Synopsis of the Nacimiento geologic nexus
— Karl E. Karlstrom, Koning Daniel J., Spencer G. Lucas, Larry S. Crumpler, Fraser Goff, Shari A. Kelley, Cameron C. Reed, Nels A. Iverson, and Laura J. Crossey, [eds.]

The 2024 New Mexico Geological Society (NMGS) Fall Field Conference will investigate the fascinating geology at the juncture of three physiographic provinces and the Jemez lineament. These provinces are the Colorado Plateau, Rio Grande rift, and Southern Rocky Mountains. We informally refer to this area as the Nacimiento nexus, for the Nacimiento Mountains. The Colorado Plateau is a Cenozoic plateau province of rugged topography and deep canyons rather than a single plateau (Kelley, 1955). Its prominent geologic characteristic is that its Paleozoic and Mesozoic strata are still flat-lying (except at the edges of sharp monoclinal bends near uplifts). Its spectacular landscapes were carved by rivers into and through the dominantly flat-lying strata. Regional differential erosion,
named the Great Denudation by Dutton (1882), has taken place via cliff retreat and vertical incision–interacting processes that are still active today. Dutton was part of a cadre of America’s most famous late-1800s geologists that included J.W. Powell (1875; the first Euro-American scientist to traverse the Grand
Canyon and second director of the U.S. Geological Survey [USGS]), G.K. Gilbert (1877; who introduced the concept of dynamic equilibrium of landforms), C. King (first director of the USGS in 1879), and C.D. Walcott (USGS Chief paleontologist and director in 1893/1894). The Great Denudation was attributed to uplift of the western United States. When uplift started in Late Cretaceous time, the sedimentary layer-cake stratigraphy that nonconformably overlies the crystalline basement consisted of (in round numbers) ~1 km of Paleozoic strata and ~2.5 km of Mesozoic strata. With the exception of the landscape that developed in late Paleozoic time during the Ancestral Rocky Mountains (ARM) orogeny, most of the strata were deposited on a low-relief landscape near sea level.
The Mesozoic sedimentary succession is capped by deposits related to the development and retreat of the Late Cretaceous Western Interior Seaway. Today’s iconic landscapes and the high average elevations of the Colorado Plateau (~6,500 ft or ~ 2 km) and Rocky Mountains (9,800 ft or ~3 km) have been shaped during a ~70-million-year “battle” between episodic uplift and erosion. The basic concept that old rocks are revealed within young landscapes seems simple, but it is an essential starting point for public geoscience education. This concept also presents persistent challenges for seasoned geologists in their efforts to interpret the cumulative history of events in these areas.