BZ - not really surprised. The key is that you are in a sand/gravel pit that is down in the water table. Movement of that shallow ground water seems to be the key. Ice is always thinner on such ponds, and even though ground water is not supposed to have dissolved oxygen, I can't help but think that there must be some in the near-surface water??
I always say that we need about 8 feet around here to be safe with a gravel pit, but 15-20 for a hill pond (dam and watershed). I'm actually encouraged that you have done so well with what is perhaps a "marginal" depth for your pond. Pretty darn cool. I may have to modify my depths, and maybe say 6-8 instead? My wife and I were just talking about this last weekend, wondering if we could find a pit to buy somewhere near town. Do I sound like the rest of the pondmeisters here??
