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Clast size controls and longevity of Pleistocene desert pavements



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Clast size controls and longevity of Pleistocene desert pavements at Lathrop Wells and Red Cone volcanoes southern Nevada Greg A Valentine Charles D Harrington Earth and Environmental Sciences Division M S D462 Los Alamos National Laboratory Los Alamos New Mexico 87545 USA ABSTRACT Formation of desert pavement and accretionary soils are intimately linked in arid environments Well sorted fallout scoria lapilli at Lathrop Wells 75 80 ka and Red Cone ca 1 Ma volcanoes southern Nevada formed an excellent parent material for pavement allowing infiltration of eolian silt and fine sand that first clogged the pore space of underlying tephra and then aggraded and developed vesicular A Av horizons Variations in original pyroclast sizes provide insight into minimum and maximum clast sizes that promote pavement and soil formation pavement becomes ineffective when clasts can saltate under the strongest winds while clasts larger than coarse lapilli are unable to form an interlocking pavement that promotes silt accumulation necessary for Av development Contrary to predictions that all pavements above altitudes of 400 m would have been reset in their development after late Pleistocene vegetation advances the soils and pavements show clear differences in maturity between the two volcanoes This indicates that either the pavements and or soils develop slowly over many tens of thousands of years and then are very stable or if they are disrupted by vegetation advances subsequent pavements are re established with successively more mature characteristics Keywords desert pavement soil scoria eolian geomorphology INTRODUCTION An important surface process in deserts is the concurrent formation of accretionary eolian soil capped by desert pavement formed of a one clast thick layer of closely spaced pebbles or lapilli A widely accepted model involves the deposition of eolian silt and fine sand around coarser clasts derived from weathering of initial rock surfaces or larger clasts or by



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