The Piedra Lumbre Formation is a geologic formation that crops out in the Picuris Mountains of northern New Mexico. Detrital zircon geochronology yields a maximum age of 1475 million years, corresponding to the Calymmian period.
Description The Piedra Lumbre Formation consists of light gray to black phyllite or schist. The formation has been subject to extreme deformation within the Hondo syncline that confuses the stratigraphic picture.Bauer 2004, p.200
The formation can be divided into three members, beginning with a lower member, about thick, of quartz-muscovite-biotite-garnet-staurolite phyllitic schist. This is characterized by light gray crenulated cleavage surfaces sprinkled with small garnet, biotite, and staurolite crystals. The middle member is interbedded light gray schist and quartzite, with the top primarily black phyllitic schist with white banding. The quartzite beds towards the base of this member contain thin calcareous beds. The upper member is of phyllitic slate that resembles the Pilar Formation but is distinguished by its stratigraphic position.Long 1976, pp.40-44
Detrital zircon geochronology yields a maximum age of 1475 Mya, similar to the underlying Pilar Formation but considerably younger than the lower Hondo Group. This suggests that the Pilar Formation and Piedra Lumbre Formation should be removed from the Hondo Group and provides evidence supporting the Picuris orogeny.Daniel et al. 2013
History of investigation The unit was originally mapped as part of the Rinconada Schist Member of the Ortega Formation by Arthur Montgomery in 1958.,Montgomery 1958, p.15 which it resembles except for having less abundant garnet.Long 1976, p.37 Nielsen recognized in 1972 that the beds were distinct from the upper Rinconada SchistNielsen 1972 but erroneously assigned them to the Pilar Formation. Long assigned these beds to the Piedra Lumbre Formation in 1976, naming the unit after the Cañada de Piedra Lumbre.
Footnotes
References
Nielsen, K. C., 1972, Structural evolution of the Picuris Mountains, New Mexico: Chapel Hill, N. C., University of North Carolina M.S. thesis.