Strelley Pool Chert

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Strelley Pool Chert in context


This is a transcript of a QuickTime movie (82.5 MB) recorded in the Pilbara in 2005. Dr Martin Van Kranendonk, a geologist with the Geological Survey of Western Australia, describes the importance of the Strelley Pool Chert in relation to probable evidence of life 3.4 billion years ago.

Transcript:

No sampling from this locality and please be as careful as possible. We hope that nobody will actually walk on the outcrops for obvious reasons. They can get damaged so just take a bit of care around.

I guess on behalf of the entire astrobiology community I'd like to welcome you to the Trendall locality. This is our baby. We've known about it for a while now and we've had a lot of people looking at these rocks now for a long time. It's a nice place to come and do geology for a number of reasons, a great camping spot and good rocks.

So, this is part of the unit called the Strelley Pool Chert. It’s a very nice exposure because it has a big area here, and as Abby [Allwood] has now found, along the ridge in both directions [it comprises] primary carbonate. And when I say primary I mean that it hasn’t been so completely overprinted by silica. The silica all occurs down in that lower part of the outcropping that is all white and hard.

But this is an area that has escaped largely that silicification process. And it has all these beautiful structures in it that people have come to look at and ooh and aah over and we’re talking about here. And as I’ve said before these rocks are about 3.4 billion years old and they sit in this tilted cross-sectional view that’s popped in that direction. And the rocks underneath that make up that hillside you can see are very strongly foliated. They’re very platey, schisty fabric, they’re composed of a metamorphic mineral called pyrophylite and they are derived from pillow basalts and they’ve been so extensively hydrothermally altered that there is almost no original material left but locally you can still see those ocelli.

Remember at Marble Bar pool we saw those round vitrification structures, you could still see ocelli in them, so we know they are derived from high magnesium basalts that have gone through a lot of metamorphosis. And that foliation comes up to the base of the Strelley Pool Chert and stops.

These rocks don’t have any penetrative fabric and then the pillow basalts that overly it also have no penetrative fabric and we regard that as marking a major time gap in geology, where the lower rocks have experienced a phase of deformation and squeezing and heating that these rocks have not experienced. And that fits with the time gap I told you about before.

The Strelley Pool here has three main components. At this locality is a very thin boulder conglomerate at the base and you can see the boulders just down below. And then this unit up through here [consists] of the layered carbonate rocks; and then toward the top you have coarse boulder conglomerates and sandstones and sediments again. And that’s shown in this geological map which is in your guidebook and we’re standing right here in the brown and dark purple units and all the flecks are just representations of these coniform structures.

I mapped this outcrop at one meter scale again right up to about where Roger and the others are standing to try and get an idea of the distribution of these coniform structures through space. And it’s fun to look at a lot of the different structures and this at least gives you a reference to see how things are put together. So you can use that and you can ask me questions about it.

We have a lot of different types of shapes here. The most common of course is the more sharply flexed coniform structures, so these kinds of examples here with the sharp crest and individual laminae that you can see passing over the crests into the flat parts around. And then there are ones that have slightly more rounded features that go back into a sharp cone. When you look around you’ll see that those cones start at one horizon and persist up through the stratigraphic record for quite a way. Then another will start at a different level and another one at a different level. There’s no sort of common individual bed or plane where they start up from.

We feel pretty confident for a number of reasons that they’re not fold structures and we can go into the details of that. And we have a little fellow who’s our favourite one, I guess. It’s up here and it shows a bit more complexity. This is one of the larger forms up through here which has a sort of wavy side and a coniform shape and a wavy side down the other way. It’s quite a large amplitude. It’s about 30 centimetres amplitude and then on the edge you see this double branching, more domical structure which we call “Mickey Mouse ears”. I just find that a very compelling geometrical argument in favour of biogenicity because it’s two quite different processes in, you know, geographically separated areas. There are a number of features through here that give you clues as to what the origin of this sediment was.

There have been alternating ideas that this is just purely a mineral precipitate crust from dissolution of seawater and that some of the structures just represent the accumulation of those crusts through time and just point-source anomalies of that material. But I have shown in a paper and we can show you here on this outcrop that there is definitely the action of wave currents here and there [are] detrital clastic grains. We see cones with little sand wedges off the side. We can see little rip up flakes of some of the laminae that are redeposited, a bunch of good evidence for wave action throughout the accumulation of this. And to me that makes whole mineral precipitate story a little bit more difficult. So that’s the main aspect of this outcrop.

As I’ve said before we also find stromatolite forms at two different stratigraphic levels of this outcrop. There’s a layer toward the top of these carbonates of almost solid black chert; but it’s replaced very fine grained sediment. We have those beautiful little domical stromatolites and I can show you those later on. And then up in the sandstones we have these thin black films beneath quite coarse sand that have a lot of ferruginous material. So it’s fun to look around this outcrop and really take some time to look at the geology of things and the different forms.

There are people with a lot of different experience here. There’s the group from ANU that’s also interested in trying to model cone shaped structures. Abby’s done some of that work as well to, and she’s talked about that at the conference, about how precipitative cones can grow outwards. These ones persist up. There’s a really nice example of one right here where the cone’s growing straight up and it’s overlaying those wavy structures.

If you look at some of the cones - Hans Hoffman pointed this out - that many are actually slightly asymmetric with one short limb and then a longer limb and it goes off just down there. If you look at them they’re slightly asymmetric. So there’s a lot of very subtle detail in the shapes of these things.

Abby [Allwood] has worked in this area and across to the north and also to the south and she’s then been able to identify several litho-classes of this kind with laminated carbonate. The only thing I’ll add to this story is that we thought, well, if these are hydrothermal precipitates - and that was the model that I pushed early on in this scenario because of these black chert veins through the outcrop I’ve described - but I’ve changed my mind on that now.

We thought that one way to test this was if these were hydrothermal precipitates, their chemistry might be different from just a normal marine precipitate of carbonate. So we did a paper with Greg Webb and Baltz Kamber to look at the geochemistry of the carbonates and in your guidebook you’ll see that we have the results there that show that, you know, they are just very, very similar to modern marine carbonates and nothing at all like hydrothermal fluids and that’s combined with the stratigraphic evidence we have, features that are described partly, to me is a very compelling argument for a rather modern geological setting.

These kinds of conical flexures are best developed here but the smaller forms are found throughout the distribution of the Strelley Pool Chert, in patches, and we don’t understand what controls that patchy distribution. We haven’t mapped that out yet but that’s over a 220 kilometre distance so it’s a big area. And the Strelley Pool Chert is on average roughly this kind of thickness, sort of 30 metres thick. It varies a bit plus or minus.

So, it has that lower clastic succession, carbonates and the overlying sandstone and then, bang, the pillow basalts and to me it really just looks like a shallow water carbonate platform. And together with the fact that it’s on an unconformity surface we can now recognise across the Pilbara, the earlier metamorphism and this later depositional sequence between, well, there was probably a sub-aerial period during the erosion and then you go through shallow water and then deeper water once you’ve got the pillow basalts erupted. It all fits together really nicely as far as I’m concerned.

In the guidebook you’ll see that we’ve got some of the pictures of the material, so these are some of the coniform shapes and here’s Mickey Mouse ears. These are a series of little coniform shapes which look very much like we saw at the top of the Dresser formation yesterday, which are just from further down the outcrop that we collected in 1999 together with a few of the people here and is now on display at the Western Australian Museum. We wanted to make sure that one of the best examples did not get destroyed by river flow or by people coming to collect. So we collected a representative beautiful pavement that’s in the museum. But these really, and there are more up the hill that I’ll take you to, are, you know, the best probably anywhere.


Glossary words

Amplitude: Height

Clastic succession: Rocks made of detritus

Conglomerate: Boulders and cobbles cemented together

Cross-sectional: As if cut across

Detrital clastic grains: Sand and bigger bits of rock

Ferruginous: Iron-bearing

Fold structures: Where the sedimentary and other layers have been squashed and bent by later pressure

Foliated: With layers resulting from later alteration and pressure

Hydrothermally altered: Where very hot waters have passed and changed the composition

Laminae: Very thin layers

Mineral precipitate crust: Rock resulting from chemical deposition

Ocelli: Spheroidal structures that form in volcanic rocks

Penetrative fabric: Where layers formed by pressure and heat cut across older structures

Platey: Breaks into sheets of rocks

Precipitative cones: Conical stromatolite-like structures perhaps formed without microbes

Pillow basalts: Volcanic lava that flowed under water, cooled quickly, and formed mounds that look like pillows

Pyrophylite: A mineral made of aluminiunm and silicon

Silica: Oxidised silicon: SiO2

Schisty: Similar to foliated

Subaeriel: Under the atmosphere. Soil is an example

Unconformity: Where a much older rock is overlain by one much younger

Vitrification: Where some process has made a glass-like rock


Contents


Introduction

Context

Early Life

Evidence

Acknowledgements

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