Are Catchment Denudation Signals Stored in Alluvial-Fan Stratigraphy? Measuring Paleodenudation Rates Using Cosmogenic Radionuclides in the Pleistocene Pleasant Canyon Complex, Panamint Mountains, California

Abstract:

Linked erosional-depositional systems such as relatively small catchment-fan systems are excellent natural laboratories for the study of signal propagation and storage in stratigraphic archives. Changing boundary conditions in erosional catchments transmit signals down system to be encoded in stratigraphy as variation in cosmogenic isotope concentrations and, potentially, patterns in depositional architecture. We examine a linked catchment-fan system on the western flank of the Panamint Mountains, California, using cosmogenic nuclides to quantify modern and paleo-sediment flux from catchment to fan in order to test coupling between sediment supply rates and preserved stratigraphic patterns.

We measure ²⁶Al/¹⁰Be in quartz from: 1) modern sediment in active channels at two drainage basin outlets, and 2) vertical transects through outcrops of mid-Pleistocene alluvial fan sediments. ¹⁰Be concentrations in modern sediment establish baseline catchment-averaged denudation rates, and allow comparison of contemporary rates to published late Quaternary denudation rates. ²⁶Al/¹⁰Be ratios from mid-Pleistocene outcrops provide an age model, while ¹⁰Be concentrations yield a record of paleodenudation. Isotope concentration variability/contribution from modern production is assessed with paired samples from equivalent stratigraphic intervals and paired shielded-unshielded samples.

We constrain the areal extent of the paleo-Pleasant Canyon fan complex using empirical relationships from modern catchment-fan systems in the region. These fan-area estimates were combined with sedimentologic facies and process interpretations derived from high-resolution characterization of the outcrop to reconstruct stacking patterns and a baselevel history utilized in a depositional model helping to contextualize paleodenudation rates.