CRETACEOUS POLLEN IN PLIOCENE ROCKS - IMPLICATIONS FOR PLIOCENE CLIMATE IN THE SOUTHWESTERN UNITED-STATES

Pliocene rocks of the Imperial and Palm Spring Formations in southern California contain reworked Cretaceous pollen that helps determine the timing of erosion of Cretaceous rocks on the Colorado Plateau. The st ratigraphic distribution of reworked pollen in the Imperial and Palm S pring Formations suggests that erosion of Cretaceous rocks in the sout hern part of the Colorado Plateau began by 4.5 Ma. Erosion of Cretaceo us rocks in the northern part of the plateau began at 3.9 Ma. This ero sional history indicates that rapid and extensive erosion of the Color ado Plateau occurred during the Pliocene and supports the hypothesis t hat much of the Grand Canyon was cut during the Pliocene, rather than earlier in the Tertiary. Rapid erosion and transport from the Colorado Plateau require the climate in that region during the Pliocene to hav e been significantly wetter than it is today.