The Kimberley Society


Geography


The Kimberley region extends from the dry red sand dunes of the Great Sandy Desert in the south through rugged sandstone escarpments of the sub humid Kimberley Plateau and Timor Sea in the north. It extends east to the Northern Territory border. Covering some 423 500 square kilometres it is nearly twice the size of the State of Victoria and three times the size of England. Numerous islands off the northern coast and the many gulfs, headlands and the irregularity of the coastline attest to the current historically high sea levels and the so-called "drowned" topography of the region.. There are more than 2500 mapped islands between Yampi Sound and the mouth of the King Edward River. In a straight line it is approximately 400km from Yampi Sound to the mouth of the King Edward River whereas it is nearly 1300 km around the actual coastline.



The region experiences a tropical monsoonal climate. The wet season extends from November to March and the dry season from April to October. Annual rainfall peaks at 1500mm per annum in the North West part of the plateau and drops off to 350mm per annum in the semi-arid south. Temperature ranges can be extreme with summer day time temperatures frequently exceeding 40C and winter night time temperatures sometimes going below 0C on the higher parts of the plateau and in the desert regions to the south.

Most of the northern portion of the Kimberley is characterised by savannah style vegetation with mature trees and grasslands. Rivers to the north are commonly lined with paperbarks and pandanus. The river patterns are commonly defined by joints or faults in the underlying sandstones and most flow north or west and commonly incise deep gorges within the sandstone. Where the rivers meet the ocean mangrove colonies are commonly developed. Throughout much of the northern Kimberley small patches of rain forest are preserved.

Reference: A synthesis of scientific knowledge to support conservation management in the Kimberley region of Western Australia.
Department of Environment and Conservation (DEC) February 2009.

(create link to document on DEC website)

Geology and Geomorphology

The rocks of the Kimberley region contain a geological record that spans the last 1900 million years of the Earth's history. The oldest rocks in the Kimberley form the Lennard Hills in the west Kimberley and the Bow River Hills and the Halls Creek ridges in the east Kimberley. These comprise metamorphosed sediments,volcanics and granites.


Geological Map of the Kimberley Region

The main part of the Kimberley, known as the Kimberley Plateau comprises of generally flat lying sedimentary rocks. These sandstones and quartzites were deposited about 1800 million years ago by major river systems that flowed from north to south across the whole region. These rocks also contain considerable volumes of concordant basalt lava flows that are a characteristic of the Mitchell Plateau. Subsequent to around 1790 million years ago the region has seen several periods of geological activity that has resulted in deposition of further sedimentary sequences, largely around the margins of the Kimberley Plateau, and there is evidence for periods of major glaciations.

The current landscape of the Kimberley has been evolving over a period of at least 250 million years. Periods of uplift resulted in peneplanation of the land surface and deeply incised rivers. A lengthy period of tropical conditions 70-50 million years ago resulted in the development of a lateritic cap, particularly over the volcanic rocks which are more susceptible to weathering. This is a characteristic feature of the Mitchell Plateau.

As sea levels rose from approximately 120m below current levels following the end of the last glacial maxima 18 000 years ago, the Kimberley coast line became drowned with the sea filling what were once river valleys. This phenomena gives the coastline its distinctive irregular outline.

Reference: Geology and Landforms of the Kimberley. I Tyler. Department of Conservation and Land Management. 2000

Marine Environments of the Kimberley
The Kimberley coast is characterised in places by extreme tidal movements of up to 11m during spring tides and less than 3m during some neap tides. In places this large scale water movement creates extensive volumes of turbid water and a large land-sea interface. The near shore and coastal environments support a diverse array of marine environments including coral reefs, seagrass meadows, mangrove forests and sponge gardens.

Terrestrial Environments of the Kimberley
In terms of its climate , landscapes and biodiversity, the Kimberley is part of northern Australia's tropical savanna biome. The Kimberley landscape supports a variety of vegetations including hummock, tussock and bunch grasslands, shrubland, tree steppe, woodlands, riverine forest, mangrove, rain forests and paperbark swamps.

Aboriginal Occupation of the Kimberley
A growing body of archaeological work indicates that aborigines arrived in Australia at least 55 000 years ago and it is possible that one of the first places occupied, given its proximity to Timor and New Guinea was the Kimberley area. Sea levels were probably about 80 metres below current levels however it is obvious from paleo-bathymetric maps that the early arrivals involved the traversing of 200-400km of open water. It is obvious from the extensive and prolific rock art (both Gwion Gwion and Wandjina styles) that occurs throughout much of the Kimberley that significant numbers of aborigines lived and travelled throughout the region for thousands of years.


Outline (blue) of the sahul (Australia-New Guinea) and Sundaland (Asia) landmasses at the last glacial maxima around 20 000 years ago.

 

Bathymetric map showing continental and approximate position of coastline at last glacial maxima.

Reference; Rock Art of the Kimberley. Proceedings of the Kimberley Society rock Art Seminar. Edited by Mike Donaldson and Kevin Kenneally. 2007
   


 
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