Seismic reflection investigations in the Willochra Basin, South Australia

Abstract

Cainozoic sediments of the Willochra Basin overlie the Delamerian Fold Thrust Belt, a north-south arcuate tectonogene formed during a major Cambro-Ordovician Orogeny. Seismic experiments were undertaken in an attempt to provide information about the large scale tectonic features of the Fold Belt below the Willochra Basin, to help resolve some of the uncertainty regarding the mechanism of formation and the internal structure of the orogen. Two small scale seismic reflection experiments successfully identified reflecting horizons in the younger Cainozoic basin sediments overlying the Neoproterozoic rocks. The acoustic impedance contrasts of these generally unconsolidated sediments resulted in significant transmission loss and absorption of the seismic signals, restricting the depth of penetration into the Neoproterozoic strata; however, useful reflections were recorded down to two-way travel times of 1/2 second. A small 1 km wide basin structure resting unconformably in the Neoproterozoic rocks was successfully mapped. The Cainozoic and Neoproterozoic reflectors were identified by correlation with geological units in a nearby stratigraphic borehole and local outcrops. The information obtained confirms that it should be possible to conduct a deep seismic profile experiment across the Flinders Ranges, perhaps quelling the decades of debate about the internal structure of the Delamerian Fold Belt.