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

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

Controls on sediment evacuation from glacially modified and unmodified catchments in the eastern Sierra Nevada, California

The degree of glacial modification in small catchments along the eastern Sierra Nevada, California, controls the timing and pattern of sediment flux to the adjacent fans. There is a close relationship between the depth of fan-head incision and the pattern and degree of Late Pleistocene catchment erosion by valley glaciers; catchments with significant glacial activity are associated with deeply incised fan heads, whereas fans emerging from glacially unmodified catchments are unincised. We suggest that the depth of fan-head incision is controlled by the potential for sediment storage during relatively dry ice-free periods, which in turn is related to the downstream length of the glacially modified valley and creation of accommodation through valley floor slope lowering and glacial valley overdeepening and widening. Significant storage in glacially modified basins during ice-free periods leads to sediment supply-limited conditions at the fan head and causes deep incision. In contrast, a lack of sediment trapping allows quasi-continuous sediment supply to the fan and prevents incision of the fan head. Sediment evacuation rates should thus show large variations in glacially modified basins, with major peaks during glacial and lows during interglacial or ice-free periods, respectively. In contrast, sediment removal from glacially unmodified catchments in this type of setting should be free of this effect, and will be dominated instead by short-term variations, modulated for example by changes in vegetation cover or storm frequency. This distinction may help improve our understanding of long-term sediment yields as a measure of erosional efficiency

Late Pleistocene glaciation and deglaciation in the Crestone Peaks area, Colorado Sangre de Cristo Mountains, USA – chronology and paleoclimate

Cosmogenic 10Be surface-exposure dating and numerical glacier modeling are used to reconstruct glacial chronology and climate in the Colorado Sangre de Cristo Mountains during the local last glacial maximum (LLGM) and the subsequent deglaciation. Twenty-two surface-exposure ages on moraine boulders and polished-bedrock outcrops in the Willow Creek valley and ten in two adjacent valleys indicate that glaciers were at or near their maxima from ∼21 ka until 17–16 ka, and then retreated rapidly, nearly deglaciating the Willow Creek valley entirely by ∼14 ka. Coupled energy/mass-balance and flow modeling of two of the glaciers indicates that, if changing ice extent was driven only by temperature and insolation changes, temperature depressions of 5.0 and 5.1 °C from modern conditions, with an uncertainty of approximately +1.5/−1.0 °C, would have sustained the glaciers in mass-balance equilibrium at their LLGM extents. Doubling or halving of modern precipitation during the LLGM would have been associated with 2.7–3.0 °C and 6.9–7.0 °C temperature depression respectively. Approximately half of the subsequent LLGM–to-modern climate change was accomplished by ∼14 ka. If the rapid main phase of deglaciation between about 16 ka and 14 ka was driven solely by temperature and insolation changes, it would have been associated with a temperature rise of about 2.5 °C, at a mean rate of approximately 1.1 °C/ky. This new chronology of the last glaciation is generally consistent with others developed recently in the Colorado Rocky Mountains. The numerical modeling, however, suggests a lesser LLGM temperature depression from modern conditions than have most previous studies in Colorado