OSL dating without sand lenses: Late Pleistocene alluvial fan aggradation in the Lost River Range, Idaho
Abstract
Along the western front of the Lost River Range (LRR) in Idaho, USA, numerous sheetflood-dominated alluvial fans extend 2-6 km from the mountain front. Despite their geomorphic significance within the basin, these fans are largely inactive under modern conditions and are hypothesized to be remnants of glacial climate (e.g. Pierce and Scott, 1982). To investigate climate and glaciation as potential drivers of fan aggradation, the first goal of this study was determining how best to apply optically stimulated luminescence (OSL) dating as deposits lacked sand lenses and were predominately clast-supported sheetflood gravels with a sandy matrix. The second goal of the study was to investigate the Quaternary climate conditions that promote alluvial fan aggradation, including the importance of glacial activity vs. general glacial climate conditions as drivers of sedimentation. Five fans that represent the variation in late Pleistocene glaciation within contributing basins (0-80% of basin area) were selected for dating and geomorphic mapping. Samples for OSL dating were largely collected by excavating material from under light-safe tarps or at night, with samples for equivalent dose estimates sieved to retain the <2mm portion. Sediment chemistry for dose rate (DR) estimation was analyzed for numerous grain sizes. Sand grain size fractions (<2 mm and 0.15-2.0 mm) produced the highest DR estimates, pebbles (>5 mm) the lowest, with results from bulk samples generally in between. Bulk sample DR estimates were preferred, likely providing the best estimate of the average dose from the sediment. Partial bleaching is not suggested by equivalent dose distributions, but overdispersion values of >20% for some samples may result from the heterogeneity in the sediment DR. Despite potential DR problems, resulting ages are consistent with fan morphology and multiple ages from single surfaces show good agreement. Ages from 31 OSL samples ranged from ~4-60 ka and with mapped fan surfaces suggest aggradation during four distinct intervals: 0-10 ka, 10-20 ka, 20-35 ka and 35-60 ka. Surprisingly, the timing of deposition is similar for all fans, regardless of past glacial extent within contributing basins. The majority of fan aggradation occurred during the late Pleistocene, with deposition during the Holocene limited to ~10% of total fan surface area. The greater extent of late Pleistocene surfaces, steeper slopes and dominance of coarser sheetflood facies suggests greater transport capacity of streams and sediment delivery to fans between 10-60 ka. Regional climate records indicate generally cold climate in the LRR throughout 10-60 ka, but more variable moisture delivery. Cold conditions may have increased transport capacity with greater effective moisture and higher spring discharges in response to increased winter snowpacks . Concurrently, sediment delivery may have increased by mobilization of stored hillslope sediment, decreased vegetation cover or increased sediment production by glaciation, enhanced weathering rates and more effective frost weathering. Overall, this research shows that glacial climate conditions, not necessarily glaciation, enhance hillslope sediment supply and fan aggradation in the LRR of Idaho.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMEP51C..05K
- Keywords:
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- 1105 GEOCHRONOLOGY / Quaternary geochronology;
- 1130 GEOCHRONOLOGY / Geomorphological geochronology;
- 1824 HYDROLOGY / Geomorphology: general;
- 1825 HYDROLOGY / Geomorphology: fluvial