Alternative hypothesis for formation of Marble Bar chert

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This is a transcript of a QuickTime movie (40.3 MB) recorded in the Pilbara in 2005. Professor Paul Knauth, from Arizona State University, presents an alternative hypothesis for the origin of the Marble Bar chert.


Another possibility here is also based on what we’ve seen down below as you’ve based. One of the things I observed down there is that as we’ve come up we’ve seen a lot of these ocelli, these spherical devitrification features getting stretched out, elongated, parallel actually to some of these dykes. And it’s clear to me that there was a deformation event that took the rock layers here, the rocky material, and there was shearing in this direction. And in fact, another way to interpret this, and I don’t know if it’s right, I’m not advocating it, I’m just exploring it, is that this is actually a little silicified fault zone. I wouldn’t call this chert, that’s a real stretch of the term. I’d call it a silicified breccia and one way to do it is to have this as a fault zone that is just, and you don’t need much displacement, and you see these kind of things everywhere in the world where you get a big rupture zone and pieces break off and some of them will jigsaw near the margin, others get wrapped up and rotated and one thing about this unit is it’s not floating in a matrix of silica. Things here are being silicified. You can see it beautifully right here. If you look at these clasts, right here you can see one of these basaltic clasts getting replaced with silica. It’s eating its way in here along these margins right here. Down here you can see one that’s almost totally silicified. There was hot silica rich fluid coming through here and if you wet it down you can see that the black so-called chert is actually a metamorph breccia, it’s just breccia of silicified fragments. It’s where the smaller ones got completely silicified. There are little indentations in some of these so that the hot silica fluid coming through this zone, which could have been a permeable breccia zone from a fault. I mean this is common. We have silicified breccia zones all over the world of all ages and it’s one of the kinds of common silicification. We simply had hot water coming through here, now to be sure, it was probably under pressure but whether it was hot water coming in and forcing an explosion of this material would not be necessary in that alternative model. So a second way to look at it is that.

A third way is the thing you were alluding to that this is a, what’s called a Neptunian dyke, where cracks open for various reasons and rocks fall down into them and they don’t look anything like this elsewhere, at least the ones I’ve seen. They tend to be more vertical although they can come out at an angle and they don’t tend to have these jigsaw puzzles so commonly although they can be there but they’re not very common.

So among the three, if I had to rank them, I’d agree with Martin that the Neptunian dyke is probably not very probable for a lot of the reasons you have pointed out. So I’d rank that one as number three. But I would put the silicified breccia zone or silicified displacement zone, or if you will, a fault, as my number one choice. I think that’s a little simpler. And I wouldn’t rule out what you’re saying. I would just rank it number two in a series of three hypotheses.

Fair enough. I can find a lot of arguments against that but we can look at more of them and discuss as we go. I guess for me a silicified fault zone is, I mean faults are relatively large structures, certainly if you have something several metres wide here and you should be able to trace that fault laterally along for a fair way. So you might say, OK, Paul’s talking about shearing of the ocelli and that you might say, OK, this is parallel to the base of the chert so it should go a long way but ….

They don’t have to. They can actually, if you have a little jag in a fault, if you have a, actually you can have a very small upset but if the fault plane comes down and makes a bend and then that goes straight again you can have a very slight lateral displacement and you open up a gap, easily this wide. We see this everywhere. This is not a problem. And we don’t need a major fault to do it. And so the difference in the two, between one and two, the significance for life and all that is that you’re using this as the conduit source fluids for what’s above and I’m saying that this has got nothing to do with what’s above it. We can probably explain those cherts without these fluids coming up whereas that’s your preferred explanation and it’s a possibility but I’ll give you two other possibilities for the origin of the Marble Bar chert.




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