Blind testing of rock varnish microstratigraphy as a chronometric indicator: results on late Quaternary lava flows in the Mojave Desert, California

Rock varnish is a manganiferous dark coating ubiquitous in desert landscapes. To test the validity of varnish microstratigraphy as a chronometric indicator, varnish samples were collected from radiometrically dated and undated late Quaternary lava flows in Amboy, Cima, and Pisgah volcanic fields (AVF, CVF, PVF) in the Mojave Desert of California, western United States. Varnish microstratigraphies show a replicable layering sequence that appears to record regional climate changes that likely correspond in time to the Younger Dryas and Heinrich events in the North Atlantic region. Microstratigraphic patterns on these volcanic fields match patterns found in varnishes from other western US sites with available radiometric age constraints. Based on this regional chronology, varnishes from the A flow, H flow, and a stone pavement surface in the Cima volcanic field were estimated to be 16.5-24, 74-85, and 74-85 ka, respectively; these ages are consistent with previously published cosmogenic He-3 ages of 18-20, 72-74, and 80-85 ka for these geomorphic surfaces. Varnishes from the I flow at Cima yielded a puzzling age estimate of 39 ka, which is consistent with an older He-3 age of 37 +/- 6 ka reported for the I flow, but inconsistent with a younger He-3 age of 31 +/- 7 ka and a cosmogenic Cl-36 age of 27 +/- 1.3 ka for the same flow. Reinterpretation of the original varnish age data, with knowledge of then available field mapping results of the I flow, suggests that the I cone is polycyclic and different flow units were probably unintentionally sampled in the field. The revised varnish ages of 30 and 39 ka. for the I flow thus may be in good agreement with their corresponding He-3 and Cl-36 ages. In a blind test of the method, varnishes from the Phase I flow at Pisgah, an unnamed flow (called here the I' flow) at Cima, and the Amboy flow were estimated to be 24-30, 46-60, and 74-85 ka, respectively; these ages agree well with Cl-36 ages of 22.5 +/- 1.3, 46 +/- 2, and 79 +/- 5 ka reported for the same flows by Phillips [Geomorphology (2002).]. These test results provide convincing evidence that varnish microstratigraphy, once radiometrically calibrated, can be used as a valid dating tool to estimate surface exposure ages of desert landforms in the western US drylands. (C) 2002 Elsevier Science B.V. All rights reserved.