Abstract

Rare outcrops of Miocene-Pliocene buried hillslopes and colluvium present in proximal basin-fill exposures in the arid southwestern United States, in settings free of tectonic influences, reveal hillslope-sediment production and delivery to piedmonts that vary as a function of climate change. A petrologic weathering inde for sand, clay-mineral data, and stratigraphic relations indicates that episodes of greater production and delivery of colluvium from hillslopes coincide with more intense chemical weathering and thus greater effective moisture. Strata recording limited colluviation, less intense weathering, and piedmonts dominated by eolian processes very likely correspond to drier conditions. The mode of climate-driven hillslope and piedmont landscape responses evident in these older stratigraphic records corroborates the conceptual model developed by Quaternary researchers for arid environments—even though climate change was operating at higher amplitudes and different frequencies in the Quaternary. This evidence of long-term continuity in the mode of landscape response to climate change provides a new level of support for using the geologic record to understand the effects of future climate change.

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