ELUCIDATING THE EARLY PLEISTOCENE HISTORY OF THE EASTERN WANGANUI BASIN, NEW ZEALAND USING MAJOR SILICIC TUFFS FOR TIME AND ENVIRONMENTAL CONTROL

Thick and sandy or gravelly silicic tuffs are a feature of Early Pleistocene deposits in the southwest of the North Island of New Zealand. They were sourced from major eruptions of rhyolitic tephra and ignimbrite from Mangakino, 50 km northwest of the most recently active Taupo volcanic centre.

The Early Pleistocene of the western and central part of the Wanganui Basin has been well documented. The stratigraphy and chronology has been interpreted in both sequence stratigraphic and climatic frameworks. In brief, low stands were terrestrial erosion or beach gravels and sands, transgressions were inner shelf sands, and high-stands outer shelf muds. No such framework exists for the eastern basin.

The tuffs have been correlated using their distinctive glass chemistry, to dated events at ca. 1.6, 1.5, 1.29, 1.05, 0.85 and 0.7 Ma. The first arrival of the water borne pumice, if concentrated and voluminous, closely follows the dated eruption.

The tuffs allow a basin-wide chronology to be developed. Prior to 1 Ma, the eastern basin was a wide wave-dominated, shallow sea in interglacial periods and a sand dominated fluvial plain in glacial periods. Although the basin was subsiding, the accommodation space was constantly reduced by rapid sedimentation. The Potaka Pumice at 1Ma was responsible for considerable progradation of the coastline. By 0.85 Ma, although subsidence continued, the sea rarely transgressed over the coastal foreland except for development of estuaries. Subsidence continued until at least 0.7 Ma, but now the environment was fluvial gravel aggradational braid-plains in the glacial periods, and fluvial over-bank sands muds and lignites in the interglacials. Uplift began soon after 0.7 Ma, and from 0.5 Ma to the present day , fluvial aggradation terrace systems have developed.

Over the last 0.7 Ma, the sediments have been rapidly uplifted, faulted, folded and tilted up to 70 degrees as several asymmetrical anticlines have grown.