Middle Cambrian high magnetic reversal frequency (Kulumbe River section, northwestern Siberia) and reversal behaviour during the Early Palaeozoic

We present a magnetostratigraphic study of Middle Cambrian sediments along the Kulumbe River (northwestern Siberia). The deposits are similar to 850 m thick and lie within the Mayan stage as constrained by trilobite fossils. Palaeomagnetic analyses reveal two magnetic components. The first component , parallel to the direction of the present day field at the site, is isolat ed in the low to middle temperature range. A high temperature component (HT C) is then isolated up to 580 or 680 degreesC, carried either by magnetite and/or by hematite. The HTC defines a sequence of 28 magnetic polarity inte rvals, some of them being defined by only one sample. Considering that the studied section has a duration of similar to5 Myr, we propose that the magn etic reversal frequency was high (similar to4-6 reversals per Myr) during p art of the Middle Cambrian, among the highest values known within the Phane rozoic. The reversal frequency may have been roughly similar during the Low er Cambrian. We further suggest a drastic decrease of the magnetic reversal frequency between the Lower-Middle Cambrian and the end of the Tremadoc (O rdovician) when a superchron probably occurred. However, this behaviour is still challenged by other scenarios which depend on the chosen Early Palaeo zoic time scale and on the reliability of some magnetostratigraphic data. ( C) 2001 Elsevier Science B.V. All rights reserved.