PLANTS OF THE MIDDLE MIOCENE COLUMBIA RIVER BASALTS IN CENTRAL WASHINGTON STATE: THE PERMINERALIZED "BOG" FLORA AND PETRIFIED WOODS FROM YAKIMA CANYON AND GINKGO PETRIFIED FOREST

Middle Miocene plant assemblages of central Washington include both petrified woods and the "bog" cherts containing anatomically preserved seeds, fruits, stems, wood and cones. A number of plants known from the petrified woods of Yakima Canyon and the nearby Ginkgo Petrified Forest at Vantage are also recognized in the chert matrix. The chert flora occurs within interbeds of the Museum Flow Package of the Sentinel Bluff unit of the Grande Ronde Basalt (around 15.6 million years old), and the Vantage woods are somewhat younger at 15.3-15.6 MY, occurring just above the uppermost flow of the Grande Ronde Basalt and within the Frenchman Springs Member of the Wanapum Basalt. Plants common to both assemblages include bald cypress (cf. Taxodium), sweet gum (Liquidambar), oaks (Quercus), sycamores (Platanus), and tupelo gum (Nyssa). Other taxa are currently known only from the chert (pines, grapes, Rhamnaceae) or from petrified wood (Ginkgo, elms, and maples). These differences in floristic composition may be partly preservational in nature or reflect the sampling of different plant habitats.