Milk River Formation
The Milk River Formation is a sandstone-dominated stratigraphic unit of the Western Canada Sedimentary Basin in southern Alberta, Canada. It was deposited in near-shore to coastal environments during Late Cretaceous time. Based on uranium-lead dating, palynology and stratigraphic relationships, deposition occurred between ~84.1 and 83.6 Ma.
The sandstones of the Virgelle Member in the centre of the formation are well-exposed at Writing-on-Stone Provincial Park in southwestern Alberta, where they bear petroglyphs carved into them by First Nations people.
The formation is fossiliferous and has yielded an extensive vertebrate fauna, as well as fossil ammonites. In some areas it hosts shallow natural gas reservoirs.
The area was probably a dense forest during the Late Cretaceous with conifers from the Yew family being common alongside flowering plants like Magnolias and Figs. The ground was covered in horsetails and sensitive ferns.
Stratigraphy and lithology
The Milk River Formation is an eastward-thinning wedge of clastic sediments that was deposited along the western margin of the Western Interior Seaway during Late Cretaceous time. It is age-equivalent to the marine shales of the Lea [Park Formation] in southeastern Alberta, and to the Eagle and Telegraph Creek Formations of north-central Montana.In Alberta it is subdivided into the following three members:
- Telegraph Creek Member, at the base, consists primarily of mudstones and shales deposited in offshore marine settings. It has produced the ammonite index fossil Desmoscaphites bassleri, which indicates an age of ~84. Ma.
- Virgelle Member, the central member, is a sequence of massive, cliff-forming, yellow to white sandstones that were deposited in shoreface and tidal channel environments. It can be seen at Writing-on-Stone Provincial Park.
- Deadhorse Coulee Member, at the top, consists of predominantly nonmarine shales, siltstones, sandstones, and coal beds that were deposited in rivers, floodplains and swamps. Most of the vertebrate fossils have come from this member.