MAGNETIC REVERSAL FREQUENCY AND APPARENT POLAR WANDER OF THE SIBERIAN PLATFORM IN THE EARLIEST PALAEOZOIC, INFERRED FROM THE KHORBUSUONKA RIVER SECTION (NORTHEASTERN SIBERIA)

Abstract

Marly limestones that crop out along the Khorbusuonka river (northeastern Siberian platform) define a well-dated Lower and Middle Cambrian sequence which was sampled for magnetostratigraphy. In the upper part of the section, from the uppermost Lower Cambrian (Toyonian stage) to the Middle Cambrian (Amgan and Mayan stages), thermal demagnetization isolates two magnetic components carried either by magnetite, haematite or both. The first is parallel to the recent geomagnetic field at the site and is removed in the low to middle temperature range. The second component is isolated at higher temperatures and has dual magnetic polarities. This component yields a positive (but partial since we consider only two sites) fold test with other Middle Cambrian data previously obtained from the distant Kulumbe river section, and was probably acquired during sedimentation or at an early stage of diagenesis. It records a very high magnetic reversal frequency, of at least six to eight reversals per Myr, during the entire Middle Cambrian (assuming a ~10 Myr duration for this epoch). A much more complex palaeomagnetic behaviour is observed within the upper Tommotian-lower Atdabanian (Lower Cambrian) part of the Khorbusuonka section. Demagnetization diagrams show large overlaps between magnetic components, which prevents the isolation of a clear magnetic component at high temperatures. Analysis of these overlaps is, however, consistent with the existence of an ill-determined primary magnetic direction for the Lower Cambrian, which is very different from that isolated in the Middle Cambrian. Although still in need of further confirmation, the possibility that a large drift in Siberian apparent polar wander occurred over a rather short time during the Lower Cambrian appears to be supported by our data.