New Mexico Geological Society Annual Spring Meeting — Abstracts

The Rio Salado is a western tributary of the Rio Grande whose valley is flanked by six major terrace levels. These terrace levels were documented by previous workers, but have not been thoroughly examined. The terrace mapping in northern, central and southern New Mexico is well documented, but the relation of these terraces to Rio Salado terraces is unknown. Additionally, the presence of several Quaternary rift-related normal faults and an active, deep, mid-crustal sill beneath the Rio Salado valley provide an unusual opportunity to investigate the effects of various modes of tectonic deformation of the terraces.

In this study, Rio Salado terraces were mapped using a commercial high-resolution DEM and digital color stereophotographs on a GIS workstation. The terraces were projected onto a vertical plane to construct longitudinal profiles. Three new soil pits were described to aid terrace correlation. We employed the terrace nomenclature of existing 1:24,000-scale geologic maps, but divide Qte into two distinct terraces (Qte₁ and Qte₂). We estimated terrace ages of 234 ± 35 ka (Qtg), 169 ± 25 ka (Qtf), 97 ± 14 ka (Qte₂), 65 ± 10 ka (Qtd), 42 ± 6 ka (Qtc), and 7 ± 1 ka (Qtb) using a net incision rate of 0.41 ± 0.06 m/ka, inferred from the correlation of Qte­­₁ to the 122 ± 18 ka Airport surface ~ 25 km south of the Rio Salado. This incision rate is >1.5 times more rapid than estimated rates nearby or in other parts of New Mexico, but results in age estimates that are in agreement with field observations of soil development. We tentatively correlate Qte₁ to the Segundo Alto terrace in Albuquerue, to Qtoc6 in the Rio Ojo Caliente in northern New Mexico, and to Qt5 in Cañada Alamosa in southern New Mexico, based on local age control and height above the stream. We also correlate Qtc to the Primero Alto terrace in Albuquerque, Qtoc7 in Rio Ojo Caliente, and Qt7 in Canada Alamosa based on the relative position of the terraces in the flight. The terrace gradients in the Rio Salado increase through time, indicating either stream response to Rio Grande incision, or footwall tilting from the Quaternary Loma Blanca fault. Additionally, terraces in the LBF hanging wall are back-tilted relative to the footwall, suggesting a listric geometry of the LBF. Two exceptions are terrace levels Qtf and Qtc, which are east-tilted relative to their footwall counterpart. Both Qtf and Qtc merge eastward with the next youngest terrace in the flight. The intermediate Qtc terrace is arched, possibly reflecting surface uplift due to the Socorro magma body (SMB).

This study is a preliminary report on the configuration and correlation of Rio Salado terraces. Future work will involve cosmogenic ³⁶Cl surface exposure dating to obtain a quantitative chronology for the Rio Salado terraces and enable the determination of incision rates and confident correlation with regional terraces. Surface exposure dates will also provide constraints on slip rates of Quaternary faults and the geologic history of the SMB.