Paleomagnetism of some Mesozoic intrusives and tuffs from eastern Australia

Abstract

Paleomagnetic studies of the Brisbane tuff and the Noosa Heads and Mount Dromedary intrusive complexes show that the stable components of natural remanent magnetization have rather uniform directions (irrespective of sign) with steep inclinations. In each case the time of formation of the rocks is known with precision, either from stratigraphic or from radioisotope evidence, to be Middle Triassic, Upper Jurassic, and late Lower Cretaceous, respectively. Field and laboratory studies suggest that the stable magnetization originated at the time of formation and was parallel to the earth's field. The observations therefore appear to provide a record of the earth's field at known times in the past. A reversal of magnetization in the Noosa Heads complex is related to a radioisotope age of 140 m.y. A detailed comparison of thermal and magnetic cleaning methods, in which rock samples from the Mount Dromedary intrusive complex were used, yields the result that the latter is to be preferred in this instance.