I agree with JohnnyBoy, probably sand mixed with clay.

What matters the most is HOW it is mixed. Based on your description, there are probably clay laminations (layers) in that soil. In some situations very thin clay layers are capable of sealing pure sands that exhibit greater than 25% porosity and excellent permeability.

Even more importantly, experimental results always trump theoretical results. If it holds water in situ, then DO NOT disturb that horizon in the places where you want it to seal your pond. (It is possible that once you disrupt the current layers, you will not be able to re-seal those layers - no matter how much compaction you use.)

Good luck on your new pond projects!