Succession and structural interpretation of the Whangara-Waimata area, Gisborne, New Zealand

Abstract

In the Whangara-Waimata area, there are over 25,000 ft of geosynclinal Tertiary rocks: pale grey, streaky, calcareous mudstone and bentonite in the Lower Tertiary, and predominantly medium grey mudstone, siltstone, sandstone, and turbidites in the Upper Tertiary. The Upper Cretaceous is represented only by occasional fault-involved slivers of argillite and by blocks of Inoceramus-bearing, impure limestone. A break in deposition at the base of the Miocene and a local unconformity at the base of the Pliocene have been recognised. Post-Miocene to early Pliocene, sinistral, north-east-south-west transcurrent faults and southward-directed thrusts have been proved. They are believed to be the surface expression of sinistral transcurrent faults in the Cretaceous “basement”, the bentonitic Lower Tertiary, having acted as a zone of decollement.