Cosmogenic 10Be and 26Al Surface Exposure Data and Ages for Middle Pleistocene Terrace Deposits Along the Rio Grande, Sunshine Valley Sub-Basin, Southern San Luis Basin, Costilla County, New Mexico

We provide new 10Be/26Al cosmogenic surface exposure ages for two samples collected from gravel terraces along the Rio Grande. Each sample consisted of approximately 6 milky quartz pebbles ranging in diameter from 3 to 5 cm collected from the surface of a pristine, unmodified middle Pleistocene terrace tread adjacent to fluvially scoured basaltic bedrock. Chemical isolation of aluminum and beryllium and isotopic analysis by accelerator mass spectrometry were completed at the Purdue Rare Isotope Measurement (PRIME) Lab at Purdue University, West Lafayette, Indiana. We estimated exposure ages using CRONUS (Balco and others, 2008) for zero and nonzero erosion scenarios; the former is described here. Sample CR-NM-S-01-13 featured agreeing 26Al and 10Be exposure ages of 395 ± 4 ka and 387 ± 10 ka, respectively (1σ analytical uncertainty). Conversely, sample CR-NM-S-02-13 had a 10Be age (147 ± 1 ka) that was significantly older than its 26Al age (120 ± 3 ka), indicating a potentially complex exposure/burial history. These ages appear to record middle to late middle Pleistocene inception and northward, headward incision of the Rio Grande across Sunshine Valley over the time interval of 400-120 ka (marine oxygen isotope stages MIS 11 to 6). Complimentary 3He surface exposure ages on the basalt and U-series ages and datasets can be found in Ruleman and others (2016), Ruleman and others (2019), and Hudson and others (2019).