This one is a funny/weird one. I'll quote the central question:
But are there any occurrence of two different cultures having similarities in their mythologies but no known interactions by which they could have transferred their mythologies to each other?
There are many pairs of cultures with similar mythologies, but rather few that never interacted with each other. I can't think of any. So, while this question seems too broad, a comprehensive answer would likely be small.
I think that there are other issues which cloud the issue (ha):
- "similar" with respect to mythologies isn't clarified
- same issue with "no known interactions" (this has since been clarified though)
- It would be hard to have a comprehensive answer, so there would probably be multiple partial answers
Now, if the question were about whole mythologies, and especially if "similar" was clarified, then I would err on the side of not too broad. However, the question title also narrows the scope to "creation myths"...which, ironically enough, makes this too broad. Creation myths (and Flood myths) tend to be surprisingly similar.
My verdict: I have voted to close this question as "too broad" pending clarification of "similar".