Palaeomagnetic Results from Palaeozoic Sediments of Northern Australia

Abstract

Summary A palaeomagnetic study was made of 52 samples from an Upper Devonian formation and 97 samples from three Cambrian formations. In most cases, the application of demagnetization procedures failed to reveal other than a recent magnetization, presumably acquired by chemical means during weathering processes. Seventeen samples, four of which had been collected a decade earlier, appeared to yield the direction of original magnetization of the Middle Cambrian Hudson Formation. Assigning unit weight to sample mean results, the palaeomagnetic pole position for the period of deposition is calculated to be 19E, 18 N with A,, = 13. The data for the Jinduckin Formation have been re-assessed using sample results, and these give a slightly revised pole position for the Lower Ordovician at 25 E, 13 S with Ags = 11. The first results from a recent palaeomagnetic survey of the Palaeozoic of northern Australia indicated the widespread occurrence of a sometimes dominant secondary magnetization (Luck 1970; McElhinny & Luck 1970). The importance of this effect was revealed during measurements on 149 samples from four other sedimentary formations collected in the same general region (Table 1). The natural remanent magnetization (NRM) of most of these samples was diiected along the Earth's present field. A similarly directed secondary component, presumably representing a viscous remanent magnetization (VRM), had been readily removed from most samples of the Jinduckin Formation by the application of partial demagnetization techniques (Luck 1970). In contrast, the present paper reports that recently acquired magnetization could not be cleaned from the Blatchford Formation, the Hart Spring Sandstone, the Cockatoo Formation or from most samples of the Hudson Formation. This hard, secondary component was taken to represent a chemical remanent magnetization (CRM) imposed by weathering processes. In favourable circumstances, the effect might be neutralized by a chemical method such as devised by Collinson (1965) for separating the red pigment from the specularite of red beds. This technique may be regarded as a means of partial demagnetization (Collinson 1967) if the secondary magnetic component resides in the pigment. Disc-shaped (3.5 cm x 1.0 cm) and cylindrical (2.8 cm x 2-8 cm) specimens were cut from most samples. The discs were investigated using an astatic magnetometer