Photos taken and copyright by Dr Seabourne Rust, unless stated otherwise.
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| Koutu Boulders, October 2014 |
Saturday
Welcome!
Greetings, welcome to a work in progress, I will endeavour to investigate some of the intriguing concretionary rock formations of New Zealand, show a collection of images, and record aspects of their geological, historical and sociological context(s)...hope you find this site interesting!
Photos taken and copyright by Dr Seabourne Rust, unless stated otherwise.
Photos taken and copyright by Dr Seabourne Rust, unless stated otherwise.
A brief introduction to concretions...
Concretions are distinctive geological features that may be simply described as often (but not always) spherical deposits within fine-grained sedimentary rocks such as sandstone or mudstone. A concretion may take thousands to millions of years to form! They are formed below the sediment surface during diagenesis by localised precipitation of minerals from solution, around a nucleus - sometimes a fossil... more on this later. The nucleus is important, as this is where a chemical reaction starts to occur. Gradually certain elements in ground water solution, magnesium or iron for example, are incorporated into the mineral-binding cement, which is often carbonate, and over time there may be a change in the ratio of these elements. If ionic diffusion occurs evenly in the surrounding sediment, the concretion can form a spherical shape.
'Septarian' concretions contain an internal network of shrinkage cracks, which widen towards the interior and may be filled with crystals, commonly calcite, sometimes other minerals such as siderite barite or pyrite.
Concretions tend to stand out in rock outcrops due to their eye-catching shape(s) and erosion resistant nature - due to cementation they are often much harder than surrounding sediments. They weather out to become prominent features in the landscape... and although found worldwide, New Zealand has some particularly good examples!
'Septarian' concretions contain an internal network of shrinkage cracks, which widen towards the interior and may be filled with crystals, commonly calcite, sometimes other minerals such as siderite barite or pyrite.
Concretions tend to stand out in rock outcrops due to their eye-catching shape(s) and erosion resistant nature - due to cementation they are often much harder than surrounding sediments. They weather out to become prominent features in the landscape... and although found worldwide, New Zealand has some particularly good examples!
Friday
Koutu Boulders
A fascinating locality on the shore of Hokianga Harbour, Northland. Spherical concretions of Punakitere Sandstone (part of the Mangakahia Complex strata of the Northland Allochthon), Late Cretaceous age (approx 80 -65 million yers old) are eroding out of the hillside in a variety of sizes from centimetre scale up to some 5 metres in diameter!
A walk along the shoreline at low tide will provide many geological features to marvel at, plus native wildlife, as well as great views of the harbour and surrounding hills. Allow at least an hour to view the large boulders around Kauwhare Point.
Interestingly there is evidence that sulphides present in the seawater solution during deposition and diagenesis of the Punakitere sandstone and mudstone, reacted with barium ions or perhaps the shells of microfossils (e.g. diatoms or radiolaria), producing the heavy crystals of barium sulphate (barite), an important mineral during concretion formation. In the deep oceans today, Barite is also the only mineral so far reported to precipitate in the water column. Some of these reactions may have been bacterially mediated; the bacteria modify the immediate geochemical environment, inducing mineral formation. Siderite can also be an additional component in some of the Punakitere concretions.
A walk along the shoreline at low tide will provide many geological features to marvel at, plus native wildlife, as well as great views of the harbour and surrounding hills. Allow at least an hour to view the large boulders around Kauwhare Point.
Interestingly there is evidence that sulphides present in the seawater solution during deposition and diagenesis of the Punakitere sandstone and mudstone, reacted with barium ions or perhaps the shells of microfossils (e.g. diatoms or radiolaria), producing the heavy crystals of barium sulphate (barite), an important mineral during concretion formation. In the deep oceans today, Barite is also the only mineral so far reported to precipitate in the water column. Some of these reactions may have been bacterially mediated; the bacteria modify the immediate geochemical environment, inducing mineral formation. Siderite can also be an additional component in some of the Punakitere concretions.
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Exposed on the shore of Hokianga Harbour - note Lucy the Schnauzer for scale!   |
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| This concretion has split revealing a central fossil - in this case the large Cretaceous bivalve Inoceramus. |
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| In this piece of concretion are numerous fragments of Inoceramus in section showing typical prismatic shell structure. Sometimes other shelly fossils are found including ammonites. Fossil shark vertebrae and marine reptile (Mosasaur) remains were also recently described from Koutu (see Rust 2022 & 2014). Bones however are rare, I am especially interested if anyone has seen, found or collected any examples, send me a photo please! Email: seabourne.rust@gmail.com |
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| Natural art... |
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| Spot the geologists! |
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| An early photograph taken at Koutu by Daniel Mundy circa 1869, from the Auckland Art Gallery Toi o Tamaki collection.
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Thursday
Waimamaku Valley, South Hokianga
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| Taita Stream hiatus concretion, Punakitere Sandstone (Late Cretaceous) |
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The surface of this 'hiatus' concretion shows evidence of bioerosion and encrustation from a shallow marine environment, probably during Miocene times. Taita Stream, South Hokianga. |
Wednesday
Sunday
Kaipara district, Northland
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| near S.H. 14, Tangiteroria |
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...in the field - near Tangiteroria |
Friday
Aurere Beach, Northland
A beautiful day in Doubtless Bay and another Cretaceous Punakitere Sandstone (Mangakahia Complex) exposure with concretions!
Wednesday
Sunday
Thursday
Moeraki Boulders
Notable for their near-perfect spherical shape and abundance in a dramatic setting, the iconic Moeraki Boulders of the Hampden area of coastal North Otago are probably the best known, most photographed and talked about concretions in New Zealand. These have formed in mudstones of the Moeraki Formation that are Paleocene age (~55-60 million years old), and are eroding out of the cliffs to become exposed on the shoreline.
Detailed studies (see Boles et al 1985) have revealed that in composition, these concretions show a calcium enrichment- magnesium depletion with growth by diffusion (from core to edge). Larger boulders at Moeraki may have taken some 4 million years to form, however new studies being undertaken by a Japanese team suggest formation may have been much quicker! (see Yoshida et al. 2018).
*More images coming soon!
Moeraki Boulders in Maori mythology:
After an epic voyage across the Pacific, the great Maori migration waka (canoe) Arai-te-Uru finally overturned at Matakaea (Shag Point), where the cargo of eel pots, gourds, and kumera were turned to stone - forming the curious spherical and ovoid boulders seen along the coast.
Detailed studies (see Boles et al 1985) have revealed that in composition, these concretions show a calcium enrichment- magnesium depletion with growth by diffusion (from core to edge). Larger boulders at Moeraki may have taken some 4 million years to form, however new studies being undertaken by a Japanese team suggest formation may have been much quicker! (see Yoshida et al. 2018).
*More images coming soon!
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| Septarian veination with crystals of calcite |
Moeraki Boulders in Maori mythology:
After an epic voyage across the Pacific, the great Maori migration waka (canoe) Arai-te-Uru finally overturned at Matakaea (Shag Point), where the cargo of eel pots, gourds, and kumera were turned to stone - forming the curious spherical and ovoid boulders seen along the coast.
Sunday
Selected references, sites of interest & further reading
Boles, J.R., C.A. Landis, and P. Dale, 1985, The Moeraki Boulders; anatomy of some septarian concretions:, Journal of Sedimentary Petrology. v. 55, n. 3, pp. 398–406.
Hayward, B.W. TREASURED SPHERES - PROTECTING NEW ZEALAND’S HERITAGE OF SPHERICAL CONCRETIONS. Geoscience Society of New Zealand 12: 24-30.
Kastner, Miriam (1999). "Oceanic minerals: Their origin, nature of their environment, and significance". Proc. Natl. Acad. Sci. U.S.A. 96 (7): 3380–7
Moeraki Boulders website: www.moerakiboulders.com
Rust, S. 2022 Report on fossil marine reptile remains (Mosasauridae?) from Koutu Boulders, Hokianga, Northland. Geocene issue 30, p14-19, August 2022 (Auckland Geoclub magazine).
Rust, S. 2014, Late Cretaceous shark vertebrae (Chondrichthyes) from Koutu Boulders, Hokianga, Northland. p 2-4 in Geocene 11; accessed from www.gsnz.org.nz/information/auckland-i-46.html#mag or you can google the title and download this freely from Researchgate.
Rust, S., Lee, D.E., Conran, J.G., Hayward, B.W., 2019. An enigmatic concretion resembling a giant puzzling palm-like fruit from Northland. Geoscience Society of New Zealand Newsletter 28, 23-28
Rust, S. et al. 2019, An enigmatic concretion resembling a giant puzzling palm-like fruit from Northland.. Available from: https://www.researchgate.net/publication/334883350_An_enigmatic_concretion_resembling_a_giant_puzzling_palm-like_fruit_from_Northland Yoshida et al. (2018) Generalized conditions of spherical carbonate concretion formation around decaying organic matter during early diagenesis. Scientific Reports volume 8, Article number: 6308 (2018)
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| Watercolour painting by D. Yanakopulos, Moirai Studio. |
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