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Evidence For Diapirism In The Arckaringa Basin, South Australia.

Jones, Paul J.

Honours Degree, 1988

University of Adelaide

Abstract

Evidence for diapirism within the Arckaringa Basin, South Australia is indicated by seismic reflection, gravity and outcrop data. Diapiric activity appears to be contained within an arcuate trending depression, the Boorthanna Trough, along the basin?s eastern margin. The basin itself is a complex sedimentary depression composed of a central platform of shallow, gently undulating crystalline basement, surrounded by a series of peripheral depressions.

Extensional tectonics and differential loading are postulated to be the driving mechanisms for initial diapiric movement and subsequent growth. Source beds for the diapirs are thought to be Callanna Group sediments (or their equivalents) known to be the source for diapiric intrusions and breccia formations in the nearby Peake and Denison and Northern Flinders Ranges. Diapiric activity probably began in the Adelaidean and continued until Cambrian times. Reactivation of diapirs has taken place in many areas as a result of tectonic activity during Tertiary times. This has affected the overlying Arckaringa Basin sediments.

A salt source for the diapirs is proposed because the area has undergone several orogenies and mild metamorphisms which would have caused shale to dewater, lose its overpressure characteristics and compact. Salt dissolution and halokinetic relationships with surrounding sediments and adjoining diapirs noted on seismic, together with gravity data, strengthen the idea of salt sourced diapirism. Diapir size and distribution can be attributed to spacing between diapirs and size of salt withdrawal basin. Shape is determined by growth stage, overburden thickness, viscosity and basin floor shape. Salt dissolution and intensity of orogeny may also have modified size and shape.

It is envisaged that the Boorthanna Trough originated as the north-west arm of a rifted triple junction whose centre lay near the crystalline basement Mt. Painter Block. Palaeogeographic reconstruction from seismic, outlines the rift structure of the Trough and well data indicates sedimentation was similar to many clastic starved rift settings (the East African rift) of today, and also the Adelaide rift valley complex, active further south during Pre-Cambrian times. Granitic intrusions, dykes with feeder sills, or wide fault zones are shown to be unlikely alternatives to the features shown on seismic reflection data within the Trough.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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