Conversation so far:
is the gulf of mexico an ocean or sea
The Gulf of Mexico is a marginal sea of the Atlantic Ocean.
what defines the Gulf of Mexico as a marginal sea
A marginal sea is a division of an ocean, partially enclosed by islands, archipelagos, or peninsulas, adjacent to or widely open to the open ocean at the surface.
where does the Gulf of Mexico end
The Gulf of Mexico extends out toward Cuba's Cape San Antonio and the Florida Keys.

Question: how was the basin formed
Self-contained rewrite: how was the Gulf of Mexico formed