Black shales, and Dino Days

I’ve been on vacation over the holidays, but things will be picking up now that we’re into the new year.

One of the first museum events of 2008 is Dino Day, on January 12. For that day only we’ll be displaying a number of specimens that are usually not on exhibit, and we’ll be opening the field jacket of one of our large dinosaur bones. If you can make it to Martinsville on the 12th, I recommend this event.

Now, for a little geology. The rock above was given to me by Grenda Dennis, and at first glance seems to be one of the most boring rocks imaginable. Later, Grenda emailed me pictures of the roadcut in Pennsylvania where the rock was collected:

More nondescript black rocks. But in fact these rocks have been of intense interest to geologists for over 100 years.

This type of rock is known as a black shale. It’s black because it contains a large amount of organic material that never fully decayed. The exact conditions that result in this type of rock has been a source of contention for over a century.

One place where you can get black shale-type deposits forming in the modern world is in the Black Sea. The unusual geography there results in water that is starved of oxygen (anoxic), preventing decay and allowing organic-rich deposits to form.

For many years the Black Sea was the standard model for ancient black shale formation, but there was a major difference. At various times in the past there were huge deposits of black shales. One of the most productive black shale periods was the Late Devonian (this is the age of Grenda’s rock.) How do you get nearly global black shales, if you need a restricted basin like the Black Sea?

It turns out that there are other theoretical models for producing black shales, that happen to be rare or non-existent in the modern world, as outlined in this paper (pdf format) by Harry Tourtelot. It’s possible that if you have an exceptionally high level of biologic productivity, there might simply be too much organic material for bacteria to break down before it reaches the seafloor and is buried.

More recent research shows that the black shales are much more complicated than previously thought. Some of the black shales are heavily burrowed, showing that organisms were living in the sediment (making anoxia unlikely.) Some deposits show evidence of storms, and other features that indicate they were deposited in shallow, oxygen-rich water. And, making them even more mysterious, there ischemical evidence that at least some parts of the black shales were anoxic. So some parts of black shale formations show evidence that oxygen was present, while other parts of the same formations show evidence that oxygen was absent.

Detailed research on black shales is continuing. What originally seemed like a simple, straightforward depositional environment turns out to be vastly more complex than was suspected.

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3 Responses to Black shales, and Dino Days

  1. Dan Omura says:

    Is there any simple way to identify black shell because I have a fossil specimen (ammonite) on a deep black sediment that comes from a locality in the Highlands of Peru (4500 meters above sea level, that´s more less 15000 feet) those deposits have been dated Late Cretaceous and based on the amount of fossils it looks like a there was some kind of local extinction because there are thousands of those ammonites there and I find it interesting how in this case maybe the color of the sendiment is related with the excess on organic material.
    Thanks for any possible answer and keep working hard on the webpage!

  2. Alton Dooley says:

    Dan has touched on two interesting features of black shales. The first concerns their distribution. Black shales are found in small amounts in pretty much all the time periods since the Cambrian. But there are a few times in the Earth’s history when black shales were forming more-or-less simultaneously all over the world, and we get thick and extensive deposits. The three most prominent times were during the Permian, the Devonian (when Grenda’s rocks in Pennsylvania were deposited), and the Cretaceous (when Dan’s rocks in Peru were deposited.)

    The other point is that the end of the Permian, the end of the Cretaceous, and the end of the Devonian, are the three largest mass extinctions in the Earth’s history. It’s not clear that the black shales have any relationship to mass extinctions, but it’s an interesting correlation. It would be interesting to see if the ammonite abundance in the Peruvian deposit continues through the entire section; if not, the Cretaceous/Paleocene boundary might be preserved in the section.

    The presence of large numbers of ammonites, by itself, doesn’t necessarily mean they all died at the same time. Black shales are thought to accumulate slowly, and since things don’t decay in them, anything that sinks to the bottom will be preserved. That way you can get a large number of shells in one bed that actually represent deaths that occurred over a period of many years.

    One other interesting point. Ammonite shells are made of aragonite, which dissolves rather quickly in deep sea water. This could indicate that the Peruvian shales were formed in relatively shallow water.

    And let me just say, 15,000 feet, WOW! I’ve never been above 11,000 feet (except in airplanes). It must be fun to collect at that altitude.

  3. Dan Omura says:

    Dear Alton,
    The localities these black ammonites come from were not that deep 65MYA, but andean orogeny is not simple though (by the way, is there any good article about the formation of the andes? We have had some many discussions here in Lima about that). Not too far away from black ammonites locality, you can find ripple marks + dino tracks also on Late Cretaceous, but they are not from the same formation. I know these two situations are not necessarily related, but still I think they give us an idea that this area was not close to deep ocean. Maybe on your next trip to Peru we can visit this area, it would be great for a Geo Field trip!!! It was very nice to work there…. but after two hours of rain and one of sleet… mmm it was not so nice!!

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