16 August 2018
A suite of deformational features in Lancaster limestones
Posted by Callan Bentley
In the Landisville Quarry, Lancaster, Pennsylvania, there is a quarry that cuts into Cambrian limestones. (The exact identity of these limestones is apparently a matter of some dispute, but that’s not going to stop us!) I visited the quarry in June on a field trip offered through the NAGT’s Eastern Section annual meeting. We witnessed multiple varieties of deformation there.
First off, there was straight-up brittle extension, resulting in bedding-perpendicular veins:
In other places, there was an element of boudinage (brittle/ductile behavior) with veins located at the boudin necks, as here:
(Note the lobate shape of the fractured bed’s contacts with its neighbors.)
The structural geologists who work on this site have interpreted that there is a thrust fault which runs through the quarry, with folding in the footwall, and mylonitization along the fault zone. Here’s David Brink-Roby showing us what they mean (red = mylonite):
Here’s some of the carbonate mylonite, a novel rock sighting for me:
Another slab of it:
And this example had some nice S-C fabric developed, with chlorite highlighting the tectonic fabric:
Here is an example of (right lateral) slickensides, a signature of brittle faulting:
Finally, there was folding, and accompanying development of axial-planar cleavage, as here:
Another view of this same site:
All told, it was a nice display of different deformational styles in a relatively small area!