Evolution of desert colluvial boulder slopes

Abstract

We employed rhyolite domes formed during the past million years in eastern California to study the evolution of boulder-dominated slopes. The slopes studied are steep (from ∼25° to ∼35°) and are made of coarse, boulder-sized blocks. These slopes include well-varnished vertically-oriented colluvial deposits that appear similar to relict periglacial stone stripes. In this case, they are the result of ongoing desert slope processes. Such deposits are common throughout the arid southwestern US, but their morphometric character, fabric and rates of formation have not been assessed systematically. Basic field and isotopic dating methods are combined with two relatively new surface dating techniques, cosmogenic chlorine-36 and rock varnish microlaminae, to produce a detailed million-year-long slope development history with broad implications for geomorphic theory. Grain size, grain shape, grain angularity, and slope deposit area do not change significantly from genesis to ∼0.5 Ma; trends in the data change markedly after that time. A geomorphic threshold occurs between ∼0.5 and ∼1 Ma. as indicated by significant changes in all parameters but grain orientation data. On-going erosion over the million-year continuum is evidenced by active debris flow features, by terminal Pleistocene ages of microlaminae on all deposits, and by 36Cl ages that are progressively younger than potassium–argon ages for slope genesis.