30 October 2014

Geomystery: what are these white lines?

Posted by Callan Bentley

Esteemed readership, I’ve got a mystery for you. What are these white lines, inclined consistently at a high angle to bedding? I picked up this sample below the “Wall of Death,” on the trail from Wapta Lake below Mount Wapta, en route to the Walcott Quarry of the Burgess Shale. The “zebra-striped” rock is of the Eldon Formation of the Cambrian section in Yoho National Park.


At first, I thought “cross-beds,” but they I realized that they were too high angle.

Here’s another side of this sample:


Tension gashes? But though they are indeed oriented subparallel to one another, they are so closely spaced, that doesn’t seem plausible.

Perhaps a clue can be found in the apparent void spaces filled with coarse dolomite crystals seen near the base of some of the white layers:


Here’s another example of that: Initially zoomed out for context…


…and then zoomed in for detail…


What about this idea?… Initial cross bedding creates small compositional variations on a geometrically regular scale. Then, during diagenetic dolomitization of a limestone protolith, the cross-beds are a site of preferential dolomitization. If shearing occurred simultaneously with the recrystallization, then that could change the orientation of the former-cross-beds in a systematic way. Perhaps this opens up new void spaces, which subsequently fill with coarse chemically-precipitated dolomite.

Problem: Dolomite is more dense than calcite, so diagenetic dolomitization should increase volume (less void space) rather than decrease it. I guess something else must be going on. Any ideas?