The Mercy Bay Member (Frasnian) is a 200-ft thick limestone unit that is present in the 4,000-ft sequence of terrigenous clastic rocks of the Upper Devonian, Griper Bay Formation of northeastern Banks Island, Northwest Territories. The limestone crops out as extensive steep cliffs and thus affords the detailed study of vertical and lateral facies variations.

In the western part of the outcrop area, the Mercy Bay Member is an areally extensive biostrome which within itself consists of bioherms, penecontemporaneous interbiohermal strata, and biostromal beds. The bioherms consist of a lower zone of corals and tabular stromatoporoids in a carbonate-mud matrix and an upper zone almost entirely of massive stromatoporoids. These 2 facies form a massive core. Bedded deposits of coral and stromatoporoid rubble form the flanks of the bioherms. Interbiohermal strata are finely bedded, unfossiliferous, argillaceous micrite. Areally extensive, thick beds, composed almost entirely of massive stromatoporoid colonies, normally overlie the biohermal masses.

A vertical sequence of environmental development can be interpreted: the lower coral zone represents a stage of biogenetic mud mounds which formed in the subturbulent zone; the massive stromatoporoid zone represents development of true organic reefs in turbulent water; the massive stromatoporoid biostrome represents a table reef which developed in turbulent water in response to a reduced rate of subsidence.

In the east the member consists of isolated bioherms with younger, coarse, terrigenous clastic rocks in the interbiohermal areas. This fact implies that a shelf-to-basin transition occurs from west to east; biostromal deposits characterize the slowly subsiding shelf; bioherms characterize the more rapidly subsiding basin.

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