History of Schopf locality

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Transcript


This is a transcript of a QuickTime movie (24.4 MB) recorded in the Pilbara in 2005. Dr Martin Van Kranendonk, a geologist with the Geological Survey of Western Australia, discusses the history of the Schopf locality.

Transcript:

The Schopf locality, all right? This is where Bill Schopf and Bonnie Packer described microfossils from, I guess the best known paper is the 1993 Science paper and he described microfossils from black chert, which he described as a sedimentary deposit with rounded sort of sand-sized grains, so sand clasts like we see in the bottom of a river there. And he had been taken previously to the spot by my boss, Arthur Hickman, who had mapped the Pilbara and Bill asked him, and you can correct me if I get parts of the story wrong, but as far as I was aware, he was asked where the best black chert was in the Pilbara. This was found to have these curious structures that were later described as microfossils.

Only one of the thirty?

Only one of the thirty-two as far as I’ve been able to be aware, yeah. That’s what he told us and we had a photograph, I don’t know who supplied that photograph but it was a glossy eight by twelve, I think it was from Bill himself, of the sample location. And Bill took us down to the river, down through there, where you’ve got nice layered rocks with bedding features and that’s a bedded chert like Marble Bar but it’s different. We’ll talk about that later. And he took us down into the valley and we had that photograph and, correct me if I’m wrong, but I think it was largely Tim had the photograph and he said, “That’s not from down here.” Tim and Malcolm, that’s right and they wandered over the hill and they came up here and said, “Yeah, that’s right” and put it together. And this is the locality where that sample comes from. And we started looking at the textures in this unit and realised that it’s not a bedded rock at all. It’s not a sedimentary rock at all. And the relationships that we have here are the bedded chert come down the main hillside and it’s parallel to the Marble Bar chert and you can see nice bedding in there, fine layering like we saw at Marble Bar. And this one comes up to it at a very low angle, about thirty degrees and that’s shown in the guide book on page seven in this figure, through here right? And on the ridge is the bedded, darker blue chert and then Chinaman Creek flowing through and then coming at a low angle is this vein, which goes right up the hillside, across the creek and goes for quite a bit further along. And off of that you can see other little veins going right up to the base of this bedded chert and again, like at Marble Bar, like at all these other ones, no veins on top of it, all right? And so we started to look at the textures in this and it has all these angular textures like we saw at the bottom of Marble Bar with black chert cutting through and breaking things off. And there are fragments of felsic volcanic rocks in here, lots of sort of brittle kind of fracturing things and all of a sudden it was very clear that this was not a sandstone and that changed the story radically because if it’s not a sandstone and you’re not at the surface of the earth, then eh, how do you get microfossils in something that’s all brecciated? And if it’s in subsurface, wow, that may mean that it’s not cyanobacteria reacting to light as was hypothesised. Maybe they’re chemoautotrophs. There’s a whole bunch of things that change under that different scenario and that really reopened the whole ball game in terms of early life in the Pilbara. So this is quite an important outcrop and it was written up by Martin Brasier and others who came, I think at about the same time and collected some samples and my map’s in that paper but that’s basically what you see here. And there are a number of things to see but just before we get into the details, just to give you a bit of a wider context, and I think it’s always good to have a wider context, there’s the page just before, there’s a map of this larger area. Some of these major veins actually sit within little faults, these dash dot red lines, that have a varied displacement as you go down them. And those kinds of faults are called growth faults and they’re active when sediment is being deposited because they’re actually dropping down the surface of the sea floor as sediment is accumulating. And you can see that this fault, it doesn’t offset this series of lenses at all, it offsets the Marble Bar chert. There are lots of other smaller veins that cut up. And they’re pure black chert. And they go up to the base of that thing and stop, so I don’t have to say that again. But fluid phases passing through rock that has been lithified and breaking it up. This rock also has quite a granular texture when you look at it in detail, all right? It’s overall grey and black and we can see lots of pieces that are sharp sided and if you go and look, you have to get the light right today, the light’s a little bit difficult today but you see lots of areas where you can see these brecciating fragment textures and when you look in detail, they’re actually multiple pulses of silicious fluids that have passed through this rock. There’s a dark phase here and that’s been broken up by a lighter grey phase, then there’s white that’s cutting through as the last one and that’s quite characteristic of hydrothermal veins. They’re just pathways for multiple pulses of fluid. But the younger generations break up the older generation and break it apart. And so that’s one way of generating this kind of “sandstone” kind of texture. But in this case you can see that it’s not sand grains it’s actually just fragments of a pre-existing silicious fluid that has crystallised and then later pulses are cutting through it.

Contents


Introduction

Context

Early Life

Evidence

Acknowledgements

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