Return to Black Mountain: palaeomagnetic reassessment of the Chatsworth and Ninmaroo formations, western Queensland, Australia

Abstract

SUMMARY Palaeomagnetic results from late Middle Cambrian‐Early Ordovician carbonate sequences sampled at Black Mountain (Mt Unbunmaroo), Mt Datson and near Chatsworth Station (southeastern Georgina Basin) are presented. A palaeomagnetic reassessment of these carbonates was designed in an effort to constrain regional magnetization ages as results from an earlier study, conducted at Mt Unbunmaroo, play a pivotal role in a proposed Cambrian inertial interchange true polar wander (IITPW) event. Remanent magnetizations within these carbonates were found to be variably developed with most specimens displaying two of the five isolated components. Component PF, for which goethite is the identified remanence carrier, is thought to reflect a chemical remanent magnetization of recent origin. Component TR, held by haematite, has a palaeomagnetic pole consistent with the Tertiary segment of Australia’s apparent polar wander path (APWP) and most probably was acquired as a consequence of prolonged weathering during this period. The A component has a palaeomagnetic pole at 54.7 ◦ S, 262.3 ◦ E( dp = 2.3 ◦ , dm = 4.5 ◦ ) after unfolding. This direction, constrained by positive fold and reversal test statistics, is consistent with Australia’s Early Devonian APWP, perhaps reflecting a remagnetization event associated with the intracratonic Alice Springs Orogeny. A Late Ordovician‐Early Silurian remanence, component B, is also described; with 100 per cent unfolding the associated palaeopole lies at 8.0 ◦ S, 216.8 ◦ E( dp = 2.6 ◦ , dm = 5.1 ◦ ). A third Palaeozoic, and presumed primary or early diagenetic, component, C, also passes applied fold and reversal tests and has a palaeomagnetic pole at 48.6 ◦ N, 186.0 ◦ E( dp= 2.1 ◦ S, dm = 4.0 ◦ ). This palaeopole is dissimilar from younger magnetizations, is consistent with Cambrian poles from other parts of cratonic Australia and falls within a cluster of Middle‐Late Cambrian (515‐500 Ma) palaeopoles from other Gondwanan continents. The age attributed to the palaeopole associated with the C component, ∼510 Ma, provides a tight constraint on the younger boundary of the proposed Cambrian IITPW event and its agreement with other Gondwanan palaeopoles is incompatible with the IITPW hypothesis. Components A, B and C are analogous to palaeomagnetic results reported in the earlier investigation of this region, and a comparison of results from the two studies, coupled with rigorous statistical analyses of the new findings, is presented.