Sedimentary structures and palaeocurrent in the Mount Eclipse Sandstone northwestern Ngalia Basin

Abstract

The Ngalia Basin is a small east-west oriented intracratonic basin centred some 200km northwest of Alice Springs, central Australia. The basin is approximately 420km long and 70km wide lying between 129 and 133 longitude and 2215 1S and 23S latitude (Fig. 1). It encompasses an area of about 16,000km2. The Ngalia Basin was first named by Tindale (1933) but apart from a few comments on the basin stratigraphy there was no systematic description of the sedimentary succession until Cook (1963) mapped the Yuendumu Aboriginal Reserve. Broader regional reconnaissance surveys and petrographic descriptions of the sedimentary succession were published in Cook and Scott (1967). The 1:250,000 scale geological mapping of the Ngalia Basin, together with gravity and seismic surveys and a programme of shallow stratigraphic drilling, was carried out by the Bureau of Mineral Resources in 1967 and 1968. This work led to a number of published and unpublished works on the Basin, e.g. Rivereau, 1965; Wells tl .91., 1968; Evans and Glikson, 1969; Tucker, 1969; Evans and Nicholas, 1970; Cooper ~ tl-, 1971; Evans, 1972; Wells, 1972; Wells tl .91., 1972; Wells, 1974; Wells, 1976, and Preiss tl .91., 1978. Deposition in the Ngalia Basin commenced in about the mid Adelaidean with a thick sequence of shallow marine quartzites. Discontinuous sedimentation continued in the Late Adelaidean with the deposition of diamictite beds (Naburula and Mount Doreen Formations) attributed to Late Adelaidean glacial episodes (Preiss et al., 1978). This was followed by a series of thin and discontinuous Early Cambrian and ?Ordovician units and capped by the thick Late Devonian to Mid Carboniferous Mount Eclipse Sandstone. From seismic information the thickest sedimentary sequence in the basin is approximately 4300m. Numerous unconformities within the sedimentary succession indicate that the basin has been subjected to several periods of minor epeirogenic movement. An important post-Ordovician period of deformation was resposible for major east-west tensional faulting of the pre-Mount Eclipse Sandstone sequence (Wells et al., 1968). This faulting produced a series of east-west horst and graben structures in the central and eastern parts of the basin, cut in places by northeast trending faults (Wells et al., 1981). Minor folding accompanied this faulting.