Reinterpretation of the glacial chronology of South Westland, New Zealand

Abstract

The glacial chronostratigraphy for South Westland established by Almond in 1996 was based on the soil stratigraphy of loess coverbeds on glacial landforms in and around Saltwater Forest. Loess sheets and morphologically identified soil features were correlated with global climatic events established by the marine oxygen isotope record. Pollen analysis and few radiocarbon ages supported the chronostratigraphy. In this study, we have used total element, mineral and phytolith analysis, and luminescence dating of loess to test the current correlation of loess/soil stratigraphic units with climatic events. Data from new sections in Okarito Forest have been incorporated. Luminescence dating was found to be of limited value. Thermoluminescence dating yielded unacceptable and low‐precision ages that were anomalously old, and in strati‐graphically reversed sequence. Optical (IRSL) dating removed some of these anomalies but uncertainty about accuracy remained a problem. Some optical ages were significantly younger than associated radiocarbon ages. We suspect the highly weathered nature of the sediments being dated in combination with sediment provenance may be the cause of these effects. Routine optical dating which targets potassium feldspars or quartz cannot be applied in Westland. Loess sheets Lla, Lib, L2, L3, and L4 are now thought to be of Last (Otira) Glacial age. L 1a accumulated in marine oxygen isotope stage (OIS) 2 before 16 ka, but after 24 ka, and Lib also accumulated in OIS 2 but between 24 and c. 36 ka. L2 accumulated during OIS 3 from 45 to c. 50 ka. Loess sheets L3 and L4 are now correlated with OIS 4. Soil alteration in loess sheet L5 indicates significant weathering consistent with deposition before the last interglacial, probably during OIS 6. On the basis of the revised loess stratigraphy, glacial advances M5 and M6, and associated landforms, which have no loess cover, both occurred during OIS 2. Two glacial advances, M4a and M4b, are both assigned to OIS 3, and are thought to have occurred between 45 and 50 ka and c. 24 and 36 ka, respectively. Moraines associated with advances M2 and M3 are conformably overlain by L4 and are correlated to OIS 4. Moraine Ml, which is draped by loess sheets L1‐L5, is thought to be of Waimea Glaciation age (OIS 6). Our present work is consistent with recent studies in Fiordland, which suggest a more complex glacial stratigraphy for the Otiran than is recognised in North Westland.