Montoya Group

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.Richardson 2008Pope 2004

Description

The group consists mostly of dolomitePray 1953 but with some limestone and a basal sandstone. The total thickness is about .Pope 2004, p.46 The group overlies the El Paso Formation and is overlain by the Fusselman Formation or Percha Formation,Pope 2004, p.52 with both contacts difficult to define in some exposure areas.Paige 1916 Biostratigraphy of the group is consistent with deposition in the Cincinnatian.Kottlowski 1963, pp.17-22

The group is divided into the Cable Canyon Sandstone, Upham Dolomite, Aleman Formation, and Cutter FormationKelley and Silver 1952 These are treated as members in most of southern New Mexico, where the Montoya is relatively thin and remains at formation rank.Pope 2004, p.47

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.

The bulk of the Upham Dolomite is skeletal packstone and grainstone arising from dolomitization of a crinoidal calcarenite. Hardgrounds are found throughout the unit.Pope 2004, p.47-48

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.Pope 2004, p.48-51

The Cutter Formation is mostly bioturbated tidal flat carbonate mudstone or dolomite.Pope 2004, p.51-52

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.Pope 2004, p.47

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.

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.Pope 2004, pp.47-52

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.

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. 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, 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.Poole ''et al.'' 1992

See also

* List of fossiliferous stratigraphic units in Texas
* Paleontology in Texas

Footnotes

References

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* 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.
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* {{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

Geologic groups of Texas
Limestone groups of the United States
Ordovician System of North America