Cainozoic sedimentary basins in the eastern Arunta Block, Alice Springs region, central Australia

Tertiary sedimentary rocks, in places over 200 m thick, occupy a series of elongate basins within and partly surrounding the Arunta crystalline basement in the Alice Springs region, central Australia. The best-known sedimentary succession is that of the Hale Basin, where four distinctive members constitute the poorly indurated Early Tertiary Hale Formation. The oldest, the Ambalindum Sandstone Member, rests on deeply weathered basement.  It is overlain by the Delaney Mudstone Member, olive-green mudstone and siltstone, deposited probably under quiet-water reducing conditions in a series of large lakes. At this time there was low input of terrigenous detritus from the nearby MacDonnell, Strangways and Harts Ranges. In places, these fine-grained sediments grade upwards and laterally into a lignite and oil shale unit, the Ulgnamba Lignite Member, probably reflecting local development of shoals and peripheral swamps. A mid to Late Eocene age is likely for this member. The Ulgnamba Lignite Member or, where it is absent, the Delaney Mudstone Member, passes abruptly upwards into the poorly sorted, coarse-grained Tug Sandstone Member. This unit is likely to have been deposited rapidly on piedmont slopes flanking the margins of the uplands.  The abrupt change in sedimentary characteristics suggests either uplift in the nearby ranges or a change in climate, or both. In the Hale Basin and other Cainozoic basins, the uppermost part of the Cainozoic succession, the Waite Formation, generally consists of green and red, silty sandstone, containing ferruginous pisoliths and a few massive chalcedonic calcrete beds. Erosion, possibly in the Oligocene, partly removed the older Cainozoic sediments as well as ferricrete formed at the basin margins, and incorporated some of the detritus in the Waite Formation. Other Tertiary basins overlying the eastern part of the Arunta Block show a broad lithostratigraphic similarity to the Hale Basin.  Palynological studies of carbonaceous sedimentary rocks and lignite in the Hale, Bundey and Ayers Rock Basins, combined with palaeomagnetic and stratigraphic data, suggest that sedimentation in these basins took place in two main phases-the first, largely in the Paleogene, may have begun in the Late Cretaceous; the other, in the Neogene, began in the Late Miocene or Early Pliocene after one or more breaks in deposition, which are likely to have occurred in the Oligocene.  Although there are large gaps in the sedimentary succession, there are sufficient data to broadly relate the depositional sequence to the pattern of uplift and sagging in the Alice Springs region and, in a general way, equate these movements with tectonism within the Australian Plate.  Palaeomagnetic dating, in some cases supported by palynology, has identified two main periods of intense chemical weathering in these central Australian basins. The younger, near the top of the Hale Formation, has a Late Eocene magnetic age.  The older is represented by relict weathered profiles developed in the Arunta basement, and may be earliest Tertiary or older.  In addition to these two main weathering periods, various oxidised, mottled or silicified rocks in several basins indicate other, lesser periods of interrupted sedimentation and weathering.

Tags: Earth Sciences, GA Publication, Journal, NT, Published_External
Organization: Geoscience Australia Data