Strelley Pool Chert Stratigraphy

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Strelley Pool Chert Stratigraphy

This is a transcript of a QuickTime movie (14 MB) recorded in the Pilbara in 2005. Abby Allwood, from the Australian Centre for Astrobiology, talks about the stratigraphy of the Strelley Pool Chert.


The Strelley Pool Chert has about 30 metres of pages of the book, so to speak, 30 metres of layering, on the average. I've recognised four subdivisions, four members within that. From the bottom for the first part of the story, the lowest layers. At the very bottom contact of the lowest layers is an erosional surface what we geologists call an unconformity and it represents a period of time where rocks were uplifted and eroded like the land surface today and it's actually the world's oldest unconformity, so that’s the world’s oldest land surface. The first evidence we have of rocks being exposed to the air and on top of that was what the Strelley Pool Chert was deposited on. And the first layers are, at least in this area, are beautiful boulder conglomerate with some boulders up to 2 meters in diameter and they’re distributed across the erosional topography that was formed beneath and they fill in the gaps and so forth. And that I interpreted as a rocky shoreline. So if you can imagine the coastline today, with not with sandy beaches but boulders and conglomerates and pebbles and so forth, pounded by the waves, smoothed and rounded boulders and all the fine sediment is carried away. It’s a very specific process that forms a particular type of conglomerate and it’s nice that you have something so unique at the bottom of Strelley Pool Chert because it really tells you something quite specific about the environment. And on top of that you have deposited the stromatolitic carbonates , so the carbonate is a mineral, and this the second member of the Strelley Pool Chert that contains stromatolites or dubio stromatolites which are the fossils or putative fossils that what all the fuss is about. And that's 10-15 meters thickness of finely laminated carbonate and locally some chert as well and that was deposited straight on top of these boulders. In the field trip yesterday I took some people to see some outcrops that really clearly showed the relationship between the carbonates and the conglomerate. Because a nice thing in Geology is, it's one thing to determine the depositional environment of the unit and the next unit, but to tie those in temporarily together and to understand how much time elapsed between the two is the next really important step and there are some features in the rocks that actually tell us that these two are really closely tied. That these boulders were deposited and then almost instantly or geologically instantly this carbonate just started depositing right on top of them and they were encrusted. The very first types of putative stromatolites are right there, right at the base so as soon as this stuff became submerged beneath the rising sea, these organisms, if that's what they were, just basically pounced right on the submerged surface and from there you have a layering that tells you of an evolution or in sedimentary sequence. The forth member is indicating an increase in volcanic activity, see lots of ash being deposited and finally a basalt on top of the Strelley Pool Chert which initiated a hydrothermal system which came through and altered a lot of the Strelley Pool Chert in places. So what you end up with is below the Strelley Pool Chert is the rest is the 12 km thick succession of volcanic and hydrothermal rocks fire and brimstone and then on top of it the same thing again and the Strelley Pool Chert is just like a little sandwich between its hiatus in all of that igneous and hydrothermal activity and this is a nice little marine period where it appears that life probably flourished, pounced on the opportunity.




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