NEW CONSTRAINTS ON THE LATE PLEISTOCENE INCISION HISTORY OF THE ANIMAS RIVER, SOUTHWEST COLORADO

Since the Late Miocene, when the Colorado River made its way to the Gulf of California, it and its tributaries have carved deep canyons through the southwestern United States. An array of fluvial terraces perched above the modern stream network provides a record of incision rate through time. Bedrock incision within this system varies spatiotemporally, and is far from being completely understood. This study aims to increase that understanding by presenting new geochronology from a terrace deposit on a major tributary of the Colorado River. This new age constraint is placed in the context of existing studies, conveying a current understanding of the variable bedrock incision in this fork of the Colorado River system.

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.