The Amoco–Imp–Skelly Osprey H-84 well, located in the Carson Subbasin on the eastern Grand Banks of Newfoundland, penetrated 2054 m of evaporites consisting almost exclusively of halite. These are dated palynologically as Carnian–Norian to Hettangian–Sinemurian and establish the occurrence of Upper Triassic evaporites on the western side of the North Atlantic. A lateral-fractionation model is proposed to explain the progressive decrease in less soluble evaporitic minerals west of the western periphery of Tethys. According to this model, slow and intermittent encroachment of the sea into the developing intracontinental graben system occurred after initial rifting. This encroachment took place in a hot and arid climatic zone and led to the brines becoming progressively depleted in less soluble salts towards the distal margins of the basin system. The similarity in composition of the Upper Triassic evaporitic sequences on the Grand Banks and in the Lusitanian Basin indicates that the two areas were close at this time. This would preclude major rotation of the Iberian Peninsula and the presence of a cratonic arch between the Iberian Peninsula and Grand Banks.