You are stuck in the same mire as the other posters. U-238 can't date things of known age accurately. Why accept it as valid for things of unknown age?
When you say "of known age" you mean things that first came into existence within the last few thousand or few hundred years. The problem with that argument is similar to the problem of weighing yourself on a scale designed for weighing tractor trailers. While such a scale is ideal for weighing tractor trailers, you are certain to get a wrong weight for a human being because that scale is inappropriate for measuring things of so little weight.
Try, also, measuring 8.4 grains of gun powder on your bathroom scale. Same problem. That doesn't mean your bathroom scale isn't perfectly fine for weighing yourself on.
