All good advice so far.
A compacted clay blanket at least 12-inches thick is necessary with suspect soils as you describe. Mixing or "processing" clay soils is a must, with proper moisture content to get adequate compaction.
Here's the analogy I use all the time, "What holds water better than anything on the planet?....a bathtub, right? Except for that little-bitty hole down at one end." Make sure your pond doesn't have any of those "little bitty holes" and you'll be successful.
The Finger Lakes are notorious for mixed soils due to glacial movement 10,000 years ago. Drumlins, round rocks, mixed soil types is common there. But, there are some fantastic manmade lakes and ponds all over that part of the country, too.
