Second-Day Road Log: From Bernalillo to San Ysidro, around Southern Nacimiento Mountains, to San Luis and Guadalupe in the Middle Rio Puerco Valley
— Daniel J. Koning, Larry S. Crumpler, Maya Elrick, Karl E. Karlstrom, Stephen G. Wells, Spencer G. Lucas, and Stephanie M. Jeffries

Today we travel northwestward from Bernalillo to the mid-
dle Rio Puerco Valley. Goals are to discuss: (1) transgressions

and regressions of the North American Interior Seaway (Late
Cretaceous), (2) late Quaternary slope and alluvial geomorphic
processes, (3) incision of the Rio Puerco, and (4) late Pliocene–
early Pleistocene basaltic volcanism of the Puerco necks part
of the Jemez lineament. We cross the geologic nexus that is
the focus of the 2024 Fall Field Conference, which includes
the Nacimiento Mountains (southern extension of the Rocky

Mountains), Rio Grande rift, and Colorado Plateau physio-
graphic provinces. Trending northeast-southwest through this

nexus is the Jemez lineament, where notable volcanism and
surface uplift occurred during the late Neogene.
The drive from Bernalillo to San Ysidro crosses the Ziana
horst and north-striking faults of the Rio Grande rift. Near the
Todilto-capped White Mesa, we cross the Jemez-San Ysidro
fault zone which marks the western extent of Santa Fe Group
rift fill and hence the traditional boundary between the rift and
the adjoining Colorado Plateau, although some extensional
strain clearly “bleeds over” into the adjoining plateau.