Asymmetric Hillslope Retreat Revealed from Talus Flatirons on Rock Peak, San Tan Mountains, Arizona, United States: Assessing Caprock Lithology Control on Landscape Evolution

Abstract

Talus flatirons (TFs) are morphostratigraphic markers of prior talus deposition that are now disconnected from the active hillslope. Three generations of TFs (TF1, TF2, TF3) exist flanking a Sonoran Desert inselberg, Rock Peak, in a welded tuff caprocks-controlled landscape bounded by pediments. TFs at Rock Peak enable estimation of slope retreat rates through the application of cosmogenic 10Be, optically stimulated luminescence dating, and catchment-wide denudation rates (CWDR). We estimate disconnection of TF1 on Rock Peak at 88.9 ± 7.8 ka (northern slope) and 29.1 ± 2.5 ka (southern slope). Rates of hillslope retreat measure between 311.6 mm·ka？1 (northern slope) and 728.5 mm·ka？1 (southern slope). Asymmetry in retreat rates is consistent with CWDR, with southern slopes denuding ∼1.5 times faster. The asymmetry is interpreted as the result of the southward structural dip of strata present (>10°). Denudation rates on the summit of Rock Peak (54.3 ± 19.4 mm·ka？1 welded tuff; 111.2 ± 15.3 mm·ka？1 sandstone conglomerate) support interpretation that removal of welded tuff caprock accelerates denudation of this landscape and amplifies the impact of the structural dip. Given this, we interpret that Rock Peak will evolve into a rounded residual hill as pediments flanking the inselberg lengthen through time, similar to landforms observed in the surrounding landscape where the welded tuff and underlying sedimentary caprocks are no longer present. Using the range of slope retreat rates from Rock Peak, we provide a first estimate for the length of time necessary for pediments to form via hillslope retreat in the Sonoran Desert. Key Words: caprock, landscape evolution, pediment association, talus flatiron, 10Be exposure dating. ？ 2019, ？ 2019 by American Association of Geographers.