Stratigraphy, structural evolution, and tectonics of the northern part of the Tawhero Basin and adjacent areas, northern Wairarapa, North Island, New Zealand

Abstract

The study area lies within the forearc of the Hikurangi margin in northern Wairarapa, North Island, New Zealand. Two elements of the forearc are described: the Tawhero Basin (which lies west of the trench slope break), and the eastern part of the forearc ridge (which lies west of the Tawhero Basin).

In the Tawhero Basin the Neogene stratigraphic succession commences with the turbiditic Whakataki Formation, which is widespread in the east coast of Wairarapa. The overlying Coast Road Formation, Otaian to early Altonian in age, comprises Lower and Upper Mudstone Members separated by the Sandstone Member. In the Altonian, the turbiditic Greenhollows Formation was deposited, followed locally by deposition of the Tutaekuri Calcarenite. However, in the south, only the Takaritini Formation formed during the Altonian. The Settlement Road Formation, of late Altonian to early Lillburnian age, comprises Lower and Upper Mudstone Members, separated by the Sandstone Member. The formation largely crops out along the trend of the Waioakura Horst, which lies between the north‐trending Breakdown and Glencoe Faults. During the late Altonian to the early Tongaporutuan, the turbiditic Tanawa Formation was deposited in two sub‐basins which were separated by the Waioakura Horst. A local break in deposition during the late Waiauan was followed in the early Tongaporutuan by a marine transgression, during which the ng_t Sandstone Member of the Ngarata Formation was deposited. Subsequently, but also in the early Tongaporutuan, the turbiditic part of the Pakowhai Formation formed near Waihoki.

In the forearc ridge part of the study area, Late Cretaceous and Paleogene strata comprising the Whangai, Wanstead, and Weber Formation crop out near Pongaroa, and Early Cretaceous strata crop out near Tiraumea. Generally the oldest Neogene strata of the forearc ridge are the ngi Sandstone Member of the Ngarata Formation (although locally near Pongaroa the Coast Road and Tanawa Formations are present) followed, in the mid and late Tongaporutuan, by deposition of the ng₄ Mudstone Member. Turbidite deposition resumed during the Kapitean resulting in the Waihoki Formation. In the early Opoitian the north‐aggrading bathyal Saunders Siltstone was deposited. Tane

Sandstone followed in the late Opoitian, filling the eastern part of the Eketahuna Basin. Following a regional unconformity, the neritic Makuri Sandstone was followed in the late Pliocene and the early Pleistocene by the Lower and Upper Pori Limestones.

The Tawhero Basin was formed by processes related to the subduction of the underlying Pacific plate. Initially, in Waitakian and Otaian time, subduction was probably normal to the Hikurangi margin, whereas in late Altonian time and thereafter subduction was probably oblique, causing periods when there was dextral faulting on the bounding faults (especially in the Tongaporutuan), but the amount of dextral displacement on them is unknown. In the late Tongaporutuan the basin became amalgamated with the forearc ridge, and there was little deposition after Kapitean time. The forearc ridge is less deformed than the Tawhero Basin; it is cut by the Saunders Road Fault.

Keywords:

• Tawhero Basin
• forearc ridge
• forearc basin
• oblique subduction
• structure
• tectonics
• stratigraphy
• basin analysis