The Montoya Group is a
group of
geologic formations in westernmost
Texas and southern
New Mexico. It preserves
fossils
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...
dating back to the
late Ordovician period.
[Richardson 2008][Pope 2004]
Description
The group consists mostly of
dolomite Dolomite may refer to:
*Dolomite (mineral), a carbonate mineral
*Dolomite (rock), also known as dolostone, a sedimentary carbonate rock
*Dolomite, Alabama, United States, an unincorporated community
*Dolomite, California, United States, an unincor ...
[Pray 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
The El Paso Formation is a geologic formation that is exposed from the Permian Basin of New Mexico and Texas to southeastern Arizona. It preserves fossils dating back to the Ordovician period.
Description
The formation is composed of gray chert ...
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 Formation
[Kelley 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
An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval o ...
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 Calcisiltite is a type of limestone that is composed predominantly, more than 50 percent, of detrital (transported) silt-size carbonate grains. These grains consist either of the silt-size particles of ooids, fragments of fossil shells, fragments of ...
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
Karst is a topography formed from the dissolution of soluble rocks such as limestone, dolomite, and gypsum. It is characterized by underground drainage systems with sinkholes and caves. It has also been documented for more weathering-resistant ro ...
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
A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The 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,
gastropod
The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda ().
This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...
s,
nautiloids, and
receptaculids. The gastropod ''
Maclurites'' is particularly prominent. The Aleman Formation is dominated by brachiopods, with some gastropods and
tentaculite
''Tentaculites'' is an extinct genus of conical fossils of uncertain affinity, class Tentaculita, although it is not the only member of the class. It is known from Lower Ordovician to Upper Devonian deposits both as calcitic shells with a brachio ...
s 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
*
*
*
*
* 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=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 , accessdate=14 September 2020
Geologic groups of Texas
Limestone formations of the United States
Ordovician System of North America