The Great Unconformity

The Great Unconformity is a paleontological mystery: there is a stratigraphic boundary where the rocks below are much, much older than the rocks above. An explanation has been proposed: it's the result of worldwide erosion by glaciers during one of the Snowball Earth episodes. It may also explain the apparent suddenness of the Cambrian Explosion, when the number and diversity of life forms on Earth rose sharply.

I remember Isaac Asimov, in one of his 1960s science columns, proposing that it was the result of massive tides from the much-nearer Moon, but that has the disadvantage of not explaining the suddenness of the transition. Still, it seems that he was in the vicinity of a better answer.