I know just about enough about this to be dangerous but you mentioned rock in the area which means not coring the dam is not even a viable option, I just had a friend help me construct a big dam, a couple years ago, that was over 200ft wide at the base, 15A lake, we never did use a sheepsfoot roller and he never does, builds a lot of huge dams, including NRCS dams.
Mine is tight as a drum, but we did hit some rock in places and had a core nearly 15 ft deep in places with some serious gravel seams that had water pouring thru them, I'm convinced it would have leaked thru that seam if we had not eliminated it. Good Luck!
