Thermal events in the Sydney-Bowen Basin as defined by palaeomagnetism

M.A. Lackie, School of Earth Sciences, Macquarie University, Sydney NSW 2109

P.W. Schmidt, CSIRO, Division of Exploration and Mining, P.O. Box 136. North Ryde NSW 2113

Summary

Palaeomagnetic studies of sedimentary and igneous rocks in the Sydney Basin identify pronounced magnetic overprinting. Secondary magnetisations are observed in sediments of the Narrabeen Group and in Permian and Early-Mid Jurassic igneous rocks, such as the Gerringong Volcanics, the Milton Monzonite and the breccia diatremes. The magnetisations are entirely of normal polarity, suggesting they were acquired during the Cretaceous Normal Superchron (118 Ma - 83 Ma). The intensity of overprinting decreases away from the coast and is consistent with a thermal event, uplift, erosion and cooling of the basin, probably related to events leading up to initial rifting in the Tasman Sea. Rocks now at the surface have been heated to about 200_ C. Fluid inclusion data and K/Ar dating of illites are also consistent with a thermal event having occurred along the sea-board at about 90 Ma. Deflection of the Cretaceous overprint directions by the Lapstone Monocline indicate that this structure post-dates the mid-Cretaceous.

Palaeomagnetic studies of coal measures in the Bowen Basin identify a consistent NRM overprint direction throughout the basin. Although the directions are similar throughout the basin, the inclination is steeper in the south (Moura region) than in the north (Goonyella Mine) of the basin. The overprint directions in the south of the basin are similar to those observed in the Sydney Basin, and a similar age is inferred (~90 Ma), whereas the direction in the northern part of the basin indicates a younger age of overprinting. Remanence directions found in Cretaceous intrusions and associated hornfels within the Bowen Basin show both reversed and normal polarities, in accordance with the ages of the intrusions (140 Ma -120 Ma), being slightly older than the Cretaceous Normal Superchron. These directions are similar to those observed in the south of the basin but differ from those observed in the north, although the intrusions outcrop in the north of the basin.