Unit Name: Orby Head formation
Unit Type: Lithostratigraphic
Rank: Formation
Status: Undefined
Usage: Currently in use
Age Interval: Early Permian (299 - 270.6 ma)
Age Justification: The few plant fossils are poorly preserved and can not be identified. Van de Poll (1989) noted that in three wells with Orby Head strata forming the uppermost part, there are Stephanian to Early Permian spores. Vertebrate fossils, including the type specimen of Bathygnathus borealis, appear to be early Artinskian. That would be younger than the Lower Permian rocks of the Hillsborough Bay area (Langston, 1963). Based on the above evidence, van de Poll (1983) gave the age of the Orby Head formation as Early Permian (Artinskian).
Province/Territory: Prince Edward Island

Originator: van de Poll, 1989

Type Locality:
Van de Poll (1989) did not specify a type section. He stated that the best section for the type section would be coastal exposures west of Cape Tryon, Prince Edward Island (NTS 11 L/5), but the absence of the Assemblage A conglomerate excluded this. According to Articles 3 and 8 of the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature, 2005), formal publication of most geologic units requires designation of a stratotype. One is required for the Orby Head formation before it can be accepted as a formal unit.

Distribution:
The formation underlies the central-northern and northeastern part of Prince Edward Island and outcrops along the coast from Profitts Point in the west to East Point in the extreme northeast. In southeastern Prince Edward Island, the core of the Kilmuir Syncline is formed from an outlier of the Orby Head formation. Van de Poll (1989) did not give a thickness for the Orby Head.

Lithology:
The Orby Head formation is the fifth and youngest, fining-upward, megacyclic sequence of the Prince Edward Island Group. At the base of the formation are basal conglomerates (Assemblage A, low rhyolite, well rounded pebbles and cobbles), overlain by coarse to medium wacke. The sequence is composed of fining-upward red beds. Fluvioclastic strata in which floodplain, channel and levee-splay facies associations can be recognized (van de Poll, 1983; Foley, 1984). The formation represents a major, fining-upward, megacycle sequence, which is made up of smaller-scale fluvial cycles also comprising channel, levee-splay and floodplain facies associations.

Relationship:
The lower boundary with the underlying Malpeque and Wood Islands members of the Hillsborough River Formation is not exposed. Van de Poll (1989) arbitrarily set it just below the basal conglomerates of the Orby Head. The top of the Orby Head formation lies to the north of Prince Edward Island in the Gulf of St. Lawrence, and presumably marks the late Paleozoic unconformity, which separates the Carboniferous and Permian rocks from Triassic strata. The Orby Head Formation is one of five formations formerly included in the Prince Edward Island Group by van de Poll (1989). In ascending order the five formations are: the Miminegash, the Egmont Bay, the Kildare Capes, the Hillsborough River and the Orby Head formations. Ryan et al. (1991) formerly abandoned the name Prince Edward Island Group, considering it to be a junior synonym of the Pictou Group. Accordingly, the Orby Head Formation is now included in the Pictou Group.

History:
The age of the red beds in Prince Edward Island has generated considerable debate. Dawson (1848) believed they could be in part post-Carboniferous, possibly Triassic. This was based on the belief that the vertebrate Bathygnathus borealis, whose type specimen was recovered from a water well dug into the Orby Head formation, was Triassic. However, Case (1905) and Huene (1905) both demonstrated that Bathygnathus was Permian. Subsequently, macroflora and palynomorphs have confirmed that the Prince Edward Island Group is Carboniferous-Permian and the vertebrates have confirmed that the Orby Head is Artinskian.

Other Citations:
Case, 1905; Dawson, 1848; Foley, 1984; Huene, 1905; Langston, 1963; North American Commission on Stratigraphic Nomenclature, 2005; van de Poll, 1983, 1989.

References:
Case, E.E. 1905: Bathygnathus borealis Leidy, and the Permian of Prince Edward Island. Science, v. 22, pp. 52-53.
Dawson, J.W. 1848: On the New Red Sandstone of Nova Scotia. Quaterly Journal of the Geological Society of London, vol. 4, pp. 50-59.
Foley, P.L. 1984: Depositional setting of the Permo-Carboniferous redbeds around Hillsborough Bay, Prince Edward Island, Canada. Unpublished M.Sc. Thesis, University of New Brunswick.
Huene, F. von. 1905: Pelycosaurier im Deutschen Muschelkalk. Neues Jahrbuch für Mineralogie, vol. 20, pp. 321-353.
Langston, W., Jr., 1963. Fossil vertebrates and the Late Paleozoic red beds of Prince Edward Island: National Museum of Canada, Bulletin 187, 36 p.
North American Commission on Stratigraphic Nomenclature., 2005. North American Stratigraphic Code; American Association of Petroleum Geologists, Bulletin, vol. 89, pp. 1547-1591.
Van de Poll, H.W. 1989. Lithostratigraphy of the Prince Edward Island redbeds; Atlantic Geology, vol.25, no.1, pp.23-35.
van de Poll, H.W., 1983. Geology of Prince Edward Island: Prince Edward Island Department of Energy and Forestry, Report 83-1, 66 p.

Source: GEOLOGICAL SURVEY OF CANADA
Contributor: G.L. Williams
Entry Reviewed: No
Name Set: Lithostratigraphic Lexicon
LastChange: 04 Dec 2007