Formation of Marble Bar chert

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This is a transcript of a QuickTime movie (22.0 MB) recorded in the Pilbara in 2005. Dr Martin Van Kranendonk, a geologist with the Geological Survey of Western Australia, discusses the formation of the Marble Bar chert.


Here the rocks have changed a little bit, right? They’re lighter coloured and you can see they’ve got a slightly more penetrative schistocity, a more penetrative fabric because we’re up close, tight behind this very resistant chert ridge. But the feature I want to show here is just underneath Hiroshi’s feet and through these outcrops you start to see cross cutting grey or dark blue, or locally, black chert veins, right? This is the start of our chert discussion. And you can see them cutting through at all angles here and they’re a little different than we see at Schopf, which are nice single big discrete veins. Here there’s more of a network of them all the way up and down the outcrop. We’ll go here and then we’ll get closer to the base. Remember we’re walking up to the bottom of the sea floor, at paleo time. So we’re going up section that way. The rocks are always getting younger going up that way. So we’re below the sea floor at the time when Marble Bar chert was deposited. And you get this network of fractures but they’re a little irregular because they’ve been squashed again, OK? So they, instead of going straight or bending in sharp angles, they go wiggly. But we’ll see nice examples of what happens and as you go up closer to the chert you get more and more of these veins. And the country rock around it is getting more and more altered by, in my model, these hot fluids passing through and stripping material from it and transporting it to the sea floor, all right? So you start to see these vein networks and some of them are white. There are some white quartzy ones, there are grey ones and then they start to accumulate as you get closer. But an interesting thing here is that you see that you get individual veins, nice sharp edges and they send out fingers, right? These are just cracks sending off fingers of fluid up into the volcanic rock above. And you can see there’s a whole network of cracks that are being filled and it goes down to a very fine scale. But the larger veins have, you know, nice sharp contacts. There one vein splits into two and then it joins itself again and you can see that there are fragments of the country rock that are being split apart by these silica veins, right? And this is just nice, very defined grained smooth chert. That’s what we’re going to be seeing a lot of over the next two days.

So this is the start of the sequence and, well, it doesn’t matter, we can talk about it a little bit further up through there. You can see when you look across to the, if we go across the road, you see more and more of the black colour, right, more and more of the silica, more and more volume of fluid accumulating in these cracks. So the rock’s getting more and more cracked up as you get up toward the base of the overlying chert.

How do you distinguish between silica that filled cracks and silica that, as you say, forced the cracks? Is there anything you can see in the rocks?

Well to me I think it’s, you know, one and the same process so when I say silica that’s the remnant product we see in the rock. But of course I mean a fluid rich in silica and the silica crystallises out of that.

Opening and then filling in with silica is not as a result of the, there’s like a tension crack, a tectonic crack.

Well I don’t think these are tectonic cracks. I think these are cracks caused by fluid pressure and we can talk it more up there because you see the relationship better.

Paul, I’d like you to come over here. So Paul asked the question, were the cracks opened up by cracking and stuff falling down into it or did the fluids cause the cracks to open and force the rocks around it to break apart? To me this is solved at this outcrop. And I’m not going to tell you why. I want you to look at it and think about it yourself, OK. And see if you can think of reasons why, one way or the other. Maybe you can’t solve it. I’ve seen a lot of rocks so maybe my bias comes into this outcrop more than others but think about it yourselves for a bit. Think about that process. If you have a sealed cap rock and you’re bringing hot fluids up, they’re cracking their way to the surface, what will that preserve? What will that produce? On the other hand if you had an old surface that was weathered and there were cracks like we see here, valleys and stuff and rubble’s falling, what will that produce in terms of texture? So this is a teaching outcrop. You guys sit down and think about it for a while in your mind. It’s a mental construct. But really geology is a bunch of simple predictive hypotheses that you solve and you solve by observing. And you observe at lots of different scales but this is one scale where I think I’m happy to observe and come to a conclusion. So I’d like to see what the rest of you guys do. But I don’t want to talk about it Paul. We’ll just reserve it for a while. We can come back and discuss it but everybody have a bit of a look, think about it and see what you feel. But come around this area, up to the top so here we’ve got the bedded chert. This is the base of Marble Bar chert. You can see the nice layering all through here, right up over. It’s a big, thick chert. We’re right at the bottom. These yellowy rocks are the volcanic rocks that underly the Marble Bar chert and the black is the chert. So let’s just have a little bit of time for each of you to think about it and see what you come up with.




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