Moeraki Boulders, Koekohe Beach, Otago

“The Moeraki Boulders are unusually large and spherical boulders lying along a stretch of Koekohe Beach on the wave cut Otago coast of New Zealand between Moeraki and Hampden. They occur scattered either as isolated or clusters of boulders within a stretch of beach where they have been protected in a scientific reserve. The erosion by wave action of mudstone, comprising local bedrock and landslides, frequently exposes embedded isolated boulders. These boulders are grey-colored septarian concretions, which have been exhumed from the mudstone enclosing them and concentrated on the beach by coastal erosion.

Local Māori legends explained the boulders as the remains of eel baskets, calabashes, and kumara washed ashore from the wreck of an Arai-te-uru, a large sailing canoe. This legend tells of the rocky shoals that extend seaward from Shag Point as being the petrified hull of this wreck and a nearby rocky promontory as being the body of the canoe’s captain.

In 1848 W.B.D. Mantell sketched the beach and its boulders, more numerous than now. The picture is now in the Alexander Turnbull Library in Wellington. The boulders were described in 1850 colonial reports and numerous popular articles since that time. In more recent times they have become a popular tourist attraction, often described and pictured in numerous web pages and tourist guides.

Moeraki Boulders are concretions created by the cementation of the Paleocene mudstone of the Moeraki formation, from which they have been exhumed by coastal erosion. Moeraki Boulders are concretions that were created by the precipitation of calcite from pore waters within the Moeraki Formation. The spherical shape of these concretions indicates that the source of calcium was mass diffusion and not fluid flow.

Studies of the percentage of magnesium and iron contained by and stable isotope composition of the oxygen and carbon comprising the calcite cement and spar comprising the Moeraki Boulders demonstrates that the main body of these concretions started forming in marine mud near the surface of the Paleocene seafloor. The isotopic data are also argued to demonstrate that the reduction of sulfate in saline pore fluids within the mudstone by bacteria caused the precipitation of the calcite forming the boulders.

The larger, 2-metre (6-feet) in diameter, Moeraki Boulders are estimated to have taken 4 to 5.5 million years to grow while 10 to 50 metres (30 to 150 feet) of marine mud accumulated on the seafloor above them. After the concretions formed, large cracks, septaria, formed in them. Brown calcite, yellow calcite, and, in rare cases, dolomite and quartz progressively filled these cracks when a drop in sea level allowed fresh groundwater to flow through the mudstone enclosing them.”

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