EAGER: Single-grain Optical Dating of Debris-flow Fans

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Debris flows are mass-movement deposits, occurring worldwide, that shape landscapes and have caused considerable damage in historic times. Knowledge of their prehistoric frequency may be obtained by use of various dating methods (e.g., dendrochronology or Carbon-14), but these are not often possible given the age, complexity, and setting of the debris flow. This proposal will test the suitability of an alternate dating tool, single-grain-quartz (SGQ) luminescence dating. Single-grain and multigrain optical dating of quartz is a proven technology but has never been attempted with debris flows. Accurate optical dating depends upon resetting the clock by exposure to solar radiation over a minimum of tens of seconds. The turbulent and rapid process by which debris flows occur may not always allow entrained quartz grains sufficient time for such exposure. Two sites have been chosen for this test. A debris]flow site near Reno occurred in 1983 over a short duration (8 to 10 minutes) and small spatial extent (0.57 sq. km.). A much larger debris flow deposit (105 sq. km.) is known in the Eastern Cordillera of Colombia that is prehistoric, but of an unknown age and duration. Thus these two settings represent approximate process]scale end members. The Colombia site has a Carbon-14 datable deposit for some age control of the SGQ results. If this project is successful, then a new dating tool will be available for such landscape deposits, a tool covering the range from historic times (last few decades) back to ca. 150 ka, well beyond the Carbon-14 dating range. Debris flows are significant and troublesome events. They usually occur catastrophically in response to another significant geological or climatic stimulus, such as a sub-glacial volcanic eruption, or heavy rains in a mountainous environment. Debris flows are often the major mechanism in the construction of alluvial fans in arid regions. In areas of human habitation, knowledge of the duration and frequency of debris flows can be essential for planning and emergency response. This project will test two end-member sites to see if debris flows contain sufficient single quartz grains that will allow determination of a reliable age profile. The Nevada site has a known age control, and the Colombian site has some organic carbon that can be used for an independent dating assessment. It is possible that the method will not be suitable, but if it does prove successful, it will provide a new avenue for establishing the chronology of these events. A successful test project will provide a new dating tool to address the relationship in time and space of prehistoric debris-flow deposits worldwide to a wide variety of likely geologic events, such as earthquakes or volcanic eruptions.