Detailed gravity and magnetic surveys confirm seismic refraction evidence of a deep (8–10 km) Carboniferous infilled sedimentary basin (the Magdalen basin) between the Magdalen Islands and Newfoundland. The basin margins are associated with gravity and magnetic 'highs' indicating shallow depths (1–3 km) to pre-Carboniferous basement rocks east of the Magdalen Islands and between Cape Breton Island and Newfoundland. The central Magdalen basin is associated with a large amplitude gravity 'low' (50–60 mgal). This can be partly explained by a 2–4 km thickness of low density Late Mississippian sediments overlain by 1–2 km of Upper Pennsylvanian or younger sediments. The western Magdalen basin is associated with large amplitude (20–40 mgal) circular and elongate gravity 'lows'. Detailed interpretations suggest they are caused by low density evaporite structures. The circular 'lows' are interpreted as salt diapirs. The elongate 'lows' are interpreted as structures in the crest of Mississippian/Pennsylvanian anticlines similar to occurrences of Windsor Group evaporites in northern Nova Scotia. The western Magdalen basin may represent one of the largest areas of salt accumulation in Eastern Canada.