Geophysical-Geologic Study of Northern Amadeus Trough, Australia

Recent seismic and gravity data from the northern part of the Amadeus Paleozoic sedimentary basin provide evidence of an origin of regional fold structures by detachment and flowage deep within the sedimentary succession. Synthesis of this information with regional geologic and geophysical surveys indicates that crystalline basement is not significantly disturbed beneath structures within the basin; however, surface structures bordering the basin involve both basement and sedimentary rocks. The principal detachment and flowage zone and the deepest continuous seismic horizon are within an upper Precambrian evaporite series; a secondary zone of plastic flowage probably is associated with a Cambrian salt series. Seismic profiles indicate that some anticlines have lenticular "salt pillows" in their cores, whereas others show abrupt wedgelike thickening of evaporite-bearing units form flanking synclines to anticlinal crests. Other profiles show concealed major thrust faults and "blind" imbricate faults associated with buried uplifts. Structures can be explained by salt flowage and gravitational gliding without major dislocation of the basement. Folds of tectonic and gravity-gliding origin may have been localized and subsequently reinforced by salt flowage. Regional gravity and aeromagnetic data indicate a regular basement which slopes northward into the deep trough underlying the Missionary Plain, where seismic and surface geologic data show the greatest thickness of sedimentary rock to be preserved. Large surface folds and major faults produce no appreciable aeromagnetic anomalies; however, they do cause sharp local Bouguer gravity anomalies, mainly near-surface effects produced by subcropping or outcropping strata of different densities.