19 August 2015

Iron oxide sweat stains in sandstone

Posted by Callan Bentley

Have you ever found a rock sample that looked something like this?


It’s got a lot of rust, but mainly around the edges. What’s up with that?

Check out my hat for a clue – this is after a long hot afternoon picking apples in the sun on Sunday:


Did you see the dirty “rind” where my sweat soaked in and moved dirt and grime along with its flow, to accumulate at the edge of the soaked zone? Flow paths depicted in the annotated version below:


In the same way, fractures (joints and faults) in rock can serve as conduits for fluid flow, and surfaces along which subterranean fluids can soak into the “fabric” of the surrounding rock.



In the case of this sandstone, the white is “unsoaked” and the brown shows the rusty signature of overlapping successive “soaking fronts”:


All the rock examples in this post are from the new Bismarck, WV, outcrop of Conemaugh Group sedimentary rocks exposed along Corridor H. Here’s an example of conglomerate overlying sandstone, with both hosting the Liesegang-like iron oxide banding:



Zooming in on the contact:


These features can be quite beautiful, making Rorschach-blot like patterns in the rock…