Quaternary stratigraphy of the Tauranga basin

Abstract

A 1:25000 scale geological map of the southern part of the Tauranga Basin is presented together with numerous stratigraphic columns to illustrate the late Cenozoic stratigraphic development and depositional patterns of the Basin. The Tauranga Basin lies 40 km north of the Taupo Volcanic Zone and immediately east of the Kaimai Range where Beesons Island Volcanics, aged 16 - 7 m.y. B.P., outcrop as an effective basement to the west and south of the Basin. The Basin originated by the northeast tilting of a large fault block, the western upturned edge now comprises the Kaimai Range and the Quaternary sequence infills the downtilted eastern margin. The tilting and hence age of the Basin mainly post-dates emplacement of the Waiteariki Ignimbrite, aged 0.84 m.y. B.P., which now immediately underlies both the Whakamarama Plateau of the southeastern Kaimai Range and the Quaternary succession in the Basin. At a few localities within the Basin Waiteariki Ignimbrite surrounds and laps onto Minden Rhyolite (4.34 m.y. B.P.) and Aongatete Ignimbrite (∼1.26 - 0.86 m.y. B.P.); these older rocks are sometimes overlain by Pleistocene deposits. The Matua Subgroup is formally erected to classify the Quaternary sequence within the Tauranga Basin. It comprises up to 240 m of terrestrial and estuarine sediments with interdigitating pyroclastic deposits both of an air-fall and flow origin (ignimbrites). The ignimbrites are classified and mapped as formations where they are subaerially exposed, and in stratigraphic order are: Wairoa Ignimbrite (∼0.6 m.y. B.P.), Te Puna Ignimbrite (∼0.4 m.y. B.P.), Te Ranga (∼0.3 m.y. B.P.) and the Waimakariri Ignimbrite (∼0.18 m.y. B.P.). The ignimbrites are unwelded and have suffered substantial post-depositional erosion thus providing the major source of the interstratified sediments of the Matua Subgroup. In many cases individual sedimentary units throughout the stratigraphic succession can be related by petrofacies studies to source ignimbrites. This enables the discrete tongues and lenses of sediments to be mapped as post-ignimbrite units. However these sedimentary units have not been formally defined as formations. A sequence of white to grey rhyolitic tephras up to 20 m thick and named the Pahoia tephras, are invariably part of the upper 40 m of the Subgroup and are intercalated with or overlie fluvial and pyroclastic flow deposits. Brown Hamilton Ash beds inturn overlie the Pahoia tephras and/or Matua Subgroup. The 42,000 yr B.P. Rotoehu Ash and younger Late Pleistocene-Holocene tephras mantle the present topography, and may overlie any of the stratigraphically older units. The pyroclastic flow deposits of the Matua Subgroup each show distinct but different textural and morphologic characteristics. These deposits are distinguished from air-fall units by their poorly sorted nature, coarser grain-sizes, and valley ponded distribution. The air-fall units are invariably thinner, shower bedded and mantle the paleo-topography. The fluvial sediments are dominated by planar stratification (Lenticular, Horizontal parallel, Horizontal discontinuous, Graded), and cross-stratification (Rho, Nu, and Alpha), and are interpreted as mainly braided river facies. The Taupo Volcanic Zone is considered the source of the Waiteariki Ignimbrite and younger pyroclastic flow deposits of the Matua Subgroup. A more northern, Coromandel Range, source is suggested for the older Aongatete Ignimbrite (∼1.26 - 0.86 m.y. B.P.). The Quaternary geological history of the region has been informally considered as three main periods: early Ignimbrite period (0.84 - 0.50 m.y. B.P.), a late Ignimbrite period (0.50 - 0.14 m.y. B.P.), and an upper Matua Subgroup period (∼0.16 m.y. B.P. - Present). The principal influences on the stratigraphic development and depositional patterns of the Basin are attributed to block faulting and tilting of the basement and adjacent Kaimai Range, volcanic activity in the Taupo Volcanic Zone, and to a lesser extent, climatic and glacio-eustatic sea-level changes.