Field Photos: Folds- Tectonic Or Soft Sediment Deformation?

During my wandering in the high valleys of Kumaon I have been noticing some intriguing boulders of sandstone which have rolled down from the high ridges in the vicinity of Panchachuli Glacier and Milam Glacier. I have access to only these blocks and for now have not been able to observe them in outcrop.

But they do show some spectacular deformation as seen in the picture below.

Is this folding due to tectonic forces that crumpled up the sedimentary sequence in to a series of small scale folds much later in the history of this region, perhaps during Himalaya orogeny?. Or, is this soft sediment deformation that took place contemporaneously with the deposition of these sedimentary layers? This occurs when semi hardened sediment moves and slides down slopes during earthquakes, contorting layers into small folds.

At this point I have not been able to decide which of the two mechanisms is at play here. What these blocks do demonstrate very nicely is how materials with different rigidity respond differently to an applied force.

Look at the sandstone boulder below.

The folding is restricted only to the central part of the block. This may happen because that layer is soft enough to flow under stress. The more rigid layers enveloping it don't warp. 

Here is another example. Notice the tiny white veins (black arrows). They occur only within a thin rust colored layer. These to me appear to be tear cracks. These small fractures open up under the influence of localized tensile forces. The rust layer was hard and brittle enough to crack while the surrounding material was too soft.

Although I haven't been able to see these sandstones in outcrop, I do get the feeling that this deformation is very localized and doesn't affect the entire sequence. This is because I have observed blocks with undisturbed thin layering, like in this cross bedded slab below. This hints that intermittent soft sediment deformation is the likely explanation, but I can't say for sure without examining the layers in outcrop.

 And this final picture of a large boulder with some crazy contorted layers. 

This is why geology field work is so much fun. You keep coming across these puzzles that make you think and which stay with you long after you have packed up camp and returned home. 

Looking once again at these pictures, I am desperate to head back. And there is more to understand about these boulders. I'll be writing about their stratigraphy i.e., their position within the thick rock unit known as the Tethyan Sedimentary Sequence, and their significance in understanding the geologic events occurring along the northern margin of the Indian subcontinent in the early Paleozoic, about 500-475 million years ago.

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