Geoscience Reference
In-Depth Information
Left bank flood plain
Channel
Right bank flood plain
0
0.5
Preflood
deposits
1.0
Trenches
Preflood surface
not reached
Figure 13.29
Flash-flood deposits of Bijou Creek, Colorado, exposed in trenches cut in the flood plain and channel. Note the
predominance of horizontal primary structures (after McKee, Crosby and Berryhill, 1967).
no opportunity for the development of avalanche faces
and their associated cross-bedding. As a consequence,
deposits are predominantly clast-supported but, because
of high bedload flux over the subhorizontal planar sur-
faces involving a wide range of grain sizes, interstices are
matrix-filled.
abundance of clay intraclasts. These have started as mud-
curls lying on the stream bed, have been entrained by
a subsequent flood and have ended up incorporated in a
superposed bed downstream.
13.6
Conclusions
13.5.3
Mud drapes and mud intraclasts
Rivers play an important role in shaping the Earth's deserts
despite the fact that they run for only a vanishingly small
fraction of time as one moves towards the hyper-arid
core regions. The thick sequences of fluviatile desert sed-
iments that range in age from Precambrian (Williams,
1969), through Devonian (Carruthers, 1987) and Trias-
sic (Steel and Thompson, 1983), to the Plio-Pleistocene
(Vondra
The fact that the concentration of suspended sediment is
often in excess of several thousand milligrams per litre,
even in the last stages of flow, that much of this is of
clay size and that the channel dries out after each flood so
that fines are not winnowed as they might be in a peren-
nial system, all mean that ephemeral stream sequences
are often punctuated by comparatively thick (say up to
0.1 m) drapes of clay that have settled from the flow
or from stagnant pools in the channel (Tunbridge, 1984;
Reid, Lindsey and Frostick, 1989) (Figure 13.27). How-
ever, not only are there intact clay drapes but also an
and
Bowen,
1978)
are
more
than
adequate
testimony.
When they run, desert rivers are more efficient ero-
sional agents than their perennial counterparts (Alnedeij
and Diplas, 2005; Reid, Laronne and Powell, 1999). This
is due to the fact that material is readily available for
