6 June 2017

Loading sags in homogeneous lithologies?

Posted by Callan Bentley

I was out in the field over the weekend, and saw something new. Readers, I’d be eager to hear what you think of it.

The feature is what appears to be a series of small “ball and pillow” type loading structures (soft sediment deformation), but the thing that’s weird about them is rather than being sand sagging into mud, we have instead silt or fine sand sagging into identical sediment. Check it out; in each case I offer a plain photo followed by a version with the base of the “pillows” highlighted:

Some of these have very compelling shapes. It just seems so bizarre that they could exist in sediments where the lower layer appears to have the same grain size (and thus mechanical properties?) as the upper layer. Where’s the differential viscosity coming from? Perhaps there was a component of seismicity that induced these structures to form through preferential liquefaction of the lower layer? Does that even make sense? Could it be that there used to be a lot more mud here, but it all got squished out of the outcrop through volume loss, and so doesn’t appear in modern cross-sections? The last one I think is most compelling as a traditional case of density inversion, with a slight difference in grain size preserved through the differential weathering profile: beds crisp at the base, then grading slightly upward to the crisp base of the overlying bed. It may be that examination under a microscope would reveal a discernible grain size difference.

It may be relevant that this site is very close (just a few meters above) the mass transport deposit I showcased previously.

Thoughts? (Thanks in advance)