effect of increasing stream power on
bed form; (b) upper plane bed form (top), dune (middle) and
ripple bed forms (bottom); (c) the form and depth/velocity
relationship of ripples.
Source: Partly after Allen (1968).