SILURIAN PINNACLE REEFS OF THE CANADIAN ARCTIC

Silurian pinnacle reefs, the first described in the Canadian Arctic Ar chipelago, are exposed on Ellesmere and Devon islands. Two main reef t rends occur, one of early middle Llandovery to middle Ludlow age and a second of middle Ludlow to Late Silurian or Early Devonian age. Reefs of both phases contain lime mudstone cores: some are stromatactoid-ri ch, and others consist predominantly of microbialite-rich lime mudston e or microbial boundstone. Facies sequences of both reef phases show e vidence of upward-shallowing overall, but in the older reefs, isochron ous capping facies are dominated either by coral-microbial boundstone, an unusual reef facies for the Silurian, or by stromatoporoid boundst one and floatstone. This difference perhaps reflects variation in wave stress and the apparent ability of a few corals thickly encrusted by, or associated with, microbial boundstone and skeletal algae to withst and greater wave energy than a stromatoporoid-coral-rich reef communit y.