page_title: Ortega Formation
section_title: Description
hierarchical_section_title: Ortega Formation / Description
caption_attribution_description: English: The Brazos Cliffs in northern New Mexico, looking east across Heron Lake.
context_page_description: 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.
context_section_description: The Ortega Formation consists of a very clean (98% modal quartz), typically bluish-white, quartzite, with some basal beds of metaconglomerate. Crossbedding is pervasive, and aluminum silicate minerals are abundant within the formation and show that its lower beds were metamorphosed to the sillimanite facies.  It is the principal ridge-forming formation of the Picuris Mountains and is a uniform 800-1200 meters thick.
  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. 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.
  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. 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.
  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. Tabular cross-bedding permits the orientation of the highly distorted beds to be determined.
caption: Massive exposures of Ortega Formation underlie the Brazos Cliffs
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