New Mexico Geological Society Annual Spring Meeting — Abstracts

Numerous Quaternary terrace levels of the Rio Grande are present at Truth or Consequences (TorC), New Mexico. The terraces are inset below the prominent Cuchillo geomorphic surface, which represents the culmination of Santa Fe Group aggradation ca. 800 ka. These terraces can also be correlated to terrace suites in tributary drainages of the nearby western Palomas Basin that include Cuchillo Creek, an ESE flowing drainage that enters the Rio Grande ~2.5 km north of downtown TorC. Building on the pioneering work of Lozinsky (1986), we map these terraces, refine their stratigraphic relations, and use eight IRSL samples and an electron microprobe analysis to determine their absolute ages. The combined dataset provides a “Rosetta stone” for correlating and mapping terraces across the larger Palomas Basin and southern Rio Grande rift. Terrace deposits are composed of sand and gravel unless otherwise noted. Tread heights are measured relative to the top of the modern Rio Grande floodplain (which lies 2–4 m above the modern channel).

The lower suite of terraces is notably gravelly, with cobbles being common. The youngest deposit in the study area (Qtr5) is relatively thin (~3 m thick) and has a tread height of 8–14 m, increasing upstream. It returned an IRSL age of 38.17 ± 3.87 ka. The deposit of the next older terrace (Qtr4b) is up to 20 m thick, and its strath height ranges from modern grade (lower than Qtr5’s strath) to 4–5 m. Qtr4b has a 22–26 m tread height and returned two IRSL ages: 78.65 ± 6.89 and 74.08 ± 6.16 ka. Qtr4a has a 25–34 m tread height and a 19–20 m strath height. It returned an IRSL age of 124.31 ± 13.12 ka. An arroyo fill sourced from Mescal Canyon, inset into Qtr4b and sharing the same tread height, returned an age of 71.97 ± 7.68 ka. When compared to the marine oxygen-isotope stages, we find that aggradation of Qtr4a and 4b may have occurred during warmer, drier intervals of the MIS 5 interglacial (MIS5a and MIS5e–early 5e).

The higher suite is notable in that the terraces appear to correlate to broad surfaces in the western Palomas Basin. Qtr3 has a 42–48 m tread height, 2–3 levels of cobbly basal gravels (2–4 m thick) whose respective heights differ by 3–4 meters, and 10–11 m of silty to sandy fill overlying the basal gravel(s). At one locality, two levels of basal gravels are capped by the same thick fill. A sample from the upper, sandy part of the Qtr3 fill yielded an IRSL age of 217.42 ± 13.12 ka. The two higher terraces are each 1–4 m thick, composed of pebble-rich gravel, and have tread heights of 49–52 m (Qtr2) and 58–61 m (Qtr1). Terrace deposits inferred to correlate to Qtr2 exhibit two exposures of interbedded ash; this ash is geochemically similar to the 630 ka Lava Creek B ash. In Cuchillo Creek, the Lava Creek B terrace level lies ~55 m below the Cuchillo surface and 46 m above the floor of the valley.

Comparing strath and tread heights between these dated terrace sets illustrates how dynamic, wide swings in base level can complicate terrace mapping and correlation. We also use our mapping and dating to calculate incision rates over 10⁵ k.y. scales. These rates were high between 800 and 630 ka (0.25–0.28 m/ka), much lower between 630 and 220 ka (0.05 m/ka), and moderately high again (0.1–0.2 m/ka) between 220 ka and present. The lower rates at 630–220 ka promoted pedimentation and mid-level benches observed in cross profiles in Cuchillo Creek. Based on our mapping west of the Rio Grande, similar benches are inferred to also represent low incision rates at 630-220 ka and can potentially be used as a correlation tool.