Late Cambrian palaeomagnetic data from the Cupala Creek Formation, western New South Wales, Australia

Abstract

The Tasmanides of south-eastern Australia comprises the Delamerian Orogen, considered to be stable relative to the craton of Gondwana since the mid Cambrian, despite the presence of the Grasmere Knee Zone, a change of structural trends in the Broken Hill area. A palaeomagnetic study has been carried out on the Late Cambrian red sandstones of the Cupala Creek Formation, a post-Delamerian formation gently folded in the latest Ordovician–Early Silurian and/or Early Devonian. Anisotropy of magnetic susceptibility (AMS) measurements show that internal strain is negligible and low Königsberger ratios indicate that lightning effects can be discarded. The magnetisation observed is complex with four components, which are sometimes difficult to isolate clearly. The orientation of the first component corresponds to the present-day field and appears to be associated with weathering effects. The second component (~185°−350°C) is of reverse polarity relative to the third (350°−575°C). These two components are interpreted to represent a chemical remagnetisation, which lasted long enough to record a reversal as the reversal test is positive and classified C. The overprint must be Early Palaeozoic in age. The last component is carried by haematite and/or maghaemite and is believed to be primary, which is supported by the positive ‘pseudo-unconformity’ test with the underlying Early–Middle Cambrian Teltawongee beds, and by the good correspondence of the pole [PLong. 351.3°/PLat.+33.9° (dp = 3.6°; dm = 6.0°); African coordinates] with other poles of the same age for Gondwana. Despite the presence of the Grasmere Knee Zone, these results imply that this area of the Delamerian Orogen did not record any movement or rotation since the Late Cambrian and can be considered as part of the craton of Gondwana.