27 February 2017
As mentioned over the weekend, last Friday was my first field day of the season. I visited a spot called “Indian Spring” in the central Fort Valley. The spot is on private property, but the landowner (Will G.) invited me and Tom Biggs (University of Virginia) to check it out. Tom’s mapping the Rileyville quadrangle, so it allowed him to put another spot on his map.
Here’s Will orienting us to the site:
Can you spot bedding and cleavage in these images?
Here, I’ll help tease it out for you:
The bedding is Devonian, when this part of Virginia was a deep marine basin, with fine silt and clay slowly sifting in, making the massive mud rock called the Mahantango Formation.
The cleavage is a tectonic “overprint” imposed on these rocks more recently, when Africa rammed into eastern North America in a profound event called the Alleghanian Orogeny. That was in the late Paleozoic, Pennsylvanian and Permian.
The bedding was laid down sometime around 392 to 385 million years ago. The cleavage developed as these rocks were squeezed almost 100 million years later, around 300-250 million years ago. You can’t deform something unless it already exists.
Bedding can be identified in these relatively unstratified mud rocks by horizons of shelly material, like this:
Most of the fossils are brachiopods, including prominent Devonian index fossil Mucrospirifer, here seen as an internal mold:
The brach-rich beds are studded with these chunky shells. Here’s a view looking up at the bottom of a bed:
Here’s the trace of another bed:
And a sample that broke out in a slab-like shape defined by the cleavage, with just a nubbin of bedding left on the bottom. It shows distorted spiriferid brachiopods, with their shape changed due to Alleghanian squeezing, the same circumstance that caused the cleavage to develop:
Here’s a wall that shows both bedding (inclined moderately to the left) and cleavage (inclined steeply to the right):
Same wall, but with sun shining on it:
Here’s a look down on a fossil-free bedding plane, and you can see anastomosing traces of cleavage overprinting it:
We were able to measure the orientation of both bedding and cleavage here, and add that data to the map. Both strike NNE, but as seen above, the two planar features have opposite dips. The bedding dips WNW, while the cleavage dips ESE. The cleavage is a tectonic “scar” that reflects the injurious birth of the supercontinent Pangaea. Rock splits into flakes today because Africa squeezed it 300 million years ago… Intense.