Rights: University of Waikato. All Rights Reserved. Published 15 November 2012 Download


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.

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