Ortega Formation
The Ortega Formation is a geologic formation that crops out in most of the mountain ranges of northern New Mexico. Detrital zircon geochronology establishes a maximum age for the formation of 1690-1670 million years, in the Statherian period of the Precambrian.
Description
The Ortega Formation consists of a very clean, typically bluish-white, quartzite, with some beds near the base of the formation composed of metaconglomerate. Crossbedding is found throughout the formation, and aluminosilicate minerals are abundant within the formation. These show that its lower beds were buried deeply enough to be metamorphosed to the sillimanite facies, at temperatures of over. The Ortega Formation is the principal ridge-forming formation of the Picuris Mountains and is a uniform 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 displacement of the Ortega Formation over the Glenwoody Formation. Structurally, the Ortega Formation tends to form stiff limbs within which weaker formations are heavily distorted. The Ortega 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, in which the individual sand grains have eroded out of igneous or metamorphic rock rather than been recycled from older sedimentary rock. 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.