The Horn Plateau Formation is a Middle Devonian limestone reef approximately 0.4 mi in diameter and 400 ft thick which crops out in the Interior Plains about 110 mi west of Yellowknife, Northwest Territories. The study is based on examination of material collected from the outcrop and 5 core-holes drilled on and near the reef.

Corals are the most important framebuilders present and crinoid ossicles contribute most to the sediment volume. Three macrofacies of this coral patch-reef are recognized: 1) organic reef, an area where framebuilders grew profusely, 2) reef flat, an area characterized by sand-sized sediments, and 3) reef flank, an area of predominantly gravel-sized sediments. Strongly agitated water conditions prevailed over the organic-reef and reef-flank macrofacies whereas moderately agitated water covered the reef flat. Subdivision of the macrofacies has produced 11 microfacies of distinctive texture-composition combinations. Paleoecologic analysis of the individual organism or group permits recognition of 6 successive biotopes from a deeper offreef position up onto the reef flat, namely; (1) bryozoan, (2) solitary coral, (3) massive coral and stromatoporoid, (4) Thamnopora, (5) crinoid and brachiopod, and (6) Stachyodes; prevailing winds were from the northeast. The Horn Plateau reef may be analogous to the Recent Sahul Shelf reefs off Western Australia.

A new interpretation of the age of the Horn Plateau reef results from consideration of the additional fauna collected. The reef is middle Givetian in age, and is correlative with the middle Pine Point and upper Ramparts formations. It “roots in” the Lonely Bay Formation, and is overlain by the black shale of the Horn River Formation.

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