Eminent sedimentary geologist Professor Cam Nelson from the University of Waikato, on site in the King Country, explains how Oligocene limestones express themselves in the New Zealand landscape. Limestones are often harder than the surrounding rocks and often stand out in a landscape, sometimes in big proud cliffs like the ones featured at this location.
PROFESSOR CAM NELSON
Well, this is a very typical expression through the Oligocene limestones that we see right throughout the King Country. Here, we are about 10 kilometres north of the Waitomo Caves, and the succession here is one in which the Waitomo caves are actually developed. The succession comprises two limestones here. We can see an uppermost one – probably about 25, 30 metres thick – exposed there, and we can see a slightly lower limestone cropping out, which is of the order of about 8 metres thick. The upper limestone’s called the Ōtorohanga limestone – that’s the one in which the caves are developed – and the lower limestone is the Orahiri limestone. They are separated by a shelf on which the trees and grass are showing at about 45°, and that’s a calcareous sandstone called the Waitomo sandstone, and that links those two limestones together.
As you move further away from this area in the King Country, the two limestones come together, and the Ōtorohanga limestone actually sits right on top of the Orahiri limestone. And then finally beneath the lower limestone is another calcareous sandstone unit at about 45° sloping down to the paddock here, and that’s called the Aotea Formation. So there are four units here of these Oligocene age rocks – the Aotea, Orahiri, Waitomo and Ōtorohanga formations, of which the Orahiri and the Ōtorohanga are the two main limestones.
Limestones tend to take on a lot of cement so that they become hard compared to the rocks that are about them and around them. So the sandstones here are a little softer – they’re not outcropping, they’re covered in vegetation – whereas the limestones are standing up vertical because they’re good hard rocks. So that’s one reason why you might expect them to stand out in a landscape, but here, the mere fact there is a cliff face here is very suspicious that perhaps there is a fault line running through here, through this location, which has up-thrown the limestones so that we now see them because of that up-throw. And indeed, if we’re to look across the road, we see no limestone, but I can tell you that if we were to drill down, you would find the limestone a couple of hundred metres or so below the road. And indeed there is a fault with a throw of around about 2 or 300 hundred metres here, which has up-thrown these limestones on this side, down-thrown on that side. So we have hard rocks that are up against softer rocks all around, and that eventually shows itself out as a cliff face that we see here.