It would be interesting to take some of the water in a jar, and see how much hydrated lime it takes to neutralize it.
What would be even more interesting is take a lump of my crushed limestone clay and mix it with the black clay and stand back. I would be willing to bet there would be a lot of heat involved, and who knows what byproducts (probably salts).
Sounds like you found old deposits from perhaps a long defunct natural sulfur spring or some other type of hot-spring. Who knows? Pretty cool for curiosity sake, but lousy for a pond. I really feel bad for the owner.
