DEFINING THE LOWER-MIDDLE PLEISTOCENE BOUNDARY AT THE MATUYAMA-BRUNHES POLARITY TRANSITION: AN AUSTRALASIAN PERSPECTIVE

The last major reversal of the Earth's magnetic field, the Matuyama-Brunhes boundary (MBB), has long been used as a chronostratigraphic marker in Pleistocene studies. The MBB is dated at about 0.78 Ma, and occurred during Marine Oxygen Isotope 19. It has been widely identified in both marine and continental sequences and is also a key time marker for the chronology of human evolution and migration.

In New Zealand, the MBB is precisely located in shallow marine sediments of Wanganui Basin, where it corresponds with the base of the New Zealand Putikian Substage. A combination of marine biostratigraphy, sequence stratigraphy, magnetostratigraphy and tephrostratigraphy permit correlation from Wanganui Basin to other on-land sections and deep-sea cores. Loess-paleosol sequences in both the North and South Islands of New Zealand all post-date the MBB, but some glacial deposits are known to be older.

In Australia, the MBB is identified in many continental sequences, particularly saline lake basins. However, chemical weathering has resulted in variable Brunhes-age normal overprints that are sometimes difficult to remove using alternating field demagnetisation techniques. In such cases, thermal demagnetisation is preferred. Where strata are strongly weathered, and secondary magnetic overprints are suspected, identification of reverse polarity magnetisation provides a minimum age of 0.78 Ma. Australasian tektites are a potential lithostratigraphic marker just prior to the MBB, but have yet to be identified in the same on-land section as the MBB in Australia. A widespread arid shift in paleoclimate succeeded the MBB in Australia, but its effects were probably time transgressive across the continental landscape.

Placement of the Lower-Middle Pleistocene boundary at the MBB would make it a widely recognisable chronostratigraphic marker in weathered continental deposits, and would facilitate correlation of the boundary between marine and continental sequences.