Calibrated, late Quaternary age indices using clast rubification and soil development on alluvial surfaces in Pilot Knob Valley, Mojave Desert, southeastern California John G. Helms, a,c Sally F. McGill, b, * and Thomas K. Rockwell a a Department of Geological Sciences, San Diego State University, San Diego, CA 92182, USA b Department of Geological Sciences, California State University, San Bernardino, CA 92407, USA c William Lettis and Associates, Inc., 28470 Avenue Stanford, Suite 120, Valencia, CA 91355, USA Received 30 August 2001 Abstract The orange coating (varnish) that forms on the undersides (ventral sides) of clasts in desert pavements constitutes a potential relative-age indicator. Using Munsell color notation, we semiquantified the color of the orange, ventral varnish on the undersides of clasts from 15 different alluvial fan and terrace surfaces of various ages ranging from less than 500 to about 25,000 yr. All of the surfaces studied are located along the central portion of the left-lateral Garlock fault, in the Mojave Desert of southern California. The amount of left-lateral offset may be used to determine the relative ages of the surfaces. The previously published slip rate of the fault may also be used to estimate the absolute age of each surface. The color of the ventral varnish is strongly correlated with surface age and appears to be a more reliable age-indicator than the percentage coverage of dorsal varnish. Soil development indices also were not as strongly correlated with age, as were the colors of the ventral varnish. In particular, rubification appears to be more useful than soils for distinguishing relative ages among Holocene surfaces. Humidity sensors indicated that the undersides of clasts condensed moisture nightly for a period of several days to over a week after each rain. These frequent wet-dry cycles may be responsible for the rapid development of clast rubification on Holocene surfaces. © 2003 University of Washington. Published by Elsevier Inc. All rights reserved. Keywords: Rubification; Desert varnish; Rock varnish; Desert pavement; Soil development; Surface exposure age; Quaternary dating techniques; Desert processes; Mojave Desert; Garlock fault Introduction Quaternary dating techniques that can be applied to desert surfaces are needed for a variety of geologic and geomorphic studies. Desert surfaces are often used as mark- ers to measure deformation in neotectonic studies in desert areas (e.g., Keller et al., 1982; Hooke, 1972; Brogan, 1991). The measurement of rates of neotectonic activity is thus dependent on age control for desert surfaces. Similarly, age control on desert surfaces is needed to constrain the timing of aggradational and incisional events in desert alluvial fan systems (e.g., Bull, 1991; Hooke and Dorn, 1992; Wells et al., 1987). Although a number of dating techniques may be applied to desert surfaces and their substrates, most of them have limitations, and so the study of potential new dating tech- niques may be worthwhile. Suitable organic matter for ra- diocarbon dating is generally sparse in arid regions. Ther- mally or optically stimulated luminescence (TL, OSL) has been used in desert regions to date the time since sediment grains were last exposed to light (e.g., Forman et al., 1989; Porat et al., 1996). While this is a powerful tool, it is not applicable to sediments that were not adequately exposed to sunlight during transport (e.g., debris flows or sediment deposited at night). Cosmogenic isotopes may prove useful * Corresponding author. Department of Geological Sciences, Califor- nia State University, San Bernardino, 5500 University Parkway, San Ber- nardino, CA 92407-2397. Fax: +1-909-880-7005. E-mail address: smcgill@csusb.edu (S.F. McGill). R Available online at www.sciencedirect.com Quaternary Research 60 (2003) 377–393 www.elsevier.com/locate/yqres 0033-5894/$ – see front matter © 2003 University of Washington. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.yqres.2003.08.002