WEST KIMBERLEY PLACE REPORT - Department of Sustainability ...

as well. Taking a longer perspective on the coast, this seaward boundary can be
refined to reflect dominant marine and terrestrial processes throughout Phanerozoic
time, due to the extraordinary stability of the Kimberley craton over the last half
billion years.
A report prepared for the Australian Heritage Council in 2009 by the Western
Australian Marine Science Institution confirms that the western margin of the
Proterozoic Kimberley craton 'lies 50 kilometres or more seaward of the
contemporary coast where the seabed is at least 30 m deep. Thus, the western margin
of the Kimberley Plateau is today inundated by the sea and the nearshore seabed is an
inundated terrestrial land surface.' (Wilson 2009a; Wilson 2009b) The seabed
topography of the submerged margin of the Kimberley craton during the Phanerozoic
eon is largely the result of terrestrial landform development rather than marine
processes to at least the 30 metre bathymetric line (Wilson 2009b). The modern inner
continental shelf of the Kimberley coast is characterised by these 'seabed landscapes'.
For example, recent imagery of the sea floor near Montgomery Reef clearly shows a
broad valley and the beds of ancient streams (Wilson 2009a) and Montgomery 'Reef'
itself is interpreted as a drowned flat-topped mesa of Proterozoic rocks encrusted with
modern coral communities (Wilson 2009b).
However, the distance inland from the shoreline that is affected by coastal processes
is subject to a number of variables apart from sea level, including climate and
topography and there is no simple metric to determine the inland extension of a ria
coast. A ria coast is the result of interaction between fluvial (river) and marine
(coastal) processes. Over long periods, the interaction between persistent fluvial
processes and persistent coastal processes produces its distinctive topography. Thus, a
ria coastal zone is best defined as that zone in which marine processes (waves and
currents) and fluvial processes (runoff and river discharges etc) have both had
substantial persistent impacts that have shaped the landscape we see today. In practice
since the Proterozoic, that means the zone from about 30 metres BPSL (LGM
shoreline) and about four to six metres above present sea level (last interglacial
shoreline), with a further one kilometre distance inland allowing incorporation of a
portion of the fluvial landscape above the limit of known marine influence.
The Kimberley coast is a tide dominated macrotidal coast which, partly due to the
extensive shallow continental shelf, has the highest tidal range in Australia (more than
10 metres in parts; Short and Woodroffe 2009). This, together with the particularly
intricate, hard-rock shore plan forms, produces rocky coast tidal current processes on
a magnitude not found elsewhere in Australia, including 'horizontal waterfalls' and
common coastal whirlpools. In comparison, on a soft-sediment coast the shore would
adjust its plan form to damp or absorb the tidal current energy; on a hard rock coast
like the Kimberley, the shoreline resists changing according to tidal currents, and so
whirlpools and horizontal waterfalls occur at narrow choke points (Chris Sharples
pers. comm. 2009). Thus, the Kimberley region's rocky coast exhibits a distinctive
and very well-expressed, tide dominated, rocky coastal, geomorphic process regime.
Comparably extensive rocky coasts:
All other stretches of Australian coast of comparable length to the Kimberley have
much higher proportions of soft sediment coast (sandy or muddy) than the Kimberley.
Other significantly rocky coastal lengths include the Albany region of southern
159