Terrace stratigraphy, ages, and incision rates along the Rio Ojo Caliente, north-central New Mexico
Daniel J. Koning, Dennis L. Newell, Andrei Sarna-Wojcicki, Nelia Dunbar, Karl Karlstrom, Anthony Salem, and Laura Crossey
We present new tephrochronologic and radiometric data that constrain the ages of terraces along the Rio Ojo Caliente. Draining the southern Tusas Mountains, the Rio Ojo Caliente is a tributary of the Rio Chama in the northwestern Española Basin. The highest terrace, labeled Qtoc1, has a strath located 107-134 m above the modern river. Qtoc1 contains ash and pumice-lapilli tephra probably correlative to the Guaje Pumice Bed of the Otowi Member of the Bandelier Tuff (1.61-1.63 Ma), although correlation to the Tsankawi Pumice Bed of the Tshirege Member of the Bandelier Tuff (1.22-1.26 Ma) is permissible. The second highest terrace, labeled Qtoc2, is 95-103 m above the modern river and correlates to terraces along the Rio Chama that contain the Lava Creek B ash (640 ka). The strath of the most laterally extensive terrace lies 19-30 m above the modern river. Called the 20-40 m terrace (labeled Qtoc6 in this study), it is older than 103 ka and probably younger than 210 ka based on stratigraphic relations and three U-series ages of an adjacent travertine mound complex. Both the Qtoc6 terrace and the next higher terrace level (Qtoc5) are characterized by having two straths (erosional terrace bases) separated by 2-7 m. Quartzite-rich gravel that overlies these strath couplets is generally 3-6 m-thick. Overlying these basal gravels is a 10-18 m-thick sequence of alluvium with higher sand content and higher amounts of locally derived alluvium. This sedimentologic trend indicates that following incision of the river to a new base level and deposition of axial gravel, subsequent aggradation was heavily influenced by input of detritus derived from nearby hillslopes and low order drainages; this locally derived alluvium was deposited as alluvial fans on the margins of the river valley. In the Holocene (present) interglacial, significant local erosion and aggradation has also occurred. Using this comparison, we agree with previous interpretations along the lower Rio Chama that locally derived, sand-dominated alluvium in the middle and upper parts of the terraces, best exposed in Qtoc6 and Qtoc5, were also deposited in interglacial climates or at the glacial/interglacial transition, and that strath formation and deposition of the quartzite-rich, basal gravel occurred largely during glacial climates (possibly at the glacial/interglacial transition). We use this process-response model to infer a preferred age of 130-160 ka for the Qtoc6 terrace deposit and its lowest strath. Consideration of terrace ages and their relative heights indicates an increase in incision rates after 640 ka, consistent with earlier interpretations.
- Koning, Daniel J.; Newell, Dennis L.; Sarna-Wojcicki, Andrei; Dunbar, Nelia; Karlstrom, Karl; Salem, Anthony; Crossey, Laura, 2011, Terrace stratigraphy, ages, and incision rates along the Rio Ojo Caliente, north-central New Mexico, in: Geology of the Tusas Mountains and Ojo Caliente Area, Koning, Daniel J.; Karlstrom, Karl E.; Kelley, Shari A.; Lueth, Virgil W.; Aby, Scott B., New Mexico Geological Society, Guidebook, 62nd Field Conference, pp. 281-300.