Gooch's Beach Concretions

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Spot the concretions hiding amongst the green. Photo/ K Pedley
Uplift of the shore around the Kaikōura Peninsula as a result of the November 16 2016 7.8 Kaikōura Earthquake has exposed (at low tide only) a number of fascinating spherical and semi-spherical rocks, similar in look to the famous Moeraki Boulders. They range in size from about 30 cm to over 2 m in diameter!
A particularly spherical concretion. Photo/ K Pedley
These spherical rocks are known as concretions. What are concretions??
Concretions can be made of a few types of minerals (e.g. iron oxide, silica, calcite) but these ones have been formed by dolomite (calcium magnesium carbonate) that formed over several million years after the mudstone they are formed in had been deposited and buried by overlying sediment. Concretions are fairly common in New Zealand marine mudstones. Concretions usually start around a piece of wood, fossil or a shell and grow radially outwards by the precipitation of mineral cement. Because concretions are much harder and denser than the surrounding mudstone, they tend to remain after the surrounding rock has been removed by erosion. This means that the boulders become concentrated in places along the shore.
These particular concretions were formed during the Late Cretaceous (>65 million years ago!) in the Herring Formation when New Zealand sediments were accumulating in the ocean off the coast of the super continent of Gondwana.
Growth rings and unusual shapes of an eroded concretion. Photo/ K Pedley
See if you can spot the "growth rings" of the dolomite cement inside a broken concretion! It will look like onion skin layers within the sphere. Some of the concretions have harder centres which will have eroded slower than the outside rings.

Take care on slippery algae and sea lettuce covered rocks. Only visit at low tide. Be aware of traffic when parking along the road side.

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Geological Age
Late Cretaceous
Zealandia Evolution Sequence
Haerenga Supergroup (Submergence): 85-35 million years ago
Stuff article by Jeffrey Kitt 27th August 2017 - "November earthquake uncovers 'dinosaur egg' boulders in Kaikoura":