A DECIDUOUS FOREST POLLEN RECORD FROM THE WILKES FORMATION (MIOCENE) OF SOUTHWESTERN WASHINGTON

The Wilkes Formation along Salmon Creek near Toledo, Washington, exposes a sedimentary section 19 meters in thickness [G. Mustoe's section 1571 (2001)]; the site shows silty-clay layers, some with peat and scattered plant fragments near the base, and tephra, silty clay and siderite zone in the upper half of the section. The formation unconformably overlies volcanic sediments that A.E.Roberts (1958, USGS Bull. 1062) considered of middle Miocene age. Ten sediment samples prepared for pollen analysis yield a well-preserved and diverse pollen flora representing a deciduous forest assemblage rich in woody dicots (16 genera) and a few gymnosperms (5 genera). Two samples from a nearby outcrop gave similar results. R.W. Brown in 1958 identified 9 megafossil plant genera and 7 species from the Wilkes Formation, and correlated the flora with the late Miocene Latah and Mascall paleofloras of eastern Washington and Oregon. Our Wilkes pollen data corroborates some of Brown's megafossil genera, and also shows similarity with late Miocene assemblages of the Pacific Northwest based on the pollen genera we have identified. Gymnosperm pollen includes members of the pine family (Pinus, Abies, Picea, Cedrustypes), and a few grains of the Podocarpaceae (Podocarpus and Dacrydium). The Cupressaceae pollen types (TCT) are dominant (~33-66% of the count) and may in part represent the leaf identifications of Taxodium and Torreya). The angiosperm pollen counts show an abundance of Nyssa toward the base of the section. Pollen of Betulaceae, Juglandaceae, Fagaceae and occasional grains of Liquidambar, Tilia and Weigelia are present. Asian genera include Eucommia, Pterocarya, Podocarpus and Dacrydium. Small percentages of herb pollen include grains of Asteraceae, Umbelliferae, Poaceae, and Nuphar. The pollen of Podocarpaceae are some of the same pollen taxa we have reported from the Miocene of the Homer and Suntrana areas of Alaska. The mixed deciduous forest flora of the Wilkes suggests a summer-wet climate similar to that of Virginia or central Korea today. The similarity of this flora with late Miocene floras published from eastern Washington suggests that the Cascades rain shadow did not yet exist.