Late Pleistocene Tephrostratigraphy of the Eastern Bay of Plenty Region, New Zealand

Abstract

This thesis has produced the compilation of a complete tephrostratigraphic record of the eastern Bay of Plenty, New Zealand. About fifty Late Pleistocene tephras (i.e. those older than the Rotoiti eruption), ranging in age from c. 600 to 50 ka, are recorded in a terrestrial sequence of loess and paleosols in the eastern Bay of Plenty. Tephra correlations are based on the distinctive physical characteristics of the airfall beds and confirmed by microprobe analysis of glass shards ("fingerprinting"). Chemical analysis of hornblendes and titanomagnetites is used as a supplementary correlation tool where the tephras are too weathered to retain glass. The eastern bay of Plenty deposits are divided into seven subgroups with their boundaries marked either by major tephras or by significant changes in the paleo-climate indicator deposits such as loess and paleosols. These subgroups, and their estimated age ranges, are: Age control on the eastern Bay of Plenty tephras has been obtained by fitting the paleoclimatic information inferred from field observations to the Low Latitude Stack (LLS) and SPECMAP oxygen isotope curves, with correlations to a few well dated eruptives providing key time planes within this record; in particular, the Mamaku Ignimbrite (correlates to the Kutarere Tephra), and the Kaingaroa (Kaingaroa), Matahina (Matahina) and Rangitaiki (Kohioawa) Ignimbrites. Tentative correlations of several eastern Bay of Plenty tephras to the western, coastal central, and Southeast-central Bay of Plenty areas (Tauranga Matata cliffs and Reporoa, respectively) have been achieved. Three additional subgroups are proposed: the Welcome Bay (with at least 6 tephras) in the west, the Ohinekoao (14 tephras) in the coastal central, and the Reihana (13 tephras) in the southeast-central Bay of Plenty; all of which overlap in time with the eastern Bay of Plenty stratigraphy. The tephras recorded in the Bay of plenty have been used to estimate the ages of formation and uplift rates for many of the landforms that are observed throughout the region. A tectonic regime of subsidence in the west towards Tauranga, block faulting on either side of the subsiding Whakatane Graben in the central Bay of Plenty, and further large scale block faulting towards the far eastern margin of the Bay of Plenty has been proposed. Activity at the Okataina Volcanic Centre is now thought to have initiated at or before c. 370 ka, with the eruption of the Paerata Tephra. This tephra has a distribution pattern consistent with an Okataina source, and contains abundant cummingtonite, which is a signature mineral within tephras from the Okataina Volcanic Centre during the late Quaternary time period. However, the much older, but less well understood, Reeves-A and Wilson Tephras—both with estimated ages of c. 0.5 Ma—also contain cummingtonite, which indicates that activity may have been initiation at a much earlier time, or that a volcanic centre other than Okataina has produced cummingtonite. Activity in the Rotorua Volcanic Centre prior to the eruption of the Mamaku Ignimbrite is also indicated, as is activity at the Reporoa Volcanic Centre prior to the Kaingaroa Ignimbrite eruption.