| Abstract |
A comprehensive understanding of subsurface geology is fundamental for successful exploration and development of geothermal systems. After more than 50 years of drilling, the petrography of the strata at the Wairakei-Tauhara Geothermal Field has been elucidated, and insights gained on physical characteristics essential for hydrological modelling in response to development. However, few studies have investigated the geological evolution and depositional setting of the strata, and none have detailed the influence of primary geological processes (e.g. explosive and quench fragmentation) on the near-surface hydrology of the geothermal system. The Waiora and Huka Falls Formations (of the Huka Group), form reservoir and capping strata, respectively, for of the Wairakei-Tauhara system. Facies analysis indicates the Huka Group represents c.300 kyr of intra- and inter-basinal volcanic, volcaniclastic and sedimentary deposition in ancient Lake Huka, the precursor of present-day Lake Taupo located south of the Wairakei-Tauhara. Over a ~100 kyr period, subaerial pyroclastic flows contributing to the 400 – 1500 m thick Waiora Formation were quenched and hydraulically reworked upon entering the lake. Subaqueously-erupted, water-supported mass flows also contributed multiple voluminous successions of highly fragmented and permeable, pumiceous pyroclastic deposits, grading from lithic breccia to fine-grained suspension cap. Contemporaneous, subaqueous lavas contain quenched-fractured breccia carapaces and contribute further coarse epiclastic sediments to the basin succession. Hydrovolcanic and sedimentary interactions resulted in fragmented vesicular deposits and fractured lavas underlain by dense breccias which facilitate permeable flow zones within the Waiora Formation reservoir. Deposition of the overlying |