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.