Unit Name: Milk River Formation
Unit Type: Lithostratigraphic
Usage: Currently in use
Age Interval: Late Cretaceous (99.6 - 65.5 ma)
Originator: Dowling, 1916.
Only the upper part of the Milk River Formation is exposed along the Milk River in southern Alberta. The lower boundary of the formation was originally placed in the subsurface at the lowest occurrence of sandstone (Dowling, 1917). This contact was difficult to map and the Alberta Society of Petroleum Geologists (1960) proposed to use instead the upper contact of the upper white speckled shale unit (Colorado Group) as the lower boundary. The upper boundary of the formation was defined by Williams and Dyer (1930) and Evans (1931) in the outcrop area along the Milk River in southeastern Alberta.
The Milk River Formation, as defined by the Alberta Society of Petroleum Geologists (1960) varies in thickness between 112.7 and 94.4 m (370 and 310 ft). It is present in southern Alberta and is a mappable unit wherever the Virgelle sandstone member is developed. In southwestern Saskatchewan and central Alberta the Virgelle sandstone is absent due to a facies change. Here equivalents of the Milk River Formation are recognizable as a lower member of the Lea Park Formation because the pebble bed above the Milk River Formation extends beyond the facies change of the Virgelle Member.
The Milk River Formation is a map-unit dominated by sandstone. Geologists working in the outcrop area (Williams and Dyer, 1930; Evans, 1931; Russell and Landes 1940) recognized a lower and an upper member. Tovell (1956), working in the subsurface divided the formation into three members. The lower member, an interbedded succession of sandstone and shale is transitional with the underlying shale of the Colorado Group and is equivalent to the informal Telegraph Creek Formation in Montana. The middle member is in mappable continuity with the Virgelle Member of the Eagle Formation; it is a light colored, fine- to medium-grained, poorly consolidated sandstone. The upper member (the Deadhorse Coulee Member of Tovell, 1956) is an interbedded unit of shale, sandy shale, sandstone and minor lignite. It correlates with the middle and upper members of the Eagle Formation. The upper contact of the Milk River Formation is selected at the top of a thin sandstone bed below a shady interval containing abundant dark colored, polished chert pebbles. The lower contact is placed at the highest occurrence of white, calcareous specks.
The Milk River Formation of the Alberta Society of Petroleum Geologists is equivalent to the combined Telegraph Creek and Eagle formations of Montana. The Milk River Formation as originally mapped by Dowling (1916) is equivalent to the Eagle Formation in Montana as mapped by Stanton and Hatcher (1905).
Alberta Society of Petroleum Geologists, 1960; Dowling, 1916, 1917; Evans, 1931; Russell and Landes, 1940; Stanton and Hatcher, 1905; Tovell, 1956; Williams and Dyer, 1930.
Dowling, D.B., 1916. Water Supply, Southeastern Alberta (Contains Geological Map 1604); Geological Survey of Canada, Summary Report 1915, pp. 102-110.
Source: CSPG Lexicon of Canadian Stratigraphy, Volume 4, western Canada, including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba; D.J. Glass (editor)
Contributor: N.C. Meijer-Drees
Entry Reviewed: Yes
Name Set: Lithostratigraphic Lexicon
LastChange: 28 May 2008