Distribution:

The region in which the Horn Plateau reefs are present is limited to the northwest by the boundary between the Nahanni and Lonely Bay formations, and to the southwest by the northern edge of the Pine Point dolostone. In the Deep Bay and Arrowhead River areas, the Lonely Bay Formation is overlain by a thick unit of fossiliferous limestone in which one can locally recognize equivalents of the Bituminous shale and limestone beds and the Undivided map unit (Sulphur Point-Watt Mountian-Slave Point). Fuller and Pollock (1972) assumed that the wells that penetrated this thick unit of fossiliferous limestone are located over a "high reef". If the fossilferous limestone is relatively thin, the well is located over a "low reef". It is difficult to report on the thickness of the Horn Plateau reefs because of the transitional lower boundary in the reef core. It is thus better to refer to the height as measured above the Lonely Bay platform. The top of the exposed reef mound near Fawn Lake is presumed to have a height of 122.5 m (402 ft.), and has a circular outline with a diameter of about 1.28 km (Vopni and Lerbekmo, 1972). If one assumes that the wells that penetrated the buried Horn Plateau reefs were drilled near the crest of the mound, then the maxium height of the buried reefs ranges from 60 to 84 m (197 to 276 ft.) (Meijer Drees, 1993).

Lithology:

Isolated limestone mounds occurring north and northwest of the northern margin of the Pine Point dolostone or Presqu'ile Barrier (Meijer Drees, 1993).

Relationship:

The mounds directly overlie the Lonely Bay limestone and are surrounded and overlain by shale. The Horn Plateau reefs transitionally overlie the Lonely Bay Formation and it is difficult to select the boundary with the aid of well cuttings and borehole logs. The boundary between the Horn Plateau reefs and the overlying shale is sharp and probably coincides with a hiatus (Fuller and Pollock, 1972) (Meijer Drees, 1993).

History:

The informal term Horn Plateau reefs is used by Meijer Drees (1993). Although the Horn Plateau reefs are mappable entities, their local and unpredictable distribution does not make them useful, subsurface map units. The transitional boundary between the Platform limestone and Mount limestone facies of the Lonely Bay Formation and the Horn Plateau reefs, and the fact that the reef mound facies is also present in the upper part of the Nahanni Formation, make it difficult to treat them as a unit of formational status. The term Horn Plateau Formation as originally defined by McLaren and Norris (1964) and Norris (1965a) needs revision. Vopni and Lerbekmo (1972) revealed that the carbonate mond near Fawn Lake, which represents the type section of the Horn Plateau Formation, probably rests directly on the Lonely Bay Formation, yet the definition of the formation by Norris (1965a) states that the reefal beds are presumed to overlie the Horn River Formation (the Bituminous shale and limestone beds). Most reef mounds in the subsurface of the Great Slave Plain directly overlie the Lonely Bay Formation, but there are reef mounds in the Deep Bay area that overlie beds belonging to the Horn River Formation (the Bituminous shale and limestone beds). Since, at the type section, it is not absolutely certain whether the Horn Plateau Formation directly overlies the Lonely Bay Formation or not, the term Horn Plateau Formation is ambiguous (Hills et al., 1981) (Meijer Drees, 1993).

Remark:

Equivalent to Horn Plateau Formation.

References:

Fuller, J.G.C.M. and Pollock, C.A., 1972. Early exposure of Middle Devonian Reefs, southern Northwest Territories, Canada; in, Stratigraphy and Sedimentology - Stratigraphie et Sedimentologie, Section 6; International Geological Congress, Report of the 24th Session, vol. 24, no. 6, pp. 144-155.

Hills, L.V., Sangster, E.V., and Suneby, L.B., 1981. Lexicon of Canadian Stratigraphy, Volume 2, Yukon Territory and District of MacKenzie: Canadian Society of Petroleum Geologists, 240 p.

McLaren, D.J. and Norris, A.W., 1964. Fauna of the Devonian Horn Plateau Formation, District of Mackenzie; Geological Survey of Canada, Bulletin 114, 74 p. + XVII Plates.

Meijer Drees, N.C., 1993. The Devonian succession in the subsurface of the Great Slave and Great Bear plains, Northwest Territories; Geological Survey of Canada, Bulletin 393, 231 pages.

Norris, A.W., 1965a. Stratigraphy of Middle Devonian and older Paleozoic rocks of the Great Slave Lake region, Northwest Territories; Geological Survey of Canada, Memoir 322, 180 p.

Vopni, L.K. and Lerbekmo, J.F., 1972. The Horn Plateau Formation, a Middle Devonian coral reef, Northwest Tenritories, Canada; Canadian Society of Petroleum Geologists, Bulletin of Canadian Petroleum Geology, vol. 20, no. 3, pp. 498-548.