The Persian Gulf, today, looks like this.
But about 4,000 years ago, it looked like this.
This extended to the point where Ur, currently an inland city, was a coastal city. During this era, no other coastlines that I am aware of have had such a dramatic change. The research I've done has not yielded many results. I do believe that a while ago, I read something saying a flood of the Tigris and Euphrates rivers deposited sediment lower down. I was unable to find this again.
So my question is, what were the cause(s) of the change of coastline?