COSMOGENIC EXPOSURE AGES OF DESERT PAVEMENTS: WHAT ARE THEY REALLY TELLING US?

Desert pavements are common surface features in arid environments. Their origin has been long debated, but in the past two decades a prevailing hypothesis has emerged of stone clast surfaces that are formed and maintained at the air-earth interface and inflated through time via air fall accretionary pedogenesis. Evidence for this mechanism stemmed from both soil studies and exposure age dating. Exposure ages of desert pavement samples on several lava flows in southern California were nearly identical to radiometric ages of the flows, suggesting uninterrupted exposure at the surface. Other research has shown that pavement continuity is closely tied to vegetative abundance and that vegetation shifts due to climate change play a pivotal role in pavement destruction and formation. This research argues that all but the lowest elevation pavements were disrupted by down slope vegetative shifts during the LGM. 3He exposure ages of desert pavements from two locations having differing elevations and source materials (Whitmore Cascade-basalt flow, 750-1120m, Western Grand Canyon National Park, USA and Johnson Mesa-debris flow deposit, 1710-1780m, Capitol Reef National Park, USA) demonstrate two end members of pavement exposure age interpretations. In the Grand Canyon, exposure ages of desert pavements on the Whitmore Cascade appear to be slightly older than exposure ages of primary flow surfaces. This suggest that the flow surfaces have been eroded and that in this case, desert pavements were likely not disrupted by vegetation during the LGM and may be the best indicator of the "true" exposure age of this surface. On Johnson Mesa, however, desert pavement exposure ages are all 30-98ka younger than exposure ages of large boulder clasts within the debris flow deposit which have an average 3He exposure age of 190±12ka (zero erosion, n=3). The average desert pavement 3He exposure age on Johnson Mesa is 127±9ka (zero erosion, n=9), which coincides with the penultimate interglacial period, suggesting a link between pavement formation and climate. This data demonstrates that not all pavements record the exposure age of the surface they are forming on and suggests that climate strongly controls the timing of pavement inception at higher elevations.