Plio-Pleistocene geology of the Lower Pohangina Valley, New Zealand

Plio-Pleistocene sediments exposed in the dissected hill country of the Lower Pohangina Valley and Manawatu Saddle provide a unique record of sedimentation adjacent to the actively uplifting main axial range, North Island, New Zealand. Located in the eastern Whanganui Basin, which has formed behind the active Hikurangi subduction zone, renowned for one of the most comprehensive Quaternary stratigraphic records in the world. The oldest beds in the succession, unconformably overlying Torlesse Group, comprise Mangapanian (3–2.4 Ma) very fine conglomerate, sandstone and mudstone of the Komako Formation. Subsequent deposition of Konewa Formation continued through to late Nukumaruan time (2.4–1.63 Ma), including large tracts of conglomerate now preserved across the Manawatu Saddle, clearly visible from the Saddle Road. Takapari Formation consists of early Castlecliffian (1.63–1 Ma) sediments characterised by a sudden influx of primary volcanic and reworked volcaniclastic material. Distinctive rhyolitic marker beds and molluscan bio-events provide a means of correlating and dating the succession from the Lower Pohangina Valley across the Whanganui Basin. We present the geology of the Lower Pohangina Valley on a 1:30,000 geological map, accompanying cross section and 3D model. A new stratigraphic framework is presented, integrating work in the Komako District, Pohangina Valley with Whanganui Basin stratigraphy through the application of tephrochronology and biostratigraphy. The Lower Pohangina Valley succession provides evidence for marine conditions prevailing across what is now part of the main axial range of New Zealand from c. 3 to 1.6 Ma. An influx of conglomerate, eroded from Torlesse Group, into the sedimentary record during early Nukumaruan time (2.4 Ma) is attributed to an early phase of tectonic uplift along the North Island Dextral Fault Belt (NIDFB), before closure of the ‘Manawatu Strait’ commenced in early Castlecliffian time (1.63 Ma).