A rich leaf and seed flora as well as diatoms and palynomorphs have been recovered from the Late Miocene lacustrine sediments at Pickett Creek. The locality is at the southern margin of the western Snake River Plain, 13.3 miles south of Murphy, Owyhee County, Idaho. The sediments have been assigned to the Poison Creek Formation of the Idaho Group (approx. 11 my). The accessible fossil beds consist of 3 m lacustrine sediments (site I), overlaid by 3 cm volcanic ash, 3.7 m pure diatomite, and 15 m of lacustrine (site II) and fluvial sediments. They contain a rich leaf flora of more than 50 species and fruits, including Acer, Ostrya, Salix, Fraxinus, Quercus , Pterocarya and Fabaceae spp. The most abundant taxa are Quercus prelobata (50%), Q. columbiana (9%), 5 Salix spp. (8.5%) and 5 Fabaceae spp. (5.5%). The composition of the flora suggests a Temperate Deciduous Broad-leaved Forest as is found in eastern China today. A high proportion of xeric leaf forms (Lithocarpus sp., Quercus hannibali, Dodonaea sp., Robinia sp. indicates a dry climate on the slopes above the lake. The statistical macrofossil count was done on a surface of 60 x 100 cm, through a total depth of 293 cm. Pollen samples were taken every 10 cm. Pollen analysis confirmed most of the leaf families and added more evidence for taxa from higher elevation and dry sites (Abies, Pinus, Amarantaceae, Chenopodaceae). The sediments contain a rich and variable diatom flora, including more than 50 known forms and some new species. Future investigations may give insight into the lake's history and may help to understand the origin of the massive deposition of diatomite above the site I fossil layers.

Key words: diatoms, fossil flora, Miocene, paleoclimate, palynomorphs, Pickett Creek