LGM GLACIER RETREAT RATES AND THE TIMING OF TERRACE FORMATION IN THE ANIMAS RIVER DRAINAGE, SAN JUAN MOUNTAINS, COLORADO

One conceptual model that links glacial and fluvial systems maintains that during glacial periods glaciers produce prodigious amounts of sediment which the fluvial system is incapable of transporting. This causes rivers to aggrade. Conversely, during glacial retreat when sediment delivery wanes, rivers incise and terraces form from the abandoned braidplains. Detailed comparison of the timing of the fluvial and the glacial systems can test this idea. In the San Juan Mountains in southwestern Colorado, neither the timing of the last glacial maximum (LGM) nor the response of the rivers are well characterized. We focus on the Animas River basin, which was fed by a complex ice cap and which contained the largest valley glacier in the range. At the LGM, the Animas glacial lobe extended 70 km down-valley and produced glacial polish that extends for over 40 km. We report concentrations of 10Be from five glacial polish sites. If all samples are cosmogenically reset, and show monotonically declining ages with distance up-valley, we may interpret their concentrations as exposure ages, yielding a glacier retreat history. Such a history is notoriously difficult to document in any other way, as errors in dating recessional moraines are large. On the other hand, if the concentrations suggest ages that greatly exceed the LGM, we must interpret them as reflecting inheritance from prior interglacial periods. We will construe this pattern to be one of varying erosion over the course of the LGM.

To characterize the fluvial system, we report the age of an inferred LGM terrace immediately downstream of its moraine complex in Durango. Based on the 10Be profile method, this will be the first absolute age of an Animas River terrace, and will constrain the timing for the abandonment of this terrace. Comparison of glacial polish and terrace ages should allow us to assess models that link the glacial and the fluvial systems, and the time scales inherent in these linkages.

To interpret more quantitatively the glacial occupation of the Animas drainage, we employ a 2D finite difference glacier model capable of treating growth and decay of ice caps in real (DEM-generated) landscapes. The model will allow us to evaluate independently the likely response times of the Animas ice lobe to imposed ELA histories.