Abstract: Stratigraphy of a Mid-Pliocene Shelf-Slope Succession, Wanganui Basin, New Zealand

The Wanganui Basin formed during the Plio-Pleistocene within the Australia-Pacific plate boundary zone, and contains a 4-km-thick succession of shelf to upper slope mixed terrigenous-carbonate sediments. This study has concentrated on 700 m of mid-Pliocene section exposed in the Mangawhero River valley, in western Wanganui Basin. The lower 300 m comprise the Matemateonga Formation and the overlying 400 m the Tangahoe Formation. The Matemateonga Formation is made up of sedimentary cycles, each comprising a lower coquina shell bed, abruptly overlain by a massive siltstone which grades into well sorted sandstone. The shell beds result from sediment starvation on a continental shelf and overlying transgressive surfaces of erosion. They correspond to Transgressive Systems Tracts, and regionally show onlap. The siltstone units downlap onto the shell beds and comprise Highstand Systems Tracts. The sharp transition from the Matemateonga to Tangahoe Formation is marked by a glauconitic siltstone and resulted from a rapid deepening of the basin. This was driven tectonically and involved a bathymetric deepening of 300 m based on foraminiferal paleoecology. The slope depth siltstone of the Tangahoe Formation is interspersed with regular packets of mass-emplaced (fluidised flow) sandstones. The sandstone bodies are a very well sorted fine sand, which exhibit a lenticular geometry, with erosional bases and contain rip-up clasts, and are considered to be possible reservoir beds. 

AAPG Search and Discovery Article #90940©1997 AAPG Foundation Grants-in-Aid