Facies architecture and stratigraphy of a basaltic-trachytic polygenetic volcano, Whakaraupō Volcano, NE Banks Peninsula, New Zealand

Abstract

Polygenetic volcanic systems typically evolve through multiple igneous and sedimentary events of construction and degradation. Understanding the stratigraphy of volcanic rocks is essential for assessing volcanic hazards and natural resources such as hydrocarbons, geothermal heat, and groundwater. It requires identifying the internal and external structures of the volcanic deposits in the geological record. We present a multiscale analysis of the development of the Whakaraupō Volcano at the NE portion of the Miocene Lyttelton Volcanic Complex (LVC), Banks Peninsula, South Island, New Zealand. We combine different stratigraphic approaches (Facies, Lithostratigraphy, and unconformity boundaries) to characterize the Volcano in time and space. Thirteen lithofacies and seven facies' associations were described and classified into four Formations (Scott Valley, Heathcote Valley, Britten Craig and Mount Pleasant). Each lithostratigraphy represents a specific depositional and eruptive environmental setting separated by unconformities. The Whakaraupō Volcano evolved progressively shifting the eruptive locus towards ENE. Hawaiian-style eruptions characterize the initial eruptive stage, forming widespread sequences of hawaiite a'ā lava flows with fan-like geometry overlapped by benmoreitic rubbly and blocky lava flows (Scott Valley Fm). Next, the activity shifted to transitional type mild explosive eruptions with numerous air-fall hawaiite to mugearite scoria cones producing ash plains around active vents, co-genetic lava domes and a‘ā lava flows fed from a NE-SW fissure system, as well as building an eruption-fed conglomeratic volcanic debris apron (Heathcote Valley Fm). The following Formation (Britten Craig) is characterised by a Fan-like a‘ā hawaiite to benmoreite flows bounded by paleosoil layers at the top and bottom, indicating a time gap between the emplacement of Formation. The final stage comprises a hawaiite-trachytic lava succession interbedded with epiclastic deposits (Mount Pleasant Fm). Multiple different ages and compositions of dikes crosscut the volcanic succession, mainly feeding trachytic lava domes with a dominant NE-SW trending orientation. Our multiscale analysis provides insights into the three-dimensional architecture of diverse rock bodies within the Whakarupō Volcano. This approach will aid the construction of prospective models that can be applied to understand the hazards and geo-energy resources of large basaltic-trachytic volcanic complexes.