{{Short description|Geologic formation in the western United States}} {{Infobox rockunit | name = Lewis Shale | image = Lewis Shale near type.jpg | caption = Lewis Shale in road cut in the type area, La Plata valley, Colorado | type = Formation | age = {{Geological range|Campanian|Maastrichtian}} | period = Campanian | prilithology = Shale | otherlithology = Sandstone | namedfor = Fort Lewis (former Army installation in La Plata County, Colorado) | namedby = Charles Whitman Cross, A.C. Spencer, and C.W. Purington | region = Mountain states, United States | country = United States | coordinates = {{coord|37.215|N|108.006|W|display:inline}} | unitof = | subunits = Dad Sandstone Member, Huerfanito Bentonite Bed, Teckla Sandstone Member. | underlies = Pictured Cliffs Formation | overlies = Mesaverde Group | thickness = 600 m | extent = | area = | map ={{Location map+ | United States#Colorado | relief = 1 | width = 250 | float = center | places = {{Location map~ | United States#Colorado | lat_deg = 37.215 | lon_deg = -108.006 | mark = Lightgreen pog.svg | marksize = 12 }} }} | map_caption = }} [[File:Lewis Shale.jpg|thumb|Hills underlain by Lewis Shale near Cuba, New Mexico]] The '''Lewis Shale''' is a geologic formation in the Western United States. It preserves fossils dating back to the Campanian to Maastrichtian stages of the late Cretaceous period.
==Description== thumb|left|350px|San Juan Basin Upper Cretaceous stratigraphy The Lewis Shale is an olive-gray marine shale with some thin beds of claystone, siltstone, sandstone, and limestone. It was deposited in the Western Interior Seaway in the late Cretaceous.{{sfn|National Park Service|2015}} The formation crops out in the Bighorn Basin, Green River Basin, Powder River Basin, San Juan Basin, and Wind River Basin.
In the San Juan Basin, the formation has a maximum thickness of over {{convert|600|m||sp=us}}{{sfn|Cross|Spencer|Purington|1899}} in the northern part of the basin, but pinches out in the southern part of the basin.{{sfn|Beaumont|Dane|Sears|1956}} It is the stratigraphically highest marine shale in the basin, and is a sandy siltstone rather than a true shale.{{sfn|Dube|Christiansen|Frantz|Fairchild|2000}} The upper part of the formation contains the Huerfanito Bentonite Bed, long recognized in the subsurface but recently discovered to crop out near Regina, New Mexico. The bed lies in the upper ''Baculites scottii'' biostratigraphic zone and has a radiometric age of 75.76 ± 0.34 million years.{{sfn|Fassett|Cobban|Obradovich|1997}}
In the Green River Basin, the formation is also up to 600 meters thick and is divided into upper and lower shale members by the Dad Sandstone Member.{{sfn|Hale|1961}} The Dad Sandstone Member contains ''hyperpycnites'', sediments deposited by floodwater from a river delta that contains so much sediment that it is more dense than seawater and behaves as a sustained turbidity current.{{sfn|Soyinka|Slatt|2008}} This is consistent with deposition of the Lewis Shale as submarine fans deposited in advance of a prograding shoreline.{{sfn|Pyles|Slatt|2007}}
==Fossils== The formation is relatively poor in fossils.{{sfn|National Park Service|2015}} However, the formation contains fossils of the ammonites ''Baculites clinolobatus'',{{sfn|Roehler|1993}} ''Baculites asper'',{{sfn|Cross|Spencer|Purington|1899}} and ''Rhaeboceras''.{{sfn|Cobban|1988}}
== Economic geology == The Lewis Shale in the San Juan Basin has historically been a minor source of natural gas. However, there is growing interest in increasing the production of this formation via hydraulic fracturing. While only sixteen wells were completed into the Lewis Shale before 1990, there were over 101 well completions by 1997.{{sfn|Dubiel|2013}}
"Dry frac", a form of hydraulic fracturing using carbon dioxide and sand rather than aqueous fracturing fluids, has been tested in the Lewis Shale of the San Juan Basin.{{sfn|Campbell|Fairchild|Arnold|2000}}
==History of investigation== The formation was named by Charles Whitman Cross, A.C. Spencer, and C.W. Purington in 1899 for outcrops near Fort Lewis, Colorado.{{sfn|Cross|Spencer|Purington|1899}}
==See also== {{Portal|Earth sciences|Colorado|Paleontology}} * List of fossiliferous stratigraphic units in Colorado * Paleontology in Colorado
==Footnotes== {{reflist}}
==References== * {{cite journal |last1=Beaumont |first1=E.C. |last2=Dane |first2=C.H. |last3=Sears |first3=J.D. |year=1956 |title=Revised nomenclature of Mesaverde group in San Juan basin, New Mexico |journal=American Association of Petroleum Geologists Bulletin |volume=40 |number=9 |pages=2149–2162 |doi=10.1306/5CEAE565-16BB-11D7-8645000102C1865D}} * {{cite journal |last1=Campbell |first1=S.M. |last2=Fairchild |first2=N.R. |last3=Arnold |first3=D.L. |title=Liquid CO2 and Sand Stimulations in the Lewis Shale, San Juan Basin, New Mexico: A Case Study |journal=All Days |date=12 March 2000 |pages=SPE–60317–MS |doi=10.2118/60317-MS}} * {{cite journal |last1=Dube |first1=H.G. |last2=Christiansen |first2=G.E. |last3=Frantz |first3=J.H. |last4=Fairchild |first4=N.R. |last5=Olszewski |first5=A.J. |last6=Sawyer |first6=W.K. |last7=Williamson |first7=J.R. |title=The Lewis Shale, San Juan Basin: What We Know Now |journal=All Days |date=1 October 2000 |pages=SPE–63091–MS |doi=10.2118/63091-MS}} * {{cite journal |last1=Cobban |first1=W.A. |year=1988 |title=The Upper Cretaceous ammonite Rhaeboceras Meek in the Western Interior of the United States |journal=U.S. Geological Survey Professional Paper |series=Professional Paper |volume=1477 |doi=10.3133/pp1477|doi-access=free }} * {{cite journal |last1=Cross |first1=Whitman |last2=Spencer |first2=A.C. |last3=Purington |first3=C.W. |title=La Plata folio, Colorado |journal=Geologic Atlas of the United States |date=1899 |volume=GF-60}} * {{cite journal |last1=Dubiel |first1=R.F. |year=2013 |title=Geology, sequence stratigraphy, and oil and gas assessment of the Lewis Shale Total Petroleum System, San Juan Basin, New Mexico and Colorado |journal=U.S. Geological Survey Digital Data Series |volume=69–F |pages=1–45 |doi=10.3133/ds69F5 |doi-access=free }} * {{cite journal |last1=Fassett |first1=James E. |last2=Cobban |first2=William A. |last3=Obradovich |first3=John D. |title=Biostratigraphic and isotopic age of the Huerfanito bentonite bed of the upper Cretaceous Lewis Shale at an outcrop near Regina, New Mexico |journal=New Mexico Geological Society Field Conference Series |date=1997 |volume=48 |pages=229–232 |url=https://nmgs.nmt.edu/publications/guidebooks/downloads/48/48_p0229_p0232.pdf |access-date=15 September 2021}} * {{cite journal |last1=Hale |first1=L.A. |year=1961 |title=Late Cretaceous (Montanan) stratigraphy eastern Washakie basin, Carbon County, Wyoming |journal=Wyoming Geological Association Field Conference Guidebook |volume=16 |pages=129–137}} * {{cite journal |last1=Pyles |first1=David R. |last2=Slatt |first2=Roger M. |title=Chapter 154: Applications to Understanding Shelf Edge to Base-of-slope Changes in Stratigraphic Architecture of Prograding Basin Margins: Stratigraphy of the Lewis Shale, Wyoming, USA |journal=AAPG Studies in Geology |date=2007 |volume=56 |url=https://archives.datapages.com/data/specpubs/study56/data/chapter-154_aapg-sp208ch154.htm |access-date=15 September 2021}} * {{cite web |title=Lewis Shale |url=https://www.nps.gov/chcu/learn/nature/lewis-shale.htm |website=Chaco Culture National Historical Park, New Mexico |publisher=National Park Service |accessdate=8 June 2020 |year=2015 |ref={{harvid|National Park Service|2015}}}} * {{cite journal |last1=Roehler |first1=H.W. |year=1993 |title=Stratigraphy of the Upper Cretaceous Fox Hills Sandstone and adjacent parts of the Lewis Shale and Lance Formation, east flank of the Rock Springs uplift, southwest Wyoming |journal=U.S. Geological Survey Professional Paper |series=Professional Paper |volume=1532 |page=6 |doi=10.3133/pp1532 |doi-access=free |bibcode=1993usgs.rept....6R }} * {{cite journal |last1=Soyinka |first1=Olatundo A. |last2=Slatt |first2=Roger M. |title=Identification and micro-stratigraphy of hyperpycnites and turbidites in Cretaceous Lewis Shale, Wyoming: Hyperpycnites and turbidites |journal=Sedimentology |date=31 January 2008 |volume=55 |issue=5 |pages=1117–1133 |doi=10.1111/j.1365-3091.2007.00938.x|s2cid=129897873 }}
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Category:Shale formations of the United States Category:Cretaceous System of North America Category:Cretaceous Colorado Category:Cretaceous formations of New Mexico Category:Sandstone formations of the United States