Numerous researchers have alluded to the existence of a Cretaceous Hudson Arm connection between the Labrador Sea and the Western Interior Seaway of North America. However, the evidence for this marine connection has been circumstantial. In this paper we present sedimentary geochemical data that indicate a marine influence in the Albian Mattagami Formation of the Moose River basin, James Bay Lowlands, Ontario. The facies associations between dinoflagellate-bearing laminated mudstones, fluvial sandstones, and early pyrite mineralization are interpreted to indicate deposition in the central basin of an estuary. We use the facies association between the estuarine fill and coeval kaolinitic paleosols in the Moose River basin, and in similar deposits in Quebec and Labrador, to reconstruct a southern shoreline of the Albian Hudson Arm to the Cretaceous Western Interior Seaway. We suggest that development of the Hudson Arm connection between the Labrador Sea and the Cretaceous Western Interior Seaway may be related to a regional extensional regime associated with rifting between Labrador and Greenland, and the passage of eastern North America over Cretaceous hotspots.