Mark 0 brain cells

Assuming the land was at or slightly above sea level prior to destruction then it is impossible. Asteroids would not cause a mass of land to sink by 1200 ft. FACT!
You either need to believe or choose to believe, either way you are not doing a lot for your cause by making such illogical statements.

If an object was to indent the earth by 400 meters it would be so massive and dense that there would be a crater miles wide, a series of smaller hits could never achieve it.
The only logical explanation for such a difference would be an earthquake.