LUMINESCENCE CHRONOLOGY OF A POST-BULL LAKE GLACIAL DEPOSIT IN THE SOUTHERN SAN JUAN MOUNTAINS: IMPLICATIONS FOR REVISITING THE GLACIAL STRATIGRAPHY IN THE DURANGO, COLORADO AREA

Optically stimulated luminescence (OSL) was used to constrain the age of a 2.3 meter high, mixed fluvial/aeolian sedimentary sequence deposited on a glacial moraine in the southern San Juan Mountains, Durango, Colorado (37.298747N, 107.876272W; 2078 m). Three OSL ages on fine-grained fluvial sediment from a stratigraphic section yielded preliminary ages of ~62±12ka, ~60±11ka, and ~82±15ka (uppermost to lowermost horizons, respectively). The OSL ages indicate that the sedimentary sequence was deposited during the interval between the last glacial maximum (~19.4ka; MIS 2) and Bull Lake glaciation (~85 to >130ka; MIS 6). Interpretation of the OSL data is complicated because the sedimentary sequence was deposited on a boulder-rich deposit previously interpreted as a pre-Bull Lake glacial moraine (glacial maximum ~250ka; MIS>6); glacio-chronologic correlation of the moraine was based on elevation, incision rates, and terrace sequences. Hypotheses to explain the processes required to deposit a ~60ka sedimentary sequence on top of a ~250ka moraine perched high above the valley floor, included: (1) insufficient OSL bleaching; (2) flawed OSL dates; and, (3) incorrect correlation of the boulder-rich deposit as a pre-Bull Lake moraine. Insufficient OSL bleaching would yield older ages, but it is unrealistic to suggest inherited dates would yield an OSL age ~four times the observed value. A complimentary field study ~4 km across the Animas River valley (Anderson and Kenny, 2005 GSA RM Section Meeting) yielded three OSL dates around 55ka on probable glaciofluvial sediment deposited at a similar elevation of 2098 m. These dates add credence to the OSL ages from this study. Although extensive work has been done on correlating moraines in the Durango area, it remains a possibility that the boulder-rich deposit is younger than pre-Bull Lake. Pierce (2003) also suggested that few terrace sequences can be tied directly to pre-Bull Lake glacial moraines without adequate age control on one or more terraces. Based on the new, preliminary OSL dates, we tentatively suggest that the boulder-rich deposit stratigraphically beneath the sedimentary sequence of this study may be erroneously ascribed as a pre-Bull Lake moraine. Quantitative data from this study indicates that the glacial stratigraphy of the Durango area should be revisited.