Sedimentology and macropalentology of the Te Wharau formation, Te Wharau, South-east Wairarapa

Abstract

The Miocene Te Wharau Basin in East Wairarapa is one of a series of Neogene flysch basins within the North Island oblique subduction system. It forms part of the East Coast Deformed Belt situated between the Axial Ranges to the west and the Hikurangi Trough (subduction trench) to the east. The basin is bounded by highly deformed Upper Cretaceous to Lower Tertiary strata of theWainuioru Anticline to the west and the Kiawhata Anticline to the east. These anticlineswere active structural highs during flysch sedimentation of the Te Wharau sediments. Basin sediments consists of conglomerates in the east grading up to graded sandstones and siltstones (flysch) in the west. The flysch strata display modified Bouma divisions ofsedimentary structures and can be ascribed to the lithofacies D division of Mutti and Ricci-Lucchi (1972). The conglomerates were deposited by gravity debris flows and the sandstones by dilute turbidity currents while the siltstones are hemipelagic sediments deposited from suspension in a continental slope environment. The Te Wharau Basin in the East Coast Deformed Belt is interpreted as a small basin (about 20 - 30 km wide) which originated on the inner slope of the Hikurangi subduction trench. The flysch sediments were ponded between thrust ridges (structural highs) formed by imbricate thrust faulting and deformation of the leading edge of the upper plate. Volcanogenic glass shards within the sediments were probably derived from Miocene volcanoes inor near Coromandel Pennisula. Deformation is by compressive folding and thrust faulting, the faults dipping north-westward away from the Hikurangi Trough and becoming steeper landwards. Strike slip faults occur adjacent to the axial ranges.