Subdividing the Pleistocene using the Matuyama-Brunches boundary (MBB): an Australasian perspective

Abstract

The last major reversal of the Earth's magnetic field, the Matuyama-Brunhes boundary (MBB), dated at 0.78 Ma, is widely identified in Australian and New Zealand Pleistocene deposits. 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 and tephrostratigraphy permit correlation from Wanganui Basin to other on-land sections and deep-sea cores. 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. 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. Identification of reverse polarity magnetisation in weathered deposits, including soils, provides a minimum age of 0.78 Ma for these materials. A widespread arid shift in paleoclimate succeeded the MBB in Australia. Placement of the Lower-Middle Pleistocene boundary at the MBB would constitute the most recognisable chronostratigraphic marker in weathered continental deposits.