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Moeraki Boulders, Koekohe Beach, Otago coast, New Zealand

Moeraki Boulders, Koekohe Beach, Otago Coast, New Zealand

The rock comprising the bulk a boulder is riddled with large cracks called "septaria" that radiate outward from a hollow core lined with scalenohedral calcite crystals. The process or processes that created septaria within Meoraki Boulders, and in other septarian concretions, remain an unresolved matter for which a number possible explanations have been proposed. These cracks radiate and thin outward from the centre the typical boulder and are typically filled with an outer (early stage) layer brown calcite and an inner (late stage) layer yellow calcite spar, which ten, but not always, completely fills the cracks. Rare Moeraki Boulders have a very thin innermost (latest stage) layer dolomite and quartz covering the yellow calcite spar.
The composition the Moeraki Boulders and the septaria that they contain are typical , ten virtually identical to, septarian concretions that have been found in exposures sedimentary rocks in New Zealand and elsewhere. Pearson and Nelson (2005, 2006) describe in detail the occurrence smaller but otherwise very similar septarian concretions within exposures sedimentary rocks elsewhere in New Zealand. Similar septarian concretions have been found in the Kimmeridge Clay and Oxford Clay England, and at many other locations worldwide.
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.

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