At the northern end of the Hudson Bay, the massive cores have been loosely described in the past as micritic, algal, or microbial limestones with metazoans. The Red Head Rapids Formation mounds are locally partly  dolomitized. The reefs contain large voids and vugs that are locally filled with bitumen and dead oil, composed of boundstone and cementstone  with various proportions of early calcified sponge tissues, microbial encrusters, synsedimentary cement and small  metazoans. The accretionary mechanisms of the Red Head Rapids Formation were mainly the result of frame building by early calcified sponges and small colonial corals and binding by calcimicrobial elements for the boundstone facies, and of marine cement precipitation near the seafloor for the cementstone facies.