OH I would NOT till an unknown mess under. You disturb the natural layers and biome and you are tilling IN any weeds. I agree with heavy mulching and clean soil AFTER you figure out what's in there (if you want) I'd just start from scratch.
Welcome to biting off a lot to chew!![]()
I totally agree @WthrLady. Tilling never worked for me. I learned from the Old Farmers Almanac planting tips that tilling actually encourages weed growth. Agricultural methods have been evolving towards "no till" and organic practices. I use mulch, and the card board is amazing. It kills the weeds and feeds your plants. Good stuff.