New Mexico Geological Society Annual Spring Meeting — Abstracts

The origins of late Holocene alluvial deposits in the American Southwest have long been debated, with climate variability being the most commonly hypothesized driver of episodic valley-fill aggradation and incision. Deposits that are correlative over multiple drainage basins are thought to form by alluviation occurring in drier periods with infrequent floods. Conversely, larger and/or more common flood events during wetter intervals permit streams to incise their valley bottoms. Such shifts between aggradation and incision have resulted in both deeply cut arroyos and low-lying alluvial benches over the past ~4000 yr or longer.

The Palomas basin of south-central New Mexico is an east-tilted half graben in the southern Rio Grande rift. Here, late Holocene valley fill occurs in three allostratigraphic units along both arroyos and perennial streams. We mapped and described these units in Rio Grande tributaries: alluvium underlying Las Animas Creek (unit Qahan) and terrace deposits along ephemeral Cañada Honda (units Qay1 and Qay2). Charcoal collected from each deposit was radiocarbon dated, and the interpreted record of tributary aggradation and incision was compared to alluvial records of the Rio Grande and the middle part of Cañada Alamosa, a Rio Grande tributary in the northernmost Palomas basin.

Radiocarbon dates constrain late Holocene aggradation in Las Animas Creek and Cañada Honda to the last ~2500 yr. The upper 1.5 m of valley floor alluvium of Las Animas Creek (unit Qahan) is incised into middle-late Holocene alluvial fans, and returned ages of 300-250 and 140-55 yr BP. The high terrace deposit in Cañada Honda (Qay1) returned an age of 2720-2385 yr BP, whereas the low-lying deposit there (unit Qay2) aggraded at 630-515 yr BP and incised shortly thereafter. Qay2 is similar in age to post-incision valley floor alluvium in Cañada Alamosa dated at 550-350 yr BP. Previous workers also used radiocarbon ages to constrain alluviation (and/or stability) along the mainstem Rio Grande in the Palomas basin at ~5000-760 and 550-260 yr BP; incision events are estimated at 760-550 and just after 260 yr BP.

Late Holocene alluviation was generally synchronous across the Palomas basin, with basin-wide aggradation at ~550-500 yr BP suggesting a common climatic driver. Regional precipitation reconstructions from tree-ring chronologies indicate the onset of sustained, interseasonal drought at that time. Incision was also consistent between the mainstem Rio Grande and Cañada Alamosa prior to 550 yr BP. Cañada Honda incised after 530 yr BP, perhaps a result of the upstream propagation of baselevel fall from the Rio Grande. Such signals appear muted in the larger Las Animas Creek drainage, where greater sediment flux could have obscured incision events related to baselevel fall. Mainstem Rio Grande incision after 260 yr BP is not observed in its Palomas basin tributaries, and may have been related to external climatic events in its headwaters (i.e. the Little Ice Age). Understanding the complex origins of these allostratigraphic units is critical for investigators who seek to understand how climate variability influences alluviation and valley cutting at the basin scale in arid and semi-arid settings.