CONSTRAINING THE TIMING AND MAGNITUDE OF DEGLACIATION IN THE DRAINAGE BASINS OF MONO CREEK AND THE SOUTH FORK OF THE SAN JOAQUIN RIVER, CENTRAL SIERRA NEVADA MOUNTAINS (USA)

Inspired by Birman’s (1964) expansive and detailed mapping, we present 38 new ¹⁰Be dates from the drainage basins of Mono Creek and the South Fork of the San Joaquin River. We find that the former San Joaquin glacier reached its greatest extent ca. 22.7 ± 1.7 ka (2σ external uncertainty). This inverse-variance weighted mean age comes from 4 dates on the lateral moraine above Rattlesnake Lake, which Birman classified as “Tahoe” (i.e., from the penultimate glaciation). Initial retreat from this position may have been rapid. Two dates adjacent to Rattlesnake Lake and 180 m below the lateral moraine average 24.2 ± 2.7 ka and four dates from a bedrock knob 11 km “upglacier” from Rattlesnake Lake average 21.5 ± 1.6 ka. This knob lies ~64% of the way from Piute Pass to the San Joaquin glacier’s Last Glacial Maximum (LGM) terminal position; the dates indicate 40% ice thickness loss at this location by ~1.2 ka after the LGM. The South Fork of the San Joaquin glacier subsequently stabilized at ~60% its LGM length at ca. 20 ka based on two moraines with mean ages of 20.5 ± 2.1 ka (n = 2) and 19.7 ± 2.0 ka (n = 2). These two moraines, combined with the LGM moraine, bracket the type location for the Tenaya ice advance (Manzanita Ridge) to between ca. 23 and 20 ka. The South Fork glacier likely remained at or near 60% its LGM length for 3-5 ka based on a series of end moraines, with the innermost moraine in that series dating to 15.3 ± 1.7 ka (n = 2) and a lateral moraine associated with the complex dating to 17.3 ± 1.1 ka (n = 6). Following this period of stability, the ice field rapidly melted. Six of eight boulders perched on bedrock between that final moraine and the cirques yield indistinguishable ages from 15.6 ± 1.2 ka to 14.3 ± 1.1 ka. These dates indicate that the Mono Creek glacier receded 23 km within the dating resolution. Some north-facing cirques hosted one final ice advance. Named the Recess Peak advance by Birman (1964), we dated it adjacent to its type location to 12.7 ± 1.1 ka (n = 4). This moraine is within 2.5 km of the cirque headwall and demarcates the final glacial advance of the latest Pleistocene in the drainage basin.

Reference:

Birman, Joseph H. "Glacial geology across the crest of the Sierra Nevada, California." Geological Society of America Special Papers 75 (1964): 1-83.