LOWER SILURIAN REEFS AND POST-REEF BEDS OF THE ATTAWAPISKAT FORMATION, HUDSON-BAY PLATFORM, NORTHERN ONTARIO

Outcrop exposures along the Attawapiskat River in the Hudson Bay Lowla nds of northern Ontario clearly reveal the morphology of Silurian (upp er Llandoverian) reefs of the Attawapiskat Formation and the relations hips between reef cores, flanking beds, and post-reef beds. These rela tionships indicate that the reefs had a syndepositional relief of at l east 8-10 m. The relief of the reefs is indicated by debris-flow lense s encased within flanking beds, a debris-flow fan at the base of a ree f, thin reef-flanking beds truncated against a vertical reef face, pen econtemporaneous brachiopod beds on the flanks of reefs, and three lar ge slide blocks that apparently slid over the side of a reef. Reefs of the Attawapiskat Formation are largely limestones with a wide range o f lithologies. from stromatoporoid-, coral-, and cement-rich boundston es to alga-cement-rich boundstones. Thick, laterally extensive Nuia gr ainstone beds, a product of widespread Nuia monocultures in supratidal to intertidal ponds, are present above the reefs. The most important diagenetic processes were early marine cementation (predominantly radi al-fibrous calcite), shallow burial diagenesis, and pervasive neomorph ism. Schematic reconstructions of the depositional history of the Atta wapiskat Formation in outcrop, constructed from observations of outcro p relationships, show a stratigraphic succession that was controlled b y relative-sea-level changes. Reefs growth was terminated by a relativ e-sea-level fall, but subsequent minor relative-sea-level fluctuations resulted in alternating deposition of supratidal to shallow subtidal sediments for a short time before the final retreat of the Silurian se as from the Hudson Bay Platform; only supratidal evaporite facies are present in the remainder of the Silurian section above the Attawapiska t Formation.