You can also use the bucket on the track loader to help compact the berm that you are building. Put enough down pressure on the berm to raise the front of the track loader up off the ground, roll the bucket back to have it roll over the berm like a rolling pin, that will will help compress the berm but still keep it in place. No need for the concrete over the top, I'd take the Geotextile fabric, and run it up and over the top of the berm and bury the end that is towards the deepest part of the pond under a foot or so of dirt, packing it down with the bucket of the track loader to keep the Geotex fabric in place. The Geotextile fabric will help keep the sand from mixing with the dirt on the bottom of the pond.
When putting down gravel driveways, putting down Geotextile fabric does the same thing, making the gravel driveways last longer by keeping the gravel from sinking down into the dirt.