The lower rocks in the Muav limestone are 505 million years old. The upper rocks in the Redwall limestone are 340 million years old. What could explain the gap?

This could be because no deposition took place or there was a long period of erosion that took away these layers of rocks. A lack of deposition would lead to a large gap in the rock strata, while a long period of erosion would have taken away whatever rocks had existed, leaving what would look like a gap that didn't have any rocks existing.