MAGNETOSTRATIGRAPHY OF THE MOYERO RIVER SECTION (NORTH-WESTERN SIBERIA) - CONSTRAINTS ON GEOMAGNETIC REVERSAL FREQUENCY DURING THE EARLY PALEOZOIC

We present the magnetostratigraphy of the early Palaeozoic Moyero rive r section in north-western Siberia. The section investigated, which co nsists mainly of reddish limestones, marls and greyish limestones, ext ends from the upper Cambrian to the lower Silurian. Palaeomagnetic ana lyses reveal two magnetic components. One is interpreted as a remagnet ization acquired during Siberian trap emplacement; the other is believ ed to be early diagenetic, consistent with the established lower Palae ozoic apparent polar wander path for Siberia. Reversed polarity domina tes from the upper Cambrian to the middle Ordovician, while normal pol arity dominates from the upper Ordovician to the lower Silurian. Only a few reversals, which are poorly defined, are observed through the se ction, in particular during the Llandeilo. These data are in agreement with previous magnetostratigraphic data from the Baltic platform and Siberia and indicate that magnetic reversal frequency was low during t he Ordovician. Because of the time-delayed process of acquisition of t he magnetization carried by haematite, we cannot clearly ascertain if a reversed superchron occurred during the lower to middle Ordovician. However, our results suggest that a 15 Myr-long reversed interval exis ted during the Arenig.