The Double Mer Formation is a sequence of redbed arkosic sandstone, conglomerate, siltstone, and shale in the Grenville Province of eastern Labrador. The formation is closely associated with a rift system extending inland for at least 300 km from the Labrador coast. Substantial thicknesses of strata of Double Mer Formation are confined to two basins, the Lake Melville graben and the Double Mer half graben. Strata correlated with the Double Mer Formation are found elsewhere in southeast Labrador; one isolated outcrop occurs within the Sandwich Bay graben, a separate, smaller basin 100 km to the southeast that trends parallel to the Lake Melville graben.The only definite age constraint on the Double Mer Formation is that it postdates Grenvillian deformation. Previously, the formation was assigned a latest Precambrian to Early Cambrian age based on lithological similarity with the Lower Cambrian Bradore Formation of the southeast coast of Labrador. On the basis of a structural model proposed here that suggests a link between graben configuration and latest Precambrian – Early Cambrian mafic dike trends, we concur with this viewpoint. The Double Mer Formation thus preserves the basin fill presumed to have accompanied graben formation, which, together with emplacement of mafic dikes and anorogenic plutons, documents latest Precambrian to Early Cambrian crustal extension that eventually led to the opening of the Iapetus Ocean.