St. Mary River Formation
| St. Mary River Formation Stratigraphic range: Early Maastrichtian |
|
|---|---|
| Type | Geological formation |
| Underlies | Willow Creek Formation |
| Overlies | Bearpaw Formation |
| Thickness | up to 762 metres (2,500 ft) |
| Lithology | |
| Primary | Sandstone, shale |
| Other | Mudstone, coal |
| Location | |
| Region |
 Alberta,  Montana
|
| Country |
 Canada,  USA
|
| Type section | |
| Named for | St. Mary River |
| Named by | George Mercer Dawson |
The St. Mary River Formation is a geologic formation of Late Cretaceous (Campanian to Maastrichtian) age of the Western Canada Sedimentary Basin in southwestern Alberta and northwesternmost Montana. It was first described from outcrops along the St. Mary River by George Mercer Dawson in 1883, and it takes its name from the river.
Fossils from the formation include remains of dinosaurs, as well as bivalve shells, plant fossils, and trace fossils.
The St. Mary River Formation is generally considered to consist of two units. The lower 30–60 meters (98–197 ft) of the formation was deposited in brackish water environments, and is characterized by fine-grained sandstones, grey shales, coquinoid beds, carbonaceous mudstones and coal beds. The remainder of the formation was deposited in freshwater fluvial and floodplain environments. It is characterized by interbedded sandstone and siltstone, with minor occurrences of carbonaceous shale and coal.
The St. Mary River Formation is part of an eastward-thinning wedge of sediments derived from the erosion of the mountains to the west. It is about 762 metres (2,500 ft) thick in the exposures along the Crowsnest and Castle Rivers, and about 457 metres (1,500 ft) thick along the Oldman River.
The St. Mary River Formation conformably overlies the Blood Reserve Sandstone, or the Bearpaw Formation where the Blood Reserve Sandstone is absent, and it is conformably overlain by the Willow Creek Formation. It extends from Glacier County, Montana to as far north as the Little Bow River in Alberta, where it grades into and intertongues with the contemporaneous strata of the Horseshoe Canyon Formation.
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