Stratigraphy and optical dating of Pleistocene coastal deposits in the Port Campbell australite strewn field, SW Victoria

Abstract

Alluvial-channel and overlying eolian deposits exposed in coastal cliffs in the Port Campbell australite (Australian tektite) strewn field, SW Victoria, were studied stratigraphically and dated by optically stimulated luminescence (OSL) for quartz sand grains to illuminate the occurrence of australites in this area. The channel deposits, termed the Dog Trap Bay Sand, yielded OSL ages from 420 ± 40 to 230 ± 15 ka for four sites, and the two eolian units OSL ages of 203 ± 16 and 93 ± 11 ka. The ages support field evidence that, contrary to the conclusions of a previous study by others, the channel deposits are stratigraphically distinct from and much younger than the Pliocene Hanson Plain Sand, which forms a regionally extensive system of marine strandlines to the north and NE of Port Campbell. The OSL ages show some preference for times of negative temperature anomalies marking the last four glacial periods of the Middle and Late Pleistocene as indicated by ice-core data from Antarctica. Eolian and at least some alluvial deposition evidently occurred when sea-level was low and rivers extended across the exposed continental shelf. Others have suggested that the Dog Trap Bay Sand contains rare australite fragments in situ. However, these occurrences may have resulted from reworking of sandy material, with the australites possibly derived from overlying younger units known to contain numerous specimens. The OSL ages imply that a discrepancy of ca 350 ka or more exists between the apparent stratigraphic occurrence, as distinct from the terrestrial age, of australites in the Port Campbell strewn field and the australite ⁴⁰Ar/³⁹Ar age of 770 ± 20 ka. Stratigraphic evidence in this area for the terrestrial age of australites is inconclusive.

我们对维多利亚西南部Campbell海湾澳洲玻陨石散落区的出露于沿海悬崖的冲积道沉积物和上覆风尘沉积物进行了地层学研究，并通过光激发发光（OSL）对石英砂粒进行了年龄确定，以证实该区有澳洲玻陨石出现。定名为Dog Trap海湾砂的冲积道沉积物的OSL年龄为420±40 至230±15Ka，两个风尘沉积物单元的OSL年龄为203±16 至93±11Ka。与前人研究的结果相反，这些年龄资料支持这样的野外证据: 冲积道沉积物在地层学上有别于上新世Hanson平原沙，而且年轻多了。Hanson 平原沙在Campbell海湾之北及东北形成了一个区域性大范围海滨线系统。OSL年龄资料表现出对负温度异常期的偏向性，负温度异常的标志是根据南极洲冰岩芯所得出的中和晚更新世最后的四个冰期。风尘沉积和至少一些冲积沉积作用发生在这样的时候: 海平面低，河流横跨出露着的大陆架。有些人认为Dog Trap海湾沙含有原地稀有澳洲玻陨石，然而，在这里出现可能是砂质物质再沉积的结果。澳洲玻陨石可能来源于上覆较年轻的地层单元。OSL年龄资料意味着，在Campbell海湾散落区，澳洲玻陨石的地层年龄与澳洲玻陨石40Ar/39Ar年龄770±20Ka之间存在着大约350Ka的差别。这个地区澳洲玻陨石陆源年龄的地层学证据还尚无定论。

KEY WORDS:

• stratigraphy
• Pleistocene
• Port Campbell Embayment
• australites
• optically stimulated luminescence dating
• climate cycles

关键词:

• 地层学
• 更新世
• Campbell海湾
• 澳洲玻陨石
• 光激发发光年龄法
• 气候周期

ACKNOWLEDGEMENTS

It is with much sadness that the results of the research by the late Emeritus Professor John R. Prescott, who determined the OSL ages, are published posthumously. We are indebted to Dr Frances Williams of the Department of Physics, University of Adelaide, for helpful discussions concerning the OSL dating. The late Dr J. G. (Jack) Douglas, formerly of the Victorian Geological Survey, participated in the initial fieldwork but sadly did not see the results of the research. We thank Parks Victoria and staff at Port Campbell for permission to sample in the Port Campbell National Park and the Bay of Islands Coastal Park. The work was assisted by grants AINGRA05136 and AINGRA06145 from the Australian Institute of Nuclear Science and Engineering and from the Archaeometry Special Fund in the School of Chemistry and Physics, University of Adelaide. We thank the two reviewers for helpful comments.

Notes

XRF, X-ray fluorescence spectrometry; TSAC, thick source alpha counting; DNA, delayed neutron analysis; NAA, neutron activation analysis; ICPAES, inductively coupled plasma atomic emission spectrometry; ICPMS, inductively coupled plasma mass spectrometry.

Gy is the SI unit Gray, where 1 Gy = 1 J kg⁻¹.

^aAverage of rates obtained by TSAC and DNA/NAA/XRF.

^bRates obtained by ICPMS/ICPAES.