Field Photos: Boudinage In Sandstone

Boudinage is a structure that is developed when rock layers are being pulled and stretched. The term is derived from the French word for sausage. A rock layer is deformed by necking and is segmented into a string of sausages. The structure is best developed when there is contrast in competence or strength in a rock pile. The stronger layer is broken up in boudins, while the weaker layer 'flows' around and fills the gaps created in the neighboring boudin layer.

I wanted to showcase examples of boudins from igneous, sedimentary, and metamorphic rocks. 

The first example has been recently documented in a paper on silica rich magmatism from Sausrashtra by Anmol Naik and colleagues. The photograph shows a rhyolite with boudinage (black arrow) developed in a lava flow band. This deformation is due to layer parallel stretching during flow of a viscous lava.

 Source: Anmol Naik et.al. Geological Magazine 2023

The second instance is from Darma Valley, Kumaon,  near the village of Philum, not so far from the Panchachuli Glacier. It is a remarkable instance of boudins developing in a sandstone. Black arrows highlight elliptical and oval knobs, fragments of a once continuous sand rich layer. Notice how the surrounding thinly layered material has flown into the gaps between these boudins. The more competent sand layer broke up, while the softer clay rich material got wrinkled and warped but did not break. 

And lastly, here is an example of the more common variety of boudinage encountered in the field. This has developed in a high grade metamorphic rock. The lighter more competent layer made up of quartz and feldspar has been deformed into a series of boudins, while the mica rich grey layers are ductile. This outcrop is also from Darma Valley, Kumaon, near the village of Baaling. 

The three examples showcase deformation which produced similar structures in very different settings. Internal forces generated during flow of a viscous lava can result in folding and boudins. In sedimentary basins, similar forces may  affect a sand rich slurry moving down slope as a gravity flow or on semi consolidated sediment shaken during an earthquake. And forces acting on high grade metamorphic rocks  produce some spectacular boudins at high temperatures during episodes of mountain building.

Documenting rock deformation in the field is fun!