Mississippian Del Padre Sandstone or Proterozoic Quartzite?

Read, A..S., Karlstrom, K.E., 1999, and Ilg, B., Mississippian Del Padre Sandstone or Proterozoic Quartzite? (a minipaper) in: New Mexico Geological Society Guidebook, 50th Field Conference, Sandia Mountains, New Mexico, p. 41-45.

Abstract:
Enigmatic exposures of white quartzite crop out throughout the Sandia Mountain region along the Great Unconformity. Some workers have suggested that this generally white quartzite is Paleoproterozoic metaquartzite (e.g. Kelley and Northrup, 1975); others have considered similar rocks nearby to represent orthoquartzite of the lower to mid-Mississippian Del Padre Sandstone (e.g. Armstrong and Mamet, 1974). An easily accessible outcrop of this quartzite and the contact between it and the Sandia Granite is the site of Stop 4. Confusion exists because these rocks can look either sedimentary or metamorphic even in the same outcrop. The stratigraphic position of these rocks (along the Proterozoic/Paleozoic unconformity), their lack of obvious foliation, and their occasionally pebbly texture indicate that they are unmetamorphosed sedimentary rocks. However, even the most sedimentary looking rocks are strongly silicified with quartz microstructures such as strongly recrystallized interlobate grain boundaries and undulatory extinction that suggest at least weak metamorphism took place. New geologic mapping and petrographic study of these rocks suggests that some are Paleoproterozoic metamorphic rocks (with garnet, cordierite, and biotite present), and some are pre-Pennsylvanian sedimentary rocks (but highly silicified, fractured, and recrystallized). The age of these sedimentary rocks is known only to be post-Paleoproterozoic and pre-Pennsylvanian. If these rocks are indeed Mississippian, hot mineralizing fluids apparently affected these rocks during late Mississippian tectonism. Alternatively, these rocks may be Neoproterozoic. A brief review of Mississippian geology in the region follows; setting the stage for a discussion of the rocks at Stop 4.