The palaeomagnetism of the Tumblagooda Sandstone, Western Australia: Gondwana palaeozoic apparent polar wandering

A magnetic polarity reversal near the top of the Tumblagooda Sandstone, in the Carnarvon Basin, Western Australia, has been intensively investigated in five coastal sections and four inland sections, all located near the top of the formation. A further four inland sections towards the base of the formation were also sampled. Of the inland sections, all but two are suspected to have been largely remagnetized in similar fashion to Late Palaeozoic and Mesozoic sediments to the south. At the remaining seven sections a zone of normal magnetization overlies a zone of mixed (coexisting normal and reversed) magnetization, and the boundary between them coincides with a unique marker horizon of pebbly sandstone. The intimate relationship of the (virtual) reversal and the pebble marker horizon lends strong support to the suggestion that erosional stripping accompanied the formation of the pebble marker. The pebble marker is best interpreted as a lag deposit. The stratigraphic coincidence does not necessarily signify the concurrence of a reversal with the formation of the marker horizon, but rather that the relationship represents a variation on the unconformity test. The record of the actual geomagnetic reversal has been physically removed from most sections. These results show conclusively that the age of the magnetization of the Tumblagooda Sandstone is very close to the age of deposition, and can therefore provide an important pole for the definition of the Gondwana apparent polar wander path. However, the suggested Silurian age must be questioned on the basis of the palaeomagnetism which best accords with an Ordovician age. The lower parts of the sequence may be as old as the Cambrian. While the age of the palaeomagnetic pole from the Tumblagooda Sandstone is therefore not well constrained, its position is well defined and in combination with other Gondwana poles indicates a period of rapid polar motion during the Ordovician/Silurian. A collision with Laurasia at the end of the Silurian and beginning of the Devonian may explain the timing of the Caledonian/Acadian Orogenies.