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.
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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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| I recently led a walk to the Koutu Boulders with the Hokianga Historical Society over Labour weekend (Oct 2014), all were welcome, the weather was fine and many came along to learn about the geology of the Punakitere Sandstone formations... |
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