Deciphering local landscape stability and surficial processes at a paleovalley margin during a pluvial-interpluvial transition, central New Mexico

Abstract

—We utilize sedimentologic and stratigraphic relations in an exposure containing the Lava Creek B ash to inter- pret surficial processes and local landscape stability during a pluvial-interpluvial paleoclimate change. The exposure of interest shows paleotopography along the southwestern valley margin of the Rio Puerco, a 250 km-long tributary of the Rio Grande that did not experience glaciation in its headwaters. Several previous studies support the premise that the 0.639 Ma Lava Creek B ash fell at a glacial-interglacial transition (Marine Oxygen Isotope Stages 16-15). Five incision-backfilling events occurred during a poorly constrained time interval (probably 104 to 105 yrs) prior to ash emplacement; the two younger fills contain coarser alluvium and more evidence of local mass wasting than the three older fills. The voluminous Lava Creek B ash induced an abrupt switch from hillslope and valley floor erosion, which had produced 15-18 m of paleotopographic relief, to long-term aggradation of the valley floor. After ash emplacment, there was a brief (~100-1000 yr) episode of relative landscape stability and low aggradation rates, in which parts of the valley bottom experienced bioturbation and weak pedogenesis. Meanwhile, a mantle of ash and ashy colluvium was preserved on northeast-facing hillslopes. After this brief period of relative landscape stability, the Rio Puerco valley bottom experienced higher deposition rates, initially accompanied by a large component of tributary-derived sediment, which on-lapped the ash-mantled paleo-hillslopes and produced >25 m of alluvial fill thickness. Tributary sediment would have been eroded from local hillslopes or upstream alluvial storage, but hillslope erosion is not evident at our site -- perhaps because of the northeast aspect of the paleo-hillslopes. If the latter two of the pre-ash cut-and-fills are related to a full-pluvial climate, then such a climate promoted hillslope mass-wasting and alluvial storage in headwater gullies. Hillslope erosion characterized the initial pluvial-interpluvial transition, possibly because of higher intensity precipitation events and/or less effective vegetative cover, which delivered more sediment to the valley bottoms than the Rio Puerco could transport downstream (even though it had more competency than the modern river). This resulted in significant aggradation during the pluvial-interpluvial transition. Finer-grained aggradation possibly continued into the drier full-interglacial, based on analogy to the Holocene, but these inferred, fine-grained deposits were later eroded.