In some cases carbon dating and radiometric dating is circular reasoning because the objective method (radiometric dating) is not used at all. Rather a fossil is dated according to which rock strata it was found in and the accepted date of that rock strata. Now the accepted date of the rock strata was dated radiometrically, but in many cases the actual individual fossils and actual individual rock strata where it was found was not dated according to the methods.
This is what creationists mean when they talk about circular reasoning. Now I'd agree with Casey that this is not the real argument, but those assumptions are used in many individual cases without data directly backing them up. tragic mishap