Miocene-Pliocene glaciations in southern Alaska

Abstract

Interbedded till, fluvial units, and lava flows, tilted and faulted into discrete blocks, form several foothills flanking the upper White River valley in the Wrangell Mts. One block has 12 tillites; others exhibit from one to five. K-Ar dates on the interbedded lavas show many of the tillites to be 9 to 10 m.y. old; one is 3.6 m.y. old; and two are between 8.8 and 2.7 m.y. old. At least four Pleistocene till sheets overlie the tillite sections; K-Ar dates show all four postdate 2.7 m.y., and radiocarbon dates show the youngest corresponds in part to Kluane glaciation. Two of the other three are more than 47,000 years old. The record from southern Alaska and elsewhere of continuous glaciation through a large part of the Cenozoic emphasizes that a definition of the Pliocene-Pleistocene boundary based on climatic cooling and glaciation no longer is feasible.