Rapanui Stream

Grey sandstones and siltstones forming the cliffs and sea stack at Rapanui Beach were laid down in an ancient sea about 10 million years ago. Some of them appear in layers that are fairly flat or dip gently towards the sea. But other layers have no bedding and in some places have folded, squiggly patterns.

These disturbed layers were formed by an underwater landslide 10 million years ago. Just like slips and rock falls on land, any part of the seafloor that slopes can potentially fail and become an underwater landslide. When underwater landslides happen, sediments start slipping and sliding downslope to the deep ocean. Along the way they are subjected to strong forces which causes them to lose their original flat-bedding and become folded and squeezed into the sorts of patterns that we see at Rapanui Stream today.

The sea stack to the south of the stream has fairly flat layers at the bottom, but above these is a very thick layer of grey rock which has lots of interesting swirls, folds and clasts - big fragments of rock - within it.

The disturbed grey rock is overlain by a sharp and striking orange layer, known as the Rapanui Surface. This surface, and the orange rocks above it are much younger than the rocks that were part of the ancient underwater landslide. We think the Rapanui Surface was formed about 120,000 years ago by wave action, and has since been uplifted by tectonic forces to its present elevation.

You can also wander up the beach to the north of the stream. The same grey, deformed underwater landslide unit is seen here to, and you can spot fairly flat, undisturbed layers right below it. What amazing patterns and shapes can you spot in the underwater landslide? Take care not to get too close to the cliffs, as you will probably see rubble littering the base of the cliffs from recent slips.