There is a ruling on Zoetic Cavern that states:
If Blood Moon is on the battlefield and a player controls a face-down Zoetic Cavern, it can’t be turned face up since it won’t have a morph cost.
This ruling comes from rule 702.36e, which states among other things:
If the permanent wouldn’t have a morph cost if it were face up, it can’t be turned face up this way.
So the Sun stops your Cavern from being turned face up.