Digging down for the new foundation, you found the melted asphalt huh.
I came across with a post hole digger when I was setting the batter boards to start the layout. I think it had been a stone lot with clean stone layer under asphalt pavement. Some of the lot was resurfaced for a drive and when that drive was taken out, whatever was left from the fire still was there about a foot underground. Kind of like gloobs of asphalt/rock.
here is a BLEVE film of a tank about a third of the size of the fuel tank in KC
http://www.bing.com/videos/search?q=famous+nevada+explosion&qpvt=famous+nevada+explosion&FORM=VDRE#view=detail&mid=8D0CE75527C6CD85F9E98D0CE75527C6CD85F9E9