NEW CONSTRAINTS ON THE LATE PLEISTOCENE INCISION HISTORY OF THE ANIMAS RIVER, SOUTHWEST COLORADO
The Animas River, a near-source tributary of the San Juan River which joins the Colorado River at Lake Powell, has incised a valley extending from Silverton, CO down to Farmington, NM. A flight of river terraces near Farmington serves as a potentially important archive of bedrock incision in this region. The ages of these deposits are poorly constrained however, limiting their usefulness as precise tracers of bedrock incision.
Here we present a Cosmogenic Radio Nuclide (CRN) burial age for the highest Animas River terrace (125 m above river) near Farmington, known to be older than ~640 ka due to its position above a terrace blanketed by the Lava Creek B ash. In conjunction with the CRN burial age, a GIS-derived longitudinal profile of compiled terrace treads and ages from Durango to Farmington offers a useful representation of the incision history along this valley. This new data set sheds light on the Animas River’s fluvial response to the onset of glaciation in the San Juan Mountains and other possible influences on incision rate including the base-level fall attributed to the integration of the Colorado River off of the Colorado Plateau.