It sounds like the culpret here is bentonite, an impure clay that generally forms from the weathering of volcanic ash. Oil well drillers love it because it absorbs large amounts of water and turns into a slurry that is easy to pump into the ground. What is strange here is that it has become active all of a sudden. I'm wondering if changing climate has mobilized it. If there was a layer of bentonite below a layer of some common type of compacted clay that began to shrink and crack due to lack of rain, then whenever the rains did come the water could find its way down to the bentonite. That could create a slippery layer for the overburden to slide on, but would be temporary as the moisture was absorbed by the surrounding sediments. At least til the next rain.
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