Professor Cam Nelson
The major limestone-forming period in New Zealand was the Oligocene, and in terms of absolute years, that’s roughly from about 30 million years ago through to 22 million years ago. Limestones were common right through New Zealand.
And so the limestones that we see here at Waitomo are much the same as those we see at Whangārei today, that we see in the West Coast of the South Island, the Pancake Rocks today, that we see around Oamaru, that we see in Southland – they’re all forming around this Oligocene time plus or minus a little bit.
And why is this so? It’s because New Zealand was effectively just a shallow platform with a sea, shallow sea covering it with little land exposed. There were islands and that here and there, but there wasn’t a lot of sand and mud pouring into the sea. And as a consequence, organisms could live happily in their zillions and die, get smashed up and contribute to carbonate sediment. And as that platform slowly subsided with time, so you built up limestone successions of the order of tens of metres to maybe a couple of hundred metres thick.
One other time in New Zealand’s geological history where limestones were very important was away back in the Ordovician, 450–500 million years ago. And Ordovician limestones are widespread in north-west Nelson, Takaka, Mount Arthur. There, the limestones, being so old, have often been cooked up by granite intrusions, and the limestones have sometimes changed over to marble, which is a metamorphic rock, but that’s another big block of limestone of much older age which is certainly important in New Zealand.
McDonald’s Lime Limited
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