North Pole Dome

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North Pole Dome


This is a transcript of a QuickTime movie (30.8 MB) recorded in the Pilbara in 2005. Dr Martin Van Kranendonk, a geologist with the Geological Survey of Western Australia, describes the geology of the North Pole Dome.

Transcript:

North Pole has so many things and one thing I forgot to mention of course is that is has, Don Lowe and others found a thin chert bed with impact spherules in them. They just look like little sand grains, but they are perfectly round, they are also floating in a silica matrix, silica is just everywhere. But they've been analysed, they’ve got high nickel content, high iridium and other characteristics which are not volcanic and these are from an impact. And they occur just about over and in through here. It also has this beautiful felsic volcano which is tilted on end and you can see the core of it. The eruptive core is in here with big breccia blocks, and fumeroles preserved, and then a hydrothermal rim of jasper and anchorite and iron retrochloride in through there. That’s preserved right in the core of this little fault bounded syncline, it’s this little canoe, and then the eruptive products go out in an apron through here and they are tilted up on their side. And then blasting through that is another volcano, but this one mafic, and you can see it through the vent here that goes like this. There’s just so many things at North Pole that makes it a special place. You can really see the dynamic build up of this area. But that's the lower part of the history and then it got a little bit uplifted by the intrusion of early granite phases over here. And then it was eroded away over about 75 million years and during that time deposition of the Strelley Pool Chert occurred and that Strelley Pool Chert is this topmost blue unit built around over here and then it comes down around on here and then it goes up around and then we lose it. But it's a very distinctive unit. It has basically three components - a lower clastic unit of coarse boulders and sandstone, then the middle unit of carbonate rocks largely silicified, and then an overlaying succession also conglomerate, sandstone and siltstone and then bang the pillow basalt again. We don't know the interval that is was deposited over, but there is a 75 million years gap of zircon ages below and above. And that’s nice, it fits in with the erosional history and all that thing. And it comes down here and you see it up through here and as I said before it occurs around the Pilbara Craton. It looks pretty similar in all the different places that we see it. One of the best places we are going to see is here at this Trendall locality, which was found by the former director of the survey, Alec Trendall, who was poking around in this area and found this outcrop down by the river, which showed these nice conical structures. He kept quite for a little while. Kath Grey looked at them and probably thought they weren't biological. There was a survey little report about that and then a picture got to Ralph Thorpe at the GSC and I saw it just before I came down to the Pilbara in 1994. Hans Hoffman got a look at it and he went gee whiz, these look pretty nice! So a trip was organised in 1999 and we went and looked at them again and we started to see a couple of really nice things and then Hans Hoffman and Kath Grey and others published a paper about it and I've written some stuff about it. Since then we've recognised those structures around, but Trendall remains probably the best place to see these things. And the continuation of the ridge to the south and to the north has primary carbonate that lies almost in direct contact with foliated basement rocks, metamorphosed foliated basement rocks. So it's a really nice unconformity. There’s a time gap there. And it took us a while to actually recognise that but it’s really quite obvious and it cuts down through stratigraphy.

Contents


Introduction

Context

Early Life

Evidence

Acknowledgements

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