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A broad area of shallow-shelf carbonate beds of the Nahanni Formation (Eifelian-Givetian) borders the Canadian shield in the District of Mackenzie, N.W.T. To the west, in the region of Deadman's Valley in the South Nahanni River area, the carbonate rocks grade into a basinal facies within about 3 mi (5 km).

Examination of the sediments and fauna across this transition permitted correlation between the shelf and the basin and recognition of several faunal communities. On the basis of (1) environmental deductions interpreted for these faunal communities and (2) lithosome geometry and the character of the sediments, it is concluded that the Nahanni sediments were deposited on a broad, slightly restricted, shallow shelf, and that their basinal equivalents were deposited in a marginal cratonic trough. The shelf and trough were separated by a moderately steep shelf edge, or epeiric slope. A shoal was developed near the edge of the shelf.

The boundary between the shelf and the deeper basin during deposition of the Arnica sediments (Eifelian and earlier) was much more abrupt, and was defined by the Manetoe barrier reef.

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