DETERMINATION OF HILLSLOPE EROSION RATES DURING THE LATE HOLOCENE BASED ON COSMOGENIC SURFACE AGE DATING OF HOODOOS, NORTHEASTERN ARIZONA

On the semiarid Colorado Plateau, NE Arizona, landscapes associated with rock types such as weakly cemented sandstones are sensitive to climate changes on millennial to decadal scales. These climate changes, hypothesized to episodically induce widespread aggradation, must have also affected the closely related, but poorly understood, processes of weathering and plant community establishment on basin hill slopes. Recent studies on the Black Mesa escarpment west of Chinle, Arizona, help elucidate the nature of these processes and impacts on slope erosion rates. Dendrogeomorphic and other studies of the long-lived pinons that thrive on north-facing slopes and that exhibit slope form consistent with diffusive slope transport show that average weathering and erosion rates during the last 450 yr rates have been rapid (average = 1.8 mm/yr). Determining erosion rates for much steeper and more spatially extensive treeless slopes that dominate these landscapes is problematical; however, the existence of more strongly cemented concretions that are comparatively resistant to weathering has locally promoted the development of “hoodoos”. Because surface erosion rates of hoodoo-capping concretions must be very slow compared to those of adjacent, generally steep slopes, we surmised that the persistence of hoodoo caps should enable accumulation of a relatively large inventory of cosmogenic radionuclides, providing a new strategy for calculating overall erosion rates of these slopes. We obtained surface ages from caps of varying heights (1-8m) that range from 0.45 kyr to 2.38 kyr. The ages are consistent with several independent geomorphic lines of evidence that constrain their relative age. Calculated vertical incision rates average 2-3 mm/yr (range: 1 -7 mm/yr) on the associated slopes. Comparison of hoodoo- vs. pinon-dominated slopes shows that, despite different aspect, weathering, vegetation community and form, average erosion rates are similar. In contrast, the observed and inferred processes of diffusive relative to advective transport and erosion rates on temporally shorter time scales (yrs to decades) on these slope forms are generally very different. We attribute these differences to spatial and temporal variation of soils and vegetation (and runoff-infiltration) on these slopes that are most strongly linked to changes in aspect and climate.