Morrissey Formation

Morrissey Formation Stratigraphic range: Late Jurassic (Portlandian) PreЄ Є O S D C P T J K Pg N ↓
Type	Geological formation
Unit of	Kootenay Group
Sub-units	Moose Mountain Member Weary Ridge Member
Underlies	Mist Mountain Formation
Overlies	Fernie Formation
Thickness	up to 80 metres (260 ft)
Lithology
Primary	sandstone
Other	siltstone, mudstone, coal
Location
Coordinates	50°30′N 114°59′W / 50.500°N 114.983°W / 50.500; -114.983 (Morrissey)
Region	British Columbia  Alberta
Country	Canada
Type section
Named for	Morrissey Ridge
Named by	D.W. Gibson[1]

The Morrissey Formation is a stratigraphic unit of Late Jurassic (Portlandian) age in the Western Canada Sedimentary Basin.^[2] It is named for outcrops on Morrissey Ridge, 16 kilometres (10 mi) southeast of Fernie, British Columbia, and is present in southeastern British Columbia and southwestern Alberta.^[1]

Stratigraphy and lithology

The Morrissey Formation is the basal unit of the Kootenay Group. It consists of massive, cliff-forming, fine- to coarse-grained sandstone, with minor beds and lenses of conglomeratic sandstone, and rare beds of siltstone, carbonaceous mudstone, and coal. The sequence coarsens upward and, in most areas, it can be subdivided into two members. The lower portion, called the Weary Ridge Member, consists of orange-brown weathering, slightly argillaceous, calcareous sandstone with rare siltstone and mudstone interbeds. The upper portion, called the Moose Mountain Member, consists of hard, medium grey to brownish grey weathering, siliceous sandstone with rare beds of carbonaceous mudstone and coal. The contact between the two members is abrupt but conformable.^[1][3][4]

Environment of deposition

The Morrissey sediments were derived from newly rising mountain ranges to the west and transported eastward by river systems. They were deposited along the edge of the Western Interior Seaway in extensive littoral, deltaic and beach environments. Thin beds of carbonaceous mudstone and coal near the top of the formation may have been deposited in interdune, lacustrine or swale environments.^[1]

Paleontology and age

Fossils, including microfossils, are very rare in the Morrissey Formation. Several genera of molluscs (e.g., Oxytoma, Modiolus, and possibly Pachyteuthis) have been collected from the Weary Ridge Member. A single impression and a few fragments of the ammonite Titanites occidentalis, which have been recovered from the top of the Moose Mountain Member, indicate a Late Jurassic (Portlandian) age.^[1] Impressions of logs are also known from the top of Moose Mountain Member.

Thickness and distribution

The Morrissey Formation extends from the Flathead River area of southeastern British Columbia near the Canada–United States border to the North Saskatchewan River in western Alberta. It has a maximum recorded thickness of 80 metres (260 ft) near Mist Mountain and the Highwood Pass in Alberta, and it thins eastward, reaching a zero edge along the eastern foothills of the Canadian Rockies in Alberta.^[1]

Relationship to other units

The Morrissey Formation abruptly but conformably overlies the "Passage Beds" at the top of the Fernie Formation, and is in turn abruptly but conformably overlain by the Mist Mountain Formation. To the north in the vicinity of the North Saskatchewan River it interfingers with and grades into the Nikanassin Formation.^[1] To the east it was truncated by pre-Aptian erosion and is unconformably overlain by the Cadomin Formation.^[2][5] Its correlation south of the Canada-United States border in Montana is uncertain.^[1]

References