Distribution and development of Middle Miocene submarine fans, Taranaki Basin, New Zealand

The Taranaki Basin was formed as a consequence of multiple geologic events. From the Cretaceous period until present, it went through rifted margin, passive margin, foreland basin, and back-arc phases. A dominantly sandy unit, the Moki Formation, was deposited during the Middle Miocene within the Taranaki Basin offshore the west coast of the North Island of New Zealand. The study area covers about 1600 km2 of the southern part of the north Taranaki graben, an area covered by a 3D seismic volume. The Moki Formation is interpreted as a basin floor fan deposit that accumulated during basinward migration of the shelf edge with supplied sediments sourced from the SSE.

Seismic profiles revealed that the mound-shape reflectors of Moki fan deposits situated between continuous reflectors of underlying Oligocene carbonates and hemipelagic muds of the overlying Manganui Formation. The reflections of the Moki sandy fan deposits locally grade laterally into interlobal deposits of hemipelagic muds. Correlation between wells Witiora-1, Taimana-1, and Arawa-1 verified the seismic interpretation, which shows an overall thickness variation of fan deposits that range from a greater thickness in the middle part of the sand lobe accumulation towards diminished thicknesses on the flanks. Gamma ray facies show clear progradation then aggradation motif that confirm the results from the seismic analyses. Depending on seismic attribute maps, paleochannels associated with the sand bodies sharing a SE to NW flow direction can be distinguished. Due to the volcanic activity in the eastern mobile belt, no paleochannels or significant stratigraphic features were recognized within the studied interval of the seismic data. Generally, in the study area, the fan deposits represent sand rich deposits that developed and prograded from south to north with variations in lateral extent driven by three major shifts in sediment pathways as the feeder channel orientations changed.