6 October 2014
Often, stylolites (pressure solution seams) are bedding parallel in susceptible sedimentary rocks. They are shaped like “beds of nails,” overall planar, but with pointy bits that poke up and down, perpendicular to that plane.
The stylolites form with an orientation that is overall perpendicular to the maximum principal stress direction, and the little “teeth” parallel to the maximum principal stress direction.
Often stylolites form in sedimentary rocks parallel to bedding (that is, horizontally) because the maximum stress is vertical (due to loading of overlying sedimentary layers).
But sometimes we find stylolites that cut across bedding at some angle, and these are inferred to have formed due to more horizontal (i.e., tectonic) stresses.
Here is an example (cross-sectional view, perspective parallel to the bedding plane) I found last month on Corridor H, West Virginia:
This is a chunk of Silurian-aged Tonoloway limestone. Closer in, you can see some additional detail of this structure:
Here’s a view of the bedding plane (“map view”), showing the trace of the stylolite cutting across bedding:
From a study of in situ examples of stylolites like these, one can infer the maximum principal stress direction (σ1) and thus potentially the direction of tectonic transport. They are subtle things, but in aggregate, a bunch of little stylolites can tell you about regional-scale tectonics.
These rocks were deposited during the Silurian, but obviously the stylolites would have formed well after that, sometime after the carbonate mud was lithified to limestone. Probably they are Alleghanian in age – due to the collision of ancestral Africa with ancestral North America during the late Paleozoic periods called the Pennsylvanian and Permian.