PRELIMINARY SURFICIAL GEOLOGIC MAP OF THE UPPER CONEJOS RIVER VALLEY IN THE SOUTHEASTERN SAN JUAN MOUNTAINS, COLORADO

We are mapping the surficial geology of the Conejos River drainage above the Platoro Reservoir in the southeastern San Juan Mountains. Previous mapping of surficial deposits focused on ice extent for the entire San Juan Mountain Range during the Last Glacial Maximum (LGM; Atwood and Mather, 1932). The goal of this study is to provide higher spatial and temporal resolution of post-LGM surficial deposits. It is our intention to examine this surficial stratigraphy in the context of a late Pleistocene and Holocene climate record that we are deriving from lake and bog cores in the same field area. The ~100 square km headwaters of the Conejos River consists of five primary tributaries which merge into a large u-shaped valley extending ~3.5 km above Platoro reservoir. Above each confluence with the main valley, the tributaries are characterized by series of knickpoints in both the valley floor and the modern channel. Generally less than one meter of glacial till overlies the bedrock u-shaped valley bottoms. In some areas, incision by the modern channel has left these till covered bedrock surfaces stranded above the modern floodplain by as much as 5 meters. In other areas the modern channel flows directly on the glacially carved bedrock surfaces. The difference between the two types of areas is possibly caused by a variety of factors including differences in bedrock strength, knickpoint retreat into hanging valleys, and variability in glacial erosion. Additionally, we recognize at least two generations of alluvial fans building onto the valley floors, and one primary fluvial fill terrace whose tread sits approximately 1.5 meters above the modern channel and is prevalent adjacent to tributary confluences. In general cirques are rare in the headwaters of drainages. Instead, divides are characterized by either large low relief areas with stagnant ice features or steep headwalls lacking flat surfaces. However, we mapped a sharp crested moraine of unknown age damming a tarn in one north-facing cirque. Future research will include relative and absolute dating of moraines, terraces, and alluvial fans as well as the examination of other high altitude, north-facing cirques for evidence of young moraines.