HR: 0800h
AN: H51C-0362    [Abstracts]
TI: Paleomagnetic and Clay δ¹⁸O Ages for Weathering in Northwestern NSW, Australia
AU: * Smith, M L
EM: martin.smith@anu.edu.au
AF: Earth Environment and CRC LEME, Research School of Earth Sciences, Austalian National University, Acton, ACT 0200 Australia
AU: Pillans, B J
EM: brad.pillans@anu.edu.au
AF: Earth Environment and CRC LEME, Research School of Earth Sciences, Austalian National University, Acton, ACT 0200 Australia
AB: There is a complex history of erosion, deposition, tectonism and weathering relating to the Surat, Eromanga and Murray/Darling Basins in northwestern New South Wales. In this study we use paleomagnetic and δ¹⁸O measurements to demonstrate that the weathering history dates extends back to the Permain. Paleomagnetic dating of hematitic regolith profiles yields ages for the terminal stages of deep chemical weathering, often corresponding to drying out of a profile. Previous work identified two major periods of weathering across the Australian continent during the Tertiary. This study extends the data in the eastern part of the continent, and reveals several stages of weathering from the Jurassic through to the latest Tertiary, including the two Tertiary weathering events, at ~60-30 Ma, and ~3-12 Ma. A possible Jurassic weathering episode is recorded from two sites at Cobar. A 100 Ma weathering event is recorded in samples from Cobar and the Broken Hill Region, which is a new weathering event recorded from Australian regolith samples. Clay δ¹⁸O compositions reflect the chemical makeup of groundwaters active during weathering and clay formation. There is a predictable relationship between δ¹⁸O and age of authigenic clays in regolith profiles in Australia. Clay δ¹⁸O ages from this study range from Permian through to Late Tertiary. The two oldest ages represent inheritance of some fraction of clay from the weathered host lithology. Samples from White Cliffs, Cobar and Lightning Ridge yield Oligo-Miocene ages. Another sample from the Cobar region yields a Mio-Pliocene age. The results presented provide evidence for multiple periods of deep weathering across NW NSW, from the Jurassic through the Pliocene. Clay formation in weathering profiles occurs under humid conditions, whilst hematite formation occurs at the terminal stages of intense chemical weathering, at the end of periods of humid climatic regimes. When the results are combined, they indicate periods of humid climates reaching terminal stages, resulting in deep, clay-rich, hematitic weathering profiles during several periods across NW NSW. This story adds to landscape evolution models derived from stratigraphic and geomorphological studies of surrounding basins.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1130 Geomorphological geochronology
DE: 1527 Paleomagnetism applied to geologic processes
DE: 1886 Weathering (0790, 1625)
SC: Hydrology [H]
MN: Fall Meeting 2005