Upper Quaternary warping and uplift rates in the Bay of Plenty and west coast, North Island, New Zealand

Abstract

Coastal terraces around the Bay of Plenty are examined from Waihi to East Cape. The tephra stratigraphy described by other authors is extended; in particular a pumiceous tephra formation immediately underlying Hamilton Ash Formation is named the Little Waihi Formation. Two terraces can be traced confidently around the Bay; the younger (BOP2) is capped by Rotoehu Ash and younger tephras and the older (BOP3) is capped by Little Waihi or Hamilton ashes and younger tephras. Wave-cut surfaces beneath the mantling deposits vary in height around the Bay, indicating warping. Near Maketu, both BOP2 and BOP3 cut surfaces are within 2 m of sea level; near East Cape they are about 50 m and 100 m, respectively. West coast terraces are traced from Kaipara and south-west Auckland into north Taranaki and thence to Wanganui. The following formations are correlated: (a) Waioneke—Waiau B—Rapanui; (b) Shelly Beach—Waiau A—Ngarino; (c) Parawai— Brunswiak; and (d) Nihinihi and upper Kaihu Formations—Kaiatea Group of Terraces. The highest littoral and marine beds in terraces correlated with the Rapanui Formation are 20–25 m a.s.l. only from Taranaki to Kaipara; the Ngarino equivalents are 40 m, and the Brunswick equivalents are 70 m. Correlation from the west coast to Bay of Plenty is subject to uncertainty about tephra relationships. It is shown that BOP2 is the probable correlative of Ngarino Terrace equivalents along the west coast, and that BOP3 is probably equivalent to Brunswick. The relationship of these terraces to sea-level oscillations which are well identified and dated elsewhere in the world is discussed. Although eight major trangression-regression cycles occurred in the last 250 000 years, only the peaks around 240 000-215 000 years and 140 000-120 000 appear to have generated widespread terraces. It is suggested that these formed the BOP3—Parawai— Brunswick set and the BOP2—Waiau A—Ngarino set, respectively. These results are compatible with radiometric age estimates of about 0.4 m.y. for the Upper Castlecliffian. Uplift rates calculated on this basis in the Bay of Plenty vary from zero at Maketu to 0.4 mm/year at East Cape. For the west coast thé rate is essentially constant from Taranaki to near Auckland, at 0.3 mm/year.