New Zealand Journal of Geology and Geophysics abstracts
Reinterpretation of the glacial chronology of South Westland, New Zealand
P. C. ALMOND
Soil and Physical Sciences Group
Division of Soil, Plant and Ecological Sciences
P.O. Box 84
Lincoln University
Canterbury, New Zealand
email: almondp@lincoln.ac.nz
N. T. MOAR
Landcare Research
P.O. Box 69
Lincoln
Canterbury, New Zealand
email: n.moar@ext.canterbury.ac.nz
O. B. LIAN*
School of Earth Sciences
Victoria University of Wellington
P.O. Box 600
Wellington, New Zealand
*Present address: Department of Physics, Simon Fraser University, Burnaby,
British Columbia, Canada V5A 1S6. email: olian@sfu.ca
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 stratigraphically 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 L1a, L1b, L2, L3, and L4 are now thought to be of Last (Otira)
Glacial age. L1a accumulated in marine oxygen isotope stage (OIS) 2 before 16
ka, but after 24 ka, and L1b 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 M1,
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.
Keywords Westland; glacial stratigraphy; loess stratigraphy;
soil stratigraphy; phytoliths; luminescence dating
New Zealand Journal of Geology & Geophysics, 2001, Vol. 44: 1-15
0028-8306/01/4401-0001 $7.00/0 (c) The Royal Society of New Zealand
2001
PDF file of entire paper: medium quality (3318K); (scanned from paper original: notes about this process)
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