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The Precambrian–Cambrian boundary problem: magnetostratigraphy of the Amadeus Basin, Central Australia

Summary A magnetostratigraphic study of Upper Proterozoic to early Cambrian sediments at 4 localities in Central Australia reveals the presence of a magnetic polarity pattern composed of 2 long quiet intervals, first of reversed then of normal polarity, which is then succeeded by a long mixed interval. Lithologic comparisons suggest that this pattern is a reflection of the behaviour of the geomagnetic field at that time, and probably is not a result of varying sedimentation rates. Two sampled sections in the Arumbera Sandstone separated by 80 km both contain the long normal interval followed by the interval of mixed polarity, the detailed comparison of which demonstrates that the Arumbera-Box Hole boundary is unconformable. Elements from the late Precambrian Mt Skinner fauna in the Arumbera Sandstone indicate that the long normal interval is entirely pre-Cambrian, and that the base of the Cambrian occurs within the interval of mixed polarity. A comparison with an additional site sampled across a fossil occurrence in the Central Mount Stuart Formation at Central Mount Stuart indicates that at least 1 of the Upper Proterozoic fossils, Hallidaya brueri, has a considerable vertical distribution in time. If used properly, the magnetic polarity pattern discussed herein offers the possibility of precise intercontinental correlations of stratigraphic sequences near the base of the Cambrian. It is suggested that a stratotype sequence for the base of the Cambrian be chosen in such a manner that the boundary position corresponds to a geomagnetic reversal.