Image of the week #6

The increasingly inaccurately named image of the "week" series continues. Since I've failed to post any of these for a few weeks, I'm going to post several images. In honour of the Boudin Centennial (which you can read more about here, but only if you subscribe to the Journal of Structural Geology), these are boudin-like structures that occur in slumps in the Albert Formation of New Brunswick, Canada:

Here's a layer beginning to break up into the boudin-like structures.

You can see a couple of the boudin-like structures here, to the right of the notebook.

This is within a slump unit, and you can see that there are fragments of fold hinges and aerofoil-shaped fold fragments that have become detached from the rest of their parent layers. I think what happens is that these isolated fragments then get rolled up as slumping continues, and you end up with something like this:

A polished section of one of the boudin-like structures, showing internal folding.

This one has a sheath-fold, if you look in the top left of the image. Not sure about this, but perhaps along-boudin extension occurs as the "boudin" gets flattened and rolls during slumpiing, analogous to rolling out a piece of plasticine.

Image of the week #3

Another one from the Suez Rift. This is my former colleague Chris Ott (hi Chris!), standing in front of a syn-rift boulder conglomerate containing various pre-rift and syn-rift clasts (the one to the right of Chris is probably a Precambrian basement basalt clast). These are actually some of the smaller clasts in the conglomerate: lower down in the same unit there are limestone blocks that are several tens of metres across, which makes mapping in the area "interesting". The unit is in the steeply dipping limb of a fault-propagation syncline in the hanging wall of a major block-bounding normal fault that was active during deposition. It is interpreted as a debris flow triggered by slope collapse.

Image of the week #2

This is from my PhD thesis. Although at first sight it probably looks like a cross-section through a fold/thrust belt, the scale is somewhat different. It's actually a scan of a large thin section. The folds and thrusts are a small-scale slump in finely interbedded sandstone and mudstone of the Albert Formation of New Brunswick, Canada. The base of the slump cuts down through the underlying stratigraphy from left to right. Having measured up a large number of folds in similar slump beds, I found that the palaeoslope was roughly perpenicular to the local faults. So these slumps may have formed during fault-induced slope instability.