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Accessibility: EASY
WHAT'S HERE?
One of the large areas of pebbles in the dunes. Person for scale.
Occurrence of a sheet of pebbles stretching between 3 and 14 m above sea level in sand dunes behind Whangapoua Beach, Great Barrier. The varied rock types present are consistent with their having been picked up off the sea floor between Rakitu Island and Haratounga, about 5 km away.
LEARN ABOUT IT
The sheet of pebbles extend to 14 m above high tide level in the dunes.
The best way to unequivocally prove that a sand or gravel deposit has been carried in by a tsunami is to see an extensive sheet located on land well above the elevation that the highest waves in a king tide storm could reach. This is the case at Whangapoua sand dunes on Great Barrier Island. Here a sheet of pebble gravel is exposed in a number of large deflation hollows over a distance of at least 1 km behind the beach and extending to the top of the dunes at 14 m above high tide level.
The rock types present in the pebbles are dominantly andesite which could be sourced from many parts of nearby Great Barrier Island. Also present are diorite and greywacke pebbles that are sourced from the basement rocks around Haratounga or to the north and diorite from 20 million year old dikes that intrude the greywackes in these areas. Also present in smaller numbers are silicified rhyolitic tuff and spheroidal rhyolite pebbles that can only be sourced from Rakitu Island 4-5 km away offshore.
This combination of rock types is only known to occur in the seafloor area between Rakitu Island and Haratounga on Great Barrier Island at a depth of about 30 m below sea level. This is the likely route of any tsunami reaching the beach. Whangapoua Beach and the shallows offshore from it are made of sand and thus the pebbles were probably picked up from the seafloor by the passing tsunami and carried 4-5 km westward before they were swept up and dumped in the dunes.
At the present time the best estimate for the age of this tsunami is about 600 years ago and possibly sourced from eruption of Mt Healy submarine volcano on the Kermadec Volcanic Arc out to the east.
The rock types present in the pebbles are dominantly andesite which could be sourced from many parts of nearby Great Barrier Island. Also present are diorite and greywacke pebbles that are sourced from the basement rocks around Haratounga or to the north and diorite from 20 million year old dikes that intrude the greywackes in these areas. Also present in smaller numbers are silicified rhyolitic tuff and spheroidal rhyolite pebbles that can only be sourced from Rakitu Island 4-5 km away offshore.
This combination of rock types is only known to occur in the seafloor area between Rakitu Island and Haratounga on Great Barrier Island at a depth of about 30 m below sea level. This is the likely route of any tsunami reaching the beach. Whangapoua Beach and the shallows offshore from it are made of sand and thus the pebbles were probably picked up from the seafloor by the passing tsunami and carried 4-5 km westward before they were swept up and dumped in the dunes.
At the present time the best estimate for the age of this tsunami is about 600 years ago and possibly sourced from eruption of Mt Healy submarine volcano on the Kermadec Volcanic Arc out to the east.
HAVE A LOOK!
Sheet of pebbles in Whangapoua dunes, inferred deposited by a tsunami.
Are the pebbles angular or rounded - what does this tell us about the origin of the pebbles?
Look for light-coloured fine-grained rocks - these are likely silicified rhyolitic tuff or sinter from Rakitu Island.
Can you see any seashells with the pebble deposit - do they occur all the way up to the top or only in the lower parts? How did they get there?
Can you see any brown garden snail shells. These were introduced to New Zealand by Europeans. How did they get in with the pebbles do you think?
Look for light-coloured fine-grained rocks - these are likely silicified rhyolitic tuff or sinter from Rakitu Island.
Can you see any seashells with the pebble deposit - do they occur all the way up to the top or only in the lower parts? How did they get there?
Can you see any brown garden snail shells. These were introduced to New Zealand by Europeans. How did they get in with the pebbles do you think?
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Directions/Advisory
Take road to Whangapoua Beach from turn off at Okiwi. Continue on to the carpark in the grass at the end of the road.
It is unlawful to disturb archaeological sites, this includes the shell midden and oven stones in the sand dunes at Whangapoua.
Google Directions
Accessibility:
EASY
Easy walk on track through sand dunes to beach and then several hundred metres south down beach and then head back into the sand dunes to see the pebble gravel sheet in a number of places.
Features
Sedimentary
Geological Age
About 600 years ago.
Zealandia Evolution Sequence
Pākihi Supergoup: 5 million years ago – present
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