Moeraki Boulders

From Wikipedia, the free encyclopedia

You have new messages (last change).
The Moeraki Boulders at sunrise.
Enlarge
The Moeraki Boulders at sunrise.

Moeraki Boulder is the local name for unusually large and spherical boulders lying along a stretch of Koekohe Beach on the wave cut Otago coast of New Zealand near Moeraki, South Island. They occur scattered either as isolated or clusters of boulders within a stretch of beach between the towns of Moeraki and Hampden, where they have been protected in a scientific preserve. Also, the erosion of mudstone, comprising local bedrock and landslides, by wave action, frequently expose isolated Moeraki Boulders embedded in it. These boulders are grey-colored septarian concretions, which have been exhumed from the mudstone enclosing them and concentrated on the beach by coastal erosion (Bole and others 1985, Fordyce and Maxwell 2003, Thyne and Boles 1989).

Moeraki Boulders have been the subject of attention starting in prehistoric times. Local Māori legends explained the Moeraki Boulders as the remains of eel baskets, calabashes, and kumaras washed ashore from the wreck of an Arai-te-uru, a large sailing canoe. This legend tell of the rocky shoals, which extend seaward from Shag Point, as being the petrified hull of this wreck and a nearby rocky premonitory as being the body of the canoe's captain. Later, 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 (Dann and Peat 1989). Moeraki Boulders were also described in 1850 colonial reports and numerous popular articles since that time. In more recent times, Moeraki Boulders have become a popular tourist attraction, often described and pictured in numerous web pages and tourist guides (Dan and Peat 1989, Mutch 1966).

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

Contents

[edit] Physical Character

The most striking aspect of the Moeraki Boulders is their unusually large size and highly spherical shape. These boulders currently found on the beaches near Moeraki exhibit a distinct bimodal size distribution. About one-third of the Moeraki Boulders range in size from about 0.5 to 1.0 meters (1.5 to 3 ft) in diameter. The other two-thirds of these boulders range in size from 1.5 to 2.2 meters (4.6 to 6.7 ft). The majority of these boulders are nearly to almost perfectly spherical. A minority of them are not spherical being slightly elongated parallel to the bedding of the mudstone, which once enclosed them (Bole and others 1985, Thyne and Boles 1989).

Neither the spherical to subspherical shape or large size of the Moeraki boulders is unique to them. For example, virtually identical spherical boulders, which are 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 2 meters (6 ft) in diameter. Similar spherical boulders, locally known as "Katiki Boulders", are also found on the north-facing shore of Shag Point some 12 miles south of where the Moeraki Boulders are found.

Large spherical concretions, similar in size and shape to the Moeraki Boulders have been found elsewhere in the world. For example, large spherical concretions, which are as large as 3 meters (10 feet) in diameter, are found along the Cannonball River within Morton and Sioux Counties, North Dakota. Large spherical concretions, which are as much as 4 to 6 meters (12 to 18 feet) in diameter, also 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 meters (18 feet) in diameter, also can be found 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 locally known as "kettles", and near the town of Mecevici and village of Ozimici, Bosnia-Herzegovina.

[edit] Composition

As determined by detailed analysis of the fine-grained rock composing the Moeraki Boulders using optical mineralogy, X-ray crystallography, and electron microprobe, Moeraki 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 Moeraki Boulders consist of much as 10 to 20 percent calcite because the calcite not only tightly cements the silt and clay but also has replaced it to a significant degree (Bole and others 1985, Thyne and Boles 1989).

The rock comprising the bulk of a Moeraki Boulder is riddled with large cracks, called "septaria" that radiate outward from a hollow core lined with scalenohedral calcite crystals. The process or processes, which created septaria within Meoraki Boulders, as in case of other septarian concretions in general, remains a unresolved matter for which a number of possible explanations have been proposed. These cracks radiate and thin outward from the center 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 (Bole and others 1985, Thyne and Boles 1989).

The composition of the Moeraki Boulders and the septaria, which they contain, are typical of, often virtually identical to, septarian concretions, which have been found in exposures of sedimentary rocks in New Zealand and elsewhere in the world. 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 of England, in the Oxford Clay of England, and at innumerable other locations worldwide (Scotchman 1991, Hudson and others 2001).

[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 is 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-meter (6-feet) in diameter, Moeraki Boulders are estimated to have taken 4 to 5.5 million years to grow while 10 to 50 meters (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 (Bole and others 1985, Thyne and Boles 1989, Pearson and Nelson 2005).

[edit] References Cited

  • Dann, C. and N. Peat, 1989, Dunedin, North and South Otago. GP Books. Wellington, New Zealand. ISBN 0-477-01438-0.
  • 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
  • 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.
  • Mutch, A. R., 1966, Moeraki Boulders in A.H. McLintock, ed., An Encyclopaedia of New Zealand. Government Printer, Wellington, New Zealand.
  • 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.
  • Scotchman, I.C., 1991, The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England, Sedimentology. v. 38, p. 79-106.

[edit] Additional Reading

  • Brunsden, D., 1969, Mystery of the Moeraki and Katiki boulders. Geographical Magazine. v. 41, n. 11, pp. 839-843.
  • Forsyth, P.J., and G. Coates, 1992, The Moeraki boulders. Institute of Geological & Nuclear Sciences, Information Series no. 1, (Lower Hutt, New Zealand)
  • 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

[edit] See Also

[edit] External links

  • Hokianga Tourism Association, nd, Koutu boulders Nice pictures of New Zealand concretions identical to Moeraki Boulders in origin and size.
  • Kitchens, O., nd, Moeraki Boulders a Photogallery of Pictures of Moeraki Boulders.
Retrieved from "http://en.wikipedia.org../../../m/o/e/Moeraki_Boulders_ecb7.html"
In other languages