Otto,
I have had 2 test holes dug that went down 10-12 ft. and there was clay starting around 2ft down with thicker clay down further. The guy who dug the clay and the second guy who came out to give a second quote both said that having clay was not a problem... plenty there. It was pretty much solid clay at a certain level.
I spent part of the day trying to find a 'home' for the left over clay so that the dike/ridge around the pond can be minimized and thus allow the pond to be pretty much at the same level as the surrounding ground. The town we live near lost a portion of a damn during this years Iowa flood. I called city hall and they contacted the engineer in charge of the project and then called me back to say "yeah he definitely is interested". That was an idea from Theo (thanks Theo) and it seems to have paid off! I may in fact talk with the engineer to see the time frame they have for the damn repair/construction... and say "hey if you can build the pond for me you can have the clay AND set your own timetable for when to do it" -- but even if they can take the clay... I'll feel great! There have been lots of ponds built recently along the 'Avenue of the Saints' highway and I'm thinking an engineer would have a pretty good idea how to build one.
Any other questions for me to answer? I really appreciate the wealth informationa and exertise and willingness to help/educate.