These layered rocks are Ordovician aged sediments deposited in Kentucky, about 450 million years ago. The sediments accumulated in a basin forming to the west of a growing mountain range that geoscientists call the Taconic orogeny – part of today’s Appalachian mountains. See the couple layers that are heavily deformed? Those layers are sediments that were churned up after they were deposited, forming what are called ball and pillow structures or, more specifically in this case, seismites. A large earthquake disturbed those sedimentary layers about 450 million years ago.
Several lines of evidence require these layers to have been disturbed in earthquakes. The disturbed layers often continue laterally for tens of kilometers, ruling out smaller-scale causes like storms or tidal flow. They also are on ground that is very flat, with slopes less than a degree, ruling out fast moving debris flows since debris flows couldn’t move rapidly for long distances along such flat surfaces. The only reasonable cause for the disruption in those layers is shaking in earthquakes, typically at least of moment magnitude 6 or greater.
Sedimentation rates for fine-grained rocks are typically slow. Therefore, the outcrop in front of your eyes probably represents several hundred thousand years of time. There are only two disturbed layers in all that time, so either earthquakes in this area were extremely rare or when they occurred the layers typically weren’t preserved; either are possible.
It is noteable that only thin layers of sediment are disturbed. The layers that are disturbed are probably the only sediment that was loose at the time of the earthquake; the layers just below the disturbed layer had already begun to lithify. it is fairly common for shallowly buried sedimentary rocks, particularly limestones, to lithify within a few tens of centimeters of the sediment/water interface.
Carbonate sequences in the ocean form something called “hardground” – a cemented layer at about this depth, due to chemical interactions with the water. Acidic water at the sediment/water interface will cause some carbonate minerals to dissolve right at that boundary, but as water moves downwards in sediments the pH changes and carbonate minerals suddenly begin to reprecipitate, hardening the ground only a few centimeters down.
Image credit: http://bit.ly/2u6atNl
Reference: http://bit.ly/2v7C454_ _
