Moeraki Boulders

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The Moeraki Boulders at sunrise.
The Moeraki Boulders at sunrise.

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, which located at 45°20′42.99″S, 170°49′33.82″EGR1. 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. [1] [2] [3] [4]

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 tell 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. [5] 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. [5] [3] [6]

A boulder, showing the hollow interior
A boulder, showing the hollow interior

Contents

[edit] Physical Character

The most striking aspect of the boulders is their unusually large size and highly spherical shape, with a distinct bimodal size distribution. About one-third of the boulders range in size from about 0.5 to 1.0 metres (1.5 to 3 ft) in diameter, the other two-thirds from 1.5 to 2.2 metres (4.6 to 6.7 ft), the majority being nearly to almost perfectly spherical. A minority of them are not spherical, being slightly elongated parallel to the bedding of the mudstone that once enclosed them. [1] [3] [4]

Neither the spherical to subspherical shape or large size of the Moeraki Boulders is unique to them. Virtually identical spherical boulders, called "Koutu Boulders", are found on the beaches, in the cliffs, and beneath the surface inland of the shore of Hokianga Harbour, North Island, New Zealand, between Koutu and Kauwhare points. Like the Moeraki Boulders, nearly perfectly spherical Koutu Boulders are as large as 3 metres (9 ft) in diameter.

Similar boulder-size concretions, known as "Katiki Boulders", are found on the north-facing shore of Shag Point some 12 miles south of where the Moeraki Boulders are found. These concretions occur as both spherical cannonball concretions and flat, disk-shaped or oval concretions. Unlike the Moeraki boulders, some of these concretions contain the bones of mosasaurs and plesiosaurs. [3]

Large spherical concretions, similar in size and shape to the Moeraki Boulders have been found elsewhere in the world. For example, large spherical concretions as large as 3 metres (10 feet) in diameter are along the Cannonball River within Morton and Sioux Counties, North Dakota. Large spherical concretions as much as 4 to 6 metres (12 to 18 feet) in diameter occur within sandstone outcrops of the Frontier Formation in northeast Utah and central Wyoming. Similar somewhat weathered and eroded giant spheroidal concretions, as large as 6 metres (18 feet) in diameter, are at Rock City in Ottawa County, Kansas. Smaller spherical concretions are found on the shore of Lake Huron near Kettle Point, Ontario, where they are known as "kettles".

[edit] Composition

As determined by detailed analysis of the fine-grained rock using optical mineralogy, X-ray crystallography, and electron microprobe, the boulders consist of mud, fine silt and clay, cemented by calcite. The degree of cementation varies from being relatively weak within the interior of a boulder to quite hard within its outside rim. The outside rims of the larger boulders consist of much as 10 to 20% calcite, because the calcite not only tightly cements the silt and clay but has also replaced it to a significant degree. [1] [4]

The rock comprising the bulk of 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 of possible explanations have been proposed. These cracks radiate and thin outward from the centre of the typical boulder and are typically filled with an outer (early stage) layer of brown calcite and an inner (late stage) layer of yellow calcite spar, which often, but not always, completely fills the cracks. Rare Moeraki Boulders have a very thin innermost (latest stage) layer of dolomite and quartz covering the yellow calcite spar. [1] [3] [4]

The composition of the Moeraki Boulders and the septaria that they contain are typical of, often virtually identical to, septarian concretions that have been found in exposures of sedimentary rocks in New Zealand and elsewhere. Pearson and Nelson (2005, 2006) describe in detail the occurrence of smaller but otherwise very similar septarian concretions within exposures of sedimentary rocks elsewhere in New Zealand. Similar septarian concretions have been found in the Kimmeridge Clay and Oxford Clay of England, and at many other locations worldwide. [7] [8]

[edit] Origin

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 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) 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. [1] [3] [9] [4]

[edit] References

  1. ^ a b c d e Boles, J.R., C.A. Landis, and P. Dale, 1985, The Moeraki Boulders; anatomy of some septarian concretions, Journal of Sedimentary Petrology, vol. 55, n. 3, p. 398-406.
  2. ^ Fordyce, E., and P. Maxwell, 2003, Cantebury Basin Paleontology and Stratigraphy, Geological Society of New Zealand Annual Field Conference 2003 Field Trip 8, Miscellaneous Publication 116B, Geological Society of New Zealand, Dunedin, New Zealand. ISBN 0-908678-97-5
  3. ^ a b c d e f Forsyth, P.J., and G. Coates, 1992, The Moeraki boulders. Institute of Geological & Nuclear Sciences, Information Series no. 1, (Lower Hutt, New Zealand)
  4. ^ a b c d e Thyne, G.D., and J.R. Boles, 1989, Isotopic evidence for origin of the Moeraki septarian concretions, New Zealand, Journal of Sedimentary Petrology. v. 59, n. 2, p. 272-279.
  5. ^ a b Dann, C. and N. Peat, 1989, Dunedin, North and South Otago. GP Books. Wellington, New Zealand. ISBN 0-477-01438-0.
  6. ^ Mutch, A. R., 1966, Moeraki Boulders in A.H. McLintock, ed., An Encyclopaedia of New Zealand. Government Printer, Wellington, New Zealand.
  7. ^ Scotchman, I.C., 1991, The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England, Sedimentology. v. 38, p. 79-106.
  8. ^ Hudson, J.D., M.L. Coleman, B.A. Barreiro and N.T.J. Hollingworth, 2001, Septarian concretions from the Oxford Clay (Jurassic, England, UK): involvement of original marine and multiple external pore fluids, Sedimentology. v. 48, p. 507-531.
  9. ^ Pearson, M.J., and C.S. Nelson, 2005, Organic geochemistry and stable isotope composition of New Zealand carbonate concretions and calcite fracture fills, New Zealand Journal of Geology & Geophysics. v. 48, p. 395-414.

[edit] Additional Reading

  • Brunsden, D., 1969, Mystery of the Moeraki and Katiki boulders. Geographical Magazine. v. 41, n. 11, pp. 839-843.
  • Klug, H., and R. Zakrzewski, R., 1986, Die Moeraki Boulders; Riesenkonkretionen am Strand auf Neuseelands Suedinsel (The Moeraki boulders; giant concretions of the beach of New Zealand's South Island) Schriften des Naturwissenschaftlichen Vereins fuer Schleswig-Holstein. v. 56, pp. 47-52
  • Pearson, M.J., and C.S. Nelson, 2006, Organic chemical signatures of New Zealand carbonate concretions and calcite fracture fills as potential fluid migration indicators. in 2006 New Zealand Petroleum Conference Proceedings, Crown Minerals Group, Auckland, New Zealand.

[edit] See Also

[edit] External links

Coordinates: 45°20′42.99 S°170′49

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