The St. Mary River Formation is a geologic formation of

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the more recent of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''cre ...

(71.9-67 Ma)

age Age or AGE may refer to: Time and its effects * Age, the amount of time someone has been alive or something has existed ** East Asian age reckoning, an Asian system of marking age starting at 1 * Ageing or aging, the process of becoming older ...

of the Western Canada Sedimentary Basin in southwestern

Alberta Alberta is a Provinces and territories of Canada, province in Canada. It is a part of Western Canada and is one of the three Canadian Prairies, prairie provinces. Alberta is bordered by British Columbia to its west, Saskatchewan to its east, t ...

and northwesternmost

Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...

. It was first described from

outcrop An outcrop or rocky outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth and other terrestrial planets. Features Outcrops do not cover the majority of the Earth's land surface because in most p ...

s along the St. Mary River by

George Mercer Dawson George Mercer Dawson (August 1, 1849 – March 2, 1901) was a Canadian geologist and surveyor. He performed many early explorations in western North America and compiled numerous records of the native peoples. Biography He was born in ...

in 1883, and it takes its name from the river. Fossils from the formation include remains of dinosaurs,Weishampel, David B; et al. (2004). "Dinosaur distribution (Late Cretaceous, North America)." In: Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press, pp. 574-588. . as well as

bivalve shell A bivalve shell is the enveloping exoskeleton or mollusc shell, shell of a bivalve mollusc, composed of two hinged halves or ''valve (mollusc), valves''. The two half-shells, called the "right valve" and "left valve", are joined by a ligament and ...

s, plant fossils, and

trace fossil A trace fossil, also called an ichnofossil (; ), is a fossil record of biological activity by lifeforms, but not the preserved remains of the organism itself. Trace fossils contrast with body fossils, which are the fossilized remains of part ...

Lithology

The St. Mary River Formation is generally considered to consist of two units. The lower of the formation was deposited in brackish water environments, and is characterized by fine-grained

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

s, grey

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

s, coquinoid beds, carbonaceous

mudstone Mudstone, a type of mudrock, is a fine-grained sedimentary rock whose original constituents were clays or muds. Mudstone is distinguished from ''shale'' by its lack of fissility.Blatt, H., and R.J. Tracy, 1996, ''Petrology.'' New York, New York, ...

s and

coal Coal is a combustible black or brownish-black sedimentary rock, formed as rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen. Coal i ...

beds. The remainder of the formation was deposited in freshwater fluvial and floodplain environments and is characterized by interbedded sandstone and siltstone, with minor occurrences of carbonaceous shale and coal.

Thickness and distribution

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 thick in the exposures along the Crowsnest and Castle Rivers, and about thick along the

Oldman River The Oldman River is a river in southern Alberta, Canada. It flows roughly west to east from the Rocky Mountains, through the communities of Fort Macleod, Lethbridge, and on to Grassy Lake, where it joins the Bow River to form the South Sa ...

Relationship to other units

The St. Mary River Formation conformably overlies the Blood Reserve Sandstone, or the

Bearpaw Formation The Bearpaw Formation, also called the Bearpaw Shale, is a geologic formation of Late Cretaceous (Campanian) age. It outcrops in the U.S. state of Montana, as well as the Canadian provinces of Alberta and Saskatchewan, and was named for the Bear ...

where the Blood Reserve Sandstone is absent, and it is conformably overlain by the Willow Creek Formation. It extends from

Glacier County, Montana Glacier County is located in the U.S. state of Montana. As of the 2020 census, the population was 13,778. The county is located in northwestern Montana between the Great Plains and the Rocky Mountains, known to the Blackfeet as the "Backbone ...

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.

Fossil content

Flora

Eighteen species of plant leaves were described from the St. Mary River Formation in 1949.Bell, W.A. 1949. Uppermost Cretaceous and Paleocene floras of western Canada. Geological Survey of Canada, Bulletin 13, 231 p. More recent work downstream from the St. Mary Reservoir increased the total to at least 32 species. The assemblage includes remains of

fern The ferns (Polypodiopsida or Polypodiophyta) are a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. They differ from mosses by being vascular, i.e., having specialized tissue ...

s, ''

Ginkgo ''Ginkgo'' is a genus of non-flowering seed plants, assigned to the gymnosperms. The scientific name is also used as the English common name. The order to which the genus belongs, Ginkgoales, first appeared in the Permian, , and ''Ginkgo'' is n ...

'',

conifer Conifers () are a group of conifer cone, cone-bearing Spermatophyte, seed plants, a subset of gymnosperms. Scientifically, they make up the phylum, division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a sin ...

s, a '' Trapa''-like plant,Stockey, R.A. and Rothwell, G.W. (1997)
The aquatic angiosperm ''Trapago angulata'' from the Upper Cretaceous (Maastrictian) St. Mary River Formation of southern Alberta
International Journal of Plant Sciences 158(1): 83-94, and at least six types of large

monocot Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are flowering plants whose seeds contain only one Embryo#Plant embryos, embryonic leaf, or cotyledon. A monocot taxon has been in use for several decades, but ...

leaves in addition to a sabaloid palm.Riley, M.G. and Stockey, R.A.(2004)
''Cardstonia tolmanii'' gen. et sp. nov (Limnocharitaceae) from the Upper Cretaceous of Alberta, Canada
International Journal of Plant Sciences 165(5): 897-916.

Molluscs

Beds of '' Ostrea'' and '' Corbicula'' shells are common in the basal, brackish water portion of the formation. The overlying freshwater beds include shells of freshwater and terrestrial

molluscs Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

. Shells of unionid freshwater

mussel Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and Freshwater bivalve, freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other ...

s are common in the fluvial sandstones.

Mammals

The mammals of the St. Mary River Formation were described by Sloan and Russell in 1974.Sloan, R.E. and Russell, L.S. 1974. Mammals of the St. Mary River Formation (Cretaceous) of southwestern Alberta. Life Sciences Contributions, Royal Ontario Museum, Number 95.

Dinosaurs

Dinosaur tracks

The St. Mary River Formation has produced relatively few dinosaur fossils from its outcrops in southwestern Alberta.Ryan, M. J., and Russell, A. P., 2001. Dinosaurs of Alberta (exclusive of Aves): In: Mesozoic Vertebrate Life, edited by Tanke, D. H., and Carpenter, K., Indiana University Press, Introduction, page 281. However, footprints and trackways have been found along the St. Mary and Oldman Rivers. More than 100 track-bearing stratigraphic units were documented in one section thick, which is one of the highest densities of track-bearing layers reported from any succession. One footprint from the formation includes the first record of skin impressions from the bottom of a hadrosaur foot.

Ornithischians

Theropods

References

{{Western Canadian Sedimentary Basin, South AB=yes Geologic formations of Alberta Cretaceous Montana Cretaceous Alberta Campanian Stage Maastrichtian Stage of North America Sandstone formations of Canada Sandstone formations of the United States Ichnofossiliferous formations Shale formations of Canada Mudstone formations of Canada Coal formations Western Canadian Sedimentary Basin Shale formations of the United States Mudstone formations of the United States