UPPER CAMBRIAN TO MIDDLE ORDOVICIAN MAGNETOSTRATIGRAPHY FROM THE KULUMBE RIVER SECTION (NORTHWESTERN SIBERIA)

We present the magnetostratigraphy of an Upper Cambrian to Middle Ordo vician sedimentary section sampled along the Kulumbe river in northwes tern Siberia. Thermal demagnetization yields the determination of two magnetic components. A component of steep inclination is interpreted a s a remagnetization acquired during the emplacement of the Siberian tr aps at the Permian-Triassic boundary. Another component with a moderat e inclination, essentially carried by hematite and sometimes by magnet ite, likely represents a syndepositional or nearly syndepositional mag netization of the studied deposits. A sequence of 17 magnetic interval s is observed between the upper part of the Upper Cambrian and the Tre madoc, three of them being poorly defined by single samples. We also f ind that the Llanvirn magnetostratigraphic sequence contains only one reversed polarity interval. Together with magnetostratigraphic results previously obtained from the Moyero river section from Siberia, our d ata suggest the occurrence of a 25-30 Myr-long period during the Lower and the Middle Ordovician characterized by a very low magnetic revers al frequency (possibly containing a superchron), and dominated by reve rsed polarity. This geomagnetic feature would be compatible with a tim e constant of about 150 Myr in the reversal process through the whole Phanerozoic. (C) 1998 Elsevier Science B.V. All rights reserved.