Geology of the Akitio area (1:50 000 metric sheet U25BD, east), northeastern Wairarapa, New Zealand

Abstract

The Akitio area, located entirely within the east coast ranges, north Wairarapa, is largely composed of Cretaceous strata which overlie the westward‐subducting Pacific plate. The oldest strata, Pahaoa Group, composed of indurated sandstone and mudstone of Early Cretaceous (Albian) age, are in the central part of the mapped area within the fault‐bounded Owahanga Block. The Springhill Formation, which follows, is restricted to one small outcrop adjacent to a north‐trending fault.Younger Cretaceous (mid and Late Cretaceous) and Paleocene strata are mapped as Mangapurupuru Group (Kipihana and Te Mai Formations) and Tinui Group (Whangai and Waipawa Formations). These strata crop out largely in the southern and eastern parts of the sheet district. However, the Whangai Formation is also mapped within, and to the west of, the Tinui Fault Complex.The early Cenozoic Wanstead Formation is present locally in the eastern part of the area mapped, and in two diapirs which lie a short distance inland, and also within the Tinui Fault Complex. The highly calcareous Weber Formation is within, and to the west of, the Tinui Fault Complex, but is absent east of the complex. The early Miocene Whakataki Formation, largely composed of Bouma sequences, is present along a northeast‐trending narrow belt parallel to the shore, and as rafts within the Mt Cadmus Diapir, and in a small area west of the diapir. Strata of early‐mid Miocene age crop out west of the Tinui Fault Complex, but marine strata younger than middle Miocene are absent from the sheet district.The most important structural element of the Akitio area is the Tinui Fault Complex in the west. The Tinui Fault, and those faults further to the west, is usually almost vertical, whereas faults to the east of the fault commonly dip c. 73° to the west. Two, low‐angle westward‐dipping thrusts are mapped near Akitio Station. Diapirs, caused partly by intense compression of the incompetent bentonitic Wanstead Formation, are mapped near the coast and within the Tinui Fault Complex near Mt Cadmus.