Moeraki Boulders

from Wikipedia, the free encyclopedia
Some of the Moeraki Boulders at sunrise.

The Moeraki Boulders are a number of unusually large spherical concretions on Koekohe Beach on the coast of Otago on the South Island of New Zealand between Moeraki and Hampden . The gray colored septaries lie individually or in groups on the coast. The erosion of the siltstone on the coast by the waves regularly exposes further spheres.

According to local Māori legends , the boulders are the remains of eel baskets , bottle gourds and sweet potatoes that were washed ashore from the wreck of the legendary Arai-te-uru canoe . According to these legends, the cliffs that extend into the sea from Shag Point , the fossilized remnant of the boat's hull, and a nearby ledge are the captain's body. In 1848 WBD Mantell drew the coast and the stones, which were more numerous at the time. This picture is in the Alexander Turnbull Library in Wellington . The boulders were mentioned in reports of the colonial administration as early as 1850 and have recently become a tourist attraction.

nature

A group of spherical boulders

The main characteristic of the concretions is their unusual size and spherical shape. The stones are clearly divided into two groups in terms of their size: about a third are 0.5 to 1 meter in diameter, the remaining two thirds 1.5 to 2.2 meters. The majority are almost perfectly spherical, a few oval shaped parallel to the layers of siltstone that once surrounded them.

Almost identical stones can be found as "Koutu Boulders" on the beach, in the cliffs and further inland under the surface of the earth on the coast of Hokianga Harbor on the North Island between Koutu Point and Kauwhare Point . They can reach up to 3 m in diameter. Similar balls are also known as "Katiki Boulders" on the north-facing coast of Shag Point about 12 miles south of the Moeraki Boulders location. These are partly spherical, partly disc-shaped or oval and, in contrast to the Moeraki Boulders, partly contain bones of mosasaurs and plesiosaurs . Similar large spherical concretions have also been found elsewhere in the world. Balls up to 3 m in size were found along the Cannonball River in Morton County and Sioux Countie in North Dakota . They reach 4 to 6 m in diameter at sandstone outcrops in the Frontier Formation in northeastern Utah and central Wyoming . Somewhat weathered balls up to 6 m in size are located near Rock City in Ottawa County in Kansas. Smaller spherical concretions occur on the shores of Lake Huron at Kettle Point in Ontario, where they are referred to as "kettles" ("kettles").

composition

A broken boulder

Analysis using optical methods, crystal structure analysis and electron beam microanalysis found that the spheres are made of mud, fine clay and clay cemented together by calcite . The degree of calcination ranges from relatively low inside the spheres to quite strong in the outer shell, which consists of 10 to 20% calcite, because here it not only holds loam and clay together, but has partially replaced them.

This bedrock is criss-crossed by large fissures or "septaries" that radiate from a hollow interior lined with scalenohedral calcite crystals. The process that created the septaries in the Moearaki Boulders and other similar concretions has not yet been conclusively clarified. The cracks lead radially outwards and become narrower in the process. They are usually lined with an earlier outer layer of brown calcite and a later layer of yellow calcite spar, which often completely fills the cracks. In a few of the Moeraki Boulders, a very thin inner layer of dolomite and quartz covers the yellow calcite spar.

The composition and septaries of Moeraki Boulders are typical of other septary concretions found in outcrops of sedimentary rocks in New Zealand and elsewhere. In 2005 and 2006, Pearson and Nelson described in detail the occurrence of smaller but otherwise identical concretions in New Zealand, but also in the Kimmeridge Clay and Oxford Clay in England and other places worldwide.

origin

The Moeraki Boulders were created by cementation by calcite from pore water from Paleocene sediments, from which they were later exposed by erosion. The spherical shape shows that the spheres were created by diffusion of the calcium and not by a flowing liquid. Investigations into the magnesium and iron content and the isotope distribution of oxygen and carbon in the calcite cement and the spat showed that the bodies of the spheres formed in sea mud near the surface of the Paleocene sea bed. The isotope distribution was also used for the theory that the reduction of sulfate by bacteria in salty pore water caused the precipitation of calcite, which resulted in the formation of boulders. The time of origin of the large 2-meter boulders is estimated to be 4 to 5.5 million years, during which 10 to 50 meters of mud collected over them. After the spheres had formed, cracks appeared, which, when the sea level fell, were filled with calcite and rarely with dolomite and quartz due to groundwater.

literature

  • Alexander Russell Mutch: Moeraki Boulders . In: Alexander Hare McLintock (Ed.): An Encyclopaedia of New Zealand . Wellington 1966 ( online [accessed December 15, 2015]).
  • D. Brunsden : Mystery of the Moeraki and Katiki boulders . In: Geographical Magazine . Vol. 41, No.11 , 1969, pp. 839-843 (English).
  • H. Klug, R. Zakrzewski: The Moeraki Boulders; Giant concretions on the beach on New Zealand's South Island . In: Writings of the Natural Science Association for Schleswig-Holstein . tape 56 , 1986, pp. 47-52 .
  • MJ Pearson, CS Nelson : Organic chemical signatures of New Zealand carbonate concretions and calcite fracture fills as potential fluid migration indicators . In: Crown Minerals Group (Ed.): 2006 New Zealand Petroleum Conference Proceedings . Auckland 2006 (English).

Web links

Commons : Moeraki Boulders  - Collection of images, videos and audio files
Wikivoyage: Moeraki  Travel Guide
  • History. Moeraki Boulders Limited, accessed November 25, 2015 .

Individual evidence

  1. ↑ a b c d e J. R. Boles, CA Landis, P. Dale : The Moeraki Boulders; anatomy of some septarian concretions . In: Journal of Sedimentary Petrology . Vol. 55, no. 3 , 1985, pp.  398-406 (English, online [accessed May 2, 2019]).
  2. ^ E. Fordyce, P. Maxwell : Canterbury 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 2003, ISBN 0-908678-97-5 (English).
  3. ↑ a b c d e f P. J. Forsyth, G. Coates : The Moeraki boulders (=  Information Series . No. 1). Institute of Geological & Nuclear Sciences , Lower Hutt 1992, ISBN 0-908678-97-5 (English).
  4. ↑ a b c d e G. D. Thyne, JR Boles : Isotopic evidence for origin of the Moeraki septarian concretions, New Zealand . In: Journal of Sedimentary Petrology . Vol. 59, No. 2 , 1989, pp.  272_279 (English, online [accessed May 2, 2019]).
  5. ^ A b C. Dann, N. Peat : Dunedin, North and South Otago (=  Miscellaneous Publication . 116B). GP Books , Wellington 1989, ISBN 0-477-01438-0 (English).
  6. ↑ Mutch: Moeraki Boulders . In: An Encyclopaedia of New Zealand . 1966.
  7. ↑ Koutu Boulders . hokianga.net.nz , archived from the original on October 14, 2008 ; accessed on May 2, 2019 (English, original website no longer available).
  8. ↑ IC Scotchman : The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England . In: Sedimentology . Volume 38 , 1991, pp.  79-106 (English).
  9. ↑ JD Hudson, ML Coleman, BA Barreiro, NTJ Hollingworth : Septarian Concretions from the Oxford Clay (Jurassic, England, UK): involvement of original marine and multiple external pore fluids . In: Sedimentology . Volume 48 , 1991, pp.  507-531 (English).
  10. ↑ Michael J. Pearson, Campbell S. Nelson: Organic geochemistry and stable isotope composition of New Zealand carbonate concretions and calcite fracture fills . In: Royal Society of New Zealand (Ed.): New Zealand Journal of Geology & Geophysics . Volume 48, Issue 3. Wellington 2005, pp. 395–414 (English, online [PDF; 13.5 MB ; accessed on February 1, 2016]).

Coordinates: 45 ° 20 ′ 43 "  S , 170 ° 49 ′ 33.8"  O