WEST KIMBERLEY PLACE REPORT - Department of Sustainability ...

coast in the country (DEWHA 2009c). Although the Proterozoic rocks of Cape
Leveque at the tip of Dampier Peninsula are roughly contemporaneous, they do not
form part of the drowned riverine system which characterises the coast from the
Helpman Islands north and east to Cambridge Gulf. Ria coasts are a major regionalscale
coastal landform type internationally (Bird 2000, p. 221). The north-west
Kimberley coast is the most extensive region of well expressed ria coast and also the
longest stretch of predominantly rocky coast in Australia (Short and Woodroffe
2009). Short and Woodroffe calculated in 2009 that the mainland Kimberley coast is
approximately 2,500 kilometres long and 80 per cent of this length is rocky. Inlets
(particularly estuarine) with muddy tidal flats, mangroves and backing salt pans are
the most important subordinate coastal landform type in the Kimberley region. Sandy
beaches are not uncommon, but most are very short (Short 2006, Sharples 2009;
Sharples et al. 2009). With the exception of localised sites such as the Cockatoo –
Koolan Island mines, there is little high-density disturbance or infrastructure on the
Kimberley ria coast. As a result long stretches of this coast exhibit coastal landforms
and processes in a mostly unmodified condition.
Bathymetric mapping has identified submarine features on the continental shelf
fringing the Kimberley coast which are evidently drowned hills, plateaux, river
valleys and gorges (for example, in the King Sound – Buccaneer archipelago region),
and hence are part of the drowned fluvial landscape which gives distinctive character
to the Kimberley coast (see for example Tyler et al. 1992). Taking a four-dimensional
perspective, the Kimberley ria coast is more than the modern shoreline. It includes
areas that have been significantly shaped by fluvial, terrestrial and marine (coastal)
geomorphic processes. These processes alternated and interacted during the
glacial/interglacial climatic phases of the Pleistocene; and over longer timescales, the
extended coast records processes affecting the ancient shores of the Proterozoic
Kimberley craton. The point of highest sea level at the Last Interglacial occurred
roughly 125,000 years ago. The point of lowest sea level at the last glacial maximum
(LGM) occurred around 20,000 years ago. Note these levels are assumed to be
indicative of high- and low-sea stands during earlier glacial and interglacial phases,
for which direct evidence is not as well preserved. However, taking the longer
timescale of the Phanerozoic eon, the 30 metre bathymetric line represents the extent
of the coast for much of the last 500 million years.
At the last interglacial high stand, global eustatic sea levels generally were less than
six metres above present sea level (APSL), although uncertainty about uplift, hydroisostatic
deformation and the erosion of evidence makes it difficult to pin down
precisely, and it varies regionally. The highest last interglacial stand recorded along
the Western Australian coast is at Lake MacLeod, which has palaeo-shorelines up to
ten metres APSL, but this and nearby sites at Cape Cuvier and Cape Range show
evidence of regional warping. There are few reliably dated north-western Australian
sites (Murray–Wallace and Belperio 1991; Kendrick et al. 1991). Because of this
uncertainty, the Kimberley shoreline at the Last Interglacial is considered to extend
inland to the six metre contour. At the LGM, the Kimberley ria coast was up to 130
metres below present sea level (BPSL), meaning the coast was a long way seaward of
its present position. Before the marine inundation following the LGM, people, flora
and fauna would have lived in an ancient riverine landscape that is now drowned
(Wilson 2009a; Sharples pers. comm. 2009; Maher and Copp 2009). This appears to
be an extraordinary sea level retreat, and to reflect processes of uplift and subsidence
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