GEOCHEMICAL CHARACTERIZATION OF NEAR AND FAR-FIELD TSUNAMI DEPOSITS: LITTLE PIGEON BAY, BANKS PENINSULA, NEW ZEALAND

Tsunami inundation events are often identifiable in the sediment record through distinct chemical signatures. Tsunamis can be near-field or far-field sourced events, but this can often be indistinguishable within the sedimentary record. In late 2016, Little Pigeon Bay, Banks Peninsula, New Zealand was exposed to and recorded inundation from the near-field Kaikoura tsunami. Recent studies of this site have since gained valuable information on the characteristics and impacts of this near-field event. Also recorded within Little Pigeon Bay are records of historic inundation events, sourced from far-field “South American” tsunamogenic sources. This study utilizes a PXRF unit to examine the geochemical characteristics of two cores from Little Pigeon Bay, surficial samples of the Kaikoura tsunami, and source sediments of the valley and offshore system to determine relationships and characteristics of near-field and far-field inundation events. The recent near-field event can be characterized by off-shore sediments incorporated with terrestrial material (soil and loess), along with higher Si concentrations, whereas far-field horizons have higher proportions of volcanics (sourced from shore platform), more abundant shell fragments, some off-shore sediments, and lower Si, K, and S concentration, and higher Ca, Fe, Sr. We postulate that far-field events derive shore-platform sediments into their inundation waves, and have limited intermixing with surrounding terrestrial sediments. Near-field events, with resulting weaker inundation, incorporate greater proportions of offshore derived sediments, and tend to have a greater ability to rework and incorporate surrounding sedimentary inputs into inundation deposits. Recognition of these subtle but discrete variances within tsunami deposits and the greater understanding of the deposition environments and source sediments can further our ability to differentiate near-field and far-field events within the geologic record, thus aiding our understanding of inundation in an area, an aspect critical in the formulation of effective mitigation strategies.