CONSTRAINTS ON THE TIMING OF FAN DEPOSITION IN DEATH VALLEY, CALIFORNIA, USING COSMOGENIC 10BE AND 26AL

Cosmogenic dating of fan surfaces potentially provides a unique opportunity to study the long-term controls on fan evolution, particularly on fans with long depositional surface chronologies over a few 10⁵ yrs. However, cosmogenic ages obtained from fan surfaces only reflect the true depositional timing if the inheritance is negligible.

Fans such as the Warm Springs fan on the western side of Death Valley, California, are ideal for the reconstruction of a long-term depositional history. The Warm Springs fan is characterized by ~25 m of incision at the fan head, which has led to a subsequent basinward shift of the active depositional lobe. Based on morphologic criteria such as the depth of fan surface dissection, we mapped three distinct fan lobes and dated these fan surfaces using cosmogenic exposure dating with the isotopes ¹⁰Be and ²⁶Al. For the fan surface dating, we measured 18 amalgamated clast samples from pavements, each containing 30-40 quartzite clasts of 5-10 cm in diameter. For the estimation of present-day inheritance, we took a total of 10 amalgamated samples from different locations within the incised, active channel.

Cosmogenic measurements on the Warm Springs fan reveal ¹⁰Be ages between ~60 and 860 ka assuming zero erosion. The oldest, deeply dissected fan surfaces on the proximal fan record deposition during ~860 and 390 ¹⁰Be ka. On the distal, undissected surfaces, depositional ¹⁰Be ages are younger and reflect depositional timing between 320 and 160 ka. The ¹⁰Be and ²⁶Al nuclide concentrations furthermore show that the fan surfaces generally underwent a simple exposure history and were not affected by complex histories of long burial and re-exposure. The cosmogenic samples from the incised channel show that present-day inheritance varies between 0.4 x 10⁶ and 1.0 x 10^{6 10}Be atoms g^-1 at the fan head and between 0.5 x 10⁶ and 1.1 x 10^{6 10}Be atoms g^-1 at the distal fan, which corresponds, depending on the production rate at the different sampling sites in the channel, to an exposure time of ~60 to 160 kyr and ~110 to 240 kyr, respectively. Although the influence of inheritance on the depositional ages diminishes with fan surface age, our results show that, assuming that inheritance in this setting did not significantly change through time, the inheritance component complicates efforts to resolve the true depositional timing.