Breccia/fractured rock boundary at Upper High Dry

[Back up to Black Rock Desert suspected impact crater page]

by Ian Kluft
e-mail:
April 21, 2008

In the Summer of 2007, we had our first chance to explore Black Rock since the formation of the theory about an impact crater.

One of the most significant findings of Summer 2007 was a recurring rock boundary in the Upper High Dry lakebed area between breccia (broken/jumbled and re-cemented) rocks above and fractured rocks below. It isn't proof. But this and related findings have given a boost to the impact crater theory. It shows rock structures which should be found in an impact site are being found at Upper High Dry.

Structure of a complex impact crater

The diagram of the cross-section of a typical complex impact crater shows what we're comparing against. If the theory is correct, then rocks in the Black Rock Desert region have to observably match that model. A boundary between breccia above and fractured rocks below is exactly what one should find in a large impact crater site. So far so good - we have found that boundary in many places on the north and southwest sides of Upper High Dry.

Based on the concentric circular pattern that we could see on satellite imagery, which I nicknamed the "southern rings", the current estimate is that Upper High Dry lakebed is somewhere near the center of the suspected crater.

In any crater with more than a 10km diameter, we'd expect it to be a complex crater structure. This is as opposed to a simple crater, such as Arizona's 1 km diameter Meteor Crater, which is just shaped like a deep bowl. A complex crater is too wide to be stable. It collapses in shelves from the rims. That pushes up an uplift or uplift ring in the middle. Black Rock appears to be at least 50 km in diameter. But even if the theory is correct, around 80% of the crater is eroded away - so it's difficult to estimate.

Mountains of breccia

In the complex crater cross-section diagram, we would expect the central uplift of a crater to have breccia rocks overlying fractured target rocks. The reason is because the fractured rocks are what got hit by the impactor. And the breccia is broken, jumbled and re-cemented rocks from the debris that landed on top of it.

So one thing that geologists who are experienced with impact craters have told me they minimally want to see in a suspected impact site is breccia. If it didn't have breccia, they couldn't take it seriously. So we took that advice seriously. But that turned out to be almost no challenge at all. As we explored some more, we found that Upper High Dry lakebed is surrounded by mountains of breccia.

I already knew there was some breccia from photos of previous visits. But once we actually started looking for breccia in Summer 2007, we realized it's there on a bigger scale than we imagined.

As you go higher in the breccia, there should be an increase in melt rocks where the hotter rocks and then melted rocks landed on top. These are not lava rocks because they don't come from a volcano. But it should look like lava because they are also igneous rocks. And as expected, the breccia has an increasingly lava-like appearance the higher you go in the mountains around Upper High Dry, especially the ridge on the east side.

We found an extensive boundary between breccia and fractured rocks on the ridges north and southwest of Upper High Dry lakebed. Limited searches on the ridge to the east side have all been breccia down to the alluvial (stream erosion debris) deposits at their base so far.

Pervasively fractured rocks

When exploring in areas below the fractured/breccia boundary, the rocks are all fractured similarly, even across different kinds of rocks. The fracturing style differs from one locality to another. We found areas where the rocks are fractured so thin that I compared them to thinly-sliced deli sandwich meat, and nicknamed them "thinly sliced". In some areas we were able to follow the fractures from thinly sliced areas to where they became so thin they were like striations in the rocks. (See an example from one of the photos.)

That pattern looked like the path to follow to look for shatter cones. But so far each time we reached the breccia boundary without finding acceptable shatter cones.

Still, that looks like an encouraging strategy to keep trying over a wider search area. Finding shatter cones would prove that it's an impact site - even the meanest volcano can't make anywhere near enough pressure to form shatter cones.

I don't have a lot of information on what to expect from the fractured basement/bedrock layers under an impact crater. But the fact that we're finding everything deeply fractured in the rocks beneath the breccia layers, is completely consistent with what the texts (such as "Traces of Catastrophe") say we should find in an impact site.

Breccia dikes

As one should expect, there is no flat boundary between the breccia and fractured rocks. It's extremely variable. In many places, deep breccia dikes intrude downward into the fractured rocks. For that reason, breccia dikes are a feature that texts say we should expect in an impact site.

We had already found enormous breccia dikes at Sulphur, Nevada on the northeastern rim of the suspected crater area. When we went to Upper High Dry, we found that breccia dikes were also pervasive in the boundary area between the breccia and fractures rock layers. They were everywhere and easy to find.

As before, these on-the-ground observations at Upper High Dry continue to be completely consistent with expectations for an impact site, and on a very, very large scale.

Are we there yet?

The observations from Summer 2007 continued to be extremely encouraging. They significantly moved forward the theory of an impact crater at Black Rock. We don't have proof yet. So, no, we aren't there yet. This will hopefully provide enough information to get more geologists on board with this study.

We have plenty of places to go look during 2008's dry season.