QUATERNARY EROSION AND COSMIC-RAY-EXPOSURE HISTORY DERIVED FROM BE-10AND AL-26 PRODUCED INSITU - AN EXAMPLE FROM PAJARITO PLATEAU, VALLES CALDERA REGION

Be-10 and Al-26 concentrations measured by accelerator mass spectromet ry in 20 samples Of quartz separated from rhyolitic volcanic ash-flow tuffs collected on the Pajarito plateau of the Valles caldera, New Mex ico, were used to model time-integrated erosion rates and cosmic-ray-e xposure histories. The model erosion rates determined for different st ratigraphic units within the Tshirege member, upper Bandelier Tuff, va ry from 0.1 cm/ka for the resistant unit to 1.1 cm/ka for the softer u nit. Intermediate erosion rates are thought to indicate earlier age co ver by post-tuff lithologies of different hardness. The geographic dis tribution of these intermediate rates allows an approximate determinat ion of the extent of material now gone (stratigraphic ghosts). Periods of burial can be determined from Al-26/Be-10 ratios. For the Pajarito plateau, burial most likely resulted from cover by soil and sediment held in place by vegetation. Our data allow us to model the duration a nd extent of thick forest cover, which today exists only