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Bioregions of the nt



Bioregions of the nt


Kerle, J. A.


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Technical Report LRD94100 Viewed at 07:02:32 on 18/02/2010 Page 39 of 275. MITCHELL GRASS DOWNS (MGD) (Barkly Tablelands) , , About one third of the Mitchell Grass Downs Bioregion is in the Northern Territory where it is more commonly called the Barkly Tablelands, This area of extensive grassland has been an important component of the pastoral industry in the N,T. since the 1880's. It lies within the monsoonal belt over northern Australia and the deep cracking clays typical of the region are treacherous during the wet. It also contains several large lake systems which, after a good wet season can cover hundreds of square kilometres and are extremely important waterfowl habitat. Location and Area The Mitchell Grass Downs occupy a band of country which runs from central Queensland, north-west to Elliott in the Northern Territory (312,788 km2). It crosses into the N.T. between 195 and 22' 30'S, follows around the edge of the Tanami region and meets the Sturt Plateau Bioregion on the western edge of Lake Woods at 134E (93,323 km2, 28.9%). It is bounded to the north by the Gulf Fall Bioregion. Climate A dry rnonsoonal climate merging into arid conditions at the south and south-west of the region and the sub humid tropics along the north. The rainfall is extremely seasonal with almost all rain falling from November to April (mostly in January and February). The remainder of the year is a long dry season which is best described as arid. May to September is practically rainless. Rainfall varies from an annual average of250mm in the south to 500mm in the north. . . At Brunette Downs in the centre of the region the mean is 412mm (median 351mm) for 89 years, at Newcastle Waters at the north-western end the mean is 532mm (median 480; 39 years) and in the dry south-east at Urandangie the 95 year mean is 294mm (median 27Imm). Temperatures are high throughout the year and relatively consistent across the region. In general, temperatures change slowly from season to season but conditions can become intensely hot in the lead up to the wet season, The mean temperatures in November and December are higher than those in January. Mean temperatures at Brunette Downs are 39C (max) - 24"C (min) in December and 27C (max) - 10C (min) in July. Geology, Topography and Soils The region is mostly gently undulating and featureless ("of low relief'), lying between about 200 to 300m (600 to 1000 ft) above sea level. The major underlying geological units are the Dunmara/Barkly Basin in the north-west and the Georgina Basin in the south-east. This has been a very stable area, geologically, with no upheavals exposing any of the metamorphosed Proterozoic rocks as has happened to the west and east, From the end of the Proterozoic period the area was covered. on three occasions by vast and expanding seas but in the Tertiary (65-1.6 million years ago) the history is one of erosion and the accumulation of terrestrial rather than marine sediments. During the Tertiary, much of the area was covered by a vast swamp and this long period of erosion reduced the region to its present gently undulating topography. , Most of the region is classified at "black soil" (heavy pedocal) plains resulting from swamp and other alluvial deposition during the Tertiary. Deposition still continues and the bluebush swamps (Sylvester Land System) and braided streams of the Georgina Land system are evidence of this, Smaller areas oflimestone with calcareous desert soils and lateritic plains with red earths Bioregions of the NT DRAFT Version 3 36 Printed:March 25, 1996

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