lbrooks, welcome to the forum. Have you contacted the NRCS and asked their opinion as to clay/location? If your county guy is not very helpful, give Sid VanDerveen a call in Houston, Mo. He's retired from the NRCS but loves to help out.
When my pond was finally sealed pretty well, under Sid's supervision, we did not line the entire pond with clay. We lined the main pool area then tied the lined clay into a strip running from hill to hill. We took the chance that the clay we tied in to covered the entire upper portion of the pond under the hard pan of gravel mixed soil. It appears to have worked.
FWIW, I'd suggest digging to the clay in the entire pool area as my pond cost 4K to build (wrong) and 20K+ to repair!
