Geoscience Reference
In-Depth Information
m
Parallel Lamination
18
Convolute Bedding
16
Cross Bedding
14
12
Intraformational Conglomerate
10
Mudstone
8
Desiccation Cracks
6
Water Escape Structures
4
Mud Draped Cross Bedding
2
Massive Sandstone
0
Figure 13.26
Measured sections of flash flood deposits of the Middle Devonian Trentishoe Formation, exposed in the sea cliffs
of North Devon, England (after Tunbridge, 1984).
mid-channel bars in the gravel-bed Nahal Zin, which
drains to the Dead Sea trough, suggesting a sediment
turnover of c. 20 % every four or five years on the ba-
sis of flood magnitude frequency and erosional trimming
and regrowth. For the same bars, Hassan (2005) gives an
average scour depth of 0.1 m for the mean annual flood,
0.3 m for the 1-in-10 year event and 0.4 m for larger floods
with a recurrence interval of 1-in-20 years.
floods in ephemeral channels, but it is more than proba-
ble that they represent upper flow regime plane beds. They
usually comprise a set of couplets, however, in which lam-
inae of coarser and finer particles alternate (Figure 13.27).
An examination of the deposits of floods associated with
one storm but in four contiguous river basins of northern
Kenya suggests that the separation of particles by size is
related to small pulses of sediment-laden water that are
superimposed on the main flood wave by staggered con-
tributions from tributary channels. From the very strong
correlations in four separate basins, it is conjectured that
each pair of laminae of the event channel-fill is associ-
ated with a single pulse (Frostick and Reid, 1977, 1979)
(Figure 13.30).
Of interest is that gravel-bed ephemerals are similarly
characterised by a predominance of subhorizontal bed-
ding (Laronne and Shlomi, 2007). The absence of cross-
bedding is attributed by Hassan, Marren and Schwartz
(2009) to the shallowness of competent flows (a part-
product of wide channels) and the low relief of the
13.5.2
Predominance of horizontal lamination
in sand beds
The planar surface of many single-thread ephemeral
streams has already been remarked on. This surface re-
flects the apparent ubiquity (Picard and High, 1973) and
prevalence (McKee, Crosby and Berryhill, 1967) (Fig-
ure 13.29) of horizontal primary structures in ephemeral
stream deposits. There is, however, some confusion over
the conditions of flow responsible for their formation. This
