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:
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.