It would be interesting to see what the content of the fill material is. That’s really a fascinating observation on your part.
There are cases where volcanic eruptions have buried entire cities but this has been nearly instantaneous. I still can’t figure out other cases except for blowing sands in the desert or jungle overgrowth in the tropics - either of which take centuries and would presuppose human abandonment altogether. Oops - I forgot that I did see one clip on tv about what appeared to be a beautiful city street in the Congo which was being totally overgrown with vegetation. Seems the Congolese didn’t give a rats ass about keeping anything working including the traffic lights after the Belgians left.