Statistically indistinguishable U-Pb age spectra for detrital zircon populations from the quartzose petrofacies of the fluvial Lower Cretaceous Bisbee Group (southeastern Arizona) and from quartzose eolianites of the Middle to Upper Jurassic San Rafael Group (eastern Colorado Plateau) support the inference that quartzose detritus was reworked into the Bisbee basin of the border rift belt from older eolianites exposed along the uplifted rift shoulder to the north. The comparative data indicate that age spectra of detrital zircons can be reproduced faithfully during sediment recycling, and the possibility of zircon recycling should be considered for many provenance interpretations based on U-Pb ages of detrital zircons. Analogous relations between U-Pb age spectra of detrital zircons in the Lower Cretaceous basal McCoy Mountains Formation (California) and in Lower to Middle Jurassic eolianites of the western Colorado Plateau to the north suggest similar reworking of detritus from sedimentary cover of the rift shoulder into the keel of the rift belt farther west.

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