Moeraki Boulders

The Moeraki Boulders are a number of unusually large spherical concretions at the Koekohe Beach on the coast of Otago on the South Island of New Zealand between Moeraki and Hampden. The gray colored septaria lie singly or in groups on the coast. The erosion of the one here on the coast Schluffsteins through the waves will periodically release additional balls.

According to legends of local Māori Boulders remains of Aalkörben, gourds and sweet potatoes that have been washed from the wreck of the legendary canoe Arai -te- uru at the coast. According to these legends are the cliffs that extend into the sea of ​​Shag Point, the fossilized remnant of the hull and a nearby rocky promontory of the captain's body. 1848 recorded W.B.D. Mantell the coast and then still more numerous stones. This image is in the Alexander Turnbull Library, Wellington. The Boulders were mentioned in 1850 reports of the colonial administration and were recently a tourist attraction.

Nature

The most important feature of the concretions is their unusual size and their spherical shape. The stones are divided in terms of their size significantly in two groups: about one-third has 0.5 to 1 meter in diameter, the remaining two thirds from 1.5 to 2.2 meters. The majority is almost perfectly spherical, a few parallel to the stratification of the Schluffsteins which once surrounded it, oval shaped.

Almost identical stones are found as " Koutu Boulders " on the beach, in the cliffs and inland below the surface of the coast of the Hokianga Harbour on the North Island between Koutu Point and Kauwhare Point. It can reach up to 3 m in diameter. Similar balls are known as " Katiki Boulders " also on the north-facing coast of Shag Point about 12 miles south of the Moeraki Boulders location. These are partially spherical, sometimes disc-shaped or oval and contain, in contrast to the Moeraki Boulders partially bone of Mosasaurieren and Plesiosaurieren. Similar large spherical concretions have been found elsewhere in the world. Thus, up to 3 m wide balls along the Cannonball River in Morton County and Sioux Countie were found in North Dakota. They reach on sandstone outcrops in the Frontier Formation in northeastern Utah and central Wyoming 4-6 m in diameter. Slightly weathered, up to 6 m wide balls are at Rock City in Ottawa County, Kansas. Smaller spherical concretions occur on the shore of Lake Huron on at Kettle Point, Ontario, where they are called " boiler " ( " kettles ").

Composition

Analyzes using optical methods, crystal structure analysis, electron microprobe analysis and found that the balls of mud, fine loam and clay exist, which are cemented together by calcite. The degree of calcination is from relatively small interior of the balls up to quite strong in the outer shell, which consists of 10 to 20% of calcite, clay and clay since it not only holds here, but it has partially replaced.

This bedrock is crisscrossed by large cracks or " septaria " that radiate from a lined with calcite crystals scalenohedral hollow interior. The process that has generated the septaria in the Moearaki Boulders and other similar concretions, is not yet fully understood. The cracks run radially outwardly and are thereby narrower. They usually come with a previously formed outer layer of brown calcite and one layer developed later and yellow calcite spar, which often fills the cracks fully lined. In a few of the Moeraki Boulders, a very thin inner layer of dolomite and quartz covered the yellow calcite spar.

The composition and septaria the Moeraki Boulders is typical of other septaria concretions found in outcrops of sedimentary rocks in New Zealand and elsewhere. Pearson and Nelson 2005 and 2006 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 around the world.

Origin

The Moeraki Boulders caused by cementation by calcite from pore water from Paleocene sediments from which they were later exposed by erosion. The spherical shape is that the spheres have been produced by diffusion of calcium and not by a flowing liquid. Studies on the magnesium and iron content and the isotopic distribution of oxygen and carbon in the calcite cement and the Spat showed that the body of the spheres formed in sea mud near the surface of Paleocene sea floor. The isotopic distribution was also used for the theory that the reduction of sulfate by bacteria in saline pore waters caused the precipitation of calcite, which led to the formation of Boulders result. The time of origin of the large 2 -meter Boulders is estimated to be a time of 4 to 5.5 million years, during which 10 to 50 meter of mud collected about them. After forming the balls, cracks, which are filled by a fall of sea level by ground water with calcite and rarely dolomite and quartz.