Tahunanui's shifting sands

01:56, Nov 14 2012
SIGNING ON: Names carved into the sandstone at the base of the graded bed at Magazine Point near Rocks Rd.
STANDING GUARD: Heritage bollards and chain top the concrete sea wall at Rocks Rd near Magazine Point.
DISCOVERY TIME: Exploring the rock pools at Magazine Point.
RIDGEWAY: People walk across the graded rock with fault lines at Magazine Point with Tahunanui Beach in the background.
ROCK POOLS: People stroll at low tide in front of Fifeshire Rock, formerly known as Arrow Rock and renamed in February 1842 after the Fifeshire went aground.
SCARRED FACE: Stabilisation work has been carried out on the cliffs above Rocks Rd.
ANCIENT SEABED: A person walks at low tide at Rocks Rd near Magazine Point on the terrestrial deposits of silt and stone layers from the port hills that are about 8 million years old.
STRIATED SHORE: Magazine Point formation of Oligocene (24 million years old) laid down seabed turbidite or graded beds. Some of the granodiorite boulders were carried along Boulder Bank from The Glen or Mackays Bluff.

Tahunanui Beach could one day lie at a different angle and adjacent rock formations buried if the sandspits building up each side of the Blind Channel collapse.

Nelson geotechnical consultant Mike Johnston said a confluence of Tasman Bay tidal currents and the effects of water flowing out from the Waimea Inlet made the area very dynamic, which in geological terms led to rapid change.

The beach which currently lies in a west-east direction could one day be aligned more north-east and become more of a shoaling beach if the sandspits gave way. Dr Johnston said that could happen suddenly, given the right conditions such as a large storm combined with a large tide which would lead to the sea breaking through the sandbacks slowly building each side of the Blind Channel.

Tahunanui beach
SANDS OF TIME: An aerial photo of Tahunanui Beach, showing erosion since 1948

He said predicting when they might collapse was a bit like predicting earthquakes: "The longer you go without one, the closer to the event they become."

The predicted build up of sand in the north-eastern corner of Tahunanui Beach could partially bury ancient rock formations at Magazine Point below Rocks Rd.

"That sand could be released relatively quickly and is likely to cover the rocks," Dr Johnston said.


Mike Johnston
ON THE ROCKS: Dr Mike Johnston, geologist measuring the trend of the rocks at Magazine Point.

He said any considerable shoaling in the area would mean that Tahunanui Beach would "not quite be the same beach it is today".

The shifting sands threaten to bury rock formations which reveal the geological history of this part of New Zealand from 23 million years ago.

"Most rock around Nelson is much older but this is the only remnant of this tertiary era.

"It gives us an insight into what was going on then. It's an important link in understanding what New Zealand was doing as it was getting near the end of a tectonically stable period, and the transition to another mountain-building period."

Dr Johnston said the Magazine Point rock formation was once sediment lying beneath deep water and had been uplifted during a time of violent upheaval.

He said earthquakes and volcanoes made New Zealand a "very dynamic place".

"If we didn't have these things we would be under water. It's these situations which give New Zealand its unique landscape of fiords, sounds, mountains and volcanoes, and its biodiversity."

Dr Johnston said challenges lay in attempts to prevent or alter the course of natural change.