The Middle Mesozoic Drift and Cooling Phase begins with the main phase of sea floor spreading, slowly but steadily opening the Gulf of Mexico basin. Initially hypersaline conditions resulted in basin-wide deposition of an original thickness of 4 km of evaporites (halite and updip anhydrite), called the Louann Salt, which likely formed with episodic seawater influx from the Atlantic Ocean. Strontium seawater analysis suggests 170 Ma as a proxy age for the Louann Salt. The arid eolian Norphlet Formation is subsequently deposited, followed by marine carbonates, evolving from ramp microbalites (Smackover) to platform margin reef systems of the Haynesville and Cotton Valley. Rafting apart of the Smackover and Norphlet in the northeast Gulf of Mexico began in this phase, possibly associated with oceanic crustal cooling which created a dip slope to the south and west. This set up a major new petroleum province which is host to several new giant oil discoveries. Periods of reduced bottom circulation resulted in at least two phases of source rock development, in the Oxfordian and Tithonian stages, that are linked to petroleum generation for both conventional and unconventional plays.