Stromatolites on ripples

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Stromatolites on ripples

This is a transcript of a QuickTime movie (21.2 MB) recorded in the Pilbara in 2005. Martin Van Kranendonk, of the Geolological Survey of Western Australia, examines stromatolites that have formed on ripple marks at the Dresser formation.


We’re at the same horizon where we could see in cross section that there were these beautiful cross beds, which is where sediment loose on the seafloor has been pushed by wave action back and forth. And as sediments accumulate, that wave action changes and so you get one set of currents cutting through and so you see cross beds because one ripple set gets cut by another as the sediment accumulates. Everybody’s seen it on the beach, you see ripples on the surface, right, where the current washes the sediments back and forth. These are ripples 3.49 billion years ago, this is an ancient beach, it’s just really simple. It’s just modern geology applied to old rocks. But these are the crests of the ripples pointing now down steeply into the earth. They have been tilted from flat to horizontal, but they have an asymmetric shape. They’ve got a low slope and then a steep slope, a low slope, steep slope. So we know that the water was going in this direction. So you can actually reconstruct the beach environment here and in the rocks underneath you can see these little star patterns, these crystal star patterns. These are when the rocks were saturated with the fluid phase and crystals from that fluid started to grow in the rock before it was perfectly lithified and it pushed the sand aside. We don’t know exactly what these mineral crystals were because they’re silicified now. But one common thing is they also look like little barite rosettes. But the important part of this feature in terms of biology or paleobiology is the fact that people have suggested all these things may be abiogenic, just structural wrinkles caused by folding or something like that. But we have this beautiful, preserved irregularity with these ripples. And at the bottom the laminates mimic those ripples, but as you go a little bit higher up, they have their own shape, they have domical shapes, and irregular wrinkly laminations in all dimensions. So it’s not inherited from the basement and the asymmetry of these structures shows that they’re not folds, because folds will generally form symmetrical shapes. That’s an oversimplification, but that’s generally what happens. But once you get up into these wrinkly laminates they have their own texture that’s completely different from these things. And you can see some of them on this surface just here, you can see some of the irregular domes that are in all directions and quite wrinkly laminations that’s got nothing to do with the shape of the ripples. And that’s a very powerful argument that you’ve got something in these wrinkly mat layers, which is different from the normal sediment and occurring under different conditions. And the only conditions that we are really familiar with, about how that formed, is biological.




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