COSMOGENIC EXPOSURE DATING AND NUMERICAL MODELING OF THE PLEISTOCENE SOUTH FORK DEEP CREEK VALLEY GLACIER, NORTHERN ABSAROKA MOUNTAINS, MONTANA

During the last Pleistocene glaciation, several ranges in the Northern Rocky Mountains were occupied by valley glaciers. Geologic dating of the record of these glaciers provides new insight on the timing of glaciation, and provides the chronological framework for comparisons of paleoclimate across the Northern Rocky Mountains. Boulder samples along the left lateral sector of the terminal moraine in South Fork Deep Creek Canyon were collected for cosmogenic ¹⁰Be exposure dating and yielded an error-weighted mean exposure age of 16.7 ± 0.7 ka. This exposure age is consistent with others from terminal moraines elsewhere in the northern Absaroka Range, and consistent with previous suggestions that mountain glaciers in this region began retreating as many as 3 kyr after the onset of global deglaciation. To infer climate conditions that accompanied the last glaciation in the northern Absaroka Range, a 2-D, numerical glacier model was applied to three glacial valleys. Modeling experiments indicate that, if precipitation was at or near modern, then a temperature depression of ~8° C accompanied the time of moraine deposition. These model results along with others from ranges elsewhere in the Rocky Mountains reveal a trend of greater temperature depressions with latitude during the last glaciation, consistent with regional-scale climate models.