A dolomite bed averaging 6 m in thickness forms the base of a limestone and shale facies of the Bonneterre Dolomite (Cambrian), southeastern Missouri. The dolomite immediately overlies the Lamotte Sandstone and covers an area of more than 16 600 km2. It is proposed that this basal dolomite formed as a result of interaction with warm, basin-derived water circulating through the underlying Lamotte Sandstone, and that the dolomitization event coincided with the emplacement of Mississippi Valley–type sulfide ores in the nearby Viburnum Trend. This hypothesis is supported by (1) petrographic evidence that suggests an origin at temperatures >50 °C, (2) cathodoluminescent microstratigraphy of dolomite cements which is correlative with that in gangue dolomite cements in the nearby orebodies, and (3) stable oxygen and carbon isotope values that are consistent with a basinal origin of the dolomitizing water. The basal dolomite bed may represent one of the most extensive epigenetic dolomites yet documented. This implies that a basinal water can alter rocks over a wide area at a considerable distance from its source.