What about existing ponds in your area? Look at some that are in the same mapped soil unit and find out how water-tight they are or how they were constructed. For instance, was clay trucked in to line them or were they built entirely with soil from the site?

I still think the only way to know for sure what you have for soil is to have some test pits dug. Get someone with a backhoe and see what you got. Maybe find local pond builder who knows what he's talking about who can dig the pits. Or see if the NRCS can offer any help.

Good luck