I would think that one of the difficulties in dating African musical traditions in the U.S. would be the probablity that African traditions were being constantly revitalized, revised and expanded by new arrivals from Africa for as long as the slave trade was in operation, whether legally or illegally. "Pure" African music heard in the U.S. in, say, 1880, might have survived in the U.S. for two hundred years, or it might have survived for a mere thirty years or less.