DEATH OF A DROWNED FOREST: TSUNAMI RISK ON THE CONWAY COAST, SOUTH ISLAND, NEW ZEALAND

Earthquake and tsunami hazards are a serious concern for much of New Zealand’s coastal community. Conway Flats on the east coast of the South Island is directly adjacent to numerous active oblique-slip faults both on land and offshore putting the area at risk for seismic activity and tsunamis. The coastal region comprises a series of unusually well preserved wave-dominated Gilbert-style fan delta and estuary deposits uplifted in a series of Late Pleistocene-Holocene terraces. The youngest terrace contains estuarine deposits with numerous, well-preserved, in situ standing tree trunks rooted along a single conglomerate horizon dated as ~8000 years old by ¹⁴C (McConnico 2012). We examined the sediments surrounding this drowned paleo-forest to determine whether a tsunami may have been responsible for its death and burial.

Numerous in situ trees are exposed for 1 km along the beach at Conway Flats and are rooted in a single conglomerate horizon and buried by well-sorted estuarine siltstone containing abundant rootlets. The layer immediately overlying the conglomerate contains abundant woody plant debris and in one instance is ramped up against a tree indicating a seaward flow direction. Four samples were collected from this layer and searched for microfossils. Diatom specimens typically found in brackish-to-deep-sea envronments were found in all four samples, but only one deep sea foraminifer was found in one sample. ¹⁴C ages from individual trees (McConnico 2012; Ota et al. 1984) yielded no clear age pattern and have a spread of ~1000 years.

The paleo-forest trees are all rooted along the same sediment horizon indicating they died in a single event. There is insufficient evidence, however, to indicate that they died in a tsunami. There are few geologic proxies to indicate tsunami inundation, as sediments are well-sorted and indicate a seaward flow. Microfossil evidence was inconclusive with only a single preserved deep sea foraminifer. Sea level data from Clement et al. (2009) indicates that sea level rose rapidly from about 15 ka until about 7 ka, approximately when the Conway Flats trees were buried. Calculated uplift rates for the region of about 1.35 m/ka are much slower than sea level rise of about 10 m/ka suggesting the trees were gradually drowned by the rising sea instead of inundated by a tsunami.