The Ngalia Basin, Northern Territory: stratigraphy and structure

The Ngalia Basin comprises Adelaidean and Palaeozoic, mainly arenaceous sediments, up to about 5 km thick, preserved in an intracratonic downwarp in Lower and Middle Proterozoic igneous and metamorphic basement rocks. The maximum known thicknesses of sediments are Adelaidean, 3200 m; Cambrian, 800 m; probable Ordovician, 300 m; and Devono Carboniferous 3100 m; but a complete sequence is not present in any one area. The basin sequence is largely concealed by Cainozoic deposits, except for a belt of folded sediments along parts of the northern margin and narrow quartzite ridges along the southern margin.Unconformities in the Ngalia Basin sequence record nine alternate periods of sedimentation and diastrophism between Adelaidean and late Palaeozoic time. The major unconformities which record these periods of diastrophism divide the succession into four early tectosomes of continental and marine sedimentation and two later tectosomes of continental sedimentation. Until Devonian time the basin was part of a much larger sedimentary or structural province. Subsequent major latitudinal uplifts to the north and south of the present basin margins formed an ancestral basin outline. These uplifts were the provenance for a thick clastic wedge, comprising the youngest sequence in the basin. A major orogeny in the Carboniferous folded and thrusted the sediments and thin Tertiary lacustrine and fluvial sediments were deposited on the eroded basin surface. Seismic data indicate that the basin is essentially a faulted easterly trending asymmetrical syncline in which the thickest sediments are preserved towards the northern margin, where north-to-south thrusting has controlled the structural framework of the basin. Major faults and thrusts outline the main structural divisions of the basin. The northern margin in the west is formed by two thrust nappes, which are partly separated from a gently basinward-inclined platform in the south by a wide fault trough. Farther east, sinusoidal horsts and grabens dominate the structure in the centre of the basin. In the east the thick sedimentary pile at the northern margin is truncated by a thrust. The large number of unconformities and the absence of demonstrated good source rocks in the sequence reduce the petroleum potential of the basin. However, a probably thick and more complete Palaeozoic sequence in the subsurface may have some potential. Uranium mineralisation confined to the Devono-Carboniferous continental sandstone is the most important potentially economic mineral deposit. Water supply is principally from bores, but quality varies considerably; Devono-Carboniferous arenaceous sediments have yielded potable water, and Cainozoic rudites have yielded large flows of stock water.