Unit Name: Ellis Group
Unit Type: Lithostratigraphic
Usage: Currently in use
Age Interval: Bajocian - Oxfordian (171.6 - 155.6 ma)
Province/Territory: Alberta; Montana
Originator: Peale, A.C., 1893. Cobban et al. (1945) designated and described the Ellis type section.
Rocky Creek Canyon, near U.S. Highway 10, 6.1 km (3.8 mi) southeast of Fort Ellis, Montana; Sec. 19, Twp. 2S, Rge. 7E.
Recognized in the southern Alberta Plains and throughout northern and central Montana. Relief on the underlying post-Mississippian unconformity affects thicknesses locally, and the group thickens in general westward and eastward from the Sweetgrass Arch. At the type section the Ellis is 88 m (289 ft) thick; it ranges from 75 to 85 m (246 to 279 ft) thick over the Sweetgrass Arch to 150 m (492 ft) in southeastern Alberta and along the mountain front in northwestern Montana. In southern Alberta the Ellis is cut by the pre-Cretaceous unconformity and is completely eroded north of Twps. 11-15.
Predominantly marine to transitional shales, calcareous shales and sandstones. The basal Sawtooth Formation consists of quartzose sandstones with a medial shale unit, the medial Rierdon Formation includes marine shales and limestones and the upper Swift Formation comprises a marine shale overlain by marine to transitional shale, siltstone and sandstone.
In the southern Canadian Plains the Ellis grades westward to marine shales, siltstones and sandstones of the Fernie Group, and eastward to marine shales, sandstones and limestones of the Vanguard Group and Shaunavon Formation. U.S. equivalents are numerous, and are summarized by Peterson (1972).
Peale (1893) mapped an undescribed Ellis Formation of Triassic to Jurassic age. Cobban et al. (1945) designated and described the type section; Cobban (1945) subsequently elevated the Ellis to group status and described and designated type sections for its constituent formations.
Cobban, 1945; Cobban et al., 1945; Peale, 1893; Peterson, 1972; Weir, 1949.
Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: B.J. Hayes
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 17 May 2004