Holocene sediments and vertical tectonic downwarping near Wairoa, Northern Hawke's Bay, New Zealand

Abstract

Depositional environments for Holocene terrestrial, estuarine, and marine sediments on the Wairoa River floodplain at Wairoa and coastal plain east of Wairoa, were determined from an analysis of stratigraphy, lithologies, and diatom assemblages obtained from auger holes and foundation and water-well drillholes. The sediments were deposited during the postglacial sea-level rise which started sometime prior to c. 9200 years ago and ended at c. 4000 years ago, when the shoreline was at least 2 km landward of the present position, and a ria-like morphology had developed at the innermost edge of the present-day coastal plain. The postglacial marine sediments usually occur slightly below the present sea level (down to -4 m a.m.s.l.) indicating net tectonic subsidence during the Holocene. Other evidence suggests that this tectonic subsidence may extend back into the middle Pleistocene.

Keywords:

• Wairoa
• Hawke's Bay
• C-14
• diatoms
• Holocene
• sedimentation
• late Quaternary
• tectonics
• change of sea level
• cores