Lowland Creek Volcanics Flow Fold

Not tectonic!

The fold above did not result from tectonic forces squeezing the rocks. It’s in the Lowland Creek Volcanics on the northwest side of Elk Park, Montana USA. While the rocks are mostly ash falls from 52- to 53-million-year-old volcanoes, the material was probably still warm enough that as it flowed down a slope it crumpled, like a pile of sheets of ribbon candy, or like thin rugs on an incline rippling over themselves.

One of the main reasons for thinking this is that there’s no obvious tectonic event in the past 50 million years in southwestern Montana that could squeeze the rocks to make such a fold, especially while leaving other equivalent, nearby rocks undeformed. In this case, the fold doesn’t even seem to continue across the narrow railroad cut where it is found, not even crossing to the rocks 40 feet away. So it had to be a pretty local feature. To deform this way, plastically rather than brittlely, it probably must have been pretty warm, even soft, but probably not actually molten.

Yes, it’s possible for extensional (normal) faults to produce drag folds, but an overturned fold like this would be really challenging to make via extension.

The ash that was falling from the volcano or volcanoes was mostly made of crystals from down inside the magma chamber plus rock fragments stripped from the volcano and other rocks the magma came through. Not much glass – so it wasn’t cooling really quickly to make obsidian, so it could have retained enough heat to allow the flow-folding described above.

At least, this is the consensus of me and the other geologist who was there with me (Mike Stickney). For those who know Butte, these rocks are about equivalent to the volcanic rocks that form Big Butte. Bicycle for scale at bottom of photo – I’m fortunate to ride my bike a lot in the complexities of the Lowland Creek Volcanics. 

I collected the fluorescent chalcedony in the specimen above about a mile from the location of the fold in the photo at top. Because volcanic rocks include a lot of gas in their magmas, it’s common to find openings lined with later chalcedony (fine-grained quartz) like the example in my photo, which is from just below the top of Sheepshead Mountain. The green fluorescence is probably caused by traces of the uranyl ion, UO₂, in the chalcedony.

You can find more about some mineralization in a different part of the volcanics in this post.