Mark Wilson May 22nd, 2010
GREENVILLE, ALABAMA — Reconnaissance is over for this part of the state, and our work commenced this morning. We want to find good sclerobiont communities above and below the Cretaceous-Tertiary boundary, so here that means we want specimens from the Upper Cretaceous (Maastrichtian) Prairie Bluff Formation and the Paleocene (Danian) Clayton Formation. That means plenty of muddy creekbeds and sun-smacked roadcuts.
Megan Innis (below) and Caroline Sogot (above) collecting bored and encrusted fossil oysters from the Prairie Bluff Formation in Mussel Creek (N 31.97259°, W 86.70387°).
Megan (in the fashionable yellow wellies) and Caroline collecting oysters from the Prairie Bluff Formation along Alabama 263 (N 32.04082°, W 86.79367°).
This would be a good time to mention that Caroline’s father is a famous magician in England with the stage name Jack Stephens. We think this is very cool. And I quickly add, Megan’s father Jeffrey is a famous pediatric geneticist at the University of Michigan. We like that too!
This is the kind of fossil we like. It is a bivalve shell from the Clayton Formation (Tertiary, Danian) thoroughly bored by sponges. Unfortunately it is also well locked into this silicified rock matrix!
Mark Wilson May 22nd, 2010
GREENVILLE, ALABAMA — The Cretaceous-Tertiary boundary at Mussel Creek, Lowndes County, Alabama, has some unusual complexity. At the southern end of the section it is simple enough, as shown in a previous blog post. Just a few meters north, though, the boundary section looks like this:
Cretaceous-Tertiary boundary section at Mussel Creek, Lowndes County, Alabama (N 31.97176°, W 086.70414°). The "K" indicates Cretaceous rock; the "T' marks Tertiary sediments; the rounded black object is charcoalized wood.
This strange stratigraphy appears to be a stream channel filled with carbonaceous (carbon-bearing) laminated sediments which were incised into the Cretaceous Prairie Bluff Formation below. Is this channel Cretaceous or Tertiary? What sort of environmental conditions does it represent? We discussed and tested many hypotheses on the outcrop this morning, which is always great fun. We finally decided that these channel-filling sediments are Tertiary, following the conclusions of some (but not all) previous authors. Still, the beds are unlike any channel-fills I’ve seen before, especially with all the carbon. My favorite idea (which has no support in the literature, I quickly add) is that this channel represents erosion of a continent devastated by the impact blast in Yucatan just across the Gulf of Mexico. The local forests were burned off in the massive wildfires (and maybe further devastated by a tsunami), leading to rapid erosion and the cutting and filling of channels on the shallow marine shelf here. All the carbon is from the enormous amounts of burned wood.
This is what we would call a romantic view of stratigraphy. It would mean that the big piece of charcoal in the section above is from a tree burned in the end-Cretaceous cataclysm. I like that idea!