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Photos of geological structures - SW Portuguese coast (#2): boudins
Imagine a geological sandwich of a competent (high viscosity) layer of rock, interbedded within two low competent (low viscosity) ones.
Fig.1 - Chocolate tablet boudinage pattern in Carboniferous sandstone (Almograve beach, SW Portuguese coast)
Now imagine that you stretch this sandwich by submitting it to extensional forces (you can imagine pulling in opposite directions both edges of our geo-sandwich, or squeezing it along the direction perpendicularly to its tabular shape).
What happens in the inner high viscosity layer? (Keep in mind that a sandwich is an ultra simplification of reality, namely because in nature we would be dealing with a much greater number of buried thick strata.)
"High viscosity" means that the rock doesn't like to flow at all. In this case it can behave elastically meaning that during a certain time it only responds to the applied extension by imperceptibly (and infinitesimally) changing its shape. During this time, if the applied extensional force ceases (i.e. if by some reason we stop pulling the sandwich) the rock completely recovers its original shape. Eventually, a limit is reached (the so called yield point) and the rock layer breaks apart in several pieces (deforms in a brittle way) - these pieces are called boudins (check Google image to learn the original meaning of this French word).
Fig.2 - Boudins of sandstone interbedded within shales in Almograve beach (Carboniferous of SW Portuguese coast)
What happens in the outer layers?
The "softer", less competent, outer layers display a completely different response (a ductile one) to the same extensional forces; as a consequence of their low viscosity they are prone to flow, i.e. to gradually change their shape along geological time. As a result, instead of breaking into pieces they get thinner, and tend to gradually contour the simultaneously forming boudins.
The result is a boudinaged deformation pattern (see photos).
In some situations the more competent inner layer isn't completely separated into several individual pieces, but instead boudins are separated by short abruptly thinned areas called “necking zones”. Concentration of fractures in these "necks" is common (since they correspond to weaker zones), and they often exhibit abundant quartz veins, tension gashes, etc.
Fig.3 - Boudins of sandstone Almograve beach (Carboniferous of SW Portuguese coast). Note that the length of the individual boudins is proportional to the thickness of the competent layers.
Boudins exhibit sharp edges and well defined geometric shapes, as oppose to more diffuse boundaries and round (barrel shaped) forms, when the viscosity contrast between the individual boudin and the matrix (i.e. the surrounding less competent rock) is extremely abrupt.
Fig.4 - A folded boudinaged layer of Carboniferous sandstone in Almograve beach (SW Portuguese coast). The fold formed before, simultaneous or after the boudinage? What is more likely to have occurred based on what is observed in the photo? (post your justified answer in the comment box below).
This is the case of the highly rotated anphibolite boudins found immersed within lower Paleozoic carbonate rocks (Ossa Morena Zone, SW Iberian Variscan orogen), which I have studied during my PhD a few years ago. We examined the rotation of these small boudins under non-coaxial deformation, and deciphered the relation of this with the generation of non-cylindrical and sheath folds in the layered carbonate matrix (here’s what we have published about it).