{{Use mdy dates|date=October 2020}} {{Infobox rockunit | name = Montoya Group | image =Timber mountain.jpg | caption = Montoya Group beds form the shelf immediately below the peak of Timber Mountain, New Mexico, USA | type = Group | age = {{fossilrange|late Ordovician}} | period = late Ordovician | prilithology = Dolomite | otherlithology = Limestone, sandstone | namedfor = | namedby = G.B. Richardson | year_ts = 1908 | region = Texas | country = United States | coordinates = {{coord|32.8519|N|105.8652|W|display:inline}} | unitof = | subunits = Cable Canyon Sandstone, Upham Dolomite, Aleman Formation, Cutter Formation | underlies = Fusselman Formation, Percha Formation | overlies = El Paso Formation | thickness = {{convert|180|meters|feet}} | extent = | area = | map = {{Location map+ | United States#New Mexico | relief = 1 | width = 250 | float = center | places = {{Location map~ | United States#New Mexico | lat_deg = 32.8519 | lon_deg = -105.8652 | mark = Lightgreen pog.svg | marksize = 12 }} }} | map_caption = }}
The '''Montoya Group''' is a group of geologic formations in westernmost Texas and southern New Mexico. It preserves fossils dating back to the late Ordovician period.<ref name="richardson-2008">Richardson 2008</ref><ref name="pope-2004">Pope 2004</ref>
==Description== The group consists mostly of dolomite<ref name="pray-1953">Pray 1953</ref> but with some limestone and a basal sandstone.<ref name="pope-2004"/> The total thickness is about {{convert|180|meters|feet}}.<ref name="pope-2004-46">Pope 2004, p.46</ref> The group overlies the El Paso Formation and is overlain by the Fusselman Formation or Percha Formation,<ref name="pope-2004-52">Pope 2004, p.52</ref> with both contacts difficult to define in some exposure areas.<ref name="paige-1916">Paige 1916</ref> Biostratigraphy of the group is consistent with deposition in the Cincinnatian.<ref name="kottlowski-1963-17-22">Kottlowski 1963, pp.17-22</ref><ref name="pope-2004-46"/>
The group is divided into the '''Cable Canyon Sandstone''', '''Upham Dolomite''', '''Aleman Formation''', and '''Cutter Formation'''<ref name="kelley-silver-1952">Kelley and Silver 1952</ref> These are treated as members in most of southern New Mexico, where the Montoya is relatively thin and remains at formation rank.<ref name="pope-2004-47">Pope 2004, p.47</ref>
The Cable Canyon Sandstone is present in the northern exposures of the group and lies unconformably on the El Paso Formation. In the southern exposures, the Cable Canyon Sandstone is not present and the Upham Dolomite has a thin basal quartz sandstone.<ref name="pope-2004-47"/>
The bulk of the Upham Dolomite is skeletal packstone and grainstone arising from dolomitization of a crinoidal calcarenite. Hardgrounds are found throughout the unit.<ref name="pope-2004-47-48">Pope 2004, p.47-48</ref>
The Aleman Formation has up to 70% chert, in the form of thin continuous beds of sponge spicules within calcisiltite beds or of diagenetic lenses of chert nodules within skeletal wackestone or packstone. The continuous chert beds appear to have been laid down in cool water, while the packstone was deposited in warmer water.<ref name="pope-2004-48-51">Pope 2004, p.48-51</ref>
The Cutter Formation is mostly bioturbated tidal flat carbonate mudstone or dolomite.<ref name="kottlowski-1963-17-22"/><ref name="pope-2004-51-52">Pope 2004, p.51-52</ref>
The contact with the underlying El Paso Formation is an unconformity corresponding to a hiatus of about 30 million years. In the Franklin Mountains, the upper El Paso Formation shows pronounced karst weathering. In other locations, the contact is sharp.<ref name="pope-2004-47">Pope 2004, p.47</ref>
The group is interpreted as having been deposited within 30 degrees of the equator on the subtidal zone of a passive continental margin during the transition to late Ordovician glaciation. The presence of abundant chert and phosphate minerals in the group is attributed to upwelling along the continental margin, possibly triggered by glaciation.<ref name="pope-2004"/>
==Fossils== The Cable Canyon Sandstone contains trace fossils (''Skolithos''). The Upham Formation is rich in fossils, including brachiopods, bryozoans, corals, crinoids, gastropods, nautiloids, and receptaculids. The gastropod ''Maclurites'' is particularly prominent. The Aleman Formation is dominated by brachiopods, with some gastropods and tentaculites but few crinoids or bryozoans. The Cutter Formation contains abundant thin packstone beds with numerous brachiopods, bryozoans, and crinoids.<ref name="pope-2004-47-52">Pope 2004, pp.47-52</ref>
==Economic geology== The formation has the potential to be an important natural gas reservoir in southern New Mexico, using horizontal drilling with hydrofacturing, with successful drilling reported by 2004.<ref name="pope-2004-52"/>
==History of investigation== The name was first used by G.B. Richardson in 2008, who did not explain the origin of the name or designate a type section.<ref name="richardson-2008"/> The formation was promoted to group rank by Kelley and Silver in 1952, who divided the group into the Cable Canyon sandstone, Upham dolomite, Aleman formation, and Cutter formation in the Franklin Mountains,<ref name="kelley-silver-1952"/> but the Montoya remains a formation in southern New Mexico, where its subunits are too thin to be mappable at the usual 1:24,000 scale.<ref name="poole-etal-1992">Poole ''et al.'' 1992</ref><ref name="pope-2004"/>
==See also== {{Portal|Earth sciences|Texas|Paleontology}} * List of fossiliferous stratigraphic units in Texas * Paleontology in Texas
==Footnotes== {{Reflist}}
==References== * {{cite journal |last1=Kelley |first1=V.C. |last2=Silver |first2=Caswell |author2-link=Caswell Silver |year=1952 |title=Geology of the Caballo Mountains; with special reference to regional stratigraphy and structure and to mineral resources, including oil and gas |journal=University of New Mexico Publications in Geology |volume=4}} * {{cite journal |last1=Kottlowski |first1=F. E. |year=1963 |title=Paleozoic and Mesozoic strata of southwestern and southcentral New Mexico |journal=New Mexico Bureau of Mines and Mineral Resources Bulletin |volume=79 |url=https://geoinfo.nmt.edu/publications/monographs/bulletins/downloads/79/B79.pdf |access-date=14 September 2020}} * {{cite encyclopedia |last1=Paige |first1=Sidney |year=1916 |title=Silver City folio, New Mexico |encyclopedia=U.S. Geological Survey Geologic Atlas of the United States Folio |volume=GF-199}} * {{cite encyclopedia |last1=Pope |first1=Michael C. |year=2004 |title=Upper Ordovician and lower to middle Silurian miogeoclinal rocks |editor1-last=Mack |editor1-first=G.H. |editor2-last=Giles |editor2-first=K.A. |encyclopedia=The geology of New Mexico. A geologic history: New Mexico Geological Society Special Volume 11 |pages=45–58 |isbn=9781585460106}} * Poole, F.G., Stewart, J.H., Palmer, A.R., Sandberg, C.A., Madrid, R.J., Ross, R.J., Jr., Hintze, L.F., Miller, M.M., and Wrucke, C.T., 1992, Latest Precambrian to latest Devonian time; development of a continental margin, Chapter 2, IN Burchfiel, B.C., Lipman, P.W., and Zoback, M.L., eds., The Cordilleran Orogen; conterminous United States: Geological Society of America, The Geology of North America, The Decade of North American Geology (DNAG), v. G-3, p. 9-56. *{{cite journal |last1=Pray |first1=Lloyd C. |title=Upper Ordovician and Silurian Stratigraphy of Sacramento Mountains, Otero County, New Mexico |journal=AAPG Bulletin |date=1953 |volume=37 |issue=8 |pages=1894–1918 |doi=10.1306/5CEADD58-16BB-11D7-8645000102C1865D}} * {{cite journal |last1=Richardson |first1=G.B. |year=1908 |title=Paleozoic formations in Trans-Pecos Texas |journal=American Journal of Science |series=4th Series |volume=25 |number=49 |pages=474–484}} * {{cite journal |last1=Zeller |first1=R.A. Jr. |year=1965 |title=Stratigraphy of the Big Hatchet Mountains Area, New Mexico |journal=New Mexico Bureau of Mines and Mineral Resources Memoir |volume=16 |url=https://geoinfo.nmt.edu/publications/monographs/memoirs/downloads/16/Memoir-16.pdf |access-date=14 September 2020}}
Category:Geologic groups of Texas Category:Limestone groups of the United States Category:Ordovician System of North America