{{Short description|Geologic formation of the Williston Basin and Western Canada Sedimentary Basin}} {{Infobox rockunit | name = Deadwood Formation | image = Fallingrock.jpg | caption = The Deadwood Formation at Fallingrock cliff in Dark Canyon in the Black Hills, South Dakota. | type = Formation | age = Late Cambrian-Early Ordovician<br />~{{Fossil range|510|470}} | period = Ordovician | prilithology = Sandstone | otherlithology = Conglomerate, shale, limestone | namedfor = Deadwood, South Dakota | namedby = Darton, N.H. and Paige, S. (1925)<ref name=Darton>Darton, N.H. and Paige, S. (1925). Description of the central Black Hills. U.S. Geological Survey, Professional Paper 65.</ref> | region = Williston Basin and Western Canada Sedimentary Basin | country = United States<br />Canada | coordinates = | unitof = | subunits = | underlies = Red River, Winnipeg & Englewood Formations & Elk Point Group | overlies = Precambrian rocks, or Lebold Formations Earlie and Pika Formations | thickness = | extent = | area = | map = | map_caption = }}
The '''Deadwood Formation''' is a geologic formation of the Williston Basin and Western Canada Sedimentary Basin. It is present in parts of North and South Dakota and Montana in the United States, and in parts of Alberta, Saskatchewan, and southwestern corner of Manitoba in Canada. It is of Late Cambrian to Early Ordovician age and was named for exposures in Whitewood Creek near Deadwood, South Dakota.<ref name=Glass>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}}.</ref> It is a significant aquifer in some areas, and its conglomerates yielded significant quantities of gold in the Black Hills of South Dakota.<ref name=Irving>{{Cite book|url= https://books.google.com/books?id=NKTmAAAAMAAJ&dq=gold+black+hills+Deadwood+formation&pg=PA11 |title=Ore deposits in Cambrian rocks: Gold-bearing conglomerates. Part 2, Chapter 2, p. 98-11, in U.S. Geological Survey, Professional Paper 26|author= Irving, J.D., Emmons, S.F. and Jaggar, T.A.|year=1904|accessdate=2014-10-04}}</ref>
It preserves trace fossils such as ''Skolithos'', and remains of Late Cambrian trilobites and brachiopods,<ref>J. Foster, 2014. Cambrian ocean world: Ancient sea life of North America. Indiana University Press, 432 p., {{ISBN|0253011884}}, 9780253011886</ref> as well as Ordovician fossils.
A 20 MW geothermal power plant is under construction, drilling 3.5 kilometers down.<ref>{{cite web |title=First DEEP Geothermal Well In Saskatchewan Exceeds Expectations |url=https://cleantechnica.com/2020/09/19/first-deep-geothermal-well-in-saskatchewan-exceeds-expectations/ |website=CleanTechnica |date=19 September 2020}}</ref>
==Stratigraphic position== [[File:Black Hills angular deformity.jpg|thumb|left|The angular unconformity between the Deadwood Formation and the underlying Precambrian metamorphic rocks near Rapid City, South Dakota.]]At the type locality in the Black Hills of South Dakota and in many other areas, the Deadwood Formation rests unconformably on Precambrian metamorphic rocks that were exposed to a long period of erosion prior to the deposition of the formation. In western Montana, western Saskatchewan and Alberta it overlies the Middle Cambrian rocks of the Earlie Formation or the Pika Formation.<ref name="Glass" />
In the Williston Basin it is overlain by the Winnipeg Formation in the center and by the Red River Formation near the margins. In the central and southern Black Hills it is overlain by the Mississippian Englewood Formation. In Alberta and Saskatchewan it is overlain by the Devonian Elk Point Group.<ref name="Glass" />
==Lithology== In the type area, the Deadwood Formation consists of a basal conglomerate and buff sandstones, overlain a sequence of by grey-green shales, carbonate rocks, and glauconitic quartzose sandstones. ''Skolithos'' borings are present in some units.<ref name="Glass" /> The Deadwood conglomerates contained significant quantities of gold in the Black Hills.<ref name="Irving" />
In Saskatchewan the Deadwood consists of fine to very coarse, commonly glauconitic, micaceous, feldspathic, slightly argillaceous and calcareous quartzose sandstones, with minor shales and conglomerates. Conglomerates are common near the base of the formation. They typically consist of Precambrian rock fragments set in finer-grained sediments, and are normally poorly sorted and unstratified.<ref name=Fuzesy>Fuzesy, L.M. 1980. Geology of the Deadwood (Cambrian), Meadow Lake and Winnipegosis (Devonian) Formations in west-central Saskatchewan. Saskatchewan Mineral Resources, Report 210, 64 p.</ref>
thumb|alt=A piece of stone about the size of a typical flag stone, held up for the camera by someone. The rock has ridges and burrows throughout, and is pinkish-red.|A slab of the pinkish-red Cambrian Deadwood Sandstone float shows clear evidence of bioturbation and worm burrows. Exposed from an outcrop off SD 44, west of Rapid City.
==Environment of deposition== In most areas the sediments of the Deadwood Formation were deposited in near shore, shallow water environments as an ancient sea advanced across the exposed and weathered landscape of Precambrian rocks. Most of the conglomerates appear to be matrix-supported debris flow deposits.<ref name="Fuzesy" />
thumb|alt=A poorly-sorted, clast-supported, pebble-to-cobble, quartzic conglomerate sits on a bed of pine needles, with a rock hammer propped in front for scale. Section shows the typical Deadwood Fm coloring of pinkish-red.|A typical exposure of the basal conglomerate in the Deadwood Formation, near Nemo, SD. Note the lack of sorting and stratification in the clasts, as well as their mainly quartzic provenance. Rock hammer for scale.
==Thickness and distribution== The Deadwood Formation is present beneath large areas of the northern plains, and its thickness can vary over short distances due largely to the substantial topographic relief on the eroded Precambrian surface.<ref name="Fuzesy" /> It reaches thicknesses of up to about {{convert|270|m|ft|-1}} in the northern Black Hills. It thins to zero eastward and is absent in northeastern North Dakota and most of eastern Saskatchewan. It is present in the extreme southwestern corner of Manitoba where it may reach thicknesses of {{convert|30|m|ft|-1}}. It thickens westward throughout Saskatchewan and Montana to the Saskatchewan-Alberta border, where it exceeds {{convert|300|m|ft|-1}}. From there it thins towards the mountain front to the west. In the north the formation extends almost to 55°N latitude.<ref name="Glass" />
==See also== {{Portal|Earth sciences|Paleontology}} * List of fossiliferous stratigraphic units in North Dakota * Paleontology in North Dakota * Black Hills Gold Rush
==References== {{Reflist}} * {{cite web|title= Fossilworks: Gateway to the Paleobiology Database|author= ((Various Contributors to the Paleobiology Database))|url= https://www.fossilworks.org/|access-date= 17 December 2021|archive-date= 19 August 2022|archive-url= https://web.archive.org/web/20220819182839/https://www.fossilworks.org/|url-status= dead}}
{{Western Canadian Sedimentary Basin|Saskatchewan=yes}}
Category:Cambrian System of North America Category:Ordovician System of North America Category:Western Canadian Sedimentary Basin Category:Geologic formations of Alberta Category:Geologic formations of North Dakota Category:Cambrian Alberta Category:Cambrian North Dakota Category:Cambrian South Dakota Category:Cambrian Saskatchewan Category:Ordovician Alberta Category:Ordovician geology of North Dakota Category:Ordovician South Dakota Category:Ordovician Saskatchewan Category:Cambrian southern paleotropical deposits Category:Geologic formations of South Dakota Category:Geologic formations of Saskatchewan Category:Geologic formations of Montana Category:Lagerstätten