Magnetostratigraphy in the Sydney Basin, Southeastern Australia

Abstract

Late Permian and Early Triassic sediments of the Narrabeen Group have been sampled in the coastal section of the Sydney Basin between Wybung Head (the top of the Newcastle Coal Measures to the north of Sydney) and Coal Cliff (above the Bulli Seam south of Sydney). An ancient magnetisation (presumed original) has been isolated only from the red claystone component of the entire section. The magnetisation of the green and buff coloured medium/coarse grained sandstones belonging to the Munmorah Conglomerate and the Tuggerah Formation has been reset during the Late Mesozoic period of magnetic overprinting. This event is primarily responsible for frustrating an attempt to describe properly the magnetostratigraphic record for the Palaeozoic/Mesozoic boundary from deposits in the eastern part of the Sydney Basin. The primary magnetisation of the red beds has survived magnetic reprinting but evidence of over-printing is present during thermal cleaning to 600°C. The detailed magnetostratigraphy of the red beds was revealed after thermal treatment at 650°C and 660°C. The high temperature directions yield a palaeomagnetic south pole for the Patonga Claystone at latitude 30.4°S, longitude 146.9°E, α95=7.8° and the difference vectors calculated from the low temperature directions yield an overprint south pole position at latitude 57.3°S, longitude 143.4°E, α95=3.6°.In the light of recent palaeomagnetic investigations in southeastern Australia and the recognition of ubiquitous Late Mesozoic magnetic overprinting, a reassessment of earlier results based upon samples collected at least in the eastern regions of the Sydney Basin may be required.