Restricted ocean or lake.
About my saying salt mine instead of salt domes:
I was in a salt MINE years ago (that is probably why I said “mine”) and that is near Grand Saline, Texas. My college class went there. There was a room as large as a convention center main auditorium and the company held meetings in this cavern of a room. All the different areas where the salt had been removed were huge. There is enough salt there to last for 20,000 more years - Morton Salt owns it.
Didn’t the salt get there because there was an ocean there at one time millions of years ago?
Restricted ocean or lake.