This is actually a crater if you look at it from the top. They still do not know how this was formed. What had happened is that the layers of rock that are normally underneath have popped to the top. There are a couple of theories. One is that there was a weakness in the layers that caused the salt layers that were underneath to pop through. This would be called a salt dome in that case. The other theory is a meteor hit that blasted through the layers. This should be called a meteor dome in that case.

https://en.wikipedia.org/wiki/Upheaval_Dome