GUEST:Stim, I can't see Richland Woman as being the source for Sweet Little Sixteen. There's not much similarity. Sweet Little Sixteen is just a twelve bar blues that starts on the I chord. Richland Woman is a sixteen bar song that starts on the V chord.

Taking a broad definition of what's "folk" and what's "pop", You could say that John Hartford's Gentle On My Mind, which started life as a bluegrass song was made pretty poppy by Elvis and Dean Martin.
Jerry Jeff's Mr Bojangles crossed over to pop when Sammy Davis Jr. recorded it.
How about this one:

Clarence Ashley - Banks Of The Ohio

Olivia Newton John - Banks Of The Ohio