New Mexico Geology

Abstract

Holocene alluvial records have been established in many parts of the American Southwest but are lacking in the central Palomas Basin of south-central New Mexico. There, east-draining streams and arroyos feature three widespread main-stem deposits that are inset into one another and have distinct surface characteristics. These are flanked by alluvial fan deposits comparable in age to those on the main-stem. We use five new radiocarbon ages and pre-existing geochronology to compare cycles of aggradation and incision between the axial river, the Rio Grande, and the lower reaches of two of its tributaries. In lower Cañada Honda, an alluvial fan at the mouth of a side drainage experienced soil development during the latest Pleistocene followed by early to middle Holocene aggradation. An adjoining alluvial fan was aggrading at approximately 2600 cal yr BP. After a poorly constrained incision event, an inset main-stem deposit aggraded approximately 600 cal yr BP and grades into an older Rio Grande terrace deposit previously dated at 5000–670 cal yr BP. Synthesizing geochronologic data for the basin, we infer probable incision during 850–550 cal yr BP in Rio Grande tributaries. This incision occurred after a relatively dry interval and during a period of enhanced summer monsoons, consistent with previously established climate-response models. Aggradation appears to have been relatively continuous along high-order tributaries during most of the Little Ice Age (approximately 500–70 cal yr BP) to the present.