Palaeomagnetism, palaeogeography and the multicomponent analysis of middle and upper Cambrian rocks of the Barrandian in the Bohemian Massif

Abstract

With a view to the importance of the Barrandian in the Bohemian Massif for global palaeogeographic reconstructions, considerable attention has been devoted to palaeomagnetic stability tests and multicomponent analysis of rocks of the pre-Variscan formations, using substantially improved laboratory techniques. This study is devoted to samples of Upper Cambrian andesite and Middle Cambrian sediments. Alternating and thermal fields were used for magnetic cleaning. A thermal demagnetizer fitted with a newly developed inductor of high sensitivity, the ROCOMA (rotating coil magnetometer), with a sensitivity of 0.1 to 0.2 nT, was used. The whole newly built and fully tested MAVACS system (magnetic vacuum control system) guaranteed a high degree of compensation of the magnetic field in a volume of 5 1, to a value not exceeding 2 nT. As numerous laboratory experiments have shown, the high magnetic vacuum was necessary to derive the palaeomagnetic directions of rocks of the older formations affected significantly by the Variscan orogenic phase. The position of the Upper Cambrian pole (28.6°S, 131.2°E) should be considered tentative because it was derived using samples from only three andesite outcrops. Two other andesite outcrops preserved Carboniferous or Permo-Carboniferous palaeomagnetic directions. The position of the Middle Cambrian pole (11.3°S, 154.0°E) was determined using samples of sediments (mainly greywackes and shales) from ten outcrops, some of which represent extensive rock cross-sections. By combining multicomponent analysis with fold tests, the mean direction of the pre-Variscan magnetization could be derived. The very low values of the moduli of the natural remanent magnetization of the Middle Cambrian sediments, and the considerable amount of magnetically soft components, required special care to be taken in the laboratory procedures. The palaeozoogeographic zones of the Barrandian are related to palaeogeographic latitudes of about 40°S, derived from palaeomagnetic data. Cold water is characteristic for the Bohemian fauna, but arctic conditions can be ruled out due to the amount of primary red sediments, mostly of continental origin.