Sandia Mountains, Albuquerque, NM
— October 8, 2021
Yesterday I saw some neat bedrock features along the northern Tramway trail in the Sandia Mountains, between La Cueva Canyon and the La Luz Trial. The trail is mostly on weathered Sandia Granite, which is actually a biotite monzogranite and granodiorite that is between 1455 +/12 Ma and 1446 +/-26 million years old (Read et al., 1995). But cutting across the granite are younger pegmatites and several faults, both of which stick up relative to the weathered granite (first photo). The bedrock faults are characterized by a glassy core (pseudotachylite or ultracataclasite) 0.1-2 cm wide with chunks of wall fragments (second photo). The pegmatites are of varying crystal-sizes, but one thing the pegmatites have in common is a lack of biotite. Thus, I think it is the weathering of the biotite that is responsible for the decomposed nature of the Sandia Granite along this stretch of trail. In the third photograph, check out the cross-cutting relations of the pink pegmatite relative to the more-weathered, grayer Sandia Granite. The pegmatite has a coarser-grained part and a finer-grained part, and you can see a minor fault between the pink pegmatite and the Sandia Granite (~10 cm left of the car keys). What do you think are the relative age relations between the Sandia Granite, the two phases (textures) of the pegmatite, and the fault? I can share my cross-cutting interpretation in the next post. By the way, the local expert on these faults and the Sandia Granite is Adam Read (also of the NM Bureau of Geology); I am just a curious observer of these kind of things. Much thanks to Adam for reading this before I posted it.
— Dan Koning, Sr. Field Geologist, NMBGMR