New perspectives on colluvial boulder deposits in the southwestern great basin, usa

Colluvial boulder fields are ubiquitous on desert hillslopes in the Basin and Range Province. Five chronometric methods indicate that colluvial boulder fields in this region have been active in the late Pleistocene and Holocene: visual appearance, radiocarbon dating of wood, macrofossil analysis, varnish radiocarbon dating, and varnish cation ratio dating. A new method is presented to constrain the ages of colluvial boulder fields, using the presence of stable rock varnish and ³⁶Cl; this method indicates that colluvial deposits at Buckhorn Mesa, southern Nevada, must be ≤ 310 ka. Rather than interpreting colluvial deposits from the traditional desert geomorphology framework of “relict” or “active,” we suggest that these features be studied from the perspective of ongoing processes using an independently established chronometic sequence.