Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 62

Figure 3.5. Major slackwater sediment terrace (arrowed) in Ord River Gorge,
Western Australia. Note the trees in the foreground in comparison to the height
of the terrace.
Sedimentary terraces occur immediately upstream of these gorge constrictions
in at least five separate locations. The terraces stand up 12 m high above the nor-
mal dry season river level and extend along stream for several hundred metres
(Figs. 3.5 and 3.6). They are flat topped and usually about 30--40 m wide and
abut the gorge walls. They can be likened in morphology to a lateral bar. They
are composed of well-sorted, fine-grained sands and silts and show no distinct
variation in colour, texture or degree of weathering down profile. Indeed for
themost part, the sediments have a relatively fresh appearance as they contain
well-preserved micas, which are normally easily weathered and removed from
sedimentary units in tropical Australia after only a few thousand years. Sedimen-
tary structures are also preserved within these units. These 'drape-like' structures
appear to be bedding planes developed from the deposition of suspended sed-
iments in relatively still water for they have high dip angles and drape over
irregularities along the contact with the underlying strata. Optically stimulated
luminescence ages from two of these terraces, about 2 km apart, suggest that
thesediments were deposited between AD 1300 and 1800. It is uncertain at this
stage whether the terraces were deposited during a single flood event or a series
of closely spaced events over this period. The height of the terraces, and hence