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!
Mark Wilson May 21st, 2010
An Alabama Creek where, oddly enough, we found superb Paleocene nautiloids in the McBryde Member of the Clayton Formation (N 31.91739°, W 086.68906°).
GREENVILLE, ALABAMA — This is the first time I’ve done fieldwork in the southern USA. The outcrops are of course very different from my favorite desert locations and oddly similar to those I visited in western Russia last summer. I’m learning once again not to pass by the muddy creek or grass-covered hillside assuming that no useful rocks or fossils will be present. Southern geologists Jon Bryan and Peter Harries have been excellent guides here because they know what treasures lurk under the vegetation and on the river banks.
A grassy hillside with beautiful Cretaceous oysters just underneath. We collected the lot by feeling for the fossils with our feet! (N 32.02580°, W 086.76788°)
Mark Wilson May 21st, 2010
Cretaceous-Tertiary boundary at Mussel Creek, Lowndes County, Alabama (N 31.97176°, W 086.70414°). Megan's hand marks the level with the Cretaceous below and the Tertiary above.
A closer view of the same boundary. My finger is thrilled to be in such a place.
GREENVILLE, ALABAMA — It is one of the most famous geological horizons. It marks the end of the Mesozoic Era and the beginning of the Cenozoic. The “K/T boundary” is dated at 65.5 million years ago (±0.3 my) and is found around the world. It is the primary datum for our work on this expedition, and we were led right to it by our friend Jon Bryan. We want to just pause a moment and enjoy the historical and stratigraphic significance of these sediments. (And yes, I know I should be calling this the more modern “Cretaceous-Paleogene Boundary” as Megan insists, but I grew up with “K/T” since my Berkeley graduate school days and it is hard to give up!)
More later from our hot and muggy day in southern Alabama!