Wood, A.M. 1977 Surface geology of the western diversions area. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 277 21 p.
Abstract: The oldest rocks in the Western Diversions area are indurated Mesozoic sediments which are exposed in the Whakapapa - Tawhitikuri Tunnel and the Okupata stream. Tertiary marine sediments uncomformably overlie the basement rocks and include: limestones, calcareous, glauconitic and carbonaceous mudstones, siltstones and sandstones of the Landon Series; silty quartzose sandstones, greywacke pebble bands and rarer limestones, siltstones, mudstones and coal lenses of the Taranaki Series. These sediments crop up in the northwest of the area, where they have been eroded into a finely dissected relief. In the Whakapapa-Tawhitikuri tunnel and the Pukehinau stream area, faults have uplifted a block of Mesozoic rock with its overlying Landon and Taranaki sediments, so that Landon sediments are jutaposed against Taranaki sediments. The major part of the Quaternary volcanic deposits are confined to the northwest and southwest of the mapped area, where they form part of an extensive ring plain of broad, subdued relief. Thick alluvial and lahar deposits occur between the Okupata stream and Wanganui River, where they may infull an old lake. Lava flows and tephras cap the alluvial sequence between the Okupata and Mangatepopo streams. Other flows and tephras infill an old valley system eroded into Taranaki Series sediments between Kapoors road and Access road 1. Tongariro Sub-group tephras, Taupo pumice and Ngauruhoe tephras mantle the present topography. Quaternary to recent faults, down-thrown to the east have left marked surface traces and scarps. (auth)
