I've seen a few versions, each with their problems. As a noncreationist projecting on a creationist theory, I would go with the Cambrian boundary. One could try to claim early life and rock were set down by God, but the major trouble is that most life Noah led up the ramp would not have evolved yet if you consider that pesky geologic column.
The only other 'logical' choice could be aroung the coal forming ages, but we have seen in debate here the problems it has. I need to kill a few brain cells and go google creationist flood theory.
ABB