The Willow Creek Formation is a

stratigraphic unit A stratigraphic unit is a volume of rock of identifiable origin and relative age range that is defined by the distinctive and dominant, easily mapped and recognizable petrographic, lithologic or paleontologic features ( facies) that characteriz ...

of Late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

to Early

Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...

age in the Western Canada Sedimentary Basin of 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 ...

. It was first described 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 along the Willow Creek, a tributary of 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 ...

. Williams and Dyer defined the type section in 1930 at the mouth of Willow Creek, east of

Fort Macleod Fort Macleod ( ) is a town in southern Alberta, Canada. It was originally named Macleod to distinguish it from the North-West Mounted Police barracks (Fort Macleod, built 1874) it had grown around. The fort was named in honour of the then List o ...

. The formation straddles the Cretaceous-Paleogene (K-Pg) boundary, which divides it into an upper, Early Paleocene member and a lower, Late Cretaceous member. Dinosaur remains are among the fossils that have been recovered from the Late Cretaceous portion.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. .

Lithology

The Willow Creek Formation is composed of non-marine varicolored

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, red beds and

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. The shales and red beds include

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

nodules and

caliche Caliche () is a soil accumulation of soluble calcium carbonate at depth, where it precipitates and binds other materials—such as gravel, sand, clay, and silt. It occurs worldwide, in aridisol and mollisol soil orders—generally in arid or se ...

deposits. The sandstones are soft, light grey, massively bedded and

cross-bedded In geology, cross-bedding, also known as cross-stratification, is layering within a stratum and at an angle to the main bedding plane. The sedimentary structures which result are roughly horizontal units composed of inclined layers. The original ...

, with harder conglomeratic sandstones toward the base of the formation.Glass, D.J., editor, 1997. Lexicon of Canadian Stratigraphy, vol. 4, Western Canada. Canadian Society of Petroleum Geologists, Calgary, Alberta,1423 p. on CD-ROM, .

Depositional environment

The sediments were eroded from the

Canadian Cordillera The Pacific Cordillera, also known as the Western Cordillera or simply The Cordillera, is a top-level physiographic region of Canada, referring mainly to the extensive cordillera system in Western and Northwestern Canada that constitutes the northe ...

, and were transported eastward by river systems and deposited in fluvial channel and floodplain environments. The caliche and red beds reflect deposition under arid to semi-arid climate conditions. This contrasts with the equivalent Scollard Formation north of the

Bow River The headwaters of the Bow River in Alberta, Canada, start at the Bow Glacier and Bow Lake (Alberta), Bow Lake in Banff National Park in the Canadian Rockies, Canadian Rocky Mountains. The glacial stream that feeds Bow Lake (Alberta), Bow Lake ...

, which includes coal deposits indicative of a more humid environment.

Distribution

The Willow Creek Formation is present in southwestern Alberta, south of the Bow River, and extends a short distance into northern

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 thins eastward from the foothills of the

Canadian Rockies The Canadian Rockies () or Canadian Rocky Mountains, comprising both the Alberta Rockies and the British Columbian Rockies, is the Canadian segment of the North American Rocky Mountains. It is the easternmost part of the Canadian Cordillera, w ...

to its limit east of

Vulcan, Alberta Vulcan is a town in southern Alberta, Canada that is surrounded by Vulcan County. It is on Alberta Highway 23, Highway 23, midway between the cities of Calgary and Lethbridge. The population of the town was 1,769 in 2021. Now known as the "Offic ...

. Thicknesses exceeding 1000 meters have been reported.

Relationship to other units

The Willow Creek Formation is overlain by the Porcupine Hills Formation, and underlain by the St. Mary River Formation. It grades into the equivalent Scollard Formation south of the Bow River at about 50° 30'N latitude, and into the Coalspur Formation in the southern foothills. The lower, Late Cretaceous member is correlated with the

Frenchman Formation The Frenchman Formation is a stratigraphic unit of Late Cretaceous (late Maastrichtian) Geochronology, age in the Western Canadian Sedimentary Basin, Western Canada Sedimentary Basin. It is present in southern Saskatchewan and the Cypress Hills (C ...

Saskatchewan Saskatchewan is a Provinces and territories of Canada, province in Western Canada. It is bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, to the northeast by Nunavut, and to the south by the ...

, and the upper, Early Paleocene member is correlated with the lower portion of the

Ravenscrag Formation The Ravenscrag Formation is a stratigraphic unit of early Paleocene Geochronology, age in the Western Canadian Sedimentary Basin, Western Canada Sedimentary Basin. It was named for the settlement of Ravenscrag, Saskatchewan, and was first describ ...

Vertebrate paleofauna

* ''

Tyrannosaurus rex ''Tyrannosaurus'' () is a genus of large theropoda, theropod dinosaur. The type species ''Tyrannosaurus rex'' ( meaning 'king' in Latin), often shortened to ''T. rex'' or colloquially t-rex, is one of the best represented theropods. It live ...

'' *

Hadrosauridae Hadrosaurids (), also hadrosaurs or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The ornithopod fam ...

indet. * Leptoceatopsidae indet. * Ornithopodia indet., most likely

Thescelosauridae Thescelosauridae is a clade of neornithischians from the Cretaceous of East Asia and North America. The group was originally used as a name by Charles M. Sternberg in 1937, but was not formally defined until 2013, where it was used by Brown and ...

Dromaeosauridae Dromaeosauridae () is a family of feathered coelurosaurian theropod dinosaurs. They were generally small to medium-sized feathered carnivores that flourished in the Cretaceous Period. The name Dromaeosauridae means 'running lizards', from ...

indet. *

Oviraptorosauria Oviraptorosaurs ("egg thief lizards") are a group of feathered maniraptoran dinosaurs from the Cretaceous Period (geology), Period of what are now Asia and North America. They are distinct for their characteristically short, beaked, parrot-like s ...

indet. *

Troodontidae Troodontidae is a clade of bird-like theropod dinosaurs from the Late Jurassic to Late Cretaceous. During most of the 20th century, troodontid fossils were few and incomplete and they have therefore been allied, at various times, with many dinos ...

indet.

Ootaxa

Numerous eggshell fragments are known from the formation; over 85% of them belong to the ornithopod oogenus '' Spheroolithus''. * ''Spheroolithus''

cf. The abbreviation cf. (short for either Latin or , both meaning 'compare') is generally used in writing to refer the reader to other material to make a comparison with the topic being discussed. However some sources offer differing or even contr ...

''albertensis'' * ''S.'' cf. ''choteauensis'' * '' Continuoolithus'' cf. ''canadensis'' * ''

Montanoolithus ''Montanoolithus'' is an oogenus of fossil egg found in Montana and Alberta. They were probably laid by a dromaeosaur or a caenagnathid.D. K. Zelenitsky and F. Therrien. (2008) "Unique maniraptoran egg clutch from the Upper Cretaceous Two Medicin ...

'' cf. ''strongorum'' * '' Porituberoolithus warnerensis'' * ''

Prismatoolithus ''Prismatoolithus'' is an oogenus of dinosaur egg from the Cretaceous (Hauterivian-Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stag ...

'' cf. ''levis'' * ''Prismatoolithus'' sp.

References

{{Western Canadian Sedimentary Basin, South AB=yes Geologic formations of Alberta Geologic formations of Montana Fossiliferous stratigraphic units of North America Cretaceous Montana Maastrichtian Stage of North America Paleogene Alberta Cretaceous–Paleogene boundary Ooliferous formations Shale formations of Canada Sandstone formations