Ortega Formation

The Ortega Formation is a geologic formation that crops out in most of the basement-cored uplifts of northern New Mexico. Detrital zircon geochronology establishes a maximum age for the formation of 1690-1670 Mya, in the Statherian period. [1]

Ortega Formation Stratigraphic range: Statherian
Ortega Formation in the Tusas Mountains, New Mexico
Type	Formation
Unit of	Hondo Group
Underlies	Rinconada Formation
Overlies	Vadito Group
Thickness	8,000 m (26,000 ft)
Lithology
Primary	Quartzite
Other	Metaconglomerate
Location
Coordinates	36°26′29″N 106°04′26″W
Region	Tusas and Picuris Mountains, New Mexico
Country	United States
Type section
Named for	Ortega Mountains
Named by	Just
Year defined	1937
Ortega Formation outcrops in New Mexico

Description

Boulder of Ortega Formation near Ancones, New Mexico

Massive exposures of Ortega Formation underlie the Brazos Cliffs

The Ortega Formation consists of a very clean (98% modal quartz[2]), typically bluish-white, quartzite, with some basal beds of metaconglomerate.[3] Crossbedding is pervasive, and aluminum silicate minerals are abundant within the formation and show that its lower beds were metamorphosed to the sillimanite facies.[2] It is the principal ridge-forming formation of the Picuris Mountains and is a uniform 800-1200 meters thick.[2].

The contact between the Ortega Formation and the underlying Vadito Group is fairly easy to trace using a regional manganese-rich marker bed in the uppermost Vadito Group.[4] This is a ductile shear zone associated either with mountain collapse at the end of the Mazatzal orogeny or tectonics of the Picuris orogeny that resulted in south-directed movement of the Ortega Formation over the Glenwoody Formation. Structurally, the Ortega Formation tends to form stiff limbs within which less competent formations are heavily distorted. The formation is quite uniform in thickness everywhere but the northern Picuris Mountains, where its thickness appears to have been doubled by tectonic imbrication. [5]

The Ortega Formation may correlate with the Uncompahgre Formation of Colorado, the Mazatzal Group in Arizona, and other Proterozoic quartzite successions associated with the Yavapai and Mazatzal orogenies.[6] These all appear to be first cycle sandstones, and their remarkable maturity may be a result of deep weathering processes acting on the original sediment beds under unusual Proterozoic conditions.[7]

The formation is interpreted as the first stage of a marine transgression on a southward-dipping siliciclastic shelf. This was likely part of a back-arc basin associated with the Yavapai orogeny, named the Pilar basin.[1][6] Tabular cross-bedding permits the orientation of the highly distorted beds to be determined. [8]

History of investigation

The unit was named by Evan Just in 1937 during his investigation of pegmatites in northern New Mexico. Just included the entire sequence of quartzite and quartz schist in the Picuris Mountains in his definition, assigning the schist to the Rinconada schist member,[3] and including the feldspathic Petaca Schist in the Tusas Mountains.[9]. Arthur Montgomery recognized the Rinconada schist as well but include the Pilar slate in the Ortega Formation[10] while assigning some of the schist and conglomerate beds to his Vadito Formation.[11] During mapping of the Las Tablas area, Barker redefined the Ortega Quartzite to include only the quartzite and basal conglomerate beds.[8]. In their sweeping revision of the Precambrian stratigraphy of Northern New Mexico, Bauer and Williams split the Glenwoody Formation from the Ortega Formation and defined the Ortega Formation to consist only of the massive quartzite and basal conglomerate beds[4] while reassigning the Petaca Schist mostly to the Vadito Group (Burned Mountain Formation).[12]

Footnotes

1. Jones et al. 2011
2. Bauer 2004, p.198
3. Just 1937, p.21-22
4. Bauer and Williams 1989, p.50
5. Bauer 2004, p.199
6. Davis et al. 2011
7. Medaris et al. 2003
8. Barker 1958, p.11
9. Just 1937, p.43
10. Montgomery 1953, p.1
11. Montgomery 1953, p.21
12. Bauer and Williams 1989, p.49

References

• Barker, Fred (1958). "Precambrian and Tertiary geology of Las Tablas quadrangle, New Mexico" (PDF). New Mexico Bureau of Mines and Mineral Resources Bulletin. 45.
• Bauer, Paul W. (2004). "Proterozoic rocks of the Pilar Cliffs, Picuris Mountains, New Mexico" (PDF). New Mexico Geological Society Field Conference Series. 55: 193–205. Retrieved 15 April 2020.
• Bauer, Paul W.; Williams, Michael L. (August 1989). "Stratigraphic nomenclature ol proterozoic rocks, northern New Mexico-revisions, redefinitions, and formaliza" (PDF). New Mexico Geology. 11 (3). Retrieved 15 April 2020.
• Davis, Peter; Williams, Mike; Karlstrom, Karl (2011). "Structural evolution and timing of deformation along the Proterozoic Spring Creek shear zone of the northern Tusas Mountains, New Mexico" (PDF). New Mexico Geological Society Field Conference Series. 62: 177–190.
• Jones, James V., III; Daniel, Christopher G.; Frei, Dirk; Thrane, Kristine (2011). "Revised regional correlations and tectonic implications of Paleoproterozoic and Mesoproterozoic metasedimentary rocks in northern New Mexico, USA: New findings from detrital zircon studies of the Hondo Group, Vadito Group, and Marqueñas Formation". Geosphere. 7 (4): 974–991. doi:10.1130/GES00614.1. Retrieved 15 April 2020.
• Just, Evan (1937). "Geology and Economic Features of the Pegmatites of Taos and Rio Arriba Counties, New Mexico" (PDF). New Mexico School of Mines Bulletin (13).
• Medaris, Jr., L. G.; Singer, B. S.; Dott, Jr., R. H.; Naymark, A.; Johnson, C. M.; Schott, R. C. (May 2003). "Late Paleoproterozoic Climate, Tectonics, and Metamorphism in the Southern Lake Superior Region and Proto–North America: Evidence from Baraboo Interval Quartzites". The Journal of Geology. 111 (3): 243–257. doi:10.1086/373967.
• Montgomery, Arthur (1953). "PreCambrian Geology of the Picuris Range, northcentral New Mexico" (PDF). State Bureau of Mines and Mineral Resources Bulletins. 30.