JOURNAL OF QUATERNARY SCIENCE (1997) 12 (4) 295–308 CCC 0267-8179/97/040295–14$17.50  1997 by John Wiley & Sons, Ltd. Uranium/thorium dating of ferricretes from mid- to late Pleistocene glacial sediments, western Tasmania, Australia PAUL C. AUGUSTINUS 1 *, STEPHEN A. SHORT 2 **, and HENK HEIJNIS 2 1 Department of Physical Sciences, University of Tasmania, P.O. Box 1214, Launceston 7250, Australia 2 Environmental Radiochemistry Laboratory, Australian Nuclear Science and Technology Organisation, Private Mail Bag 1, Menai, NSW 2234, Australia Augustinus, P. C., Short, S. A. and Heijnis, H. 1997. Uranium/thorium dating of ferricretes from mid- to late Pleistocence glacial sediments, western Tasmania, Australia. J. Quaternary Sci., Vol. 12, pp. 295–308. ISSN 0267-8179. (No. of Figures: 9 No. of Tables: 6 No. of References: 32) Received 22 January 1997 Revised 5 May 1997 Accepted 7 May 1997 ABSTRACT: The uranium/thorium (U/Th) dating method was applied to pedogenic/diagenic ferricretes developed within glacial drifts from the northern West Coast Ranges of Tasmania. The dates derived from the method are, by definition, minima for the sediments, with the ferricretes being secondary mineral deposits which appear in the form of bands, horizons or nodules comprised of indurated masses of iron oxy(hydr)oxides developed post-depositionally within the host sediment or soil. The absence of such deposits in well-identified drift sediments of the last glaciation suggests the development of ferricrete horizons in glacial drifts occurred during earlier interstadials and interglacials. This has been confirmed by the uranium/thorium dating of ferricrete bands within glacigenic sediments obtained from three drill cores from the Boco Valley. The ages derived were found to be consistent with estimates for the timing of glaciation derived from palaeomagnetic analyses, radiocarbon dating and relative dating tech- niques. The results show that the uranium/thorium method can be used to successfully date ferricretes obtained from within glacial sediments of ages 350 000 yr and provides valuable information regarding the minimum age of the host sediment.  1997 by John Wiley & Sons, Ltd. KEYWORDS: uranium/thorium dating; ferricrete; glacial drift; Tasmania Colhoun, 1991). The youngest event, known as the Margaret Introduction Glaciation, is the best constrained and has been dated by 14 C to ca. 18 800 yr BP (Colhoun, 1985). This glaciation did not penetrate west of the West Coast Ranges and was The glacial history of the West Coast Range of Tasmania has been reviewed in a number of recent studies (Colhoun, restricted to the higher peaks. However, more deeply weath- ered drifts deposited by the Boco and Bobadil Glaciations 1985; Colhoun and Fitzsimons, 1990; Kiernan, 1990). How- ever, none of the middle Pleistocene sequences are radio- are much more extensive and beyond the range of the 14 C method (Augustinus and Colhoun, 1986; Augustinus et al., metrically dated. Nevertheless, a relative glacial stratigraphy has been constructed for the Pieman–Boco valley area in 1994). These drifts have been assigned to the middle Pleisto- cene on the basis of normal polarity recorded in glaciolacus- the northern West Coast Ranges, but the absolute dating of the sequences is poor due to a general lack of datable trine silts (Augustinus et al., 1995). Reversed polarity recorded in the most deeply weathered and extensive Bulgo- material (Augustinus and Colhoun, 1986). Reliable dating of the glacial sequence is required for the development of bac drift indicates that it is older than 783 000 yr BP (Augustinus et al., 1995). regional, as well as Southern Hemispherical correlations of the recognised glacial episodes. A range of post-depositional weathering data help con- strain the age of these glacial drifts. These criteria include: A partially dated glacial history for western Tasmania has been developed largely on the basis of relative dating criteria thickness of weathering rinds on clasts of chert and Jurassic dolerite; and percentage absorption of water and specific (e.g., Augustinus and Colhoun, 1986, 1995; Fitzsimons and gravity of the volcanic clasts from the drifts (Augustinus and Colhoun, 1986, 1995; Table 1). The drifts are also observed * Correspondence to: Departments of Geography and Geology, University of to contain differing degrees of ferricrete development, with Auckland, PB 92019, Auckland, New Zealand. the last (Margaret) Glaciation drifts showing minimal Fe/Mn ** Present address: Forbes Rigby, Consulting Engineers and Planners, PO Box oxide staining and no ferricrete development. This contrasts 5125, Wollongong, NSW 2500, Australia. sharply with heavy Fe/Mn oxide staining and the develop- Contract grant sponsor: University of Tasmania Contract grant sponsor: Australian Institute of Nuclear Sciences and Engineering ment of variable thickness ferricrete bands in the pre-last- interglacial Boco, Bobadil and Bulgobac drifts (Table 1).