FR,

Knowing you are in tall grass prairie country, you may be surprised at just how much organic matter and nutrients are in the topsoil. Sounds like you may have a hard pan underlying. To me its all good. However be careful of have much topsoil remains in the pond. Organic percent greater than 3% is borderline hyper-eutrophic and any shallow areas inundated that have that level or greater organic matter content will grow vegetation prolifically.

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers