Moeraki Boulders, Koekohe Beach, Otago Coast, New Zealand

Moeraki Boulders are concretions created by the cementation the Paleocene mudstone the Moeraki Formation, from which they have been exhumed by coastal erosion. Moeraki Boulders are concretions that were created by the precipitation calcite from pore waters within the Moeraki Formation. The spherical shape these concretions indicates that the source calcium was mass diffusion and not fluid flow. Studies the percentage magnesium and iron contained by and stable isotope composition the oxygen and carbon comprising the calcite cement and spar comprising the Moeraki Boulders demonstrates that the main body these concretions started forming in marine mud near the surface the Paleocene seafloor. The isotopic data are also argued to demonstrate that the reduction sulfate in saline pore fluids within the mudstone by bacteria caused the precipitation the calcite forming the Moeraki 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) 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.