Elk Point Group
{{Short description|Stratigraphic unit in the Western Canada and Williston sedimentary basins}}
{{Infobox Rockunit
| name = Elk Point Group
| image = Blue halite (Prairie Evaporite Formation, Middle Devonian; Potash Saskatchewan-Lanigan Mine, Saskatchewan, Canada) 2.jpg
| caption = Blue halite (Prairie Evaporite Formation, upper Elk Point Group, Middle Devonian) from Saskatchewan
| type = Geological formation
| age = Early to Middle Devonian {{Fossil range|400|385| }}
| period = Devonian
| prilithology = dolomite, halite, anhydrite
| otherlithology = Limestone, shale, potash
| namedfor = Elk Point
| region = Northern plains
| country = Canada, United States
| coordinates = {{coord|53.9053|N|110.6304|W|display=inline,title|name=Elk Point Group}}
| unitof =
| subunits = Upper and Lower Elk Point Group
| underlies = Beaverhill Lake Group, Manitoba Group
| overlies = Precambrian to Silurian formations
| thickness = up to {{convert|610|m|ft|-1}}
| extent =
| area =
| map =
| map_caption =
}}
The Elk Point Group is a stratigraphic unit of Early to Middle Devonian age in the Western Canada and Williston sedimentary basins. It underlies a large area that extends from the southern boundary of the Northwest Territories in Canada to North Dakota in the United States. It has been subdivided into numerous formations,{{Cite web|url=http://ags.aer.ca/reports/atlas-of-the-western-canada-sedimentary-basin.htm|title=The Geological Atlas of the Western Canada Sedimentary Basin, Chapter 10: Devonian Elk Point Group of the Western Canada Sedimentary Basin|author=Mossop, G.D. and Shetsen, I. (compilers). Canadian Society of Petroleum Geologists and Alberta Geological Survey|year=1994|access-date=2016-06-20}} number of which host major petroleum and natural gas reservoirs.{{Cite web|url=http://ags.aer.ca/reports/atlas-of-the-western-canada-sedimentary-basin.htm|title=The Geological Atlas of the Western Canada Sedimentary Basin, Chapter 32: Oil and Gas Resources of the Western Canada Sedimentary Basin|author=Mossop, G.D. and Shetsen, I. (compilers) Canadian Society of Petroleum Geologists|year=1994|access-date=2016-06-20}}
Lithology
The formations of the Elk Point Group are composed primarily of carbonate rocks (dolomite and limestone) and evaporitic rocks (halite, anhydrite and potash), with lesser amounts of dolomitic mudstone and shale.Glass, D.J. (editor) 1997. Lexicon of Canadian Stratigraphy, vol. 4, Western Canada including eastern British Columbia, Alberta, Saskatchewan and southern Manitoba. Canadian Society of Petroleum Geologists, Calgary, 1423 p. on CD-ROM. {{ISBN|0-920230-23-7}}.
Paleontology
Some of the carbonate formations of the Elk Point Group contain rich assemblages of marine invertebrate fossils, including many species of brachiopods, gastropods, bivalves, cephalopods, crinoids, ostracods and corals. The evaporitic formations are unfossiliferous or contain a few spores and algal remains.
Environment of Deposition
The formations of the Elk Point Group were deposited in a marine embayment that stretched from an open ocean in the present-day Northwest Territories of Canada to North Dakota in the United States, covering an area roughly half as large as that covered by today's Mediterranean Sea. At times of low water levels and excessive evaporation, halite and other evaporite minerals were deposited in sabkha, supratidal flat and coastal lagoon environments, and at times of higher water levels carbonate platform sedimentation and reef growth were dominant.
Distribution and Thickness
The Elk Point Group extends from the southern boundary of the Northwest Territories through northwestern British Columbia, Alberta, Saskatchewan, and southwestern Manitoba in Canada, and continues into eastern Montana and North Dakota in the United States.Meijer Drees, N.C. 1986. Evaporitic deposits of western Canada. Geological Survey of Canada, paper 85-20. It reaches a maximum thickness of about {{convert|610|m|ft|-1}} in eastern Alberta.
Stratigraphy
The Elk Point Group was named for the town of Elk Point, Alberta by J.R. McGehee in 1949.McGehee, J.R., 1949. Pre-Waterways Paleozoic stratigraphy of Alberta Plains. Bull. American Association of Petroleum Geologists, 33:4, p. 603-613. Core from a well that was drilled near Elk Point has been designated as the type section (Anglo-Canadian Elk Point No. 11, 2-11-57-5W4). The group is subdivided into the Lower and Upper Elk Point Group, each of which is further subdivided into formations according to the dominant lithologies, as shown in the tables below.
The Lower Elk Point Group comprises all strata lying below the Winnipegosis Formation (in the south) or the Keg River Formation (in the north) and is present only in the deepest parts of the basin. The Upper Elk Point Group, which is present throughout the basin, includes those formations and all overlying formations to the base of the Manitoba Group (in the south) or the Beaverhill Lake Group (in the north).
=Subdivisions=
In northern Alberta and central Alberta, the Elk Point Group contains the following subdivisions, from top to base:
;In southern Alberta
The Elk Point Group is dolomitic and is not differentiated.
;In Saskatchewan, Manitoba and Montana
| class="wikitable" | ||||
| Sub-unit | Age | Lithology | Max. Thickness | Reference |
|---|---|---|---|---|
| Dawson Bay Formation | Givetian | dolomitic mudstone, crystalline limestone, argillaceous carbonate, bituminous limestone, dolomite, anhydrite, halite | {{convert|50|m|ft | |
| 1|abbr=on}} | ||||
| Prairie Evaporite Formation | Givetian | halite, anhydrite, dolomite, dolomitic mudstone, limestone, potash | {{convert|218|m|ft | |
| 1|abbr=on}} | ||||
| Winnipegosis Formation | Givetian | dolomite, bituminous carbonates, anhydrite | {{convert|100|m|ft | |
| 1|abbr=on}} | ||||
| Ashern Formation | Eifelian to Givetian | argillaceous dolomite and dolomitic shale and siltstone; minor anhydrite | {{convert|55|m|ft | |
| 1|abbr=on}} | ||||
| Meadow Lake Formation | Eifelian | dolomite with mudstone interbeds, limestone and sandstone at base | {{convert|56|m|ft | |
| 1|abbr=on}} |
Relationship to Other Units
The Elk Point Group is conformably overlain by the Manitoba Group in Manitoba and Saskatchewan, and by the Beaverhill Lake Group in Alberta. It rests unconformably on Precambrian basement rocks in northern Alberta, on Cambrian strata in northeastern Alberta and in Saskatchewan, and on Ordovician to Silurian formations in western Alberta, Saskatchewan and southwestern Manitoba. In the Northwest Territories, some of its uppermost units are exposed at surface or are unconformably overlain by Cretaceous strata.
The Lower Elk Point Group is equivalent to the Stone Formation and its equivalents, and the Headless and Nahanni Formations, in northeastern British Columbia and the southwestern Northwest Territories. In the same areas, the Upper Elk Point includes the Pine Point Group, and is equivalent to parts of the Horn River Formation, Besa River Formation, and others.
Petroleum and Natural Gas
The porous carbonate rocks of the Elk Point Group host major petroleum and natural gas reservoirs. As of 1994, the Initial Established Recoverable Petroleum Reserves and the Cumulative Petroleum Production for the group were estimated at 339.3 and 240.4 million cubic metres, respectively. For natural gas, the Initial Established Marketable Reserves and the Cumulative Production were estimated at 142.7 and 79.5 billion cubic metres, respectively.
References
{{reflist}}
{{Western Canadian Sedimentary Basin|Northeast_Plains=yes|Northwest_Plains=yes|South AB=yes|Saskatchewan=yes}}
Category:Geologic groups of the United States
Category:Oil-bearing shales in Canada
Category:Oil-bearing shales in the United States
Category:Western Canadian Sedimentary Basin
Category:Devonian British Columbia
Category:Devonian North Dakota
Category:Devonian Northwest Territories
Category:Devonian Saskatchewan
Category:Geologic groups of Montana
Category:Geologic groups of North Dakota
Category:Geologic groups of Alberta
Category:Geologic groups of British Columbia
Category:Geologic groups of Saskatchewan
Category:Devonian System of North America
Category:Devonian southern paleotropical deposits
Category:Lower Devonian Series
Category:Middle Devonian Series
Category:Devonian southern paleotemperate deposits
Category:Geologic groups of the Northwest Territories