some thoughts from an old excavater. Sand, gravel and ponds don't mix. Just finished building one two weeks ago with the same problem. Dug test holes and from good clay. Was just about done and the dozer started pumping up water. That is a bad sign of whats below. I found just about the same mixture you found. I say don't go deeper, it will get worse. I did my best to line the bottom with clay. But once it starts to pump water up from the weight of the dozer, things get worse. I got about a foot of clay over most of it. I am still praying.
