THE MIO-PLIOCENE MASHEL FLORA (PIERCE CO., WA) AND THE LATE PERSISTENCE OF “TERTIARY RELICT” TAXA

In the Paleocene and Eocene, many Northern Hemisphere plants had circumboreal distribution, due to warm, equable climate and persistent land bridges connecting North America to East Asia and Europe. Cooling and drying through the Cenozoic resulted in the incremental modernization of flora in western North America, including the loss of most warm-adapted hardwood taxa, so-called “Tertiary relicts”, sometime in the Neogene. In the Pacific Northwest, our understanding of the modernization of the flora is limited by a paucity of fossil floras from the Late Miocene and Pliocene, especially on the windward side of the Cascade Range. Multiple localities of the Mashel Formation (Pierce County, WA), with maximum depositional ages of 4.1 and 6.0 Ma, contain pollen and macrofossil remains, filling an important knowledge gap. We interpret their depositional environment as proximally volcaniclastic alluvium deposited on flood plains with lakes and streams, all within a piedmont setting. Pollen spectra are dominated by early-successional riparian taxa, including Salix (willow), Alnus (alder), and Pinus (pine) in response to Cascades volcanic disturbance. The late successional component consists of upland/montane conifers, although Pseudotsuga (Douglas-fir) is notably absent. The fossil palynoflora contains “Tertiary relicts” Ilex (holly), Liquidambar (sweetgum), Pterocarya (wingnut), and Ulmus/Zelkova (elm) today native to East Asia and eastern North America. Results are similar to the 6.1 Ma Wilkes flora (Lewis Co., WA) and the 11.4 Ma Vasa Park flora (King Co., WA) and which include similar hardwood taxa in limited abundances. Recent paleobotanical data suggest that the modernization of Pacific Northwest flora was a long process, still incomplete in the Mio-Pliocene.