B Bug,
You are correct, your soil should still be workable. However, even if you only get a 1/2" of frozen damp clay material overnight, your equipment will easily work through it - but it will not compact correctly when piled into the core trench.
The other thing is that pits can really attract cold air. I had frost all around my lowest pond one fall morning when I went out to work early. Just happened to mention it to my farmer neighbor that day. He has a max/min thermometer, and said it only got down to 34F that night. It was a windless night, so I am pretty sure the depression of my pond drew in the cool, sinking air from the surrounding area, and actually got colder than the ambient temperature. If you start your pond in a low spot on your property and make an even deeper spot, you may slightly observe the same phenomenon.
Of course, IMO your biggest problem is probably to have a big precipitation event get your pond excavation project really wet when you are only half done. If you see rain on the way, please send it to someone on the forum that has low ponds.
Good luck on you pond build!
