Quaternary stratigraphy, tectonic geomorphology and long-term landscape evolution of the southern Sierra Nacimiento
— Meri Lisa Formento-Trigilio and Frank J. Pazzaglia

The geomorphic expression of the Sierra Nacimiento-Colorado Plateau margin in northern New Mexico is consistent with an active tectonic landscape. In light of recent studies that have documented a protracted period of deformation, we investigate the relative roles of active tectonism and erosional exhumation in shaping the landscape of the Sierra Nacimiento. We mapped the southern Sierra Nacimiento from Arroyo Penasco in the west, around the southern portion of the range, and east to the town of Cañons in the Jemez River drainage basin. Based on correlation of post-Bandelier Tuff terrace deposits in the Jemez River drainage basin, we establish a Quaternary stratigraphy with relative ages for surficial deposits based on limited ¹⁴C radiometric ages, the occurrence of Lava Creek B ash, dated at approximately 620 ka, and soil stratigraphy and morphology. In the Arroyo Penasco drainage basin, three mapped units, including the youngest Holocene terrace, are offset by faults that generally exhibit down-to-the-west, normal components of slip and stratigraphic separations ranging from 1.2 to 4.2 m. Two of these Quaternary faults are coincident with the Nacimiento range-bounding fault. In the Jemez River drainage basin, Quaternary faults offset the oldest terrace deposit dated at approximately 620 ka, based on a single bed of Lava Creek B ash that occurs within the Qtl terrace deposit. All faults in this basin display down-to-the-southeast, normal components of slip with stratigraphic separations ranging from 1 to 8.2 m. We suggest that the Quaternary landscape of the southern Sierra Nacimiento has responded to neotectonism related to extensional deformation of the adjacent Rio Grande rift. Neotectonic deformation has reactivated pre-existing structures, increased local relief and played a partial, but important role in fluctuations in local base level. Variations in local base level destabilized surficial deposits and accelerated the process of erosional exhumation. Relatively high incision rates, (0.16 m/1000 yrs), since approximately 600 ka, and low average rates of Quaternary fault offset (0.03 m/1000 yrs) indicate that the long-term evolution of the southern Sierra Nacimiento is predominantly one of erosional exhumation rather than tectonic uplift.