Timing and patterns of debris-flow deposition on Shepherd and Symmes Creek fans, Owens Valley, California, deduced from cosmogenic 10Be.

Abstract

Debris-flow fans on the western side of Owens Valley, California, show differences in their depths of fan head incision, and thus preserve significantly different surface records of sedimentation over glacial-interglacial cycles. We mapped fan lobes on two fans (Symmes and Shepherd Creek) based on the geometry of the deposits using field observations and high-resolution Airborne Laser Swath Mapping (ALSM) data, and established an absolute fan lobe chronology by using cosmogenic radionuclide exposure dating of large debris-flow boulders. While both fans and their associated catchments were subject to similar tectonic and base level conditions, the Shepherd Creek catchment was significantly glaciated while that of Symmes Creek experienced only minor glaciation. Differences in the depth of fan head incision have led to cosmogenic surface age chronologies that differ in the length of the preserved depositional records. Symmes Creek fan preserves evidence of exclusively Holocene deposition with cosmogenic 10Be ages ranging from 8 to 3 ka. In contrast, the Shepherd Creek fan surface was formed by late Pleistocene and Holocene debris-flow activity, with major deposition between 86-74, 33-15, and 11-3 ka. These age constraints on the depositional timing in Owens Valley show that debris-flow deposition in Owens Valley occurred during both glacial and interglacial periods, but may have been enhanced during marine isotope stages 4 and 2. The striking differences in the surface record of debris-flow deposition on adjacent fans have implications for the use of fan surfaces as paleoenvironmental recorders, and for the preservation of debris-flow deposits in the stratigraphic record.