Limestone secrets revealed

Transcript

Limestone defined

Professor Cam Nelson

Limestone is sedimentary rock, so it must have formed at the Earth’s surface.

It starts off as sediment, and it must have more than 50% calcium carbonate in it to qualify as limestone. Rock such as mudstone or sandstone – where the grains have come down rivers and been delivered to the sea – they can have a highish calcium carbonate content perhaps as well, but less than 50%, and so we could call those calcareous mudstones or calcareous sandstones to indicate that carbonate content.

So limestone specifically must have more than 50% calcium carbonate, and the little grains that give you that calcium carbonate are typically the smashed up shell remains of a whole variety of organisms. These can be molluscs, gastropods, echinoderms, bryozoans, brachiopods, worm tubes – most invertebrate animals secrete some type of skeleton, and very often that’s a calcium carbonate skeleton.

So in New Zealand, they are the main contributors to the formation of carbonate sediments, which, once they are changed into rock, become limestones.

Acknowledgements: McDonald’s Lime Limited Dr Roger Grace National Institute of Water and Atmospheric Research (NIWA) Ken-ichi Ueda

Limestone diagenesis

Professor Cam Nelson

Diagenesis is the hardening of loose sediment into sedimentary rock, so in the case of carbonate sediments – skeletons that make up carbonate sediments – they go from being loose into a hard rock which is a limestone.

In this particular sample from the quarry here of this Ōtorohanga limestone, we can see it’s thoroughly cemented and hard. It has undergone diagenesis. If I get a hand lens on this and have a look at it, it’s very difficult to see what it’s made up of.

But I can show you what it’s made up of. This is a pottle of sediment taken off the seafloor in the vicinity of Three Kings Island to the north of New Zealand, and that is about 98% calcium carbonate. It will become a limestone one day, but right now, it’s a loose sediment made up of bryozoans mainly – there are also some bivalves and other things here, but dominantly bryozoans. And when we cut a thin section and look at this hard rock under the microscope, you see exactly that.

This is zero years old. It was dredged just a few years ago, so it’s modern. This is 25 million years ago, and the big difference is hardness. That’s diagenesis. How do we go from this to this? By simply burying the sediment, and as it’s buried, these little grains of calcium carbonate are in contact with one another and they get pressed into each other and calcium carbonate dissolves under pressure.

So at the points of contact, you actually dissolve the calcium carbonate. It goes into the surrounding pore spaces where it’s no longer under pressure and it can reprecipitate as a cement, as a calcium carbonate cement. And that cement is what ultimately creates a very tough tight hard rock like this. That process is called pressure dissolution – pressure dissolving the carbonate – and the pressure dissolution kicks in typically at about 100 metres of burial. So when the sediment is down below, about 100 metres below the seafloor buried by other carbonate sediment, it’s starting to dissolve and starting to form a cement in the rock. And by about a kilometre of burial, you’ll end up with a rock looking like this.

Acknowledgements: McDonald’s Lime Limited

Limestone characteristics

Dr Steve Hood

So here we have a lovely boulder of hard well cemented Ōtorohanga limestone of latest Oligocene, earliest Miocene age. I just want to point out some of the key characteristics.

Clearly, this is dominated by burial diagenesis and burial cementation. It’s extremely hard, well indurated. There’s no visible porosity in the rock. That’s all been plugged up during pressure dissolution – burial of skeletons, skeletons pressure dissolving into each other and then that material going to produce or precipitate a cement.

A key characteristic of these limestones is the flaggy appearance, and the flags here are separated by thinner less carbonate-pure seams. Now we envisage that the amount of time that’s encapsulated in one of these flags is perhaps as much as you know 500, 1000 years.

So one of the key characteristics of the carbonate rocks or the carbonate minerals, so that’s limestones in particular, is they fizz in association with the application of acid. If I take a piece of Ōtorohanga limestone, hard indurated non-porous limestone, what I’m going to do is pop it in a glass beaker here, and if I take some 2 molar hydrochloric acid and pour that on the rock, we will see that we get quite vigorous fizzing there.

So we see with the addition of the hydrochloric acid that the calcium carbonate – the bonds are broken – we’re liberating CO2 to the atmosphere producing water and calcium chloride.

Acknowledgements: McDonald’s Lime Limited

Limestone history

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.

Acknowledgements: McDonald’s Lime Limited Certain photos in this video are the copyrighted property of 123RF Limited, their Contributors or Licensed Partners and are being used with permission under license. These images and/or photos may not be copied or downloaded without permission from 123RF Limited Dédélagodasse Bevan Thrower Dave Schumaker Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic Andrew Purdam Creative Commons Attribution-NonCommercial-NoDerivs 2.0 Generic

Modern limestone formation

Professor Cam Nelson

In New Zealand, we can say the heyday of limestone formation was in the Oligocene. That doesn’t mean to say that we don’t have some limestone in the making around New Zealand at the modern day. We certainly don’t have it off the eastern and western sides of New Zealand in general, because there’s too much sand and mud being poured into the seas, the shallow seas, from the rugged countries, the mountains that we have forming a backbone of North Island and South Island.

But away to the north around Three Kings, and away to the south around Stewart Island and Snares Island, the shallow seafloor is covered in smashed up shelly material whose composition is exactly the same as we find in Oligocene limestones. The only difference is they’re loose and modern, forming today and are slowly accumulating, whereas Oligocene limestones have undergone diagenesis – they’ve been converted from loose sediments into hard rock as a consequence of burial.

Acknowledgements: McDonald’s Lime Limited Dr Roger Grace

Uses of limestone

Professor Cam Nelson

Scientifically, you know, limestone is important because it contains many fossils, and those fossils can be used to date the rock so that the geological period of time in which the limestone’s formed can be determined. The same fossils can tell us a lot about the environment in which the limestone formed.

There’s an economic importance hugely tied to limestones. Limestones, being built up largely of calcium carbonate, calcium carbonate has so many uses – let’s just touch on a few of these. New Zealand agriculture, farming, so very important, but many of our soils are a little acid and you have to decrease that acidity. The addition of lime to pastures has been absolutely fundamental in New Zealand’s sort of agricultural position. And of course that comes from the crushing of high-grade limestone to very fine sizes and the spreading of that onto pastures.

We all know how important concrete is in this day and age, and concrete to harden needs cement, and cement is made by mixing limestone with clay minerals. So the manufacture of concrete through cement is important.

We can move into petroleum. The world’s largest petroleum and gas fields are contained in limestones, and this is because limestones can have lots of holes in them, and any gas and petroleum that’s arising from deep down can actually end up being caught up in these holes. So the Middle East, for example, most of the hydrocarbons, the petroleum that is there, you are drilling into limestones to capture those hydrocarbons. And the same is true for some New Zealand limestones – they are known to contain petroleum.

There’s also the tourism aspect, caves, of holidays on a coral atoll, coral reef, of abseiling in countries with big steep cliffs which limestones often form. So there’s a huge tourist industry which is based effectively on limestones.

And then we could name other industrial applications that limestone gets into, you know, things that range from steel making to toothpaste to glass making – it goes on and on and on.

Acknowledgements: McDonald’s Lime Limited Certain photos in this video are the copyrighted property of 123RF Limited, their Contributors or Licensed Partners and are being used with permission under license. These images and/or photos may not be copied or downloaded without permission from 123RF Limited Lloyd Homer, GNS Science Waitomo Caves Discovery Centre

A typical limestone quarry

Professor Cam Nelson

Here we’re in the McDonald lime quarry, one of the largest limestone quarries in New Zealand, which is situated near Te Kūiti. The lime that’s being quarried here is of high grade, better than 85 and certainly mainly better than 95% calcium carbonate, which is why the quarry is here.

There’s a very large reserve, and in particular, we’re quarrying the Ōtorohanga limestone, which is typically a very pure limestone everywhere. The Ōtorohanga limestone thickness can be anywhere between 50 metres and perhaps as much as 200 metres, and this quarry I suspect is getting closer to that latter value of about 200 metres. So there’s a huge lime resource here of high-grade, high-quality calcium carbonate.

Acknowledgements: McDonald’s Lime Limited

Limestone and oil

Dr Steve Hood

Well the Tikorangi Formation is a very important limestone rock in Taranaki Basin. It’s got similarities to what we’re seeing behind us in the quarry face here, which is the Ōtorohanga limestone. The difference with the Tikorangi is that it’s currently sitting 3 kilometres below the surface, to the east of Stratford.

This Tikorangi is an oil reservoir rock. This is a section of the Tikorangi Formation. They’ve drilled down and extracted this length of core. So it’s muddier, finer and compositionally a little bit different than the Ōtorohanga. But very importantly, it’s of the same age. So the key characteristic that makes this limestone an important reservoir rock is the fact that, due to tectonic compression, the rock, already being cemented and hard, was brittlely fractured.

So here I have a section of core that intersects one of these fractures. One side of the fracture is seen here, and the fracture then introduced space or porosity – it’s called fracture porosity – into the rock. And it’s because of these major fracture systems that we see here that the otherwise tight non-porous limestone – there’s no holes or porosity in the limestone itself, they’ve all been plugged with cement during burial diagenesis and pressure dissolution – but these fractures have been introduced later on due to tectonic compression, and these fractures have again opened up space in the rock that have enabled it to act as a hydrocarbon reservoir.

And given we had a source of hydrocarbons further down – hydrocarbons being less dense migrates up – is able to move through the fractures, and if there’s a structure or a bowl feature with capping mudstones that are impervious, then the oil can be trapped in the fractures in a structure where you’ve got a seal over the top.

All of the reservoirs bar one in Taranaki Basin are sandstones, and sandstones have naturally occurring pores that haven’t been filled with cement in them. Now, the Tikorangi is the only carbonate or limestone reservoir in Taranaki and therefore the only one currently producing in New Zealand.

So we might see behind us in the quarry face some of the fracture-type features that we commonly get in hard competent limestones, which are called joints here in the quarry, but otherwise, outside of those joints or fractures, the limestone has no ability to host because the porosity has all been filled with cement during burial diagenesis.

Acknowledgements: McDonald’s Lime Limited Ministry of Economic Development Lloyd Homer, GNS Science Grant Pearson