Friday Field Foto #64: Nice ‘Z’ fold in Patagonian outcrop

Here’s another one for the structural geology fans out there.

This is a photo of a cute little ‘Z’ fold in some Cretaceous strata in Patagonia (from this place).

Cretaceous Tres Pasos Formation, southern Chile (© 2008 clasticdetritus.com)

Sedimentologists and stratigraphers will typically try and find outcrops that lack structural deformation. This is because one of our goals is to precisely reconstruct the distribution and layering of sedimentary rocks. If the rocks are all faulted, folded, fractured, or otherwise FUBARed this becomes very challenging.

But it’s important to point out that, the whole reason the outcrops are there in the first place is because of structural deformation. In the case above, a series of mountainside exposures are uplifted and dipping to the east (in photo above, note the overall tilt of strata to the left). Associated with that uplift was some localized and relatively small-scale reverse faults. In other words, we need some deformation to produce the outcrops (but I’d also point out that structural geologists love sedimentary rocks because it allows them to very precisely map structures).

In the example above, you can quite easily put that sandy package (which is about 20 m thick, by the way) back together. The other thing I like about this photo is how you can see the ‘Z’ repeated in the gully in the background.

Happy Friday!

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