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Chronology of Pliocene and Quaternary bioevents and climatic events from fission-track ages on tephra beds, Wairarapa, New Zealand |
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| Authors: | Phil Shane Paul Froggatt Tasha Black John Westgate |
| Institution: | aDepartment of Geology, University of Toronto, Toronto, Ont. M5S 3B1,Canada bCentre for Continuing Education, Victoria University of Wellington, P.O. Box 600, Wellington ,New Zealand |
| Abstract: | High-resolution Pliocene and Pleistocene sequences exposed on land in New Zealand are some of the few detailed records of widepread marine bioevents and paleoclimatic changes in the Southern Hemisphere. Marine biostratigraphy calibrated in deep-sea cores by paleomagnetic reversals has been the primary basis for the chronology of these sequences. We have determined ages for several tephra beds which now provide an independent numerical age calibration for a well-studied marine and terrestrial section in Wairarapa. By using the isothermal plateau fission track (ITPFT) method on volcanic glass we have overcome the problems of partial track fading and detrital mineral contamination, which hindered earlier studies, to reveal a new chronology extending back to nearly 5 Ma. Our ages for the Hikawera Tuff (4.91 ± 0.25 Ma) and Spooner Tuff (3.44 ± 0.13 Ma) are consistent with the appearance and disappearance of many early Pliocene foraminiferial species, validating their age calibration in New Zealand. However, some fossil occurrences, including coccoliths, differ temporally by as much as 0.55 Ma, perhaps due to local tectonic-induced recycling. Four Pleistocene tephra beds (Potaka tephra (1.00 ± 0.03 Ma), Kaukatea tephra (0.87 ± 0.05 Ma), Rangitawa tephra (ca. 0.35 Ma) and Kawakawa tephra (ca. 0.22 Ma)) are now recognised in the Wairarapa sequence via stratigraphic and new geochemical and age data. These beds allow direct correlation to other marine and terrestrial basins, as well as volcanic regions in New Zealand, and will ultimately aid in a regional paleoenvironmental reconstruction where bioevents are absent. The tephra ages indicate that the marine sediment accumulation rates varied from 90 to 250 m/Ma between different sections of the Pliocene and reached ca. 350 m/Ma in the last 2.4 Ma, when the sequence displays pronounced glacioeustatic cyclic deposition. In the terrestrial realm, the oldest loess in New Zealand is now constrained to between 1.00 and 0.87 Ma. |
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