Abstract
Okains Bay, New Zealand, provides a sequence of 48 defined beach and 10 dune ridges through
a process of progradation with the deposition of sediment derived from the continental shelf
(Stephenson and Shulmeister, 1999). Investigation of past sea-level variation in this area
shows that the sequence of beach ridges dates approximately back to the beginning of the
Holocene when the infilling of the Bay began 7.5 Ka. The Canterbury coastline has not seen
significant sea-level work performed in the past, creating a major gap within New Zealand’s
Holocene sea-level history. Looking at past sea levels can further help comprehend the
impacts of localised sea-level variations in the present day and in the future. A transect cut
through the middle of Okains Bay over 1.7 km allows for accurate data collection. The use of
Ground Penetrating Radar (GPR) enabled a look into the subsurface using electromagnetic
reflective pulses. Radiocarbon dating (14C) from shell fragments and thermoluminescence
(TSL) dates from quartz grains are supplied from Stephenson and Shulmeister’s (1999) paper
which deliver further observations into the timing of events at Okains Bay. The results from
GPR show past sea-level changes through the prograding sequence of where the high and low
stands occurred, as well as providing an insight of erosional events through scraping and storm
surge of beach ridges. 14C dates are 672 ± 63 and 1674 ± 76 BP, and TSL dates are 7.5 ± 1.2
ka and 13.5 ± 1.6 ka BP respectively. However, these dates are not in chronological order
irrespective of dating methods, potentially due to the mixing of sediment, lag time between
death and deposition, or the re-exposure of grains to light. The primary data collected within
this research contain a fragmented history of localised sea-level on the Canterbury coastline,
aiding in covering an unstudied area within New Zealand's sea-level history. Sea levels dated
back to 7.5 ± 1.2 ka show that average sea levels were 1.2 m higher than present day.
Furthermore, sea levels varied between present day sea-level and 1 m in height between 7.5 ±
1.2 ka and present day. Such can ultimately contribute to providing a relevant framework for
understanding mid to late Holocene sea levels, which in turn, can help with future management
of coastal regions.