Backside ridge at Dresser

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Backside ridge at Dresser

This is a transcript of a QuickTime movie (14.8 MB) recorded in the Pilbara in 2005. Martin Van Kranendonk, of the Geolological Survey of Western Australia, describes the context of the backside ridge at the Dresser formation.


That old sea floor that we were looking at is above us, but it’s gone right above us, but it forms the tops of all the hills right around us and this is the bedded sedimentary unit here. What we did from the panorama which was right over the other way, we drove around the outside and then cut back in and around back under. And just to show you, the stromatolites were on the outside of this geological map. So the light grey or the chert units dipping off at about 30 to 45 degrees and then we drove outside, around here, back in and we’re right up in the guts. And as you have heard me describe, the bedded chert units are underlain by this swarm of black chert and barite veins here and when we look out over the country side, we’re looking down stratigraphic section. But in those hills there and where we parked the trucks, you can see there’s a boxwork or irregular pattern of these dark chert walls. Those are all silica veins here. And as you go down further along strike, there are several hundred of these veins. Some are up to 30 metres wide and a kilometre and a half long. That’s a very big system here. And they mined the barite veins. Some of those single veins are 12 metres wide with crystals this big growing in from the walls under repeated steps of fluid pulses moving through them. And the purpose of this stop, remember we are right underneath where the stromatolites were at the top of the sedimentary sequence of this Dresser formation, and that unit is anywhere from 15 metres to about 200 metres thick. It varies across growth faults and the depositional environment changes from place to place with more or less currents. So its not a huge stratigraphic thickness that we are looking at. But what we are going to do is we are going to go into the bottom of the system here, and look at the intensity of the hydrothermal veining that affects the sediments and is incorporated into the sediment and then overlain by the youngest component of the sedimentary sequence. And then just along the edge of the ridge we actually have formed in the ridge, one of the veins that cuts up through altered basalts and goes up to the bedded sedimentary sequence and we’ll look at the relationship between the vein and the bedded deposit. But this gives you a bit more of a regional overview. Again in the centre part of the flats is that younger North Pole monzogranite. So there, rocks are about 3490 (by old), that monzogranite is about 3460. So it intruded younger. That’s out on the flats. And it’s pushed all this stuff up and now this granite is more able to weather. It’s flat. So granite domes are flat and greenstone belts are ridges.




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