An investigation of the Rapanui and Ngarino marine terraces in south Taranaki : coverbed stratigraphy, distribution and tectonics : a thesis presented in partial fulfilment of the requirements for the degree Master of Science in Quaternary Science at Massey University, Palmerston North, New Zealand
Uplifted marine terraces, formed by successive marine onlap during interglacial and interstadial sea level maxima are preserved parallel to the South Taranaki coastline between Kakaramea in the southeast, and Hawera to the northwest. The objective of this research was to investigate the formation, deformation and subsequent landscape evolution of the Ngarino and Rapanui Terraces in the area between Hawera and Kakaramea The northwestern part of the study area occupies the southeastern margin of the Egmont ringplain where terraces that extend from the southeast sector are progressively buried by volcaniclastic sediments west of the Tangahoe River. Ages of the wave-cut platforms have been inferred from the coverbed stratigraphy and subsequent correlation to sea-level and oxygen isotope curves. Mapping the height of wave-cut platforms also allowed calculation of marine terrace deformation rates and patterns in this area. In the past, studies have tentatively concluded that between Kakaramea and Hawera the younger 120 ka Rapanui Terrace is cut out at the coast along with the 100 ka Inaha Terrace, and that to the west only the Ngarino Terrace is preserved. Examination of coverbed stratigraphy has established that both the Ngarino and Rapanui Terraces (210 and 120 ka respectively) are present in the study area. The terrace coverbed stratigraphy presented here indicates that next to the coast the terrace is younger than 210 ka. On this terrace loess units, corresponding to oxygen isotope stages 2,3,4 and 5 are found and gives a maximum age of 120 ka. Inland, the terrace stratigraphy reveals loess units corresponding to stages 2,3,4,5, and 6, providing a maximum age of 210 ka. Structural interpretation has demonstrated that underlying tectonics associated with the Taranaki Fault Zone has subsequently deformed the wave-cut platforms from their shallow angled seaward-dipping profiles. Southeast of the Tangahoe River, uplift rates between 0.3 - 0.55 mm/yr have formed well-defined terraces. The doming of the terrace surfaces is the surface expression of the Patea-Tongaporutu High, which is east of and parallels the Taranaki Fault Zone. The Tangahoe River is interpreted to overlie the Taranaki Fault Zone. This Zone marks the eastern boundary of the Taranaki Basin where uplift rates decrease to below 0.3 mm/yr. Stern et al. (1990) suggest that at this point broad bending of the lithosphere is induced by west-moving thrust sheets within the Taranaki Fault Zone and this is thought to be responsible for the increased uplift rates in this area. The uplift pattern displays the differential between the Patea-Tongaporutu High to the southeast and the Taranaki Basin to the west.