A reasoning is logically sound if it applies valid reasoning rules on well formed propositions. Young earth can be both false and logically sound, in that case

A reasoning is logically sound if it applies valid reasoning rules on well formed propositions. Young earth can be both false and logically sound, in that case it would also be the case that some of its assumptions or models need to be wrong. Another example of a logically sound yet wrong theory is the Ptolemaic model of the solar system, if nothing else because by Fourier analysis it follows that you can approximate well any periodic orbit using enough epicycles.