Travis, that is going to be a VERY challenging environment to build a pond. I have a similar situation on our farm, but at a much smaller scale - so it is more manageable.

A 15' thick layer of unconsolidated (loose) sand is going to be tough. As the dragline or excavator removes sand below the water level, adjacent sand will just flow into the newly created low spot. You may have seen sand pit operations in your area that have created a deep pond. Usually they use a dredge to pump out wet sand and water. Only after they have sold thousands of tons of sand does a pond with significant depth get created.

I believe you should try again to get samples with the backhoe or mini-excavator. You did not need to quit digging when you hit water last time. Keep excavating in the same slot trench next time. If the sand is mixed with some clay material it may stay in place and you can dig down to the max depth of your equipment and see if you encounter the underlying clay layer. If the sand is flowing, then you will not be able to dig down to maximum depth. The sides of your slot trench will start collapsing and your hole will get wider. Try several spots on your property. If the sand layer has a variable thickness, or has some areas with significant clay mixed in (to make it stable), then you may be able to construct a pond in that area. (If the clay layer is 15' deep everywhere, then you are beyond the maximum depth of a typical backhoe or mini-excavator. If that is the case, you will need to pay a contractor with a larger excavator to dig your sample sites.)

If your sand layer will flow when wet, then the only feasible way to create a "sealed type" of pond is to drain your working area and work with dry sand. The method would be to create the deepest sump possible and pump out water to a distant location so that it did not just return to your pond by re-entering the sand layer. (If your have a 15' thick sand layer, it might not even be possible for you to pump your pond area dry.) After you get the water level down to the clay layer, you can pull clay up with an excavator and start lining and compacting the sides of your pond with clay to eventually seal the sand layer. If you can create a sealed pond to the depth you desire, you will then have to have a means of keeping if full of water. On your flat ground, I don't believe the watershed will direct enough water into your pond. However, you would still have an excellent groundwater supply of water in the areas outside of your "sealed" pond. You could use your little areas that are 3' deep and pump water into your pond. That is not much cost since you are barely "lifting" the water and not pumping it very far. However, you must determine if that is legal in Michigan. I believe that type of operation is legal in some states and illegal in other states.

Sorry for the bad news, I think it is going to be difficult and/or very expensive for you to build the pond you are describing, IF I am understanding the ground conditions correctly. (Hopefully I am wrong.)

However, the good news is that your neighbors do have ponds. Obviously there is some viable way to build ponds in the area. Have you struck up a friendship with any of the neighbors yet? Asking them how they constructed their ponds, or asking them to show you their pond site might be a huge benefit.

Perhaps they are on lower ground and they only had a foot thickness of the sand layer. In that case, they just needed to dig their pond in the clay and the groundwater in the sand layer keeps their pond mostly full. Perhaps their sand layer is absent, and they just excavated into the clay and created a surface run-off (watershed) pond. You should be able to determine if they have that type of pond from the satellite images. The pond should be in a topographic depression and have a more or less straight dam on the low side.

Keep gathering more info and posting on Pond Boss. We will try to help you as much as possible. (I am a geologist, not a pond building expert. So the more accurate information you can give me about the ground materials, the more accurate estimates I can give you about how they will react to your construction operations. However, remote information given over the internet is never as valuable as the advice given by a good pond-builder that is on location. The hard part is determining if you have a "good" pond builder!)

Good luck on your future pond! I hope you can manage to pull off some favorable outcome.