Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 61

150 E

Katherine Gorge

10 S

Gunlom

Cape York
Peninsula

Kimberley
Region

Darwin

Cairns

Wangi Falls

Fitzroy R.

Herbert R.
Burdekin R.

Ord R.

Broken R.

Indian Ocean

Lennard R.
Margaret R.

Brisbane

Per th

Pacific Ocean

400 km
N

Tasmania

Figure 3.4. Location map of palaeoflood sites in northern Australia.

into a sedimentary stack where multiple, individual flood events each leave a
sedimentary layer. The law of superimposition applies in such circumstances so
theuppermost layer is the youngest and each successively lower layer becomes
progressively older. When dating the layers, the accuracy of the technique used
can be partly tested by the stratigraphic integrity of the dates i.e. each layer
should return a progressively younger age with upwards stratigraphic order.
Layers are often separated from each other by a thin layer of leaves and other
organic debris and/or a drape of clay. Gillieson et al .(1991)found four such flood
stacked slackwater deposits in alcoves of the walls of the limestone Windjana
Gorge (Lennard River) in the west Kimberley region of northwest Western Aus-
tralia (Fig. 3.4). The age of the layers was determined by radiocarbon dating
freshwater mussel shells at the base of layer 2 and thermoluminescence dat-
ing of quartz sand within other layers. The four flood events occurred between
AD 1962 and 2800 years BP.
While many slackwater deposits are composed of stacked relatively thin
(<1 m) layers, other deposits can be made up of one very thick layer possibly
representing a single very large flood event. One such sequence occurs in the
Ord River Gorge of northwest Western Australia. The gorge is about 20--50 m
deep and is carved into Proterozoic sandstones. It does not maintain a uniform
width along its length; in places the width decreases from about 300 to 200 m.