GEOCHRONOLOGY OF LATE PLEISTOCENE GLACIAL DEPOSITS NEAR RIDGWAY, COLORADO, NORTHERN SAN JUAN MOUNTAINS

New geologic mapping, weathering characteristics, and luminescence dating provide the first absolute age estimates of glacial deposits in the northern San Juan Mountains of Colorado. Prior studies have assumed that glacial moraines located near Ridgway, Colorado correlate with Pinedale and Bull Lake glacial episodes. A combination of optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL), however, show that moraines represent marine isotope stage (MIS) 4 and older glaciations. At Ridgway, Pinedale glaciation is represented by a single outwash terrace.

Field mapping shows that there are a total of 4 outwash terrace and 3 moraine levels in the study area, and glacial landforms are 4 to 50 m higher in elevation than the modern Uncompahgre River. Older outwash terraces and moraines have thicker weathering rinds and more advanced stages of pedogenic carbonate accumulations (maximum is Stg III). Initially, OSL dating of quartz was used to evaluate landform ages, and results produced an age of 23.3 +/- 1.6 ka for the youngest outwash terrace. This age indicates that the terrace records the LGM and Pinedale glaciation. The next oldest terrace, which is correlated with the youngest preserved glacial moraine at Ridgway, produced an infinite OSL age of >76 ka based on the luminescence properties of quartz grains. Due to the volcanic-rich composition of the outwash sediments, post-IR IRSL was used to date feldspar grains from the same terrace deposits, and this effort produced ages that range from approximately 50 to 80 ka, which correlates broadly with MIS 4. Older moraines in the Ridgway area remain undated.

In summary, mapping and new age estimates of outwash terraces and associated glacial moraines near Ridgway suggest that the landforms and deposits primarily represent MIS 4 and older Bull Lake glacial episodes. The youngest outwash terrace is the only Pinedale-age deposit at Ridgway. This study highlights a record of MIS 4 glaciation, along with older Bull Lake events, in the northern San Juan Mountains.