This is not an antiform

Tim and I have spent the last several days driving across the US, heading for the Society of Vertebrate Paleontology meeting in Las Vegas. Along the way we’ve been making as many stops as we can at museums and geologic sites, including the northern Arizona feature shown above. I’ve often said that “Nature will tell you a direct lie if it can”, and this feature is a good example.

At a glance, this seems like a pretty straightforward eroded antiform fold, basically a fold that curves upward (we would have to know the relative ages of the sediments to know if it’s an anticline or a syncline). The tiled beds are clearly visible on each side:

It’s pretty easy to sketch in a possible shape of the fold:

However, if we climb to the top of the ridge, we can see that this interpretation is completely incorrect:

This is the famous Barringer Crater (or Meteor Crater), a meteor impact site formed during the Pleistocene. Approximately 50,000 year ago an iron meteor around 50 meters in diameter smacked into the ground here, with an impact speed of almost 13 kilometers per second. The impact punched this hole in the ground, and disturbed the Permian and Triassic sediments that covered the area.

In fact, there is a fold here, but not the antiform it looks like from the side. The sediments along the rim and stretching out for some distance on each side of the crater are overturned, so that the crater’s rim is a synclinal fold. The impact causes the upper sediment layers to immediately fold over on themselves; essentially, they splash out around the crater. An overturned rim sequence such as this one is a key feature of impact craters (Kring, 2007).

Tim and I should arrive in Las Vegas this evening. As usual, I’ll be posting updates both here and on Twitter about the SVP conference, and hopefully will have a few more posts about some of the things we’ve seen along the way.

Reference:

Kring, D. A., 2007. Guidebook to the Geology of Barringer Meteorite Crater, Arizona (a.k.a. Meteor Crater). Lunar and Planetary Institute Contribution No. 1355.

 

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