The Hudson Bay Basin is the largest intracratonic basin in North America, but remains a frontier area for our knowledge of its stratigraphy and sedimentology and its hydrocarbon potential. Large domal reefs (up to 10 m thick and 500 m wide) in the Upper Ordovician Red Head Rapids Formation on Southampton Island developed on the margin of this shallow-marine evaporitic basin in which physical and chemical seawater parameters were distinct from the open ocean and in which a diverse community of

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... ef-building and dwelling metazoans was unable to flourish. The main reef facies comprise boundstone and cementstone composed of various proportions of early-calcified sponge tissues, microbial encrusters, synsedimentary cement and small colonial metazoans. The accretionary mechanisms of the Red Head Rapids reefs were mainly the result of framebuilding by early-calcified sponges and small colonial corals and binding by calcimicrobial elements for the boundstone facies, and of massive aragonitic cement precipitation near the seafloor for the cementstone facies. These Upper Ordovician reefs, in which microbialites dominate but coexist with metazoans, were more widespread in the Early Ordovician immediately prior to the Middle to Late Ordovician expansion of skeletal-dominant reefs. The Upper Ordovician reefs on Southampton Island, porous and locally bitumen impregnated, underwent early marine, near-surface and progressive burial diagenesis; reducing its primary porosity but significantly increasing its secondary porosity. They represent one of the major untested petroleum play types identified in the Hudson Bay Basin.